Mystery iron for a 36watt build?

[titanicslim]

Hi

I've got a Stancor 25watt OT and an unidentified PT that seem like they would be good candidates for a 36watt build, but I'd like to get some opinions from you before I start.

The OT says A 3313 on the label- I think- as the last digit is not all there. Maybe someone has an old catalogue? Clearly legible, however, are 10,000 ohms CT primary, 70Ma. DC and 4, 8, 15, & 500 ohms secondary.

The primaries are not a problem, but the 2ndaries have none of their color left except for the black common. From it to the four leads I measure appx. 0.8, 1.0, 1.2 and 19 ohms. May I safely assume they are, respectively, the 4, 8, 15 and 500 ohm taps?

The PT I bought on fleaBay and it was represented to me as being from an old Magnavox hi-fi. A shoe tag affixed to the PT indicates that it powered four 6BQ5s, one 5U4 and three 6EU7s and that it provided 315-0-315 VAC with no load from 120VAC in.

Well, there you have it. Frankly, although I don't have a scale that is small and accurate enough to weigh these, I can say that the PT doesn't seem to be much more substantial than the commonly available 18watt transformers. The OT, on the other hand, is fairly hefty- about like a 30watt unit, it seems to me.

Marriage made in Heaven? Bletchley? My initial plan was to use the OT in a Deluxe Reverb kind of rig, but this seems to be a much better fit for EL84s, even though the stated output is 11watts less. 25 would suit me much better.

Any ideas, alarms, suggestions are welcome and very much appreciated.

Dave

[Phil_S]

You seem to have a Stancor A-3311, rated at 25W. It sold retail for $6.85 in 1948, so it is an expensive piece. Stancor has a good reputation. Nice iron. You have identified the taps correctly. The primary on this is 10K, which will work fine for a pair of 6BQ5 or 6V6, but it is high for the 18W type, which should be 8K. If you want to build a 36W, this PT is wrong. You need about 4K primary for a 36W.

The tube compliment on the PT, if accurate, is very nice. Filament supply should be more than adequate. That 315 will drop probably 4-6% giving you around 300-0-300.

If this were my iron, I would probably build something besides an 18W type amp. Of course, opinions will differ. I'd look for a circuit that is designed around a 10K primary. I think your initial instinct (for the Deluxe) on the OT was more appropriate. You know, these fetch a good price in eBay...maybe between $50 and $100. For that price you can take the cash, maybe add a little, and get the right thing for a 36W build.

[titanicslim]

Thanks for responding, Phil!

The primary on this is 10K, which will work fine for a pair of 6BQ5 or 6V6, but it is high for the 18W type, which should be 8K. If you want to build a 36W, this PT is wrong. You need about 4K primary for a 36W.

OK, I finally found those Heyboer specs. I had thought for some reason that the primaries would be more in line with the 18watt's. Hey- there's also an old A-3800 in my stash! How do you figure 5k primaries would do in the 36watt? Worth a try? It's a little closer to being in line with the 36watt output. I think it's 30w(?) but this is quite an old unit- pre yellow label- and I don't have a spec sheet....

[Phil_S]

If I had that A-3800 at 5K, I'd give it a try with 4x 6BQ5. I bet that's some good tranny. I don't think you'll quite get the traditional Marshall grit, but I'd expect you'll get a sweet sounding amp. I think you'll like what you get.

Yes, according to the catalog, it's 30W, with 500, 250, 15, 8, and 4 ohm secondaries, 80mA.

[titanicslim]

Thanks for the encouragement, Phil

If I had that A-3800 at 5K, I'd give it a try with 4x 6BQ5. I bet that's some good tranny. I don't think you'll quite get the traditional Marshall grit, but I'd expect you'll get a sweet sounding amp. I think you'll like what you get.

That's the plan! Since my recent 18watt TMB/EF86 I just built came out so well, I drilled the chassis for a six position switch as well as a socket for an EF86. I have also discovered that the 3800 is one of the models that Ken Fischer used in his Trainwrecks. If I don't like the way this sounds, I can try it in a Liverpool clone!

