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Post subject: Fender TV Bassman Head Cutting Out
Posted: Mon Jan 24, 2011 1:17 pm
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Hey guys,

This amps head has been making crackling noise for some time, probably bad tube. I can live with that, however I recently noticed after it's nice and warm, at least 30mins at pass mid volume it cuts out. No sounds at all, dead silent but power remains on. Bad tube or bad cap somewhere? I checked the soldering and nothing looks dry. Any idea?


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Post subject: Re: Fender TV Bassman Head Cutting Out
Posted: Tue Jan 25, 2011 12:16 am
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All a WAG... Some kinda ultrasonic instability rearing its ugly head after amps been on for awhile? Does the power tubes seem to get hotter as the volume drops? This is want happened to a couple of amps that cut out after use, in my experience. Try contact cleaner on all the sockets and tube pins. Have you tried changing out rectifier and power tubes?

If cleaner & tube change doesn't work, usually need an amp tech to go through section by section to find the site(s) of potential instability.

Are the trannies getting really hot, just before the amp cuts out? Could be shorted core lammies (rusted out or some such thing).

Good luck. Prolly need to find a good techie.


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Post subject: Re: Fender TV Bassman Head Cutting Out
Posted: Tue Jan 25, 2011 6:11 am
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Well funny you mentioned that, I cleaned the socket and tube pins with contact cleaner and played last night and it has not cut out so who knows maybe that did it. Tubes are definitely past their prime though, power tube is the one crackling when I tap it. If it starts cutting out again I'll re-tube, starting with rectifier. It's my best amp and what I record through so I want to keep it alive. Thank you for the input.


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Post subject: Re: Fender TV Bassman Head Cutting Out
Posted: Tue Jan 25, 2011 8:54 am
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Could be a few things

Low Power or Loss of Volume
Back to the top of the Amp Debugging Page Back to the GEO Home Page

Faulty Preamp Tube
Faulty Power Tube(s)
Bad preamp cathode resistor
An unbypassed cathode resistor has drifted upwards (to 5K-10K or over)
Faulty phase inverter
If for some reason the phase inverter input side is good but the inverted side is bad, the power amp will still work, but power will be very low. This can be a bad 1/2 tube, a faulty socket contact, a broken or open plate resistor or coupling capacitor to the output tube, or a bad solder joint on any of these.
Open cathode bypass capacitors in preamp
if they go open, the stage they're in loses gain, but does not otherwise fail. If they short, it dramatically shifts the bias point, and may cause distortion as well as low volume.
Faulty vibrato circuit on neon/LDR vibrato Fenders
If there is a dummy plug in the footswitch hole, or a bad footswitch so the vibrato is always active, sometimes the vibrato tube turns on and stays on, not oscillating. This keeps the neon bulb on all the time, shunting lots of signal away. Same thing can happen if there is a shorted vibrato tube (rare) or a bit of wire or solder shorting the vibrato tube. Check the plate voltage on the vibrato tube to be sure it's oscillating.
High voltage isn't high enough for some reason

* Failing rectifier tube - try swapping in another one
* Failing power filter capacitors
* Failing or open series dropping resistor in the bypass networks leading to preamp stages
* Failing bypass capacitor - treat as in power filter caps.

Open screen resistors on power tubes
Amp cuts out or "goes dead" when the volume control is turned up higher than "X" or when you hit a specific note
You have a parasitic oscillation above hearing range. This can overheat an output transformer, and really needs to get fixed fast. It can often be fixed by tube swapping, but you often need an oscilloscope to see what's happening in the electronics.


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Post subject: Re: Fender TV Bassman Head Cutting Out
Posted: Tue Jan 25, 2011 9:15 am
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Will keep an eye on it, as I said, if it happens again I'll do a total re-tube. If that doesnt help it may be beyond my capacity and it could be tech time :(

I wonder if it only cuts with certain pedals, like overdrive, wah, fuzz or anything that could overload the power.


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Post subject: Re: Fender TV Bassman Head Cutting Out
Posted: Tue Jan 25, 2011 1:31 pm
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As was stated by JonCarr, could be a number of things and/or any combination of two or more. If overdrive pedals cause it to crap out... I'd suspect a leaky coupling cap.

Cleaning the pins often helps because bad contact can not only lose signal but also give you iffy contact to the bias power supply and potential loss of idle bias setting on the power tubes. Also, screen grid input, yaddy-yaddy-yaddy. It's why cleaning sockets, pins, and even tightening the socket contacts (tangs) helps 50% or more initial problems in tube amps (esp old ones) IMHO.

If the amps craps out with pedals applied, you prolly should have a good amp tech go through the thing front-end to power section (power tubes).


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Post subject: Re: Fender TV Bassman Head Cutting Out
Posted: Tue Jan 25, 2011 1:42 pm
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Thanks to all who gave their valuable inputs, I will update this thread if the situation develops. :)


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