One-Tube Opto Comp

[Wavelength]

There are 12DJ8 that run at 12.6V
I would replace the 10K cathode follower resistor with either a Vishay 4.7ma current source J510 or semitec E-452. Both available at Mouser.

 

[OneRoomStudio]

This is what I had been thinking for the PSU (for those of us in North America anyway). This is a $20 transformer, and I'm using one of the two "primaries" as a 115V tap.

 

[OneRoomStudio]

There are 12DJ8 that run at 12.6V
I would replace the 10K cathode follower resistor with either a Vishay 4.7ma current source J510 or semitec E-452. Both available at Mouser.

Do you think the current source is better than the updated arrangement in post #12?

 

[rock soderstrom]

I would replace the 10K cathode follower resistor with either a Vishay 4.7ma current source J510 or semitec E-452. Both available at Mouser.

Interesting. What would be the concrete advantages of a current source in this place?

 

[rock soderstrom]

This is a $20 transformer, and I'm using one of the two "primaries" as a 115V tap.

This will not work on my side of the great pond.

 

[Wavelength]

Do you think the current source is better than the updated arrangement in post #12?

Yes, keeping the gain control ground referenced is always a better idea. The semitech are pretty cheap and the tube will run more linear since it looks like ~1M than the 5K or 10K originally.

 

[Wavelength]

Also that filament supply is going to need some work. 18vac even single rectified will be really high and the 317 is going to be smoking. Plus remember a regulator removes x amount of noise. 1000uF into R into 0.1uF is going to have tons of ripple. Maybe 1000uF R as needed 1000uF->LM317->6.3V. Running the 6922 a little hotter like 6.5V will lower the Rp and increase the current and in something like this it will work a bit better.

 

[OneRoomStudio]

Thank you for the feedback! I’m learning a lot. I’ll add an updated schematic in the morning.

 

[ruffrecords]

If only E88CC's could be powered by 12.6V, this would be even easier....

There is a 12V heater version called the 8416 but all the audiophiles bought them up years ago and now sell them on eBay for insane prices.

Cheers

Ian

 

[OneRoomStudio]

Also that filament supply is going to need some work. 18vac even single rectified will be really high and the 317 is going to be smoking. Plus remember a regulator removes x amount of noise. 1000uF into R into 0.1uF is going to have tons of ripple. Maybe 1000uF R as needed 1000uF->LM317->6.3V. Running the 6922 a little hotter like 6.5V will lower the Rp and increase the current and in something like this it will work a bit better.

Ok, added the extra 1000uF and upped the resistor slightly. My simulation shows a little over 10V at the input of the LM317, which seems good for a ~6.5V output. Any less than that and the LM317 won't work properly (it needs a minimum of a 3V differential). The 15R resistor needs to dissipate about 1.5W, but that's ok.

 

[OneRoomStudio]

Yes, keeping the gain control ground referenced is always a better idea. The semitech are pretty cheap and the tube will run more linear since it looks like ~1M than the 5K or 10K originally.

I'm not super familiar with using CRDs with cathode followers. Out of curiosity, why did you choose 4.7ma? Based on the curves, it seems like something closer to 10-14ma would be more ideal. Maybe I'm misunderstanding something? Thanks!

 

[gyraf]

This is what I had been thinking for the PSU (for those of us in North America anyway). This is a $20 transformer, and I'm using one of the two "primaries" as a 115V tap.

Please don't do this, even though it's tempting: You run a real risk of a live mains chassis: PureAnalog M/SLevel Vari Mu Compressor (manual + schematic)

/Jakob E.

 

[rock soderstrom]

Isn’t that too little ripple filtering for B+?
I would increase the two resistors

 

[OneRoomStudio]

Please don't do this, even though it's tempting: You run a real risk of a live mains chassis: PureAnalog M/SLevel Vari Mu Compressor (manual + schematic)

/Jakob E.

In this case, I contacted Antek (the manufacturer of the transformer) and asked. I was told, "All output coils are insulated. You can use one of the inputs for 115V and other coil 115V as output."

 

[OneRoomStudio]

Isn’t that too little ripple filtering for B+?
I would increase the two resistors

Probably too little, yes. I'm trying to find the balance of not losing too much voltage with having enough filtering. I should have around 160V after the FWB rectifier, and I'd like to keep B+ as high as possible (ideally with very little ripple). I'll have to do some tweaking to find the right balance.

 

[OneRoomStudio]

Ok, according to my simulation, this B+ arrangement should give me ~110VDC with less than 100mV peak-to-peak ripple.

 

[OneRoomStudio]

I *think* this is how to implement the CRD/CCD. I chose to use a 12mA one (Semitec E-123), since with a B+ of 110V, that seems to put the tube in a linear range. I tweaked a few other resistors for the new lower B+, and added a "slow attack" switch to the sidechain (C10). Thoughts?

 

[ruffrecords]

Ok, added the extra 1000uF and upped the resistor slightly. My simulation shows a little over 10V at the input of the LM317, which seems good for a ~6.5V output. Any less than that and the LM317 won't work properly (it needs a minimum of a 3V differential). The 15R resistor needs to dissipate about 1.5W, but that's ok.
View attachment 116307

I would be tempted to raise the 47uF HT smoothing caps to at least 220uF and add a third RC filter stage. Class A tube circuits have a poor PSRR so you need to reduce ripple as much as possible.

Cheers

Ian

 

[OneRoomStudio]

I would be tempted to raise the 47uF HT smoothing caps to at least 220uF and add a third RC filter stage. Class A tube circuits have a poor PSRR so you need to reduce ripple as much as possible.

Cheers

Ian

Agreed. In the latest version I posted (post #36) I increased them significantly.

 

[Wavelength]

I'm not super familiar with using CRDs with cathode followers. Out of curiosity, why did you choose 4.7ma? Based on the curves, it seems like something closer to 10-14ma would be more ideal. Maybe I'm misunderstanding something? Thanks!

I have used 4.7ma in the past as I had a reel of the Vishay parts with 6922 and they worked well into 600 ohm transformers.
Also you don't need the 100 resistor there.