The HODS project

[romberg]

hods-title.png

Years ago, I bought a Weber Halle kit with the intention of tweaking it slightly to be more in line with a "real" ODS. The tweaking process turned into a full
on make over as I decided to make my own boards and create a circuit based on a 102 drawn up by ic-racer from this thread. The schematic I'm using as a base for my build is ODS-102-HRM.pdf.

I created a schematic with ki-cad and a layout with librecad. The main deviations in my build will be that the HRM tone stack will be mounted in the front panel
and I've added a PPIVMV because I've never installed one in an amp and not liked the extra control they offer in over driving the PI. Because space is an issue in
this chassis, I've combined the pre-amp, power and HRM boards into one. The design is a mix of layouts from this site and Merlin's book. All my plans for this
build can be found here:

• hods-1.0.2.pdf
  Having finished sourcing all the parts and then burning out on amps for a bit (lord I hate buying parts), I've finally got some time to assemble
  all these into a finished amp. The cabinet was covered during the first bit of this whole pandemic stay at home thing. Now it is time to melt some
  solder.
  
  parts.jpg
  If there is interest, I'll post a few pictures as I go.
  
  Mike
  
  EDIT: Update schematic

[romberg]

Soldering has begun and the first 20db of gain are up and running. Thanks Martin for the excellent
thread on setting the JFET bias. I
first installed the guts of the circuit into a breadboard and used two 9v batteries to confirm that
the 2.2k resistor would drop 1/2 the voltage.

fet-board.jpg

In my case, the JFET would be exactly 1/2 with a 2.29k resistor (I wired in a pot to dial it in exactly). Since
this is kinda an oddball value, I've decided to let sleeping dogs lie and stay with the 2.2k.

Since all looked good on the breadboard, I soldered in everything but the voltage divider and fired it up.

fet.jpg

Looks like it is giving me about 10x (or 20db) of gain. Which should be enough to juice up lower output pickups
a bit before hitting the first gain stage.

fet-test.jpg

[JazzGuitarGimp]

Following with great interest.

[ChopSauce]

Man, this (also) sure looks like a great project! Thanks for sharing...

[rootz]

Subscribed!

On thing that catches my eye is the power supply in the 102 HRM schematic. It ends with a 10k right before V1. IMHO this can only be right if there is no Fet board. I think, but correct me if I’m wrong, this resistor was increased to 10k to emulate the higher voltage drop you’d see there when you use a Fet. I’ve seen 2.2k frequently on HRM pictures I have. The Hoyt Axton Dumble uses a 2k7 there. But that’s about it. From the top of my head the Fet draws about 1mA with a 2k2 source resistor, V1 about 1.5mA and the FET power node another 1.5 to 2 mA. So a 10k resistor would drop at least 40V. A bit to much I think. Also the resistor before this one I mentioned should be 22k. Isn’t 15k more of a Bluesmaster value?

Anyway, different values are easy to test with your board layout. I think the supply cap placement on v1 and V2 makes it very easy to test different values.

[romberg]

On thing that catches my eye is the power supply in the 102 HRM schematic. It ends with a 10k right before V1. IMHO this can only be right if there is no Fet board. I think, but correct me if I’m wrong, this resistor was increased to 10k to emulate the higher voltage drop you’d see there when you use a Fet. I’ve seen 2.2k frequently on HRM pictures I have. The Hoyt Axton Dumble uses a 2k7 there. But that’s about it. From the top of my head the Fet draws about 1mA with a 2k2 source resistor, V1 about 1.5mA and the FET power node another 1.5 to 2 mA. So a 10k resistor would drop at least 40V. A bit to much I think. Also the resistor before this one I mentioned should be 22k. Isn’t 15k more of a Bluesmaster value?

Anyway, different values are easy to test with your board layout. I think the supply cap placement on v1 and V2 makes it very easy to test different values.

Thanks roots! I really do appreciate an extra set of eyes. I spent some time trying to retrace my thought process in creating this. Because there is limited space in the chassis I'm using, there was no way I had the room for all the caps in a precision power supply. So, it looks like I used the following schematic from this site (forget exactly where it came from) to use the early power supply (less caps).

101-hrm.gif

However I kept the dropping resistors from the precision power supply (and what ic-racer used in his 102 schematic). I don't know why I did this and your estimate on the voltage drop sounds reasonable to me. Since I've seen ODS dropping resistor values of 3k-22k-2.2k and 3k-15k-10k on multiple schematics I decided to run the numbers and see what they do (in theory at least).

