ODS 124 - revisited for the 21C

[didit]

I'm having trouble viewing the latest update in the first page.

Here's the latest FET Boost/Pedal schematic: ODS FET FS 1.3.pdf

The chassis FET-activated LED now looks correct.

Best .. Ian

[didit]

On a sidenote, I put the relay supply board together last night. This is the Tube Town kit... ...This is what I'm calling B+6 in the schematics I'm drawing.

I'd recommend that you not call the relay supply B+6. That designation should be assigned to the FET filter cap node, which is fed from B+5 on the main power rail.

Agreed. Was going to comment earlier but time crunched.

Call it something unrelated to the HT power string. B+Relay works. I chose the label +VRelay. However, also somewhat sloppily called HT input B+1 and didn't label the node after the dropping resistor at the filter node.

Best .. Ian

[Stephen1966]

On a sidenote, I put the relay supply board together last night. This is the Tube Town kit... ...This is what I'm calling B+6 in the schematics I'm drawing.

I'd recommend that you not call the relay supply B+6. That designation should be assigned to the FET filter cap node, which is fed from B+5 on the main power rail.

Agreed. Was going to comment earlier but time crunched.

Call it something unrelated to the HT power string. B+Relay works. I chose the label +VRelay. However, also somewhat sloppily called HT input B+1 and didn't label the node after the dropping resistor at the filter node.

Best .. Ian

Thanks for the suggestions, I'll implement that with the next version. Final, I hope

[Stephen1966]

[Edit: Houston, we've had a problem! A heavy-handed approach to frequently editing the first post and inserting then replacing files for download has resulted in a broken first post in which the contents are all jumbled up and it doesn't display what is shown in the editor here. To avoid any possible future problems, I'm going to use the first post only to post files for download. To avoid a string of duplicate posts I'm saving a copy of the first post here so that the references are preserved. From now on, I will keep the project updated using the normal "reply" sequence of posts and that should keep the burden of editing in the first post, to a minimum. If you've already read through the following previously, my apologies.]

Greetings tubaholics,

Let's consider this, the start of my build... Before delving into the deep and arcane mysteries of the ODS #124, those of you who've been following will know of my other post, 'ODS - any advice?' https://ampgarage.com/forum/viewtopic.php?f=4&t=33582. I would like to begin with a sincere thank you to everyone who contributed to that post. Your deep knowledge and vast experience has set the standard for beginners like me and in this present post, I hope to bring your generous, wide-ranging insights to bear on the specific task of building a new amp.

It's my intention to use this thread to collate the various nuggets of information and document the build as it progresses.

The process began in July '20, and from the vague idea of building an ODS, I've progressed to the stage of knowing what I hope to achieve and I'm at the stage of drawing up the schematics and the BOM, ready for the orders.

For my money, there is no better place to start than the ODS Skyliner of 1988. It is reputed to have the best documentation of all the models, and to my imagination, the best sound.

Having discussed the viability of including a Dumbleator onboard the amp, I took a leaf from Mr Fuchs' book and studied the topology of the 124. My initial layout and schematic were found in ic-racer's post: A Collection of #124 Information (Schematics, Layout, etc.) https://ampgarage.com/forum/viewtopic.php?f=5&t=5719. The schematic (1.6) seems particularly faithful to the layout, and appears the most up-to-date and correct. I took the liberty of using this layout and schematic, and reworking them with the aim of identifying each component and of garnering a topological understanding of the amp with layers, each representing a discrete section of the circuit:

• FET Preamp

• Clean Preamp/Tone Stack

• Master Volume/FX loop

• OD Preamp

• Phase Inverter

• Power amp

• Output

• Footswitch (ampside)

• Power Section

• Footswitch pedal

Dumble ODS with layers-optimised.pdf

The schematic was treated differently, with minor alterations to reflect some updates for my amp and with each component being given a matching RefDes for ease of identification. [Edit: the 1.6 version of the schematic has been updated with labels for the two inputs of the LTP of the PI.]

#124 Schematic_1988_Skyliner [marked up_2].pdf

The goal is to make this circuit the base for an amp which includes both a buffered FX loop and reverb onboard, using more up-to-date circuit refinements and components. Some of these have been highlighted in the spreadsheet I'm using to keep track of this project and source alternatives. Things like the convenience outlet, the ground switch and death cap will go. Martin Manning's eyelet boards also include a redesign of the power section and deal with the problem of sourcing the harder to get hold of can caps. Also, the relay power section can be reformed with more compact kit components which I will detail as I get to them.

