1987 50watt, 6550 or 34's?

  • Thread starter Thread starter Capulin Overdrive
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Capulin Overdrive

Capulin Overdrive

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got a modded 1987 50watt MK II, and trying to figure out if i'm safe to run 6550's in it?


i've read that the MK II came with 6550's? i can't find a build date anywhere on it.


if it came with 6550's and was modded for 34's, where would i look to figure that out?
 
Bias resistors are on the top (depending on the direction you're looking) of the output coupling caps. 220K was stock for EL34's

DSC03269.jpg


What do you have in there?
 
thanks Brian! :rock:

got 6CA7's in there now, but even with SED 34's something just seems a little bit off.


if you don't mind, what's the colors for 220K? just in case i can't find my cheater wheel! :lol: :LOL:
 
Capulin Overdrive":2lcnlv3c said:
thanks Brian! :rock:

got 6CA7's in there now, but even with SED 34's something just seems a little bit off.


if you don't mind, what's the colors for 220K? just in case i can't find my cheater wheel! :lol: :LOL:
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sorry a bit off topic, but what kind of 6CA7's do you have in it, and what do you think of them Martin? I recently put a quad of the new JJ 6CA7 in my Wizard, and while I'm really liking the results, they crackle a bit when I first warm them up coming out of standby. It only lasts for the first few seconds, but it's definitely noticeable when I first start playing. I'm wondering if it's normal, or if I should be worried. Once it's warmed up, it sounds killer, and the crackling goes away. The wizard is a bright amp to begin with, so the darker 6CA7 work well with it's topend, and they also add a bit of bass response. I'm really liking the results so far, but I'm a bit leary of what they are doing electrically with that crackling.
 
i have EH 6CA7's in there. they're pretty cool and can give an amp a EVH vibe, i run them sometimes just for something deferent and fun. suppose they have a 6L6ish vibe to them, no big spikes in bass, mids, etc. they're pretty even across the tone spectrum, maybe slighty more bass than a 34, and a little less mids.

no crackling or anything like that. :thumbsup:
 
Brian,


in your pic the two black caps left of the bias pot are the coupling caps, correct?


does this mean i'm gonna have to pull the board to get a look?


220k = red red green?
 
my board is deferent, but i have the 15k and 56k in the same general area?
 
Coupling caps are the red squares kinda in the middle. What colors are on the 2 resistors towards the top between the orange and green wires?
 
Once you know the values of the bias splitters, the last piece is the actual bias supply resistor which will probably need to be altered to get the tubes to bias up in the correct range.
 
I'd go with 150K to 100K on those. You could just parallel a 220k with each of those pretty easily which would give you 110K. Your bias feed resistor is probably a 47K and probably needs to be ~ 56K. I assume you know all the dangers of the high voltage and how to drain the caps. :)
 
Capulin Overdrive":gr76l54x said:
red red yellow! :lol: :LOL:

220K :D


what would they be for 6550?
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I think alot of them were 150K.....I use 100k.

Not alot of hope for 50watt tops, 6550's help and lots more filtering. :)
 
100K

I think 150k can be used for both types
the zakk wylde and kerry king JCM's have stock 220k's :aww:
 
briango2000":jmj7oqtt said:
I'd go with 150K to 100K on those. You could just parallel a 220k with each of those pretty easily which would give you 110K. Your bias feed resistor is probably a 47K and probably needs to be ~ 56K. I assume you know all the dangers of the high voltage and how to drain the caps. :)
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if it's set for 34's, which it appears to be. think i'll leave it alone for awhile.


i'll look for the 47k, if it happens to be 56k, then i might think about it.


i know how to drain caps, but that's about it! :lol: :LOL:

i work with 480 3phase at work quite a bit, don't know the tech stuff, but can get crap to working. ;)


thanks Brian! :rock:
 
My 75 has 6550 stock and has the 150k bias res. I dont care much for the 6550 and prefer the el34 as they crunch up faster then the 6550 at less volume. Only the marshalls shipped to the usa market came with the 6550, they still had el34 in uk and elsewhere during those years. Marshalls had 6550 stock starting around 74 up into the early jcm 800s of the mid 80's. Its a fairly easy swap and u can find info on the net fairly easy.
 
Capulin Overdrive":3twsldm0 said:
my board is deferent, but i have the 15k and 56k in the same general area?
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If you have those values there, the bias supply is set up for 6550's. That may be why it sounds off, as it may not be possible to get the EL34's biased properly with those range resistor values in there.

It's also correct that 6550's used 150K grid leak (often called the bias splitters) resistors instead of 220K which is standard for the EL34's.

If you are looking to keep the EL34's, I'd do three things:

Change the 15K to 27K.
Change the 56K to 47K.
Clip the wires connecting Pins 1 & 8 on the power tubes to the ground tag on the mounting bolt and replace them with 2 watt, 1 ohm resistors. This will allow you to measure bias via cathode current by setting your meter to millivolts and measuring the voltage drop across the resistors, and then simply converting it to millivolts (since it's a 1 ohm resistor, milliamps will be the same as millivolts, so 40mV = 40mA).

If you use a bias-rite (I hope not. Bias Rites are notoriously inaccurate) or other external bias probe, you can disregard that last step.
 
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