6AU6, 6U8, 6CL6, plus UTC A-10 make Mic Pre? - diyAudio
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Old 2nd October 2005, 08:36 PM   #1
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Default 6AU6, 6U8, 6CL6, plus UTC A-10 make Mic Pre?

I bought this intesting looking device built by RCA. It is a Sound Diplexer Modulator, MI-26493-B Type TSD-3A. Its built almost inside out, all the electronics are mounted to a single plane. No lids or front, or rather the front panel IS the back panel. Very odd. Anyway, it comes with a UTC A-10 transformer which I know is fantastic for mic pres, but the tube complement is 6AU6, 6CL6, OA2, and 6U8 x3. It also has what appears to be an output plug-in transformer labeled merely IN2389 and lastly an unlabeled power transformer. I am interested in what this device was originally for. I bought it for the UTC xfmr anyway (plus it has a nice meter) so I have no desire to use it for its intended purpose. I would like to make it into a channel strip. Ideally I'd like to construct a mic pre/DI with an EQ and maybe a compressor limiter section, even an internal headphone amp. Here is my question(s)... Can I do this with the tubes that it already has? If so does anyone have some schematics I can use? If it would be easier to just swap out for more common tubes and gut the device (I will probably gut it anyway unless some collector catches me before I do that) let me know. Thanks in advance.

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Old 3rd October 2005, 06:31 AM   #2
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Default Re: 6AU6, 6U8, 6CL6, plus UTC A-10 make Mic Pre?

With the input & tranny the 6AU6 might well be enuff.

6CL6 is a small power amp tube (is the other trafo an OPT?)

Click the image to open in full size.

The OA2 is a regulator tube....

and the 6U8 is a triode/sharp cutoff pentode -- don't how useful that is.

community sites t-linespeakers.org, frugal-horn.com, frugal-phile.com ........ commercial site planet10-HiFi
p10-hifi forum here at diyA
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Old 3rd October 2005, 08:05 AM   #3
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The Sound Diplexer Modulator was actually an FM signal generator. It generated a frequency modulated RF subcarrier which provided the audio on a microwave television signal.
The subcarrier frequency was above the highest expected video frequency. Today, the audio subcarriers are in the range of 5.8 MHz to 6.8 MHz.
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Old 3rd October 2005, 04:00 PM   #4
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Default MI 26493 B

This is an audio subcarrier modulator and sub carrier generator.
In other words it takes the audio input ( 600 ohm balanced ) thru the A10, amplifies it ,and then FM modulates it with the sub carrier which is generated on board. The subcarrier freq. is somewhere between 5.8 and 6.4 MC. and may be adjustable, but it should be on a label on the board.
It was used on RCA Video microwave units for closed circuit TV broadcasts from a remote pickup site to the main studio.
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Old 5th October 2005, 06:20 AM   #5
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> it comes with a UTC A-10 transformer which I know is fantastic for mic pres

I know I'll offend somebody, but not so very long ago we figured an A-10 was just good enough for a classroom intercom, hardly special. When even that market died, the local electronics shop closed them out at $0.50 each. I took two, knowing I'd never need more than that.

Actually, for the reasons you would want a transformer-tube preamp, the A-10 is fine. And I now wish I'd bought the entire box and salted them away for my retirement. (Who knew?)

> 6AU6, 6CL6, OA2, and 6U8 x3

Buncha tubes.

6AU6 is a great RF/IF amp. It was often used as an audio amp, but not specified as such. Some samples will be fine, others will have excess audio-band noise. Common as dirt.

6U8 is a fine little tube, good for a lot of utility work. Billions were made, so they are worth about a buck. Don't obsess about ordinary TV tubes.

As dave says, 6CL6 is a small power tube. Specifically, a video amplifier, meaning it can make around a Watt at past a MHz (enough to slap the stray capacitance of a CRT grid). In fact a search at http://frank.pocnet.net will pull-up the data on all these very-common tubes: 6CL6 can make 2.8 Watts into 7.5K, and only 3V peak on the grid. Or at 4MC bandwidth: 132V peak-peak into 3.9K, with 3V p-p on the grid. Lots of gain. Still, it is just a better-grade TV tube, worth a couple bucks.

You do seem to have a power supply, which can surely be used for an audio amp.

You have way too many tubes for a sane mike-amp. You don't have anything useful for a tube compressor. Anyway a studio-interface box will need a costly output transformer, and a tube limiter needs at least one and very preferably two very well balanced interstage transformers.

At a glance, I can see a dozen ways to get to a mikeamp, maybe with a mono headphone output. Unless you can settle for hi-Z unbalanced output, all will need expensive iron. And while I like to potter with ideas, I'm not inclined to spend hours or weeks working around an odd lot of tubes. Find some existing plan using an A-10 and a 300V power supply and see what more you will have to get. Some of the tubes might be similar enough to be substituted, but no big loss if they aren't. The big expense will be Output Iron.

As a starting place.... wire the 6AU6 as a Triode, except with Plate grounded and G2 working as "Plate". This gives a low-power voltage-amp triode with good shielding (because the outside plate is grounded) and Mu around 40. Plate resistor like 100K, a K or so of cathode resistor to set plate voltage about half the stage supply voltage. Then you probably have to go to a 250K pot to control gain: even working Triode you will have a lot of gain. Then on to the 6CL6 wired conventional triode with a 10K:600 output transformer and bias like dave shows.

A-10 has voltage ratio of... google... 1:18???? (kinda steep, which is why it is flavorful). The 6AU6 triode will give gain over 25. 6CL6 triode, about 15. 10K:600, about 4:1. Total gain is 18*25*15*0.25= 1,700 or 64dB. Input overload will be too low for German Broadcast condenser mikes: this is a dynamic/ribbon mike preamp (the A-10 would be distressed by the output of my 414s). Because both stages work as triodes, distortion, bandwidth and output impedance will be fine.

At prodigy-pro.com/forum there are some folks who play with creative studio gear, and some who know the color of the insulation inside each of the several A-10 production runs.
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