# Inductive Coupling



## Paul Running (Apr 12, 2020)

Any builders here, experiment with transformer or choke coupling to the output stage in guitar amps? I am interested in your discoveries. I noticed that Gibson used an inter-stage TFM in their GA-20RVT model, other than that it seemed to lose popularity as time past. I did some experimenting years ago, attempting to completely eliminate blocking distortion. I recall that I had to pay close attention to the location and orientation of the TFM...they love EMI.


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## jb welder (Sep 14, 2010)

Garnet used a center-tapped choke as phase inverter in early versions of Pro and BTO.
From what I was told, he always updated units that came into the shop to be the newer, more conventional LTPI arrangement. I got one of those chokes from Pete at Garnet several years back, but I've never come across a choke with center-tap anywhere else.


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## Paul Running (Apr 12, 2020)

Interesting circuit, using a tapped grid-choke. By the style of his symbols, it looks like an early design. Trying to imagine if the coupling cap to the choke will have less effect to blocking distortion, as opposed to RCR coupling, when operated in AB2 mode? Not sure on all the pros and cons in a functional sense. I have a Stancor A-153-C, a small inter-stage TFM. Not much iron to it, rated 10mA on the primary...I might have to keep the DC off it to reduce saturation. May I ask the size of the iron on your CT choke, used in the Garnet design? Just trying to sort this out in my mind before going to the bench.


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## nonreverb (Sep 19, 2006)

If I recall, Acoustic used interstage driver transformers on some models/eras of their SS bass amps.


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## nonreverb (Sep 19, 2006)

This is one of the most eaborate uses of an interstage transformer driver. I've worked on a couple of these. Uses a 6L6 as the driver for the transformer!!


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## Paul Running (Apr 12, 2020)

I located another Garnet model, BTO with inductive coupling however, I can't figure out how Gar configured the driver stage to the output tubes...looks like he is using a TFM winding that is not tapped on the grid side; unable to source another schematic for the power amp to verify changes.


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## Paul Running (Apr 12, 2020)

nonreverb said:


> This is one of the most eaborate uses of an interstage transformer driver. I've worked on a couple of these. Uses a 6L6 as the driver for the transformer!!


A HD amp. I wonder what special characteristics those 6550s possess?


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## nonreverb (Sep 19, 2006)

Paul Running said:


> I located another Garnet model, BTO with inductive coupling however, I can't figure out how Gar configured the driver stage to the output tubes...looks like he is using a TFM winding that is not tapped on the grid side; unable to source another schematic for the power amp to verify changes.
> 
> View attachment 341585


Is that schematic drawn correctly?


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## nonreverb (Sep 19, 2006)

Paul Running said:


> A HD amp. I wonder what special characteristics those 6550s possess?
> View attachment 341586


GE 6550A's. I heard tell they were designed with this amp in mind. I've seen pics of what this amp does to Sovtek and Chinese 6550's...it ain't pretty.
_and Tung Sol_


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## Paul Running (Apr 12, 2020)

I was hoping to find a production schematic, unless Gar added mojo.


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## nonreverb (Sep 19, 2006)

nonreverb said:


> GE 6550A's. I heard tell they were designed with this amp in mind. I've seen pics of what this amp does to Sovtek and Chinese 6550's...it ain't pretty.
> _and Tung Sol_





The GE-6550A, and The Fender 400 PS Amplifier. A most Amazing Amplifier!!!


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## Lincoln (Jun 2, 2008)

Fender tried it also, on the Music Master Bass


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## jb welder (Sep 14, 2010)

Paul Running said:


> I located another Garnet model, BTO with inductive coupling however, I can't figure out how Gar configured the driver stage to the output tubes...looks like he is using a TFM winding that is not tapped on the grid side;





nonreverb said:


> Is that schematic drawn correctly?


It is not drawn correctly, and not a 'real' Garnet schematic. Gar would freak if anybody even suggested adjustable bias. And with that 10K off the HV winding, feeding the bias diode, bias caps would go boom. And bias can not cross from primary to secondary of interstage TX  .
I think it was drawn by someone who did not understand the tapped choke.
BTO (PA) version with tapped choke attached.



Paul Running said:


> May I ask the size of the iron on your CT choke, used in the Garnet design? Just trying to sort this out in my mind before going to the bench.


Looks very similar to your Stancor, but I can't guess the size there. So core lam. thickness about 1/2", height 1&3/8", width 1&3/4".


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## nonreverb (Sep 19, 2006)

Paul Running said:


> A HD amp. I wonder what special characteristics those 6550s possess?
> View attachment 341586





jb welder said:


> It is not drawn correctly, and not a 'real' Garnet schematic. Gar would freak if anybody even suggested adjustable bias. And with that 10K off the HV winding, feeding the bias diode, bias caps would go boom. And bias can not cross from primary to secondary of interstage TX  .
> I think it was drawn by someone who did not understand the tapped choke.
> BTO (PA) version with tapped choke attached.
> 
> ...


lol...I was asking that rhetorically as it looks like a dog's breakfast! 
Can you imagine someone trying to build an amp from that schematic?? Scary...


