First post Auction Amp help

[okguitarman]

 Mike is my name I am a Electrical contractor in OKC that loves tube amps and guitars.
Just picked up a old tube amp 10.00 At auction (Commex brand) also Voycall 8TWD label, it has 1- 12ax7,1- 6l6 and a 5y3. All RCA tubes!
any help identifying power tran. no name part # 2182A.
Would like to convert to guitar amp any info on this amp would be appreciated! Cable input and output 3 wire.

                   Thanks okguitarman!

[sluckey]

Show us the back side and inside. Where is the OT (output transformer)? Any idea what this was part of? Is the grey shielded twisted pair cable connected to it?

It's probably not necessary to identify the PT. Just measure the secondary voltages. There should be 5V and 6.3 volt windings and a high voltage winding, maybe 250-0-250, or 300-0-300, something like that.

Could be a good candidate for a big Fender Champ. Gonna need an OT for any guitar amp.

Are you affiliated with the FAA or the Aeronautical Center?
 

[PRR]

It probably already IS a guitar amp, near enough.

You *know* the power transformer, output transformer, and 6L6 were all designed around each other. You know the 12AX7 is there to bring small signals up so they bang the 6L6 good.

If it has a MIC input, adapt the jack.

However I am suspecting a Muzak amp. Background music for a store arrived  by phone line, this boosted it to store speakers. The odd not-transformer transformer thingie by the 12AX7 may be a teeny transformer in a large tranny case; the phone line interface. Also PA amps often had some style, this was meant to hide out of sight.

But all that can be modded with a few snips. It's clean. Please don't hack it apart.

Gut-shots as Sluckey says.

[casssax]

http://www.geofex.com/article_folders/old2new.htm

Have you seen this paper about converting old amps into guitar amps?

I helped me on my conversion project.

[okguitarman]

Thank you guys!  yes the output trans. is inside and the input and output are the grey cables both are 3 wire shielded cable with black,clear,and bare.
The on/off/volume pot needs replacing but it powered up has weak output I'm thinking bad 6l6.
looks like 10 or 11 watt output. I need to do some measuring and math for output impedance
I used a 8 ohm speaker for test. I think the bare ground is a drain. It also has a ground lug on back labeled connect to water pipe has 3 wire power cord, but might have came with a 2 wire power cord.

[okguitarman]

Inside.

[sluckey]

If that was mine, it would most likely be a 5F1 Champ or a 5F2A Princeton next week. Maybe even a Vibro Champ.

[okguitarman]

What all would I need to do? Champ was my thought but would you change to 6v6

[sluckey]

What all would I need to do? Champ was my thought but would you change to 6v6

Stay with 6L6. Follow the link at the bottom of this page to the "Tube Amp Library of information". You will find a Champ project there. BTW, the Princeton is just a champ with a tone control.

[okguitarman]

Cool, Much appreciated! Been to college for electronics and have repaired many amps built a few guitars also.
Some off my woodwork.

[eleventeen]

Boy that looks just great, and a great deal too. Congrats!


PRR probably has it exactly right. It wants a biggish input signal that a bare guitar cannot supply. It's possible, as you suggest, that the tubes are weak, after all, it was probably left on continuously for years. But sometimes tubes operated well under their capacity can have astounding lifetimes of service. I just bought a Peavey 100 watt head for $50 where a filament fuse had popped. Solid state preamp & reverb driving 4 x 6L6's, Sylvanias. So it was a 10 cent fix. I actually have no great use for the amp, I bought it for the 4 x 6L6 tubes which ended up testing 95% new, excellent, even though they are marked Peavey and probably the original tubes and maybe 20 years old.


Prowl around this site and you'll get some excellent clues as to what to do. With an absolute minimum of work you'll be able to make that a 6L6-fired Champ. You might change a grand total of 2-3 resistors in the preamp stage to adjust the gain.


The power transformer on that is huge, it could handle a push-pull pair of 6L6 to make a 45-50 watt amp with no problem if the volts are high enough. The current capacity is obviously enough. It even looks like there is a cover plate covering a hole which would accomodate another tube socket. You'd have to change the output transformer which is obviously for single-ended as it sits. That would possibly force to change driver (1/2 of the 12AX7) configuration and then you would be getting into "real" mods, but nothing NASA-grade.

