Looking for help with understanding output tranformer!

[kmardell]

I could really use some advice about an output transformer on an old tube amp chassis which I hope to use as the basis for a guitar amp build. The amp is a Rauland Borg RA100A 100watt amp with 4 x 8417 octal power tubes (missing). I am unable to find any schematics or other info on this amp. It's a good size chassis with massive PT and OT and lots of tube sockets. The OT is like something I have never seen. There are 6 wires on the input side. 3 of them are the usual 2 leads from the plates with centre tap (which I will use) and the other 3 seem to be part of a "unity coupled push pull circuit" which I hope I can just ignore on my guitar amp build. On the output (speaker) side of the OT there are 6 wires, 3 pairs with no connection between them i.e. 3 separate windings. The turn ratio for each output pair calculates to around 14.5 when tested with a variac connected to the main input windings of OT. So, hooking up one pair to an 8 ohm speaker should give about 1682 ohms primary impedance. I hope this is Ok for a 4 x 6L6 configuration amp? My dilemma is what to do with the 3 pairs of OT outputs. Can I just use one and leave the others disconnected? If I hook up more than one speaker using the other windings pair(s) does this change the impedance seen by the primary side? I hope my questions make some sense! I'm a bit out of my league here so any advice would be much appreciated. Thanks, Kevin.

[Latole]

First time I heard this name

 

[Latole]

Delete to avoid polluting the subject.

[kmardell]

Here's afew pictures of the actual amp. It is a rack mount. The only output connection is a 6 pin socket, which seems to be 3 seperate windings rather than different taps (such as 70v, 25v, 16, 8,4). Got me puzzled!

[kmardell]

Pic2

[Latole]

With lots of search, I can't find any schematic.

[uki]

Nice amp, you pretty much have an amp there already!
Maybe just some tweaks to convert to something else,
look those electrolytic caps, the blacks ones, do I see a bubble there?
it is a good idea to replace the electrolytics caps, they are probably gone.

here is the power tube https://frank.pocnet.net/sheets/168/8/8417.pdf
from the tube data sheet you can gather some info about the amp.

Let me ask are you new to amp building?

[AlNewman]

It's probably some high end recording or PA gear.

I'd try to build a schematic of the output section as built before doing anything.  Cross reference it with data sheets, etc. 

A guitar amp isn't always the best solution.

[kmardell]

I agree that this is probably some kind of high end amp as it seems to use a "Unity Coupled" circuit as in McIntosh design. Given the unusual power tubes and unknown application I did not see much point in trying to rebuild it to original. I was really just thinking of stripping it down to the bare chassis with PT, OT, tube sockets and heater wiring, then building a guitar amp turret board for something with 4 x 6V6s or 4 x 6L6s. I was trying to make sense of the output transformer winding ratio / impedances to figure out what I could do with it when I discovered this odd 3 winding arrangement on the output side. One suggestion I was given is that you link together the segments for different impedence speakers, eg one winding for 16 ohm, 2 windings for 8 ohms or 3 windings for 4 ohm speakers. Thoughts?

[Latole]

Let me ask are you new to amp building?

 And the answer is ;

[Latole]

 I did not see much point in trying to rebuild it to original. I was really just thinking of stripping it down to the bare chassis with PT, OT, tube sockets and heater wiring, then building a guitar amp turret board for something with 4 x 6V6s or 4 x 6L6s.

I agree

You have to make some tests to see if both transformers are working.

[AlNewman]

I'm not saying you shouldn't build a guitar amp with it, but I am saying it may be beneficial to map out the original schematic to discover what the OT actually does, or is supposed to do.
For one, you won't really know if it's a candidate to build a standard push/pull amp, and secondly, that transformer may be much more valuable in a different application.
If you can map out what's there, others can help in the detective work.

[Latole]

I'm not saying you shouldn't build a guitar amp with it, but I am saying it may be beneficial to map out the original schematic to discover what the OT actually does, or is supposed to do.

I agree.

And complete OT and PT tests . If they are no good, project could fall through

[trobbins]

An aussie 100W PA amp (AWA PA1001) also used a unity coupled output stage with quad 6L6.  There was also a 50W PA amp (AWA PA 1002).
https://www.dalmura.com.au/static/AWA%20PA1001BY%20amplifier.pdf

+1 for AlNewman's suggestion to spend the time and effort to prepare a schematic of the output stage at least - then go from there.

