Dumble Steel String Singer inspired Clean King Amp

[Svempan]

This is my first real post on this forum. I have followed some threads here with great interest. So, now I hope to get some interesting comments from the people here .

(By the way: I live in Sweden, Scandinavia, Europe. I'm a tube amp tech and I also play guitar and sing in the blues band Red’n’Blue.)

OK. Now to the point.

First: For several years I've wanted to do something in an amp for myself with EF86 tubes in the preamp. I like the shimmering highs they seem to make/deliver (even undistorted).
Second: For a long time I have, in a way, liked the sound of 6550 as output tubes.
Third: A year, or so, ago I serviced a Marshall VBA 400 bass amp and was absolutely knocked by the sound it gave for guitar.
Fourth: I've wanted to tweak the Fender blackface reverb for a better reverb sound and especially a better dry sound.
Fifth: One of my favourite guitar sounds is Stevie Ray Vaughan’s sound on the DVD "Live in Montreaux 1985" (not the one from 1982) where the two miked amps appears to be his Dumble Steel String Singer with a tall Marshall speaker cabinet and one of his Fender Vibroverbs.
Sixth: I saw the schematic for a Dumble Dumbleland (or if it was for a Steel String Singer) that had EF86:s in the preamp.

These things, and some other thoughts I had/have, led me to make an amp for myself that I, a little jokily, call "Clean King". It's based on a Silverface Bandmaster Reverb that I‘ve had for several years as a 3x10 combo. The output transformer is from a blackface, or early silverface Fender Super Reverb.

Please look at the attached pdf file here below for schematics of how the circuit is right now. I guess (and hope) that there are interesting things in each circuit stage (or at least things that are worth to comment or to question)...

I probably should mention that my main purpose with this amp is to have a clean sound. I use it together with one (or more) other amp which purpose is to have a dirtier sound. The sounds from the different amps shall blend together to a god match.

Other than some little tweaking that still can be done, there are some more things I want, and probably will try in the amp:
- Input stage with a paralleled double triode (12AY7, 12AX7 or E88CC), instead of the triode connected EF86.
- To put the amp back in it's piggy back format and use it with a closed back 4x10 speaker.
- Try a Mercury Magnetics blackface Twin Reverb output transformer I've got, or an output transformer from a Dynaco Mark III I've also got.
- Try a slightly bigger reverb drive transformer. Maybe one from/for a black- or silverface Fender Champ or Vibro-Champ.
- Try another type of screen grid supply for the output tubes (maybe with a separate transformer).
- See what a cathode follower connected double triode between the phase inverter and output tubes would do to the sound.
- Maybe a totally new chassis and cabinet so that I more can optimise the transformers, power filters, tubes and so on...

It would be really nice to get comments on my thoughts and on the amps circuit .

By the way again: Is this "Dumble discussion" forum the right place for this post, or do you think that it should be under the "Dumble files" forum?

Thank you,

Sven-Johan

PS: I hope my English isn't too bad…

[Sven]

Greetings,

This looks like dumble schematics. I wander (besides already explained), what are the advantages of EF86 valves in the CL1 and CL2 positions? How are they different from two halves of 12AX7 used by other Dumble designs?

Second, can one and how could one implement ALSO an addition to this schmatics by adding an OD pre-amp stage with OD1 and OD2 valve (two halves of an 12AX7, I suppose, as everywhere in other shcematics), so one could have both clean (as it is now) and overdrive pre-amps in the same amp, switchable between the two, of course. Could someone draw such a schematics, with BOTH circuits included, or would it be sufficient just to merely add the usual OD pre-amp stage pasted from other schematcis found elesewhere on this forum?

Third, how does this amplifier sound? Are 6550 valves the ones that give it more clean headroom than 6V6, 6L6 or EL34, as it is expected usually?

Fourth, are the values the same in all pre-amp, tone stack, PI and power amp circuits if one merely skips building a reverb into this amp?

Fifth, is there a layout for this amp?

Sixth, how would ¨returning¨ to use of 12AX7 valve(s?) in CL1 and CL2 position differ from the existing arrangement with EF86? Are two EF86 valves, as seen on this schematics, better than one 12AX7 used as CL1 and CL2, better (or how they differ)?

Seventh, why would one use paralleled 12AX7 in the clean pre-amp, besides the obvious 3dB increase in signal strength?

These are rather rough questions, excuse ...

Is there a clip with the sound of this particular amp recorded, available for posting here?

Thanks.

Anyway, this is very interesting schematics.

All the best,

Sven

[Svempan]

Isn't Sven a Swedish, or at least a Scandinavian or Nordic name?

Exceuse that I haven't ansered your private message to me. I rather answer here in public...

As I've mentioned earlier, my purpose with the EF86 was to get the shimmering treble and high mids that I think that they give to the sound. Connected as in my amp they probably also give more headroom than the conventional triodes (12AX7 for example).

Sorry, but I probably wont make a schematic with an overdrive circuit. Sorry! Please excuse! My goal with this amp is that it shall have a good clean sound.

