Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: smackoj on January 28, 2018, 06:53:40 pm

Title: Help requested for Fender Champ 12 (single-ended 10 watt with 6L6 pwr)
Post by: smackoj on January 28, 2018, 06:53:40 pm

Hi fellas; I could use a suggestion or two re: Champ 12 tube amp. I bought this in As-Is shape and luckily there was very little wrong that a few pwr caps and some contact cleaner couldn't fix. But I believe the design allows the high voltage and current to hit the 6L6 before it has the heater warmed (the schemo is attached so maybe Fender has something in the B+ line that slows down the juice?). I would like to add a Stdby switch if adviced and looking for suggestions on the best route to take?

Thanks amigos,  Jack D

Title: Re: Help requested for Fender Champ 12 (single-ended 10 watt with 6L6 pwr)
Post by: SILVERGUN on January 28, 2018, 07:39:51 pm

Read this:
http://www.valvewizard.co.uk/standby.html

Title: Re: Help requested for Fender Champ 12 (single-ended 10 watt with 6L6 pwr)
Post by: PRR on January 28, 2018, 07:59:41 pm

> allows the high voltage and current to hit the 6L6 before it has the heater warmed

So??

This is not a HIGH voltage application like a radio transmitter. The 6L6GC was designed for bang-start TV duties. Is not a problem.

Title: Re: Help requested for Fender Champ 12 (single-ended 10 watt with 6L6 pwr)
Post by: smackoj on January 29, 2018, 02:03:26 pm

Hi again; Well I always thought that not having the heater warmed shredded the cathode? Good to know that it's alright to not use a Stndby switch. Good article too Silvergun.

thanks amigos

Title: Re: Help requested for Fender Champ 12 (single-ended 10 watt with 6L6 pwr)
Post by: 92Volts on January 30, 2018, 07:59:46 am

There is some truth to what you describe, known as "cathode stripping", but only at extreme voltage... like in radio transmitters where you might see 3000 volts on the plate. In other words, the problem does not exist in guitar amp or similar audio circuits.

As the Valve Wizard page mentions, standby switches have continued to be used just because amp manufacturers started including them, and everybody assumed there was a good reason they did that. It's possible a Fender engineer at the time was thinking about cathode stripping when they decided to add a standby switch... but that doesn't mean it was a legitimate concern.

Title: Re: Help requested for Fender Champ 12 (single-ended 10 watt with 6L6 pwr)
Post by: PRR on January 31, 2018, 11:48:24 pm

It is a Stand By switch, not a "Start" switch.

It is for between sets, take-a-break. Especially the old guitars and amps, there was a huge racket when left on; and in older days (even now in church service) the crowd could be quietly annoyed.

Your transistor Peavey, you just turn-off, cuz it comes to life quicker than you can find your pick again. As you know, the tube amps take some/many seconds to come alive from off. If the barkeeper or preacher is waiting, that's too long.

Of historical note: "Instant On!!" TV sets appeared about the same time. These killed the HV after Milton Berle was done, but left the heaters running (half or full) all night, so the set would come-up "instantly" at 7:59 the next night.