Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Rgeebee on April 14, 2022, 01:25:25 pm

Title: Unusual fault
Post by: Rgeebee on April 14, 2022, 01:25:25 pm

My fender  cvr guitar amp is dead on both channels until I give it a really strong input signal!
Then it works perfectly.
All tubes not microphonic when working.
I cleaned all the tubes pins but still the same problem.
Next time I use it same thing happens.
Any ideas?

Title: Re: Unusual fault
Post by: EL34 on April 14, 2022, 01:26:35 pm

You posted in the wrong board
Moving

Title: Re: Unusual fault
Post by: Rgeebee on April 14, 2022, 01:29:03 pm

Sorry, new member

Title: Re: Unusual fault
Post by: PRR on April 14, 2022, 04:18:37 pm

Stick a plug in/out the Effects loop many times, clean the contacts.

Title: Re: Unusual fault
Post by: Rgeebee on April 15, 2022, 04:02:30 am

This model doesn't have an effects loop.
Could this be a problem with the phase inverter tube?

Title: Re: Unusual fault
Post by: mresistor on April 15, 2022, 09:28:16 am

The first thing to do is try swapping tubes  - if you suspect it might be the PI then swap it out with a new/known good one. You should always have a couple of spare 12AX7 12AT7 tubes. Wouldn't have hurt to have gotten a spare 6L6/5881 duet before now due to the tube shortage.   Which are you using in the amp? and has the bias been checked? If your power tubes have drifted then the bias could need adjusting. It's also possible one of your power tubes has failed.

If you don't have another 12AT7 you could use one of the 12AX7s from one channel as the PI temporarily.


I think those PCB amps also have an Inrush Current Limiting Thermistor and I've read they can fail but I don't know what the symptoms are when that happens for sure - I suppose it would be low global voltages in the amp.

Title: Re: Unusual fault
Post by: mresistor on April 15, 2022, 09:45:55 am

Here's the schematic  And if one of the moderators or Doug could put it into the Fender schematics list that would be cool. 


https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Custom-Vibrolux-Reverb-Reissue-Schematic.pdf

Title: Re: Unusual fault
Post by: tubeswell on April 15, 2022, 03:07:44 pm

Are you certain it’s the amp? (And not guitar/guitar cable?)

Title: Re: Unusual fault
Post by: Rgeebee on April 15, 2022, 06:05:35 pm

Thanks everybody, I could kick myself!
I thought it was an internal problem.
Turned out to be a break in the moulded speaker jack plug!
Replaced the plug and all fine now.
Amp is fantastic now.