Iincrease signal to output tubes in Fender Super Champ XD?

[Steve A.]

A blues guitarist friend of mine is looking for more volume from his Super Champ XD when the preamp is set for clean sounds. I was looking at the schematic from the Fender site and there is a 10k grid load resistor on the predriver ahead of the cathodyne PI. Damn, no wonder his amp is so anemic- that 10k grid load is like sucking all of the life out of that amp. I was thinking of replacing that resistor with a multi-turn cermet trim pot, probably 100k for starters, adjusted to whatever seems like an appropriate setting. Of course if we boost the output like that he will need to keep the preamp volumes down with the high gain distorted sounds. (I think that Fender was going for a fail-safe design so that it won't explode with all of controls set to 11 and a few cascaded OD boxes also cranked up to 11. My philosophy about guitar amp design is that they are like fine test equipment which needs to be adjusted carefully not an appliance that you just plug in and set to 10. Of course I guess that was just an excuse when the amps I built would oscillate like a mofo with everything turned up to 10.)

Here is a link to the schematic:

http://support.fender.com/schematics..._schematic.pdf

Any thoughts on this or other suggestions? My friend has a room full of guitar amps of various sizes but he got the Super Champ XD as something easy to pack around for low volume jams.

Thanks!

Steve Ahola

P.S. I was thinking it might be cool to add a Power Amp In jack on this if he wants to bypass all of the digital and ss crap in the preamp by plugging in a multiprocessor.

[Bruce / Mission Amps]

Steve, keep in mind that the device driving that grid is a low zed solid state device too. Who knows what the stability of it would be by going crazy with R5 (10K) ... Hmm, looks like R5 (10K) and R25 (15K) are kind of an AC voltage divider.... there's a 25%-50% loss right there.

[Chuck H]

I was thinking the same thing. It's not a tube driving the PI. If the output impedance of the driving device is low enough, that 10k might not even be loading the signal significantly. Try changing the divider ratio and staying close to the same circuit impedance. As for the added PI drive making the OD channel too agressive... I have trouble reading the schem with all the allocated leads and the IC's in there. But you could trace the signal to the mix for the OD and clean channels and perhaps pad the OD channel there.

[hasserl]

How about increasing the value of the plate load resistor for V1B to increase the gain of that stage? Say 220k.

[Steve A.]

Steve, keep in mind that the device driving that grid is a low zed solid state device too. Who knows what the stability of it would be by going crazy with R5 (10K) ... Hmm, looks like R5 (10K) and R25 (15K) are kind of an AC voltage divider.... there's a 25%-50% loss right there.

Thanks for pointing out the R5/R25 voltage divider! It seems to me that if the sum of the two is 25k then the SS output will be seeing essentially the same load so it should be happy- right? And I imagine that if the SS circuit is "happy" with a 25k load to ground on the output, you probably could boost it up a little bit without it becoming unstable (but I have no what a "little bit" would be= 35K? 40K?)

How about increasing the value of the plate load resistor for V1B to increase the gain of that stage? Say 220k.

If you raise the impedance of plate resistor from 100k to 220k on one of the initial preamp stages you will usually get more gain but the sound won't be as clear and you may lose some of the clean headroom. Would that also apply to V1B which is the pre-driver gain stage? Or does it not make as much of a difference further down in the signal chain? My friend is looking for a louder clear sound for blues. He was wondering why his Deluxe Reverb RI is so much louder than the Super Champ XD. Of course the transformers are much smaller and the DRRI has 391vdc on the plates as opposed to 375vdc as well as a larger speaker but you'd think it would have a little oomph with the clean sounds like the original Super Champs... Thanks!

One question concerning phase inverters- will you get more gain from a LTP or the pre-driver/cathodyne PI arrangement as in the Super Champ XD or the Peavey Classic 30's and 50's?

Steve

P.S. Here is a link to the 13 page service manual which has a much more legible schematic:

http://music-electronics-forum.com/a...r_champ_xd.pdf

[kleuck]

You have twice the gain with a cathodyne with the second triode used as a preamp.
You can make R5 way higher, i would make R26 a 33K, and R5 a 100K.
You can too replace R21 for a 220K, and R16 for a 1K8 to 2K2 for more gain, and lower the NFB (lower R23 or higher R17) but you'll have to test by ears.
You can simply try to remove the NFB, or make it switchable, but it would be another amp.

[Enzo]

WHat speaker is it playing through now? Swap that for a speaker with a few more db efficiency, and you will add a few more db of volume.

[Chuck H]

This seems like a good idea. For one, there's no risk of instability on the OD channel due to added PI drive. And for another it's non invasive. And for another it offers the chance to adjust the amps voice.