ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

previous: Charles I was toying with running a Champ (... -- 11/12/1997 9:31 PM

Re: EL84 in a CHAMP?

11/17/1997 12:07 PM
Doc	Re: EL84 in a CHAMP? 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 12:07 PM

Doc

Re: EL84 in a CHAMP?

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

Replies:

Charles Thanks Doc, for the reply.R... -- 11/17/1997 4:39 PM