Hammond 154EA might work I dont think the gain and driver stage draw so much current.
The first thing junked were those so called Rubycon Fotoflash caps. They make the amp sound REALLY BAD.
There is space under each output transformer to do all sorts of stuff. Even a Pete Millett MAIDA regulator if you want.
I did end up bridging them mono by tying the 12AX7 cathode and grouding 1 grid and using the 2 + posts as output.
The result: a Muscle triode monoblock.
The first thing junked were those so called Rubycon Fotoflash caps. They make the amp sound REALLY BAD.
There is space under each output transformer to do all sorts of stuff. Even a Pete Millett MAIDA regulator if you want.
I did end up bridging them mono by tying the 12AX7 cathode and grouding 1 grid and using the 2 + posts as output.
The result: a Muscle triode monoblock.
The PS is driving both channels so I think the choke need to be around 350mA to 400mA where it's really big to fit inside the chassis. I think my PT is 500mA.
So you separated to 2 - 12ax7 and wire them parallel in order to run as mono for each channel? Can you post the picture what it looks like.
The original design schematic is 0.1uF coupling caps. I put 0.47uF and sound better.
Mine is everything on the PCB so it was hard to desolder the caps.
So you separated to 2 - 12ax7 and wire them parallel in order to run as mono for each channel? Can you post the picture what it looks like.
The original design schematic is 0.1uF coupling caps. I put 0.47uF and sound better.
Mine is everything on the PCB so it was hard to desolder the caps.
Connect pin 3 to pin 8 and connect rca right channel to ground using left channel input. Then connect speaker wires to L 4ohm red and R 4 ohm red.
Replace 12ax7 with 5751.
Regarding choke. Replace that 1k drop resistor with choke.
Regarding the Fotoflash caps, just snip them and replace with anything decent. improvement is way more than just crowding the amp with boutique coupling caps.
Replace 12ax7 with 5751.
Regarding choke. Replace that 1k drop resistor with choke.
Regarding the Fotoflash caps, just snip them and replace with anything decent. improvement is way more than just crowding the amp with boutique coupling caps.
Not yet, I have 2 - 100uF/250v Nichicon caps that I may swap out or order the panasonic 500v square film caps and the choke as you mention, that would give the 6sn7 around 18v more to where the schematic is. Need to see what else I need to order.
I have extra Vishay 50uF/550V square film caps that I used with preamp, I think it's the same as panasonic
I have extra Vishay 50uF/550V square film caps that I used with preamp, I think it's the same as panasonic
Last edited:
No longer has hum, it'd dead quiet now. Hum was a nasty one to fix on this one. It caused by many other issues combine.
Broskie 2 diodes approach was a first try that didn't affect anything. Even tried later still no solve problem. Even tried Broskie's Ground House.
Tried many different GND point but not the problem either.
The previous owner:
-Converted 12ax7 and 6sn7 to DC filament at 6.3vAC so the 3 tubes got around 5.0vDC. I removed and put back to AC voltage.
-He used 290vAC line with Bridge for 300B and 310vAC with Half Wave for 6sn7 and 12ax7. This was one of the issue of the hum problem. I ended up convert everything to run at 340vAC with Bridge diodes for all tubes at 120vAC input. This also reduced a lot of AC ripple. The PT has 10v, 290v, 310v, and 410v at 110vAC, so when I push to 120vAC, I was able to use the line 340vAC
-6.3 C.T. didn't work out so I removed and add 2-100ohm to the filament to the GND
So the 2 steps to get rid of the hum were PS supply and 100r resistors on 6.3vAC. I walked backward to any GND loops that I have tried before and Broskie 2 diokie, it's really not affect to any hum problem.
I think my OT and PT are different from yours KT88.
It does help a lot to run at 120vAC. The 5.0vAC filament can run at 4.8vAC. If I run it at 110vAC, the filament is around 4.4vAC. Use resistors for 6.3vAC voltage drop from 7v to 6.4vAC. No longer need variac for 110v
I actually like this amp very much where I got with all tubes for around $150 or $200. I did look to find another one but no success.
Broskie 2 diodes approach was a first try that didn't affect anything. Even tried later still no solve problem. Even tried Broskie's Ground House.
Tried many different GND point but not the problem either.
The previous owner:
-Converted 12ax7 and 6sn7 to DC filament at 6.3vAC so the 3 tubes got around 5.0vDC. I removed and put back to AC voltage.
-He used 290vAC line with Bridge for 300B and 310vAC with Half Wave for 6sn7 and 12ax7. This was one of the issue of the hum problem. I ended up convert everything to run at 340vAC with Bridge diodes for all tubes at 120vAC input. This also reduced a lot of AC ripple. The PT has 10v, 290v, 310v, and 410v at 110vAC, so when I push to 120vAC, I was able to use the line 340vAC
-6.3 C.T. didn't work out so I removed and add 2-100ohm to the filament to the GND
So the 2 steps to get rid of the hum were PS supply and 100r resistors on 6.3vAC. I walked backward to any GND loops that I have tried before and Broskie 2 diokie, it's really not affect to any hum problem.
I think my OT and PT are different from yours KT88.
It does help a lot to run at 120vAC. The 5.0vAC filament can run at 4.8vAC. If I run it at 110vAC, the filament is around 4.4vAC. Use resistors for 6.3vAC voltage drop from 7v to 6.4vAC. No longer need variac for 110v
I actually like this amp very much where I got with all tubes for around $150 or $200. I did look to find another one but no success.
I wonder what R6 R7 are used for. R6 is the plate resistor but R7 with a cap makes little sense to me.
R6 is the plate load for the input tube.
The series network of R7 and C4 are the high frequency dominant pole.
They are there to set the high frequency gain and phase, so that the high frequency gain and phase of the rest of the circuitry will make the global negative feedback have good stability.
Without the dominant pole, the amplifier might turn into an oscillator.
The series network of R7 and C4 are the high frequency dominant pole.
They are there to set the high frequency gain and phase, so that the high frequency gain and phase of the rest of the circuitry will make the global negative feedback have good stability.
Without the dominant pole, the amplifier might turn into an oscillator.
Much appreciate it gents. The NFB resistor of a mere 4.7k seems odd.
Is it because the gain of the 12ax7 is so darn high that they are applying a lot of global NFB to suppress it and to prevent the inevitable oscillation from too high NFB it uses a double dose of high frequency stabilization w the paralled cap as well as that low pass filter on gain stage plate?
Is it because the gain of the 12ax7 is so darn high that they are applying a lot of global NFB to suppress it and to prevent the inevitable oscillation from too high NFB it uses a double dose of high frequency stabilization w the paralled cap as well as that low pass filter on gain stage plate?