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Just an FYI...I swapped a pair of JJ 6V6's into my Triwatt last night & I have this same bias problem. I thought I had this licked last year but it appears I was mistaken. The range I'm getting is similar to norton's...about 8-17 mv. I tried the old tubes in there with the same result. I must have an error in there somewhere but I'll be damned if I can see it.

Stephen or Phil, can you post a close-up of the correct wiring for the bias supply & the 6V6/KT66 switch? We must be missing something here.

Darrin

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SG + Brit tube head = Nirvana

So the pair of 6V6 that were in there biased OK? and the replacements dont?

I can post close up pics but that's a strange one. Assume you are measuring only across one tube not two?

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Stephen
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Are we sure that in both cases the KT66/6V6 switch is actually in the 6V6 position? What's being described here means that either the bias voltage is too high (usually due to the switch being in the wrong position) or that the tubes are badly out of spec. The correct wiring of the switch is shown on page 40 of the build guide. Note, with more recent Triwatt kits, the zener is mounted on the circuit board, rather than directly on the switch - but the way it works is still the same. What's important to note about the bias circuit is that only the 180k resistor should be in circuit for the 6V6s, while the additional resistor (normally 620k) gets placed in parallel with it to raise the bias voltage for KT66s.

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Great tones come in small glass jars!

With the switch pointing towards the 12ax's the low voltages are present. When the switch is pointing towards the power transformer then the range is from 12.5mv to 25.7

The diode and switch is set up as in the layout.

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dig it

errr...ummm...(cough)...never mind. Problem turned out to be between my ears...

It seems when I built the thing I had labeled the switch in permanent marker on top of the chassis...incorrectly...but had it in the correct position when I biased it. I must have hit the switch accidentally when I swapped the tubes, but thought it was in the right position because the label said so... One of those days.

My range is from about 17-32 mv. I currently have the JJ's biased at around 20. Judging by norton's last comment, sounds like a similar situation.

We now return you to your regularly scheduled program...

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SG + Brit tube head = Nirvana

Which is the range it should be in - ie with 20mA somewhere roughly in the middle. The exact position will vary slightly between different kinds of 6V6s.

FYI the original Hiwatt amps didn't have any bias pot of course. Just a fixed resistor, so you just plugged the power tubes in and hoped for the best.

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Great tones come in small glass jars!