Tube Decibel Meter into Mic Preamp

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tablebeast

Well-known member
Joined
Feb 8, 2005
Messages
145
Location
Forest City, NC, USA
Hi, I just got back from Hamfest in Shelby, NC. I did get one big score, this 4U Rack Mount Military-Grade Decibel Meter Model ME-22A/PCM. I was told my one guy at the fest that it could be made into a Mic Pre easily. Is he right?

Here is what I have:

There are 8 tubes, as follows:
3x 6SJ7WGT as Audio Frequency Amplifiers.
1x 6H6 as the Meter Rectifier
1x 5881 as Voltage Control
1x OC3 as Voltage Control Regulator
1x 6SQ7 as Voltage Control Amplifier
1x 5Y3WGTA as the Rectifier

All the tubes are JAN. Every Part is top quality and it was built by Borg-Warner. I think its from 1959 by the date stamped in the input transformer. The Freed input transformer has a 600 ohm primary for balanced input with a switch that flips to bridging (for unbalanced input?). It is a HUGE transfmormer, check out this picture of the input transformer!
The power transformer is equally huge. Check out this picture of the power transformer! A pretty hefty unit overall and one heck of an overbuilt Decibel Meter.

So, here is what I want to do. I would like to make it into not just a mic pre, but if possible a channel strip with compressor and EQ circuits that I can switch in and out of the signal path. I don't want it to be any normal kind of mic pre. I'd like to be able to adjust the gain on each of the AF amplifier sections seperately for more control over distortion. I'd also like to use the Voltage Control circuit possibly as a compressor of some sort. The transformers are Freed so I will probably be looking for a similar era Freed output transformer to go along with this project.

My questions are:
Is this going to work as a mic pre? How about the other stuff like EQ and compression? I have the time and willingness to pull this off if the existing circuit (Here is the full size schematic) can be relaced and remade as a Mic Pre. How feasible is this? What parts need to be replaced? What needs to be removed/added/moved around? How much of the exisiting circuit can I reuse for my purposes? Can I reuse this decibel meter's meter to guage audio levels?

Even if I have to gut it and completely rewire a whole new circuit that's fine with me, I'm just looking at the shortest distance between this thing and a mic pre. I figure it has a meter, power transformer, input transformer, audio tubes, and the case. Even if I have to replace everything else that would be fine. I would prefer to reuse as much of the exisiting parts as possible, though.

Thanks so much for your help in advance. Here is the page with all the pictures and a copy of the schematic. DB Pre Project Page

dbschem02.jpg
 
nice "find" :thumb: should make a fine micpre - but you need to find a nice output transformer for it - something like 40k to 600 ohm. try connecting the primary to c7 and give it a listen (disconnect from the meter rectifier for now). you might think about adding some feedback to the circuit. ahh, and a pot for output level in between v1 and v2.
 
Anything can be a mike preamp.

This box is very stable and very accurate, but not lowest noise, and does not have a Line Out or even a good place for one.

Remove the 6H6. Connect a 5uFd 400V cap to V3 Plate: the other end is a ~10K line out with max output maybe 60VRMS.

Gain from V2 grid to V3 Plate is maybe 150; from V1 grid to V1 plate maybe 70, overall gain in tubes maybe 10,000 or 80dB. Voltage gain in input transformer is 1.58, or 4dB. You will almost certainly need a 10K:600 output transformer, gain of 0.25 or -12dB. Overall gain 4dB+80dB-12dB= 72dB.

From there you can go down in 10dB steps to 32dB, BUT you are throwing away signal to noise ratio. This is a meter amp, meter only shows 23dB, the amp only needs 23dB S/N, tape needs S/N of 40dB, 50dB or more (to 96dB if you believe in digital).

The attenuatior is very high precision but only makes sense where it is: at the input, wasting signal.

> a switch that flips to bridging (for unbalanced input?).

It is, in either position, a floating input. It can be connected to balanced or unbalanced, makes no difference. (Some funny little thing going on with C2, but it will not matter.) In "600", it puts a perfect 600 ohm load on the source. In "Bridging", it appears as a load much higher than 600 ohms (about 7,800 ohms).

> I'd also like to use the Voltage Control circuit possibly as a compressor of some sort.

???? That's a Voltage Regulator. Is nothing like a compressor.

I'm baffled why it has a 6L6GAY. Yes, the input and feedback stages need regulated supply for +/-0.1dB stability.... but the 0C3 takes more of the 6L6GAY's regulated output than the two 6SK7s do. I suspect it is over-designed to jack-up the price on a no-bid contract.

