Fender Twin - steps to reduce hiss?

[lonote]

The tank originally lived in the bottom of the combo, but now that I've 'beheaded' it, the tank needs to be closer to the rest of the amp.

Somehow I missed that you have it in a head cab. In that case, I think you are doing really well with noise!

It IS pretty crazy how the tank orientation/proximity is so variable with generating noise & seems somewhat random to the specific tank & amp.

I built a Champ circuit with 2-tube reverb & tried to install it in a head cab. Tank on the bench it was fine, but anywhere near the amp it was an unacceptable level of noisy.

I tried all kinds of shielding, then isolating the reverb internals & shielding the shell, etc. but finally gave up as it was a lost cause & it became a combo.

[daxliniere]

Thanks @lonote
Here's a rough video I filmed while documenting the reverb tank placement process: https://youtu.be/1gFeXflm-Pg?si=WMk6AdAzqxx8UI4I

Also, I realised the attachments didn't 'stick' to my previous post which was supposed to have photos, so I've corrected that.

[daxliniere]

..separating the two cathodes of the driver tube and installing seperate grid resistors on it is often enough

Ahh! I wasn't aware ofthe separate grid resistors mod. I added separate cathode to ground resistors+caps, but didn't know about this.

[Roe]

..separating the two cathodes of the driver tube and installing seperate grid resistors on it is often enough

Ahh! I wasn't aware ofthe separate grid resistors mod. I added separate cathode to ground resistors+caps, but didn't know about this.

try and see if it helps. use 22k or higher values on each grid

[daxliniere]

try and see if it helps. use 22k or higher values on each grid

I'm a bit confused. There isn't current a series resistor feeding the parallel grids. The pairs of 470K resistors seem to just be isolating each channel output from the other's output. (But this is just a guess.)

!reverb driver.jpg

If you're suggesting they're a new thing that needs to be added, that's fine; very easily done in-line with the wires going to pins 2 and 7. (with appropriate heashrink insulation, of course).

[Roe]

try and see if it helps. use 22k or higher values on each grid

I'm a bit confused. There isn't current a series resistor feeding the parallel grids. The pairs of 470K resistors seem to just be isolating each channel output from the other's output. (But this is just a guess.)
!reverb driver.jpg

If you're suggesting they're a new thing that needs to be added, that's fine; very easily done in-line with the wires going to pins 2 and 7. (with appropriate heashrink insulation, of course).

each grid needs a dedicated grid resistor placed as close to the pins as possible

[daxliniere]

So this would be an addition to the original design, yeah?
What value would you suggest?

Also, I just discovered that if I have the reverb turned to maximum, the HF of the amp is rolled off. It sounds like it's creating a highly resonant low-pass filter. The difference is massive. Any ideas?

Edit: fixed typos

[daxliniere]

Here's the spectrum.
Reverb = 0

Screenshot_20250123-112238.png

Reverb = 10

Screenshot_20250123-112316.png

[Roe]

So this would be an addition to the original design, yeah?
What value would you suggest?

with those two 470ks in front, I'd start with a smallish value such as 10-22k on each grid

[Roe]

So this would be an addition to the original design, yeah?
What value would you suggest?

Also, I just discovered that if I have the reverb returned at maximum the HF of the amp is rolled off. It sounds like it's creating a highly resonant low-pass filter. The difference is massive. Any ideas?

perhaps try to rewire the reverb pot in the SSS manner: swap hot and wiper (not ground). This may help perhaps unless you t turn the reverb to 10

[daxliniere]

Cool, thanks Roe.
So my to-do list is:
1) Replace 470R 1W carbon resistors on output tubes with ultra low noise Vishay metal films. (waiting on stock)
2) Swap reverb pot in and out wires
3) Add 10-22k resistors in series with reverb drive grids


For the 'Miller Effect' caps (Cgk) I added to the first stage triodes for both channels, I used 100V caps because that's all I had to hand at 100pF. Are these fine or do they need to be changed for a higher voltage rating?

[Roe]

Cool, thanks Roe.
So my to-do list is:
1) Replace 470R 1W carbon resistors on output tubes with ultra low noise Vishay metal films. (waiting on stock)
2) Swap reverb pot in and out wires
3) Add 10-22k resistors in series with reverb drive grids


For the 'Miller Effect' caps (Cgk) I added to the first stage triodes for both channels, I used 100V caps because that's all I had to hand at 100pF. Are these fine or do they need to be changed for a higher voltage rating?

100v is ok between grid and ground or cathode but not if you connect it to the anode/plate.
1) won't matter much but keep the 1w rating since these work as fuses if tubes short

[Stratscratcher]

On the reverb noise, I have two of these amps that are in head cabinets. I replaced the discrete wires leading up to the reverb knob with a shielded cable connected directly to reverb pot. I cut the now unused copper patterns on the board right at the pot so they would not act as antennas. Helped quite a bit reducing feed back and noise.