Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BlackCrowe604 on May 09, 2025, 01:45:03 am
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Hey everyone,
I recently recapped an old Hylight-era Hiwatt DR504 & replaced OPT. All filters & bias cap.
I checked my wiring many times and can't find any mistakes. OPT test ok with AC signal connected to B+ node & plates of both output tube sockets and makes a tone.
When I take the amp off standby, I just get a super loud buzz with nothing plugged in and all pots at zero. I shut the amp off right away. Numbers are all over the place on my bias probe for PV & PC.
I've only ever heard this sound once before... on a JTM45 reissue Marshall. Same sound, and bias probe numbers went wild.
Except, on the JTM45, I measured the caps with a capacitor wizard, and found the caps in it had dead shorts. I replaced them in that amp and it came back to life.
Which is why I am scratching my head on this one. I used all new TAD caps.
The buzz goes away if I remove V3 - 12AX7 and take the amp off standby.
It is NOT blowing fuses.
I brought new caps up gradually on variac & current limiter. All seemed fine.
I have measured the voltages at the pins, they seem ok.
Going to take another crack at it over the coming days. Hoping someone might be able to steer me in the right direction :)
One thing that stands out to me is: V2 12AX7 Pin 1 @ 0v as well as V4 12AX7 Pin 2 @ 100v
Much appreciated, and thank you!
Voltages are per image below.
Should I try replacing the diodes in the half wave?
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I have measured the voltages at the pins, they seem ok.
Voltages are:
12AX7s:
PIN 12AX7(i) 12AX7(ii) 12AX7(iii) 12AX7(iv)
1 543 0 566 557
2 0 0 0 100
3 0 0 0. 0
4 0 0 0 0
5 0 0 0 0
6 543 556 565 550
7 0 0 0 0
8 0 0 0 0
9 0 0 0. 0
The preamp voltages are not okay. Going by the above numbers, you probably made a mistake with the ground return when you recapped the amp- as none of the tubes appear to be conducting because the plates are all sitting at B+.
Suggest you take the amp to a tech
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Those were the voltages at the pins with the preamp tubes removed, so nothing conducting.
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I have measured the voltages at the pins, they seem ok.
Voltages are:
12AX7s:
PIN 12AX7(i) 12AX7(ii) 12AX7(iii) 12AX7(iv)
1 543 0 566 557
2 0 0 0 100
3 0 0 0. 0
4 0 0 0 0
5 0 0 0 0
6 543 556 565 550
7 0 0 0 0
8 0 0 0 0
9 0 0 0. 0
The preamp voltages are not okay. Going by the above numbers, you probably made a mistake with the ground return when you recapped the amp- as none of the tubes appear to be conducting because the plates are all sitting at B+.
Suggest you take the amp to a tech
Those were the voltages at the pins with the preamp tubes removed, so nothing conducting, yes they are sitting at B+.