Standel driver transformer replacement help.

[Anthony]

Hello. I have a Standel Studio model amp I'm fixing up and it needs a new driver transformer. I haven't the slightest idea about any of the specifications of the original, or if Hammond(or anyone else),has one that will work in this application. [URL="http://www.standelamps.com/support/schematics/documents/1967/ar_and_smodels_serv.pdf"]

Thanks,
Anthony

[R.G.]

First, are you absolutely sure that it's the driver transformer that's faulty? How did you test it?

These are very like the Thomas Vox driver transformers. They were never an industry standard thing, and there are simply no replacements. However, I've spent a few decades pursuing how the Thomas Vox amps work, so I know a fair amount about how the driver circuit works. If it's really, really, most sincerely dead after some detailed testing, the only option is to rewind it. I can provide pointers.

[Anthony]

Hey R.G., I traced signal up and to the driver transformer. I then switched over to the secondary side and the signal was gone. I suppose something else could be up.
I'm kind of dumb-founded on how there isn't one driver transformer out there that would complete the task. I guess much of my confusion comes from a lack of understanding when it comes to the driver transformer. I wish I could just pop one of about 10 Hammond driver transformers available and get close.

[R.G.]

Here's how you test a trannie.

1. get the transformer's schematic, and figure out which wires are to be connected by the windings, and which must be separate
2. get out your ohmmeter and see if in fact there is winding-level resistance between the appropriate wires. Some windings may be a few hundred ohms, but if one of them reads over a megohm, it's open; be sure and test a couple of times and be certain your probes are making good contact
3. If the windings are continuous where they're supposed to be, check for shorts from winding to winding. Again, the ohmmeter, but clip it onto one winding, then probe one wire of the other windings in turn. If you see less than 1M, there's an internal short.
4. If all that looks good, try the shorted-turns test here: Shorted Turns Test
If the windings are continuous where they're supposed to be, isolated where they're supposed to be, and there are no internal shorted turns, the problem is almost certainly elsewhere.

Note that you must take the transformer out of circuit and ensure that all windings are completely disconnected from anything else for the tests to mean anything.

If it passes all that, try putting a small signal on a winding and looking for a proprotionate signal on the other windings. If that happens, there are only very, very rare things that could be wrong.

[R.G.]

I wish I could just pop one of about 10 Hammond driver transformers available and get close.

Sadly, it's not to be. I went off looking at the Hammond drivers, and they won't work, at least in what I think your amp is. This amp is about 30-60W, right? That's similar to the Vox Buckingham and Royal Guardsman models. The driver in this circuit has to handle a significant amount of power. These are about 2W and 4W for the amps I mentioned. Hammond's drivers seem to be limited to about 1W max. And the load impedance on them is about 16 ohms. Hammond's are all higher than that, in the 100 ohms and up range if I didn't miss one.

The thing is, these were never standard items. At the time they were made, transformer makers would make a custom job for a manufacturer in quantities of a few hundred units. I think all the ones I've ever seen are custom/house brand ones.

I have been studying the Vox ones for a long time, and as soon as I can cobble together a coil winder again, I'm going to have a go at making a driver transformer for a Vox Beatle. I doped out how many turns and so on, and have deduced how the windings are wound (how many turns, what sections, which over which other, how they're connected inside) and I think I can do it.

You could probably do a reasonably facsimile of the one you have by finding a same-size small power transformer and using it as a parts donor. See http://geofex.com/FX_images/Salvaging A Transformer Kit.pdf at my website, geofex.com. That set of pictures was taken from the dissassembly of the donor for my Beatle driver.

If you find through one of the tests that your driver transformer is in fact dead, that's a back-handed advantage. If it's dead, disassembling it to count the number of turns in the various windings and noting how they were wound is not a loss. Transformer winding is not horribly difficult, just obscure.

