kt88 SE

Andy Le Blanc · Post by **Andy Le Blanc** » Sat Jul 30, 2016 3:14 pm

Been having a grand time with a SE kt88 rig.

I scrounged an old silver face champ, and a mystery SE opt from a junk radio.

champ PT, sand rectumfryier..., its a diode board from an old twin.
Cathode bias, un bypassed chathode resistor, screens have there own supply tap with a bleeder resistor and screens voltage is set at approx -5v to plate voltage.

the 12ax7 pre is a simple zero bias stage... vol./tone followed by a second stage again biased with an unbypassed cathode resistor,... and just a touch of feed back, and oil-n-foil Jensen coupling caps thru out.

Nice rig... loud...and great tone color with the un-bypassed cathodes.

Its a modded champ... www.triodeelectronics.com... has an affordable 15w champ up grade OPT that looks real flexible. #40-18031 5k/8k primary...4/8/16 secondary
next step is to source parts for a BOM.

I have it on Jensen 12's... think MODs would be ok... I'm eye balling webers for it.

haven't been jazzed about a rig in quite a while

dorrisant · Post by **dorrisant** » Sat Jul 30, 2016 6:11 pm

Schematic?

I have a bunch of KT-88s here. Was thinking SE would be cool.

Stevem · Post by **Stevem** » Sun Jul 31, 2016 10:44 am

What the heck is your heater voltage at with that Champ PT being used for a 88, as that tube needs over 1 full amp more Heater current than a 6V6 does!

I would venture a guess that the voltage is well below the minimum needed 6.1 volts?!

Leo_Gnardo · Post by **Leo_Gnardo** » Mon Aug 01, 2016 2:26 am

Stevem wrote:What the heck is your heater voltage at with that Champ PT being used for a 88, as that tube needs over 1 full amp more Heater current than a 6V6 does!

I would venture a guess that the voltage is well below the minimum needed 6.1 volts?!

Let's not forget Champ & Princeton PT's are the same item. In a PR you need 2.25A to run all the tubes + pilot. Transformer should be able to deliver 2.5 maybe 3A on the filament winding. Might just squeak by OK with a KT88 plus a couple pre tubes. If not, look for that wisp of smoke coming out.

FWIW I've put 6L6's into Princetons for 25+ years now, that's presumably 3.15A off the filament winding, haven't had any PT meltdowns. OTOH lots of happy crustomers.

Stevem · Post by **Stevem** » Mon Aug 01, 2016 11:17 am

Even though they are seperate windings , still a Princeton is not idling in class A ?

Leo_Gnardo · Post by **Leo_Gnardo** » Mon Aug 01, 2016 11:53 am

Stevem wrote:Even though they are seperate windings , still a Princeton is not idling in class A ?

A "modern" Princeton, that is 1960 on, furnished with two 6V6's? No class A here, unless you dial the bias current needlessly way up. Just buttoned one up, 20 mA per 6V6 with about 415V on the plates, a long way from class A. No crossover distortion at clip, sounds terrific. FWIW when it came in its bias filter was failing, bias voltage running low, that sent the bias current to 35 milliamps per 6V6, plates were 385V. Lucky there were JJ 6V6's in it, they survived the extra plate power without a complaint.

Tweed Princetons were just more or less Champs in a bigger box. One output tube = class A, hard to avoid it.

tubeswell · Post by **tubeswell** » Tue Aug 02, 2016 9:45 am

dorrisant wrote:Schematic?

I have a bunch of KT-88s here. Was thinking SE would be cool.

Sorry to hijack. AX84 has/had a KT88 SE schematic a few years back. I snagged a copy back then, Here you go.

Andy Le Blanc · Post by **Andy Le Blanc** » Thu Aug 11, 2016 7:38 pm

so far so good with the heaters, ill recheck for sure

Andy Le Blanc · Post by **Andy Le Blanc** » Thu Aug 11, 2016 7:53 pm

quick scheme of the pre... I'm at a gig waiting for the act

real simple pre, the second stage has a trick I opted for instead of global feedback.

the idea comes out of magnatone amps... un-bypassed wire wound cathode resistor with a resistor to the power rail.

it stabilizes the voltage at the cathode and sounds neat.

everything else is champ except for the screens source for the pentode.
I have that set below the plate voltage with its own bleeder and bypass cap

certainly check the fil. volts but like I said so far so good

RJ Guitars · Post by **RJ Guitars** » Mon Aug 15, 2016 3:32 am

I'll add my version of a modified Champ with a KT88 and follow along...

Andy Le Blanc · Post by **Andy Le Blanc** » Wed Aug 17, 2016 3:45 pm

champs are fun.... always liked the simplicity

the heaters on me rig appear just fine

I like the zero bias as the first stage, the gain charts for this type are given at 5% harmonic distortion

Synchu · Post by **Synchu** » Wed Aug 17, 2016 7:10 pm

I came up (certainly not the first, but...) with marrying Champ pre with EL84 power stage for a friend. He did the soldering and handed it to me. After fixing some stuff (got terrible hum and oscillation) and changing/adding some components and features (tuned Power filtering, switchable bright cap, gain boost, power tube grid stopper, etc) it turned great.
So great that I am building one for me.
It could be the vintage radio power and output transformers we used, or some luck (dare I say, I know what I am doing?) during me fixing/tunning it, but it worked out.
After doing another weird SE some time ago, that I posted about, I am quite convinced that these can be made fabulous sounding units with some work, persistence and informed choices.

Niki

P.S. Here's a quick sample of the EL84 "champ"
https://soundcloud.com/synchu/jabs-champ-2

Tube Amp Archive

kt88 SE

kt88 SE

Re: kt88 SE

H

Re: H

H

Re: H

Re: kt88 SE

Re: kt88 SE

Re: kt88 SE

Re: kt88 SE

Re: kt88 SE

Re: kt88 SE