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Old 25th March 2007, 02:14 AM   #11
rdf is offline rdf  Canada
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Nice shots. Measure the DC resistance of the transformer primaries and the no-signal voltage across them under normal operation. It's an easy way to calculate standing current.
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Old 25th March 2007, 03:23 AM   #12
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I see what looks like a 2nd set of I/P jacks. By any chance, is there a power connection for an external tuner?

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The original caps were ceramic discs, if you can believe that.
Yeah, I can believe all sorts of corners were cut in cheap construction.

BTW, before you go to separate EL84 bias networks, try an inexpensive pair of well matched Russian 6p1p-ev/EL84M tubes. The shared bias network will compensate for minor imbalances in the "finals".
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Old 25th March 2007, 04:22 AM   #13
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Quote:
Originally posted by Eli Duttman
try an inexpensive pair of well matched Russian 6p1p-ev/EL84M tubes. The shared bias network will compensate for minor imbalances in the "finals".
You mean 6P14P-EV right?
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Old 25th March 2007, 08:00 AM   #14
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The DC resistance of the output transformers don't match. One is 300 ohms, and the other 287.

They're both fed from the same 331V supply. On the plate side of the transformer's primary winding, one is 316V and the other is 319V. Unfortunately, I don't recall which was which.

By the math, either the one tube is pulling 50 ma and the other 42, or one is pulling 40 ma and the other 52.

Maybe one of my OPT has a shorted winding or two? Can I just stick 13 ohms resistance in series with it?
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Old 25th March 2007, 08:13 AM   #15
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I just asked the same thing in another thread, it's due to normal winding differences. That's in fact a far better match than my transformers. In that case I added a few ohms to the lesser side to even them up. In yours though the difference is pretty much meaningless, pretend it's 295 and call it a day.
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Old 25th March 2007, 01:23 PM   #16
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Quote:
Originally posted by sorenj07


You mean 6P14P-EV right?

6p14p-ev is correct. I dropped the "4", sorry.

FWIW, I intentionally use lower case letters in Russian types to indicate that phonetic transliteration of Cyrillic to Latin has occurred.
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Old 28th March 2007, 01:40 PM   #17
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Quote:
Originally posted by Eli Duttman
[B]I see what looks like a 2nd set of I/P jacks. By any chance, is there a power connection for an external tuner?
No, this unit never supplied power to an external unit. There was one set of inputs labelled "Tuner" and another set labelled "Phono". There is no phono pre-amp built into this unit. I do not recall there being any difference between the two sets of inputs.

There is a third set of jacks which are connected to the output side of the OPTs. There is some kind of voltage divider circuit or something around them which I've mostly ignored.

I think I'm getting to closer to understanding why my tube is starting to glow. I've calculated between 40ma and 52ma idle current on the EL84. I thought this was OK, based on the tube's data sheets. Well, 48ma is the specified plate current - at 250V. Mine actually have more like 320V at the plate. I found "Duncan's Amp Pages", where he has an Excel based tube load calculator. At 52ma, I'm running my EL84 at about 135%. I'm betting this is why it's starting to glow.

Should I just put a bigger resistor on the cathode?
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Old 28th March 2007, 03:04 PM   #18
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Is that 320 V from plate to cathode or plate to ground? The first determines plate dissipation. I say go for it. My preference is about 35 ma along with higher B+ voltages on an EL84, though I haven't tried it with feedback. Every brand I measure shows quickly increasing high harmonics in the distortion with currents above that. Plus the higher grid voltage generally results in a higher Class A power out, and that 12ax7 won't do anything other than Class A. The downside is more 2nd harmonic and a very slightly less damping factor.
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Old 28th March 2007, 09:13 PM   #19
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The 320V is plate to ground. The cathode is about 8.7V (also with respect to ground).

So again, I suppose I can just swap the 80 ohm cathode resistor for something a little bigger? Is there any sense in trying to put some resistance on the plate side instead, or does that just mess up the loading on the tube?

I'm wondering why it's too high in the first place... the resistors around the output tube all seem to be more or less in spec. The only one that seems really off is the 2.1K at the tail end of the power supply. It's a lot closer to 3K.

Could my line voltage throw things that far out of whack? Or maybe they just designed it too hot to begin with?
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Old 28th March 2007, 11:44 PM   #20
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Quote:
Originally posted by Ty_Bower
So again, I suppose I can just swap the 80 ohm cathode resistor for something a little bigger?
That's the way to go. 120-150 isn't a bad place to start. The STC data sheet is very comprehensive.


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Is there any sense in trying to put some resistance on the plate side instead...?
Nope.

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Could my line voltage throw things that far out of whack? Or maybe they just designed it too hot to begin with?
Could be a number of things. I have plenty of transformers designed for 115 VAC nominal while my local utility is 120 VAC. Filament windings here generally need some series resistance to keep from cooking tubes. Could be your power supply transformer was designed for a heavier current load too. You'll probably find B+ rises further still when you reduce the current through the EL84s. Personally I like EL84s run high voltage low current so for me it's not a bad thing, just keep an eye on plate dissipation.

Edit: Sorry, didn't see both channels share a single cathode resistor. 120-150 is probably too high but not a bad thing, the possibility of damage is reduced starting from that end.
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