Cut control limiting output power

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Gaz
Posts: 1145
Joined: Tue Aug 25, 2009 6:27 am

Cut control limiting output power

Post by Gaz »

While testing for clean output in a new Plexi style build, I noticed I was only getting about 60W instead of the expected 100W.

I soon found that disconnecting the Vox style cut control (250K pot/.0047uf), yield the expected 100W of power. I tried the same experiment on an AC30 build, but disconnecting the Cut control did not change output.

I don't know if it has to do with drive requirements for the power tubes, fixed vs. cathode bias, or something else. Any direction anyone can offer would be great. Thanks.
Gaz
Posts: 1145
Joined: Tue Aug 25, 2009 6:27 am

Re: Cut control limiting output power

Post by Gaz »

Well, I solved my own problem - That's what I get for posting after midnight and staying up 'til 2:30.

The problem lied in not being able to drive the power tubes to full power, which is an annoying problem when you're going for loud.

In a preemptive move to stop blocking distortion, I reduced the power tube grid leaks to 100k from 220k. With the addition of the Cut control loading the PI, I presume, it was just enough to prevent the EL34s from being driven to full power. Changing them back to 220k fixed the problem.

I should also note that using a larger pot for the Cut almost brought full power back, but not quite, and at the expense rotation vs. amount of cut.

Anyway, hope this helps someone one day!
Gaz
Posts: 1145
Joined: Tue Aug 25, 2009 6:27 am

Re: Cut control limiting output power

Post by Gaz »

Not sure if increasing the grid leaks was a red herring, but the real fix was increasing the phase inverter plate voltage to about 300vdc on each side, just like the Komet 60... I'm going to start or more general discussion about the K60' PI in the TW section...
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