Silvertone 4784 to Guitar Amp Conversion

[dorrisant]

Ok... Mark wanted more build posts so here's one to consider.

I have a 4784 chassis I recently acquired via trade. Someone had attempted a rebuild... let's just say that the poor amp will have a much better home. I am building this as a surprise gift for my brother in law. It was in rough shape so I gutted what was there to make way for something new. I have some NOS can caps that test fine so I installed a 50/50/30 @ 350v in the old slot. I will change it if needed. I will keep the tube rectifier although I'm opening this up to suggestions about which one to use. I want to keep the 6AQ5 power tube. I put the dropping resistors in to match the original schematic but I will change those too. New 7 pin and 9 pin sockets as well.

So single ended 6AQ5 output (I have a bunch of 'em) a 5 volt rectifier and two novel sockets to fill. What can I put in there?

Bryan likes to play rhythm (mostly clean to medium breakup) and sing... I want this to be a useful tool for his songwriting so I thought the Pedal Mule type circuit would be a good fit for the future owner but it wouldn't need the LTPI and I just cannot waste a perfectly good triode. Also, how would you pull off a cut control on a SE output. Maybe a low loss treble control like a 5E3? I thought the extra harmonics you can get with a paralleled input stage combined with the lack of harmonic cancelation from a SE output might be interesting to hear. Thoughts?

Anyway, this could all go out the window for another idea... Just trying to build something really cool for a really good guy who deserves something very nice.

[Stevem]

A number of years ago I came across a circuit on the ax84 site, it was just a tone control and it sounded interesting to try out, not that I have yet though!
It was called a wrap around tone control, or circuit if I recall right.
If you can not turn it up and want to check it out let me know and I will dig it out of my files .

[Phil_S]

I may be mistaken, but I think the 5F2 or 5F2A circuit is a one knob tone control that is really a cut control.

[dorrisant]

Phil... 5F2A - that was the exact one i had drawn in my notes, not from a 5E3 as I said. Spot on!

Steve... I found the Comanchero Club. Is that what you were thinking? It looks interesting. Lots of different tricks to pick apart and try to wrap my mind around. Has anyone here built one, or used parts they could report about? I couldn't find any clips to speak of.

I am posting both of these schematics for ease of consideration. I want to build the basic circuit and test different ways to skin this cat. I could always do loss measurements and post them. I'm going to start with the 6AQ5 setup like the Warped Octal One schematic and adjust as needed. That's the plan anyway.

Thanks for the answers guys, I will put aside some time in the next day or so to research the Comanchero a bit.

[Phil_S]

You might think about the Gibson GA-20 circuit with a bit of modification. You only need to build one channel. It has the 6SJ7 input followed by a 12AX7 triode and then the output stage. IMO, you want to get the 6SJ7 plate voltage up to around 150V. You might consider input 1 to the pentode and input 2 to the triode (bypassing the pentode.) You can hang whatever output stage you want on it. I'm thinking SE 6AQ5 is a nice choice.

I'm thinking that 6SJ7 amp you are showing us will have too much dirt for a rhythm player.

[Stevem]

I have to dig more tonight, as I went to where I thought I filled it away, but no luck!

[sliberty]

The current model AC4C1 circuit is essentially a Top Boost preamp with a single ended power stage (EL84). Even though the tone stack is not a single pot, the way the circuit is configured, the bass and treble controls work well, and the gain makeup allows the amp to have guts. It's a great rhythm guitar circuit, and can do lead well too. The Vox doesn't' use a rectifier tube, but certainly could.

[Phil_S]

The Vox doesn't' tush a rectifier tube, but certainly could.

LOL, you think you can elaborate on the procedure for doing this?

[sliberty]

I hate autocorrect.

[dorrisant]

You might think about the Gibson GA-20 circuit with a bit of modification. I'm thinking that 6SJ7 amp you are showing us will have too much dirt for a rhythm player.

Sorry for the confusion Phil, I just wanted to use the output section from the above schematic. I like the 6SJ7 and have a few. I will probably build something dirty with one in the near future.

I am still thinking that rj's Pedal Mule preamp will work... Volume/gain control and the bass control. I will build it up this way and try a couple of different ways to apply treble control.

I couldn't find too much on the Comanchero... So I'm gonna just try that one and hear. I will post a schematic of my idea for clarity.

[dorrisant]

Been a little while...

I used rj's Pedal Mule schem as a start. Cut it off at the Bright switch. Instead, I used a 5E3 type treble control then added a 12AX7 parallel gain stage... I just wanted something to replace the gain of the original PI. Not sure about the gain there. It may need to be knocked down a bit. The amp can get up and scream past Rocket type gain... don't know if I will keep that gain level or not.