I may find there are mistakes in the components marked either their values or their purpose, in which case, I will update this first post with the latest files. This spreadsheet is derived from the BOM posted by Joost in collected 124 information linked above. My list largely void of any order numbers - I figure if anyone wants to use this for themselves, they will probably want to mix and match the components for their own purposes. If you find a mistake though, please let me know and I will correct it.

Every part is accounted for, and I've added descriptions of its function. You'll also find parts numbers and in many cases a link to the part in a store...

ODS components.xlsx

POWER SECTION

Here then is the schematic for the revised power section.

ODS Main Power Supply(2).pdf

As I mentioned previously, this incorporates Martin Manning's design of the circuit https://ampgarage.com/forum/viewtopic.php?f=5&t=29630 which, in turn, looks a lot like the power section of the #183. Accordingly, there may be some value in increasing the 470R resistors from Martin's recommended 1/2W to 1W or even 2W. A larger power rating could possibly reduce the chance of contact noise. Wirewound resistors, here and elsewhere in the power section may mitigate contact noise entirely. I am assuming carbon film resistors were used in the 183 and Martin's design. Aiken amps pages are clear and informative: https://aikenamps.com/index.php/resisto ... -it-matter

As promised, the death cap and ground switch have been omitted. Polarity doesn't appear to be an issue with European grid supplies. Also gone, is the convenience outlet. The power switch and fuse (2A slow blow for 240VAC) can be incorporated in an AC power module such the Ohmite AF-C10-SD which is filtered as well. The IEC-C13 (with ground) seems pretty standard.

The power transformer, a subject of some debate in the first post I made, is the Hammond 290FEX. The specs are very similar to Fender issue 022756 (at least according to the Classic Tone version of it). Likewise, the choke is also a Hammond, the 194B which at 4H and 90mA is close to the 022699 specified in the original.

The 290FEX requires the heater centre tap resistors having no CT of its own so there is bound to be some fidelity to the original circuit here.

I did consider the 6.5V supply voltage of the heater windings but if the 6L6's draw 900mA each and the 2025's draw 300mA the voltage drop of 0.2V would require a resistor of 0.0416 Ohms. Blencowe on 'Heater / Filament Supplies http://www.valvewizard.co.uk/heater.html mentions a +/-10% nominal value as acceptable. So on balance, I don't think I will bother with a resistor here.

For a couple of days, I also considered the possibility of a DC heater supply using a full-wave rectifier though when I look at the figures for a power factor loss (Blencowe again) I don't want to overload the transformer. Even with the addition of the FX loop tube I am within the rated 5.5A at 4.8A. With a power factor loss of 0.5 that would shoot up to 9.6A for DC. Reading around, it seems DC in this equipment is hardly necessary so I'm not including it.

The power node (B+1 - B+5) voltages are entirely speculative at this stage, derived as they were from the observed voltage measurements taken by some of the other builders in this forum. At some point, I will try modelling the circuit in Duncan's PSU designer II and see what numbers it throws out. For the moment though, I am guessing these voltages are somewhere in the ballpark.


OD, MASTER AND FX LOOP SECTIONS

Is [the onboard Dumbleator] possible? Sure!

There are a number of members who have successfully incorporated the D-lator into their amps but as Tony (talbany) pointed out, it comes with a few caveats. To paraphrase Tony:

• The CF impedance load alters the frequency response

• It can also generate 2nd order harmonics as well as soft clipping when driven hard

• The added gain stage (in series) reverses the phase of the preamp

A Pandora's box of features in short. I'm immensely grateful to Tony for pointing this out to me now, before I go to all the trouble and expense of finding these things out for myself, six months down the line. I haven't given up on the idea entirely, I will as Marcus and Tony suggested build a standalone D-lator first. And then, armed with oscilloscope and spectrum analyzer I will study the problem further. Let me echo Tony when I say, don't let this deter you if you want to try it for yourself. It seems to work out okay for a few builders here. Personally, I see this as a great learning opportunity though, and so it makes sense to build it as a standalone unit first and this way, I can focus on the issues around the signal without having to factor in the physical issues of what goes where.

While researching this I saw several versions of the FX loop but I couldn't find a schematic of Normster's design which comes highly regarded. The layered pdf of the layout has been downloaded a number of times in just a few days so I include it here again, updated to correspond with the new schematic.

