6550's in a marshall superlead.

[Jelle]

Hi everybody,

I have a 560V superlead with an 100w EL-34 OT and I really like the idea of replacing the el-34's which are currently in it with 6550's.
I have a couple of questions about putting 6550's in here:
-The 6550 data sheet tells me to reduce the 220 bias splitters to 47k. Should I do this?
-Should I change the NFB?
-What value of bias current should I go for, using the 1 ohm cathode method?
-Do you recommend 1.5k or 5.6k grid stopper resistors?
-What is the minimum screen grid filtering that you would recommend? I use 32//32uF right now.

Thanks,
Jelle

[mac1amps]

FWIW, you should know that [as a general rule of thumb] when you convert a Marshall to 6550's: Your high end will be harsher (though some would just call it "brighter"); The overall response will be "flatter" (not neccessarily good for a guitar amp).....In my experience [of 33-years] as an amp tech., I've found that most "classic" rock players prefer the "warmth" and mid-range of EL34's whereas, the "Metal" players (who usually add some form of "processed" distortion, in addition to the amp's break-up) tend to prefer the "linear" sound of the 6550's, which are also about 20%-30% more expensive......Another factor to be aware of is that if you use alot of master volume (i.e. operate your amp with the preamp gain high and master volume set low) the power tubes you use will be a minor factor in your overall sound (I realize that S.L.100's didn't originally have master volumes, but [unfortunately] MANY of them have had them installed).......BTW, the "bias splitters" you referred to should be (2) 150K resistors, not 47K.....good luck!

[Jelle]

Mac1amps,
Thanks for your reply. I realize that there will be a difference in sound because I have played a fender AB763 homebrew with 4x6550 and the Zakk Wilde JCM800 head and I like it a lot compared to the original AB763 and JCM800. I hope it will be the same improvement for the superlead. What would be a safe bias current for the 6550 at 560v plate and screen voltage?
Thanks,
Jelle

[Ray Ivers]

Jelle,

Another tube type you might consider is the KT88, which IMO has a slightly better overdrive tone than either the 6550 or the EL34.

47K output grid resistors will greatly load down your phase inverter output, although they may increase bias stability and reduce bias shift at full power. I would recommend 150K.

I would move your NFB connection to the 4 ohm tap with the 6550 conversion. I would also try removing it completely, and see how you like that.

I would try several different idle current settings between 25mA and 40mA, and see which you like best. I would recommend not exceeding 43mA at this B+.

Either 1.5K or 5.6K grid stoppers should be fine; I would use 1.5K myself.

64uF of screen filtering sounds like plenty to me... OTOH, if you meant 32uF (plate) and 32uF (screen), I would bring them both up to at least 50uF, and I personally prefer 100uF or more at the plate B+ supply in this kind of amp.

As always, YMMV.

Ray

[Jelle]

Ray,
Thanks for your answer.
I have the 6550's and kt88's lying around. I'll try both. I'm shooting for a Hendrix sound with this amp.
For the filtering I meant two caps of 32 uF in parallel making it 16 uF total for the screen filtering. BTW, I am currently using 100uF for the plates.
I'll convert to 50 uF for the screens.

Does increasing the grid stoppers also increase the bias stability?
Thanks,
Jelle

[Ray Ivers]

Jelle,

I meant two caps of 32 uF in parallel making it 16 uF total for the screen filtering.

Two 32uF caps in parallel = 64uF, and two 32uF in series (I assume this is what you meant?) would be 16uF. I guess you are using 450V or 500V caps, so for a 500V+ screen voltage you would need to connect them in series for reliability.

Does increasing the grid stoppers also increase the bias stability?

No - increasing the grid stopper values will decrease the bias stability - but in your amp the stopper values are relatively small, so the grid bias-feed resistors and bias-supply impedance will have far more effect on stability.

Ray

[Jelle]

Ray,
Ooops, you are right...I put the caps in series making it 16uF....
I now have all the info to convert the amp.
Thanks,
Jelle