Plexi KT88?

[MHProd]

I'm about to drop a nice pair of KT88SC in a 1986 circuit. I changed the bias Rs to 120K. The Penta KT88SC specs sheet shows a Max anode dissipation of 50W. My B+ is 420VDC. Should I treat the tubes as a 25W EL34 or would it be OK to go higher?

[Stevem]

You can go higher if you like and that get the output stage to clip faster if that's what you want!
The main difference you will hear between the two tubes is the dynamic range is less with the 88s when you listen close,this is due to them being a beam type tube which the 34s are not.
One way of looking at it is that the 88 will rev up faster if you will at a certain drive level .

[pdf64]

Should I treat the tubes as a 25W EL34 or would it be OK to go higher?

Yes, bear in mind the capacity of the PT when setting the static current draw.

The main difference you will hear between the two tubes is the dynamic range is less with the 88s

Is that an observation / perception comment, or something that's quantifiable?
They seem to have similar gm and ra characteristics at 250V.

[Stevem]

Empirical!
Plus the 88 being a beam type which the 34 is not means all things being equal the 88 needs less drive signal for a givin ma output and as such has less dynamic range!
May be not much with a amp that uses feedback, but just do a A/B session in a amp that does not and if you the cannot hear and feel it when the amp is into clipping than I'am sorry what can I say!

[rp]

If the OP doesn't know a KT88 Plexi exists, it's the Park 75. Marshall just stuck KT88s in a stock early 70s 1987 circuit with stock PT and OT. I've never come across one and can't say anything on sound but you might want to research it and see if there any discussions of it out there. Here's a start: http://www.vintageguitar.com/15454/park-75/

[MHProd]

Yes I'm well aware of the Park 75.

But I'm not sure how they were biased though... Some reports I've found so far imply the 88s should be biased like 34s (17.5W).

Unfortunately, one of the tubes I received went bad quickly. It didn't fail, but the bias was unstable and kept going up. It was working fine for a short time and sounded pretty good (big and round).

I'm waiting for a replacement.

I also noticed the amp was working hard and sagging quite a bit. I have to double check the specs. I know the heater current is 6A so I'm good with that part. I can't remember if the PT secondary is 150mA or 200mA.

[Stevem]

If you have less than 500 volts on the plate and only 150 ma out of the PT it will unlikely to even make 50 watts of rms power!

[rp]

If you have less than 500 volts on the plate and only 150 ma out of the PT it will unlikely to even make 50 watts of rms power!

Which begs the question, what did Marshall have in mind when they made the Park 75? Granted, you'll get a different tone but the PT was the 1202-118 or 133 which should give ~420 or ~450V on the plates @150mA. Was the Park 75 just pure marketing BS on the part of Marshall or maybe they re-specd the PT and kept the serial numbers? Anyway, I always wanted one as I never had a KT88 amp of any type, and if I had some old KT88s and a JMP50 I'd sure try them.

[fusionbear]

A JTM 45/100 has a OT with 4.2K Primary and about 560B+ on the Plates. This is a perfect candidate for a Pair of KT88's in the Place of a Quad of KT66's. Some say its heresy, but I prefer it this way. My JTM45/100 build I call "Igor" is exactly that. A JTM45 front end into a pair of KT88's!

My next build may be a Super Bass with a pair of KT88's....

[martin manning]

A JTM 45/100 has a OT with 4.2K Primary and about 560B+ on the Plates. This is a perfect candidate for a Pair of KT88's in the Place of a Quad of KT66's.

I don't like this substitution at all. The load line is far below the knee and peaks at over 200% plate dissipation (42W KT88EH). The situation wasn't good with 4x KT66, and this is even worse!

[rp]

Martin, in case I do something like this what is a better load line for KT88 @ over 500V? I was going to use an A470 @ 4.3K as the OT in a JMP50 for either EL34s or KT88s. That OTs big brother the A431 also @ 4.3K was made by Dynaco specifically for KT88s in their MK3 and was what Sunn used.

http://store.triodestore.com/a4dyoutr43oh.html

While you have the datasheet in hand how about the optimum load @ 470V, 500V, and 550V?

[fusionbear]

A JTM 45/100 has a OT with 4.2K Primary and about 560B+ on the Plates. This is a perfect candidate for a Pair of KT88's in the Place of a Quad of KT66's.

I don't like this substitution at all. The load line is far below the knee and peaks at over 200% plate dissipation (42W KT88EH). The situation wasn't good with 4x KT66, and this is even worse!

Please elaborate, because I used the Dynaco PT specifically for this amp.

I have not had any issues with the amp whatsoever in a year and a half of hard use...

[rp]

I was reading up on KT88s if anyone's interested here's some fun info:

http://www.jacmusic.com/KT88/kt88.htm

[martin manning]

fusionbear what plate and screen voltages are you running?

The dissipation is probably okay (I was looking at the MOV data sheet before), but the screen voltage wants to be reduced. With 560 on the plates, around 330 would be good. If the screens are up near the anode voltage the knee is way above the load line, which means high screen dissipation and reduced headroom.

[MHProd]

Received a replacement for the tube that was funky.

Bias is not the most stable yet as the pair seems to be drifting up quite a bit.

I noticed the tubes need to warm up for quite a few minutes before I can get a proper reading.

Anyway, I have around 405V on the plates and the tubes are biased around 65ma which is around 55% plate dissipation (considering the absolute max. is 50W). If I go less, the amp is brighter and feels anemic. With this setting, the amp is very articulate, great separation between the notes. It does the Zeppelin thing quite well. NOS JAN 5751 in the PI.

Oversized NOS 50W Triad OT with 3K3 primary.

LOUD!