Normster - 97_ODS_HRM_Loop[marked up].pdf

I've removed the 'ODS OD Master FX loop' sketches I made since as erwin-ve kindly pointed out, I had entertained entirely the wrong idea about the parallel RC network just after the master in the 124. I've fixed this now and put Normster's original design alongside my own ideas on a single sheet. Our circuits are essentially the same except I include a bypass switch and control on the front face of the amp (no reaching around the back to find the new Master!) Mine also includes a couple of options for the master, with TYPE A utilising the existing master in the loop circuit, and Type B swapping out the master for a fixed voltage divider in its place. The schematic is (probably) self-explanatory. I still can't remember who came up with the idea of a fixed voltage divider in place of the master but I'm grateful to them for it – it seems like a neat idea and it's probably what I would have done.

The rest of this section was musing and mulling but I will keep the relevant outside references here. You don't need a blow-by-blow account of what's going on in my head. Or maybe you do, in which case, buy me a beer and I'll talk all night.
Here's the schematic:

Normster's FX loop.pdf

One last thought on this… the loop as I designed it with bypass switching, seems to require its own lead to R43 (rather C26) next to the presence pot which constitutes a second input to the LTP (the feedback input); similar to that from the OD section. It is included in the schematic for my loop but its position is entirely speculative. The 124's FB is grounded to the chassis through the SEND and RETURN jacks but it is also reacting with the cathodes of the OD tube when this channel is active and of course carrying a phased signal from the OT. Something to bear in mind... It might be ok just to hook it up to the Loop jacks.

The parallel RC network after the master…

...There is a very old thread discussing this https://tubeamparchive.com/viewtopic.ph ... 4&start=60. Structo explained its purpose, thus: "The way somebody told me how it works is: that resistor and cap helps with the loading on the PI. The cap across the resistor is mainly used to keep the low end focused after the signal is amplified in the loop. It also lowers the impedance helping with the loading on the PI. It works great w/.022 PI input cap." … There is a handy impedance calculator for playing with these values, here: https://keisan.casio.com/exec/system/1258032649

The voltages are assumed, but I gather they are correct…

FET SET UP

Martin Manning and Charlie Wilson have recently been updating the FET circuit and this includes several options for setting up a high Vd with centre bias for clean boost or a low Vd with FET distortion which is closer to the original circuit.
The pages are here:

FET Questions=Martin?
https://ampgarage.com/forum/viewtopic.php?f=4&t=34108

Method to set FET bias and supply voltage
https://ampgarage.com/forum/viewtopic.php?f=5&t=18389

CLEAN PREAMP/TONE STACK

Here is my redraft of the schematic:

ODS_Clean Preamp Tone Stack 1.4.pdf

This is a faithful (as faithful as I can get it) rendering of the 124 layout along with a study of the photos (linked later in the post). My aims are to build a version of this for myself, so you will notice that it is different in a couple of places. The logical layout of the traces has been a challenge but fruitful, as (among other things) I now understand how the switching in particular affects the dynamic of the signal path. It's also clear that when I draw up the final draft of the whole amp, I will need to re-designate the RefDes of components as the relays and the pedal are integral to the tonestack. Because of redesign for modern components and mods, there will be a top-to-tail restructuring of the logic of component numbering, but for now I haven't changed anything. All the RefDes of components still matches the BOM linked above.

I present this schematic as a near faithful rendering of the original circuit but I will post another, in which the input jacks are on a NOR/FET relay and Manual/Pedal switch. As I studied this circuit, it became clear to me that I couldn't have both jacks active, which would have been handy I guess with a stereo output guitar. But what would be the point? I don't need two jacks with the Constellation: I can route both piezo and mag outputs through a single jack. It's one less cable!

Kudos to whoever drew the 1.6 schematic. It's a marvelous rendering of the entire amp but if I understand the workflow on that, Tony and Martin started with the photos, then a schematic with a start by odourboy [a fragrant fellow - no doubt] and then - finally - the layout. As I studied the photos though, I found a high(er) degree of correspondence between the layout and the photos, such that I have a high(est) degree of confidence that the layout is the most up-to-date and correct. Wire colours are often changed, but the layout, is a real work of art with the smallest details included from the photos.