The regulated supply voltage is probably 300V.

> insane, look at the output voltage (750V)

750V CT. 375V each side. With 5Y3 and cap-input filter at 20mA-40mA, about 470V-500V DC output. That tells me C11 and C10 are oil caps, worth a pretty penny. However they may not be making DC clean enough to listen to. If you need more filtering, electrolytics are much cheaper, but are scarce above 450V.

Oh, golly. It isn't even a VU meter.

> a channel strip with compressor and EQ circuits

Well then, it isn't "easy". You have a box with sockets. Maybe you can leave the heater and main rectifier wires alone. Almost everything else has to be re-thought and re-worked. And there isn't enough "stuff" there for your plans, unless you want an exceptionally lame EQ and limiter.
 
Really, man...

This one has all it takes to become a nice vari-mu compressor. I like using high gain vari-mu circuits with a high ratio transformer at the input, so I can feed it with a microphone. It becomes my micpre-varimu comp in the same box.

It´s almost a varimu compressor, with all these tube sockets, 4U rack, big power transformer. There are many nice projects you could use, like the RCA 6BA6, or the Gates STA level, or anything else...

You would have to rewire it all, for sure. Some people uses single ended transformers as pushpull tube ciruit inputs. Even Langevin did it, what the hell... It should work just fine with very simple little changes in the original RCA circuit.

Or you could just turn that input transformer around, with the 2kCT facing the grids, and the 5k facing the input, and feed the thing with hot line level signal. That way, you have a balanced secondary and a center tap to feed the CV. Also, a step down at the input would be nice, because most vari mu circuits have too much gain to work at line level inputs without attenuation...
 
Wow, thanks for the information so far. You guys are infinitely helpful. By the way I am going to take pictures of all the steps of this project and build a page on my tablebeast.com web site around it. I figure if people can re-lace tube PA's and organ amps into guitar amps, then why not turn this kind of stuff into pre amps. I'm looking for unique sounds, not necessarily pristine or even 'good' by normal standards. I have plenty of clean, clinical pre amps. For vocals I like double tracking and using one clean pre along with another pre that has a rawer, dirtier sound and then blend the two to taste.

Ok, so let me reflect on the responses to this project so far (correct me if I'm wrong on any of these points)...

1. It CAN be made into a Mic Pre. Maybe even a very good one.
2. It can't be made into a channel strip with a mic pre/EQ/comp, but...
3. It can be re-made into a vari-mu compressor/mic pre.
4. These oil filled caps are valuable and the whole thing is overbuilt to the point of ridiculousness.
5. The voltage is not insane.
6. The impedance switch is useful for switching to two different loads.
(I like the idea of using it as a mic pre/vari-mu comp in the same box. That is exactly where would like to take this, I guess I was just overreaching by trying to add EQ.)

Ok, I have some new questions as this project starts to take shape.

1. What does everyone think should happen here: Build a Mic Pre using mostly what is already there or have it completely remade into a Vari-Mu Comp/Pre?
2. The Gates STA Level schematic that I found Gates STA Level Schem. uses a 6386 tube and these are not only impossible to find but expensive. I'd like to keep this project on the cheap as much as possible. My question is can I use a 5780 or two matched 6BA6/12BA6 tubes instead?
3. Do these oil filled caps need replacing like old electrolytics?
4. How valuable are these oil filled caps? Could I sell them, replace them with electrolytics, and have better sound with some money left over for more parts? (I like the idea of selling off superfluous parts of this device to fund the purchase of new parts, I'm a recycler at heart which is why I'm doing this to begin with)
5. When I originally discussed using the voltage control circuit for compression I was thinking more of a simple coloration from an overdriven power tube. Since the 5881 is there I'd like to use it for some kind of effect. Is anything like this possible?
6. How do do this conversion to a vari-mu? (I don't expect step by step instructions, but a pointing me in the right direction to get started would help me out immensely.)
7. Where can I get a good output transformer for this at a decent price? (Besides ebay. And I'd prefer a Freed to match the others, but any decent 10k to 600 ohm 50's/60's era output iron would work).
8. Are there any sites on the net that have projects like this already documented?

Thanks again in advance. The wheels are turning...
 
1. What does everyone think should happen here: Build a Mic Pre using mostly what is already there or have it completely remade into a Vari-Mu Comp/Pre?

Complete remade into VariMu comp. A tube mic pre can be built in a much smaller case. This one is perfect for a bigger project like a vari-mu. It´s not easy to find a good looking big rack case just like this one.