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## Latole (Aug 11, 2020)

It is a phase inverter


Paul Running said:


> Any builders here, experiment with transformer or choke coupling to the output stage in guitar amps? I am interested in your discoveries. I noticed that Gibson used an inter-stage TFM in their GA-20RVT model, other than that it seemed to lose popularity as time past. I did some experimenting years ago, attempting to completely eliminate blocking distortion. I recall that I had to pay close attention to the location and orientation of the TFM...they love EMI.


It is a phase inverter.


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## Granny Gremlin (Jun 3, 2016)

Paul Running said:


> I located another Garnet model, BTO with inductive coupling however, I can't figure out how Gar configured the driver stage to the output tubes...looks like he is using a TFM winding that is not tapped on the grid side; unable to source another schematic for the power amp to verify changes.
> 
> View attachment 341585


That doesn't look like a diagram drawn by Gar, especially for an early BTO.

In the Garnet book the diagram for "Final and Power Supply L190D -B190D" shows a centre tapped choke being driven by a 6SN7 with negative feedback. Really weird that he didn't just use the 6SN7 to invert as it's wired up halfway there except then he sums the 2 plate outputs to drive the choke, vs driving a power tube each (like later Pro/BTOs did on subsequent pages) . The choke's centre tap is actually the return of the trem circuit and a voltage off a dedicated supply (separate from the B+; tapped off the PT primary) labelled "800 P.I.V."

Pretty much the same thing on the next page (B260D and L260D) which are just the 4 power tube version of the Pro/BTO vs 2 as with the 190.

So modding an older choked PI to be chokeless would be easy as the 6SN7 was already there with cathode1 feeding Grid 2. I wonder if the only reason for updating the PI was a safetey concern over the (non-isolated) 800 volt (?) PI voltage to the choke's centre tap. Considering the extra parts and power supply required I don't see why he woulda done it in the first place if it wasn't better (performance/tone wise). Also don't understand why the PIV wasn't taken after the PT, but perhaps it needs to be isolated from the other B+ voltages for some reason.


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## Paul Running (Apr 12, 2020)

That diagram is all over the internet, I located it at EL34 World and it's on the tubestores site too and several other small hits.


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## Paul Running (Apr 12, 2020)

Granny Gremlin said:


> In the Garnet book the diagram for "Final and Power Supply L190D -B190D" shows a centre tapped choke being driven by a 6SN7 with negative feedback. Really weird that he didn't just use the 6SN7 to invert as it's wired up halfway there except then he sums the 2 plate outputs to drive the choke, vs driving a power tube each (like later Pro/BTOs did on subsequent pages) . The choke's centre tap is actually the return of the trem circuit and a voltage off a dedicated supply (separate from the B+; tapped off the PT primary) labelled "800 P.I.V."


If the amp is biased to AB2 mode, I believe that it was incorporated to reduce blocking distortion, when grid-current may occur however, I believe that a TFM would provide more isolation. I don't know the man or his habits, it could be in his tech notes.


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## jb welder (Sep 14, 2010)

Granny Gremlin said:


> I wonder if the only reason for updating the PI was a safetey concern over the (non-isolated) 800 volt (?) PI voltage to the choke's centre tap.


That voltage going to the center-tap is not 800V, it is the bias (point G). The trem modulates it.
The voltages at either end of a choke are opposite phase, so it's a very simple PI.



Paul Running said:


> That diagram is all over the internet, I located it at EL34 World and it's on the tubestores site too and several other small hits.


That "RCB" diagram? Yikes!


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## nonreverb (Sep 19, 2006)

jb welder said:


> That voltage going to the center-tap is not 800V, it is the bias (point G). The trem modulates it.
> The voltages at either end of a choke are opposite phase, so it's a very simple PI.
> 
> 
> That "RCB" diagram? Yikes!





Granny Gremlin said:


> That doesn't look like a diagram drawn by Gar, especially for an early BTO.
> 
> In the Garnet book the diagram for "Final and Power Supply L190D -B190D" shows a centre tapped choke being driven by a 6SN7 with negative feedback. Really weird that he didn't just use the 6SN7 to invert as it's wired up halfway there except then he sums the 2 plate outputs to drive the choke, vs driving a power tube each (like later Pro/BTOs did on subsequent pages) . The choke's centre tap is actually the return of the trem circuit and a voltage off a dedicated supply (separate from the B+; tapped off the PT primary) labelled "800 P.I.V."
> 
> ...


800 PIV is the rating for the diode I believe.


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## Granny Gremlin (Jun 3, 2016)

LOL I didn't mean to post that, was gonna edit, but then walked away to do other stuff and I think my kid hit enter. I figured out that was the diode rating (how would you get 800 volts from half wave rectified 120), and that for some reason, this is the only schem where Gar actually showed the bias supply.


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