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

[84GTIRookie]

Thank you Helmholtz - this is something I could learn --- so you are saying adding another 0.005 cap in series next to C11? OR adding another 0.0005 cap in series with C12 (which is parallel to the 500k vol pot)? Would one of these to methods act as a high pass filter? Is there a way to calculate/get a sense of what size cap to add to cut say... X frequency? Thank you!

[Helmholtz]

Thank you Helmholtz - this is something I could learn --- so you are saying adding another 0.005 cap in series next to C11? OR adding another 0.0005 cap in series with C12 (which is parallel to the 500k vol pot)? Would one of these to methods act as a high pass filter? Is there a way to calculate/get a sense of what size cap to add to cut say... X frequency? Thank you!

C11, C13, C14 are already parts of HP filters. Those might already be sufficient. Just try.
If necessary reducing the value of either of these caps will lower bass response.
Frequency calculation is complicated as the filters are interactive.
Tweak by ear.

[84GTIRookie]

Cool - very simple - Thank you! I think I'm good to go on all the questions on my mod! Should be fun learning from these changes!

[Chuck H]

Here's what "I" would do...

1) Add a bypass cap across R4 (as per Helmholtz). I'd start with 22uF and go down from there if the tone is too bottom heavy. Five or six dB added to the first stage is actually quite a bit and if you're happy, you're done.

2) If that's not enough gain then remove the bypass cap and try the cascade circuit proposed. It will be A LOT more gain. Circuit tuning may be needed.

3) If that's still not enough gain then reinstall the bypass cap across R4. With both the bypass cap and the cascade mod the amps gain would be increased about 200 times and have gain higher than an early plexi Marshall. Again, some circuit tuning for tonal balance (and at this point circuit stability) may be needed. Ergo my consideration of an additional high pass filter regarding the tonal aspect..

[84GTIRookie]

Yes, you're saying the same thing I was thinking --- trying each of the mods by themselves to see the impact of each... and also cause as you're saying... both together will likely be --- either over the top gain wise, or unstable, or both! Obviously, when I do the cascade mod, I'm gonna just hard wire it initially to try it out... easy... then if it does what I like I'll get a push pull pot to make it permanent.
Again, thank all you guys for taking me under your wing to work me thru this mod! It's rare to find a forum where people are so knowledgeable and helpful at the same time!

[Helmholtz]

With the cascade mod the bypass cap across R4 might be necessary, because any heater buzz of the input tube will be increased by a factor of maybe 60.

[84GTIRookie]

Good to know. That's helpful thank you!

[Chuck H]

With the cascade mod the bypass cap across R4 might be necessary, because any heater buzz of the input tube will be increased by a factor of maybe 60.

Good catch and observation.

And so, 84GTIRookie,
If the cascade with the first stage cathode bypassed is TOO much gain there are ways to bring that down to sane levels. Though there really aren't enough gain stages here to get insane for a preamp it may be over the top for a cranked amp. I get the impression that LOUD is how you're using these old sweethearts.

EDIT: OTOH...

Most modern very high gain preamps use only partial bypass on the first stage cathode. There IS more noise. Mostly in the LF of course. Hum and emission noise. But is it so much as to be intolerable? Sometimes, maybe. And that would accound tor the trend toward DC filament circuits in a lot of those designs. But even a cascade Marshall has only partial bypass when it's the "bright" channel used for the first stage. I guess my point is just that modern amps have A LOT more gain than this design would even with the cascade modification. So partial bypass or even no bypass probably isn't a deal breaker here. Though I do think it's more ideal.