2. The Gates STA Level schematic that I found Gates STA Level Schem. uses a 6386 tube and these are not only impossible to find but expensive. I'd like to keep this project on the cheap as much as possible. My question is can I use a 5780 or two matched 6BA6/12BA6 tubes instead?

many tubes will do. very easy to get substitutes for 6386, if you are going for two tubes instead of one tube. I can send you a list later. You can find these tubes anywhere, for very low values, like $3-5 each. So, you can just buy a bunch to get good matches.

Or maybe go for an RCA circuit, without 6386s. You see, you have some octal sockets in there. You must use octal tubes. Federal and RCA used the 6SK7. You could also try an octal pentode straped as triode, if you wanna do a 6386, or even a fairchild-like circuit. There´s a thread about a 670 with a solidstate side chain.

3. Do these oil filled caps need replacing like old electrolytics?

There are cheap oils caps that go bad all the time, but yours are canned. Should be allright.

4. How valuable are these oil filled caps? Could I sell them, replace them with electrolytics, and have better sound with some money left over for more parts? (I like the idea of selling off superfluous parts of this device to fund the purchase of new parts, I'm a recycler at heart which is why I'm doing this to begin with)

You got the DIY bug now. Save it for another project, if you do not use it.

6. How do do this conversion to a vari-mu? (I don't expect step by step instructions, but a pointing me in the right direction to get started would help me out immensely.)

There´s no easy way. Look at the schematics you will be following. Make sure you try to understand it as much as you can. It may not look so easy in the beginning. As much as you can understand it, the better for building it. Make sure you understand grounding, PSU, etc at least... Then, unsolder all the wires and parts from the unit. Buy new parts for your project and solder it in place. Make sure you get some basic layout hints. Read the forum.

I think the expensive part in the ´project is done already. OK, maybe you do have to buy some transformers, some caps and resistors, but this is nothing. The expensive part is the case, and all the tube sockets already in place, and this you got. You also have what looks like a pretty healthy power transformer, already in place. In DIY, the expensive part is actually your work, not the parts. Say you will spend $200 in parts, and that includes a all the resistors, caps and transformers you need for a full working varimu comp. That´s nothing. The metal work is all there. the hard part is done. Buying good parts and soldering it in place is actually the fun of all this.

7. Where can I get a good output transformer for this at a decent price? (Besides ebay. And I'd prefer a Freed to match the others, but any decent 10k to 600 ohm 50's/60's era output iron would work).

As you have all set up already, go expensive for the transformers. I would buy a complete audio transformer kit from Sowter. They make great iron. You could buy all the iron you need for a RCA BA6A, a Gates STA Level, or a Fairchild from sowter site. If you are in USA, use our hosts, Prodigy Pro. They are the ones that makes this place possible.

And you still save some dollars, because you do not need the power transformer. You have a nice looking shielded one.

8. Are there any sites on the net that have projects like this already documented?

I´m sure there is lots of them. You gotta have a lok at the forum. There are METAs.

Ok, I would use it for a gates STA level output:

http://www.sowter.co.uk/acatalog/SOWTER_PRO_VINTAGE_TRANSFORMERS_5.html#a190

The sowter 9185.

And for the input, if you want to feed it with a mic go for this one:

http://www.sowter.co.uk/specs/9045.htm

These are the ones you want.

If you wanna feed it with line levels eventually, just include the original STA level pad before the iron, with a switch.

There´s also a simple mod in the STA-level. I´ll draw it up for you. It make attack-release and ratio all variable. It´s very simple.

Spend the money on good iron. It´s not really LOTs of money. You end up with a very nice micpre/varimu on par with a $3.000 dolars unit. And remenber, soldering it all together is the fun part, but you have to get a clue on what you are doing...
 
Another thing you need to check is the frequency response of that input transformer. I once bought a Hewlett Packard patch panel attenuator system designed to feed a voltmeter. And the frequency response is atrocious. Some of those military decibel meter units may only have a 5 kc frequency response!

BK
 
I'm getting more excited about this with every post. Ok, so its decided. I will build this into a wicked mono Vari-Mu Limiter with line and Mic inputs. I will use all octal tubes and reuse as many of the original parts as I can but not skimp on the new parts I need. I just bought some Neutrik XLR's. Next will be to get a ahold of the RCA BA-6A and Federal Schematics. Then I can get the rest of the parts and dive right in. So, now for some new questions.

