Tube Power Transformer Questions

[SirCVH]

Hello
I have a nice tube power transformer from a tube cb  (International Crystal Executive ctz100c) that I would like to use in a project but I have a few questions before I do. I have attached a circuit description of the transformer from the original circuit if it helps.

Transformer Characteristics
Primaries: 1 x 120v, 2 x 12v Center Tapped (This is to be able to run the unit on 120v, 12v, or 6v. There is extraneous circuitry on the attached schematic that converts 6 or 12vdc to ac prior to the 12v primary windings.)
Secondaries: 1 x 600v Center Tapped, 1 x 12v

My Questions
1) The 120 volt primary leads come out on the secondary side of the transformer for some reason. Can I trust that these are attached to the primary side internally, or is there any chance that it is running the secondary unisolated?

2) When voltage is applied to the 120v leads (how I would like to use it), the secondary voltages come out good, but also the 2 12v primaries show 12 volts as well (actually 15v unloaded). Is this because of inductance on the primary or could the 120v be  connected to the 12v windings internally somehow?

3) Could I get the maximum current out of the transformer by running 120v into one primary winding plus 12v into the other primaries? I am worried to try this because of the issue in question 2.

Thanks for any input you may have!

 

[PRR]

The wires come out any which way internally. If there are 2 exit holes, they "can" be routed randomly; there's no strict pri or sec here.

It's got 13 wires, 8 pri, 5 sec. Bringing the two 120V wires with the 5 is slightly more "even", or may suit the wire layout better.

> running 120v into one primary winding plus 12v into the other primaries?

Where are you going to get a useful amount of 12VAC? And it better be EXACTLY phased and ratio-matched to the 120V AC, or they will fight. Also those "12V pri" are probably for SQUARE WAVE of a 12V battery, not sine-wave from a wall transformer.

The core is designed for the load, with either primary energized. The only "downside" of the 120V/12V feature is more copper winding. If you specified an exact-right part for 120V only, it might be a pound lighter and $3 cheaper. But the over-frilled PT in the hand sometimes beats the just-right part after ordering and shipping.

The PT was designed to deliver good load power with just the 120V winding to the wall. Don't get clever. If you do, do it outside.

> voltage is applied to the 120v leads ...the 2 12v primaries show 12 volts as well

That's how a transformer works. Feed electricity, the core shakes with magnetism, all windings on the core shake with electricity.

The windings are shown fully isolated, and that would certainly be true for the primaries for safety reasons. If you are in doubt, use your ohm-meter.

A "CB" will be a 5 Watt input RF stage and a 2 Watt audio power amp. Basically you got 10 Watts of DC power there. This would be suitable for like 4 Watts SE or 5 Watts push-pull. A "small Champ" or a pair of video-amp bottles. Not anything monstrous.

AND... a CB radio would only be in Transmit mode for short time. There were actually limits; CB was not for non-stop yak. The PT designer probably took that into account. Short gigs, probably fine... that lump will take many-many minutes to get hot. 24/7 work, it may smell and die young. (Not that it is young now.)

 

[PRR]

If you have the whole corpse---

6CL6 is the final RF. Rated 7.5W Pdiss, though under FCC CB regs it could only get 5W of DC, output 3W of CB, leaves 2W heat in the tube (very conservative selection). This tube (if good) might be a sweet SE audio amp.

6AQ5 is your audio power amp; speaker on receive, modulator on transmit. A small-jug 6V6 with slightly lower ratings.

This rig was around $200 new in 1962.

 

[SirCVH]

PRR,
First, thank you so much for taking the time to give such a detailed response! I do have the whole unit and am hoping to reuse as much of it as I can. I plan to use the case for a guitar amp based on this transformer. I was hoping to add a 5 volt transformer for the rectifier and make a 5e3 deluxe, as the total current load of the cb tubes was a bit more than the 5e3. It sounds like you are saying it might not pull off the 13 watts necessary though , so I will try for something smaller.

Again, thank you for your responses they are very helpful and much appreciated.

 

[Bo Deadly]

My understanding is that rectifier tubes don't contribute to supply sag much. A simple diode rectifier and some series resistance is equivalent. If you want supply sag then you need to pull a little too much current from the HT secondary so that hysteresis starts to limit current. But you would need to load the HT just so and the core material will affect what the hysteresis looks like (sharp or gradual). And it would probably need to be PP and not SE.

 

[PRR]

The existing power supply will be fine as-is (bad caps replaced). It comes up near 420V. The resistors will give it "sag" not so different from a tube rectifier. Small power amps don't invoke "sag" like large over-volted bottles.

