SP 12 SE

[laatsch55]

I still don't know how to evaluate the filament +/- thing here guys. going to the phono board I could have them reversed. On the pots, every other input acts fine John. Those pots are quality stuff. The only thing I've seen track better in a stacked or ganged pot is the volume pot in a Spec 1.

 

[Skratch]

Lee, Pins 4 & 5 are + Pin 9 - if the hearer voltage is 6.3 V

 

[laatsch55]

Heater voltage is 11.7-12

 

[Skratch]

Lee are the 2 tubes heaters in series? You should have 6 volts across pins 4&9 and 5&9 on each tube

 

[laatsch55]

Oh, don't know Mark, will check that, Thank You.

 

[Skratch]

Lee what tube are these?

 

[laatsch55]

6922's

 

[Skratch]

Lee If you have 12 volts across 4&5 it is series and polarity makes no difference

 

[laatsch55]

OK, that helps a lot, thanks Mark.

 

[speakerman1]

If they were NOS they came in the original boxes. I will shop for another pair. If in doubt change tubes. LOL

Larry

 

[speakerman1]

I can get some EH 6922s. Think I know where my tester is. Will test the M8 tubes and send to you if you want to borrow it. Do you want another pair of tubes?

Larry

 

[laatsch55]

No, I might after I get the new phono board populated and tested.

 

[soundude]

Lee, see if you have any stray ac on the dc line, since those tubes are high gain you could be picking up stray ac...try coupling cap from filament to chassis see if the noise goes away... .01-.47.
The tubes have a high gain so any little signal will amplify alot. also check to see if there is a 47k or higher at the input to ground...this is the matching resistor for the cartridge loading...if its not there put one there, will have to be changed later to match cartridge...

 

[laatsch55]

OK. That still doesn't explain why the low channel's pot is full up and the high channel is full down and they aren't even close yet. I will try what you have suggested.

 

[soundude]

lee check the inputs at the selector, if the tubes aren't out of balance, has to be a reversed wire some where...or the pot is wired wrong...if you turn the pot the other way does the oppisite happen? if you need to check the balance and wiring checks out throw in a 12ax7, it is close and should not harm anything in the circuit...also if those filaments are wired in series put the coupler off pin 9 to ground. and is that a fet in the cathode of the first gain stage?

 

[soundude]

[attachment=0:101ncnty]p6dj8_cir.gif[/attachment:101ncnty]

 

[soundude]

Ya know one of those tubes could have a bad filament in one of the triode section, do they all light?
That would explain the low output too...just a fyi.
Don't like the current source inputs, I sent a schematic that is very close to the one you posted with out the SS in the cathose circuit.

 

[laatsch55]

Supposedly the FET was to reduce noise because of the no negative feedback topology??

 

[laatsch55]

Yeah, that schem looks pretty familiar.

 

[laatsch55]

There are gain adjust 3386 Bourns in the board, that's the pot i was refering to. And they are one wayers, can't get em wrong. And turning them the other way does make the opposite happen.