Amp issue - PI distortion and low volume - request for help
Moderators: pompeiisneaks, Colossal
Amp issue - PI distortion and low volume - request for help
Hello friends,
I have a small (built in a Vibro Champ chassis) amp I built for a friend that now has an issue I haven't successfully diagnosed. I'm hoping you can help. My friend said it suddenly lost volume and he "smelled something" so he shut it down immediately.
The topology is: gain stage; volume; gain stage; tone stack; PI, power. (2 12AX7s and 2 6V6) Solid state rectifier, runs ~420V on 6V6 plates.
When I input a sine wave and check the signal on a scope, input to V1a is good, output of V1a is good, input of V1b is good, output of V1b is good. Input to PI is good. Output of either side of the PI starts to distort at volume=3 and just gets worse from there. No headroom, *terrible* tone.
All voltages seem fine, both plate and cathode. I've subbed all the tubes with no change. Also subbed a new power supply cap for the PI. Nothing looks wrong except the signal post-PI.
I can show pics this weekend. In the meantime, any ideas?
Thanks,
Michael
I have a small (built in a Vibro Champ chassis) amp I built for a friend that now has an issue I haven't successfully diagnosed. I'm hoping you can help. My friend said it suddenly lost volume and he "smelled something" so he shut it down immediately.
The topology is: gain stage; volume; gain stage; tone stack; PI, power. (2 12AX7s and 2 6V6) Solid state rectifier, runs ~420V on 6V6 plates.
When I input a sine wave and check the signal on a scope, input to V1a is good, output of V1a is good, input of V1b is good, output of V1b is good. Input to PI is good. Output of either side of the PI starts to distort at volume=3 and just gets worse from there. No headroom, *terrible* tone.
All voltages seem fine, both plate and cathode. I've subbed all the tubes with no change. Also subbed a new power supply cap for the PI. Nothing looks wrong except the signal post-PI.
I can show pics this weekend. In the meantime, any ideas?
Thanks,
Michael
Wife: How many amps do you need?
Me: Just one more...
Me: Just one more...
Re: Amp issue - PI distortion and low volume - request for help
Was it being cranked when it lost volume?
My band:- http://www.youtube.com/user/RedwingBand
Re: Amp issue - PI distortion and low volume - request for help
Nope. It was not cranked. But the owner did say he "smelled something" and then turned it off immediately. I suspect the PT, but voltages still look okay.
Wife: How many amps do you need?
Me: Just one more...
Me: Just one more...
Re: Amp issue - PI distortion and low volume - request for help
I'd guess the power tube failed, burning something in the process. Screen resistor?
I build and repair tube amps. http://amps.monkeymatic.com
Re: Amp issue - PI distortion and low volume - request for help
Is there a global feedback loop?
Scoping signals within a loop (eg phase splitter plates) can give strange results, due to the NFB error correction mechanism.
It can be best to open the loop when fault finding with a scope.
Scoping signals within a loop (eg phase splitter plates) can give strange results, due to the NFB error correction mechanism.
It can be best to open the loop when fault finding with a scope.
My band:- http://www.youtube.com/user/RedwingBand
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Hhh
How much does that 420 volts of V+ pull down when the amp starts to drop volume?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Amp issue - PI distortion and low volume - request for help
Thanks, folks. I'll be able to investigate and take pictures this weekend, perhaps this evening. I'll get back to you.
Wife: How many amps do you need?
Me: Just one more...
Me: Just one more...
Re: Amp issue - PI distortion and low volume - request for help
One side of the output xfmr measures 88 ohms, the other side measures 72 ohms. Voltages aren't very off, but this just looked weird.
Powered up -
Brown wire/power tube - Blue wire power tube
No signal - both at 437V
@3 on vol - both at 383V
@10 - brown=362V, blue=360V
Screen resistors are fine. Power tubes are new. Voltages are steady.
I can open the fb loop because I installed a switch to do just that. Pics that follow are open-loop. Volume setting is the pic name. Starts to square on 3, worse on 4, BAD worse with ringing on 10. These are all at the higher voltage PI plate.
New PT is on order. (One of the mounting screws was stripped. No idea how that happened. It was delivered to me that way.) Do I need a new OT also? Open to all suggestions. Again, thanks. Also, if I can provide any other pics/info, just let me know.
Powered up -
Brown wire/power tube - Blue wire power tube
No signal - both at 437V
@3 on vol - both at 383V
@10 - brown=362V, blue=360V
Screen resistors are fine. Power tubes are new. Voltages are steady.
I can open the fb loop because I installed a switch to do just that. Pics that follow are open-loop. Volume setting is the pic name. Starts to square on 3, worse on 4, BAD worse with ringing on 10. These are all at the higher voltage PI plate.
New PT is on order. (One of the mounting screws was stripped. No idea how that happened. It was delivered to me that way.) Do I need a new OT also? Open to all suggestions. Again, thanks. Also, if I can provide any other pics/info, just let me know.
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Wife: How many amps do you need?
Me: Just one more...
Me: Just one more...
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- Posts: 4730
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Hhh
You can run it open loop but your gonna have at least 6 db more signal than you need feeding the PI and that will carry over to too much feeding the output tubes to have much clean volume range.
Your big drop off in power supply voltage with the amp on only 3 is a sign that you have too much drive signal hitting the output tubes,I would try out a at7 or ay7 in the Pi socket.
Double check the resistor values in the PI section.
Can you post a schematic ?
Your big drop off in power supply voltage with the amp on only 3 is a sign that you have too much drive signal hitting the output tubes,I would try out a at7 or ay7 in the Pi socket.
Double check the resistor values in the PI section.
Can you post a schematic ?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Amp issue - PI distortion and low volume - request for help
Here's the schem
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Wife: How many amps do you need?
Me: Just one more...
Me: Just one more...
Re: Amp issue - PI distortion and low volume - request for help
The output xfmr was blown. I subbed in a known good one and all was well. Just installed the real new one tonight. Rockin' once again.
Just FYI - the sound of a blown output xfmr is horrible.
Just FYI - the sound of a blown output xfmr is horrible.
Wife: How many amps do you need?
Me: Just one more...
Me: Just one more...
Re: Amp issue - PI distortion and low volume - request for help
IMHO, transformers should always be overspecced. Very little sonic goodness comes from pulling too much current through an OT and its an expensive "fuse." PTs, too. If you want to simulate poor regulation, a power resistor works fine.
Re: Amp issue - PI distortion and low volume - request for help
The original was a Hammond 1608. Definitely not underspec'd.Firestorm wrote:IMHO, transformers should always be overspecced. Very little sonic goodness comes from pulling too much current through an OT and its an expensive "fuse." PTs, too. If you want to simulate poor regulation, a power resistor works fine.
Wife: How many amps do you need?
Me: Just one more...
Me: Just one more...
Re: Amp issue - PI distortion and low volume - request for help
Probably infant mortality. Happens in the best of families. If nothing untoward happened, it should be on them.