Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: J. Ignacio on January 30, 2025, 11:05:47 am
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Hi
First of all, I hope you can understand my english. I'm from Argentina.
My friend has this amp. He give it to me because don't sound. I have some knowledge in electronics. I built pedals and tube amps
The amplifier not sound in the clean channel. Only have a little sound with vol and tone at top. Dirty channel sounds at middle vol when i heat the guitar. It sounds "choopy".
My first thought is the tube amp is dead. I don't have instruments to test
I made some proofs
- Guitar in Return Fx. Not sound at all
- Headphone conect. It sounds good. Reverb works. Tonestack works
- Send Fx to another amp, sounds good. Reverb works. Tonestack works
- new 12ax7. Same problem
For me the problem in the power section but I can't found it.
Buy a new 12bh7 is not cheap here. So, if i would buy a new one is because it's confirm tube issue
Thanks. I hope your answers
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Hard to say without a schematic, but the headphone out likely comes after the power section and output transformer, so possibly look for a bad speaker/connection first? Can you find a schematic? Pictures of the board?
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Hard to say without a schematic, but the headphone out likely comes after the power section and output transformer, so possibly look for a bad speaker/connection first? Can you find a schematic? Pictures of the board?
Speaker works fine. Also the conection.
(https://www.tdpri.com/attachments/buggerit_g5-1-jpg.1127154/)
I add Schematic down here.
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Wow, what a nightmare.
You can clean all connections, check all ribbon cables, etc. Check voltages at your power supply rails.
You can check voltages at the 12BH7. Make sure it warms up, use a scope or listening amp to check signal along the signal chain. Maybe it is just the power tube, but you can do some tests to confirm before you buy a new one.
Good news is, you know the pre amp works. Maybe you'll get lucky, and it will be something simple. Finding where you lose signal in the chain is probably key.
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power down, wait 20 minutespull the PA tubes
ohm R114 to ground. Ohm R151 to ground
post values
with tubes OUT, power up
Measure Pin 2 VDC to ground
Measure Pin 7 VDC to ground
post values.
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Wow, what a nightmare.
You can clean all connections, check all ribbon cables, etc. Check voltages at your power supply rails.
You can check voltages at the 12BH7. Make sure it warms up, use a scope or listening amp to check signal along the signal chain. Maybe it is just the power tube, but you can do some tests to confirm before you buy a new one.
Good news is, you know the pre amp works. Maybe you'll get lucky, and it will be something simple. Finding where you lose signal in the chain is probably key.
Thanks! I will re-check welds and clean all the connections
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power down, wait 20 minutespull the PA tubes
ohm R114 to ground. Ohm R151 to ground
post values
with tubes OUT, power up
Measure Pin 2 VDC to ground
Measure Pin 7 VDC to ground
post values.
If i understand right here, are the voltages
R114 and R151 to ground. 100mV
Pin 2 and 7. -18v
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those look reasonable
so, like AInewman suggested jacks, cables, connectors.
a serious visual inspection, flashlight, magnifying glass, looking for "burnt traces", cracked solder joints
the areas I see on the schematic so far are input jack, FW jack, attenuator switch, channel switch
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New test
- I connect they guitar in Return and use the Headphones. It's sound good.
So.... OT transfo? Tube?
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If I read the schematic correctly, then you've got a 12BH7 as a power tube - did you try replacing that one?
Also, I see there is a power attenuator switch - you should check to see if that is working correctly.
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If I read the schematic correctly, then you've got a 12BH7 as a power tube - did you try replacing that one?
Also, I see there is a power attenuator switch - you should check to see if that is working correctly.
No. It's expensive tube in Argentina. So I want to be sure that is the problem to buy new one
Attenuator switch works fine
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Do you have a 12AU7? You should be able to use that as a substitute for the 12BH7 - unless someone else on here on the forum knows a good reason not to. It should be fine to try because the 12AU7 does not draw as much heater current as the 12BH7, but is similar in most other ways.
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read all about them
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I was also going to point out this valuable thread of information provided by HotBluePlates:
https://el34world.com/Forum/index.php?topic=28043.msg308667#msg308667
Edit; I changed the link so it is at the start of the thread, not the end. Willabe
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I tried a 12ua7. Didn't work
Maybe 12bh7 is fine and problem became from another sector...
OT?
I hace to recheck welds
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I suspect headphone jack X5.
It should connect the speaker when the headphones are unplugged.
It should disconnect the speaker when the headphones are plugged in.
Jack X5 has two internal switches.
Test them by unplugging the headphones.
Measure resistance between points 3 and 5. It should be 0 ohms.
Measure resistance between points 2 and 4. It should be 0 ohms.