Pre-Amp Build

marantzfan

Administrator
Staff member
I believe so. Trying to ignore that I built it, is seems way more transparent than my little Wright 6SN7 pre and more dynamic than the Cary SLP-98. The preamp creates a whole brighter more expansive view into the music. I'm back to a little more hum in one channel but since I've had it quieter I'm assuming some wire drifted closer to something hum producing. Overall, I'm very happy and have begun ordering parts for a final build. You might see what @jmathers has to say. He listened a little bit on two different occasions.
I certainly will look back at those post from @jmathers.

Really excellent work and determination to see it through to the end. I might have been tempted a few times to just chuck the whole thing out the window as my patience isn’t that good. ;)
 

Wntrmute2

Junior Member
Running into another issue that I'm sure is a brain fart. I wanted to check my cathode to heater voltage since upping the B+ to 210V. I have 57.7V elevating the heaters but my cathode voltage is only about 4VDC. I'm measuring from audio ground (bus) to pin 4 on the #76 and pin 8 on the 2C22. Something seems off. I thought the cathode voltage would be in the hundreds of volts. What stupid thing am I missing here?

Screenshot (6).pngScreenshot (8).png
 
Edit: oops, my bad. I misread your post as stating your cathode to ground voltage was 57.7.

Your heater supply is referenced to a voltage divider in the power supply comprised of a 470K top resistor and a 100K bottom resistor. 100K/570K = approx. 17.6%. 17.6% of 340V is approx. 60V. So your heaters are elevated approx. 60V above 0V ground , give or take based on the resistor tolerances. But you can measure the heater elevation by measuring the voltage between the middle of the voltage divider and chassis ground. Based on your measurements, I'm pretty confident that actual heater elevation is closer to 61.7V. Your cathode is elevated . 57.7V and you're measuring the 4V difference.

The similarity of heater elevation voltage and cathode bias voltage seems too close to be an accident -- but I have no idea why one would want the two voltages to be the same.

@ Paul: can you explain? Or is this just a coincidence?


cheers, Derek
 
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Wntrmute2

Junior Member
Well, in reading the data sheets there was mention of not exceeding 90 volt difference in cathode to heater voltage I also thought that was one reason for the elevated heaters. To keep the cathode to heater voltage from exceeding that. So when I checked to make sure the increased B+, I was surprised to see only 4V. Other amps I seem to remember 60-70V on the cathode. Just seemed out of the ordinary.
 

paulbottlehead

Active Member
The cathode voltage on both tubes for rev 3 will be 4-5V.

Elevating the heaters is helpful for the rev 1/2/4 preamps where one cathode is at high voltage and the other is close to ground. This isn't so helpful with rev 3, but it also isn't hurting anything.
 

Wntrmute2

Junior Member
Cool. Thanks for relieving my anxiety. Just as I think I have a handle on some concept, I realize just how little I know!
 

Wntrmute2

Junior Member
"Final" layout. Still trying to decide on 3.25" height vs 4.25" the cardboard box is 3.25" tall. The taller chassis would give me more room between the attenuators and the socket bases as well as between the inputs and the rest of the circuit.

PXL_20201017_185613536.jpg
PXL_20201017_213138106.jpg
 

paulbottlehead

Active Member
I'd also throw a vote in for not using that type of tube socket. I have found that the spring tension clips on the pins fail and fall off, then socket becomes useless. They are totally OK for prototyping, but I would never build one into anything.
 

Wntrmute2

Junior Member
I'd also throw a vote in for not using that type of tube socket. I have found that the spring tension clips on the pins fail and fall off, then socket becomes useless. They are totally OK for prototyping, but I would never build one into anything.
Damn, I bought double the amount I needed. Oh well.
They were just a few bucks each.
 

Wntrmute2

Junior Member
That's a beauty! I am thinking that I could give up aesthetics for an improved build. The heater wiring, attenuators and the LED readouts would be just that much further away from the signal carrying stuff. But almost everyone is suggesting the 3 incher so.....
 
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