A schematic without the reverb circuit but with a loop circuit wouldn't be too hard to do. For example the schematic for the Dumblelator is available here on the Dumble files and it would be rather easy to implement it in a circuit like my Dumble inspired such. I don’t promise that I will do it, though . It will, among other things, depend on how big the interest becomes here. Same thing with a layout. I haven't had any thoughts of making that... I hope you understand me.

Even in the type of output circuit my amp has, the 6550's give more headroom than, for example, 6L6's. I haven't A/B tested but it feels as the amps is almost as loud undistorted as a blackface Twin Reverb. 6550's also have there own kind of sound, I think. It's hard to describe sound, and this is just my personal thoughts, but beefy, clean, good presence and shimmering is some words that I want to use. As I hear it, they also give the tone a nice kind of compression. The output circuit in my amp could probably be much more optimized for 6550's.

An amp tech friend of mine, who has worked on Hi-Fi tube amps for many years, thinks that a triode connected pentode (like an EF86) as input stage, is very good for Hi-Fi amps, but that it’s rather "lifeless" (sound wise) in guitar amps. That's one reason I want to try a paralleled double triode there. One more purpose is to get more headroom than a single triode. Before I had EF86 here, I had a conventionally connected 12AY7 triode and liked that.

At this moment I don't have any sound clips of this amp that I'm enough satisfied with, and that I think makes justice to it's good sound. If the interest becomes great on this forum, I maybe will fix clips…

Take care,

Sven-Johan

[Svempan]

If someone's interested, here's a picture of the amp in question.

[Darkbluemurder]

That picture is great! It looks almost like my Bandmaster. I had a custom 3x10" cabinet made for it in rough white tolex and gold sparkle grille. I replaced the OT against a Bassman OT which gave it a lot more punch. The speakers are Jensen Neodyms.

My signature still shows it as an HRM amp but meanwhile I ripped out the HRM circuitry. It just did not work for me. Right now it is a clean only amp just as yours but with cathode follower before the tone stack, a master volume and a variable negative feedback control (aka soul control). However it still runs on 12AX7 and 6L6 tubes as it always used to. The photocell vibrato is also still in it (maybe this is the reason that the HRM circuit never sounded right to me) as is the original fender reverb circuit (the surf circuit).

I think it's a great idea to try new things such as EF86 and 6550 tubes in an otherwise familiar circuit.

[Svempan]

...I think it's a great idea to try new things such as EF86 and 6550 tubes in an otherwise familiar circuit.

Hmmm... I'm a litte surprized that you think that the reverb circuit is familiar. Could it really be that?

Sven-Johan

[Bob-I]

That looks amazing. I'd love to see a gut shot of that amp.

I really like the treble and mid shift idea. How effective are the controls? I'm thinking of maybe replacing the switches with small 3 pisition switches.

[Svempan]

That looks amazing. I'd love to see a gut shot of that amp.

I really like the treble and mid shift idea. How effective are the controls? I'm thinking of maybe replacing the switches with small 3 pisition switches.

I gut shot? Is that a picture of the inside, the circuit board and the wiring?

The treble and middle switches are rather effective to tailor the voice of the respective tone controls. I guess you have tested to change a treble cap from 220 pF to, let's say, 330 och 470 pF or a middle cap from 0.047 uF to 0.022 uF? That changes things quite a bit, doesn't it?

I absolutely think that the switches are useful, but maybe you're right that you don't need five positions. I wanted to have four position switches but the five positions were what I had in my shop.

With four positions I probably would have chosen capacitors to a total capacitance of:
Treble: 150 - 270 - 320 - 480 pF.
Middle: 0.010 - 0.020 - 0.032 - 0.050 uF.

With only three positions I think I would have chosen:
Treble: 270 - 320 - 490 pF or 270 - 370 - 540 pF
Middle: 0.010 - 0.032 - 0.050 uF.

Note that there is one cap always in the circuit and that the switch parallels different caps with that one.

To avoid pops when switching it also would have been good to have high resistance resistors (several mega ohms) between each position or between ground and each position. I don’t think that the pops are a big problem though.

I’m a little surprised that no one comments the reverb circuit. Is it just Dumble’s own reverb circuit or the "one tube" reverb that counts here now?

Take care,

Sven-Johan

[skyboltone]

The speakers are Jensen Neodyms.

DBM. How do you like the Neos? Can you make a comparison to anything familiar?

Oh and great looking amp Svempan.

[glasman]

I’m a little surprised that no one comments the reverb circuit. Is it just Dumble’s own reverb circuit or the "one tube" reverb that counts here now?

Take care,

Sven-Johan

Your reverb is pretty unique, The use of the EL85 for the driver is a nice idea, it has a very low plate resistance and matches well with the standard Fender reverb tranny. Fender did some thing like this with the Vibroking, in that amp, they used the EL84.

The ECC832 is also a good choice and I see you are using the low gain side for the mixer. I am curious why the negative feedback around the mixer, did you find some issues here or just wanted to limit the gain of the stage.

