Vari-mu tube selection

[rotation]

Hi,

i'm helping a friend fixing his vari-mu compressor (he bought it as a kit) which is similar to Altec 436.
I think we fixed all mistakes (schematic and instructions came from someone who don't know electronics ) the only thing left is input tube. We are going to use 6BZ7 and i have some question about it;
How many tubes, more or less, do we need to buy to make work one channel?
Is 6BQ7 electrically really the same as 6BZ7 when used in vari-mu compressor?
Anyone knows good tube dealer for this kind of tubes in Europe? We would like to buy them matched.

Miha

 

[ioaudio]

hi miha,

you cant buy matched tubes for vari mu work (except maybe the new jj 6386) because you have to match them for the various grid bias points happening during compression. i dont think tube dealer offers this service.
-max

 

[Jonte Knif]

They seem to very closely the same tube if transfer characteristics are compared.

Can I ask why you chose that particular tube? The original has 6BC8 which is better for variable mu purposes and they are not expensive.
I retubed and calibrated two UA176:s just a couple of months ago, and bought only 8 tubes (also 6BC8). I can not remember the brand but they were very good. Perhaps RCA. I only checked with transconductance tube tester for good pairs and didn't care to match any better. And they happened to be perfect. Good luck this time.

Before I got my current test gear I used a simple test setup to check for currents and voltages at two operating points. Extremely crude method, but now when I have good tube tester and test also for gain at several operating points I'm not getting substantially better matches. Frustrating for me, good news for you. Nowadays I even test for distortion at different operation points, but that starts to be neurotic.

If you just have the time and energy (and as DIY:er you have ) build something simple to test the tubes yourself. Go as far as your abilities allow. You can use the ready compressor as a test setup with minor mods. Connect a negative adjustable supply for control voltage and measure the gains (separately for each half) with a couple bias points.

In the original comp there seems not to be any cathode balance trimmer. This makes the matching very critical. Do put some trimmer there to adjust the cathode resistor balance if (and when) necessary.

Now, whether or not you did have the time to do all this, the final step is to check how well thumping (side chain signal) is rejected. You can do it with music, but better to take a modest signal generator and adjust it to produce 1 kHz sine with dc offset, so that the peak to peak voltage is about -4 to 0 volts. Or any value as long as it's not positive i.e. the offset has to be at least half of the peak to peak value. Insert this into side chain output. (the center tap of the input trannie) Then listen or use scope, AND now if you have the cathode trimmer minimize the residual.

I'm tired, so the explanation might need some fine tuning. Don't hesitate to force me to improve it later It has been a long way for me to realize all the details, but you get very far with these methods only.

 

[rotation]

hi miha,

you cant buy matched tubes for vari mu work (except maybe the new jj 6386) because you have to match them for the various grid bias points happening during compression. i dont think tube dealer offers this service.
-max

Hey Max,

i thought it will not be so easy. But i got good reply below and will try to do it this way. I never finished vai-mu compressor so i'm kind of lost. Beside this there were some BIG mistakes in schematic...

They seem to very closely the same tube if transfer characteristics are compared.

Can I ask why you chose that particular tube? The original has 6BC8 which is better for variable mu purposes and they are not expensive.
I retubed and calibrated two UA176:s just a couple of months ago, and bought only 8 tubes (also 6BC8). I can not remember the brand but they were very good. Perhaps RCA. I only checked with transconductance tube tester for good pairs and didn't care to match any better. And they happened to be perfect. Good luck this time.

Before I got my current test gear I used a simple test setup to check for currents and voltages at two operating points. Extremely crude method, but now when I have good tube tester and test also for gain at several operating points I'm not getting substantially better matches. Frustrating for me, good news for you. Nowadays I even test for distortion at different operation points, but that starts to be neurotic.

If you just have the time and energy (and as DIY:er you have ) build something simple to test the tubes yourself. Go as far as your abilities allow. You can use the ready compressor as a test setup with minor mods. Connect a negative adjustable supply for control voltage and measure the gains (separately for each half) with a couple bias points.

In the original comp there seems not to be any cathode balance trimmer. This makes the matching very critical. Do put some trimmer there to adjust the cathode resistor balance if (and when) necessary.

Now, whether or not you did have the time to do all this, the final step is to check how well thumping (side chain signal) is rejected. You can do it with music, but better to take a modest signal generator and adjust it to produce 1 kHz sine with dc offset, so that the peak to peak voltage is about -4 to 0 volts. Or any value as long as it's not positive i.e. the offset has to be at least half of the peak to peak value. Insert this into side chain output. (the center tap of the input trannie) Then listen or use scope, AND now if you have the cathode trimmer minimize the residual.

I'm tired, so the explanation might need some fine tuning. Don't hesitate to force me to improve it later It has been a long way for me to realize all the details, but you get very far with these methods only.

Hi John


This are some good suggestion, thanks a lot. So, i wanted to use 6BZ7 because on schematic it was listed as a second possible option, beside another tube that a friend bought and doesn't work for vari-mu at all. Later i found out some compressors also use 6BQ7, compared its datasheet to 6BZ7 and thought it doesn't matter which tube of this two i use...

I will try your setup for testing and matching tubes. I have both cathode trimmer and i think also trimmers around anodes (have to check schematic).

I find your idea for testing distortion at different working points very interesting. I've done it with a friend on preamp and found out some combinations of harmonic distortion sound better than others.

I'm ok for now but will come back when i have new tubes.

Bye!

 

[Jonte Knif]

Hi,

The datasheets show that the plate resistance and Gm are pretty close between 6BC8 and the tubes you mentioned _at one point_, but looking at the curves it seems that 6BC8 is more remote cut of tube than the others. This just means that you get more compression with the same control voltage -> less thumps. In a compressor without interstage transformer this is a good idea.

Yeah, the distortion test. I got frustrated by the fact that even after careful matching I could not finds pairs to make the distortion equal between channels in a stereo comp. So, it could happen that the other channel has more distortion at no GR, and the other at some other GR and then at still more GR the other way round. The differences were big enough (even for me, I don't care too much about minor amounts of 2nd harmonic) to try to find a solution. I made some progress, but the only way to get "perfect" results would be a tube curve tracer _with very sophisticated matching algorithm_. Because you can not find perfect pairs, you should be able to find pairs which behave the same _after the trimming procedure_. I'm not capable of doing this. So after the distortion measurements I still have to try which pairs finally do work. Making a mono compressor is pure joy, stereo units for mastering use sometimes less so.

-Jonte

 

[jensenmann]

Rotation seems to be talking about the famous hidden VK1 project which Purusha has stolen. Tat was not able to make his VK1 work properly and contacted several persons involved in this project to get support. All I can say about this is that Tat never got the latest revision of the VK1 schematic which makes Rotation think that the originator of this project has no clue about electronics. (Mine is working properly, btw, with the right set of tubes - matched) Rotation seems to try to help Tat or another member to bring his clone to work.

Everybody here has his own opinion about that subject and should decide what to do.....