1. Should I plug this thing in as it is first and test it for its stock functionality? Can I do any harm this way?
2. Is 10k:600 my only output transformer option? Or was that just if it was solely a mic pre? What kind of output transformer do I need for the Vari-Mu? (I know rafa pointed me to the new Sowters, but I mean more specifically what do I need to look for in old iron? I'd like to keep this as period as I can with the iron if I can and use the Sowters as a backup plan.)
3. The input transformer that's already in there, can it be used as a line input? Can it be used as a Mic input? Can it be used for both at the same time with a pad for line input?
4. Does anyhone have the Federal AM864 and RCA BA-6A Schematics they could share? PM Me.
 
Another thing you need to check is the frequency response of that input transformer. I once bought a Hewlett Packard patch panel attenuator system designed to feed a voltmeter. And the frequency response is atrocious. Some of those military decibel meter units may only have a 5 kc frequency response!

Ah, good Idea. I figured it would have a wide response for wide testing ability. How do I check this? I think maybe the one side of the transformer that I can't see has some info on it. I will try to take it out and see.

Jesse
 
[quote author="tablebeast"] Does anyhone have the Federal AM864 and RCA BA-6A Schematics they could share? PM Me.[/quote]
You sound like you're gonna drive yourself crazy with this thing, man.
http://www.waltzingbear.com/Schematics/RCA/BA-6A.JPG
http://www.collinsradio.org/html/manuals.html
Search around for the vari-mu compressors in the Collins link.
http://www.groupdiy.com/index.php?topic=11681&start=0&postdays=0&postorder=asc&highlight
And this whole thread is dedicated to the Federal AM864/u comp, schemo and all.

analag
 
I think you should still give a good look at the STA Level schem.

The collins schem is nice for a simple limiter too, but has only two stages, and less gain than the Gates.

Federal is all octal and you have a 6SQ7 there alresdy, so might also be a good plan for your case.

RCA is too complex for a first time project. really.

Tell me your email and I´ll send you schems.
 
Since it is already a 'Federal' piece of gov't sanctioned gear and because it seems to be a better project, I am going to build the Federal AM-864/U circuit into this case. It already has the octal sockets so what the heck. I went ahead and ordered some tubes from ebay. I still need a 6SN7WGT, but I have all the rest AND they are all JAN. I want to use all JAN tubes. I want this guy to be all US military as much as possible (modern USA made caps and resitors will be good enough for that).

Ok, so now I know that I want to put the Federal AM864 circuit in here, but that takes up 5 of the 8 sockets, two of which have the tubes already in there. Then I was thinking, why not ALSO use the three 6SJ7's (which are already there) as a full fledged Mic Pre Amp Stage? Thay way I could have the Vari-Mu and Mic pre all in one box. I would have XLR inputs and outputs for each circuit, with transformers on each I/O. I would also have a switch that would jumper the circuits together. That way I can leave the Federal Line Level and if I'm buying a mic input transformer anyway, why not make a seperate circuit (I'm thinking like some UA-LA610 kind of thing)? Then I will have my 'super-channel' and I will use it for vocals, guitar, or keys when tracking live and as the go-to input channel for overdubs. I can even split it up for live tracking using the Pre in one place and the Limiter in another.

Now for a few questions?

1. Is there any reason why I shouldn't utilise the 6SJ7's that are already there ALONG with building the Federal Limiter circuit next to it?

2. Is there an American manufacturer that makes the caps and resitors I wil need? Even affordably priced NOS would be fine. Links?

ALSO, a little background on me. I know I just jumped into this forum and started up a seemingly big first project. Since we haven't been formally introduced, hi my name is Jesse and I am better known on the net as Tablebeast. I have been messing with electronics my whole life and I build circuit-bent keyboards for a living. That affords me a lot of time to experiment. I have tons of time to read and study new stuff and over the past few years I have been reading more and more about tube based elctronics. So, I am hip to a lot of the science, its just the actual physical application that I am doing for the first time. I have built, repaired, and modified plenty of solid state stuff from Casio SK-1's to my friend's MOTU 828 mkII (after getting my 828 done by Black Lion and seeing how easy it was). I have a real desire to learn this tube stuff to not only rebuild and clone old stuff, but to eventually design and build my own gear. Anyway, thanks so much for the advice and the help, having gurus like you guys around is a godsend. I have to warn you guys that once I start rolling with this tube thing, the projects will get more frequent. I get a kick out of making stuff do things it was not meant to do. Not to get ahead of myself, but next on the chopping block another 4U gov't beast. A Signal Corps US Army Frequency Meter BC-638-A made by Bendix in Army Green...