Assuming it doesn't smell burned, you might consider building what we call a "light bulb limiter" which is an easily built thing I am sure you have the parts for laying around. This lets you plug in new-to-you stuff without concern that a blown e-cap will blow something up. Or you could plug it into a variac and gradually bring up the line voltage. Either way. In the case of very clean gear like that, I often just plug it in and hope for the best.

Once it stays lit up without flaming;

I would first try to power it with a tuner; which would ordinarily supply a hi-fi or stereo amp with nominally 1 volt "line out".  If that works and drives the output, you are in business. Flip a couple of resistors and you have your 6L6-fired Champ.

[PRR]

I think this OT is wound for a "25V" output.

Ass-uming 10 Watts output, the load should be 62.5 Ohms.

Do you have an ohm-meter? Meter across the "output cable", or the OT secondary.

An 8 Ohm winding will measure 1 Ohm or less. A 62 Ohm winding will measure 5 or 10 Ohms.

In real life, the "load" was many speakers, each with a transformer, to take the desired power (1W or 2W) from a 25V line.

The higher voltage means you can use thinner copper (saves money over long runs). The added expense of a transformer at each speaker is balanced against the convenience of adjusting each speaker's loudness by tapping the transformer. (If it is too loud in the office and not loud enuff in the stockroom, you can change the taps and make it better.)

The problem is: you probably don't have any 62 Ohm speakers.

An 8-stack of 8 Ohm speakers in series would be a fine fit, and loud; but way out of proportion to this amplifier.

You could go back to the store and snag all the speaker transformers; but they probably gone now and anyway these things suck.

A single good 25V:8 Ohm 10W transformer would be nearly the price of a known-good SE OT, without the double-losses.

I'm afraid you should find another OT. Since the PT is clearly ample for 6L6, you may as well look for around 5K impedance 10 Watts SE output. While our host may not have such (since Fender didn't go that way), I know I have seen such at other OT vendors.
_______________________

Why do I think it is 25V? Note the two 270r 2W across the output wire. Apparently center-tapped to ground. That makes little sense for a 4/8/16 output. However 540r is a fine "open load" protector for a 10W 25V (62 Ohm) output. (There's also 70V lines, but 540r on 70V would suck 9 Watts, leaving little for the speakers, until the 2*2W resistors burned-off from their 9 Watts.)
_______________________

Interesting build. Very for-this-job. Note the chassis has elaborate punching for at least one more power tube, here blanked-off. It is a universal chassis, not being used to the fullest.
_______________________

The "input cable" *appears* to go to two 0.1uFd to ground. If we ass-ume a 600 Ohm phone-line from the music supplier, this is about 5KHz low-pass. Reasonable to cut stray crap off the phone line. Background music rarely extends any higher than that. I assume from there it goes to the transformer(?) near the 12AX7, then into either the volume pot or the 12AX7 (depending how weak the signal was from the phone line). Much of this must go. Hold the small transformer: it may be a great "balanced line out" part some day.

[eleventeen]

You say you got another goodie at your auction? Let's see it!


The chassis and cage on the Commex amp you showed is quite reminiscent of several 5E3 (Deluxe) builds that a number of forum members have executed. This is a popular amp; I myself am not a big tweed fan, but most other people are.


You just have to decide what you wish to do with it. PRR says "don't gut it"....but it will never ever be a Muzak paging amp again. I think he means don't strip every single part out of it leaving a bare chassis. And if his notion that the output tranny is no good, then you are compelled to start spending some money (should be OK since you got it so cheap) and then which direction do you go? 6L6-fired Champ? 5E3 (push-pull 6V6? The good news is, it will wait patiently while you ponder it out.


You could wait until you find a piece of junk radio or phonograph and swipe the OT. Those types of thing aren't quite the common pieces of junk they used to be, that's for sure.


Given the size of that power transformer, I have no doubt you could even build a dual-6L6 something if you wanted, like a single channel Bassman or similar.   


IMO, the ideal thing would be to find a junked stereo integrated amplifier. Steal one OT for this, steal the PT and OT for another build.

[okguitarman]

Thank you again great info!
Auction amp =Output trans 368vac on 4&6
5v on 8&2
1.9 ohms across O.T.
408vdc B+/on 8

The other goodie Leather chair And new file cab $16.00 lol

[okguitarman]

 Change 1st stage add second stage and change output trans. to this output?

