First time down the amp design rabbit hole. Ampeg V4 variant. Need some guidance

[jacobarber]

Howdy folks. After having fiddled with tiny electronic circuits for a while, I decided to start thinking about building an amp. I've had a 1974 Ampeg VT22 (non-master, non-distortion) for many years and love it to death. But, as anyone who knows this amp knows it is an absolutely loud and heavy beast.

So I had an idea to design something based on the V4 with a few changes that I wouldn't mind seeing. The changes can be summarized as follows:

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POWER AMP
2x 6V6 instead of 4x 6L6 / 7027. This is meant to bring the overall power down from 100-120w to 20-25w. I know this won't bring the amp down to bedroom volume, but it can help a little. And the reduction of components can bring the overall weight down. My thought on powering the output is to utilize a Hammond 1650FA.

PREAMP
The original V4 preamp is a wild mix of 12AX7, 6K11, 12DW7, and 12AU7 tubes. The reverb section holds an additional 6CG7 and 12AX7 section but that's a different story.

The 6K11 is a 3-section tube where section 1 is half of a 12AU7 and sections 2&3 are a full 12AX7. This redesign will replace the 6K11 with those two tubes.
The 12DW7 is half a 12AU7 and half a 12AX7. 12DW7s are still manufactured, but there are way more options for 12AX7s and 12AU7s, so I'm proposing to use those instead of any 12DW7s. This means that all non-power tubes are 12AX7s and 12AU7s.

REVERB
This is where diehard Ampeg fans will revolt. I'm removing the reverb altogether. I love the reverb in my old VT22, but I also love flexibility of my reverb pedals. The 6CG7 goes away along with its associated circuitry. The whole amp moves a little closer to the V4B with this but is till geared towards guitar instead of bass.

CHANNELS
Only 1 channel. I'm removing the 2nd channel completely, as I never use it and I want to simplify things as much as possible. I think some people would try to use the 2nd channel to create some gain stages, but I really don't want higher gain. From everything I can see/tell/read, both channels are identical and independent. I think in reality they do interact a little, but I don't think there's any harm in removing one channel. I might keep the input sensitivity switch, but I really haven't decided. Its neat, but I honestly have never used it on my VT22.

OTHER CHANGES
FX loop: I'm changing the "Ext amp" jacks to a real FX loop. This has been documented relatively well with these amps.
Master volume: I need to study how to make this work, but I'd love to give it a MV control. My VT22 uses a pedal plugged into one of the 'Ext amp' jacks in the back which behaves sort of like an MV control. It lets me saturate the poweramp a little and get some fun compression without blowing my eardrums out. I really just want the same thing; not preamp distortion. Still need to research this.

POWER TRANSFORMER
I don't know what power transformer to use at all. None of this is my domain, so all I've done is based on other peoples' works. The changes in tubes and circuitry would probably qualify other available power transformers than what is already in my VT22. No clue.

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And the big reveal for my progress thus far is attached. I provided a PDF for your viewing pleasure.

I built the schematic idea in KiCad over a few days. I started with the available non-MV, non-distortion V4 schematics and then went to town modifying.

Here's the thing. I have no EE background and don't have much regarding electrical knowledge. I know what components do and can solder til my hands hurt, but I couldn't begin to design a circuit from scratch with any intent. I'm not going to go ordering parts and assembling things without some confidence that this whole project might actually work.

I would really love some eyes on this. Tell me I'm nuts, tell me I have every single resistor or capacitor value wrong, or tell me how you might go about accomplishing the goal. Any feedback and input is better than nothing. I'm a blank canvas and open to what the collective knowledgebase here can give me.

[PRR]

> love some eyes on this.

Blurry. Maybe it is crystal-clear in your program, but this image is badly degraded. Letters are rendered at 6x4 dot resolution, which was poor on a TV Typewriter in in 1984. What is a "33t" resistor? (Yes, I know tera, but you can't buy a teraOhm, and would not put it there.) Do I see more than two smudges which may be "V3"? Not helped by zig-zag-zig routing which could be direct (yes, I  know this is hard, but help us help you). See attached. Grabbed verbatim and scaled 2X but otherwise unmolested.

Before my eyes quit I noted that the Bass/Treb network probably needs a ground, and the second "V3" (?) driver needs a cathode ground return.

[jacobarber]

Yeah, the PNG didn't turn out how I wanted. The PDF is readable but I don't want to force people to open a separate file. I'll take some better images of specific sections and post them instead.

I'll take a look at what you posted regarding the grounding and see how its handled in the original V4 schematic.

[sluckey]

The PDF is readable but I don't want to force people to open a separate file.

Why??? I much rather look at a clear pdf. Everyone should be able to open a pdf on any device. I post all my full schematics and layouts in pdf format. Never heard a complaint.

