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In Reply to: Re: el34s in a mac mc60 posted by lovetube on July 28, 2006 at 07:11:19:
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.
Follow Ups:
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.
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