[eleventeen]

It's all a matter of personal choice, but if it was me, by the time I was making that many changes and buying that tranny (easily $60 with freight) I'd be changing the whole thing to a 5E3 (Tweed Deluxe, w/2 6V6) or even a Bassman (2 6L6)


With that tranny, you are going to end up with a clean hi-fi Champ and maybe that's what you want, though it seems kind of conflicting, to me.


Though the amounts of money involved are not large, these are not easy decisions because you have to gather your parts and those parts commit you to your decision.


I again think that if you like building amps, you should look around for a junked stereo tube amp, steal one OT for this one and steal the PT and one OT for the next build. If you search ebay for "integrated tube stereo amp" you'll find these. People want too much money for them now that tube amp building is a "thing", but I'd reco just taking a look. I'm not advocating tearing up a nice old Scott or Sherwood or Pilot or Fisher or even a Heathkit, but surely there is a local guy who buys and sells old stereo stuff near you that you can find on your local Craigslist who has a junked stereo tube amp with reasonable looking transformers you can make a deal on.

[PRR]

You don't need a 30-Watt OT for a 6L6, especially for a guitar amp.

EDCOR: XSE15-4-5K, GXSE15-4-5K, GXSE10-4-5K
Weber VST : re-did their website and it is impossible to find stuff now... maybe WSE15 Transformer
www.tubesandmore.com : PT-31

[okguitarman]

Thank you very much is there info here on sizing output trans.
PT-31 and change input
Old multi cap has shorted filtercap
two 8ohm 450v  and a 16ohm 475 sound right?
Your right I don't want Hifi  could you explain why the 30 watt trans would make it hifi?
Hz?
 

[eleventeen]

"Your right I don't want Hifi  could you explain why the 30 watt trans would make it hifi?Hz?


More or less, yes. We associate a big PT with current capability/ampacity. That's why, looking at the size of your PT, I have no hesitation opining it could handle dual 6L6. That thing is easily the size of a Bassman/Super PT. Have I measured it? Of course not. I can tell by the size, having done this a few times and seen what size PTs you find in dual-6L6 amps.


With an OT, the bigger, the more power it can transfer and hand-in-hand with that is bass response. That of course assumes the power supply can handle that much current. In this case, is sure can. A Champ Amp, I have always thought of as a 5 watt record player kind of thing. It has a very small OT. If I replaced a Champ OT with a bigger one and made no other changes, I would expect better bass response, automatically. If I also boosted the power supply, MUCH better bass response and power handling. If I now replace the 6V6 with a 6L6 (nearly twice the power capability of a 6V6) AND this bigger OT is rated 30 watts vs the 19 or so watts a 6L6 is rated, assuming I can drive the 6L6, I would expect massively more power, more freq response, more everything thru that output section.  There is nothing in the output section you should expect to over drive and so you ought not to expect breakup. Hence, a hi-fi amp.

This is "rule of thumb" kind of stuff, to me. Of course it can be calculated if you know the specs of the elements and care to do the math. I just see that with that OT, you'll have a fully powered 6L6 able to really drive a 12" speaker. That's not to say it would not be an interesting amplifier! But it for sure would be very hi-fi and it will not break up like a Champ, and if you build a Champ, breakup is probably what you want, I would guess. 

[sluckey]

Your right I don't want Hifi  could you explain why the 30 watt trans would make it hifi?Hz?

It's not so much the power rating. But the frequency response of 20Hz to 20KHz @ +1db at full power and the 40% screen taps make it very hifi-ish.

[eleventeen]

You could do a lot worse than buying this Mangavox from forum member DummyLoad for $60 + frt.


with tubes, bunch of controls you'll need, OT & PT for your next build.


http://el34world.com/Forum/index.php?topic=18778.0


That's less than the value of the parts, plus you have another 2 x 6V6 amp 70% built....w/chassis VERY suitable for experimentation, AC switch, all drilled & punched. Sockets mounted & wired. Pots, few 1/4" jacks. Hella deal, no two ways about it. 4 good clean 6V6s are worth a solid $10 each and he says they're new. You should grab this.


It's only after you've built a few amps that you realize how much time it can take to carve out the AC entry and drill holes in a reasonably straight line.

[okguitarman]

 I need to get my books out!   trying to grasp sizing O.T. trans and impedance
the reason a 15 watt 5k output transformer was recommended?
 I'd like to say thank you people again I Will Post my Project as It goes.
Can I use any wattage as long as i stay around 5k and how far can i vary from rated load resistance?
Impedance=tube rated load resistance?