I've started working through the photos and this week, its been capacitors. Some, are just too obscured or out of focus to be clear, but after sifting through the posts about the various builds of the 124 I think I have found a consensus on those elements. These are marked in my schematic. I had my Zen moment with the relays but I think I figured it out. I've left notes in the schematic for my own benefit more than anyone else's but here's hoping it finds some use with you guys.

Edit [14/03/2021]: the schematic has been updated to clarify the GND on the NOR jack and includes the BRIGHT switch #183 mod mentioned later in the post.
Edit [20/03/2021]: Updated to clarify Input grounds and switching on PAB and OD circuits.

FET BOOST

Here is the schematic for the FET on a relay to the pedal.

ODS FET FS 1.1.pdf

I called on Martin Manning's verified design in his post: Footswitch-able FET Boost https://ampgarage.com/forum/viewtopic.php?f=5&t=31084. Martin's layout is great but I prefer a toggle switch over the push/pull (or push/push) switching pots. Martin's switching pot appears to be operating as a Manual or Pedal switch as in the OD and PAB switches on the back panel of the amp. I would therefore keep the Manual or Pedal switch on the back panel to keep them all together. Martin's set up though, appears to only allow switching between the NOR and FET via the footswitch, and switching the PED/MAN switch in the Manual position, appears to provide an open circuit back to the negative pole on the relay power board. The circuit would appear to be only closed by the Footswitch. The G5V series 2 non-latching relays are nc (normally closed) when there is no ground reference (no polarity) so it would mean - I suspect - that the FET circuit would be engaged in the Manual toggle position. So, I have placed a circuit breaking switch just before the relay so that I can cut it's power and send the relay into the normally closed (NOR channel open) position. And, for visibility and good measure, I have added an indicator LED just after this switch so that when the power to the relay is open (and the FET engaged), the light indicates it is on. The complication is that if the footswitch is in the off position, no light will display but putting the Manual/Pedal toggle in the Manual position, the circuit will be open. The enable switch is perhaps unnecessary, the front panel switch would be performing a similar function, just from a different place in the circuit. I might simplify this so that the GND wires are static as described in the PEDAL DETAIL. [Watch this space for version 1.2 ]

I am using TubeTown's 12V relay kit for the relays which has 166mA of current available. Using the low energy LEDs I calculate, I can get away with 480R resistors for all the LEDs for each, and the relays, if I recall correctly, pull around 30mA which appears well within the limits of the power board. The FET indicator LED pulls its current almost directly from the power board and it doesn't have the filtering capacity of the original relay power board so for this reason, it is probably a good idea to keep the LED indicator net's GND seperate from the input GNDs to avoid any coupling: the LED indicator grounds back at GND 5 on the map in the layout, and the input is tied to GND 1. I have designated the 12V supply coming off the board B+6. I've also used DPDT switches for redundancy along the lines of the original 124. For reference, I've included a schematic sketch of the layout indicated in Martin's PEDAL DETAIL from the post mentioned above.

Here's the updated and simplified schematic:

ODS FET FS 1.2.pdf

So, to recap, I reconsidered the idea of the extra front panel switch with Martin's Manual/Pedal switch on the back panel. Martin was right of course, it isn't especially useful to do it this way and there's no need to reinvent the wheel. The one small change I would like to implement though is to have the Manual/Pedal switch as a separate, more easily visible toggle switch and to augment this with an LED indicator to show when the FET circuit is engaged. I also like the interoperability of the FET and OD with Martin's ENABLE switch on the pedal, though of course, that might be omitted and replaced by an even simpler ground scheme with all the foot switches connected to GND at pin 1, the 12V supply and any LED net connecting to pin 2, and pin 3 being left n.c. (not connected).

Good health!
Stephen

[Stephen1966]

I'd recommend that you not call the relay supply B+6. That designation should be assigned to the FET filter cap node, which is fed from B+5 on the main power rail.

Agreed. Was going to comment earlier but time crunched.

Call it something unrelated to the HT power string. B+Relay works. I chose the label +VRelay. However, also somewhat sloppily called HT input B+1 and didn't label the node after the dropping resistor at the filter node.

Best .. Ian

Thanks for the suggestions, I'll implement that with the next version. Final, I hope

Just a quick update, I've had to rebuild the first post to include just the files for download. Many thanks to ~Phil for working the problem with me. In addition, I've updated the FET Boost schematic to correct the power designation and posted it there (Thanks again to Martin and Ian). At the moment, all the RefDes component figures are all over the place as the logic of the final circuit and its topography becomes clear. Just a reminder then, for the moment, the parts list spreadsheet still corresponds to the schematics I'm posting but like almost everything else, it will need revision and consolidation when all the mods are in place.

