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November 26, 2021, 04:17:22 PM

Author Topic: Voltage Check Weirdness: T19, T20, T21  (Read 133 times)

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Offline TubeGuy12

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Voltage Check Weirdness: T19, T20, T21
« on: June 15, 2021, 12:52:22 PM »
My Crack is sounding great, but I've wondered about the following discrepancies between my voltage readings and those in the manual:

T19=86v (instead of manual's 0)
T20=0v (instead of manual's 206v)

Also: T21=212v (no voltage specified in manual). This must correspond to the function of the manual's 206v for T20.

I look forward to hearing from those in the know.

Offline Paul Birkeland

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Re: Voltage Check Weirdness: T19, T20, T21
« Reply #1 on: June 15, 2021, 12:54:14 PM »
What's the date on the manual you're looking at?  We haven't specified voltages for those terminals for 5+ years.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline TubeGuy12

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Re: Voltage Check Weirdness: T19, T20, T21
« Reply #2 on: June 15, 2021, 02:52:21 PM »
6/4/10, believe it or not. (It unfortunately sat unbuilt in storage for 10+ years.)

Offline Paul Birkeland

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Re: Voltage Check Weirdness: T19, T20, T21
« Reply #3 on: June 15, 2021, 02:59:29 PM »
Yeah, there were some errors in some manuals long ago.  If the voltages on terminals 1-10 are OK, the rest of the amp is OK.  That's what we went with in the newer manuals to simplify things.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline TubeGuy12

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Re: Voltage Check Weirdness: T19, T20, T21
« Reply #4 on: June 15, 2021, 03:06:24 PM »
Many thanks, Paul. Yes, other voltage readings are as listed in the manual, including 1-10.

(On a side note, one more question, if you don't mind: what's the official line on using a 12BH7A without resistor swap but with speedball? I've searched for this on the forum but due probably to some error on my end have failed to find an official answer.)

Offline Paul Birkeland

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Re: Voltage Check Weirdness: T19, T20, T21
« Reply #5 on: June 15, 2021, 03:15:41 PM »
The 12BH7 in the stock amp will have lower plate voltage than the target of the original design.  The simple approach is to just adjust the current to bring the plate voltage up.  You could also replace the LEDs with different ones that have higher forward voltage which would also raise the plate voltage a bit and I think would potentially sound better as well.  Try a pair of HLMP-QB00-S0000. 
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline TubeGuy12

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Re: Voltage Check Weirdness: T19, T20, T21
« Reply #6 on: June 15, 2021, 03:26:23 PM »
Thanks again for all your help, Paul. Cheers.