Increasing gain of the drive channel

[GERPUD]

Hi
I ve done several mods on my amp. See schematic.

My problem: the drive (hot) channel has now about the same amount of drive than the cool channel. I guess the problem is the 120k load resistor I have added, but I'd like to have confirmation before doing corrections.
I have also added a 1M leak resistor at V2-B to avoid switch "POP". (I've also added a 1M pot leak resistor ar V3-A for the same reason, but used it as a tone button )

From my understanding, the stock load equivalent resistance of V2-A is:
R19 (470k)+R16 (100k)+ (R15 (1M) and cool pot (250k)) = 770k (when cool volume is 100% on)

The stock drive gain was fine.

My guess is that 120k need to be 250k. What do you think?

Again, from my understanding, V2-A, V2-B, V1-B have the all same gain. V1-A is used by both. So why the drive channel after going through V1-A and V2-A has the same gain that cool channel after V1-B? (from what I hear).
Maybe it has something to do with the input that is shared b both channel? This divide the current not the voltage so should not do any differences...I don,t know!

Other question:
Can I now remove R15 and bypass C7? If not, why?

Thanks!



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[matt h]

(deleted)

[GERPUD]

I'm not sure how V2a gets its signal, I assume there's just nothing drawn there. However, the 10k resistor to ground (R17) does not look right. Unless that's not going to ground, but instead going to the wiper of the "hot volume" pot.

R17 is going to the HOT pot wiper. The schematic is hard to understand...


What should I do with the 120k I added? Just remove it? I put it so the signal has somewhere to go when the hot channel is off. If the output of the tube can have infinite resistance, I could remove it?

[matt h]

(deleted)

[John_P_WI]

+1 to Matt's comments. Sorry, I didn't spend enough time to evaluate the schematic, try making the 120k to ground added resistor 10 or 100 times larger. I think you will want something there to prevent the cap from "popping" due to a dc level shift when switching (assuming it is connected right after the coupling cap / resistor on the scheme).

[GERPUD]

ok, I think the combination of the very hard to follow Fender Schematic over my "not so well drawn" modification is not optimal.

The 120k is part of the new load resistance of V2-A since it is not connected anymore to R16. So load of V2-A is R19 + 120k = 590k

After R30 I added a 1M leak resistor to avoid pop.

So, input and load resistors of all stages is about the same
guitar input: 1M
First stage:
Hot load resistor: 470k V1-A (r32 + pot)
Cool load resistor: 470k V1B (r18+pot)

Second stage:
Hot load resistor : 590k V2A (r19+120k)
Cool : no stage

Third stage:
Hot load resistor: 250k V2-B (2 500k pot in paralell)
Cool load resistor: same V2-B

So, why Hot channel is not hotter than cool when having one more stage?

I'm sure I'M missing something but I don't know what!!

[matt h]

(deleted)

[GERPUD]

You got a point. I forgot to take into account the voltage divider...

So from:
http://amps.zugster.net/tools/triode-calculator

I get:
Stock:
V2-B had a load of 670k, voltage divider reducing to 70%, gain = 38.9

My wrong mod:
V2-B ad a load of 577k, voltage divider reducing to 20%, gain = 11

So, I should replace my 120k with 300k to get the same voltage divider and gain...

However,
Hot channel
Stage 1: gain = 28.7
Stage 2: gain = 10 (instead of 22)
Stage 3: gain = 50
Total: 89.7

Cool channel
Stage 1: gain = 28.7
Stage 2: gain = 50
Total: 78.7

So is it normal that a voltage gain difference of 10 sounds similar in terms of amount of distortion?

[matt h]

(deleted)

[GERPUD]

thanks matt,

I don't think I can remove the 120k completely because there won't be any load resistor when drive channel is off, and the load resistor when drive is on would be 1.47M...
I think my mods are really not clear enough on the schematic. (R30 goes directly to R30 after the mod, so I added a 1M leak resistor and a (wrong) 120k load resistor.)

Splitting the circuit at this location was needed to separate the interaction of both channel on each other. Cool pot was modifying voltage divider and load resistor of V2-A. There was also a little bit of noise from the unused channel on the used channel. The splitting give a better control and less noise.

However, you are right (again) on the 300k value. My goal is to get the same gain than stock amp. 300k was good without the new 1M grid leak. The 120k need to be 430k...
That would double the gain of that stage (10 to 22). I have no idea how much I need to get enough overdrive, but theoretically, that should work (if I'm not making 1000th error... by the way, I'm happily surprised that after doing all these mods and errors, my amp is still working!!)

Thanks for pointing out my (novice) errors. Without your help (I talk to everybody in this forum that are trying to make helpful comments), my amp would have go to garbage!

[matt h]

There isn't really a huge reason you need to maintain that load on the triode when you're not using the channel... i mean, it's one of those "in theory vs in practice" sorta things that the argument that can be made for it to be important in theory turns out, typically, not to matter one lick a bit in practice.