PP 6V6 Amp Conversion

[TIMBO]

Hi guys, Got a neat PP 6V6 amp (no name  ) but it has the local built Ferguson transformer on board.
I would have liked to rebuild this in to one of our own name vintage amps, but 6V6s seem to have been left off there their build list.


So after some googling the valve lineup a couple of great amps popped up.
Gibson GA20
Valco Model1212
Valco Supreme Model 510-1B
Supro Supreme 1950/51

The Gibson grabbed me the most much smoother, but the Valco/Supro had some real balls.

With the lineup of valves from the original amp this is what I have got.

OR I could just mod what is already there......





There is lots of surface rust, so the old ecaps and coupling caps will go and the rust treated.

 

[92Volts]

I'm confused what's going on with the screen of that 6J5 in the current configuration.

It looks like it's triode-connected, that cap to ground almost looks like a pentode screen filtering cap but might just bleed off some high-frequency (for any number of reasons). Converting it to pentode mode is an easy way to get a LOT more gain.

The phase inverters in the 2 new schematics are interesting, I think those are paraphase designs?

I'm not sure what benefits there are to reworking the phase inverter that way, but a 6SL7 will provide way more gain than a 6SN7. If you wanted to use that other PI design with a 6SN7 however to keep the original tubes, you'd need to figure out how it works and tweak the values to accommodate a 6SN7.

[TIMBO]

Thanks 92, I do believe the 6J7 is a "triode strapped"

I'm thinking I might try the circuit as is and replace the caps and see what it sounds like and then tweak from there.

[PRR]

> lots of surface rust

It's pushing 70 years old. It's freckles, not tumors. If that is all the rust it has, it won't eat-through for another 700 years.

And I'm sensitive to rust this month. I had a car brake failure. Upshot was 2 brake lines, 2 fuel lines, $420. Turns out the standard brake lines are expected to have a 5 to 15 year service life. There's a better type, lasts far longer, but costs a few bucks more in mass production. Three car companies use it; Honda did not. (Of course I also got the exploding airbags...)

I say the stock circuit has merit, but is intended for an external preamp.

[TIMBO]

Thanks PRR.
Doing a bit of an overall check of the remaining components is not a great result.
Tranny's check good 
So complete gut.

I have done a few mock layouts on the existing tag board that will allow some changes without too many problems.
I am thinking a 2x10" combo with this chassis sitting on the bottom of the cab, with a second chassis at the top for the controls.

I can't find a PT model number that will give me an idea the HT mAs or the heater current.
It has two heater taps.
6.3v no CT for the 6V6s
6.3v CT for the preamp.
The umbilical cord socket had both HT supply and heater supply.

So I'm hoping that both could supply at least one valve.
REVERB!

This is the rectifier valve....


Valve measures 3" long not including pins.
Branded Mullard.
Any idea the type.

[PRR]

> an idea the HT mAs or the heater current.

If you run with two 6V6 and a reasonable number of preamp valves, it will be fine.

If that socket on the end is intended to power a separate preamp, you can do similar.

[TIMBO]

Merry Christmas PRR, A big thanks for your ongoing help with my projects over the last year.

My thoughts at this stage is to settle on the amps circuit.
The GA-20 is an awesome sounding amp, so I'm guided by it for basic circuit.
I have a 6SL7 so I'm able to swop with the 6SN7, components on the board are easy to change.

[TIMBO]

New components in.

I have added in grid and screen resistors to the 6V6s and a temporary volume pot.(no tone as yet)


As is, got some good sounds.
Screen voltage is a bit high?? better to lower it for the 6V6s sake.

I am assuming the low plate voltage on the 6J7 is because there is no bypass resistor??

[PRR]

> low plate voltage on the 6J7

Ratio of plate resistor to screen resistor on these pentodes is usually 1:4 or 1:5. You have 1:2.

I think you want plate to sit higher than 13V.

I think 470K is awful high for a signal coupling resistor. Try 220K.

[92Volts]

Looking good so far!

The 6SN7 draws 600ma heater current while the 6SL7 draws 300ma. Even if you didn't believe this transformer powered a separate preamp, this change alone gives you another 300ma to play with.

