More Pwr and Headroom for SF Champ

[JPH]

Hi-

I'm new here - Happy to have found this site.

I have a SF Champ. I am looking for ways to increase the power AND headroom. Most mods seem to make things dirtier - I want to go the other way.

Heres what i have going currently (btw my guitar is an G&L ASAT w/ the p-90 style MFD pu's).

Stock 8" alnico spkr, 5V4 rectifier(for more headroom) 5751 preamp and RCA 6V6. Original filter caps - soon to be replaced.

The amp sounds great but the bass response isnt happening and the spkr while it sounds good lacks bass response and is a bit farty

I know replacing the cap can will tighten things up.

What about removing the feedback loop? With it add more pwr or just make things dirtier?

Same for tone stack? I like to be able to adj the EQ - at least the treble
does running the tone knobs at 10 achieve the same things as removing the tone stack?

I've heard some guys replace the 6V6 with a 6L6 for more pwr - but I've heard that its not good for the Pwr xfmr - I know to increase the voltage rating on the bypass cap to 50V if I do this.
Any other tube substitutions for more pwr & headroom?

I plan on replacing the spkr w/ either a 10" weber or 8" Jensen - are the MOD series decent?
I want to stick with the combo package - ie no separate cabs.

What about biasing? I know these amps are cathode bias but I suss that installing a variable bias control or biasing it a certain way is probably key to my achieving my goal.

Any info/help much appreciated.

Thanks in advance!

JPH

[stokes]

I tried the 8" Mod Series speaker and hated it,I am a big fan of Weber speakers and have a few of them in different sizes,including the 10F150 in a Champ type clone,with a 6L6GC.When changing the filter caps I would suggest getting rid of the can cap and using 40mf's for the main and screen caps to tighten things up.A solid state rectifier will also help,as well as an adjustable cathode bias resistor.

[JPH]

Thanks for the info -

Couple questions - have you tried the 8" webers? I hear the 10"s are good but that the 8"s are cheap spkrs - I know 8" spkrs dont offer alot of bass response anyway. Just curious as they cost $75 new.

I think I remember how to install a variable bias control - I put one in a Super Champ years ago - is there any place on this site that has the info?

Cheers,

JPH

[stokes]

I have never used the Weber 8" spkrs,but they are reputed to be of lesser quality than his others.They are much less than $75,I believe the alnico version is only about $35.The Champ is a cathode biased amp so to make it adjustable just replace the cathode resistor with a 500ohm adjustable wirewound.If you want to make it an adjustable fixed bias,you will need to create a negative voltage supply from one side of the high volt windings.

[MWJB]

+1 for Stokes' fixed bias suggestion. Look here...

http://area51tubeaudiodesigns.com/Me...ry_Code=cat004.

SF Champs use the same PT's as the Princetons (2A heater current), they will easily handle a 6L6 power tube. If switching to 6L6 with cathode bias, use a 10W cathode resistor and a 100v cathode bypass cap. In cathode bias, you'll really need to up the B+ (SS recto, maybe try a 660ohm cathode resistor) to realise more power.

Removing NFB, try it and see...should sound like more power but less smooth frequency response?

Removing the tone stack will increase gain and reduce headroom. Try tweaking middle resistor value (another 15K in parallel, or a 25K pot in #2 input) for more scooped mids?

[JPH]

Thank you both for the info!

Wow - more options than I thought.

-But that raises more questions

Re the Weber alnico 8" - their website says the alnico will break up sooner than the ceramic one they offer - I think thats where I got the $75 price tag. what about Kendrick replacement spkrs - any good?

I'm going to takes Stokes' advice and replace the cap can with discrete 40 MFDs before I change spkrs to see how much the bass response improves.


Good to know about the PT - but if I go with a ss rectifier will I have to upgrade to a larger PT?
I have Gerald Weber's book - Desk Ref for Hip vintage guitar amps - He says not to put a SS in a blackface deluxe or prinecton as doing so will raise the plate voltages another 30V and that the design already exceeds a 6V6 voltage capacity by 70 - 100V - he says it will work but it most likely blow out the output tubes - my question is this the same for SF champs?

Stokes talks about using a 6550 for an pwr tube and upgrading the OT.
How much pwr can be gained by doing this? And what kind of OT would I need to install?

I read a thread on this site about using a 6L6 but that the sound difference is barely audible - true?


I will check out the link before I ask more questions....

Thanks!

[stokes]

The reason Gerald recomends not using the SS rect in those amps is because they already have "too high" a plate voltage for the 6V6's,so using a ss rect will take the plate volts over the top.To make a fixed bias feed is quite simple and much cheaper if you buy the components seperately,although the easier way would be to order the kit MWJB pointed out.I wouldnt recomend using a 6550 without upgrading the PT and OT.It would be simpler to use the 6L6 with the same trannies,you may pick up a couple of output watts which will be realized in some extra headroom,along with the other mods mentioned,will tighten up the overall response,especially in the bass.Removing the tone stack is a good way to improve gain,the tone stack will suck a lot of the signal out and the stage afterwards brings it back up.Without the tone stack you will see a lot more gain.

[JPH]

Thanks -

Cool!

Ok, so 1st replace caps with 40mfs, convert to fixed bias, use 6L6, remove tone stack and look at upgrading the spkr.

2 Questions - is there a way to partially remove the tone stack - ie, the bass control so that there is still a treble or tone control? If so, I was thinking of installing a switch on the treble pot so it switches btw no tone stack - higher gain, no tone control and partial tone stack - some gain increase w/ tone control.

Can a DPDT switch be added to the back of the stock pot or does it have to be replaced with a pot/switch combo?

Thanks!