High Power Champ

[plaidzebra]

Hey all,

I'm considering starting up a new project for a friend of mine. What are everyone's ideas on how to get more power out of a Champ circuit? Ideally I'd like to make something like a 15-30 watt amplifier using the circuit from a champ (I.E. just a volume knob), modifying the power section with a 6L6, 2 6V6s, or some other combination of tubes.

Is there precedent for this that anyone can redirect me to?

[kagliostro]

If you do a search with google to 6L6 Champ you can find much more

Franco

[labb]

Get your transformers right and use a KT 88 tube.

[Paul1453]

The Champ's SE output has a very different harmonic distortion signature than a PP output.
The PP configuration cancels or greatly reduces the even order harmonic distortions.
PP output won't really sound like a Champ.

Don't get me wrong, I really like the PP output for these smaller tubes 6V6/6AQ5/EL84.
But for the SE output, like a Champ, I've found I like a bigger bottle like a 6L6.
I wish I had a KT66 and KT88 to try.

They have posted some good SE ideas for you.

I'm quite happy with the sound of a simple EF86 / 6L6  6.5 Watt power output circuit, I just made.

Just a single volume knob, with B+ @ 250V, is much cleaner and louder than a Champ with the same low watt SE OT.

When dimed it is just starting to get the 6L6 to break up.

If you want some dirt put a distortion/overdrive pedal on the input.

Then you can make her as dirty as you want.

I don't know if that is loud enough for you, but it is louder than a regular Champ you just put a 6L6 in. 

[plaidzebra]

Thanks everyone for the ideas!

So far I'm loving the idea of a KT88-powered champ. I am very new to amp design, though, so I have no idea what I would need to change in the original Champ schematic beyond the transformers. With those, I'm not sure about what transformers I would need. I'm also concerned about the layout of the amp; I know that the physical position of the components is very important in order to reduce hum.

I'd love to learn a lot with this build, and my guess is that I'll need everyone's help just about as much as I can.

[jjasilli]

Dan Torres has simle mods to hotrod a Champ.  Those mods & more are in the schematic below.

[kagliostro]

I forgot to link this





Franco

[pompeiisneaks]

Another thing you could think of, if you like the tube in the champ would be to parallel single end it.  I'm not finding any super easy schematics, but basically you need to ensure the output transformer is set to have the different impedance, the champ uses a 6v6, I think if you're using the two with the norm being about 5k for a single tube, it should be doubled at 10k right?  (Someone correct me, maybe due to the output tubes being in parallel they become 2500?)

At any rate, that way you can double (or quadruple) the output but keep it single ended and keep the tone similar, just louder.

~Phil

[pompeiisneaks]

I should clarify, that 5k is from the 6v6 datasheet I was looking at in class A for a single tube at 250V you would need to check the voltages you're using of course.  I do think I found that I was right in my second statement, two tubes in parallel halve their impedance so a 2.5k OT should handle it.  Please don't count that as gospel, though, I'm still learning :)

~Phil

[plaidzebra]

No worries, friend, I'm still learning too! And by the sounds of it, you seem to know a fair bit more than I do.

I would like to keep this as simple (and hopefully as cheap) as possible, and thus my idea is to keep this limited to a single-ended amplifier (if this is neither cheaper nor easier, please let me know!). My limited knowledge thus leads me to believe that beefing it up to a 6L6 would increase the volume of the amp, and beefing it up to a KT88 would do so even more. I've been kicking around the idea of a SE KT88 for a while now, so I think that's the path that I would most like to stumble down.

Is it as simple as changing the transformers so that they can handle a KT88, or are there more circuitry shenanigans that I would have to deal with w.r.t. the 5F1? Does Doug have any transformers that I would be able to use for this application, or would I need to head somewhere like Hammond to get a transformer specific to this project?

[labb]

take a look at this. Might help you. Just ignore the master volume control.


http://www.ax84.com/static/corepoweramps/20W_SE/AX84_20W_SE_Poweramp_Schematic.pdf

[pompeiisneaks]

Going to a bigger tube will definitely increase output, but the tubes tone changes a bit too.  If you're after the tone of the champ, the other output tubes will be a little different, but experimentation is the best part of this :)

Phil

[PRR]

Just for a dose of reality: an SE power stage won't do over 40% efficiency. Plate idle heat is 2.5 times the maximum power output. For 30 Watts output you need 75 Watts of plate(s).