Then there is just a simple 6AQ5 single ended output.

I tried this with a 5Y3 rectifier but the voltages just looked too low. Switched to a 5U4GB and all is well... kinda. I'm not so sure of the 1R dropping string resistor to the last stage. Are ther any rules I'm overlooking with this? I want to change it but don't know why. Someone should tell me ok, change it or just smack my hands... Maybe just list a page in Merlin's PS book.

Deviations: I had to put the pots in with the terminals on the bottom for clearance sake. Someone before me drilled the holes. Couldn't find any 3M pots in my stash, used 2M instead. I used a 10uF cap on the last filter stage... sounds fine so far. There is a 5E3 type Treble control in place of the cut control. Also... I didn't do anything parallel in the dropping string.

Observations: Sounds great with all three knobs pointed straight up noon. With the volume up past 1:00-2:00 it starts getting into some serious overdrive. I don't think this circuit should break up this much but I kinda like it. Gonna keep it that way for now. Bass and Treble controls work great, although I wonder what the 3M Bass pot would do. I'm not noticing any change with the deep switch but that could be due to the bench speaker I'm testing with. Any thought on that?

[Phil_S]

Nice job on this little amp. I like it!

I noted your comment on breakup at about 1:30 on the dial. I am thinking the 470KΩ you have at the end of the B+ ladder is bleeding down the plate voltage on the two 12AX7's. If removed or changed the value to allow plate voltage to rise, I think you'd get more clean headroom and that might be a good thing for this little amp. I wouldn't be concerned at all if the plate voltage on the 6AQ5 rises. I think it is safe up to about 310-315, though I really like how you were able to dial it in so close to 250.

[dorrisant]

Nice job on this little amp. I like it!!
I wouldn't be concerned at all if the plate voltage on the 6AQ5 rises. I think it is safe up to about 310-315, though I really like how you were able to dial it in so close to 250.

Thanks Phil! It was easy to dial in... kinda. I used he same dropping string values that were in the original amp's circuit. The voltage was pretty low so I subbed in the 5U4GB and viola! It is what it is. V1 was at like 84v before the rectifier swap. I think I will move the 470k bleeder to the first filter stage and change it to 1M while I'm at it. Everyone could take advantage of a little extra headroom. Increased headroom was the reason that I used a 1R resistor at the end of the dropping string. Are there any disadvantages to doing this?

Funny how I just inadvertently learned how to get more dirt from stage one next time I need it... No clipping diodes involved.

Side note: I labeled the plate pin wrong on V2a... I will update the schematic with the changes. The voltages at V1 & V2 went up one volt... If a butterfly flaps it's wings...

Hope rj doesn't get pissed that I borrowed/stole his schematic and proceeded to butcher it!

[Phil_S]

Tony, I always try to remember to tell folks I'm an amateur so they won't take what I say as absolutely correct, though I don't try to mislead intentionally. Sometimes I'm wrong.

The 1Ω in the B+ ladder isn't enough to decouple the last stage. You might as well just remove it and feed the two tubes from the same node. 1KΩ is probably near the bottom of the range to get adequate decoupling, though I recall seeing schematics that use lower values. The posted schematic shows a drop of 243 t0 240 over 1001Ω, which is 3mA for both the PI and the preamp tube. Because the current draw is so low, you need not be afraid to use a large enough value to properly decouple both the V1 and V2 stages. Go for about 2K, remove that 470kΩ. I'm doubtful it will affect the B+ very much.

I don't know how you can use a bleeder with a can cap as there is a common ground, but maybe you can strap a 220K from the 1st B+ node to ground and that will work. I'm guessing 1M is way overkill. The bleeder isn't essential. Why not just omit it?

You might also think about increasing the value of the cathode resistor on V1 (maybe just a little to 1.8K or 2.1K)to increase gain, though it might introduce noise and maybe unwanted tonal qualities.

Hey, try something. See what it does!

And to make the Ω symbol, use Alt+234!

[dorrisant]

No worries, Phil... I appreciate the intelligent conversation and the ideas that you are presenting.

After a thorough test last night I am thinking of lowering the gain a little but not too much.

I will experiment with the last dropping resistor value and see what it brings. I am pretty well stoked with the way it plays and sounds right now so not too much to do other than that.

Thanks again for your ideas and thoughts! Especially the Ohm symbol shortcut... I never knew!! ΩΩΩ