Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jwhitmore on December 21, 2022, 05:11:35 am

Title: Fender 59 Bassman LTD - Noise problem
Post by: jwhitmore on December 21, 2022, 05:11:35 am

Hello all,
    I went and got a pre-owned fender 59 Bassman LTD and it's got a noise issue. I guess I will have to get the scope out and download the schematics, but kinda hoped that maybe wiser, more experienced, heads than mine might be able to short cut some of the searching.

I've got all the pots on the control panel zeroed so there is no signal going through the Amp, however when it warms up there's an occasional clicking put of the speakers and it appears to coincide with the power valves brightens fluctuating. I'm not sure which is the cause and which the effect. So does a click hit the input of the power tubes and cause the brightness to dim slightly, given the controls are at zero I don't think that's the case. So I'm assuming that there's an issue with the heater voltage rail and a fluctuation in heater voltage is causing the click.

The click is one thing but added to that there is a sustained crackle out of the amp occasionally. That might need scope work to find, but perhaps it's the same issue as the click.

So for the minute the plan is to put the scope on the heater voltage rail and start there. I'd imagine that the output transformer for the heater voltage rail is fairly basic, in terms of the number of components being used but given that basic circuit maybe that's not going to be the issue. Off to look for the schematics. Thanks for any advice.

Title: Re: Fender 59 Bassman LTD - Noise problem
Post by: pdf64 on December 21, 2022, 05:23:25 am

I suspect the heater idea is a non starter.
Things happen slowly with heaters.
Bad  / intermittent connections in the heater circuit result in slow fades out and in, not clicks.
Are you sure that the lower brightness that coincides with the click isn’t similar to the brightness they have when in standby mode?
Clicks might be a bad valve, resistor or cap, or connection to them.
Do the treble or mid controls affect the click?

Title: Re: Fender 59 Bassman LTD - Noise problem
Post by: jwhitmore on December 22, 2022, 04:16:04 am

Thanks a million for that pdf64, I'm currently away and with the festive period fast approaching I might be delayed, but as soon as I can I will fiddle with the EQ settings to see if that effects the noise an report back.

Thanks again