aa764 Champ first build voltage issues

[dcline]

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 give this a break for a couple days. I've run out of Kerosene to keep my shop warm and I've been spending so much time out there that I've been ignoring my household duties.
Will get back to it in a couple days and will post the results by the end of the week.
Thanks again

[Colossal]

Gonna have to give this a break for a couple days. I've run out of Kerosene to keep my shop warm and I've been spending so much time out there that I've been ignoring my household duties.

It's good to walk away as well when/if you're exhausting all ideas and still coming up short of the root cause. Often changes to parts can masked or even make a problem worse. Come back at it with a marked up schematic with all DC and AC voltages at every point in the amp and a fresh perspective. Oh, and some more kerosene too.

[jjman]

Have you tried disconnecting the NFB to ensure it's not positive?

[rdjones]

Coming at this from a different angle, what output transformer are you using ?
What is the impedance of your load (speaker) ?

Something else that may help is to operate the amp in the dark and watch for arcs, sparks, and weird tube flashes during the bad behaviour.

rd

[surfsup]

Its hard to see what's up with your tone stack stuff due ot the two large orange caps in the way. I see the red lead off the plate of the first stage that runs under the pots to the term strip and then I see what looks like the 100k off that where it should be the 100k/250p. Can't see where the 250p is connected.

Also the 100k, is it 100k? It doesn't have an orange band like the two 100ks on the plate load resistors.

[dcline]

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 that came with the schematic and the voltages I'm getting now. The only strange voltages seem to be in the rectifier tube. Pin 8 has to be 400 volts plus because it blows the circuit breaker in my desktop meter. Voltages on the pre-amp and power tubes look good.

I used the exact OT that the amp called for. 7000 ohm primary and 4 ohm secondary with a brand new Jenson 4 ohm speaker.
What does NFB stand for? Not ringing a bell at the moment.

I ran a 450 Htz tone at 1 volt into the amp. Pin 7 showing 1 volt AC and pin 6 showing 46 volts AC. Not sure why but pin 1 and 2 only show voltage for a second and then dissipates into nothing. "0"

Once again I'l like to point out that I'm very happy with the sound of the amp. The only problem happens when I turn the treble past 3/4's when the volume is all the way up. It's almost as if it's getting too much high frequency but I'm not really sure what is going on here. Don't let it do it for long because I already blew one OT. I wonder if I went to a smaller value pot if it would solve the problem?
I'm giving this amp to a female friend once it's done so it's gonna be right.

[Phil_S]

The amp called for a 600v 50ma PT and the closest I could find was a 650v 70ma PT.

...voltages seem to be in the rectifier tube. Pin 8 has to be 400 volts plus because it blows the circuit breaker in my desktop meter.

I get 410V expected. That's [650*sqrt(2)]-60 = 410. It's a bit high for this amp. Even though you can get the preamp tube in line with the spec, your power tube is running very hard. You might consider doing something to drop the screen voltage more. I think you were on the right track changing to 5K. Try 10K or even 20K. You may want to lower the next dropping resistor so preamp voltages will come up after you reduce screen voltage. Also, since this is a cathode biased amp, changing the cathode resistor won't do that much, but I think I'd go up a step or two, maybe to 560R to see what that does. Raising the Rk may have the effect of lowering Va, which is one of the goals here, IMO.

A better solution would be a 600V PT, but that's not what you have. There are all manner of tricks people employ to reduce output voltage, but I am not fond of any because the all make a lot of heat.

[dcline]

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

[rdjones]

You have a high voltage conspiracy.
You schematic calls for 117V supply but I bet it runs high like 125V.
As you noted your HT secondary is higher than spec'ed.
The rectifier is not a 5Y3, the Sovtek tube marked "5Y3" is a Russian 5C4M or whatever the Crylic translation is.
It has a indirectly heated common (single) cathode that is more efficient than the directly heated individual filament/cathodes of a real 5Y3.
This type of construction is nothing at all like a 5Y3, the "X pattern" plates and single shared cathode are the dead giveaway.
It does raise the voltage out compared to a proper 5Y3.
You schematic is vague as to which pin is supplying B+ from the "5Y3".
It should be pin 8 on a tube with a real cathode, the cathode is connected to pin 8.

You may have a problem with your meter.
If it's blowing a circuit breaker that means it is drawing current, since a fuse or circuit breaker is rated in amperage and is designed to open when there is a current overload not voltage.
Are you measuring voltage or current ?
Most meters have a fuse to protect it from damage when measuring current but this should not effect voltage readings.
You might look into getting a meter that can measure the voltages you are dealing with.

I can't say for certain that the voltage issue is causing the sound you are hearing, but getting voltages under control should be near the top of the list.
Are any of the tubes arcing internally when the offending sound is heard ?

NFB = negative feedback
here's your schematic with the feedback path highlighted.

[Phil_S]

I was avoiding the whole thing that RD got into, but now that I read it, I think it has much more weight than my initial impression.

I was looking at a simplified approach to the problem, one that might get the amp running better, though it might not have the sound you were after initially. I am thinking that you can abuse the plate with high voltage, but you really have to control the screen voltage when you do that. I figured that lowering the screen voltage would allow the tube to operate better in such a high voltage environment. I'm not positive this is the case, but it is relatively simple to modify the B+ ladder and see.

Your fundamental problems with the build are more structural in nature:
-Transformer secondary should be 300-0-300 not 325-0-325
-Rectifier should be a real 5Y3, not Sovtek; if you must use the Sovtek, then PT secondary should probably be around 275-0-275 to compensate for both the rectifier and the high voltage conspiracy.

Until you address the real problems, you might not be able to get this amp to do what you want. I don't really know.

[dcline]

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 what happens.

[dcline]

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.