New project

[gui_tarzan]

PI pin 6 reads 251v and when I put a signal on pin 7 it goes to 351v. A hundred volt increase? If I put the signal on pin 2 of V2 or pins 2 or 7 on V1 it raises it one volt on the PI pin 6.

[Tom Phillips]

PI pin 6 reads 251v and when I put a signal on pin 7 it goes to 351v. A hundred volt increase? If I put the signal on pin 2 of V2 or pins 2 or 7 on V1 it raises it one volt on the PI pin 6.

That seems like a meter / measurement problem. You want to be measuring the AC signal component. Maybe your meter isn't good at measuring a small AC signals riding on a large DC offset. Consider the following:
1) You need to state the signal amplitude you are injecting so that we have a reference starting value.
2) The meter should be set on volts AC. You can either measure down line of the DC blocking cap or put a small cap (.01 to .1 uF) in series with your meter lead to block the DC component.
3) The DC level at the plates of the preamp tubes should not change much, if at all, when you inject a signal.
4) You can leave the injection point at the input jack as you trace the signal through the circuit path using your AC meter. As a first guess I'd set the input signal level at 100 mV and the volume control at about 1/3 rotation.
5) Even a small signal should be pretty loud in the speaker. I don't think you have a dummy load. Correct? If you don't you can remove the power tubes while tracing the signal through the amp. The B+ voltages will go up but it shouldn't hurt anything.
6) If you do change the signal injection point be careful not to connect the output of your sig gen to a point of high DC voltage.

As you trace the signal through the circuit you will be evaluating the gain of each stage by comparing the input vs. the output level of the AC signal. If you post those values then we can isolate the problem to the malfunctioning stage of the amp.

[gui_tarzan]

I think I have a meter problem. I'm not getting voltage out of anything now. This is frustrating. I'll check back in when I get my hands on another meter.

[g1]

Well partly, but what I meant was that yours is shown in series with the wiper, it's actually supposed to go across the pot, from hot to wiper, not in series at the output of the pot.

Just to clarify, I'm referring to your updated schematic from post #56. The schematic in post #42 showed the 100pf wired correctly at the volume pot: http://music-electronics-forum.com/a...custom-amp.png

[gui_tarzan]

Yeah, it's across lugs one and two of the volume pot. Here's a pic, don't mind my messy bias circuit or wires, I'll clean it all up once it's working right.

Edit - that's odd, why did I change the .0001 like that in #56? Here's the corrected schem too.

[g1]

In case you get impatient while waiting for the new meter (or if it doesn't give better results), try Tom's trick with the cap in series with the probe (and the rest of what he said in post #77).

[gui_tarzan]

You guys aren't going to believe this... I checked the heater voltage just out of curiosity when I couldn't get good readings on the power supply and I measured 2.3 volts. Yes, the second and third meters read the same. So I checked the 120v coming in and it was 50v. So I checked my power strip, outlet, other outlets in the house, breaker box and they ALL read 50v. So I called the power company and they said I might have a bad leg on the power pole and they'd send someone out. I've had power problems here years ago but not in several years. We have all florescent bulbs in the house so it didn't dawn on me because they don't dim on low power.

Wouldn't that just be the shit if that's all my problems have been with this amp? It would certainly explain why it worked, then it didn't.

[Tom Phillips]

That is wild.
I have had power issues that caused the house feed to be low (about 60 V) However, at that low voltage florescent lights that were off would not turn on. The ones that were on at my house were not full brightness. You must have some real efficient lights. Some appliances started making noises of distress so I turned off the breakers to anything I thought would be damaged by trying to run on the low voltage such as the refrigerator.

[gui_tarzan]

Power company was here, they fixed something on the pole and I have 12.6v across my heater leads now. My tubes are all glowing brightly, here we go!

[gui_tarzan]

I should clarify that. I was concerned about the lack of "oomph" from the heater in my PT so I wired in a separate 6.3-0-6.3 transformer just to heat the tubes. They glowed very brightly just like my rectifier tube did with that addition. When the heater was wired to the PT they were very dim. Now I'm wondering if my PT has a heater tap that's not up to snuff? It is out of a Hammond organ and was powering eleven tubes, now it's only powering five. I'm only getting 6.2 across both PT heater leads and 2.8v to the heater CT from each one. That doesn't seem right.

I just realized what I did with the separate transformer. Twelve volts going to the heaters was not right. but boy did they look pretty!!!

[Tom Phillips]

...I'm only getting 6.2 across both PT heater leads and 2.8v to the heater CT from each one...

6.2V powering the heaters is fine. However, if you are measuring only 2.8v to the heater CT from each side that only adds up to 5.6V. It should be equal to your total 6.2V. I think it's a meter problem again.

[gui_tarzan]

I don't think so at this point Tom, I've got three different meters (my original, my backup and my son's) and my original meter was reading accurately according to the other two. I know that doesn't make sense but I have a hard time believing all three would be equally as wrong. I am confused as to why the CT reading on each would be so low.

Given the power problems we had previously it doesn't surprise me now that I had such wonky readings before.

[Tom Phillips]

Did you make the 6.2V full heater winding measurement and the 2.8V from each side to center tape under the same conditions? That is, with all the tubes installed and same line voltage? If so, I can't think of any explanation for those readings if all three meters are giving the same results.

[gui_tarzan]

Yes, all tubes installed and a consistent 120v line voltage now. I think just out of curiosity I'm going to put that other 6.3v transformer on these heaters (just 6.3, not the full 12.6) and see what happens. I'm really skeptical of that PT right now and this will at least satisfy my curiosity there.