PP 6B4G Mono Block Issues

[Blind Lemon]

New Build: Peter Millet Design PP 6B4G mono blocks.

The Issue that I'm having is, with 1v 1000K signal on the inputs of both amps I'm getting different signal levels out at the speaker jacks. (.843mv vs .279mv) The sign at the grid of the power tubes is the same amplitude on both amps. Plate voltage on the power tubes is very close on both amps < 10v. Bias on both amps is about the same ~45v.

The only differences in the 2 amps is one has a new 1625 opt and the other has an old one I had on hand. I'm starting to wonder about the heath of the old 1625 on the lesser performing amp. Double checked both are wired for 8 ohms.

Thought I'd check in with the brain trust for any ideas before I jump out and buy a new 1625.

BTW first time building DHT amp.

BL

 

[rkoonce]

Have you tried moving the output tubes to the other amplifier to see if the issue follows the tubes? You can also swap the output transformers to see if the problem follows the transformer.

 

[grovergardner]

Have you measured the primary resistance of the two OPTs?

 

[rayma]

Are you measuring the amplifier under proper loading?
It would be easy to miswire the OT secondary. Otherwise swap the OTs between amplifiers.

 

[Blind Lemon]

I have swapped all tubes. Have not swapped the OPTs. Guess that might be next.

Thx for the responses.

BL

 

[rayma]

If you have swapped the tubes and checked the wiring, especially the secondary connections, then swapping the OTs is the next step.
But if that points toward the OT, the question then is which one is bad, or else are they both good but electrically different?
You can check the OT turns ratio with a signal generator. Also the secondary is not grounded.

 

[jhstewart9]

C10 in the Schema is interstage +ve FB.
The finished amp might be unstable.

I've used that sparingly to get some extra gain when I needed it.
But that was inside a -ve FB loop. 🙂

 

[jhstewart9]

Looking at that circuit again, the 0.25 cap C11 increases the common mode gain of the stages one & two.
That can result in Power supply problems as electrolytics age. The evidence is usually motorboating of the
complete amp.

We would have the ask Pete M why he put C10 in the cct.

The amp could benefit from bootstrapping the final stage to make driving easier.🙂🙂

 

[Blind Lemon]

Replaced the above link with an image of the schematic.

Heres a pic of the mono blocks.

Getting ready to swap OPTs side to side.

BL

 

[Blind Lemon]

Looking at that circuit again, the 0.25 cap C11 increases the common mode gain of the stages one & two.

Don't find a C11 i n the crct.

We would have the ask Pete M why he put C10 in the cct.

Isn't it to keep 40vdc off the grid #1 of V5.


Wellllllll, I found a wiring error, huh go figure. All my fault. So the good news is the transformer is just fine. And everything seems to be working.

Thx, for the suggestions and support. Sometimes just verbalizing an issue helps solve it. 🤣

BL

 

[grovergardner]

Good!