Tube Amp Archive

Interesting... Thank you!

I'm working on a 76 JMP 50 watt master volume model. Someone has modded it with the post PI, dual pot master volume. I always thought these amps sounded great with the stock pre PI master volume. I have yet to put new tubes in it to come to a conclusion. Sounds pretty buzzy right now. Just wanted to...

Another thing RD,
The AC from my wall socket is about 125V.
After changing the rectifier tube, I'll plug it into my variac set to 117 and see if I still have the problem.
Thanks again everyone... this little amp has turned into way more of a learning experience then I expected.

I do have some old 5Y3's in the pile. Guess I'll throw one of those in and see what I get. As far as my meter goes... you're breaking my heart. It's a newer desktop and I paid good money for it. I was measuring voltage and did find it strange that it blows the breaker... Thanks very much! I'll post ...

Phil_S
So you believe the problem I'm having is a voltage related issue?

Here is the schematic I used to build the amp with the mods I did. Merely just added a power resistor to bring voltage down and raised the 1k resistor on the filter cap to 4.7k. The amp called for a 600v 50ma PT and the closest I could find was a 650v 70ma PT. Then a comparison of the voltage chart ...

Will do...great advise! Thanks again! Makes total sense. I have an old tube Eico audio generator that I recapped. It's a bit noisy but should do the job. Till now I've only used it for checking bias with my scope. Another tool that I really need to learn how to use to the full extent. Gonna have to ...

Thank you Colossal. I will do that. :) This problem I'm having is driving me crazy to the point that I couldn't sleep last night. I ended up out in my shop at 2 in the morning going through my cap pile. I found a couple ceramic caps that I twisted together to get 500pF. I tried that and the problem ...

Very cool! Thank you!
Unfortunately I use mac and they don't seem to have a calculator for mac.
Guess I'll just raise the value of the 250pF cap and see what I get.
Thanks again

After staring at the schematic... I'm gonna assume that lowering the value of the 250pF cap will bring down the high end?
And what about the 100k and 15k resistors in the tone stack. Will changing the value of them alter the high end?
Sorry if I'm thinking out loud here....

Checked all the pots. They seem to be fine. The bass pot is reading a bit higher then 250k at 259k. I just played through the amp for a bit. It is sounding really good! Has a nice smooth bite to it. The bass and treble are working... it just seems like the highs are going overboard if you crank the ...

Well, didn't have a 5k resistor but had a 4.7k which put all my voltages in tolerance so I'm good there. Plus now G2 is slightly lower than plate like it should be. So voltages on both tubes look good now. I do the same thing and check the resistance and capacitance of all components when installing...

Cathode voltage on the 6V6 is 19.3 which I think is pretty good. The schematic I have says it should be 20. There is about .15v on pin 5 which should be 0. 12AX7 voltages are as follows: Pin 1= 206.9 2=0 3=1.62 4&5=3.42 AC 6=211.8 8=1.49 I've looked over my work for hours checking everything with th...

It's a new Sovtek 5Y3 which I've heard kick out more voltage then the NOS 5Y3's.
Hopefully these pics help...
Thanks again

I finally built my first amp. An aa764 Champ. When completed, I eagerly fired it up and felt like Frankenstein when it actually worked. And at curtain volumes it sounds pretty good! But when I turn the volume and treble control up past 3/4's at the same time it breaks up horribly! I had several sugg...