78 Champ Mod

[2deaf]

I bought this Champ new in 1979 and have modded it numerous times over the years.  The only things that are original in this amp now are the cabinet and the faceplate.  I still have the original chassis and PT with a different mod in it that is still evolving.  I put the original OT into a customer's Champ after I sizzled his OT by putting the 6V6 in wrong.  The socket was so worn that the 6V6 would go in without the alignment key aligned.  That was the one and only time that I have ever destroyed an OT.

This is hands down the loudest Champ that I have ever built.  I took it down to the studio and the guitar player liked it so much that he wouldn't stop playing it.  Drummer got pissed after awhile and I had to pry the amp away from the guitar player so we could play some tunes.

The thing sounds good wide open even though it has a cathodyne in it.  Thanks, Merlin.

[2deaf]

Gut shot.

[SILVERGUN]

Very nice work and thanks for the schematic.
Do you know if the player preferred input 1 or 2?
What speaker do you have it paired with?
Voltages?

[shooter]

I like the zener drops, might be a consideration for tubes that get all pissy about G2 voltage.

a loud Champ is a good thing 

[Ed_Chambley]

Very nice work and thanks for the schematic.
Do you know if the player preferred input 1 or 2?
What speaker do you have it paired with?
Voltages?

Both each for different things Dave. Darn son!

[2deaf]

Very nice work and thanks for the schematic.
Do you know if the player preferred input 1 or 2?
What speaker do you have it paired with?
Voltages?

Thanks, nice of you to say that but I know it's still not up to the quality that you guys do.

The player never tried input 2.  I figure the drummer must have a New Years gig coming up and just wanted to get some practice in.  He was already making exaggerated watch-checking actions and he probably would have blown a gut if we moved on to input 2.  He doesn't even own a watch.

Input 2 is very Marshall with the treble peaker and the cathode follower.  The player commented that input 1 sounded like a big Marshall and that it sounded like early Billy Gibbons.

The speaker is a 10" Eminence Red Coat Ramrod.  Both transformers have like a quarter inch clearance to the speaker frame.

 

[2deaf]

I like the zener drops, might be a consideration for tubes that get all pissy about G2 voltage.

It not only gives the 6V6's a break, it also gives that poor current-challenged PT a break.  Especially when the amp is obviously designed to be played flat out.

[SILVERGUN]

Very nice work and thanks for the schematic.
Do you know if the player preferred input 1 or 2?
What speaker do you have it paired with?
Voltages?

Both each for different things Dave. Darn son!

The player never tried input 2....
...The player commented that input 1 sounded like a big Marshall and that it sounded like early Billy Gibbons.

The speaker is a 10" Eminence Red Coat Ramrod.

Mr Chambley, you should be thanking me for getting him to give up the goods, not mocking me. You know that If Im gonna steal it I wanna make sure I get it right. Maybe mix this Billy Gibbons channel with a nice tweed grit second channel. Always thinking......


Thanks for the voltages 2d. This is a great example of how to share an amp design.
I am hoping to get better with my schematic drawing this year. That sounds like a good resolution for Tues. night.

[jojokeo]

Nice use of 6 triodes and 2 pentodes in (basically) a 3-hole faceplate 2d. - but calling it a Champ? I’m a little surprised with the choice of using the cathodyne set up rather than the usual LTPI?


I really love the Ramrod speaker, to me it’s got something I can’t explain that makes me want to play more.


This combo makes for a fantastic grab n go anywhere light little powerhouse.


What’s the PT secondary 325v?

[jjasilli]

Interesting design features!


Can you explain the theory & results of:


* rather large filter caps for nodes A & B
* reduced & "nearly regulated" screen voltage
* the dual 500k pot & filter circuits


Thanks!

[2deaf]

I’m a little surprised with the choice of using the cathodyne set up rather than the usual LTPI?