This could be six 6V6/EL84, three 6L6/EL34, or two 6550/KT88.

Or an HK-257(?) or similar transmitter tube. (813 "can work" but will need a 6V6 to drive its hungry grid.) (The common 4X150 makes a terrible audio amplifier.)

[shooter]

fwiw, I built a KT88 sort-kinda Champish amp, schematic below.  It specs around 16W clean, 23W full distortion.  It's been passed on,  2 gigging guys now and the 2nd guy won't take it out of his house!, It's his main practice amp.  It's not a 1 trick pony, but both musicians had to *learn* how to dial it in.

[labb]

I have a little SE amp that is set up with Parallel tube sockets and external bias.We have run El84 up to the KT88. We have tried them all. Guess what? We always end up going back to either the EL84 or the 6V6. Granted we have to cheat the bigger tubes on the available plate voltage.

[plaidzebra]

PRR, maximum wattage isn't as big of a deal as the principles behind the experiment. I knew from the start that I wouldn't be able to push a KT88 up to full power without some serious iron, something which I don't anticipate I'll use for this build. Maybe someday I'll build a Marshall Major clone 

Shooter, that amp looks just about perfect! I'm definitely going to add that to a list of possible amps that I have going, seeing as how it doesn't look too challenging to build. Do you happen to have a layout for it? And where can I get that power transformer?

[Paul1453]

This one has caught my attention.

It's in this thread:

http://el34world.com/Forum/index.php?topic=17669.0

[shooter]

Yup;

I made it a *wood-box* combo, it was mated to a celestion hot100.  but it sounded best through a 2X12 with an Eminence legend and celestion cream back from 80s?

EDIT: double, triple, check the layout against the schematic, (which should be correct)
I'll try and find the build# since I had Doug make the board.

[kagliostro]

Ciao Shooter

Thanks for sharing the layout


Franco

[Ed_Chambley]

fwiw, I built a KT88 sort-kinda Champish amp, schematic below.  It specs around 16W clean, 23W full distortion.  It's been passed on,  2 gigging guys now and the 2nd guy won't take it out of his house!, It's his main practice amp.  It's not a 1 trick pony, but both musicians had to *learn* how to dial it in.

Questions on this design.  Looks different and I like different.  First, why are you using the cathode to drive the grid?  Just curious.  Why did you decide no to use a choke?  I like octal preamp, did you have any noise problems?


If you had to guess at an amp with a similar tone, what would it be?

[Paul1453]

fwiw, I built a KT88 sort-kinda Champish amp, schematic below.  It specs around 16W clean, 23W full distortion.  It's been passed on,  2 gigging guys now and the 2nd guy won't take it out of his house!, It's his main practice amp.  It's not a 1 trick pony, but both musicians had to *learn* how to dial it in.

First, why are you using the cathode to drive the grid?  Just curious.  Why did you decide no to use a choke?

Do you mean on the KT88?

It looks to me like the Grid leak resistor tied to Gnd,
but they are normally located on the other side of the Grid Stopper aren't they?

I'm guessing he didn't have a decent choke in stock at the time, or he probably would have used one.
I would rather use the LC Choke filter for the B+ if I had the choice. 

[shooter]

why are you using the cathode to drive the grid?

It didn't start out that way, I was aiming for a 4 gain pre, but after 3 stages I had more than enough to get the 88 bent slightly, I passed on my *notes* and didn't digitize them but IIRC, I was just over twice bias @ 3gain stages and when I did the CF it *cleaned up* the tone.  I was shooting for lite jazz, clean.  I re-tuned it with my Son (the 1st owner) to straight 60's-70's rock sound.  That's what some of the *'s are in the schematic.

I do have a couple big-'ol chokes from Edcor, but I didn't think in a SS PS they would be money well spent.

I like octal preamp

Look at Sluckey's rock-ola, that's where I stole the original idea from