Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fpolak on May 03, 2023, 06:45:15 pm

Title: Fender Vibrochamp 1979 problem... sounds like delay decay
Post by: fpolak on May 03, 2023, 06:45:15 pm

Hi guys,

I have a Fender vibrochamp 1979 with a strange problem. it sounds like a delay when volumes goes off and distorted. it is perceived more with a low volume 4/5.

This is not a tube problem (I replaced all) and not vibrato problem, because i put off vibrato tube and problem persist. All electrolitics caps and tone was changed for sprage atom few years ago.

I dont know what to test...  I upload video with this problem

eature=share

Title: Re: Fender Vibrochamp 1979 problem... sounds like delay decay
Post by: AlNewman on May 03, 2023, 07:59:20 pm

You could try setting up a listening amp and go from there.

Title: Re: Fender Vibrochamp 1979 problem... sounds like delay decay
Post by: fpolak on May 03, 2023, 08:15:59 pm

Quote from: AlNewman on May 03, 2023, 07:59:20 pm

You could try setting up a listening amp and go from there.

How can I do this?

Title: Re: Fender Vibrochamp 1979 problem... sounds like delay decay
Post by: AlNewman on May 03, 2023, 11:45:41 pm

http://musicfromouterspace.com/analogsynth_new/HOT_TIPS/images/amp_signal_tracer.gif

Build that, but use a 630v or higher cap instead of a 50v cap on the signal side.  Then plug it into another amp and start listening, starting at the front of the chain and working towards the back.

Adjust volumes every time you change connections, and be careful of high voltage.  A set of clip on probes is really helpful, and at the very minimum use standby whenever you change connections.

Title: Re: Fender Vibrochamp 1979 problem... sounds like delay decay
Post by: tubeswell on May 04, 2023, 12:18:28 pm

Besides tube swap*, 1st things to check are power supply voltages at each filter cap and plates, screen, cathodes. Need to know if any of these look ‘odd’, which might give clues about if/where parts are failing.


*did you try swapping all tube’s (including rectifier)?



Also, you say filter caps were changed ‘a few years ago’, but what else has been done to the amp? (Rewiring?, modifications?).

Title: Re: Fender Vibrochamp 1979 problem... sounds like delay decay
Post by: pdf64 on May 04, 2023, 12:36:53 pm

The video reminds me of the ‘knackered speaker’ syndrome.