I wanted the higher open loop gain of the gain stage/cathodyne as compared to a LTPI in order to recover from the effects loop and overdrive the output tubes.  Cathodynes don't usually sound good for overdrive, but Merlin has some suggestions on how to make it work.  I wanted to try it out, so I put them into this amp.  He was right, again.

I really love the Ramrod speaker, to me it’s got something I can’t explain that makes me want to play more.

Yeah, I was pleasantly surprised and it really puts out some SPL.  I looked over the frequency response curves before I selected this one and maybe that actually works (never can rule out dumb luck, though).  I selected a CV-75 for that little DeLuxe that I built because its frequency response was essentially the same as the more expensive Allessandro and that worked out pretty good, also.

What’s the PT secondary 325v?

I didn't measure it, but 423V at the first node is certainly consistent with 325V on the secondary.  I can pull the chassis and measure it if you want.   

[2deaf]

Can you explain the theory & results of:

* rather large filter caps for nodes A & B

Most of my life I have been dealing with large guitar amplifiers.  I learned early on that you need a lot of filter for the power tubes if you're going to be overdriving them.  Otherwise you get hum while overdriving that is particularly annoying on leads.  I was putting 470uF for the first node on some of those amps.  I don't have much experience with overdriving 6V6's, but their power demands are much lower and I decided to try 100uF to see what happens. 

* reduced & "nearly regulated" screen voltage

There has been a lot of discussion about grossly exceeding the published voltage limits on a 6V6 and I have actually had some trouble with 6V6's at 450V.  Some of the bigger bottles can take higher plate voltages as long as the screen voltage is reduced, so what about 6V6's?  I'm trying it out. 

That 22772 PT is rated a little low for this amp as well as for stock Princetons.  Moving all of the lines down on the graph with reduced G2 voltage in order to lower the current consumption seemed like a good idea.

* the dual 500k pot

I only have three holes in the faceplate, so the volume control needs to work with both Input 1 and Input 2.  I always put a volume control after the first stage because distortion created by the interaction of the first and second stages cannot be eliminated with a volume control after the second stage.  If I just used a single 500K volume control after the second stage,  I would lose some versatility.       

[DummyLoad]

have you experimented for max. cathodyne outputs swing to try to make it even more transparent? i have an experimental amp on my breadboard now that's using a 200K load (100K+100K) instead of the fender-ish values of 112K (56K+56K) and i tuned Ib for for max. input threshold.   

fiddling with your numbers, the best input threshold  i can come up with running 350V B+ is around 115V. so 100K under Rbias and 100K Ra with Rbias of 820R. 




--pete

[2deaf]

have you experimented for max. cathodyne outputs swing to try to make it even more transparent?

Naw.  I just used the Fender version.  I did draw it out, though, and felt pretty confident that it would swing way more than the 31V_p needed to clip the output tubes.

fiddling with your numbers, the best input threshold  i can come up with running 350V B+ is around 115V.

 

If I understand this correctly, then it should clip at 57.5V_p.  I estimated that it would bottom out around 55V and the actual cathodyne in the Champ clipped at 52V with 350V B+ and the 6V6's pulled.

i have an experimental amp on my breadboard now that's using a 200K load (100K+100K) instead of the fender-ish values of 112K (56K+56K) and i tuned Ib for for max. input threshold.

How's that working out?  I'm going to have to play with this after the New Year's dust settles.

[DummyLoad]

How's that working out?  I'm going to have to play with this after the New Year's dust settles.

loving it so far. i'll post the schema when i'm done tweaking it. i'm also experimenting running the power amp in UL mode. loving the results so far. not hearing hardly any of the blocking/doubling distortion typical with a cathodyne when pushing the preamp into hard clipping.

i am also running with the merlin recommended large-ish cathodyne grid stopper to limit g1 current/freq. doubling and lowering the high corner freq. response. with a 470K part it's about 100KHz with a 12AX7. 

--pete

[Willabe]

Thanks, nice of you to say that but I know it's still not up to the quality that you guys do.

No, that's a very nice looking build!