Strange Power Amp Problem

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Strange Power Amp Problem

Postby Hugh's Wattwerx » Sat Feb 01, 2014 10:28 pm

Can't even think of a succinct search term to describe this, but in a nutshell . . .

Having fried all but two of my available EL34s, I'm trying to test my build with a mismatched pair -- 1 is an EH, the other a Mullard reissue that I had bought for a SE project (this is push-pull, cathode biased) If I have them in one orientation, say, mullard on the left, EH on the right, it sounds fine. B+ is 440V, For testing I'm trying to keep on the safe side, and measure around 22.7 mA from each. If I switch them, however, the Mullard is drawing 222mA, and the EH is drawing zero. I get very little volume, and B+ dives. I turn it off immediately, of course. Switch, back, and it sounds fine. I've attached the full schematic. I thought maybe something was funky in my balance/RMS configuration, so I bypassed all that, and hooked both cathodes up to a single 270-ohm resistor and a 220u cap -- same thing happens.

This may or may not be related, but that 250R resistor (R24) was added to keep the wattage down to where I wanted it (~36W total max), and seems to make the overall cathode resistances way higher than I would have thought correct.

Have I done something obviously stupid here?

Also, parenthetically, that 'Rx' 68k resistor in the preamp, shown in red, was also an ad-hoc addition. When in 'M'(arshall) configuration (12AX7->EF86->tone->PI), everything sounds fine at all volumes. In the 'F'(ender) position (EF86->tone->12AX7-PI), I was getting a high pitch squeal when the volume was turned up. Adding the resistor eliminates it, although the overall volume is decreased slightly compared to the other setting. (I tried a variety of values: 1K still squealed, as did 91k and 82k. 68k stops it, but I have a sense I'm addressing the symptom but not the cause.

Suggestions? Pointers?

(Oh, btw -- the (see inset) note refers to a removed diagram -- it just showed the layout where I mount the tone controls on a tiny perf board directly on a dual concentric pot.)

Thanks,

Joe
A Little Loud.pdf
full schematic
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Re: Strange Power Amp Problem

Postby Hugh's Wattwerx » Sat Feb 01, 2014 10:33 pm

Hmm -- I just stuck an EH 6CA7 (fat bottle) I had around in place of the Mullard, and it seems okay, so maybe the Mullard is bad? Can't understand why it would work in either position. Sometimes it take a little while for the problem to start, and don't want to wake the wife, so will experiment further in the morning.

Joe
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Re: Strange Power Amp Problem

Postby Hugh's Wattwerx » Mon Feb 03, 2014 7:33 am

Not so strange after all -- frayed wire coming from the cathode, intermittently shorting to ground . . . . =:o
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Re: Strange Power Amp Problem

Postby Shannon Parks » Wed Feb 05, 2014 5:26 am

Hugh's Wattwerx wrote:Not so strange after all -- frayed wire coming from the cathode, intermittently shorting to ground . . . . =:o


Occam's razor strikes again! Glad you found it. Sometimes it takes forever to catch things like this.

Shannon
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Re: Strange Power Amp Problem

Postby Gingertube » Sun Feb 09, 2014 11:00 pm

Also that additionof R24 to keep dissipation under control is not too unusual.

For separate cathode resistors on the push pull pair total resistance to 0V for each cathode should be about 470 Ohms (470 Ohms is what the datsheets say). Without R24 you are way below that.

The post PI Master Switch Common connection should go the otherside of that top 27K.

Cheers,
Ian
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