Code: Select all

Suppose:
B+3 = 400V (just a guess)
V1 and V2 draw 1.5mA each
FET board draws 3mA

Then with 22k and 2.2k we get:
    B+4 = 268V
    B+5 = 258V

And 15k and 10k yields:
    B+4 = 310V
    B+5 = 265V

So, the 15k-10k string runs V1 and V2 a little hotter than the 22k-2.2k. V1 is about at the same voltage but V2 is a bit higher. Maybe I should switch to the 22k-2.2k string? the schematics I used as a reference are from a while back. Maybe more information was learned since then. Of course, like you point out, it would not be super difficult to switch them out after it is up and running and I can see real (rather than theoretical) numbers .

Mike

[norburybrook]

I've always aimed to get V2 slightly higher voltage than V1. The original #102 is 200v V1 and 205 V2.



M

[rootz]

You're welcome Romberg. I know the 102 HRM schematic, I have it in my archive too. I do not know if it is based on an actual amp. I think Marcus has the prefect guideline for the voltage difference between V1 and V2. I think most tend to shoot for 190-200V on the plates of V1. That should be 300-320V on the last node (B+5) IIRC (not the 258V you get).

I'd change the 10k to a 2k2 and adjust the 15k to taste to get V1 and V2 in the right spot. You could also adjust the dropping resistor before the PI if you'd need some more voltage down stream. It all largely depends on your B+ voltage. If it really is just 400V, you should adjust somewhere. #102 B+ is 465V according to the layout. You won't get the same V1 and V2 voltages without adjusting. Like you said, easy to experiment with. But I'd definitely keep the 2k2 at the end of the string.

[romberg]

Thanks everyone for the feedback! I've decided to use the 22k/2.2k values at the end of the power supply string and have updated my schematic in the first post. I dug around a little more on this site and found that it looks like ic-racer used these values when he actually built his 102. I also ordered some more 2w resistors (which are now literally on a slow boat from China). So, I should be able to tweak things after it is built to get around 200V on the plates of V1 and V2.

I have done a bunch of drilling and have test fit most everything. Next time these parts go in it should be for keeps:

fit-guts.jpg

fit-front.jpg

fit-back.jpg

Things are going to be a bit tight. This area in particular should be interesting (but doable I think):

fit-tight.jpg

Mike

[romberg]

I made some progress in that the power transformer is now wired in and tested. The filaments, main rectifier board, 5vdc (relay) power and bias supplies are now all connected. Here you can see that I'm using my breadboard to temporarily wire in the rest of the bias circuit in order to tweak resistor values.

guts-1.jpg

The orange wire carries the rectified 330v and will be connected to the standby switch. I have not yet plugged in the two HT fuses into the rectifier board (not 100% certain running the rectifier without a load is a bad thing or not). Anyway this side of the amp is very close to done.

pwr-boards.jpg

With the primary wired up I was able to fire it up and check some voltages. The old ArcherKit meter is one of the first electronic projects I ever built. I think I was like 12 or 13 at the time. It still works!

I ended up adjusting the bias supply resistors to give me a range of voltages between -55.8 and -27.8 volts. I would like to run either el34s or 6l6 power tubes in this amp. I think this range should do it. I think 330 vac should give me around 450VDC on the plates. So, the cutoff for el34s would be around -450V/10 (-45V) and 6l6s 450/8 (-56V). I've read (on the interwebs) that a good upper bound for bias voltage is around 0.6 times this amount. So, something in the area of -27V? I know you use current/power to set the actual bias point. But you need a range of voltages capable of doing it. I'm crossing my fingers that I have enough range. Because this amp is going to be kinda tight packed and replacing some parts after it is built will be a chore .

voltages.jpg

Since the heater string voltages looked ok, I plugged in some tubes and verified that all the filaments light up. Here it is all glowy. I'm running the pilot light off the 5vac line that was over in the area to power the colossal relay board. The 5v tones down the pilot lights brightness a bit and was a happy accident.

glow.jpg

[romberg]

So, life intervened and threw me a few things to do more important than building amps. But that is now over with and I have finished wiring up the front control panel.

I plan to put in the board last as there is not going to be much extra room.

ctrl-back.jpg

guts-2.jpg

At this point the relays, footswitch and front panel overdrive switch operate.

relays-on.jpg

The front panel contains a panel mounted tri-color led (red/green/orange). The idea is green is on when the PAB is engaged. Red is on when the overdrive relay is engaged. When both are on the LED is orange.

