federal comp 6sk7 balance meter

[diggy fresh]

Hi,

i am in the studying phase of building a federal am-864 compressor, i read every thread and most everything on the net.

I've read the importance of matching tubes or balancing the 6sk7 tubes current to minimize thumping.

I only have two nos 6sk7 and would like to implement a scheme to "balance" them.

i found this thread were rafafredd is proposing this :
https://proxy.imagearchive.com/3bf/3bf1c1cc348372688965c8496f0a7cb2.gif
thread : https://groupdiy.com/threads/6sk7-balance-pot-to-federal-limiter.27372/

Anybody succesfully implemented a balance scheme like that?

I've seen someone also recommend adding a "zero centered current meter" to make it easier to balance.

I have this meter (attached image) and would like to add it but im kinda clueless as to where to connect it.

Should it be connected across the cathode balance pot lugs 1 & 3? or in between the the two grids? (pin 6)


im still green with tubes and don't want to blow things up.

Do i need to implement both schemes proposed by rafafredd or just the grid one? also what about the wattage of the trims?

Any help clearing this out for me would be greatly appreciated.

thanks

 

[Rompstomp89]

Most other vari-mus have a plate balance and a cathode balance. Since the feds cathodes are hard grounded I've implemented a plate balance trimmer where the B+ feeds the two 12K plate resistors. Im sure there might be a better way to do it but Ive had success with this method.

 

[emrr]

i found this thread were rafafredd is proposing this :
https://proxy.imagearchive.com/3bf/3bf1c1cc348372688965c8496f0a7cb2.gif

what that doesn't show is any end stop resistances. I'd probably do something like a 22K / 100K pot / 22K string to limit the range. 22K would give extreme plate value settings of 13K4/8K9; if that isn't enough range you need other tubes. 12K4 would be the centered plate resistance, totally fine, probably more accurate than originals with aged carbon resistors.

 

[diggy fresh]

Thanks for the reply!

I will try the plate balance with stopper resistors where the 120k trimmer is drawn.

Do the grids need balancing too?

And can a current meter like i have (zero centered and 1ma scale) be used? across the outer lugs of the trimmer pot?

thanks!