KT88 Champ

[wcrankshaw]

I posted this under the build your own section, but I know a lot of the people who will have quick info on this are probably centered around this section of the forum.

So, this may seem ridiculous, but I want to build a champ style amp with a KT88 output. I like the idea of the AX84 SEL, but I'm pretty sure I'm not so interested in 3 preamp stages. Is it possible to modify the 5e1 circuit to accommodate a KT88 in the output and only have one 12AX7 in the preamp? I'm sort of 50/50 on whether or not I care about master volume and a 3 band tone stack. If it's not a big deal I'll go for it, but I also like how the champ is just a volume knob. I'm good at following schematics and doing builds, but I'm not into the prototyping stage yet. That being said I understand most direction given by people on here. Thanks in advance.

[Chuck H]

So, this may seem ridiculous, but I want to build a champ style amp with a KT88 output.

I think it's a great idea. How else can you get the Champ to a usable volume. I have planned to do this for years but something else always seems to get priority.

Is it possible to modify the 5e1 circuit to accommodate a KT88 in the output and only have one 12AX7 in the preamp?

Yes.

You typically need more drive for a KT88 than a 6V6 to get the same level of OD so there are some circuit consideraions to be made. The 5E1 doesn't have a cathode bypass cap on the second gain stage. The extra gain you need might be possible by just adding a cathode bypass cap to the second gain stage.

I'm sort of 50/50 on whether or not I care about master volume and a 3 band tone stack.

If you want to get distortion and EQ with a master volume you will definitely need additional gain stages. Nearly all of the distortion you hear from "Champ" type amps is from the power tube. With a standard "Champ" type circuit a master volume becomes a distortion control that is still gain/volume dependant... Useless.

but I also like how the champ is just a volume knob. I'm good at following schematics and doing builds, but I'm not into the prototyping stage yet.

Easy peezy. I might start with a 5E1 and add a 22uf cathode bypass cap to the second gain stage. That may provide enough drive for the KT88 without the need for another gain stage.

You could probably skip using a "choke" (expensive part for this type of amp) and just use the 5F1 schem instead but bypass both preamp cathodes with a 22uf cap.

Make sure to look at the tube specs for the KT88 and get a PT of the appropriate mA rating and voltage and an OT of the appropriate mA rating and primary impedance. Nothing more to it than that.

Oh, a really efficient 12" speaker would be a nice touch if added volume is what your after. Speaker efficiency is a great way to add volume without extra watts.

Chuck

[Steve Conner]

I think it's a great idea too! Quite a few people have built these, and I seem to remember a guy who made a double version with two KT88s in parallel.

As Chuck H points out, the distortion in a Champ comes from the power tube, so changing it out for a KT88 will alter the tone and feel of the thing. It'll go louder and won't break up so easily, if at all!

You'll need a much bigger set of transformers than in a standard Champ, though, and the bigger the OT the better. I think Hammond has something suitable in their line, but maybe someone else can recommend a good set.

You can skip the choke, but the amp will probably hum more.

[wcrankshaw]

Wow, thanks! Let me make sure I've got this straight. If I just strap a 22uF/25V across the cathode resistor of V1B there aren't anymore circuit modifications? The transformer I have is from weber. It's a champ PT and says it has a 660V tap - that seems high enough, but it only does 100mA which doesn't seem high enough.

It sounds like my intentions might be a little misguided. I'm interested in the supposed earth shattering overdrive of a KT88, but it sounds like this won't give the desired results. Am I better off just doing the ax84 SEL, or will that just create a hi-gain 12ax7 amp?

[Chuck H]

Wow, thanks! Let me make sure I've got this straight. If I just strap a 22uF/25V across the cathode resistor of V1B there aren't anymore circuit modifications?

Well, there could be. Like I said before, it takes more voltage to drive a kt88 than a 6V6. The extra gain from the added bypass cap may not even be enough to get the same level of clipping out of a kt88. That isn't to say it won't work. If you built it and wanted more drive you would need to add a tube so perhaps leave room on the chassis for that if it comes up.

The transformer I have is from weber. It's a champ PT and says it has a 660V tap - that seems high enough, but it only does 100mA which doesn't seem high enough.

Nope. A kt88 is probably good for as much as 12 watts in class A. Most amps of this wattage are fine with a 100mA PT but in class A the tube is biased at or close to max dissapation at idle. Probably about 35 watts for a kt88. So your probably better off with a PT in the 200mA range.The OT could be in the 100mA range though.

This only applies if you intend to go after max output power too. If you ran the kt88 at typical Champ voltages you may be able to get away with stock trannies but the output won't be much louder than a normal Champ.

It sounds like my intentions might be a little misguided. I'm interested in the supposed earth shattering overdrive of a KT88, but it sounds like this won't give the desired results.

Well, since any of the kt88 OD tones you've heard are most likely from class AB1 amps, this would be different, yes. But it could still be great sounding. I've never heard a kt88 in class A.

Am I better off just doing the ax84 SEL, or will that just create a hi-gain 12ax7 amp?

I don't know that amp but the way you describe it in your first post it sounds like another preamp distorto machine as many of the ax84 projects are.

Chuck

[sherpakahn]

I actually just completed converting a red-knob Champ 12 into a KT88 SEL champ. I used many existing parts, as well as some capacitors salvaged from TVs, projectors, other amps, etc.

If you look around, you'll see a lot of people dissatisfied with the Champ 12, and after trying a few mods, I decided to try the SEL instead. I've tried it with a tired old 6l6gc, a Sovtek 6550WE, and then a JJ KT88. The KT88 is my favorite sound so far, with an incredible amount of bass coming out of the tiny open-backed cabinet. The departures my build takes from the SEL design are as follows:

10-watt 5.3K: 7.4 fender 024038 OT
stock Fender champ 12 mains transformer
James (Passive Baxandall) tonestack
Rhythm/Lead Switch (From SEL options guide)
Original Champ 12 power supply/voltage doubler

I wanted to see how clean and loud I could go with the stock transformers, and have been very pleased so far. I doubt the KT88 is really distorting though; until I secure a fan and beefier PT/OT I'm running the bias cold at 54mA cathode current(Rk=680R, Vp-k=420V).

I like your idea, and am thinking if I can wire in some switches to change my build to 5F1 mode. If your OT has multiple taps you could probably run a variety of octal tubes with bias adjustments, sounds like a great project.

[MWJB]

I'd really be looking for 450vdc-ish on the KT88 plate (use a 1K5W screen grid resistor & explore 6550 as well), so as Chuck says a 200mA-ish PT would be good, or even a typical Fender style 40-50W PT (might be <160mA depending on who you get it from). Fixed bias would be a good idea to fully explore the power available.

If you go for a single volume pot you may have to explore a fixed tone shaping circuit to shelve some bass (or limit coupling cap & bypas cap values) & help fidelity?

[mooreamps]

ya, when I built an SE 36 watt using a KT88 for one of the power tubes, it was too much bass ; at first. I really had to clamp down on the signal coupling caps. Some as low as 4.7 nF, but in hindsight I could have went lower. It was a big sound. I power braked with 600 volts on the plate and up to 450 volts on the screen. Probably could have used a SGR bigger than 1.5K, but thats a good starting point. Even running SE, it was a lot of good clean headroom. Keep the plate coupling caps small, and it would make a good jazz amp.


-g