I have a '66 AC50 that has been my main amp for years. I'm having an issue where it loses punch & gets fuzzy after a couple hours of playing. Any tips as the best place to start looking for the cause?
The tubes are fairly new and I've never experienced this problem with tubes before.
I'm recording several hours a day lately. At first I thought it could just be hearing fatigue but I compared tracks I did early in the day to ones a few hours later and could definitely hear a difference.
Thanks
Kirk
AC50 loses punch & gets fizzy after a couple of hours
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silverjet89
- Posts: 6
- Joined: Sun Dec 07, 2014 3:09 am
Re: AC50 loses punch & gets fizzy after a couple of hours
When's the last time it was serviced? How old are the electrolytic caps?
It's true i've lost my marbles and i cant remember where i put them
Re: AC50 loses punch & gets fizzy after a couple of hours
Speakers get hot with use, especially vintage types with unvented magnets.
Voicecoil resistance and its related losses then increase, and the power that the amp can push into it will be reduced. Same process applies to the amp’s iron but speakers can get way hotter.
The above is my take as being a common cause of the effect noted in post #1.
To mitigate it, use larger arrays of speakers that have heavyweight, vented magnets. If cabs are open back, train a fan on to them, using the principal that anything is better than nothing.
For the amp, simply remove the front panel, instantly far better ventilation.
Here’s an AC50 MkII large box I did recently https://youtu.be/2GX-cHFQmR4
Some later AC50 versions had ventilation holes in the front panel.
Note that with the above amp, in addition to all ecaps, maybe 75% of resistors and 33% of other caps required replacement, due to excessive drift, noise, leakage etc.
eg an input stage plate resistor had risen from its 100k nominal to nearly 1M; I don’t expect that the drifted value was particularly temperature stable.
The power compression effect caused by the rise in voicecoil resistance with temperature is a big issue for PA systems, due to much high power levels and the much lower output impedance of solid state amps.
The high output impedance of tube amps somewhat mitigates the effect for guitar amps.
Voicecoil resistance and its related losses then increase, and the power that the amp can push into it will be reduced. Same process applies to the amp’s iron but speakers can get way hotter.
The above is my take as being a common cause of the effect noted in post #1.
To mitigate it, use larger arrays of speakers that have heavyweight, vented magnets. If cabs are open back, train a fan on to them, using the principal that anything is better than nothing.
For the amp, simply remove the front panel, instantly far better ventilation.
Here’s an AC50 MkII large box I did recently https://youtu.be/2GX-cHFQmR4
Some later AC50 versions had ventilation holes in the front panel.
Note that with the above amp, in addition to all ecaps, maybe 75% of resistors and 33% of other caps required replacement, due to excessive drift, noise, leakage etc.
eg an input stage plate resistor had risen from its 100k nominal to nearly 1M; I don’t expect that the drifted value was particularly temperature stable.
The power compression effect caused by the rise in voicecoil resistance with temperature is a big issue for PA systems, due to much high power levels and the much lower output impedance of solid state amps.
The high output impedance of tube amps somewhat mitigates the effect for guitar amps.
Last edited by pdf64 on Sat Jul 11, 2020 10:58 am, edited 5 times in total.
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Stevem
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Re: AC50 loses punch & gets fizzy after a couple of hours
You have resistors and signal caps that are old and heat build up changes there values, so the amp needs a overhaul!
For now I would place a small fan on the rear of the amp to add some positive pressure and vent the heat out faster and you just might get away with getting done with your recording and getting the amp worked on when your done.
Place the fan closer to the side of the amp that the EL34 output tubes are on.
For now I would place a small fan on the rear of the amp to add some positive pressure and vent the heat out faster and you just might get away with getting done with your recording and getting the amp worked on when your done.
Place the fan closer to the side of the amp that the EL34 output tubes are on.
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Not screaming like the passengers in his car!
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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!
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silverjet89
- Posts: 6
- Joined: Sun Dec 07, 2014 3:09 am
Re: AC50 loses punch & gets fizzy after a couple of hours
Thanks for the replies everyone!
This gives me somewhere to start. I'm fairly good at doing repairs but not always the best at diagnosis so I appreciate the input.
Some more details I should have mentioned:
The amp was gone through and all electrolytic caps were replaced about 10 years ago. Some resistors were changed as well. I suppose this could still be an issue so I'll look into it.
There is a fan in the cab that pulls hot air out through the top vents so it stays relatively cool for an AC50.
Speakers are Celestion Gold & Weber Alnico Blue Dog. Both rated 50 watts. Since I'm doing a track & listening back/editing, the cab isn't getting signal constantly for hours but the amp is on.
I do have another cab with a Gold & Eminence. I'll try swapping them next time I notice the problem and see if that helps.
This gives me somewhere to start. I'm fairly good at doing repairs but not always the best at diagnosis so I appreciate the input.
Some more details I should have mentioned:
The amp was gone through and all electrolytic caps were replaced about 10 years ago. Some resistors were changed as well. I suppose this could still be an issue so I'll look into it.
There is a fan in the cab that pulls hot air out through the top vents so it stays relatively cool for an AC50.
Speakers are Celestion Gold & Weber Alnico Blue Dog. Both rated 50 watts. Since I'm doing a track & listening back/editing, the cab isn't getting signal constantly for hours but the amp is on.
I do have another cab with a Gold & Eminence. I'll try swapping them next time I notice the problem and see if that helps.