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Old 26th April 2012, 04:03 AM   #1
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Default HELP: Eimac 75TL amp high frequency roll off

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Last edited by HP8903B; 27th September 2013 at 06:13 AM.
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Old 26th April 2012, 08:19 AM   #2
tricomp is offline tricomp  Denmark
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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
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Old 26th April 2012, 08:49 AM   #3
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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.
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Old 26th April 2012, 08:54 AM   #4
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Schematic
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File Type: jpg 75TL Schematic.jpg (244.5 KB, 283 views)
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Old 26th April 2012, 12:28 PM   #5
hooman is offline hooman  Iran
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your shematic is not so clear . used feedback ?
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Old 26th April 2012, 02:44 PM   #6
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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.


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Old 26th April 2012, 05:27 PM   #7
tricomp is offline tricomp  Denmark
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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,

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Old 26th April 2012, 05:55 PM   #8
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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.
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Old 26th April 2012, 05:57 PM   #9
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I removed the headphone jack and the capacitor.
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Old 26th April 2012, 06:00 PM   #10
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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.
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