Marshall jtm45-2245 Drake transformers

[acdsee]

Hi i am the owner of marshall jtm45-2245 reissue model produced in 1989.The amp came with Drake 789-84 P.T. Drake 784-389 O.T. and sovtek 5881 tubes.I would like some technical info or shematics about these transformers (i could not find anything on internet) and basically if they can handle kt66 tubes.I tried a set of golden lions kt66 and bias at 41ma with 410v on plates but the amp doesnt break up,i tried full volume but is complete clean and punchy but not even crunchy.Thanks a lot!

[neikeel]

Yes they are for KT66s

The amps came with 5881s.

I had one (it has an Arcol resistor instead of choke.

How are you biasing?

Full hot bias is 42-43mA using direct (shunt method) if using 1R on ground then you can go hotter.

What preamp tubes you using. They typically break up and crunch at 5-6 on the dial.

What guitar? Single coils or hum buckers? Using the high channel 1 input?

[acdsee]

the amp came with 5881s,at the past i want to put some el34 in but my my tech told me that output transformer is at 8k so i put some kt66 in.After all i sent a mail to marshall about power transformer cause i thing there is a difference in current between kt66 and 5881 (1.3a-0.9a) and i dont want to burn my iron.Marshall did not replay at all.So i'm a little bit confused.I read that 450+ volts are ideal for kt66 but my amp is lower than this.The amp came with a resistor instead of choke so i put a choke in, lower the filtering to dual 32 and 16 caps,change mixers to 270k and put kt tubes to come closer to original circuit.I bias with 1ohm to ground and tried a few preamp tubes that change tone a little but not overdrive the amp, now i'm using Ei long plate preamps.As for my gear i use a 57 cs strat and a 58 vos les paul through 1x12 creamback.The problem is that i have too much headroom and punch and the amp dos not break up at all.Thanks a lot for your fast replay!

[neikeel]

Ok.
Disappointing that Marshall did not reply, but the amp. is fine for KT66 (the head cab is a bit tight for clearance?) the originals used KT66 in the mid 60s. Raising the plate voltage to 450v is not necessary and will give even more headroom and punch.
So using your 1ohm method raise the bias to 45mA and see if it sounds any better.
Here is my old '89 that I reworked ages ago. What choke did you use (3H is best, IMO).
You will need to find the sweet spot on the amp but with the mods you have done I would expect crunch and break up in the 5-6 vol with the Les Paul humbuckers.
You will also find the EQ useful. Dial the mids to 10 and the bass to 1 or 2. I find that I need the treble up and presence around 6 a t least on a JTM45 (new or old).

[bill bokey]

Do you have any pics ?
It should overdrive a lot more than you describe, maybe a bad cathode bypass cap ?

By the way, you can try EL34s if you want, lowish B+ and hi OT impedance will only result in less volume and more harmonic content. Or set the amp to 4 ohms when used with an 8 ohm cab (or 8 with a 16 cab).

[neikeel]

If you want more break up and sensitivity I have a couple of pairs of Mullard EL37s that really rock and grind in a JTM45

[acdsee]

I will take and post some pics and see if there is circuit or wiring issues.