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Topic: Stumped 5F6-A issue! No volume, heavy distortion! |
Dave Zielinski
From:
Pennsylvania, USA
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Posted 22 Mar 2007 7:11 pm |
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This one has got me pretty bad... I'm usually pretty good figuring things out, but this one has gotten me...
1960 4-10 Tweed bassman, 5F6-A. The holy grail.
Was at a gig with it and all of a sudden it got mushy, distorted (like ACDC!) and the volume sucked. I had to turn the amp up to hang with the band (this NEVER happens with this amp!) The amp used to play real tight and clean- I never had to crank it, always had it at about "4" which is plenty loud and toneful...
So I tore into it over the past few nights...
Filter caps and all electrolytics are fairly new, I did these in 10/06. Tubes are good- I swapped them all with me clone 5f6-a and still the same issue. I checked the gain stage, cathode resistors, plate resistors, solder joints, tube sockets, all connections. replaced the dropping resistors int he power supply and replaced some drifte resistors.
no avail..
Bias is 34mA per tube, B+ is 466V. usually this sounds heavenly.
I swapped the OT with a mercury magnetics-no dice..phew...I was hoping the OT was good and it is..
It sounds like if you took out one power tube- I rebuilt the phase inverter- still no dice...It also sounds like the filter caps are REALLY bad, but I know they are good- and I measured B+ and watched it while I played, it stays pretty solid.
I am worried that I have a conductive circuit board. I've heard about this freak issue. How do you check for it???
I blew out all the dust- those amps get a notorious amount of funk from the input jacks, I cleaned up everything and still no dice!
I checked the Astron caps for dc leakage by lifting one leg and measuring to gnd. They all seem ok, but I realize they still might be dripping dc. I swapped each one, one at a time and STILL no dice!
I know it can be fixed...it might take a whole pile of new parts, but I'd like to avoid that... If it happens, it happens...an amp not worth playing is an amp not worth taking up space....well, not exactly...
Anyone ever had a similar issue and fix???????
Thanks for any help at all! |
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Jay Ganz
From:
Out Behind The Barn
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Posted 22 Mar 2007 7:51 pm
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.....and you checked all the voltages on the preamp tubes  You might to have to trace the signal
through the amp stages and see where the problem starts. At least try to narrow it down somewhat.
Use another amp as a signal tracer and couple the audio into that with a capacitor on the end of your probe. |
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Glenn Austin
From:
Montreal, Canada
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Posted 22 Mar 2007 8:06 pm
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Hi Dave, Try posting your question on the Metropoulos amp forum,
http://forum.metroamp.com/
This is by far the best amp building, repairing forum on the internet. These guys really know their stuff. |
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Jim Sliff
From:
Lawndale California, USA
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Posted 22 Mar 2007 8:54 pm
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Another site to post on is the Amp Workshop. These guys aren't focused on one type, and most of the regulars are guitar tube amp techs who make a living at it.
http://p210.ezboard.com/bampworkshop
Another suggestion - call Ron Veil (Uncle Spot) in Modesto CA. he might be able to fix it on the phone in seconds.
Good luck!
_________________
No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional |
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Ken Fox
From:
Nashville GA USA
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Posted 22 Mar 2007 9:28 pm
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| What Jay said! You got get down to trouble shooting, not parts swapping! |
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Scott Swartz
From:
St. Louis, MO
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Posted 23 Mar 2007 7:12 am
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With an oscilloscope you can see the parasitic oscillation tha can be caused by a conductive board (often a higher freq than you can hear) easily, of course assuming you have a scope.
You could also inject a signal at the phase inverter and isolate to preamp or power sections.
IMO if you want a reliable gigging amp changing out the signal caps is the way to go (you said you already did the electros)
I was havung weird intermittent issues with my 68 Twin, new board and caps and it has worked perfectly ever since.
_________________
Scott Swartz
Steeltronics - Steel Guitar Pickups
www.steeltronics.com |
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Marvin Born
From:
Ohio, USA
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Posted 23 Mar 2007 9:51 am amp problem
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You may have a leaky or open coupling cap. or
a bad cathode bypass cap. You have checked most of the other possible problems.
I live in Columbus, OH if you are in western PA, I can help you fix it. Or you can ship the chassis.
Marvin |
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