SP 12 SE

[laatsch55]

Never used a trace pen Larrt.Are the 12au7's what??

 

[soundude]

The 12s of that year were not as solid as the newer ones...if I recall I think the clear tops were from the 40's...the grid rods werent anchored well and the filaments were straight instead of spiral. so they were flimsy like a light bulb filament. Not 100% about the filaments design have to check the data on those..Think the clear tops were trials, but sound great..awesome tube for later staging in amplifiers as phase splitters and second gain stages...thats where I have them in my amp.

 

[speakerman1]

Never used a trace pen Larrt.Are the 12au7's what??

Are those 12AU7s. What tube number are they?

Larry

 

[jbeckva]

Man all you tOObers out there are piquing my interest in those things. I can remember back in me teen days listening in on some garage band neighborhood sessions, and for the most part the heads on the amps were tubes. I *think* I can recall what you guys refer to as tube warmth.. hmmm...

 

[Gepetto]

Guitar guys nearly always have tube front ends because the tube rectifiers produce a naturally weak HV source which produces a great sustain that dies gracefully, the fast attack slow decay sound.

SS amps do not do that so they sound much different.

There is a book about this that I read a long time ago, something about the art of tube amplifier design. It discusses this power supply related feature of guitar amps.

 

[laatsch55]

12AU7A's Larry. The level of hum is still to high ,at least to me, some is at floor level and some increases with an increase in volume level. I have not up to this point used shorting rca's or tried the Turntable. The flu has kicked my nass for the last few days and just today started feeling human again.

 

[speakerman1]

Glad you are felling better.

Larry

 

[laatsch55]

Thanks Buddy

 

[soundude]

Lee does the hum go away when you short the inputs?
Need to drink more alcohol kills everything...lol

 

[laatsch55]

Kinda skimpy on brain cells latelyu. I'll check.

 

[laatsch55]

Shorting the inputs does not kill the hum, BUT it does kill the traces of crosstalk that is present when the gain is set way high. Nor does grounding the amp to the preamp.

 

[soundude]

which inputs did you short? was it the phono or the preamp, if it was the phono the hum could be after the board, between the phono to the input of the preamp. if it was the preamp inputs shorted then its in the powersupply, how many filtering stages are there? check dc caps and parallel a bypass if need be. also if negative side of PS is grounded to chassis lift it, if it isn't then try connecting it....is the plug ur rack is plugged in on the same electrical circuit as the preamp..ground loop there. try the preamp and the amp out of your rack on the floor plugged in the same outlet as the record player...check to see that the cartridge is got good connections and that the arm and body of the record player have no resistance with the grounding wire...also check continuity from the wire to the body and tonearm...

 

[laatsch55]

Haven't tried the TT yet John. I shorted the phono inputs, and every input that I selected after that, while leaving the phono inputs shorted. The pre is plugged into a different circuit than the rack. There may be some other issues you are noy aware of. There was some swapping of caps to SIO's and they were a lot bigger and had to mount them differently. I will post a pic to let you see them.

 

[soundude]

ok, first stage of filtering the cap shouldnt be to large, like 20-30mf of the cap won't discharge fast enough to be a efficient filter...if to big ripple will bypass the cap and enter next stage and can enter the circuit... You have SS rectifier?

 

[laatsch55]

It's a hybrid, I'll post the schematic of the power supply. The pic is of the cap I'm concerned about. Standing right next to the 12AU7.

 

[soundude]

Yes that cap is a concern, cany you take a good pick of the entire inside so I can see the grounding scheme? Also the power supply and filtering section...

 

[soundude]

those tubes by the plate design look like RCA's...?

 

[laatsch55]

Yep, RCA's.

 

[laatsch55]

Schematic

 

[soundude]

first is there is no shields around the input wires or the outs from the phono. these are the 2 most important areas for shielding because of the amount of gain and rf pickup. anything that the wires pickup is amplified, 12au7 about 60 times higher, also the volume pots without the shield and grounds connected act as a antenna because of the high resistance and will pickup any kinda noise, rf, and hum. check the 12x rectifier too they were known to induce hum, bypass cap from cathode or B+ to ground. measure the hum at the outputs of the preamp with your fluke and set for HZ and see what the frequency is. Look at 2 of my preamps one has the phono stage and other is just preamp. power supply setup and input wiring should be far away and shields all the way to selector and volume controls...just some guidance
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