6v6 red plating on OTS 20 Lunchbox

Sam Slick
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« on: July 27, 2013, 01:07:11 AM »

I finished building my lunchbox 20w OTS and when I fired it up, the power tube closest to the mains fuse started glowing red when I flipped the standby switch to on. I swapped tubes with another set of 6v6s with the same result. I hear a low hum from the speaker cab when I flip the standby switch to on. As far as I can tel, the wiring looks good to both power tubes. Any suggestions?


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wyatt
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Re: 6v6 red plating on OTS 20 Lunchbox

« Reply #1 on: July 29, 2013, 03:21:13 AM »

What are the voltages at the tube sockets?

Have you properly biased the tubes?

If you are red plating the tubes are dissipating probably more than 150%. That ever means they are biased way too hot or the plate voltage is excessive.


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Sam Slick
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Posts: 20

Re: 6v6 red plating on OTS 20 Lunchbox

« Reply #2 on: August 17, 2013, 02:27:38 PM »

Thanks Wyatt...

The problem wound up being that I had the polarity reversed on the 47uf / 450 volt cap nearest the first 12AX7 tube.
That problem being resolved, now the amp is oscillating. Biasing does not resolve the oscillation. I am troubleshooting that now.

Also I have a question about the spare parts that were sent with the kit... There is a 1M resistor, a .22uf cap and a.1uf cap. Any ideas on what these parts are for? I'm still learning from the masters like you!

Grasshopper


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