[HotBluePlates]

... advice about an output transformer on an old tube amp chassis which I hope to use as the basis for a guitar amp build. The amp is a Rauland Borg RA100A 100watt amp with 4 x 8417 octal power tubes ... There are 6 wires on the input side. 3 of them are the usual 2 leads from the plates with centre tap (which I will use) and the other 3 seem to be part of a "unity coupled push pull circuit" which I hope I can just ignore on my guitar amp build. ...

I hate to kill hopes & dreams, but you will not be able to ignore the "unity-coupled" aspect of the output transformer.

"Unity-coupled" works like this:
   - A normal Class AB 6L6 amp might have an output transformer with a "4kΩ plate-to-plate" primary impedance.

   - When that Class AB 6L6 power section is driven to max power output, one side turns off.

   - The remaining side that is still on sees a primary impedance that's 1/4 the total "plate-to-plate primary impedance."  So 1kΩ for our example.

   - "Unity-coupled" operation has 2 primary windings: one for the plates, and one for the cathodes.

   - The winding for the plates of our "6L6 unity-coupled amp" will be 1kΩ plate-to-plate, or 1/4 a "normal 6L6 output transformer."

   - The winding for the cathodes of our "6L6 unity-coupled amp" will be 1kΩ cathode-to-cathode, or 1/4 a "normal 6L6 output transformer primary winding."


Bottom-line for you is this means the output transformer is not equivalent to anything you would normally use, even if you "only use the plate wires."  That means your choices are to buy a new output transformer (skipping the point of salvaging IMO), or you keep the power section (and very likely the bias supply, phase inverter, and power supply) intact as Rauland-Borg did it.


8417 is a special/weird tube.

Plate dissipation is like the 6L6GC or 7581A of the era, but it has 4x the transconductance of a 6L6GC or 7581A.  That means the 8417 will develop the same power output as a 6L6GC or 7581A, but only needs 1/4th the drive-signal to get there.  It also means a much smaller voltage biases the tube, so the bias supply is incompatible with 6L6GC/7581A as-is.

... I was really just thinking of stripping it down to the bare chassis with PT, OT, tube sockets and heater wiring, then building a guitar amp turret board for something with 4 x 6V6s or 4 x 6L6s. ...

IMO, you're barking up the wrong cadaver.

   - Using that OT means you're locked into the Unity-Coupled output section.
   - A Unity-Coupled output section means a large drive-signal (which is partially offset here by using 8417).
   - The large drive-signal needed for this type of power stage & output transformer means you need to keep all the phase-inverter/driver stuff exactly as Rauland-Borg did it.
   - The large drive-signal needed might also mean a need to stay with 8417 tubes (which are built in a way they really want to blow themselves up, even if they weren't expensive).


"Happy Conversions" mostly happen when you leave the amp 90% the same as it original was, replace worn-out parts, and maybe tweak a preamp or tone-stack to be more guitar-like.

Conversions get frustrating and/or expensive when you try to make something radically different than what it was originally, unless you always planned to
rip & replace everything."

[AlNewman]

So what would be the advantage of a unity coupled transformer?

And what is the purpose of the 3 separate windings on the driver side?

[trobbins]

So what would be the advantage of a unity coupled transformer?

You may have to do your own homework on that, such as read everything technical you can find on McIntosh, and unity coupled, including chasing down patents, and obscure forum threads.  Perhaps then technically review what you have come across, and identify queries you don't quite understand, and then repeat the google search process to try and clarify queries.  Maybe identify some McIntosh forums and technical dudes and query them on your concerns.

There may be a good reason why the primary windings can't be connected as per a typical PP configuration, and I'm recalling that has something to do with insulation rating between winding sections.

[HotBluePlates]

... what is the purpose of the 3 separate windings on the driver side?

There aren't 3 separate windings; there are 6 wires:
  - Center-tapped winding for the output tube plates.  (3 wires)
  - Center-tapped winding for the output tube cathodes.  (3 wires)

So what would be the advantage of a unity coupled transformer?