[shooter]

The way I *get there* is with tube data sheets  http://tubedata.milbert.com/sheets62.html

Schematics of *like amps*, PP 6L6, 400vdc plate, etc.

Impedance=tube rated load resistance?

The wise here can give the correct verbiage, if the data sheet says 5k @ 400vdc plate, 250vdc g2 and max plate dissipation for tubes is 18w.  I'm gonna get a tranny that yields 5k primary and hopefully multi tap outputs.  8ohm on 8 ohm tap gives me the 5k, but so does 4ohms on the 4ohm tap.
Again the wise can help with power ratings, there is some interaction between under or over rated OT and tone.
Your impedance will change with frequency, so I'm guessing a +-20%? for optimum?

[eleventeen]

The other approach is: Magnavox designed the amp to use those OTs with 6V6's and they probably made tens of thousands of them. They were the right impedance in 1958 and they are the right impedance today.

[okguitarman]

Just wanted to say thanks for all the info!
Installed PT31 (Thank you PRR) changed feed back resistor to 22k
Changed input resistors to 33k and 1m
Installed power switch
Old multi paper cap all 3 were good come to realize my cap meter had bad battery.
The original problem (Low volume) I discovered was a bad solder on pot never seen that before looked like someone had tried to repair amp at one time.
Have not wired tone control, thought I'd ask you guys For advice.

Final Results dead silent no hum tone dripping, loud for 8 watts great headroom and turn it up and it just sings, very touch sensitive. Tested with vintage 30 8 ohm
Lindy Fralin strat pickups vintage hot

I get a chance ill post some of inside amp PRR I didn't Butcher it LOL

 I have to start on another amp build this project has made me addicted!

[HotBluePlates]

Congrats on the completed conversion!!

...  trying to grasp sizing O.T. trans and impedance
the reason a 15 watt 5k output transformer was recommended? ...

The reason for the suggestion of a 15w single-ended OT for this (assuming you were going to stay with single-ended 6L6):

The 6L6GC has a 30w plate dissipation rating. A single-ended output stage must operate in class A, and the theoretical maximum power output for a class A stage is 1/2 the tube's plate dissipation rating. 30w/2 = 15w.

Usually, you don't get the full 50% power output. Further, the amp may have been designed for an earlier 6L6-variant with a lower plate dissipation rating. The 6L6 data sheet (which sticks to lower-dissipation conditions even on the 6L6GC sheet) shows 8-11w for a 6L6 in single-ended operation. So that's why PRR started off recommending a 10w SE OT, and ultimately guided you to a 15w part (which may be as much about available transformers as anything else).

As for the 30w SE transformer you first found, I wouldn't look at the issue as hi-fi vs not; since we established a single 6L6GC could only manage 15w of output in the best-possible case, you'd be paying for a much bigger hunk of iron than you need, so wasting your money. Secondarily, as Sluckey noted it's a 20Hz-20kHz OT at full power, while guitar is only 80Hz to ~5-6kHz (your speaker is likely rolling off significantly by 6-10kHz). So a smaller, cheaper hunk of iron is typical in guitar applications.

Anyway, that's all moot since you've got it going. Congrats again!!

[shooter]

Nice amp n work!

Have not wired tone control

If you want to stay simple but useful try a 1 knob from like a fender 5F2 schematic, tweak the caps to your liking.

welcome to the addiction

[okguitarman]

After installing tone! Sounds great
Like to build another on a board how do you go about figuring out a layout on a board?
It is like finding a piece of a puzzle in that maze.

[shooter]

how do you go about figuring out a layout

Is there a simple amp you like?  at the bottom of this page look at the schematics library for the *classics* they sometimes have board layouts, study them, see what they did back when, try and "get in their head".  before long you'll have 12 sheets of paper, pencils ground to the nib, less hair, but you'll have a pretty good idea on how layouts are done.  Go to dougs page of turret boards, they  have layouts also.  The wheel isn't worth re-inventing, stealing from others... 

[okguitarman]

Thank you! Kinda what i had in mind

[shooter]

A key point to remember, when you lay out your board make absolutely sure it's laid out the way it will be inside the chassis, it's real easy to get this wrong!, my last board I had to ask doug to install the turrets on the "bottom" of the board because I messed up.