Nice that the 3800 has the same footprint as the other one. It's in the chassis and most of the hook-up is done. Like I said, the 3800 is old enough that it had a little metal plate with the model no. stamped on it instead of the later yellow sticker. I had never really noticed before, but all the cloth-covered leads are about nine inches long. I doubt it's ever been used, aside from testing.

If I understand correctly, the PT had to provide 0.3amps to each of the 6v heaters on the Magnavox's 6EU7s, and my 12AX7s and 'AU7s will draw half that @ 12v, right? So, that should ensure that there is enough current to run the EF86 in addition to the other seven.

Thanks, Phil- I am getting that weird feeling again. Adrenaline?

Dave

[Phil_S]

I know that blood pumping through the veins feeling. Be careful that you don't throw caution to the wind. It can be intoxicating.

I'm not sure what you're doing there with filament supply. Are you saying the donor PT has a 12.6V winding? Or, are you using a separate filament transformer?

What I'm getting at here is that there is no free lunch. The winding supplies amps * volts = watts. The watts must be treated as a constant; if you half the volts, you do get twice the amps and vice-versa. So, where are you getting the 12.6V supply? It is not unheard of, but a little out of the ordinary. The ultimate point is that I don't understand how you will squeeze out what you need to supply one more tube.

[titanicslim]

Sorry, Phil. I tend to get tangled up in my words.

I'm just saying that, in this PT's former life, it ran three 6.3v preamp tubes which drew 0.3amps each. The rig I'm putting them into will have three 12Ax7s which will draw half that much current @ 12.6v, right?

So, as I understand it (?) since the Magnavox drew 0.9amps for its 6EU7s, my three 12AX7s should need a total of 0.45amps, leaving me more than enough current to fire the EF86, which requires an additional 0.2amps.

Or is that the way it works at all?

Dave

[Phil_S]

No, Dave, I think you need some help here. You will not be running anything at 12.6V unless you've got me fooled.

The donor had 3x 6EU7 + 4x 6BQ5. 6EU7's need 0.3A ea. 6BQ5's need .76A ea. So, we know the filament winding had at least (3*.3)=(4*.76)=3.94A....let's call it 6.3V @ 4A.

12AX7's can run on either 6.3V or 12.6V. But to run them on 12.6V, you need to supply them from a 12.6V winding and wire them appropriately. Your PT doesn't have a 6.3V winding, unless there's something you're not telling. So you need to run your 12AX7's on 6.3V, which is what *everyone* does.

So, if you think the PT only has 4A available, then you are capped at three preamp tubes and four power tubes or some combination of 4A. I am not a betting man, but I'd be surprised if you couldn't milk another .2A out of that winding.

If this were mine, I'd just go ahead. It's about a 5% overload, if there is no excess capacity, and there nearly always is. If you want to know how to test it, I'll try to help you.

[titanicslim]

Thanks for your help and patience, Phil. I'll go ahead with the build, then get a small filament transformer if needed for the EF86.

I must admit, however, that I am a little perplexed at your explanation. I had always thought that 12xx7 heaters, by virtue of their higher filament voltage, actually drew half the amperage of their 6v counterparts.

I guess it's time for me to do some homework on the theory. Is there a text (preferably online) that will illustrate this principle, hopefully with graphic aids?

Dave

[Phil_S]

Dave,
Go here:
http://tubedata.itchurch.org/index.html
Find and download one of the 12AX7 data sheets. You'll see it says .15A at 12.6V or .3A at 6.3V. Both ways, it's 1.89W and that's what is constant.

Here, I'll make it easy...this one is very much on topic...
http://tubedata.itchurch.org/sheets/127/1/12AX7.pdf

There are two ways to wire a 12AX7.

The common 6.3V way to do it in guitar amps is to jumper pins 4&5; then one filament lead goes to 4&5, the other goes to pin 9.

The other way to wire a 12AX7 is to omit wiring pin 9, and run one filament lead to pin 4 and the other to pin 5 (no jumper). When you do it this way, you need to supply 12.6V.

Since common guitar amp trannies don't supply 12.6V, "no one" does it this way.

Transformers are built to deliver a certain voltage, and UP TO a limted number of Amperes. So, a transformer with a 6.3V winding will supply that amount of voltage until you hit it's rated limit for Amps. After you exceed the Amp rating, voltage will start to sag below 6.3V, the transformer will heat up, and eventually, the transformer will fail. You don't get the option of drawing 12.6V from a 6.3V winding or vice-versa.