You may have to do your own homework on that, such as read everything technical you can find on McIntosh, and unity coupled, including chasing down patents, and obscure forum threads.  Perhaps then technically review what you have come across, and identify queries you don't quite understand, and then repeat the google search process to try and clarify queries.  Maybe identify some McIntosh forums and technical dudes and query them on your concerns.

Winding Capacitance is reduced/controlled in a way that allows much more negative feedback around a power section using the Unity Coupled Transformer without making the amp unstable.
And then a special winding-style is employed to achieve that lower-capacitance.
And then a special circuit-configuration is used to attack the capacitance-issue from a different angle.

There may be a good reason why the primary windings can't be connected as per a typical PP configuration, and I'm recalling that has something to do with insulation rating between winding sections.

I don't know, but it would make sense there is a lesser insulation-resistance between windings in the OT, and the "special circuit configuration" I mentioned aims to keep voltages between adjacent wires "substantially the same," which is the "attack the capacitance-issue from a different angle" bit I mentioned above.



I only kinda-sorta remember the Unity Coupled stuff from years ago when I got interested in the McIntosh MC30s I have.  So I could be wrong on some details, but I still would stay away from trying to make this amp into anything else.

[AlNewman]

Hm, neat.

I pulled up the MC-30 schematic, looks like the screens are tied to the plates of the opposite tubes, and there's a separate output from the cathode winding of the transformer.  As well as another winding specifically for the NFB loop.

I wonder if there's a special crossover or something built into the speaker cabinet for the 2 separate outputs?

https://i.ibb.co/drjH0Sz/Mc-Intosh-MC30-phase-splitter-2880x1920.jpg

Kinda out of my pay grade anyways, I suppose if I ever own one I'll go down the rabbit hole.  Would likely make a pretty awesome (and expensive) hi-fi setup.

[trobbins]

Would likely make a pretty awesome (and expensive) hi-fi setup.

That may not be the situation for your Rauland Borg RA100A, unless you can confirm it started out as an awesome hi-fi setup.  The output transformer may be far from hi-fi, with minimal low frequency and high frequency capability, given it could have been designed for PA use and lowest cost.

[HotBluePlates]

I pulled up the MC-30 schematic, looks like the screens are tied to the plates of the opposite tubes, and there's a separate output from the cathode winding of the transformer.  As well as another winding specifically for the NFB loop.

I wonder if there's a special crossover or something built into the speaker cabinet for the 2 separate outputs?

There are not "2 separate outputs."  There is only 1 secondary winding, and it gets connected to 1 speaker (or cabinet, or portion of a bi-amped/tri-amped speaker cabinet).

This setup uses 2 Primary Windings that are tightly-coupled by virtue of the method of making the transformer, and there is 1 Secondary for the speaker(s).  At times, there is also a separate secondary winding for feedback.


At this point it is appropriate to do what trobbins said earlier, and research McIntosh circuits and patents separately.  We've gone far afield of the Rauland-Borg that was the subject of this thread.

[AlNewman]

Yes, you're right, didn't mean to hijack the thread.  The separate outputs I mentioned were the 2 leads coming from the cathode winding to the output jack in the schematic.
Anyways, I don't anticipate seeing a transformer like this anytime soon, I was just asking out of curiosity. 

[HotBluePlates]

... The separate outputs I mentioned were the 2 leads coming from the cathode winding to the output jack in the schematic. ...

The MC30 manual says you can have a 600Ω output across pins 5 and 8 of the Output socket, or a 70.7v output (also termed "166Ω" output for this 30w amp) across pins 5 and 6.

   - McIntosh tapped the cathode-load to get these impedances by auto-former action.
   - These output lack the DC-blocking of the 4/8/16Ω outputs.
   - Like most other amps, McIntosh didn't envision the user employing multiple outputs at the same time.

Sorry I overlooked those, cause despite having a pair of MC30s since the late-90s, I've never seen anyone use the Output socket before, and have never had an Audio Transmission Line (600Ω) or "PA system with per-speaker distribution transformer" (70.7v) situation where I've used those impedances.

[AlNewman]

Wow, I was looking at your service manual, and it's a pretty wild design.  It looks like one big feedback loop to me.  Neat stuff.