C3M for one bottle pre, etc?

[ihscoutlvr74]

Gents,

Was wondering if anyone has used these tubes for any preamp projects, etc.  Seems like a very high quality audio specific tube for a decent price and was curious.  Thoughts???

 

[Jonte Knif]

A good tube for many purposes, but for pre amp? Mic pre amp? _single tube_ pre? No way, the gain is low.

Used a lot for driver stage for big triodes like 300B.

There are a lot of other SQ or Post tubes with ridiculously low prices compared to the awesome quality, which might serve you better and save from that 20V heater "problem" in C3m. Many of them have very high Gm and/or amplification factor. I have some interesting types and many of the pentodes work as superb triodes too. D3a for example is a robust pentode with mu around 75 and is _very_ linear. Hard core tube hifi DIY:ers know them all.

-Jonte

 

[PRR]

C3m:

> No way, the gain is low.

As Pentode, 1:10 input iron and 6K:600 output iron, one stage can do gain of 100 or 40dB, which is not too shabby for modern studios. Noise is low enough. It will do nearly +30dBm, which is pretty good. That assumes your mike can deliver -10dBu, which many condensers do. But output impedance is very-very high, which distresses the output transformer and makes output level VERY load-dependent.

As Triode, loaded in 2.5K (matched), gain is 32dB in 600 or maybe 38dB in hi-Z load.

If you run two of them, 1:10 tranny R-C loaded triode then transformer loaded triode, you can arrange gain of 10*14*9*0.5 or 650, 55dB, which is decent.

D3a is quite interesting. The triode Req is so low (at 24mA!) that you could get decent NF with a 1:1 transformer. Stage gain could be 50, and another triode-transformer stage could get you to very good gain and ample output. But this could be 50mA total demand, 10 or 20 times more than "usual" designs. No Free Lunch.

 

[Jonte Knif]

One of my friends wanted an "old school" pre with no FB. I settled for D3a. I have a lot of them. Never built it, just tested input and output stages and left it as an exercise, but here is what you could get with 2 of them in triode mode:

input trannie 1:5. High input C rules out 1:10 trannies, but 1:5 is fine. Gain pot between stages. Output stage single ended transformer coupled with 4:1 trannie.

Max gain 74dB
input noise near theoretical minimum, ie. mostly transformer resistance.
Z out 140 Ohms.
Max out 33V peak into 600 Ohms.

Distortion low enough to be inaudible at high loads and single sources. (my estimate. At some gain setting the stage distortions will cancel each other)
Power supply 170V 30mA per channel. In two channel pre this is still not an issue.
And see what you can get with it.

You could record a harpsichord with ribbon mikes and drive a vintage limiter if the instrument is too dynamic

Other tubes of interest include E180F and E810F. And there are more.
Drawbacks in high Gm tubes include fairly big spread in Gm (1/3 of my D3a went into thrash), high Cga (in triode mode) and the fact that they are prone to oscillate and require fairly low grid leak resistors. And because some are quite microphonic 1:1 input trannie is not a good idea. But when you know what they require the rest is not rocket science.

And excuse my arrogance, but I still don't get the idea of "One tube pre". Why leave other tube out to make the pre less versatile? (you can always bypass one tube if needed) If one is obsessed with a single tube clapping electrons in the woods, then OK.

Well, umm...the topic was C3m..ooops..sorry. I get carried away when somewhat obscure low cost high quality tubes are mentioned.

-Jonte

 

[PRR]

> "One tube pre". Why leave other tube out

I believe NYDave's plan is a DUAL-triode, and thus a good all-round design, and a wee bit easier to build than say a classic RCA two-1620 preamp.

Because the selection of dual-unit tubes is limited, you can't meet every wish-list spec at once, but it sure can function in any modern studio. Many-many "professional" preamps were either twin-triode or two identical bottles.

These one-unit telco tubes are not the same idea. They are very good, but since cascade gain multiplies, using just one unit instead of two gives you Square-Root performance. And as you say, that means it will not work well in every situation. And to get the most out of them, you need large cathode and high current.

> 170V 30mA per channel. In two channel pre this is still not an issue.

No. In small-studio DIY, that is trivial.

I (and maybe you) have worked in studios with a dozen or more mike preamps. When you have trays and more trays of tube preamps, the power supply becomes awkward and the total heat coming out of the box can become a problem. That's why so many console modules run the first tube at part-mA and the second tube at few-mA. That's enough to get -10dBm output, and -10dBm is enough to drive a fader and a mix network. Then another -10dBm mike pre, a master, and your +30dBm line-out module.  

The last large tube console I hacked came out of a TV truck. In parts of the USA, the sun on a truck roof is already more heat than humans can stand. Since this truck was also full of tube TV (including two Ampex 2" videotape machines), I don't think the audio heat was the biggest problem. But you also found quite large tube consoles jammed into quite small booths.

> High input C rules out 1:10 trannies

Ah, yes, I overlooked that.