[Svempan]

Your reverb is pretty unique, The use of the EL85 for the driver is a nice idea, it has a very low plate resistance and matches well with the standard Fender reverb tranny. Fender did some thing like this with the Vibroking, in that amp, they used the EL84.

The ECC832 is also a good choice and I see you are using the low gain side for the mixer. I am curious why the negative feedback around the mixer, did you find some issues here or just wanted to limit the gain of the stage.

Thanks for your comment

Yes, I know that Fender used EL84 in the first Vibro-Kings, but, as I see it, that reverb circuit is otherwise more as their stand alone reverb 6G15 (except for the higher voltages in the Vibro-King). I neither like the sound of the 6G15 or the Vibro-King reverb .

The EL85 wants about 11 - 12 kohms load at the voltages I use. I will soon try a different reverb transformer that is a little bigger than the silverface 125A20A transformer I want to see what that does to the sound. I'm also a little nervous that the silverface trany can not handle the primary current...

The main reason to use EL85 was in fact it’s low filament current (I still have the original 125P5DX power trany in the amp…). I would rather use EL84 (because they are commercially more available) and a different reverb trany.

It's less than a week or so since I put the negative feedback around the reverb mixer. Therefore I think I want to play the amp a little more before I judge (the gig this weekend was a little strange...). My initial purpose was to lower the distortion in this stage, but when I checked, it seemed that it almost didn't distort at all, with or without feedback, even at very high volume settings. I first made a local feedback from the plate to the grid, but then just for curiosity I tried it like it is now, and liked the sound. That was just my first thought, so I maybe will do another solution here. For the feedback, I was also (of course) inspired by the schematics here of the Dumblelator and your Reverb-A-Loop...

The purpose with the ECC832 (12DW7) was to use the low gain side for the mixer so that I could use a rather low value on the series resistor (the 3.3M/10pF on Fenders). I want to go even lower than the 330k I now use and will probably tweak a little more for that. When I think of it I haven’t even tried this stage without the cathode decoupling cap... Hmmm... Comments?

The high gain side of the ECC832 is used for the reverb recovery to get a good blend between dry and wet signal.

The mix between dry and wet signal is now very good. The Reverb Send (Dwell) and Return (Reverb) controls also works in a good and, as I hear it, musical way.

The power amp section in the amp distorts much earlier than the preamp. I now want to make changes in the power amp for less distortion. That, together with a bigger output trany and a closed 4x10 speaker cabinet, is probably now the highest priority for this amp. Any comments on that (less distortion in the power amp section)?
- Maybe a 12AX7 as phase inverter but another double triode (12AU7) as cathode followers between the phase inverter and power tubes?
- Maybe bigger power filters?
- Maybe a different screen grid supply (perhaps a separate trany)?

Thank you,

Sven-Johan

[heisthl]

Here's another unusual phase inverter:

[skyboltone]

Here's another unusual phase inverter:

This cleans up the PI? Never seen one. (though that shouldn't be surprising)

[Darkbluemurder]

The speakers are Jensen Neodyms.

DBM. How do you like the Neos? Can you make a comparison to anything familiar?

Before the Neos I had two Alnico Webers (10F125) and one Ceramic Weber (10F150) in that amp. While the clean tones were good, all of the D-style circuits sounded like a chainsaw massacre in a beehive. The same amps through a Neo 12 sounded sweet and singing. The Neos have a lot more low end and are better balanced IMHO. They are still a bit raspy on the high end. I attribute this to the fact that they are still new and don't have a lot of mileage on them (I haven't taken this amp to a gig with the Neos yet). What is really great is that they weigh a lot less which makes this amp a lot easier to carry.

It would have been nice to try some Eminence but noone in my area carried the 10" models. Another complication was that the OT has a 4 Ohm tap so I needed 16 Ohm versions (3 of them in parallel make 5 1/3 ohms) which are not offered by Eminence. There is no useful way I can wire 3 8 ohm speakers to get close to a 4 ohm load. So all I can offer for comparison is that to the Webers.

BTW this is not to diss the Webers. I believe they would sound great in a Tweed Bandmaster where the extended highs could complement the midrange of the amp. So if anyone is considering such project I could offer speakers that fit They just don't seem to suit D-style amps well.

[skyboltone]

Before the Neos I had two Alnico Webers (10F125) and one Ceramic Weber (10F150) in that amp. While the clean tones were good, all of the D-style circuits sounded like a chainsaw massacre in a beehive. The same amps through a Neo 12 sounded sweet and singing. The Neos have a lot more low end and are better balanced IMHO. They are still a bit raspy on the high end. I attribute this to the fact that they are still new and don't have a lot of mileage on them (I haven't taken this amp to a gig with the Neos yet). What is really great is that they weigh a lot less which makes this amp a lot easier to carry.

Thanks. It sure seems from the science that you'ld get an accurate driver with the neos. Some times with hype though all you get is the hype. Anyway, I want to build a 4 10" open back cab and will probably use the Eminence Legends for that. Still, the neos are tempting, if expensive.