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Plate current and red plate problem
Plate current and red plate problem
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Old 22nd November 2012, 05:24 AM   #21
gingertube is offline gingertube  Australia
diyAudio Member
Join Date: Oct 2004
Location: Adelaide South Oz
It depends upon how the output tranny is wound. Each half primary needs to have the same number of turns but if the most basic winding structure is used (that is, 1/2 primary, then secondary then the other half primary) then the second half primary being on the outside of the bobin may have have significantly more length of wire to achieve the same number of turns, that would mean more resistance.

The fact that you measure very nearly the same resistance on each side shows that your tranny wiring is well interleaved. (meaning the primary windings and secondary windings are split up into smaller sections and then wound in an order from inside to outside such that each primary half ends up with about the same length of wire to achive the identical number of turns). I have seen some where I saw something like 60 Ohms on one side and around 80 Ohms on the other.

So is it normal? - YES for a good quality, well interleaved output transformer which you would normally associate with HIFI. For guitar amp output tranny I would think that equal resistances are probably less usual than un-equal resistances. Resistance measurements of say 2 is to 1 would be highly un-usual and in most cases would suggest that one primary half has some shorted turns and the tranny is faulty.


Last edited by gingertube; 22nd November 2012 at 05:29 AM.
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Old 22nd November 2012, 05:35 AM   #22
Conrad Hoffman is offline Conrad Hoffman  United States
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Join Date: Mar 2007
Location: Canandaigua, NY USA
I just worked on a Peavey TripleXXX amp with almost the exact same circuitry. Best advice is to make or buy a bias adapter, just a male and female tube socked wired straight across, but with a resistor so you can measure the bias current. You can also buy them ready made from Eurotubes for $25 or so. FWIW, the inductance of their output transformer was equal per side, but the DC resistance was different by about 10 ohms.
I may be barking up the wrong tree, but at least I'm barking!
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Old 22nd November 2012, 06:06 AM   #23
dscottguitars is offline dscottguitars  United States
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Join Date: Mar 2011
Thanks Ian, I bought the OT on ebay as a 180 watt. I don't remember what it was from or for but it's a big piece of iron. I'm glad to know it's a quality one.

About the adapter: I've seen those and may get one at some point. But in my future amps I plan on having a quality 1 ohm resistor with a test point by each tube and rubber grommet. One can then pull the tube slightly to measure the plate voltage w/o having to take the thing apart and measure the cathode voltage to bias it correctly and easily.
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