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5703 tube vs. AC701(k)


I few months ago I won the auction for JAN 5703 tube and the seller send me the instructions how this tube can be interchangable with AC701. What are your opinions on that people except that this tube runs on 6.3V ? Does someone try it in microphone project and how does it compare with 5840 and of course why is 5840 wired as triode and no one using 5703 as natural triode ?
Thanx and best regards.


anonymous Wed, 01/28/2004 - 07:09
Not that I want to pollute your conversation with talk of hi-fi...

The follow-up product to Musical Fidelity's "Nu-Vista" (sic) range is the "Tri-Vista" which, you guessed it, has 5703 triodes buffered by transistors. Apparently they bought several thousand 5703 as they did with the 6CW4 (Nuvistor used in the Ampex mr70 btw), with a spare pair for every customer.

That's my piece of trivia for the day.


anonymous Wed, 01/28/2004 - 13:50
From reading what I can find on the ac701. It appears this tube was made for microphone use. The grid leakage current is very low. The grid exits the other side of the tube. It has double mica washers for lower grid current. A certain oxide on the cathode etc. Problem for me the good info is in german.

This would be a good tube to have made again. It might be the best tube for tube microphones ever made.

anonymous Thu, 01/29/2004 - 06:21
Originally posted by Fred Forssell:
the 6CW4 (Nuvistor used in the Ampex mr70 btw)
FWIW, while the 6CW4 Nuvistors were used in the mike preamp section of the MR70, the record and repro sections did not use them. Those sections used 7897 triode and 7587 pentode nuvistors.

Fred Forssell [/QUOTE]Thanks for that info Fred. I have a mr70 schem, should've checked it before posting here!

Out of interest, have you used the mr70? Does it live up to the reputation?


fred01 Thu, 01/29/2004 - 09:04
Out of interest, have you used the mr70? Does it live up to the reputation?
Back in the mid to late 1970's we had several MR70 machines in a couple studios in which I worked when I wasn't on the road. We had a couple of MR70 2-tracks, a 1/2" 4 track and a rare(even for MR70s) 1" 8 track. (An aside, someone told me at the time that the 8-track MR70s were built for the United Nations to record multiple translations simultaneously, and that there were something like 30 of the machine built for the UN. I don't know if that is true or not, but it sounded reasonable at the time.)

Sonically, the machines were really, really good. We never used the mic preamps and truthfully, I don't even remember if any of those machines had mic preamps installed.

When properly aligned and maintained, the transports were really cool to use as well. But the transports needed lots of continuous tweaking to keep the running well, and it was kind of a pain. This was back in the days when studios had full time people that did nothing but maintainance and tech work (our studio had 3 full time techs), so it was too big of a deal to keep the MR70s, the Studers and the other Ampex machines running well.

But the sound of the MR70 was really great. I don't know what ever happened to those machine after I moved on.

Fred Forssell

anonymous Thu, 02/26/2004 - 18:12
one mic that used the 5703 originally was the capps 2001 system- a sort of mid-level American tube condenser from the 60s (i think). had an aluminum diaphragm. i've never seen a real schematic, but based on some commentary on by Scott Dorsey, i've drawn up a rough one:

the thing that is interesting is that because the voltage at the grid is so high, the capsule can be directly polarized with it. no pull up resistor or blocking cap needed.

scott recommended maybe bypassing the 47k resistor to reduce feedback going into the grid. he warns that getting the right combo of resistors to get a good polarization voltage on the grid while keeping the tube running as a follower may be tricky. also, it might work better with a 1Gohm resistor subbing in for the 200Mohm.

best of luck to anyone who tries this circuit. this is everything i know about it. i'll probably get around to trying it eventually, and report back if i do.


rmburrow Mon, 05/27/2013 - 19:07
Capps mic using 5703

I own a Capps CM2250 condenser microphone and p/s and I have the paperwork. The aluminum diaphragm collapses after a few years. Guaranteed. I had another Capps years ago and that one had a collapsed diaphragm. The diaphragm is spring loaded so when you reskin it, you need a fixture to compress the diaphragm material against the backplate, which will simultaneously load the spring. I want to experiment with gold sputtered mylar (if I can find some), but experiments with electrolytically grown thin nickel give nice results.

The nice thing about the Capps is that the diaphragm tension is predetermined when the diaphragm material is compressed onto the capsule and the clamping ring affixed. You don't have to pre-stretch the diaphragm material and "tune" the tension for resonance at 1500 Hz or some other frequency before clamping it...

The power supply is a full wave 6X4 circuit supplying 225 volts DC for the B plus, and there is a 6 volt DC supply for the 5703 heater. The heater supply is "floated" through a divider off B plus to stay within heater/cathode voltage rating of the 5703. The mic cable uses the big 6 pin Cannon plug and Belden 8426 type cable. Wonder if you can still get Belden 8426? The Mogami cable should also work...

The 100 megohm polarizing resistor in the Capps is carbon...replace with metal film...same deal with other resistors in the preamp for better noise. Do NOT change the values of the 390 ohm and the 47 k ohm resistors (in series) in the cathode circuit. The 47 k ohm resistor determines the bias voltage for the capsule, and the 390 ohm resistor determines the grid bias for the 5703. The capsule is polarized by the IR voltage developed across the 47 k resistor; namely, the product of the 5703 cathode current and the 47 k resistance. Note....the capsule polarizing voltage in the Capps is significantly higher than the nominal 60 volts used to polarize most tube mic capsules.

Nothing like nine years later on this thread but later is better than never....Capps has been out of business a while, but these mics can still be repaired....



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