HELP: Eimac 75TL amp high frequency roll off

[HP8903B]

.

 

[tricomp]

Hi,

Schematics and operating conditions...?

Running this monster like you do (Class-A SE?) seems impossible.
Is the Lundahl undersized for this in any case?
What kind of voltage drive do you have with a 7788?

Class AB2 push-pull is when it delivers ..at a price.
Driving power is something you get from a FET or a suitable PP driver transformer and a reserve of 5-10 watts.

I'm no pro, just curious because what you are trying to do is very far from what I expect to be possible.

rgds,

/tri-comp

 

[HP8903B]

It is single end.

The Lundahl LL9202AM 50mA is perfect for this amp.
I run the Eimac 75TL at 45mA.

7788 voltage is 250V.

This amp was once worked very well.

Also, I figured that the driver stage response is fine.
It must be the output stage.

 

[HP8903B]

Schematic

 

[hooman]

your shematic is not so clear . used feedback ?

 

[bear]

Hey, you have just one channel?? Mono?

Both channels do the same thing??

HF noise? Could be arcing?

Got scope??

Replace the output transformer with a suitable power resistor of the same impedance as the primary and look at the signal...

No scope, tougher to find the problem.

Either that or something is shunting the highs to ground, or the like.


_-_-bear

 

[tricomp]

Headphone output socket shorted?
That would put 680nF from the 7788 anode to ground and thereby remove all high-frequencies.
Not much else left except the output transformer.
At least when you say this configuration worked in the past.

rgds,

/tri-comp

 

[HP8903B]

No feedback.

It is a stereo amp. Only one channel is shown on the schematic.

Both channels do the same thing.

I do have a tektronix scope.

 

[HP8903B]

I removed the headphone jack and the capacitor.

 

[HP8903B]

I disconnect the ground wire to the negative of the OPT secondary.
The high frequency is a lot better like this. It is still a little weak.

 

[djn]

Would it change much if you went higher with your coupling cap.. Maybe .100 uf?

 

[HP8903B]

It actually has .1 uf cap

 

[hooman]

stopper resistor on g1?

 

[HP8903B]

there is one resistor there.

 

[HP8903B]

I swap the OPT and the high frequency is fine.

Perhaps the Lundahl needs an 8 ohm load on the secondary to have proper response.

 

[Michael Koster]

I swap the OPT and the high frequency is fine.

Perhaps the Lundahl needs an 8 ohm load on the secondary to have proper response.

From this information I'd say the HF noise being due to transformer insulation breakdown (arcing) is a definite possibility. You should have a load on your OPT. I hope arcing hasn't damaged your expensive amorphous Lundahl

Where does the other tap on the OPT secondary go? On your schematic there's a note but I can't read it.

 

[HP8903B]

I highly doubt there is insulation breakdown.

On the secondary, one tap goes to positive speaker terminal, and one tap goes to negative speaker terminal.

 

[HP8903B]

Could someone take a look at the Lundahl datasheet for the LL9202 ?

I wired it in "C" configuration.

I understand the primary connection, all windings in series.

I don't understand how the secondary is wired for "C".

Could someone take a close look and make sure that "C" is a proper way to wire the secondary ?

 

[HP8903B]

"C" is for 11k : 8 ohm

 

[Michael Koster]

I highly doubt there is insulation breakdown.

On the secondary, one tap goes to positive speaker terminal, and one tap goes to negative speaker terminal.

So you do have a speaker connected? It occured to me that with nothing connected to the secondary, the 75TL plate resistance might not be enough to damp out some high voltage overshoot. You did mention sound coming from the OPT?

C is a good connection for an 8 ohm speaker.