[Anthony]

Hey R.G. My interest in these amps came about when I repaired a broken headstock on a classical guitar for Pat Metheny. He was on tour here in Austin and I got a call from his tech. Anyway, later that evening I met him and we began discussing amplifiers. He told me that the best jazz amp he ever heard was a Standel that he saw Wes Montgomery play about three months before his death. Pat said he bought one just like it and that it was sitting in someone's shop for about 3 years. It had the epoxy covered modules in it. In 2012 Standel still hadn't posted the schematics to the inner workings of these modules on their web site. They're up there now. I wanted to reverse engineer his amp and build him another one. However the other tech seemed to become motivated to figure it all out once they came calling for it. So I never got a hold of the amp.
Two years later, a co-workers Thomas Vox amp broke and I couldn't find a schematic for it anywhere. It really was one of the best solid state amps I'd ever heard. Original speakers and everything.
I don't want this technology to die. I think people should hear these amps again in this time. There were, and still are transistor amps designed out of a specific need and goal for musicians.
I suppose to harness this I need to get into these driver transformers.

[52 Bill]

I think that RG is singlehandedly keeping hope alive for all solid state amps that use that power amp design. Eventually he will solve the puzzle and then we can all repair our dead VOX, Standel, Kustom, Acoustic and Peavey amps.

Wes Montgomery played a solid state Artist model with a 15 inch speaker. I heard him play through a tube Ampeg (I think) once, and he sounded amazing. I don't think that any amp had anything to do with how great he sounded.

[loudthud]

Going back to my high school days... I think it was Triad (possibly Stancor) that made a driver transformer with split secondaries. IIRC the number was TY-160X with a 6:1:1 ratio. There was a project in Popular Electronics for a power booster that used the transformer driven by the speaker output of another amp and drove a couple of power transistors. For a power transformer a couple of filament transformers were connected in series.

Edit: Whoa Nelly. Bing found this: http://www.ralphselectronics.com/Pro...ber=TRI-TY160X

[Enzo]

Cool, except for the last part:

0 on hand, email for availability

[g1]

I read elsewhere about mercury mag having the one for the royal guardsman available by request. You could check with them about availability of that or something similar, maybe they have even cloned the Standel by now. Won't be cheap though.

[loudthud]

Cool, except for the last part:

0 on hand, email for availability

Ya, I saw that, but it's a long way from never being available.

[52 Bill]

That's the transformer for the Delco 100 watt power amp design that has be discussed here before.

www.updatemydynaco.com/HistoricDocuments/Delco100Watt.pdf

Edit: This link seems to be dead.

[silverfox]

Isn't there some what to determine the turns ratio and rebuild the transformers or is this one of the epoxy coated deals?

Silverfox

[R.G.]

My day job for some years was designing transformers (among other things), so I have little awe left for them. They're tricky, but fun.

Here's how you design the transformer in the first place.
1. figure out how much voltage and current your output transistors have to put out into the speaker; peak, RMS, etc. to get the required output. For most of the amps of this era, that was +/-15Vdc to +/-35Vdc. I think the transistors didn't exist to do anything else. The UK Vox Supreme used bigger voltages by stacking transistors in series and getting them to split the total voltage across the, but that's somewhat risky business. and I digress.
2. Once you know the output voltages and currents, figure out how many volts and amps you need into the bases of the output devices to drive them to those levels. In the Beatle for instance, you need something like 7V and 350ma available to supply enough current and such. This will vary depending on the bias resistors and the gain of the transistors at peak output voltage and current.
3. Knowing the output voltage and currents on each secondary, you know the supply voltage on the primary, so you know the turns ratio. You also from the amount of base current on the secondaries know the idle current in the primary. The primary idle current times the turns ratio into the secondaries is the peak base current.
4. From that, you know the DC current the primary will have to withstand without saturating; there is another requirement on primary saturation resistance, and this is the lowest frequency that will be run through it. Also, the lowest frequency sets a requirement on the primary inductance. So from all three of these, you can pick a core size, a gap for DC saturation resistance, and a number of primary turns. This step requires an afternoon or so of iterative calculations until you converge on a number of turns for a core size. Once that is done, the number of secondary turns is simple multiplication.
5. Near as I can tell, all transistor drivers are wound the same way: half the primary, two secondaries bifilar, then the other half-primary. So you can calculate wire sizes from the core window you got from the core you picked in (4).

It's not too hard.

[g1]

RG I think you've done this before, maybe this will ring some bells/provide some numbers:
Vintage Amps Bulletin Board ? View topic - Vox Royal Guardsman Driver Transformer Recipe