Marshall KT88 amp PI's.. vs. LTP?

[satsloader]

Hi all,

After being away from tweaking amps and this forum for a long spell, I've come back to the fold ( A lousy excuse for being away really, work.. and say it's not so, radio control airplanes. oh the horror! )

I also started playing Bass with some friends, and have decided to build a 2xKT88 amp. Volume isn't an issue, as we'll never play anything more organized than BBQ's and Birthdays.

I'll be using hardware I already own ( MM 50 4550JT-G1 PT, Dyna-Clone A431S OT, Weber 100 watt chassis, nice 100 watt Marshall style cabinet, and the usual suspects for the rest. None of my collection of Mustards will be in this one, I'll take the clean route for caps and put in Orange Drops.. Not that Mustards aren't clean.. Saving them for Marshall clone projects ).

My original plan was to build a 5F6-A, and sub KT88's.

Looking at Marshall's schematics ( the closest thing to the 5F6-A with KT88's I could find ), I saw that the phase inverter is tail-less, and the tone stack is moved to the middle of V2.

One obvious point here is that the Marshall is a 4xKT88. Another is KT88's take a good swing to operate, or so I've read.

Since I'm only driving two KT88's, could I get away with simply applying them to the original 5F6-A circuit ( given I'll be going the UL route, utilizing the Dyna Clone's taps and modifying the power supply accordingly )?

Would it be better to build a Marshall 1966, but with two KT88's?

I should mention the rectifier will be 50 watt style full wave, not a bridge, and I'll have a good chunk of filtering, 400uf for plates/screens, 200uf for the PI, 50uf each for V2 and V1.

Your asking, why not just build a 4x, since we all know Bass needs all the power you can get?

Purely economics I'm afraid. I don't normally compromise on projects, but recent changes in my sweetiepie's employment dictate That I go on the cheap for a while. I've got the stuff mentioned above, not using any of it for anything, and as mentioned above, don't need gobs of volume.

Whata Ya'll think?

Many thanx,

Bill

[satsloader]

Bump..

[loudthud]

The Sunn Model T (1974 version, not the Fender re-issue) used 4 6550's U/L and a LT PI. The 1st generation is real close to a 5F6a. The interesting thing is the 22K feedback resistor from the 16 ohm tap. I've tried adding a resonance pot to that circuit and there isn't much gain to spare. It takes a lot to drive those big tubes in U/L mode.

In the Marshall Major (ie model 1966) the 12AT7 driving the KT88's is just a differential amp, there is a concertina PI just ahead of it. The reason for that IMHO is the low grid leak resistors (68K) required by the KT88's (per the Genalex data sheet). A longer tail would improve balance but Marshall didn't see a need for it.

Some of the Fender amps with six 6L6's use 68K grid leaks. I think all of them use the extra differential amp but I'd have to check. It would be good for me to review some of those Fender UL designs.

[satsloader]

Thanx LT! And thanx for the tip on the model T. That's one neat looking amp. The parallel chokes in the Pi section are interesting.. I'm surprised larger value filter caps weren't used in the Pi section as well.

As for my project, Since I'm only using two KT88's I'm thinking I can get away with the LTP. It's certainly is more familiar to me .

Regarding the bias circuit, do you think there would be any problems utilizing the Marshall 1987 bias circuit, subbing in the lower value 68k bias feed resistors ?( I'll be using one of Mercury Mag's 50 watt PT's, 500vdc @ 120Vac input, no bias tap on the secondary of course, thus my desire to do it the 1987 way ). I'd most likely use 10uf filter caps in it as well. If necessary I could go with a dedicated bias transformer and circuit instead.

Thanx again,

Bill

[mooreamps]

Kt-88 Se

I've read that as well ; about the KT-88's needing a wider swing to make them work. I guess I'll find out. Right now I'm doing a 36 watter with one KT-88 and one EL34 running SE. Planning on running the KT-88 plate at 700 volts with the screens at 350. Also, seems to me the only practical way to get 36 watts SE is going with two output transformers; one for the KT-88 and one for the EL34. As such, I'll just go ahead and make a sterio pre-amp as well.

-g

[loudthud]

There are two versions of the 1987 from the Unicord days. The Mk-II is the American version with 6550's and they changed that first resistor to from 220K to 150K and the caps to 10uF. On both versions the bias supply should be on the transformer side of the standby switch so that there is full bias that first second you flip the standby to on. The 150K should be a 2W FP type. In any case, make sure you can adjust the bias to at least -80V. Some 6550's require that much bias.

Sunn used the same chokes in the Model T as they did on their 60W UL amps. They needed two in parallel for the current draw. Their original amps (ie 200s et al) used KT88's with a Dyna A431 OT. After a couple of years they switched to 6550's and released a 120W (ie 2000s) version with 2 chokes. Those amps all used a 6AN8 PI like the Dynakit Mk-III they were modeled after. The chokes are something like 1.5Hy 300mA ~50ohms. Originally they all had GZ34 rectifiers but eventually went solid state.

I need to get up to speed with Mercury. I haven't heard any bad things about them and I have a Model T that needs a new PT.

[satsloader]

Thanx all,

MM's transformers are nice, but until 5 units have been purchased, their pricing is painfully high. Some folks have formed temporary group buyer's clubs to get around that. I inherited mine from a friend who used it until he could get his original Marshall PT re-wound.

My OT however was a pretty good deal.

http://www.triodeelectronics.com/a4dyoutr43oh.html

Gotta love the old Dyna design, eh?

Bill