[TIMBO]

Thanks guys, Checked both heater voltages and they are just sitting above their nominal voltages.
I temporarily connected a 6BM8 to the preamp heater (6.3v CT) and this dropped to 3.2v on either side.
Connected to the other 6.3v and this actually dropped from 6.7v to 6.6v.
So either tap has enough supply for the 6BM8.

[TIMBO]

Hi guys, Built the cab yesterday, looking for a retro feel.
http://www.scottymoore.net/goldtone.html
Might even try the 10/12" combo.

[Glenn]

Wow, wish I could do stuff like that. It looks great

[tubenit]

This is a VERY cool project!  Enjoy watching your progress. Thanks for sharing.

With respect, Tubenit

[TIMBO]

Thanks guys.
Glenn, with help from the guys here you can do stuff like this.
When I joined many years ago, I was wanting to do this and now I can but I still need a hand with the more technical stuff.
Plenty to learn.
Good luck. Tim

[DummyLoad]

nice work tim. looking forward to seeing the finished piece.


--pete

[TIMBO]

Hey pete, The reverb had me a bit stumped.....
I thought it would be an easy addon.
For whatever reason beyond my knowledge the 3.3m mix resistor was causing all sorts of problems.
A rethink has it sorted.

I tweaked up the power supply just to increase the voltages on the preamp.

The next job to sort is the TONE as this just does nothing.
Might investigate the 5E3....

[TIMBO]

Guys, got the last few things finished and did a mock fit together.
All fits nicely.


Had a quick play and there is no excess in noise due to the umbilical cords.
Also added a 5E3 style tone stack, works ok.

[kagliostro]

Ciao TIMBO

Can I suggest to put a layer of rubber foam between the Chassis and Reverb Tank ?

--

May be I asked before, but my memory is fault


do you bend by yourself those "chassis" (the one on which are pots) or you have a craftsman that do it for you ?

Franco

[tubenit]

What a great job you have done with this!  Bravo!

With respect, Tubenit

[TIMBO]

Thanks T, Trying for a real retro feel with the cab.
Just love these old amps.

Franco, I have a sheet metal guy do all my chassis.
I usually get a few bent at the same time cheaper that way.
The top chassis was not meant to be position that way, as it was a design for another amp.

[kagliostro]

Thanks

Franco

[TIMBO]

Tolex is done...


Face plate will be similar to this...

[kagliostro]

NICE !

Franco

[DummyLoad]

awesome work!   


--pete

[tubenit]

Tim,

That really looks sharp!  Great job.

With respect, Tubenit

[TIMBO]

Thanks guys, I think this cab will be a "one off" took about 4 hours to do.

But very happy with the end result.

[TIMBO]

Hi guys, This one has been sitting for way too long and needs to be finished.
The new faceplate has been sitting on the shelf and gathering dust.........
Got myself reacquainted and I was not really impressed, seems to be lacking in a bit of sting.
This is what I have come up with.

[bmccowan]

Hey Timbo,
I also like that Gibson tone. for V1 have you tried different plate and screen resistors? For the BR6F and early GA-20 (nearly the same amp) they used 470K & 1M, or 270K & 2.2M I think. The P90s help too.
Nice work!

[TIMBO]

Hi guys, Dusted this one off again and to be honest not really impressed with the overall sound.
So back to the drawing board...
I'll start at the beginning and rewire to the original circuit...
Part of the dislike of the modified circuit was the low level hum when the volume was down and no guitar plugged in.
I believed this was because the 6J7 had a shielded wire from the input jacks that was causing problems.
So i think adding another valve (6SN7, 6SL7 or 6SJ7) as the first preamp valve and place in the top chassis.
Any thoughts appreciated.

[trobbins]

Isolated input jacks, and well screened cable through to input grid.

Last schematic shows 2 inputs effectively shorting the hi-gain input stage's output.

The mixer 6J7 has a TC that is mixing high impedance pot wiper circuits that need careful shielding from TC to wipers, and cable screens connected to 6J7 0V node, and all metalwork needs to be linked to keep pot cases at 0V.

You may want to add a grid stopper input to the PI to see how that sounds, as you can likely overdrive the PI stage fairly easily.


