Champ Biased Too Hot?

[sunnydaze]

Hello everyone,

First, wanted to thank the folks that take the time to keep this forum up and running. Really enjoy reading the forum, what a great resource and wealth of information here.

I've done several searches on this and other forums, kind of mixed results. Here is the situation, I have a Fender Bronco and my friend has a Fender Champ. My PT has the code P022772 and his is coded L010020 and is a little larger than mine. Both are cathode biased using a 470 ohm w/ 25uf cap. Voltages on the powertubes are:
Item.................Bronco........Champ
Plate DCV.................. 334......... 368
Cathode DCV................22........ 23
Bias current ma.............42....... 46

The bias current is the actual observed measurement, although both are close to the result I get using tube dissipation spreadsheet I down loaded from this forum (thanks). My friend been using an NOS Tung Sol 6v6 in his Champ for quite ahwile with no problems, but this seems really hot to me. Should I suggest he cool things down a bit. I read on another forum that doubling the resistor and cap value is the way to go. However, based on the tube dissipation sheet, it seems like that would reduce the bias current too much. I'm using a JJ 6v6 - we both thought his amp sounded sweeter.

Would appreciate any comments.

Thanks
mike

[Firestorm]

Well, neither amp is "really" hot; the Champ is a little hot. These amps have to run in Class A so they idle close to the max dissipation of the tube and swing around that when signal is applied. The Bronco is running at 13.1W (334 - 22 x .042) and the Champ is running at 15.9W (368 - 23 x .046). The 6V6GT is a 14 watt tube (though often run hotter in guitar amps). So the Champ is a little hot. You could try replacing the cathode resistor with a 560 (or a 510 if you can find one), but it's a bit of a Catch 22: when you raise the Rk, less current flows, so there is a proportionately smaller voltage drop across the resistor and you're almost back where you started. Instead you might try rolling a few 6V6s and see which one the amp likes best. NOS will sound sweeter (IMO) and aren't too expensive if you stay away from "Holy Grail" versions.

BTW, Champs and Broncos technically share the same PT (the 125P1B/022772), but the L010020 is a "universal" version that could be used both in domestic models and in export models (by changing the way the primaries were connected). This shows up in some '70s Broncos right before they were discontinued and in Champs from that era and later.

[sunnydaze]

Thanks for the quick reply Firestorm. I guess my friends NOS tung sol is a pretty tough tube, doesn't seem to red plate, and like I said, my friend has been using this tube awhile with no trouble. About the L010020 PT, I wonder if that one had a 100 v primary option and if my friends champ is wired that way. I live in Taipei and the wall voltage is 110. Just trying to figure out why his B+ is so much higher than mine. I tried switching the rectifier tubes, didn't do much at all, both are old US 5y3's and seem to be working fine, very similar efficiency.

Thanks
mike

[Firestorm]

That PT can be wired for use with a wall voltage 117V or 234V (essentially 120/240). At 110VAC, the secondary voltages will be tamed a bit, but I think these trannies were a bit hotter than the original 125P1Bs so that likely explains the voltage difference.

[sunnydaze]

That PT can be wired for use with a wall voltage 117V or 234V (essentially 120/240). At 110VAC, the secondary voltages will be tamed a bit, but I think these trannies were a bit hotter than the original 125P1Bs so that likely explains the voltage difference.

Thanks Firestorm, make sense that L010020 PT puts out a little more juice, it is a little bigger than the one in my Bronco (P022772). Also, the fact that the Bronco has the extra tube for the trem circuit - probably reduces plate voltage a little, too.

thanks
mike