As always...

Good health!

Stephen

[Stephen1966]

(Deep breath!)

Here is my second draft of a reverb mod for the 124.

ODS RVB 2.0.pdf

This is implanted wholesale from heisthl's design for the Carvin X-60A to clone project https://tubeamparchive.com/viewtopic.php?f=5&t=2165 which in turn, is a derivative of the Fender Prosonic reverb, and it's the second design I considered. I mused over others including a digital reverb, kits, pedals, single tube reverbs but the one I landed on first with any intent was the Blackface reverb. Reading around though, it doesn't seem that this circuit does so well in a Dumble... muddy, and with high impedance affecting the circuits upstream and downstream, possibly because of the 3.3M mixing resistor and also, quite possibly because it is using only half a recovery tube so it suggests there may be phasing issues, overall the impression of a heavy cost on the tone of the amp.

This schematic shows my intended topology. The reverb is placed after the overdrive and Master volume and before the FX loop jacks just before the PI. Again, from reading around, the post overdrive seems to yield the better results. Also, I would route the signal to the reverb through a bypass relay so that when the reverb isn't engaged it is a straight through signal to the FX loop and PI. This extra relay will probably require the heft of Mr D's original relay power design (with the FET boost as well, I'm up to four relays and a handful of LEDs). I don't want to overtax the relay transformer when my recently completed 12V kit only puts out 166mA. Along with the extra relay, I will need to go with a 7 pin din on the back panel and pedal as well.

The design has been around a long time. Does anyone have any past or current experience of this reverb in a 124? Are there any refinements along the lines of current thinking that might be added to it? Or if anyone has any better ideas, I'm always glad to hear them.

Good health!

Stephen

[Stephen1966]

Here is the same reverb circuit with some small changes.

ODS RVB 2.1.pdf

There were a couple of corrections of my own: the upper leg of R102 is placed after the coupling cap (C62) not before; the polarity of the transformer has been marked correctly to agree with the Hammond schematic. Heisthl's (Henry's) B+3 supply is 422V, this seems close to the B+3 supply in the 124 precision power board as well. It may well take some adjustment of the load resistors to bring it into the range of voltages Henry recorded and which I've included here, either that or taking it from another HV node.

There was also a discrepancy between Henry's schematics and his layouts: R105, the resistor he marks "FROM TANK" is designated R304 in the Prosonic schematic and parts list and this is listed as a 10k CF, 1/2W, 5% resistor therein. Henry marks the part 100k in his schematic but in both the layouts from the post linked above, it is marked 10k, in agreement with Fender documentation. There are photos of the amp there as well, it looks like a four band resistor brown/black/grey/gold but that doesn't make sense. To be 10k it would need to be brown/black/orange/gold and 100k would be brown/black/yellow/gold. It isn't clear but it is probably a mistake in his schematic rather than his layouts. To be fair, there are discrepancies between the Fender schematic and parts list as well.

Tweaked - detail.jpg

Good health!

Stephen

[Stephen1966]

I've just uploaded the updated schematic of the main power supply in the first post. This includes a B+Relay circuit as per the original. After including the Reverb (a fourth relay) I decided it was safer to go with a higher current transformer. The Triad Magnetics F-105Z can be wired in series (for 12V) or parallel (6V) and in series, at 12V it has a current rating of 1A which is perfect for the relays and more LEDs and the circuit in general. So it's back to square one in a sense but this relay supply has plenty of overhead and should run cool. It stands 6omm high, so it should fit nicely into a 65mm deep chassis.

Good health!

