Amp stuttering at full volume

Killcrop

Silver Supporting Member
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11,528
I have a tweed repro that stutters at near maxed out volume. So if I almost dime the amp and slam it hard with a Les Paul it cuts out for a split second. I haven't really pushed it beyond that. Also if I have it at half volume and hit it with my '70 Fuzz maxed out it does the same thing.

Does this sound like a tube issue?
 
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525
What model is your amp a repro of?

It could be a couple of things. Depending on the model, it could be location of the choke in circuit that is causing this issue. And the main filtering may need to be beefed up.

Need more info to be sure though.
 
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525
It is a repro of a narrow panel 5F4 tweed Super.

In the stock configuration, the power supply can not handle the power demands of the power amp when the amp is dimed with humbuckers. The choke is in the circuit in such a way that it resists the current draw of the power tubes. When dimed with humbuckers this causes the stuttering you speak of. It's similar to a person gasping for air while running.

For your requirements, the center tap of the output transformer needs to be moved to the hot side of the choke and possibly another 20uF filter cap, or more added to the main filter.
 
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525
That's interesting. Can it harm the amp? Thanks for the help btw.
Depending on the actual components used in the amp, it is probably ok.

What you are experiencing is sag in biblical proportions.

If you play the amp like that regularly, you should consider having the mods done that I mentioned, and possibly switching to a GZ34 rectifier. This way the amp could keep up with you without "gasping."

You're welcome.
 

mxvin

Member
Messages
1,259
very interesting.....good stuff to know. Almost like a rev limiter on a car or a motorcycle...
Is that a poor design or is it a safety kind of built in so you dont blow a speaker or burn anything uP???
 
Messages
525
very interesting.....good stuff to know. Almost like a rev limiter on a car or a motorcycle...
Is that a poor design or is it a safety kind of built in so you dont blow a speaker or burn anything uP???
Well, that arrangement was popular in audio stuff at that time too. And it does work to limit. So it's hard to know if they were just copying typical circuits of the time or trying to keep from blowing output transformers and speakers or both.

The vintage 5F4, 5E5A, 5E7 had small output transformers that may have been so to limit power so as not to blow the speakers, or just to save money. Problem is, those amps tended to blow those output transformers.

I'm sure the OT in your Victoria is rated form more current than the vintage ones. So probably no need to worry there. But the "gasping" for power is annoying when trying to rock hard.
 

Killcrop

Silver Supporting Member
Messages
11,528
It only does it if I am using my Les Paul R8 that has a 14ohm humbucker in the bridge or if I slam the front end with the 70's Fuzz with the gain on 10. Its pretty extreme conditions either way.

So a GZ34 rectifier may help solve this? I have some laying around I could try.
 
Messages
525
It only does it if I am using my Les Paul R8 that has a 14ohm humbucker in the bridge or if I slam the front end with the 70's Fuzz with the gain on 10. Its pretty extreme conditions either way.

So a GZ34 rectifier may help solve this? I have some laying around I could try.
Probably best turn down some with that LP and the Fuzz.

Well, the choke /filter cap arrangement is the biggest player here causing the huge sag. Changing that will allow the amp to "open up" and "breathe" freely. After that change, the GZ34 would boost the B+ some (requiring re-biasing) and reduce the sag a bit more.

Just switching to a GZ34 in the current configuration will raise the B+ some and reduce the sag a tiny bit, but it probably won't be very noticeable when cranked with that LP as the choke arrangement will still be causing the amp to gasp. If you do change to a GZ34 under any circumstances, the amp will need to be re-biased.
 
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