It turns out that tri-color LEDs are more complicated that I first imagined. The original part I had worked great but had a common cathode. This common cathode did not work with the relay boards I'm using. I needed a common anode led. These seem to be more rare. But I found one. However to work as a tri-color LED the two individual LEDs need to be somewhat balanced in their luminosity vs current characteristics. My new Chinese common anode LED was not well balanced. So, I messed around with the current limiting resistors a bit and got something working that looks sorta orange when they are both on. But you have to be looking straight on axis for it to look orange. Otherwise it looks red. I continue to look for a real tri-color common anode LED. But this works ok for now.

green.jpg

red.jpg

Next up, I'll mount the output transformer, speaker jacks and impedance switch. Then create, populate and mount the circuit board and it should be working (fingers crossed). I then plan to tweak the voltage divider for the FET board and put it in last. After everything else is working, I may add in the effects loop.

Mike

[romberg]

Most all of the wiring in the chassis is now done. It seems that every build I do I oscillate between wishing I'd done the leads to the board first or the leads to the off-board parts first. In this case, there are certain parts that would be next to impossible to attach wires to once the board is in place. I really, really hope I got them right and don't have to later tear apart stuff to get at them. Here is the chassis at the peak "rat's nest" phase:

rnest.jpg

I'm planning on hooking up the loop jacks and FET board after the amp is up and running. I *think* there will be enough room to hook up the preamp sockets once the board is in place. Speaking of which...

Here is the main board all drilled out and turrets installed. I'm going with turrets instead of eyelets as I have an easier time dealing with them.

board-empty.jpg

Now on to the part of a build I enjoy the most. Populating the circuit board. With luck, I measured the components more of less correctly when drawing up the board and everything fits!

Mike

[daniboy79]

Real nice project! Following this one for sure! Keep up the great work!

[romberg]

Real nice project! Following this one for sure! Keep up the great work!

Thanks! So, far most everything is falling into place. Of course this may mean that Murphy and his dang law will show up at the end .

I've populated the main board. I decided to add a few leads to it. These leads would be tricky to mount after the board is in the chassis:

board.jpg

top.jpg

I ran all the under board wires using the technique Steve Lucky demonstrated in the trainwreck discussion section. This technique works pretty good (as long as you don't try and run too many wires into the center of the turret. I plan on triple checking these under-board wires before putting it into the chassis.

bottom.jpg

Mike

[romberg]

Getting close to the end now. Been working on wiring the board into the chassis. Started at the power amp end and have been working my way towards the pre-amp side. Just one tube left to wire in.

guts3.jpg

I left the negative feedback wire disconnected and the wires off of the PI plate caps loosely tacked on with long leads for now. Once it is up and running I can check the phase and then permanently connect them. I'm also leaving the effects loop out of the picture for now.

At some point I may try and find a small effects loop board to jam in there (there is one free piece of chassis left on the back side! . So, far I've only found effect loop kits that have the board with jacks attached (with a hole spacing wider than I can live with). Maybe I'll make my own loop board. Not sure.

I've been using my signal generator and scope to verify things. So, far I'm able to pass signal from the PI outs to the power tube grids. And I can also pass signal from the plate resistors on the overdrive tube through the HRM tone stack and overdrive pot. Signal makes it from the output of stage two to the PI input (through the relay). And I can pass signal into the first stage of the over drive tube.

signal.jpg

The voltage divider into the first stage of the overdrive (with the OD trimmer) sees a little extreme to me. It is cutting the signal down to only around 4% of what is coming out of the clean channel. This matches what is on the 102 schematic I'm using. So, maybe this is what is suppose to happen. I think I should check some other ODS schematics and see if the voltage divider into the OD channel is similar.

As I was checking the HRM tone stack, I found that the mid control seemed to have no effect on the signal no matter what frequency I was sending through the stack. It was also pretty stiff when turning it. On further inspection it seemed that the center wiper was not making contact. Which would explain why the control did nothing. This pot is in a terrible spot to replace. I was kicking myself for not checking it before I put it in. Then the cavalry arrived!

magic.jpg

In desperation, I gave it a shot of the majic fluid and it started working! Of course this may not last. But for now I'm gonna proceed as if the incident never happened .

Mike