Variable-mu lives!!!

I just got my variable-mu compressor going. Pictures later this week. It's roughly based on a single channel of a 670, but very roughly. The biggest challenge was matching the tubes. Some parts values needed to be tweaked to use parts I had around. I just swapped the 6BA6 tubes around 'till I got it to work without any control voltage feed-through (compression thumps). In hindsight, I should have got more 6BA6's and had an easier time matching them. I got 10 and used 8. Should have got 20 of them.

Tube complement:
6BQ5, 6U8, 0B3 - voltage regulator
6BA6 x8 - main part of compressor
12AX7, 12BH7, 6GW8 x2 - control amp

Transformers:
Hammond 850N x2 (signal amp in and out)
Hammond 850G (control amp in) - should change this to 850N or something like that.
Amveco 62025 (control amp out) - not ideal, it's on my list of things to change out too.

All but the 0B3, 6BA6's and tube sockets were salvaged parts or stuff I had lying around. Project cost was about $200CDN (not including stuff obtained years ago and sitting in my garage).

It sounds pretty good. Different than any other compressor I've used. It really gives vocals a 'presence' in the mix - a solid like a brick outhouse sort of sound.

Oh yea, the purpose of the powerful control amp is to charge up the CV capacitor with an attack of .2 msec. I always wondered why there was such a powerful amp in a Fairchild when Altec and other variable-mu compressors didn't do that.

-Dale

> the powerful control amp is to charge up the CV capacitor with an attack of 0.2 msec.

Alternatively they could have used a smaller cap.... except with massively-parallel grids they have to keep the leakage resistance low. So in the end it comes down to the Release resistance. With up to 16 grids (in stereo) parallel, you need 8 times the control power of a simple mono 2-grid VCA.

The 10 watt amp is just about right, for their topology.

If you had a good DC amplifier you could do it different. But they didn't have good accurate DC amplifiers in the 1950s, or not simpler than they way they did it. Today you could use a 1-watt control amp, scale the time constant parts to 10X impedance, and stick a high-voltage op-amp between the time-constant and the grids. (But then you need a large negative supply voltage... it isn't really simpler than the 660.)

Good one Dale. Please post some pictures soon !

Congrats! :thumb:
Must be a good heating for the wintertime. :grin:
Pics would be nice, and a sound file.

Congrats to you Dale, especially for $200! Look forward to the pics.

Justin

Any pictures? This sounds like in might be a very cool build. Are you using pearl tube coolers to help with the life of the tubes?

I have to borrow the digital camera from work to get photos. I might do that in the next day or two. Also, I'll post an MP3 with the vocals brought up too. The track was recorded with a TLM103 and a Tascam preamp - not ideal. I think the processed vocal has more presence and a nicer sound when mixed. I now have a better mic (DIY C12-like 6072 with C414ULS capsule) and a variety of DIY pres.

No, no tube coolers, I live in Canada. The tubes run fairly cool - you can touch them for a few seconds without getting burned.

The bottom is a bit messy so far, I need to do some cleaning of the 'patches'.

Here's the photos:
www.telusplanet.net/public/dulan/VariMuBottom.JPG
www.telusplanet.net/public/dulan/VariMuTop.JPG

I've found an instability when using a power transformer as the output of the control amp, so I wound a new transformer using a Hammond 125E transformer core, but I didn't have the proper finishing tape. The other thing I need to address before neatifying the compressor is to possibly look at a way of gain balancing when the control voltage dips really low - below -70 volts. I've got some ideas that should work quite simply.

Here are a couple of other shots:

www.telusplanet.net/public/dulan/VocalBooth.JPG
www.telusplanet.net/public/dulan/StudioCtrl.JPG

I think I'm going to finish fixing the few glitches I've found then make up some audio files for your listening pleasure.

i'm so jealous. your dr16 still works. mine died last month. beyond my knowledge to fix...........

It's actually a DR8 (if it was a DR16 I'd probably still use it) - mostly I use it to record just scratch stuff and mic shootouts.

dale-

kinda OT, but have you ever thought about mounting your monitors on the wall and moving your console back about 3-4 feet? If the tweeters are above your ears and you arent sitting right on top of those monitors, Ive found them a lot easier to work on. Perhaps you are happy with the setup, but if you are still tweaking to find the right spot for everything, consider giving that a shot to see if thats any improvement.

dave

I know that ebay auctions should be posted at the black market, but as some of you are working on this BEAST, I though I had to put it here, so that everyone can see it.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=3284&item=5710731424

I don´t know if it´s true, but it says those are the orinal 670 input trafos...

With buy it now...

us canadians always have tonnes of hammond iron laying around. curious to hear some samples. looks cool. congrats.

:sam: :thumb:

Yea, I'd like to build a tube bass amp using the 1650W transformer, but the tubes are a bit pricey. My girlfriend is going on a vacation over the next couple weeks so I should be able to finish it and get some audio clips up.

One thing I found is if you're nailing the thing (getting more than 30dB of gain reduction) it clicks and pops, otherwise it sounds nice. Some vocals sound sweet through it, some vocals are sort of indifferent, and it's just the thing for bass (upright at about 5dB of gain reduction, or electric at about 20 dB).

Hey Dale,

You wouldn't be willing to post a schematic of this unit would you? I am interested in building a stereo Vari-Mu. I just finished my record using the Manl@y stereo version and would like to have one of them in my own studio.

I was looking at Kent's version of PPR's idea as well but he said himself that this may or may not be "Pro" quality.

I would rather do this project on a PCB as well. But a schematic would get me 1/2 way there. Thats if you think making this into a DIY project is worth while.

Thanks

ChuckD

I used the following schematic:
http://www.vintagedesign.se/schematics/670schem.pdf

I thought point-to-point wiring worked well for this - especially for the heater lines because there is a lot of current there. One thing to remember is that they only show half of the input tubes on the schematic. I made my mods just in my head, but they're not hard. I used 6BA6 tubes in place of the 6386, and that worked fine. Connect with the screen and suppressor to the plate for a triode connection. I also added 470 ohm resistors in series with each grid to prevent oscillation.

Darn Dale, you beat me to it!

As a side note, Brian Sowter has told me that he is putting the BA6a transformer set on his site shortly! He has to figure pricing first. He has sold a set already, so maybe wait for the verdict before ordering.
cj

Did you happen to take some Max current measurements?
Is it possible to put the circuit into a PCB? Also what stages can be reduced to solid state?...

What is your opinion on this?

-ChuckD

hey chuck,

sombody on the site.has put together a hybrid varimu (really can't remeber who, check the vari mu meta) using a 12au7 i think, and solid state side chaind and makeup gain.

The filament current for all the tubes was about 5 amps at 6.3V. One problem with having heaters on a PCB is if you use AC heating, the hum gets into everything because all of the wiring is on the same plane. The heater traces also like to split any attempt at ground plane fills.