KT88 Plexi

[jckid649]

I noticed a company called The Royal Amp company that makes an amp they call the MSG 100. From my research and talking with them they basically say it's a bone stock 1969 10,000 series Plexi with "minor changes" to the circuit with 4 KT88 tubes making just a little over 100w.

My question is: If I were to build a basic stock 69 Plexi with KT88's what kind of changes in the circuit would be necessary? I have been advised that the screen grid resistors may be something to look at, but what else?

[martin manning]

I think you could drop a pair of KT88's into the 1959 schematic with the same 5k1 grid stop and 1k screen resistors, but perhaps lower the bias feed resistors to 100k. I would definitely use a 2k2 OT, and keep the plate voltage down around 450 if possible.

[jckid649]

I think you could drop a pair of KT88's into the 1959 schematic with the same 5k1 grid stop and 1k screen resistors, but perhaps lower the bias feed resistors to 100k. I would definitely use a 2k2 OT, and keep the plate voltage down around 450 if possible.

So would the standard 1.75k OT not be acceptable?

[martin manning]

1k7 would mean more plate dissipation, and it would only take ~370V on the plates to hit 200% of Pa max (42W). In push-pull operation that is a semi-soft upper limit. With 2k2 and 450V, the max dissipation on the load line is 220%, which should be ok. 1k7 would be 284%.

[jckid649]

1k7 would mean more plate dissipation, and it would only take ~370V on the plates to hit 200% of Pa max (42W). In push-pull operation that is a semi-soft upper limit. With 2k2 and 450V, the max dissipation on the load line is 220%, which should be ok. 1k7 would be 284%.

I have a tech friend that suggested that maybe I consider upgrading the PT to provide a B+ of 700-750v because that would provide the best performance for the 88's. But then again I'm not trying to make a metal amp. I am more interested in the kind of tone Royal is getting from that MSG 100 that just has mountains of headroom with balls.

If I am upgrading the OT should the PT remain stock or should I look at that too? How far do I go to provide the proper power before I end up needing to make more changes in the circuit to accommodate it and when is it just too much for the type of tone I'm trying to achieve?

[martin manning]

I have a tech friend that suggested that maybe I consider upgrading the PT to provide a B+ of 700-750v because that would provide the best performance for the 88's.

That would require a very different power supply with screen voltage around half of the plate, and 3x higher primary impedance.

I am more interested in the kind of tone Royal is getting from that MSG 100 that just has mountains of headroom with balls.

If I am upgrading the OT should the PT remain stock or should I look at that too? How far do I go to provide the proper power before I end up needing to make more changes in the circuit to accommodate it and when is it just too much for the type of tone I'm trying to achieve?

If this amp is something very close to a 100W Plexi with each pair of EL34's replaced by a KT88, and screen voltage essentially equal to the plate like the original, then you want to stay with the Marshall specs. Some had very high plate voltages (~550), but I would use a PT like Classic tone 40-18053 with 350V secondary. That should yield ~470V B+ at idle.

[jckid649]

If this amp is something very close to a 100W Plexi with each pair of EL34's replaced by a KT88, and screen voltage essentially equal to the plate like the original, then you want to stay with the Marshall specs. Some had very high plate voltages (~550), but I would use a PT like Classic tone 40-18053 with 350V secondary. That should yield ~470V B+ at idle.

I got ya'. Yeah I was actually looking at their site and saw they had the 2k2 OT for the JMP 100 style amps. I'm not trying to deviate too far off of the original specs. Just accommodate the KT88 bottles.

[M Fowler]

What about the screen voltage that may need to lowered.

Take a look at the TAD 150w Bass amp using Marshall circuit as a basis.

[Roe]

1k7 would mean more plate dissipation, and it would only take ~370V on the plates to hit 200% of Pa max (42W). In push-pull operation that is a semi-soft upper limit. With 2k2 and 450V, the max dissipation on the load line is 220%, which should be ok. 1k7 would be 284%.

you're discussing a pair of 88s while he asks about a quad (or did I misunderstand). I've seen several plexis that do ok with kt88s or 6550s if the screen voltage isn't too high (usually the power supply sags alot in these amps - the plate voltage sags roughly 100v and the screens even more)

[martin manning]

There is some confusion... Checking the the Royal site:

MSG100: 100 watt, KT88, ECC83 tubes, 4/8/16 ohm taps. Grounded AC operation. Custom transformers. Custom Choke.
MSG50: 50 watt, same as above less two KT88s and different transformers.

So apparently they are using 4 KT88 and only getting 100W in the MSG100. This seems a little misguided.

With a robust 470V power supply and the appropriate load (~1k), that configuration would produce well over 200W. With 100V sag, it would still do ~125. If the load is 1k7 or 2k2 (just drop KT88's into an EL34 amp), it would come down to ~100W, but the load line would be far below the knee. Removing a pair of KT88's would result in a much better impedance match, and produce only a bit less power.

[Reeltarded]

The Majors have 1k7 ots. This amp is nothing but a 100 with 88s.

It's nothing but a 100 with 88s.

[martin manning]

Majors have higher voltages- 590 on the plates, and 585 on the screens (Marshall voltage chart on 200W PA schematic). The 1k7 impedance is is pushing the plate dissipation pretty hard at that voltage, but the UL (or DL if you prefer) OT pulls the screen voltage down at high signal levels. That (almost) keeps the screen dissipation under control. ;^)

[Roe]

the major has a different driver and very different transformers. even the primary impedance is lower, around 1k4 reportedly