Tube Compressor

[J M Fahey]

[facepalm] I've seen worse !!

[soundmasterg]

I'd recommend to get the "Recording the Beatles" book as they go into extreme detail about the recording setup and equipment in use at Abbey Road at the time. By that time they were using the solid state mixing desk, but still using lots of glorious tube mics and the Fairchild compressors were used on just about everything, but EMI modified them so who knows what the real schematics of them are. Anyway the book is super excellent and will probably help you on your quest to build a tube compressor.

Harrison most likely used his 1957 Les Paul with PAF's known as Lucy on that recording for the solos, and by then they were largely using Fender amplifiers, probably the '68 Twin Reverb or maybe the '63 Bassman. They were recording to an 8 track tape machine too by then.

Greg

[Austin]

From here: Circuits


Saw these yesterday and reminded me of this post.

[exclamationmark]

I think I accidentally made a tube compressor whilst trying to work a Heptode a long time ago... I didn't actually know what I was doing, but it sounded pretty neat!

[Enzo]

Heptode - a beatnik amphibian?

[Jazz P Bass]

The Heptode.
What a wild application specific tube.
(the way it was intended to be used by the designers.)
5 grids.
Yeaha.
The Heptode

[Enzo]

We used to call them pentagrid converter tubes, as they were used mainly in things like IF circuits .

[nickb]

Out of interest, and because I'm too lazy to calculate it by hand, I did a simulation of a 12AX7 Voltage Gain vs Grid Voltage (Plate Load= 470k, Vsupply=250v). Interesting wide range. I had no idea it was so big. Hope I got it right...



Makes you think, hmm...

[lowell]

Whoa wild!

Austin, I do not for the life of me get how the 2nd stage in that "electronic compressor limiter" schem works at all. There is no DC supply on the anode pin1 !? How can that tube conduct at all with that being the case? Or am I missing something.. most likely.

[exclamationmark]

Whoa wild!

Austin, I do not for the life of me get how the 2nd stage in that "electronic compressor limiter" schem works at all. There is no DC supply on the anode pin1 !? How can that tube conduct at all with that being the case? Or am I missing something.. most likely.

I think the 1Meg cathode resistor is probably indicative that the currents involved are really quite low. The HT supply appears to be an AC voltage that is coupled from the first triodes cathode (it's a cathode follower, though it doesn't appear to be biased at all! - it probably doesn't need to be since it should have adequate headroom for low level signals, <2v). The second triode then sees an AC voltage on it's plate and grid that are identical, except that the plate source actually has some current driving capacity. I think the first triode is purely some sort of current buffer, though it may have some compressive (clipping?) qualities due to the lack of bias. As to what happens when you have an identical signal on plate AND grid? I don't actually know... Furthermore, the signal on the plate of the second triode would only be perhaps one or two volts, which probably necessitates that 1 meg cathode resistor. It then goes on to another cathode follower, DC coupled to gain stage (that again has no bias!), that mixes the signal with the original signal. Weird design for sure.

[paggerman]

We used to call them pentagrid converter tubes, as they were used mainly in things like IF circuits .

Ooh. I picked up a box of valves a while ago to get some 'useful' amp building ones - and there were several heptodes and pentagrid converters. They're in a box under the stairs right now. But I'm interested to see where this thread goes....

cheers

[nickb]

I think the second section of the first triode acts as a diode. The second section of the second triode acts as a variable resistance.

[J M Fahey]

Out of interest, and because I'm too lazy to calculate it by hand, I did a simulation of a 12AX7 Voltage Gain vs Grid Voltage (Plate Load= 470k, Vsupply=250v). Interesting wide range. I had no idea it was so big. Hope I got it right...

You did !!
Simply, tubes compress on their own, even if you don't want to.

[twiggybush]

Ever put a photo resistor across your input jack attenuated from a light bulb driven by the speaker output? It ain't La2a but it works quite well and it's clean.

[lowell]

Huh? Wha? Sounds interesting. Question, do you use a opto-coupler as in fender vibrato circuits? And can explain in a bit more detail the connections? Is the bulb in parallel with the speaker? Or series?