KT88's in a JCM800 (2203)?

[freakshow__]

I was asked about building someone a 2203 100 watt with KT88 power tubes like what Billy Corgan (smashing pumpkins) did with his in the early 90's.

My questions were:

Will it be bad for the OT or the power tubes with just the standard OT that is intended for EL34's? KT88's really want to see a different primary impedance on the OT correct?

I assume you would want to change the grid resistors as well?

Possibly the bias resistor to get bias in the correct range?

[Stevem]

To me it seams like a expensive waste since even just two kt88s with more power supply voltage and current can make darn close to 100 watts!

Also if this amp will not in use be driven ( played loud enough ) to clip the output stage even when fitted with 34s ( 25 watts ) , then why the heck fit it with KT88s which can do 42 watts?

If the owner wants a tighter bottom end response out of the amp then install a quad of Groovetubes EL34S ( JJ brand) with a number 10 rating ( 10% later clipping then a number 1 rating ) and then add another 100 Uf of filtering strapped on to the first filter node.

[pdf64]

…when fitted with 34s ( 25 watts ) , then why the heck fit it with KT88s which can do 42 watts?
…

I recall this ‘apples to oranges’ comparison being made previously?
The argument presents a rather inaccurate impression of the relative capabilities of the 2 valve types.
Note that under suitable conditions, a pair of EL34 can also put out 100W, see p3 of https://frank.pocnet.net/sheets/030/e/EL34.pdf
To recap, the 25W of EL34 is under the design centre rating system.
Whereas the 42W rating of KT88 is an absolute max rating, which is a dissipation that a proportion of functional valves won’t be expected to accommodate.
A likely design centre rating of KT88 is perhaps 30W. So really it’s only 20% beefier than EL34, not the 68% beefier that the previous post suggests.

https://frank.pocnet.net/sheets/086/k/KT88.pdf

To get your head around the different rating systems, please read http://www.one-electron.com/Archives/RC ... 0Tubes.pdf

[wpaulvogel]

I built a 1992 Super Bass Marshall clone and a DR103 Hiwatt clone for a guy in Texas. Both amps are running KT88’s. He likes KT88’s and 6550 tubes. They are AWESOME in these amps. You do have to tweek the bias voltage and swap the resistors (220k) that feed the bias voltage to the power tube grids to 100k in the Marshall. The output transformer is fine and the big tubes don’t even break a sweat in the lower voltage lower current environment. You are able to bias the amp around 25 watts dissipation per KT88 and this eliminates the crossover notch during full clean rms output. The notch only appears after a decent amount of clipping is occurring in the output section. Output power usually is equal if the EL34’s are strong. New production Tung Sol EL34B’s put up a good fight with KT88’s for power in a Marshall but KT88’s sound fuller. Definitely recommend!!

[freakshow__]

Thanks wpaulvogel.....very useful info!

[pdf64]

...You are able to bias the amp around 25 watts dissipation per KT88 and this eliminates the crossover notch during full clean rms output. The notch only appears after a decent amount of clipping is occurring in the output section...

I think people put too much significance on that. It's perfectly normal for AB1 valve outputs to bias themselves colder when signal level approaches / exceeds grid clipping, pretty much inherent to their operation.
Hence it can be an issue when setting bias with scope only.

[Colossal]

I built a 1992 Super Bass Marshall clone and a DR103 Hiwatt clone for a guy in Texas. Both amps are running KT88’s. He likes KT88’s and 6550 tubes. They are AWESOME in these amps. You do have to tweek the bias voltage and swap the resistors (220k) that feed the bias voltage to the power tube grids to 100k in the Marshall. The output transformer is fine and the big tubes don’t even break a sweat in the lower voltage lower current environment. You are able to bias the amp around 25 watts dissipation per KT88 and this eliminates the crossover notch during full clean rms output. The notch only appears after a decent amount of clipping is occurring in the output section. Output power usually is equal if the EL34’s are strong. New production Tung Sol EL34B’s put up a good fight with KT88’s for power in a Marshall but KT88’s sound fuller. Definitely recommend!!

Paul,

I can't remember, but did you change the primary on that Super Bass or leave it stock at 1k7? That was a sweet amp. Probably make a couple of cabinets bounce around the room

Thanks,
Dave

[wpaulvogel]

I built a 1992 Super Bass Marshall clone and a DR103 Hiwatt clone for a guy in Texas. Both amps are running KT88’s. He likes KT88’s and 6550 tubes. They are AWESOME in these amps. You do have to tweek the bias voltage and swap the resistors (220k) that feed the bias voltage to the power tube grids to 100k in the Marshall. The output transformer is fine and the big tubes don’t even break a sweat in the lower voltage lower current environment. You are able to bias the amp around 25 watts dissipation per KT88 and this eliminates the crossover notch during full clean rms output. The notch only appears after a decent amount of clipping is occurring in the output section. Output power usually is equal if the EL34’s are strong. New production Tung Sol EL34B’s put up a good fight with KT88’s for power in a Marshall but KT88’s sound fuller. Definitely recommend!!

Paul,

I can't remember, but did you change the primary on that Super Bass or leave it stock at 1k7? That was a sweet amp. Probably make a couple of cabinets bounce around the room

Thanks,
Dave

I used a stock Merren C1998 output transformer in the Marshall and in the Hiwatt I used a stock Heyboer after talking to Phil.

[Colossal]

I used a stock Merren C1998 output transformer in the Marshall and in the Hiwatt I used a stock Heyboer after talking to Phil.

Nice, thanks Paul!