SP 12 SE

[Gepetto]

Hmmm means something is off by an order of magnitude here Lee and I know that you know that

I thought Larry's "FFFFFFF" was the equivalent of your "M&^%$# F&^$$^%^&"

I just finished helping a guy in Canada (good guy) debug his PL700 over the internet and know some of what you feel. With his permission, I will eventually try and post the debug path that took us down.

 

[speakerman1]

Yes Joe it was. LOL

Larry

 

[laatsch55]

So true Joe, so true.

 

[laatsch55]

So, how easy is it to take a tube down?? In the phono section the Heater circuit is not labeled +,. or -. When it leaves the main board to feed it it is marked H+, H-. Is this an issue? I could not see any polarity markings on the phono board, and knowing as much about tubes as I do, can't tell if they should be.

 

[speakerman1]

Don't know. Is the gain all the way down? Do they make noise with the gain down. You turn the gain up gradually. At some point you are going to get noise. If I am wrong someone please speak up. If they are making noise. I think they are bad. Haven't found the tester yet.

Larry

 

[speakerman1]

Tubes have been shipped. May be here friday. So take a break and relax till they come. LOL

Larry

 

[laatsch55]

Back on it Larrt. sorry was awayn from it this long. One question, are the tubes matched, any certain order of placement?

soundude, if you read this, check the ground scheme on the paper there and tell me if it's right. Was having some Hum problems on this one.

 

[speakerman1]

Lee nope. They aren't power tubes so it isn't as much of a deal.

Larry

 

[laatsch55]

Hookay.

 

[mlucitt]

Lee,
Are you becoming a tube guy? Just heaters, grid, and plate just like collector, base, and emitter. Microphonics probably means a loose connection or a bad cap somewhere.

Skratch,
Did you mention Sound Values? I have a VTA-70 that I put together as a kit, it had Mullard tubes and they still sound great.

Class is over for awhile. I am back in the sandbox.

Mark

 

[soundude]

Actually larry it matters more with preamp tubes because of there used as gain, power tubes doesnt matter to a extent...lee what kinda preamp is ti? Phono or line stage... you need to blow up the schematic can't see the writing...
H+, H- matters depending what end of the filamentis closest to cathode, if its the positive cound induce hum...try reversing heater feeds. If its high gain like a 6922 or 6dj8 which is a high transconductance frame grid tube, it very sensitive to emf and bucking or chokes and with amplifiy the 60hz. to determine use a hz meter and see whats coming out of the outputs.
Also if you put your hand close to the tube and it gets louder the filament is wrong polarity, if it gets quiet then your inducing hum. They call it biasing the filament +/- to the cathode.
try grounding the negitive supply of the filament to the chassis to bleed off. also in the rca jacks common to chassis?
Take a larger resolution pic of the schematic so I can blow it up, like what i do with my pics.

 

[laatsch55]

Dude!! Didn't stay out long did ya?? Swapped tubes, the noise floor dropped like a 200 story elevator.

Distortion is pretty good. Frequency response??? Have to let a tuibe guy answer that one. As far as an SS having a response like that, we would be looking to improve that.

I have 60 hz on the outputs when the phono section is online.

 

[soundude]

Ok I see one problem, the ground from the line in is feeding the isolated grounds of the rca jacks. Lose it and use a bypass cap...second is the rca's that are at the back to the left and come off phono stage is not sharing the common ground..they might be the culprit, try connecting to the ground of the other rca's ground. By the way love those clear top rca 12au7's and by looking at the internal tube structure of the phono stage they look like 6dj8's.
Lee what is that metallic stuff on the bottom of the chassis? looks like foil barrier, that can act as a antenna to the circuit. 6dj8's are finiky tubes and noisy and very microphonic.
also are you testing it on a phonograph, you will need a ground wire from the chassis to the phono to shield rf pickup.
With a phono stage line grounds have to be isolated from common, I usally install a bypass cap .1/100v between line and common.
And the leads coming from the volume pot to the board need to be shielded, twisting them does cancel rf noise only ac hum.

 

[laatsch55]

The rca's are common to chassis. EXCEPT the phono in.

 

[soundude]

At work, nights this week...really busy changing out a 450 ton air handler friday, lotta prep work.
got helicopter and crane doing lift off roof....try get some pics to ya...

you changed the 12au7's...the clear tops didn't have spiral filament so they will have hum, usually grounding or bypass one of the filament leads to ground makes the play nice. If phono is open loop you will get noise.

 

[laatsch55]

6922's in the phono section. The Phono inputs are isolated from the ground bus bars.

 

[laatsch55]

I have a thread titled " So ,What does that Latsch feller do for a livin...." you can post in there or start your own. Pretty interesting what folks do outside this hobby.

 

[laatsch55]

The phono section is a touchy bastard. Just touching the shielded cables makes a lot of noise.

 

[soundude]

cool I'll do that, you have to hook up a phono to really test it for noise and hum, you also have to have the ground wire for the record player.
Also one thing I have learned with phono stages is that they have a load resistor for the inputs at a normal spec of 47k which is considered cartrige loading...well this resistor should be easily swappable since most of the high end is cut through this...if its there remove it and replace with a higher value say 100k and you won't believe the difference in clarity....I have grado cartride on mine and it say's to load it to 47k, but the highs were so low, loaded it with a 80k and sounds fantastic..your spec1's should have cartridge loading selectors pf and resistance...awesome to have that selector...

 

[soundude]

Yes, it needs to be loaded with a cartridge, very high gain tubes and picks up everything.
If you hook up record player and ground the phono ground wire to chassis, and you still have hum, trust me go to chassis and rca grounds with bypass cap, common is a .1mf. but if you want to test anything .01 to .1 should work. Those cables coming from phono ins to board input really should have shield, that area is most induced with noise.
Leaving to go fight the deer on the way home, catch up with you tomorrow when I get up.
Good luck...