Restoring an ampeg. When playing there is a fizzing that intensifies if I play harder. It also has some slight treble sweeping in the background. I tested all the preamp tubes and they appear to be good. I replaced the 7591 power tubes . The rectifier 5y3 checks out good. I measured all voltages and they seem good, however the V1 preamp tube which handles the trem circuit has an obscene amount of voltage on pin 10 (plate). It says per schematic that it should be at around 130volts- it measures around the full B+ 340volt. The plate resistor gives appropriate value when measured and the coupling cap I think is rated far below the voltage it's seeing. Could that be the issue? It doesn't seem burnt or physically damaged. Any pointers here???
I am wondering why the voltage is 340v before the plate resistor and about the same after it??? Could the value measure correctly and still not perform??
Thanks for the help.
I am wondering why the voltage is 340v before the plate resistor and about the same after it??? Could the value measure correctly and still not perform??
Thanks for the help.
An externally hosted image should be here but it was not working when we last tested it.
Above is the variation on the schematic. Part values are Identical. Voltages are within 10%
That'd be favourite IMHO
A resistor with no voltage drop says to me the other end ain't grounded.
Chris
FUnny voltage on the oscillator tube plate won;t likely make fizzies on your sound. Have you connected the amp to a different speaker? A rubbing speaker voice coil can be described as making a fizzy sound.
However, just to be sure, ground pins 3 and 5 - should already be wired together - of V1 to eliminate any influence from the trem circuit.
I just hate those Joe Piazza drawn AMpegs schematics. I see no voltage references. Are you using a different schematic?
With the trem switch OFF, the tube can;t conduct, so no voltage drop across its plate resistor. Any 130v plate reading would have to occur when the trem was running. With switch off, full B+ voltage would be normal there.
The trem switch shows a 270k cathode resistor. Really? I'd believe a 2.7k. Is yours actually 270k?
However, just to be sure, ground pins 3 and 5 - should already be wired together - of V1 to eliminate any influence from the trem circuit.
I just hate those Joe Piazza drawn AMpegs schematics. I see no voltage references. Are you using a different schematic?
With the trem switch OFF, the tube can;t conduct, so no voltage drop across its plate resistor. Any 130v plate reading would have to occur when the trem was running. With switch off, full B+ voltage would be normal there.
The trem switch shows a 270k cathode resistor. Really? I'd believe a 2.7k. Is yours actually 270k?
Enzo: I tried a different speaker with the same problem. So I am pretty sure it isn't the speaker. I do wonder if the OT could be responsible.
I do have a schematic on the inside of the amp but it is pretty tore up so I have been piecing it together from different sources. The Schemo internally has most of the voltages posted so I am pretty sure Im close in all the B+ voltages. Should I just start swapping parts? It's a real phantom problem, with my experience being a newb.
I kind of think it might be related to the cathode resistors. It's that weird under/over biased preamp tube sound (fizz). However, I am not sure what that whirling sound in the background is?
The Resistor you speak of is I believe a grid resistor (270k), the cathode in my amp has a 3.3K resistor. And funny you mention that resistor because it is the only non stock part in the amp. So it for sure has been serviced at that point. Should I try to swap it?
Thanks for all the insight everyone.
I do have a schematic on the inside of the amp but it is pretty tore up so I have been piecing it together from different sources. The Schemo internally has most of the voltages posted so I am pretty sure Im close in all the B+ voltages. Should I just start swapping parts? It's a real phantom problem, with my experience being a newb.
I kind of think it might be related to the cathode resistors. It's that weird under/over biased preamp tube sound (fizz). However, I am not sure what that whirling sound in the background is?
The Resistor you speak of is I believe a grid resistor (270k), the cathode in my amp has a 3.3K resistor. And funny you mention that resistor because it is the only non stock part in the amp. So it for sure has been serviced at that point. Should I try to swap it?
Thanks for all the insight everyone.
I swapped both the cathode resistor and cap. And now it is at the proper voltage per schematic. Problem persists......
When the trem is bypassed I still get a distorted signal when playing moderately hard.
Also the Trem is chopping hard (ticking) when intensity is past 3 on control.Could it be the whole trem section or just the caps? What do yall think???
Could it be the tubes???? I checked them on a tester and they tested good.
Trying to narrow it down with voltage inspection. I will tinker more tonight and see if i can track it down.
When the trem is bypassed I still get a distorted signal when playing moderately hard.
Also the Trem is chopping hard (ticking) when intensity is past 3 on control.Could it be the whole trem section or just the caps? What do yall think???
Could it be the tubes???? I checked them on a tester and they tested good.
Trying to narrow it down with voltage inspection. I will tinker more tonight and see if i can track it down.
Pulled that tube and played same ills.
Hmmn?????
Even though my voltages are in line with specs at the filter caps. Could the can be causing the problem.
It has that slight wave phasing in the noise floor. Similar to the sound of tuning an old am radio.
And of course the fuzz when I strike hard on the strings. What could it be??????
Hmmn?????
Even though my voltages are in line with specs at the filter caps. Could the can be causing the problem.
It has that slight wave phasing in the noise floor. Similar to the sound of tuning an old am radio.
And of course the fuzz when I strike hard on the strings. What could it be??????
I worked on a bunch of Ampegs. They use some of the worst random surplus caps I have seen in gear of this quality. FIRST thing I would suspect is coupling caps. What is in there for caps?
The green transparent Wimas are usually ok but Tiny Chiefs and those green plastic Sangamos are very dubious.
Also if the filter caps are shot, which is a very good bet, you could have all sorts of odd oscillatory effects due to deficient decoupling. This seems to be in accord with the symptoms you report.
The green transparent Wimas are usually ok but Tiny Chiefs and those green plastic Sangamos are very dubious.
Also if the filter caps are shot, which is a very good bet, you could have all sorts of odd oscillatory effects due to deficient decoupling. This seems to be in accord with the symptoms you report.
Fix one thing at a time. Turn off the trem and fix the distortion. THEN go back and fix the trem.
A rule in my shop is FIX the amp first, THEN go back and update parts. If you start swapping out a lot of stuff "as long as I am in there" but without having made the amp work, you are just adding that many more opporunities to cloud the original problem and its solution.
A rule in my shop is FIX the amp first, THEN go back and update parts. If you start swapping out a lot of stuff "as long as I am in there" but without having made the amp work, you are just adding that many more opporunities to cloud the original problem and its solution.
After much, much tinkering the problem persists. I change the filter caps. To no avail. Still producing a fizzy fuzz when played hard. There is a small window of clean tone before it gets thrust into cutoff and distortion. Has anyone ever experienced a OT acting like this. I get a good volume and definitely audible amplification. So assume the OT is fine. Not sure where to proceed except rebuilding the preamp in hopes that it is the caps in that section.
It produces fuzz then when signal amplitude goes down it hits a gate point where it is no longer fuzzy.
Hmmnnn. Any thoughts
Thankyou
It produces fuzz then when signal amplitude goes down it hits a gate point where it is no longer fuzzy.
Hmmnnn. Any thoughts
Thankyou
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