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

[84GTIRookie]

Here is what I think is the actual schematic. I don't see an extra 12AX7 stage on this diagram? So, not sure I could put IN a series gain stage, BUT... is it possible with this type amp to do a "1 Wire MOD" like they talk about on a JTM45... so that the two sides of the first 12AX7 are run in series instead of parallel?!!! Thank you.

[Chuck H]

You already identified the amp as a 1624T. The schematic you just posted is for a S6424. Now, a lot of things changed often with these early designs. Sure it's possible your amp could be marked as a 1624T and actually have a circuit like the S6424. Maybe some kind of transitional design using existing chassis and face plates, etc. But...

Have you discovered that your amp matches the above schematic? How? With a question like yours every piece of information you recieve will depend on having the correct schematic.

And, yes you can cascade stages the amp is already using. In fact that is the nature of the Marshall preamp schematic that g1 posted. When you plug into one input the amp is using one stage like you have now. When you plug into the other there are two series stages. This could also be done with a panel switch rather than a jack switching.

[84GTIRookie]

Thank you Chuck H - Yes, you are totally right - I need to clarify. I originally posted the 1624T schematic - that's the WRONG one... I HAVE the later 1624T which is designated the S6424 (with the TWO Volume & TWO Tone pots!). So the 2nd schematic I posted called the S6424 is the correct schematic, which is my actual amp. Thus, as I said, I don't have an extra 1/2 of a 12AX7... so what I do have... is the dual channels with the dual volumes & dual tone controls. Not being any type of expert... I can build amps, pedals, and all kinds of things, but I do not know the theory and details required to do a custom job outside the normal SOP... like a "One Wire Type Mod" on the Valco in order to run the channels in parallel??? If you or someone could show me a markup of the schematic for where to splice in, and what if any components should be removed (or values modified?) to accomodate the mod... I would be a super happy camper! I'm thinking that would make the amp absolutely amazing! Thank you!

[Chuck H]

This is a very basic alteration. You can only use the top channel when the mod is switched in. When the mod is switched out the amp is stock. You may need to make adjustments or add a couple of components for stability and tone. The DPDT switch is common to push pull pots and most of this should be do-able at the control panel. I'd probably change the first channel volume control to a push pull pot with the switch. Keep leads as short as practical.

[Helmholtz]

Also you could bypass R4 with e.g. a 25µ cap for a boost of up to 6dB.

[Chuck H]

Also you could bypass R4 with e.g. a 25µ cap for a boost of up to 6dB.

I saw that with this particular model. I addressed stacking the channels as requested. Might not need additional gain after that, but want and need are different things If R4 was to be bypassed AND you did the cascade mod you'd almost surely have to add a series capacitor with the mod to trim LF.

I'm wondering if C2 and/or C3 might need to be bumped into the uF range to keep stability with this cascade mod. Once you start cascading stages it doesn't take much interaction on the HV rail to cause problems. I think the 100k resistors (R6 and R8) are probably enough and at circuit impedance I think it's safe from HF oscillation, but upping the DE-coupling is an option if it wants to motor boat.

[Helmholtz]

If R4 was to be bypassed AND you did the cascade mod you'd almost surely have to add a series capacitor with the mod to trim LF.

I'm sure you have much more practical experience with such mods than I, but why would that have to be?
Adding the bypass cap would boost all frequencies above the corner frequency and not increase bass response.

[Chuck H]

I'm sure you have much more practical experience with such mods than I, but why would that have to be?
Adding the bypass cap would boost all frequencies above the corner frequency and not increase bass response.

Well this could fall into the "subjective" area of tone, but as gain continues to increase LF (especially in the early stages) needs to decrease or things get mushy. There are exceptions but they still trim LF in the second stage. Like Marshall mods that stack the normal channel in front of the bright channel or the Trainwreck circuits. Both trim some LF in the second stage. Where most cascade designs trim LF from the first stage. Either way LF is trimmed before things get TOO clipped. Adding as much as 6dB to the first amp stage is actually A LOT more gain with the cascade so I'm just putting it out there as a probability.

Regarding bypass on the first triode in the cascade... You've probably read my take on this but I think that designers have been making an error here for decades by using "partial bypass" in the form of lower value capacitors. This allows any noise occurring between the cathode and grid below the bypass caps frequency to enter the signal chain. In my own experiments I've found this to be a significant consideration. Especially with modern preamp tubes. There are other ways to shape signal than partially bypassing the cathode so relying on this as a tone shaper only invites the possibility of extra noise.

