Tube Amp Archive

First off, thanks again for the great help on this one. I couldn't have done it without you. I've uploaded an updated schematic. What I did: o Eliminated V1b gain stage. o Removed 470K grid stopper from V2b-7 to ground. That snapped the V2 voltages right back in spec. o Removed standby. o Tweaked fi...

Thanks rd. I will look into that. I was just removing the standby yesterday. I *think* B+ is connected to pin 2. Can you elaborate on why this is bad? I realize "it seems to be working" isn't always a good rationale for anything. Aside from all the other good suggestions I have to point out a mistak...

OK, so if I understand you correctly, the lower measured plate voltages are just due to a larger IR drop across the plate resistors? That probably explains why Fender schematics with 12AT7 PIs run the plate voltages on the pins at ~315V, instead of 260V for a 12AX7. Thanks. A 12AT7 draws about 3-4 t...

I decided to take rawnster's suggestion about trying to get the voltages back to spec on the original Fender schematic. I reinstalled the original 27K/4K5 dropping resistors. I decided to be more thorough with my measurements this time (measure plate voltage at all 4 plates). It turns out that I thi...

I don't have a 5751, but that would be interesting to try.

rawnster wrote:Mike, I would want to get the voltages back to where they were, or per schematic, and attempt to fix the issue. Have you tried a 5751 in V1?

I'm pretty darned happy with the amp after changing to the 12AT7. Early breakup, too much or farty bass doesn't seem to be a problem now. Originally, the power stage coupling caps were .1uf like the AA964. But rawnster and I changed them to .022uf to try to tame the bass a bit. However, now I wonder...

What would that do?

Jana wrote:You might also want to try a 1uf bypass cap on the first stage instead of the 25uf.

I decided to swap in a 12AT7 in the PI. Instant major improvement. Bass tightened up dramatically. Amp is now clean through the entire volume control. I see from bassman schematics that 12AT7s can take up to 315V on the plate. So I may monkey with the dropping resistors trying to find the optimum vo...

Yup.

Firestorm wrote:

Atmospheric wrote:Next, I jumpered around V1b (wired volume control wiper to V2-2).

You also disconnected the existing connection at V2-2, right?

Changed the dropping resistors to 18K and a jumper wire. Voltages are: V1/V2 plate=247V B+ to V1/V2=371V I got a tiny bit more volume out of the amp, but also kind of a nastiness on the top. I suspect that 247V is probably too high. Next, I turned the bias down to 18mV. That helped a fair bit. Monke...

First, thanks one and all for the responses. You need the voltage at the node as well. Well, knowing the size of the resistor and the voltage at the pin you can work backwards... but the pin voltage only tells you half the story. Unfortunately, I had already removed the dropping resistors before I p...

all plate voltages measured at tube pins.

Also, power tubes biased at ~22ma.

I've got this homebrew Princeton-oid thang than rawnster helped me build last Fall. I basically use it as a pedal platform for absurdly low stage volume situations. However, the amp just breaks up way too soon for me (about 1/2 up on the volume, after that it sounds pretty ratty). It's also puts out...

We replaced the bias tap resistor and the diode and 100uf cap on the bias board.

Thanks one and all for the help. Much appreciated.

@martin: With my DMM at the bias test points, power on and Standby off, voltage starts around zero and gradually gets more negative. With Standby on, it reads ~20V DC. At no time is there any appreciable AC present (seems like bias diode is rectifying). @jjman: This build is a Princeton derivative. ...