As a least-case, keep the 6AQ5 audio power stage and its output transformer to the internal speaker. Is there a 12AX7 in this one? That's the rest of a "Champ", only near 2 Watts and the lame 4" speaker in a rattly case; add Ext Sp jack for serious use. 2 Watts in a good Twelve will call the cops fine.

 

[SirCVH]

This project is coming along nicely, it does have the original 12AX7 preamp tube, so I am building it as essentially a 5f1 Champ / 5f2a Princeton, but I have a question about the output transformer I grabbed out of the unit. The schematic is attached, but basically the primary of the transformer is center-tapped like a push-pull transformer and the primary has the 6AQ5 plate on one end, 6AQ5 screen to the center tap, and 6CL6 plate on the other end. Can this be safely used on a single-ended amp, or should I purchase a new output transformer? Alternatively I guess I could try to grab another 6AQ5 and try push-pull? Any ideas?
Thanks for your time!

 

[PRR]

Don't see a 6CL6 in that snippet.

OK, this is an AM modulator. Not gonna teach that obsolete theory tonight. Tape up the blue lead and pretend it is a normal output transformer. If bass is wicked muddy, get a proper SE OT.

 

[SirCVH]

Yeah, the full schematic is pretty big so I didn't include the 6CL6. If you'd like to check it out the full schemo is here:

http://www.cbtricks.com/radios/international_crystal/ctz_100c/graphics/ctz_100c_sch.pdf

I took measurements on the transformer last night, and using the brown to red works out to 6400:4 ohms, so not quite the 5k output of the 6AQ5, but close enough I suppose? I will try it out to see how it sounds and grab a new OT if it doesn't work out.

I should be able to otherwise finish this without buying anything new which was my original goal, I am just using pieces of the radio and parts and resistors from around my garage. I did buy new filter caps to match the fender designs more closely though, I thought using the dual 80uf can cap from the radio might be a bit much for the Fender mojo (correct me if I'm wrong), though the can capacitor esr still measures fine so that might work nicely for a project in the future.

Thanks for your continued help PRR!

 

[PRR]

> 6400:4 ohms, so not quite the 5k output of the 6AQ5, but close enough

Note that Champ OT is usually rated 7K.

The "ideal" match depends on the V and I the final tube runs. 6AQ5 book-spec is 5K at 250V? If you go a higher voltage, you *must* reduce current (to stay within Pdiss limit), and the "ideal" match is a higher Z. Fender was running over 300V, and not really reducing current much; also a 7K was a standard part in the early days.

I would just assume some pocket-protector guy and his boss designed it good-enough.

For understanding, here is the Transmitter current path without all the crazy transmit/receive switching and condensed so you can see it.

Main B+ goes to the OT (and G2) then the 6AQ5 plate.

6AQ5 G1 gets about a dozen Volts of audio, plate makes big swings.

For receive, 6CL6 is cut-off by a switch, speaker is connected, you hear. This is how it must work for you.

For transmit, the second OT primary winding feeds the 6CL6 plate tank circuit with audio-wobbled B+, so the transmitted carrier wobbles with audio. This is the 2nd oldest way to put speech on radio.(Older is to put a Carbon Mike right in the RF plate circuit, but even a faction of a Watt of RF burns-up the mike.)

Very old AM transmitters used only one OT plate winding. This had to carry the DC current of audio +plus+ radio power amps. With the 2-winding (probably not exactly center-tap), the DC flux in the core is the -difference- of the two currents, which may be a lot close to zero. However this OT is clearly designed to function fine with _just_ the 6AQ5 plate current for Receive, so it will be OK.

Guitar wants bass down toward 80Hz (not so much when using small speakers that just flap bass). Speech can extend to 50Hz BUT when you want CLEAR speech with LEAST power, you can cut a LOT of bass off, turn-up, and still understand every word. It is possible this OT is bass-shy, 150 or 250Hz bass loss. It is likely acceptable for small-speaker work (8 inch or smaller). When you hook up your JBL D-130, or your Full Stack, and don't get the THUD you expect from serious air-whackers, then you can go find a fatter OT.

 

[SirCVH]

I was finally able to get back to this project and wanted to give the thread some closure. I did a little write up of the project here: https://www.reddit.com/r/diytubes/comments/72nuvb/my_6aq5_guitar_amp_from_parts/, and posted pictures here: https://imgur.com/a/uAYAT.

Special thank you to PRR for your input on this project, and for your posts on this and various forums where your tube knowledge has been saved for those of us on our learning path.