Does this make sense to you?

[titanicslim]

Phil

OK, so you're saying that, from my 6.3v winding, all I can put on my 12xx7 heaters is 6.3v. In order to give them 12.6v I have to have a 12.6v coil on my PT, right? Wiring them to 6.3v coil will just give my triodes 6.3v each, but in series. So, when I look at the tube manual and it says that the same tube draws half as much current @ 12.6v, that only applies in a circuit with a 12.6 winding on the PT- is that right?

Dave

[CurtissRobin]

The 12A_7 tubes have center-tapped 12.6V heaters. A center tap (CT) is a connection at the midpoint of the heater. The two ends of the heater connect to pins 4 and 5 while the CT is on pin 9. The center-tapped heater is simply a two-resistor network. You can connect two resistors in series or in parallel. From there it's all Ohm's Law.

Given a PT with a 12.6V winding you'd connect that winding to pins 4 and 5 of a 12A_7 tube. The two halves of the heater make a series circuit and the center tap on pin 9 is not used. According to the tube data sheet the heater's current draw is 150 mA at 12.6V so the heater consumes 1.89 VA of power.

Given a PT with a 6.3V winding you'd connect both heater ends (pins 4 and 5) to one lead and the heater's center tap (pin 9) to the other lead. This puts the heater halves in parallel and each half sees 6.3V just like in the series connection described above. With each half seeing 6.3V and the current draw of each half being 150 mA, the total current draw is 300 mA. The power consumption is still 1.89 VA.

Nearly all the PT's we use in guitar amps use 6.3V windings because the power tubes typically have 6.3V heaters and the preamp tubes that have 12.6V heaters have center taps allowing a single 6.3V winding to serve all the tubes. If you have a PT with a 12.6V winding you'll have to do something about those pesky 6V power tubes. Wiring them in series is possible according to Ohm's Law but in practice it's a bad idea.

KennyO

[titanicslim]

You know, I was evidently taught that load of crap (somehow getting 12v out of a 6v coil) about forty years ago, and, as it's not yet presented a practical problem (always use big iron) I've just accepted it and never given it much thought. It's because I lack basic theoretical knowledge.

But, thanks to you all I am learning more and more all the time, I love this place. You guys are the


Dave

[Phil_S]

You know, I was evidently taught...(somehow getting 12v out of a 6v coil) about forty years ago...

I've been wondering...now it's no wonder it's taken so many words to move you off this idea. Old habits die hard. I think you've got it now. You're gonna do just fine. Remember, the only stupid question is the one you don't ask. Amp work can be dangerous. Err on the side of looking foolish and being safe. We can't make you undead and we can't put the smoke back in the parts.

[titanicslim]

We can't make you undead and we can't put the smoke back in the parts.

That's right- only Mme. Laveaux can do that and human flesh gives me gas. Seriously, I take the safety precautions... seriously...

Anyway, I got this rig all hooked up and it does sound sweet. The configuration is just like the Ceriatone (I really need to buy something from this guy- I'm starting to feel guilty about taking advantage of his Interwebular largesse) 36watt EF86 with the exceprion of the Master Vol. replacing one of the TMB inputs, as on his 18watt EF layout. Truthfully, I miscounted the holes when Laying out the chassis.

Isn't it strange that, as long we keep making these things, they nearly always surprise us with how nice they sound? Odd...

At first trials, I was a little underwhelmed by its soft output and lack of headroom until I realized I had installed an old 5Y3 of questionable provenance, so I swapped it with a new JJ 5AR4. That's what it needs, isn't it?

So far, I'm not disappointed with the transformers. There seems to be plenty of current everywhere and the thing just roars. Now I shall have to make another one, this time with the Heyboer iron, so I can compare... Hey- How do you stop this thing anyway? All my money is going to iron mongers!

Dave

[Phil_S]

... Hey- How do you stop this thing anyway?
Dave

You don't. It's a monkey on your back. You'd be better off with substance abuse, for which there are good 12 step programs, rehab clinics, etc. I think I'm about to start #11. They are all different. They all sound awesome.