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EL84 in a CHAMP?

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11/12/1997 9:31 PM
Charles	EL84 in a CHAMP? I was toying with running a Champ (silv.) with an EL84 as opposed to a 6V6. No problem making the adapter, but was just wondering if anyone else had tried this before? Thanks! Charles

11/17/1997 12:07 PM

Charles:

You'll need a different cathode resistor when using an EL84 in the Champ circuit. Since it requires a lower value than 6V6 does, you can't just wire one in series inside the adapter socket. ( You could if the situation was reversed.)

A few years ago I built a champ amp using a gutted Gorilla GC-25 chassis & cabinet. There is a separate tube socket for a 6V6 and for an EL84, each wired with the appropriate cathode resistor. I put a mini toggle switch on the back to be able to switch from one to the other. The switch also intrrupts the heater power to the unused tube, since the power transformer I used (from a Hewlett-Packard VTVM) wouldn't carry the continuous load of both output tubes. My buddies named it The Chimp! Since the gorilla had 3 tone controls plus gain & master volume already labelled on the chassis, I wired my circuit like a BF champ with midrange & mv added, so all the controls are functional. There was a boost switch, which got wired as a standby. On the back panel I converted the headphone jack to a line out, off the speaker, with a level adjust pot next to it.

The thing sounds pretty good, and there is a slight but noticeable difference in the tonality from one tube to the other.

If you don't mind puching a new hole in your Champ's chassis for the 9-pin socket, and a small one for the swich, you could make this mod rather easily. When you size the cathode resistors, do it by measuring the idling current. Your champ may have voltages fairly high for a good 6V6, and they may be too high for a russian EL84 (but maybe OK for a 7189). Also, make sure your amp has screen dropping resistors.(If not, put them in.) I've come across some SF Champs that did not, and they were running plate & screen at 425 volts; not good in my opinion.

Hope this tale was of some help. It was a fun project.
Doc

11/17/1997 4:39 PM

Thanks Doc, for the reply.

Right now the Champ idles at about 45ma, then goes up to about 52ma when really crankin'. I think the EL84 should do about the same in the same situation, but of course, the resistor will need to be chosen to match.

I've heard about 400+ volts in Champs, but I've never seen anything like that. My '76 one only does about 360vdc. That's a little high for an EL84, but I was hoping to drop that with a zener diode (a la Weber suggestion).

The whole idea was to get the amp to sing a little better, break up a little better and at lower levels. Plus, I've always thought that the EL84 sounded better than the 6V6G

11/18/1997 8:06 AM

Charles:

In your amp, from the data provided, the 6V6 is dissipating in the range of 15 watts. (360 volts plate supply, minus 45ma through the 470 ohm cathode resistor, about -21 volts, or 339v X .045=15.3)

You can run the EL84 in the same area. Try a cathode resistor around 250-300 ohms. This should put you in the range of grid bias & plate current resulting in 15w of heat.

The manufacturer's design rating for 6BQ5 is 12 watts plate dissipation, but that's for long life/reliable service, not guitar amps & tone.

Just be sure that the plate structure isn't getting too hot (reddened color in dark background lighting), or the plate voltage (or idling current) will have to be reduced as you've suggested. I have read that EL84's sound better for guitar when the plate voltage is around 275-300 volts.

11/18/1997 8:14 PM

Yes, about 15 watts is what I came up with too, which means that the cathode resistor is probably going bad. Actually, there's just a tad under 350vdc at the plate, but that still gives about 15 watts.

The 6V6 gets nasty awful hot. I really should have checked the resistor when I replaced the bypass cap, but... you know how that goes. The cap was bad BTW. I really don't know why they always mounted them actually touching the cathode resistor. Seems like a poor practice, especially since there's all kinds of room to move it over out of harms way. I did notice the cathode current actually rises to about 55ma after about a half hour of idling...

Looks like another Saturday project!

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