Marshall JTM45/100 550V!!!

[Jelle]

Hi guys,

I'd like to put 6550's in a 560v '71 superlead. I already modded it to JTM 100 specs except for the EL-34's and OT. Can you guys please help me to convert it to 6550's?


1) Should I change the NFB? Or should I leave it at 27K from 16 ohms tab?
2) What value of bias current should I go for, using the 1 ohm cathode method? 70%?
3) Do you recommend 1.5k or 5.6k grid stopper resistors?
4) What is the minimum screen grid filtering that you recommend? I use 32//32 right now. I suppose 16uF will cause it to hum?

Thanks,
Jelle

[jaysg]

Sounds like a candidate for the JJ KT-77's.

[Jelle]

Hi Jaysg,

Currently the amp is running with Philips Holland EL-34's and I think it sounds great with humbuckers.....but since I'm mainly a strat player I'd like to put in 6550's that I already have. I know they will not run for a decade in this amp but I'd like to try it.

How much bias current and how much screen filtering do you advise?

Thanks,
Jelle

[ampcession]

I was reading that Hendrix gear link Steve provided and was struck by this:

Hendrix’s Marshall of choice was the 100-watt Super Lead driving two 4x12 cabs, and his standard backline would quickly grow to three Super Leads and six 4x12s. He plugged his guitar into one amp, and linked it to the others by running a cable from an adjacent input (the Super Leads had four inputs) to the second amp’s input jack, and so on.

This was not the first time I heard of this but to put it in context, at my last band practice I used my rebuilt Bassman 100 with my 4x12 dimed with no attenuation, and it was real loud.

I like loud, but I can't imagine what three Super Lead stacks was like.

Back in the 70's and early 80's I used three 100 superleads. I retired from playing in 1985. My hearing has slowly come back(somewhat) over the past twenty years. The ringing is still there but it's not as loud as it used to be and I can actually tolerate being in a quiet room now. It used to drive me nuts cuz the ringing was so loud.....SO LOUD I thought other people must be a ble to hear too if they got too close to me. It's gotten better enough that now I can actually hear a little of what people are saying to me in a noisy room. Any sharp loud noise, like the sound of a book dropping on the floor can set my hearing back ten years or so and it takes about 5 days to get back to hearing above the RINGING. Earplugs are great.

[Enzo]

I am with you ampcession. Not as bad as yours, but the ringing never stops. Every now and then I can beat a test tone against it to determine what the freq us at the moment.

Protect your ears, you only get one set.

[rocket]

Is this not plainly a warranty issue?

[Jaicen]

Apparently, he used to record with full stacks as well.
Maybe he killed himself because he was deaf?

[dai h.]

if the guy can afford to buy one of those heads, then he can probably spring for a quad of GEC KT66s which will probably hold up better. Turning down the AC (10% down from 6.3V at heaters) will probably help too. I'm assuming since he bought that amp he is going for authentic old Marshall sound (hence the old tubes suggestion), but if he's not that hung up on that, then maybe beefier tubes such as 6550s (or maybe a quad of Philips/Sylvania 7581s?) as suggested by others. Also, reducing the grid load/bias return Rs should help but might want to wait 'til the warranty runs out.