Jesse
 
Ok, so today I took the mounting nuts off of the input transformer to see what was on the side I couldn't see! Here is the input transformer. Looks pretty damn good as far as frequency response. After someone said it may have 5kc bandwidth I had to check. So, there it is 200 to 35kc within .5db. This may actually be perfect for vocals and guitars as I usually use a hi pass at 150 to roll off the bottom anyway. And look at that high end! So, this discovery leads me to a few more questions.

1. Since this has a center tap would it be best to keep this tranny and flip it around with the CT facing the tubes for the line input of the Federal? The schematic says it wants a 600/10kCT, will mine work? What will I have to adjust to make it work?

2. Would I maybe be better off using this transformer as the output of the Limiter, again flipped so that the CT side faces the 6SN7?

3. Is it at all appropriate as an Input transformer? I mean isn't it a bit of overkill? Wouldn't it work better as an output transformer considering its +25 dbm power level and frequency response?

I know I keep asking questions before the previous ones are answered, I'm just stoked about this and ready to get going with it! Now that I have the rough ideas worked out its time to hammer out exactly what I plan to do so that I can order all the parts and build it!

Jesse
 
1. Since this has a center tap would it be best to keep this tranny and flip it around with the CT facing the tubes for the line input of the Federal? The schematic says it wants a 600/10kCT, will mine work? What will I have to adjust to make it work?

yes, it will work very well, and you will probably have much better Frequency response, cus you will feed it with much lower impedances than 5k anyway. It will certainly work very well as you line input. Just make sure it´s inverted, with the CT facing the tubes. You need the center tap for the control voltage and good balancing/cmrr.

2. Would I maybe be better off using this transformer as the output of the Limiter, again flipped so that the CT side faces the 6SN7?

Nope! You need to buy an output transformer. I would really go high grade here, but a big edcor may work great. I would try a 20k:600 instead, but that´s just me. It will make the 6SN7 tubes more confortable and you get a lower level, more into what we use as line level today.

3. Is it at all appropriate as an Input transformer? I mean isn't it a bit of overkill? Wouldn't it work better as an output transformer considering its +25 dbm power level and frequency response?

It might work as an output transformer, but you wiould have too high output impedance, and too high level for this circuit. Use it as an overkill line input. I bet it will work GREAT! What´s wrong with overkill? :green:


The mic pre idea is nice. I would´t include a swtich for jumpering the circuits together. If you want to use it together, just patch it together in your patchbay like you would do with two separate units.

If you still have to buy some 6SN7s for the project, you could also get crazy and buy four 6SN7 total, and then have a look at the NYDave 6SN7 one bottle micpre and go fopr a full unit, with three mic preamps (each preamp uses just one 6SN7, and 10-12mA of current from your power supply. make sure your power transformer can handle it... Whatever you do. Maybe you will need to ad another power transformer just for the mic preamp HV... Maybe you will need more power for heaters. 6SN7s are heater hungry, at 600mA. This is becoming a big project. Gota study and think about everything...
 
I think using some 6SN7's for the pre section are a good idea. I just got a few, they're not JAN, but they are nice Raytheons and they are matched with identical readings on both sides of the bottles and the four of them are matched to each other, so I could do some very interesting things for a Mic Preamp. Since I have three octal sockets to use for the preamp, does anyone care to suggest a circuit? I was considering doing the 'three bottles, two pres' 6SN7 circuit I saw on another thread. I'm open to alternate suggestions. As for transformers I was looking at the Edcors and couldn't believe the prices, freaking cheap! So cheap I wonder if they're any good, but I hear people on here praising them so I'll give it a shot. I'd rather save some dough and try Edcors for now, then maybe get some Sowters later once I work everything out with the cheaper transformers. I'm going to use the Freed in there as the line input for the Limiter. So, for transformers I'm looking at Edcor XSM600/10k for Mic Inputs(s), XSM10k/600 for both the mic and limiter outputs. I noticed that their WSM series had a 20-30k response and the XSM had a 20-20k but were more expensive. What's the difference in these two series? Pros and cons of each? Which ones should I go with? Thanks for the help so far.

Jesse
 
I was considering doing the 'three bottles, two pres' 6SN7 circuit I saw on another thread

Post links so that we know what you are talking about...

You need good nickel core shielded transformers for mic inputs, not edcors.

edcos wsm will do a wonderful job at the outputs.
 

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