Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Lectroid on October 24, 2025, 10:08:00 am
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About a year ago I finished an 18W amp from by as circuit by tubenit. After initially getting it running, I documented the voltages at that time on the schematic below. Those numbers on the schematic are a year old.
I re-measured the voltages yesterday:
A=334
B=323
C=288
D=271
Plate Cathode Rp Rk
------------------------------------------------------------------------
V1b: 90 2.87 100K 1.5K
V1a: 82 2.85 100K 1.5K
V2b: 312 12.06 Reverb 1.5K
V2a: 82 2.85 100K 1.5K
V3b: 194 1.77 82K (1.5K eff/shared 820R)
V3a: 82 2.85 100K (1.5K eff/shared 820R)
V4: 328 11.53 - 213R
V5: 328 11.53 - 213R
Here's my question. When I compare the original voltage numbers from a year ago (schematic) with the numbers I got yesterday, they are close to what I recorded a year ago. Overall, I'm happy with it.
EXCEPT--on the first stage tube V1. V1b's plate voltage is 90 volts, or 38 volts lower than it was a year ago. Likewise V1a's plate voltage is 82, or 40 volts below what it was a year ago. See the schematic above for the original numbers.
Why would the voltages for the V1 first stage now be so far below the voltages I recorded when the amp was just built? All the other tubes' voltages are back close to those original values. I've rolled a few tubes through there but voltages don't change with that. I'm probably missing something obvious. Can anyone point it anything else can I test?
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That is hard (for me) to understand. While your plate voltage is lower, your cathode voltage is higher, which points to the tube, I guess. It is always possible the initially recorded voltages were wrong, but your cathode voltages are otherwise in line, so maybe not likely. I guess you could check the 5K dropping resistor between C and D, but that would not account for what you're seeing.
I'm stumped; hopefully not chumped.
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That is hard (for me) to understand. While your plate voltage is lower, your cathode voltage is higher, which points to the tube, I guess.
That's normal. The plate voltage is lower because of higher current through the tube causing more voltage to be dropped across the plate resistor and that leaves less voltage to be dropped across the tube. Higher current means the cathode voltage will be higher.
Those resistor values are normally used on a 12AX7, not a 12AU7.
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SEL49 wins the prize this morning.
I missed that V1 is a 12AU7 on the schematic.