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 Post subject: Triwatt - Bias and OD
PostPosted: Tue Jul 14, 2020 9:11 am 
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Two quick questions for you guys. When engaging the OD I get quite a bit volume jump, is that the case for you guys?

More importantly, I'm confused with the bias switch. I'm running JJ 6V6S and I'm a little lost with which way to put the bias switch. Looking from the top of the chassis down at the switch, its pointing towards the preamp tubes and I measure 20mv bias and 470v on the plates. Switch it towards the caps I get 30+mv and 440v on the plates. Does this sound right / look right in the pic. I've gone through the wiring and nothing is jumping out at me.


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PostPosted: Tue Jul 14, 2020 4:42 pm 
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jjsoybean wrote:
Two quick questions for you guys. When engaging the OD I get quite a bit volume jump, is that the case for you guys?


Yes, unity with the OD engaged is around 9:00 for me (give or a take a bit), and above that, volume goes up rapidly, and then distortion kicks in pretty quickly. The OD mode is a beast!

jjsoybean wrote:
More importantly, I'm confused with the bias switch. I'm running JJ 6V6S and I'm a little lost with which way to put the bias switch. Looking from the top of the chassis down at the switch, its pointing towards the preamp tubes and I measure 20mv bias and 470v on the plates. Switch it towards the caps I get 30+mv and 440v on the plates. Does this sound right / look right in the pic. I've gone through the wiring and nothing is jumping out at me.


I was confused by that as well, see this thread: Triwatt bias switch clarification.

When I initially built my Triwatt, I actually had the bias switch backwards. See this post in my build thread where I elaborated on the mistake I made.

Hopefully those two threads have enough info to clarify everything for you!


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PostPosted: Tue Jul 14, 2020 7:15 pm 
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Matt - i read through those and got some info from stephen and it makes more sense. With that knowledge took the amp put it on 6v6 which gave me the right plate voltage and turned the bias pot a hair (no longer short to ground) and blew the fuse. Seems like i got a wire off in there somewhere....now gotta chase it.


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PostPosted: Wed Jul 15, 2020 10:45 am 
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Yeah the OD, when engaged, produces a pretty big jump in volume as Matt mentioned. When I use it, I typically have it between 10 and 2 o'clock with the channel volume around noon, and use the MV to tame things down. With those settings I get a really great OD tone out of the amp. Not sure how engaging the OD with a footswitch in a live setting would work due to the huge increase in volume. In any case, it sounds really good!

The bias switch layout had me scratching my head for a while. When looking down into the chassis, the "wired" side of the switch is the KT66 side. This allows 2 things...the B+ can bypass the Zener diode since it has a pathy through the switch (i.e. the Zener is shorted), and 2) it engages a parallel resistor (R44) for the bias supply. The no-connection side is 6V6, so when this side is selected the B+ has no where to go except through the Zener, which drops the voltage, and R44 is not added in the circuit.
Therefore, from the outside of the chassis, the handle of the switch will be towards the OT when in KT66 mode, and towards the PT in 6V6 mode.


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