Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bullkelp on January 31, 2025, 03:42:56 am
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Spent the day testing my 6g6b bassman build
All seemed good
I decided to fit some output tube clamps which arrived late in the post so I drained the caps, removed tubes, took the screws out of the output sockets and replaced everything.
When I fired the amp up again I got a loud honking sound so I switched it off and plugged it in thru the current limiter .
Now I have no output at all. Not even hiss.
I measure plate voltages (a little higher than before) but zero plate current.
What’s the likely issue here ? Both out put tubes gone ? I can’t see a reason as I didn’t change any wiring .
The current limiter isn’t heating up .
Just seems odd that it was working fine earlier ….
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Check that the power tubes are properly grounded (pin 8)./Leevi
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Check that the power tubes are properly grounded (pin 8)./Leevi
No issues there
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Spent the day testing my 6g6b bassman build
All seemed good
I decided to fit some output tube clamps which arrived late in the post so I drained the caps, removed tubes, took the screws out of the output sockets and replaced everything.
When I fired the amp up again I got a loud honking sound so I switched it off and plugged it in thru the current limiter .
When it was "working fine earlier" then you messed up something, or something is touching or something broke loose. I have had far too many amps come in for service where
the wire inside the cloth/insulation broke from the customer moving something but visibly it was connected. Check continuity (with the amp off) on each wire coming off of your tubesocket to the other end of the wire. I have a sneaking feeling when you moved the sockets something is loose, visible or not.
When you do that, check from the socket pin to the other end of the wire (e.g. do not check the solder side of the socket to the other end of the wire, you could be fooling yourself
and not realize there is a broken socket pin, happened to me on many occasions).
Again, when you say "zero plate current" then it stands to reason you lost bias, or lost cathode connections (you probably have them common, I would certainly check that continuity
first)... so I would triple check my connections to the other end of the wire and check the resistance of each to ground as a sanity check.
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Always show schematic
No plate current at each power tubes ;
1- no ground at cathode.
2- -many bad contacts on tubes
3 - Bias negative voltage very very high , almost impossible.
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Always show schematic
No plate current at each power tubes ;
1- no ground at cathode.
2- -many bad contacts on tubes
3 - Bias negative voltage very very high , almost impossible.
Or perhaps the screen grid resistor/s have blown.
0V on the screen grid results in 0mA anode current.
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Always show schematic
No plate current at each power tubes ;
1- no ground at cathode.
2- -many bad contacts on tubes
3 - Bias negative voltage very very high , almost impossible.
Or perhaps the screen grid resistor/s have blown.
0V on the screen grid results in 0mA anode current.
Blow resistor may be easy to see with eyes
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Righto
Two issues identified:
Firstly, cracked the solder joint to pin 6 of the first power tube that feeds B+ tp the screen grid via the screen grid 470 ohm resistor
Once I reflowed that I had current to one tube only. I switched the tubes over and the issue followed the tube. When i shake it is has a slight rattle and I can see a little of the silver reflective internal coating has chipped off. No other visual clue.
I changed the svetlana 6l6 tubes out for a set of JJ 6l6. Both tubes draw plate current now. Ive had to bias them up as compared to teh svetlana the current was much lower. Also I see the plate voltage is signifacantly lower than with teh svetlana. Is that an issue.
Back story to this. I wired the PT for 230v (NZ). I was finding my voltages on the high side (plates 495 and other voltages thru the circuit on the high side) so I connected to 240v priamry instead which settled everything nicely. Plates about 455.
Now the plates (with the JJ) are sitting around 420 - 425. The schematic says 458.
I could easily change the primary tap again, but on quick inspeaction through the rest of the board/preamp sockets the voltages numbers still look good.
I guess I can assume the plate voltage to drop as a reduce negative bias for more current flow. Is that expected with JJs?
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For a given mains voltage and anode current, anode voltage should be the same whatever valve is in there.
Maybe you mains voltage is fluctuating?
I think it's beneficial to check it several times during an amp test session, as it affects pretty much every voltage in the amp. Doing that avoids chasing red herrings.
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Yep good call.
Its early here so It could be that the voltages are lower over night. I'll leave it on the bench and check thru the day.
It was 236v mains when i checked last night. Ill have a look now
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Mains 236v this morning , same as yesterday
But! I’m an idiot
I still had the amp thru the current limiter
I’ve rebiased
The plates are as they should be