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In Reply to: el34s in a mac mc60 posted by MIchael Samra on July 27, 2006 at 20:52:15:
did you rebias the amp ?
i can't believe you can run the EL34 on the 6550 designed amps
Follow Ups:
Love
The mac MC series amps arent ones you rebias..The output circuit is different than any other amp in that it amplifys current in the output stage and most tube amps amplify voltage.It sets itself in the amp and its proportional to plate and cathode voltages at any given time.The tubes arent glowing cherry red in anyway but what Im going to do is put a steady signal in the amp and see if the tube draws excessive current.
I have a friend who had his 6550 Jadis amp rewired by the manfacturer to use EL34s which he prefers, but can still use Kt-88s some how. I don't know what they had to do. But it was simply rebiasing the amp.
I'm sure the EL34 have a nice sound compare to the 6550 and I know Jadis amps was designed to run 6550/Kt88/KT90 and EL34 which need rebias for each type ,however the question here was the MC60 Mcintosh that was not design to run EL34 but only 6550 or KT88 .
I wonder how Mike he can run EL34 on these amps without rebias the amps ?
Love
I was just making light of the fact that the el34s sounded good in the mc60s..You keep mentioning bias.These amps amplify current in the final stage and being the el34 can handle up to 700vdc on the plate,it doesnt really pose a problem in short duration..I will however open the cathode circuit and see what current Im drawing at 46 watts maxium power that the el34s were making and if its within its curve you are ok.As I said I am running an additonal neg 2 v on the grid of the el34s so that could be the difference.
Hi Mike.
how can you run an additonal neg 2v on the grid of the el34s ?
let me know so I can do it.
Love
When I changed the original diode I put in a uf4007 and I had to put a 5.1k resistor in series with it to get the proper neg bias..The mac service lit calls for 435vdc on the plate and -47 on the grid at idle of course..When you change from the selenium to the silicon you have less drop and a high voltage out so the 5.1k limits that..I put in a 3.9k to raise it so now I have minus 49v on the grid.
Mike.
I did the same to mine and do you know trying difference resistor ranging from 3.3K to 5.1k made a difference in sound ? I think i like the 4.7k best and that is what got , so you might want to try this out . it will varies the negative voltages to the grids of the output tube by doing that and I think the diffrence is notable .
LOve
your are correct..Our line voltage may be a little higher than yours and thats why I run the 5.1k and you are correct in that if the tube doesnt conduct quite enough,it will be harsh.
Im running 2v additional neg bias on mc60s so that could be part of it but there is no bias adjustment on this amp other than changing the resistors on the control grid.The tubes arent drawing excessive current and they are making nominal power levels for that tube.
yes Mike.
that is what I means about rebias , changing the resistors after the dios , it set the negative voltages to the control girds .
do you note anythings after you install the EL34s on the Mc60 other than better sound ?
i will going to try it my self so i better made sure
Pete
That will be good if you try it on your mc60s..I put the amps on the ar3s which have good paper in oil caps in them as well as a nice piezo extension tweeter on top in a box.
I can try it on the imf sacms to but the real test will be the electrostatic martin logans.The female vocals were outstanding and thats what I used for reference.
Can you email me at merccougar89@yahoo.com?
Mikey
LOve
Im not sure its better sound but for an el34 it was very impressive..Typcially EL34s to me sound peaky,punchy and kind of brittle but this wasnt and the vocals had nice clarity and the bass definition was wonderful as well as midrange..
One time a friend of mine put 6l6gcs in his mc60s and they sounded fabulous but after 10 min it sucked in the sides of the tube envelope and warped the plate structure and curled the mica insulators LOL.That was arc over tho..Now the kt66s did fine.
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