wad 300b pse monoblocks feedback problem

[Princo]

Just finished building 300b monoblocks, all is fine when FB not connected, but when FB connected the bias voltages are all over the place.(see attached )

Have checked wiring numerous times and all seems fine, both amps have the same issue so it must be something i've done but i just cant figure it out.

The rectifier is a 5AR4 so the voltages are a little higher than design

[John Caswell]

As far as I can see the circuit is correct. It is quite possible that you have the feedback connected the wrong way around. Have you placed a loudspeaker on the output to listen to the sound. With the feedback incorrect you will possibly get a "howling" or "motorboating". If motor boating you can do one but not all of these.
Swap the feeds from the cathodes of the ECC82, swap the green and purple leads from the o/put tx, or swap the leads to the primary of the o/p tx.

John

[Princo]

Thanks John,

I put them in my system and run them for a few hours with NFB, they sound fine, very quiet.

When i connect the FB and power up on test with a 10w 10r dummy load, the load resistor gets very hot in a relatively short time (with NFB the dummy load resistor is barley warm), so i am reluctant to leave it switched on, i haven't listened through speakers with FB connected.

When you say swap the leads from the cathodes of v2 do you mean like this(attached)?, is this to do with phasing of the feedback?

Do you think the o/p tx is wound in opposite direction?

Jim

[A Stuart]

Originally Posted by Princo

with NFB, they sound fine,

I would not wish to stick my oar in and confuse the issue, but I sense that you are writing "with NFB" to mean "with no feedback", whereas I (and I think the rest of us) would take it to mean "with Negative FeedBack".

Alastair

[Princo]

Yes, by NFB i mean no feedback.
thanks

[John Caswell]

Hi Jim,
Yes exactly as you have drawn - it changes the output phase 180º, which will reverse the feedback from positive to negative.
It is quite possible that the output transformer is wound differently to the original (assuming it is an replacement/clone).

John

[Princo]

Hi John

Thanks for the feedback, i first tried swapping the cathode connections on V2 as per the sketch, this didnt work so i put them back and swapped the o/p tx primary wires and this worked.

You are correct the tx is not an original, its a 3rd party make, all else is fine only the primary.

I will probably not use feedback at the end of the day but im very happy to understand what the problem was.

Many thanks to you and Richards help

Jim

[John Caswell]

Hi Jim,
Mea maxima culpa. Not only that I am an idiot. The swapping of the cathode leads would not/could not work because it is a PSE amplifier and the ECC82 is just used purely as a driver, so both cathodes have the same phase signal on them, whereas it is used as a phase splitter as in the 300b p-p.
Put it down to senile decay
At least swapping the primary leads has cured the problem - the secondaries would have done the same of course.
The difficulty with "clones" is that often the clone manufacturer does not have the original drawings to hand so can identify the readout wires as he wishes.

John

[Princo]

Thanks again for the help