Stephen

[talbany]

Stephen

I took a quick look at your 2.1 Pdf in the last thread and it looks like your using the input to the reverb after the overdrive. By doing this method you are sending a distorted signal into the reverb tank in OD, Is this what you want to do?
IMO a better way would be to grab a lower level cleaner signal after the tone stack use that as your input to your reverb amplifier then mix that in with the distorted signal with some kind of mixer amplifier before the master volume..This is how Dumble approached his ODSR's and has proven to sound good with the ODS circuits

Nothing against Henry although his designs posted here were before anyone knew anything about the ODSR..
Tony

[Stephen1966]

Stephen

I took a quick look at your 2.1 Pdf in the last thread and it looks like your using the input to the reverb after the overdrive. By doing this method you are sending a distorted signal into the reverb tank in OD, Is this what you want to do?
IMO a better way would be to grab a lower level cleaner signal after the tone stack use that as your input to your reverb amplifier then mix that in with the distorted signal with some kind of mixer amplifier before the master volume..This is how Dumble approached his ODSR's and has proven to sound good with the ODS circuits

Nothing against Henry although his designs posted here were before anyone knew anything about the ODSR..
Tony

Thanks Tony, I'd like to look into that. Any suggestions where to start looking for examples? Something like the SSS #002 perhaps? The send signal coming from just before the grid of the clean channel, second triode returning to the grid of the last triode of the OD?

Good health.

Stephen

[talbany]

Stephen

I took a quick look at your 2.1 Pdf in the last thread and it looks like your using the input to the reverb after the overdrive. By doing this method you are sending a distorted signal into the reverb tank in OD, Is this what you want to do?
IMO a better way would be to grab a lower level cleaner signal after the tone stack use that as your input to your reverb amplifier then mix that in with the distorted signal with some kind of mixer amplifier before the master volume..This is how Dumble approached his ODSR's and has proven to sound good with the ODS circuits

Nothing against Henry although his designs posted here were before anyone knew anything about the ODSR..
Tony

Thanks Tony, I'd like to look into that. Any suggestions where to start looking for examples? Something like the SSS #002 perhaps? The send signal coming from just before the grid of the clean channel, second triode returning to the grid of the last triode of the OD?

Good health.

Stephen

We did a whole discussion about it last week
#13 ODSR

https://ampgarage.com/forum/viewtopic.php?f=4&t=34339

Tony

[Stephen1966]

Stephen

I took a quick look at your 2.1 Pdf in the last thread and it looks like your using the input to the reverb after the overdrive. By doing this method you are sending a distorted signal into the reverb tank in OD, Is this what you want to do?
IMO a better way would be to grab a lower level cleaner signal after the tone stack use that as your input to your reverb amplifier then mix that in with the distorted signal with some kind of mixer amplifier before the master volume..This is how Dumble approached his ODSR's and has proven to sound good with the ODS circuits

Nothing against Henry although his designs posted here were before anyone knew anything about the ODSR..
Tony

Thanks Tony, I'd like to look into that. Any suggestions where to start looking for examples? Something like the SSS #002 perhaps? The send signal coming from just before the grid of the clean channel, second triode returning to the grid of the last triode of the OD?

Good health.

Stephen

We did a whole discussion about it last week
#13 ODSR

https://ampgarage.com/forum/viewtopic.php?f=4&t=34339

Tony

Really??? Of course, I was following, but I thought i had invented fire over here...

I gave some thought to what you were saying and you have a good point about sending a distorted signal into the RVB (probably not a great idea!) so I had a look again for something that offered a mixing amplifier. The problem is partly one of space, if I'm to use a standard sized chassis I can't see there being room for a three or four tube RVB. The closest thing I saw to a suitable 2 tube RVB was the High Plains Drifter circuit, but sans the filters. I would ideally like to preserve as much of the original timbre of the signal as it passes through the RVB circuit and if I have all my ducks in a row the High Plains Drifter/#005 is a take on the Tweed RVB. Someone, I can't remember who, said it was the best thing since sliced bread

And so, I know you said, "DON'T ASK," but what do you think of this...

RVB HPD-005 v2.2.pdf

I really don't think I need the filters of the SSS so the problem for me is how to calculate the resistor on the Dry portion of the signal coming from the CL2 plate of the preamp. Or even if, I should preserve this whole network of resistors and cap to ground as well. The .001 coupling cap is a given - I don't want +300VDC charging the grid of the 12AX7b. I see the 820k + 220k resistors in series in the HPD as a mixing resistor/attenuator block, but with a 100k resistor to GND as well, perhaps I also need to think of the net as a form of high pass filter? Is a high pass filter really what I want on the dry portion of the signal entering the mixing amp 12AX7??

[talbany]

Thanks Tony, I'd like to look into that. Any suggestions where to start looking for examples? Something like the SSS #002 perhaps? The send signal coming from just before the grid of the clean channel, second triode returning to the grid of the last triode of the OD?

Good health.

