Tuberolling in a Vibro Champ

[Jahn]

Hi all, my new '71 Vibro Champ just arrived last night and I love it! I have a box of nice tubes I'd like to experiment with, but I wanted to run it by you first.

http://www.ampwares.com/ffg/schem/champ_vibro_aa764_layout.gif

That's the layout, so it goes Rectifier-Power Tube-Tremolo Tube-Preamp Tube I think.

I'm going to leave the rectifier tube alone. But for the others, will a 6L6GC/12AT7/12AU7A combo be ok instead of the 6V6/12AX7/12AX7? Is this a self-biased amp? Thanks for the help all!

[mc5nrg]

I was hoping a real tech was going to respond. Keeping in mind that I'm not...
Its possible that the trem circuit won't work too well with something that isn't a 12AX7
A 12AU7 is pretty low gain to replace the V1 tube so you may not be real happy with the result
According to some threads on the front page you can sub the 6L6GC for the 6V6 but the amp would then prefer a 2 ohm load. I don't know how well the current draw of the tube switch would agree with the voltage available from the Champ transformer.
The amp is cathode biased so usually you are OK. Some folks suggest pulling the 12AX7 for the trem if you never use it- I've forgotten just what this is supposed to accomplish.

[Faustine Amps]

Hi all, my new '71 Vibro Champ just arrived last night and I love it! I have a box of nice tubes I'd like to experiment with, but I wanted to run it by you first.

http://www.ampwares.com/ffg/schem/champ_vibro_aa764_layout.gif

That's the layout, so it goes Rectifier-Power Tube-Tremolo Tube-Preamp Tube I think.

I'm going to leave the rectifier tube alone. But for the others, will a 6L6GC/12AT7/12AU7A combo be ok instead of the 6V6/12AX7/12AX7? Is this a self-biased amp? Thanks for the help all!

The Vibro-Champ is a great little amp. I have a '72 Bronco, which is a Vibro-Champ in disguise, and it's a little monster!

Now, regarding your tube subs... you can't use a 6L6GC as a drop-in replacement for a 6V6. The 6L6 has a different plate resistance so you'll have an impedance mis-match. That's not such a big deal as most Fender amps can tolerate a 100% impedance mis-match, but it will have an effect (not necessarily positive) on the tone. It won't really be at the optimum bias either. The more critical issue is the filament current. A 6L6 draws twice as much filament current as a 6V6 so you will be stressing the power transformer beyond its intended design limits and could possibly burn out the filament winding.

The tremolo oscillator tube (the middle 12AX7) must remain a 12AX7. A lower gain tube, such as a 12AT7, 5751, 12AU7, etc. may not provide enough gain to drive the feedback loop for the oscillator. There's not much point in "rolling" this tube anyway, since it doesn't affect the tone.

The preamp 12AX7 tube can be rolled to your heart's delight. I've tried several NOS Euro and American brands and eventually settled on a NOS GE 12AX7A. There are lots of 6V6 options as well. I'm using a NOS JAN RCA VT-107 (mil-spec 6V6) and it sounds bitchin'!

I think you could improve the tone a lot more by replacing the speaker. I have a Weber 3.2 ohm alnico in mine and it made a definite improvement.

I would recommend contacting Mike at KCA NOS tubes for some good advice regarding tube subs (he also hangs out here on this forum).

That's my $0.02. Have fun with your Vibro-Champ!

Regards,
Tim Gregoire

[Jahn]

Thanks, perfect answer! I'm off to roll that last 12AX7 then, and search for some 6V6 goodness!

[Blue Strat]

Actually, 6L6s do quite well in these amps. The bias is actually worse for 6V6s in the stock configuration. It's recommended to swap out the 470 ohm power tube cathode resistor with a 1K to make it run cooler/safer for a 6V6.

[Faustine Amps]

It's recommended to swap out the 470 ohm power tube cathode resistor with a 1K to make it run cooler/safer for a 6V6.

True... if you are so inclined to do any "mods", you should replace the bias resistor with a 1K 2W, and you'll also need to replace the 25uF, 25V bypass cap next to it with a 50V cap to compensate for the higher voltage drop on the cathode resistor. I did these mods (and several others) to my own Bronco. The last time I mentioned these mods, I was barraged with e-mails asking for specifics, schematics, etc. so I'm reluctant to open that "can o' worms".

Mike's post (regarding the suitability of 6L6 tubes in this amp) got my curiosity up, so I got out my Fender schematics. As it turns out, Fender used the same power transformer (125P1B / 022772) in the Vibro-Champ and the Princeton Reverb amp. That being the case, the Vibro-Champ power transformer should be able to handle the additional current demands of the 6L6 without any problem. I stand corrected!

Thanks for pointing that out, Mike. The impedance mis-match not withstanding, I'm curious to experiment with some 6L6 tubes myself now. I can't imagine that it's going to buy me any more gain, but I'm anxious to hear any tonal differences.

Regards,
Tim Gregoire

[Blue Strat]

Yep, the Princeton PT is the key. You won't get any more gain (no matter how you define it) but will get slightly different tone and possibly another watt or 2. The power supply and OT pretty much limit the power you're going to get.

[Jahn]

Great, I put my RCA 6L6GC in the VibroChamp with confidence, and it changed this thing into a mini-Twin. So clean! I actually prefer that to breakup - I crank it to 5 for as loud as bedroom level needs to get, and it's still clean! Great!