[jacobarber]

> love some eyes on this.
Before my eyes quit I noted that the Bass/Treb network probably needs a ground, and the second "V3" (?) driver needs a cathode ground return.

So while I'm figuring out a better way to post the whole thing, I did find where I'd missed the ground connection in the tone circuit. Thanks for pointing that out.

I am looking at your point how the cathode needing a ground return. It shows that even in the original (seen here or here) there is no ground return in the way you mention.

Regarding the zig-zag routing, I genuinely tried to start by replicating the actual schematic of the amp and make modifications as needed. It certainly isn't perfect, and this is kind of new ground for me. Again, I'll repost the images once I find the best way to get you something clearer. In the meantime, thanks for the help.

[jacobarber]

The PDF is readable but I don't want to force people to open a separate file.

Why??? I much rather look at a clear pdf. Everyone should be able to open a pdf on any device. I post all my full schematics and layouts in pdf format. Never heard a complaint.

Well then. The PDF is in the original post, so folks can check that out if they care to. In the meantime I do need to fix an issue or two and will update that PDF accordingly. Might just take the PNG down altogether.

[pdf64]

Get rid of that nasty standby, either totally or in favour of an arrangement that doesn’t treat the output valves like crash test dummies.
eg scroll down to the bottom of http://www.valvewizard.co.uk/standby.html
The V4 standby, like 50W Marshalls of the 70s, enegises the both slow charging bias supply and the fast charging HT supply simultaneously. So the valves are momentarily unbiased and pass very high cathode current.

Connect the rectifier negative output to 0V common 

[jacobarber]

Get rid of that nasty standby, either totally or in favour of an arrangement that doesn’t treat the output valves like crash test dummies.
eg scroll down to the bottom of http://www.valvewizard.co.uk/standby.html
The V4 standby, like 50W Marshalls of the 70s, enegises the both slow charging bias supply and the fast charging HT supply simultaneously. So the valves are momentarily unbiased and pass very high cathode current.

Connect the rectifier negative output to 0V common 

Heck yeah, this is the good stuff I'm looking for. I moved the standby switch according to Valve Wizard's recommendation for a bridge rectifier setup.

I also found 2 additional grounds that I hadn't connected: The common on the OT and the #2 lug of the hum balance pot. Fixed those.

[glass54]

+1 with Sluckey. PDF's are universal, you can even access them on your mobile (sometimes need your magnifier BUT available  )
Just for clarity, R33, R35 etc, have component ID, Value and Tolerance and "1M" attached. Is this an error? See attached partial.
As far as readability goes, I often split my schematics into Preamp and separate page for Power Amp + Power Supply.
You could consider this, as your grounding for Preamp and Power Amp/Supply will be different. On the Preamp you would simply note your supply rails as "D", "E", etc. If you do 2 pages, I would recommend splitting at C10 and making a note at R24 and V4A as "Preamp Out"
My thoughts and all the best with the project.
Kind Regards
Mirek

[jacobarber]

Great feedback so far. I've been coming through and fixing issues as I find them. Few grounds, a slew of mislabeled resistors probably because I copied / pasted a bunch, and the improved standby.

Here's something that I want to learn more about: How do I choose a proper power transformer? I landed on the Hammond 1650FA output transformer because it is listed as appropriate for a compliment of 6V6 output tubes. But there isn't really the same qualification for power transformers.

So what do you all use to decide on a power transformer?

[pdf64]

Yes, the 1650fa looks good, I was thinking of suggesting something from that range, as they look higher spec (probably interleaved) than the typical sub 30W guitar amp OT.
For the PT, maybe the Pro Jnr replacement type https://www.hammfg.com/files/parts/pdf/290XXA.pdf to get a HT of around 400V DC. I’d rather try to find something with a higher HT current rating though.

[PRR]

....even in the original ... there is no ground return in the way you mention.

Pink:

[jacobarber]

Pink:

Yar, got it. I actually caught that in the last round of fixing missing connections. I read your initial response wrong maybe, which is why I was confused. Got it now. Thanks!

[TitaniumValhalla]

OP, any updates on your project? I started looking into building a single channel, non-reverb, 2x6V6 version of the Ampeg V4 then I see you beat me to the punch! I figured a Deluxe Reverb cab/chassis/transformers would be easy to repurpose for this.

[jacobarber]

OP, any updates on your project? I started looking into building a single channel, non-reverb, 2x6V6 version of the Ampeg V4 then I see you beat me to the punch! I figured a Deluxe Reverb cab/chassis/transformers would be easy to repurpose for this.

No, nothing to report yet. I'm still learning a lot about what goes into design of the circuit and constantly tweaking the schematic. I did start fiddling with a turret layout, and have to say that the number of components in this thing is going to eclipse many of the kit amps out there.

I'm glad I'm not the only one who thought about this. If you end up making any progress, please share it. I'll do the same. I'm not sure if you use any software for your schematics but I can post my KiCAD file if needed.