I have a 1966 Valco 1624T... what mods would best increase the gain of the circuit?

[84GTIRookie]

As the title says. Mine is a Valco Airline model. I currently use the highest gain 12AX7's I have, I use early breakup speakers to help, play with humbuckers... but looking for additional gain without resorting to pedals! (((NOTE: I just bought a NEW Supro Comet... which has a little more gain than my old Valco... and that's what got me thinking of adding gain to the Valco!))) Thank you!

[84GTIRookie]

I also jumper the two channels together... which makes a noticeable difference in growl/gain!

[Helmholtz]

Schematic?

[84GTIRookie]

Here is what I found on the Internet. Mine is a 1966 with 6973 power tubes...

[Helmholtz]

For a noticeable increase in gain you will need to add a tube.
What's wrong with using a clean booster (pedal)?

[84GTIRookie]

I currently use a 12AX7 pedal that I built myself... powered by 12Vdc, but I was thinking I could get more gain out of the amp? Would there be a benefit to putting a bypass resistor & bypass cap on the cathode of V1A?
Also, I notice it says that V2B is not used. How difficult would it be to wire that as a switchable--- additional gain stage? Thank you!

[Helmholtz]

Would there be a benefit to putting a bypass resistor & bypass cap on the cathode of V1A?

No, there's no cathode degeneration and the stage essentially uses all gain available.

Also, I notice it says that V2B is not used. How difficult would it be to wire that as a switchable--- additional gain stage?

That's certainly a possibility.

[xtian]

You can cascade the gain stages, instead of two parallel input stages.

[84GTIRookie]

Thank you Helmholtz - that answers the cathode bypass question. I'm not even close to an expert --- so I can learn from you xtian --- I currently plug into the LOW of V1A then use a jumper to come out of the HIGH of V1A and plug the other end of the jumper into the LOW of V1B. Is this what you mean by cascading... or would it require a wiring change?

[g1]

Cascading means putting the two stages in series. When you jumper the 2 channels together, it just puts them in parallel, which does not give near as much gain boost as wiring them in series.
Since you have an extra unused triode hanging around, I think that would be the best bet to add in series with whichever channel you prefer.
The extra triode could be made switchable via the jacks, like Marshall did with the later 2203 Master Volume amps.

[84GTIRookie]

So, g1, I have built a few amps, but I'm not super technical so I can be confused pretty easy... can you explain how the signal path works if I plug into the Low input. Same question if I plug into the high input? Thank you!

[Tom Phillips]

... I currently plug into the LOW of V1A then use a jumper to come out of the HIGH of V1A and plug the other end of the jumper into the LOW of V1B...

As an easy experiment, I would try plugging into the HIGH of V1A then use a jumper to come out of the LOW of V1A and plug the other end of the jumper into the HIGH of V1B. It won't add as much gain as the cascade mod but may get you a bit closer to the sound you want.

[84GTIRookie]

Thank you Tom - you are correct --- I've tried all combinations of jumpering... and there IS ONE WAY that I've found that has the most grit... but to be honest, I can't remember off the top of my head which which way it is. I was quoting the way I wrote it above for example purposes, but it's not necessarily the way I've found to be best.
To be clear, the amp sounds AMAZING... and the 6973 tubes have a kind of Roar/Sustain that the new Supro Comet 1610RT doesn't have, but since I just got the Comet, and have found it has a little more raw grit than the Valco, I've started thinking... how can I get the Valco perform with that extra bit of mojo? As you can imagine, I play Led Zeppelin I & II songs with the amp. The Comet is totally plug & play of those tones plus early '70's ZZ Top... but it doesn't have the roar/sustain of the Valco... so trying to get the best of both worlds out of the Valco!

[84GTIRookie]

I think this is the actual amp schematic.
https://www.prowessamplifiers.com/sc...upro_s6424.pdf

[g1]

So, g1, I have built a few amps, but I'm not super technical so I can be confused pretty easy... can you explain how the signal path works if I plug into the Low input. Same question if I plug into the high input? Thank you!

The arrows are switch contacts of the jack, normally closed when nothing is plugged into the jack. So when you plug into low, the connection to the high jack is broken, and the signal goes straight on to the V1a circuit, bypassing V1b.
When you plug into high, the switch contact on the ground side of the high jack is opened. Signal goes into V1b and through the .022 coupling cap on to the low jack. There it goes through the low jack switch contacts and on to the V1a circuit.