• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

My Mullard 5/20

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Family_Dog said:
OK, that puzzles me a bit too, as the VTVM should have negligible loading effect on the EL34 cathodes. Have you access to a circuit diagram? There was a circuit diagram in one of the links I referred you to earlier.

I do have a circuit diagram, and pin outs for each tube, but they do little good if I read them carelessly.

Bill, I have just checked... Pin 5 on the output tube (EL34) is the control grid (g1) of the valve. That is a high impedance circuit, although your VTVM should still have negligible loading effect. You will have to check the circuitry there, including the 0.47uF coupling caps, and the values of the grid resistors, which should be 470k matched to within 5% of each other, while the ECC83 anode resistors should be 180k, matched to within 5% of one another. I would suggest measuring all resistors for accuracy.[/B]


You are correct, of course; pin 5 is the control grid. The 0.47 µF caps are new. The grid resistors measure 468,467,467, and 466 ohms. Originally, the builder had installed trimmer caps on two of the EL34 tubes. They looked ancient, and were not included in the original schematic, so I removed them. Should I consider putting them back in?
The four ECC83 anode resistors measure within 6 ohms of each other. BTW, I am running NOS Westinghouse 5751 tubes because I was trying to lower the sensitivity of the amp, and their output is purportedly lower than ECC83.


Have you tried removing all the tubes from each amplifier in turn while powering up, ie you only have one amplifier running at a time. Wind the voltage up slowly and measure all the cathode & other voltages with only one amp in circuit. I wonder how good your original EL34 cathode caps are? For a rough test, remove one amplifiers EL34 caps & parallel them temporarily with the opposite side and see what happens. Also, swap the tubes around.[/B]


I didn't know that it is OK to run an amp without some of its tubes. Can all of them be removed? Is there a recommended sequence?
The high frequency noise appears in both speakers. In this case, would tube swapping help pinpoint the problem?
 
diyAudio Senior Member
Joined 2002
Hi,

The grid resistors measure 468,467,467, and 466 ohms.

Wouldn't those be the cathode resistors?

BTW, I am running NOS Westinghouse 5751 tubes because I was trying to lower the sensitivity of the amp, and their output is purportedly lower than ECC83

Not by all that much.
I doubt you'd notice the difference in an amp with global feedback.

Originally, the builder had installed trimmer caps on two of the EL34 tubes.

Where exactly on the EL34s were these installed?

The high frequency noise appears in both speakers. In this case, would tube swapping help pinpoint the problem?

I doubt it.

Have you replace the cathode bypass caps on the EL34s yet? Their likely to be faulty because of age.

Cheers,;)
 
HI Bill,

I agree with what Frank states above. The trimmer caps might be there to compensate for O/P transformer shortcomings if they're connected to the anode (Pin 3). Disconnect them and see what happens, they might be objecting to the high voltage across them.

You certainly can remove all the tubes from one of the amps, but make sure there is no sharing of HT voltage between the two halves of the amplifier, otherwise you would have the HT electrolytics charged with some 450V DC.

The resistor values you mentioned are presumably the resistors between Pin 5 and earth of the EL34s, and are measured in kilo-ohms. That being the case, they are spot on.

I get the feeling that your problem is most likely a capacitor problem, but find it strange that it appears in both channels of the amplifier. Is there any common coupling between them? You did state that there are two GZ34 rectifiers, is the DC path completely independent for each amplifier? It seems to me that something is drawing too much current when you advance the variac. Have you noted the various voltage readings on HT, anode, cathode of each tube just before the high frequency noise starts, and also after it appears?

Bill, have you checked that the feedback components are suitable for your speaker impedance? Do you still get this instability when you disconnect the feedback path? Mullard specified different values for the feedback components for 4,8 & 15 Ohm speaker loads. If you do not use the correct values rated fopr your speaker impedance, frequency instability could result. These components should be within 5% of their rated values.

-Eric
 
Bill,

Another thought - your electrolytic filter caps might have gone high-resistance due to their age. This would have a marked effect at higher frequencies. You might benefit from fitting 0.47uF capacitors across each of the filter caps in the HT line (4 per channel). Ensure their working voltage is higher than your HT voltage.

Also, the grid leak resistors of the EL34 (Pin 5, 2200 Ohm) should be mounted directly on the valve base to prevent parasitic oscillations, ie keep these leads as short as possible. Like wise, the Screen grid resistors, (Pin 4, 1000 Ohm).

-Eric
 
Hi, Eric,

Thanks for checking in with me and my amp. I've been away from it for the last few weeks due to work and travel. I need to try your suggestions. Now that the weather is warming up, I'll be able to take it out on to the terrace - at a distance from family members who ask "are you still working on that thing?"

Bill
 
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