Fender Twin - steps to reduce hiss?

[daxliniere]

Hey everyone,
I have a 1987 Fender 'The Twin' (not Twin Amp aka Evil Twin). It sounds great, but there is the usual level of hiss associated with Twins. As it is to be used in a studio environment, I would love to see if I can address the hiss, minimising it as much as possible without major detrimental effects on the tone of the amp.
I recapped all the electrolytics and gave it a thorough once-over including bias & balance of output tubes. Tubes have not been changed. Previous owner had some cheap Chinese 'Sino' 6L6GCs installed, preamp tubes are probably all original. No signs of microphonics. Increasing reverb level makes the hiss worse again.

I may be barking up the wrong tree mentioning all the tube info, but this amp has 11 of them.

To be clear, this isn't the amount of hiss that would make me consider this amp has a problem, just that it would be nice to minimise wherever possible.


Thanks in advance,
Dax.

Fender the Twin Red Knob schematic

[Colossal]

What goes into the front end, gets louder. If not already metal film, change the input grid leak/pull-down resistor (the 1M to ground, mounted on the input jack(s)) to a 1-3W metal film resistor. I like 3W. Reduce the V1a input grid resistor (usually 68k in Fenders) to 10k (or even less, if your amp does not live in a noisy space) and tack on a 100-220p capacitor from Pin 2 to Pin 3. This small value cap increases the Miller capacitance of the tube and works with the reduction in the input grid resistor value to achieve a similar -3dB high frequency rolloff point (usually 25-50kHz is fine).

From Blencowe...

Blencowe Input.png

and

Blencowe Input 2.png

and for a 10k input grid resistor with an additional 100pF in Miller Capacitance...

Input Cap.png

You can add more capacitance if you want a lower frequency cutoff and/or want to intentionally roll off more high end. The length of your instrument cable and any pedalboard (if applicable) play a role in this as well.

[daxliniere]

Wow, wow, wow! Thank you so much, @Colossal! I guess you live up to your name.

Really pleased to hear all of this. I'm away at the moment, but have some photos of the internals, so will take a look and start the plan of attack.

Can you tell me a little more about your 3W MF resistor preference? I wouldn't have thought there would be any difference considering current at that point would be miniscule.
Also, perhaps this would be the best place for a n ultra low noise Vishay 3W MF?

[Colossal]

Yes, I was going to say that the 3W 1% Vishays are good for this task. Takman metal films come in 1W and are excellent and very low noise as well (although 910k is the largest value I have seen available of late, but they did used to offer a 1M). I have a stash of the now obsolete IRC GS3-0 3W metal glaze resistors which I usually keep for input jack duty. The current is minuscule yes, but it is my understanding that the larger substrate of a 3W vs. a 1W resistor results in lower noise for resistors of the same value. I learned this from Mark Huss, an emeritus member here. These small increments in performance add up.

[daxliniere]

Maybe I'll string together some 100W wirewounds.

[Colossal]

Maybe I'll string together some 100W wirewounds.

Two series and two parallel and make sure they are the non-inductive NS types for 10x the cost!

[daxliniere]

Great idea! Though because of the length, I will likely need to shield the resistor array. But then I'd have all the extra capacitance that would be created!

[Roe]

consider the 3m3 mixing resistor and the reverb circuit

[daxliniere]

Hey Roe,
Yes, when the reverb knob is turned up, the hiss is increased considerably. But as it is a return level, setting it to zero should avoid all of that noise.

But I'm curious about the 3M3 resistor you mentioned. I scanned over the schematic, but couldn't find one of that value yet. Any tips where abouts to look?
EDIT: Oh! By any chance, are you talking about the two 8M2 that appear above the reverb select switch? I guess they look like they are mixing the two input channels. (I'm learning a lot about amps as I dig further into this and I'm loving it. )

[daxliniere]

Should I just replace every carbon resistor I find with metal film of the same power capacity or higher?

I found a photo from before I started my refurb. The electro caps marked green have been replaced.

R101 and R103 (pink box) are the 8M2s that I believe are channel mixing.

[lonote]

You might find this Hoffman thread relevant &/or useful.

https://el34world.com/Forum/index.php?P ... #msg349210


From following this, I ended up going with some 100uF bypass caps in a noisy reverb section of a Princeton-ish amp, which eliminated nearly all the noise. I used them for both the driver & recovery sections.

[daxliniere]

Thanks Ionote. I read about halfway down the page and it was all getting a bit over my head.
But basically what you're suggesting is to replace C105 with 100uF and add 100uF in parallel with R56?

I'm definitely keen to try this, though reverb noise is lower down on my list of noise concerns for this amp. But, again, thank you.

[Roe]

Hey Roe,
Yes, when the reverb knob is turned up, the hiss is increased considerably. But as it is a return level, setting it to zero should avoid all of that noise.

But I'm curious about the 3M3 resistor you mentioned. I scanned over the schematic, but couldn't find one of that value yet. Any tips where abouts to look?
EDIT: Oh! By any chance, are you talking about the two 8M2 that appear above the reverb select switch? I guess they look like they are mixing the two input channels. (I'm learning a lot about amps as I dig further into this and I'm loving it. )

vitually all twins use 3m3, but here it is 8m2. the mixing circuit is noisy. I redesigned mine, using separate cathode resistors and caps on v4 a/b

[daxliniere]

Alright, cool, I am primed for this.
Looks like a pretty easy mod, too: un-parallel the cathodes, add bypass caps to each and use 1.5k resistors for both? Or is the rule different for parallel triodes?
I'm assuming your v4 is my v9 (reverb driver tube).

[Roe]

Alright, cool, I am primed for this.
Looks like a pretty easy mod, too: un-parallel the cathodes, add bypass caps to each and use 1.5k resistors for both?

this works for my v4 which is the reverb recovery and mixing tube.

seperate cathodes also work for the reverb driver, making it less prone to oscilliate