Stephen

We did a whole discussion about it last week
#13 ODSR

https://ampgarage.com/forum/viewtopic.php?f=4&t=34339

Tony

Really??? Of course, I was following, but I thought i had invented fire over here...

I gave some thought to what you were saying and you have a good point about sending a distorted signal into the RVB (probably not a great idea!) so I had a look again for something that offered a mixing amplifier. The problem is partly one of space, if I'm to use a standard sized chassis I can't see there being room for a three or four tube RVB. The closest thing I saw to a suitable 2 tube RVB was the High Plains Drifter circuit, but sans the filters. I would ideally like to preserve as much of the original timbre of the signal as it passes through the RVB circuit and if I have all my ducks in a row the High Plains Drifter/#005 is a take on the Tweed RVB. Someone, I can't remember who, said it was the best thing since sliced bread

And so, I know you said, "DON'T ASK," but what do you think of this...


RVB HPD-005 v2.2.pdf


I really don't think I need the filters of the SSS so the problem for me is how to calculate the resistor on the Dry portion of the signal coming from the CL2 plate of the preamp. Or even if, I should preserve this whole network of resistors and cap to ground as well. The .001 coupling cap is a given - I don't want +300VDC charging the grid of the 12AX7b. I see the 820k + 220k resistors in series in the HPD as a mixing resistor/attenuator block, but with a 100k resistor to GND as well, perhaps I also need to think of the net as a form of high pass filter? Is a high pass filter really what I want on the dry portion of the signal entering the mixing amp 12AX7??

The Idea behind the H.P.D reverb is to mix the reverb in before the filter recovery. So you would want to take the signal coming from the OD switching feeding the Master and mix this with your reverb before the gainstage the HPD uses as a filter recovery send that to your master vol then off to the PI..
here you are using this gain stage to help recover the losses from the loading effects from the mixing resistors usually 220k ala Fender style. I am not really a big spring reverb guy and have not used this type reverb circuit..Obviously it would sound better IMO if you could implement the complete mixer/amplifier like 3 tube one in the ODSR. So your going to have to play around with it or if someone else wants to jump in and has done these types of reverbs can give you their opinion..
BTW.The tweed mixer circuit uses 3 1/2 tubes..The 2 tube reverb in the HPD is more like a Black face Fender type
Good Luck!

Tony

[Stephen1966]

Here the RVB is developed to include a switching relay.

ODS RVB 2.3.pdf

This is a bit easier to follow since it includes the last stage of the Clean channel, the OD section, the RVB with switching, and then the Master+FX loop which leads to the PI.

The mixing resistor is speculative but I would go with a 220k for a start. I've shown the position of the filter circuit as well. I'm not looking to develop the latter but if it is really indispensable to the RVB I could find a way to include it. If that were the case, I would follow Martin's version: https://ampgarage.com/forum/viewtopic.php?f=5&t=30118

Would anyone else like to jump in with suggestions?

Good health!

Stephen

[rootz]

This looks very different from what I did when I crossed the 005 and 124 (or at least did an attempt). Can you explain why you'd want reverb on only the clean channel? You use an extra relay for the reverb, when you could easily use an extra relay to switch reverb level pots. With a DPDT relay you could actually switch reverb send and return controls. The Coral does that IIRC.

One thing that occurs to me as an error in your latest reverb schematic: how much signal will the reverb mixing triode see? There essentially is a 1:1 voltage divider on the grid of that tube, so it will see half the maximum output of the clean preamp! I think this means it will be very easy to overdrive the grid of the mixer stage. You need to dump more signal, like a filter section would.

It's been a while since I've been tinkering with different reverb ideas. Last time, I had a good look at many pictures of the Steel String Sultan from AN, which I was told is a clone of the HPD, which in turn is a clone of the 005. IIRC the attached picture is a schematic of the best I could make of all the pictures, including the lack of bypass caps for the mixer, rather strange cathode values for the 2nd clean triode, unbypassed ververs driver cathode resistor. Just ran some quick sims of it: 100mV input signal of 1kHz with all controls on noon gets me a whopping 42V peak-peak at the anode of the second clean triode. You do not want that on the grid of the reverb mixer; it would make your amp useless. After the filter I get 300 mV peak-peak on the grid of the reverb mixer triode, which translates to 8V peak-peak at the output. More than enough to drive the PI pretty hard with the master on 10 .

So to summarise: dump much more signal before the reverb mixer triode, just like the filter would do.