Is the output stage contributing any hum, or is it just the input stage that dominates?

[DummyLoad]

V1 is seeing about 20K load. e.g., 33K || 68K || 330K || 500K vol ctrl. + TS, so V1 becomes anemic.

suggest inserting and recover effect after tone shaping. please see attached.

--pete 

[bmccowan]

Timbo - the work you've done is too nice to let sit. I just reconfigured an amp I built many years ago and had never liked. It was a good feeling to salvage a likable amp out of it.
Anyways - with 2 6J7s in the mix you may want to look at two schematics that use multiple 6J7s or 6SJ7s and sound great in my opinion. The original Gibson EH185 which uses multiple 6J7s - I built one from an old PA, changing from the original inductor bass control, It sounds great on the mic channel, not so much in the inst channel like so many early amps. Folks here helped me a lot with that project.
And the DanElectro Special 12 which uses all 6Sj7s in the preamp. Repaired an original - nasty in a good way.
Not suggesting you build either circuit but the way the pentodes are configured might provide some ideas. I look forward to seeing what you come up with, and wish I had the patience and talent to fabricate chassis like you do.

[TIMBO]

Thanks guys, I think I'll start at the power amp end and work back to the preamp...
I saying this because the OT is quite large for 15w compared to a fender 5E3 size OT.
I found specs on the OT, 10k to 15ohm
Could this be part of the lack of response and could the PI be redesigned.

[shooter]

10k to 15ohm

fwiw;
my last build was PP 6V6 using a 10k, got good results, my B+ is ~ 40vdc less
tubes biased HOT at ~ 13W each.  DL did help me rebuild my PI and that DID help

[tubeswell]

...could the PI be redesigned.

It would only take a couple of minor mods to make it into a T-bolt paraphase. (T-bolt has one preamp stage and paraphase driving cathode-biased 6L6 PP and has decent overdrive, albeit being not a very high-powered amp)
(But of course, an extra gain stage adds more gain possibilities)

[66Strat]

Thanks guys, I think I'll start at the power amp end and work back to the preamp...
I saying this because the OT is quite large for 15w compared to a fender 5E3 size OT.
I found specs on the OT, 10k to 15ohm
Could this be part of the lack of response and could the PI be redesigned.

The PI is REAL close to a 5C5 PRO. Voltage gain of the non-inverting triode is about 42. It is not the limiting factor IMO. Swap out the 4.7K grid resistor of the inverting triode to one in the 5.8K range and by-pass the cathode resistor.

I like tubeswell's idea of cathode biased 6L6s. If the transformers have the current capacity, I would do that. An 8 ohm load would result in a 5.3K reflected load. Primary impedance of the 5C5 Pro was in the 6K range.

[TIMBO]

Thanks guys, This is the circuit ATM.

Adding another gain stage IS an option.

[tubeswell]

Here's that sort of t-bolt topology - to give you an idea of comparative gain structures


(The input stage has a switchable single/parallel V1, which is kind of comparable to the gain from a small signal pentode if you sort out the output impedance - so you'd do the tone stack differently. The rest is basically the same, but you'd have a different OT impedance ratio and bias for 6V6s)

[TIMBO]

Thanks mate, This is what i have put together.

The preamp and reverb circuits will be on the top chassis.


Not sure i'll stick with the 12AX7 in the preamp, may change it for a 6SL7/6SN7 keeping it all octal.
I know the gain will drop a bit, but i'll have a spare octal socket on the bottom chassis i could stick a single triode/pentode to pump it up....... maybe

[TIMBO]

Hi guys, Another option on the preamp to give the MIC. it's own volume control.

[TIMBO]

Hi guys, This amp has had a few changes...


So if you have a suggestion, give it to me.....
What I was finding..
Using a 6SN7 or 6SL7 in V1 made no real difference.
Using a 6SN7 in V4 was less noisy than the 6SL7 in the previous circuit (have not tried it in this circuit)
The tone works well on both channels, but because they are after the volume, they kinda work a bit like a volume as well (increases volume with brightness)
Instr.2 has more volume than Instr.1 i'm thinking lower loss TS in Instr.2