My Mullard 5/20

[billinchile]

I recently purchased a home-brew amp here in Santiago and I need some help to get it up and running. I suspect that it is a stereo version of the Mullard 5/20 design. The tube layout is GZ34 (2), EL34 (4), EF86 (2), and 12AX7 (2). It is hefty (35 lbs) spaciously laid out on a 21"x 9" chassis. I brought it up on a variac and got a loud, high-pitched screech from the speakers. What additional information is needed to diagnose the amp? Where do I begin to look?
Thanks for any help.
Bill

[SY]

Check the power supply first. You can disconnect the power stages and power up the supplies with a dummy load and see if they're functioning properly.

How old is the amp? If it's a veteran, plan on replacing all the electrolytic caps and the tubes.

[billinchile]

Originally posted by SY
Check the power supply first. You can disconnect the power stages and power up the supplies with a dummy load and see if they're functioning properly.

How old is the amp? If it's a veteran, plan on replacing all the electrolytic caps and the tubes.

I'm not sure how old it is, but it has obviously been around for a while. A label inside the chassis indicates that the builder was a professional, since it reads "Juan A. Zahbla V. Radio Técnico. Ex-Laboratorista, S. Karlson, Brasil". But the amp has been through other, more and perhaps less skilled hands. Most of the resistors and many of the caps look new. But the two multi-section cans look very old and were sourced in Germany.
The speaker taps contain only a ground and hot tap per channel, and I'm not sure what the impedance for them is. Any way I can check? How do I disconnect the power stages and power up with a dummy load? How do I know if they are functioning properly? Thanks for your patience; you can see that you are dealing with novice.

[SY]

Tube amps are different than transistor amps in that they carry a MUCH greater shock hazard. If you don't have experience working with these voltages, make sure you do this under supervision. Death is very permanent.


That said, you can estimate that the output stage will draw about 100 ma per channel at idle. The nominal B+ will be something like 400-450 volts. If you can't figure out how to calculate the resistance and power of a dummy load from this information, and how to disconnect the output stages and substitute the dummy load, you'd really, really do yourself a favor by not digging around in there without a seasoned tube veteran standing next to you.

[billinchile]

Originally posted by SY


Tube amps are different than transistor amps in that they carry a MUCH greater shock hazard. If you don't have experience working with these voltages, make sure you do this under supervision. Death is very permanent.


That said, you can estimate that the output stage will draw about 100 ma per channel at idle. The nominal B+ will be something like 400-450 volts. If you can't figure out how to calculate the resistance and power of a dummy load from this information, and how to disconnect the output stages and substitute the dummy load, you'd really, really do yourself a favor by not digging around in there without a seasoned tube veteran standing next to you.

Thanks for the warning. Unfortunately, we are talking 3rd world here, with no "seasoned tube veterans" in sight (imagine a city with 2 million inhabitants without a single tube supplier or vintage equipment repair shop) and the only source of components or help being the internet.

[SY]

There is an active ham community in Santiago- I'd start there first.

BTW, it's a great city. I love the place.

[billinchile]

I have nothing against Santiago when the smog goes away after a rain. Then you get a nice view of the mountains. Besides, there is decent wine reasonably priced. As for a "a seasoned tube veteran standing next to you", Apparently mistakenly, I was under the impression that is precisely what this forum is about.

[EC8010]

The problem is that a seasoned electronics engineer would only need to glance at your amplifier for a couple of seconds to spot something glaringly dangerous. If you are able to post some photographs of the inside of your amplifier, that would be very helpful.

The screech sounds like positive feedback. Have the output transformers been replaced? Is there evidence of fresh soldering? Does it definitely screech out of both channels?

The Mullard 5-20 circuit is published all over the place. A web search would probably find it.

[SHiFTY]

Try this:

1) with the amp unplugged and disconnected, put a ~100ohm resistor temporarily across the high voltage capacitors (from + to - of each) to discharge the high voltage. You might also want to read the safety thread in the forum...

2) Then have a look at the output transformer connections. You will see two wires that go to the speaker connectors for each channel ( the secondary of the transformer). The positive speaker terminal will have another wire to a resistor which will go back to the preamp stage. find the value of this resistor.

3) disconnect this wire for each channel and tie it off (use some tape or something) temporarily.

4) Try bringing the amp up on a variac again, see if it squeals.

5) If it is quiet, then try reversing the output transformer secondary leads and reconnecting the resistor.

[billinchile]

Thanks very much for the information. I'll get to work on it. Meanwhile, I think I need to better understand what kind of beast I am looking at here. Because of the tube complement, I assume it is a Mullard 5/20 design. But looking at the schematic for that amp and the parts list, I see some significant differences. Three of the EL34 output tubes have a 500 ohm cathode bypass resistor and one has a 700 ohm. Each of these tubes has a .1µf 1000V oil cap connected to pin 6. Two of the output tubes have a strange component mounted on on the base. I have attached a couple of pictures. Hopefully, someone can tell me what I'm looking at here.
Thanks.