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hey everybody- I just finished my TC-15 kit and fired it up. Sounds beautiful with minimal hum/noise. However, as I was checking my voltages, I noticed that my EL84 plates are getting 365 VDC on them while the screens are only getting 218 VDC. My AC voltages on the GZ34 are pretty high as well- 305 VAC per side. My B+ is also high (with all the tubes in) at 371 VDC. I am getting great tones from both channels, but I especially like the EF 86. I have only built a few amps before (Trainwreck style- one express and one liverpool), so I would appreciate any input on this issue. On the bright side though, I have no redplating on the tubes and no smoke. The heaters are all OK at 6.4VAC. My other voltages on the PI and preamp side are with about 10-15% of the voltages in the guide, I am just confused about these really high plate voltages. Hmmmm...
Also, secondly, I am getting a fair amount of oscillation when I am playing distorted. Is this due to wire placement? Any advice on this would also be welcome. Thanks. Mark

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R.Y.N.O.

Does seem extra high. What is the line voltage? Might require a simple Zener to drop that.

You did use a Tube rectifier, correct?

Cant say I've heard that oscillation when distorted though

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

yes- I used a JJ GZ34 tube rectifier as per the build directions. I guess I'll have to take the board out and check all the under board wiring on the power side. It seems like I'm not getting enough voltage drop to the plates and too much to the screens. I'm using EI EL84's which I believe can handle these plate voltages, but something is definitely wrong here. The amp though sounds great.
On my TW style amps, I had a fair amount of oscillation because they are very high gain amps, and I cured most of it by putting in grid resistors. But this amp already has the 68K grid resistors, so I may just rip out some wiring and try moving some things around.
The line voltage where I live is 123 VAC from the wall, a little high.
If I used a zener to drop the voltage, where would I place it? Thanks for the feedback.

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R.Y.N.O.

You should be able to just check the jumper continuity w/o ripping out the board. 123 VAC is a bit high but 3% mains voltage adds about 10 or so secondary volts.

If you're concerned, the zener would go in the B+ after the rectifier, before the first filter cap. We use 18V 5W zeners on the TRIWATT for the 6V6 tubes. that would be excessive here.

If you leave the voltage high, consider a higher value cathode resistor for the power tubes in the 150 ohm range.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Stephen. do you ever do the "zener in the power tranny centertap" deally?
Stew

On the TRIWATT, 18V 5W zener is in the B+ after the rectifier, before the first filter cap.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I was wondering about that method, too.....I went ahead and gave it a try on my Chimp (SE, mongrel design). So far so good. All voltages came out where I expected. I'm far from being an expert, however, and I don't know what potential downside may exist.

YMMV

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Thanks for the feedback coco- I think I will just go ahead and put a 150 ohm cathode resistor because I have some extras from my liverpool. They are 25watt but I think I can squeeze it in there. Thanks!

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R.Y.N.O.

I personally don't like anything that interferes with ground connections. So I avoid putting zeners, resistors, standby switches or any other stuff in the connection between the PT's high voltage centre tap and ground.

FYI when you place zeners in the B+ line, you can get a slightly different voltage drop between having the zener before the filter cap or after the filter cap. This is because the zener is not fully ON all of the time when it's being fed a rectified AC voltage coming straight off the rectifiers. So after the first filter cap does it's work, you get slightly less voltage drop than the diode's spec zener voltage. In the TRIWATT, with an 18V zener, we get exactly the voltage drop we want. With the zener(s) after the filter cap, you get it dropping much closer to the spec zener voltage.

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