[84GTIRookie]

First of all - Thank both of you for taking the time to consider my questions - I greatly appreciate your insights! It is so nice to have people that really understand, have the experience and have the confidence to make sound decisions!

Next, I have a few questions, because I don't understand things on the level that you two do. I'll do separate posts for the different questions I have regarding your bypass cap at cathode at R4, and your suggestion for additional series capacitor to trim LF (I'm assuming that LF abbreviation means Low Frequencies?).

BUT first things first... QUESTION #1: I just want to make sure I understand the routing of the guitar signal thru the cascade so I can wrap my head around the suggested cascade mod: My interpretation is that the guitar signal leaves V1A plate at pin 6... thru C11 & C12/Volume Pot at R25... then the new (upper) switch activation shown with the red arrow facing down would swing to the left to make contact and then the guitar signal would move to the new (lower) red arrow switch activation, which, at the same time would swing to the right to make contact... so the signal would then go thru R10 to the grid at V1B at pin 2... then thru the plate at pin 1... then thru C13 & C14 to... C16 /Volume pot R26 then thru tone pot R28... and the on it's way down the rest of the circuit...! Is this a correct interpretation of the Cascade Mod? That's the end of Question #1.

[84GTIRookie]

I have experience with adding bypass caps --- having done so many times. It definitely adds a raw/grit/gain... whatever you call it... to the sound. I modded a Hammond Organ into a guitar amp, and used Rob Robinette's switchable cathode bypass cap mod, so I know that will help - Thank you Helmholtz for the suggestion, I will definitely try it!

QUESTION #2: Some of what you guys are talking about is beyond my understanding, but It seems you two are debating the value of the bypass cap, and the best value in order for it to best compliment the Cascade Mod...??? Maybe the size of the bypass cap at R4 ??? From my experience, using a lower size cap (smaller than 25µ) will be a little less full in the spectrum of frequencies that are gained and may reduce that thick "fartiness" while still adding something to the overall gain. I can definitely play with the value there - but thank you Helmholtz for the suggestion, I will definitely try that using one cap value or another! If you guys have a suggestion for a better value based on some modeling data... please make suggestion. Thank you.

QUESTION #3: I have heard thru the Internets that cascading channels can create oscillation issues. I have no understanding of how this issue comes about, nor how to solve it. Is that why you are suggesting a "series capacitor to trim LF"? ALSO --- where would the series capacitor be added? and what value? Thank you!

[g1]

Q#1, that is correct. When upper switch is to the right and lower switch is to the left you are in stock configuration. It is a ganged DPDT switch so just one toggle to throw. In cascade mode it routes through as you described.

[Chuck H]

Sorry for the confusion. I actually did mark the switches with a dot for like position but I guess it wasn't clear. I changed the schematic to represent switches as they would be "seen" on a pot switch and re routed traces so they are in like position in the diagram as they would be when wired. I hope this is sufficient for understanding.

[Helmholtz]

Well this could fall into the "subjective" area of tone, but as gain continues to increase LF (especially in the early stages) needs to decrease or things get mushy. There are exceptions but they still trim LF in the second stage.
.

Might have to do with artificial LF content produced by IMD at larger signal.

Regarding bypass on the first triode in the cascade... You've probably read my take on this but I think that designers have been making an error here for decades by using "partial bypass" in the form of lower value capacitors. This allows any noise occurring between the cathode and grid below the bypass caps frequency to enter the signal chain.

That's clear.
The larger the bypass cap, the lower the noise voltage between cathode and ground.
And the first stage noise gets amplified most.

[84GTIRookie]

No sorry required - I'm very appreciative of you guys help! My thing about tracing the guitar signal path was to confirm that I had my head wrapped around the mod.
Q: Is it possible for you guys to comment on my 3rd Question above about my lack of understanding of the what is meant by the phrase:

• Originally posted by Chuck H View Post
• "If R4 was to be bypassed AND you did the cascade mod you'd almost surely have to add a series capacitor with the mod to trim LF."

[Helmholtz]

I understand Chuck means that if you use a large value bpass cap across R4 you might need additional high pass filtering after the first stage, using a low value series/coupling cap.
OTOH the amp already utilizes 3 consecutive high pass filters.