Anode resistor values on 436/RS124 variable mu tube.

[MaxDM]

I've noticed that most vari-mu compressors either use an interstage transformer as a load, or a value of 10K or even 4K7 on the Urei, as opposed to 47K

What possible reasons would have been behind this choice, do you think? Gain? PSU current?

Also, is the 100r balance pot on the RS124 enough, considering the lower current draw, compared to other designs, and should it maybe be upped in value?

Has anyone had problems balancing tubes with the EMI design?

 

[emrr]

Out of a half dozen tubes, 100Ω has been fine for all.

There are old posts here from long-departed smarter people about B+ coupling in vari-mu. Many reasons for varying approaches.

A Fairchild is actually a push-pull 50K winding, but if you followed the parallel logic of most being (2)10K, you'd see (2) 2K5 in a Fairchild. Apples to oranges, being R versus L. The higher R value allows greater gain swing, but also greater B+ voltage swing. A transformer minimizes the B+ swing. (2) 10K looks pretty funny to me, it appears most are trying to lock in a minimal B+ swing without the expense of a transformer. Then, Collins 26U, (2) 10K loading an interstage transformer - that locks in the interstage response through pre-loading, so it doesn't swing with varying GR tube impedance under compression. The 436 is a sloppy general purpose compressor. The EMI addition of plate feedback resistors increases B+ and lowers the total resistive value in relation to the B+ source.

 

[dino_dave]

In a past life I built dozens of units (50?) from the RS124 scheme. I used 6ES8 valves and, as with most similar units, some valves end up in the trash as they won't balance well.
On the whole though, I didn't find it any more finicky than other vari-mu designs in that regard. Maybe it was 1 in 10 valves that wouldn't pass balance tests.

The standing, no compression current draw of the 6ES8 in the RS124 scheme isn't particularly low - anode voltage sits at about 50V in that condition. It is a bit higher than a 6BC8 where you'd typically see 75V or thereabouts, so I don't quite understand that part of your post.

I recently built an RS124 and have been using it quite a bit. However, I was getting some microphonic issues around 10KHz with the 6BC8s. I tried a drop in replacement to the 6ES8 which did seem to be less noisy but gives a helicopter type sound that seems to be based on input level. I tried messing with the balance pot, but I was unsuccessful. Have you encountered this issue with the 6ES8s?

 

[gyraf]

..remember, the 6ES8 / ECC189 was made for UHF front-end duties, and will often misbehave and run off into oscillations if you don't give it a (e.g. 470R) grid-stopper resistor. And I agree with the general 1:10 ratio of unusables to usables

/Jakob E.

 

[emrr]

6ES8's I have are all just fine in a modified original Altec 436, FWIW.