interstage transformer

[uneumann]

I'm mulling over a project that needs a single triode (12ax7) CC stage with gain and I expect it to be clipping. The problem is that I need the stage to be non-inverting since I want to mix it's attenuated output with a copy of the input signal and I need them to be in phase. The option that seems most interesting and simple to me is to use a small interstage transformer to invert the triode output. Is there any example of an amp out there that uses an interstage transformer in an application even remotely like this? Example schematic pointers?

[g1]

Seeing as you said "remotely" , some older Garnet amps used a centre-tapped choke to invert phase for the phase splitter. Not a transformer but similar.

[soundguruman]

I'm mulling over a project that needs a single triode (12ax7) CC stage with gain and I expect it to be clipping. The problem is that I need the stage to be non-inverting since I want to mix it's attenuated output with a copy of the input signal and I need them to be in phase. The option that seems most interesting and simple to me is to use a small interstage transformer to invert the triode output. Is there any example of an amp out there that uses an interstage transformer in an application even remotely like this? Example schematic pointers?

Fender Music Master Amp...inter-stage coupled.
fender musicmaster_bass_schem.pdf
BUT...distortion does not pass...will turn a square wave into a sine wave. Inductive coupling tends to do that.


The best example of combining original signal with transformer coupled signal is the Super Champ.
But even that uses the primary audio, not secondary...
It's kind of tricky to combine signals cause of - phase lag...resulting in cancellations of audio frequencies.
The super champ is more complicated than it first appears.
It is using signal from the reverb drive, and combining it with preamp signal.

[Leo_Gnardo]

Fender Music Master Amp...inter-stage coupled. BUT...distortion does not pass...will turn a square wave into a sine wave. Inductive coupling tends to do that.

Wayll... depending on the quality of the transformer. You can depend on the bargain brands to "round of the corners" but something really good like Dean Jensen iron and/or nickel core is very good at preserving waveform integrity. Not that anyone would bother spending Dean Jensen prices in a guitar amp.

The best example of combining original signal with transformer coupled signal is the Super Champ.
But even that uses the primary audio, not secondary...
It's kind of tricky to combine signals cause of - phase lag...resulting in cancellations of audio frequencies.
The super champ is more complicated than it first appears.
It is using signal from the reverb drive, and combining it with preamp signal.

That reverb-drive trick is in some other Fender amps from 70's-80's. "Pull for icky tone" switch. Just awful.

[uneumann]

THANKS - some cool examples. The super champ uses a coupling transformer, as many reverbs do, and phase control is important. The Musicmaster uses it in a PI configuration. The Garnet is pretty cool - just a tapped choke as a PI. Thanks - these are all pretty cool and useful ideas for transformers.

I'm wondering if I can capture some of the tone and behavior of an SE output stage (on a mini-scale).
My idea is shown below - don't pay too much attention to the component values or even the exact configuration. It's still in the idea stage.
I do want the output level of the transformer stage to match (roughly) the level of the stage feeding it so the two level controls will mix them about evenly. So I want gain (for clipping) but then I'll attenuate back to the input level. The funky switching is to add some voicing options - that's going to take lots of testing and listening to get right, but you can see where I'm going... some loading, Eq., and feedback variations.

[soundguruman]

Wayll... depending on the quality of the transformer. You can depend on the bargain brands to "round of the corners" but something really good like Dean Jensen iron and/or nickel core is very good at preserving waveform integrity. Not that anyone would bother spending Dean Jensen prices in a guitar amp.



That reverb-drive trick is in some other Fender amps from 70's-80's. "Pull for icky tone" switch. Just awful.

Yes Leo, I agree. It does sound absolutely awful.
Vomit-tone.

[Leo_Gnardo]

I'm wondering if I can capture some of the tone and behavior of an SE output stage (on a mini-scale).
My idea is shown below - don't pay too much attention to the component values or even the exact configuration. It's still in the idea stage.
I do want the output level of the transformer stage to match (roughly) the level of the stage feeding it so the two level controls will mix them about evenly. So I want gain (for clipping) but then I'll attenuate back to the input level. The funky switching is to add some voicing options - that's going to take lots of testing and listening to get right, but you can see where I'm going... some loading, Eq., and feedback variations.

Might also have a peek at the old "Gibsonette" I think GA-5. If my memory serves correctly this amp that's otherwise Champ-like used a fingertip-size IT to deliver signal to the output tube. I had to fix one & my best guess was a tiny IT from Antique/CE. It did work well, and the trans was rated for a 3:1 stepup ratio. Don't have the part # hand OTOH Antique/CE don't have very many IT's of this sort, I think maybe three. It was @ $15. You might be able to do something with that one.

[kg]

Does it HAVE to be common cathode?

[uneumann]

Does it HAVE to be common cathode?

Well, I need gain (at least 20 - more is probably better) so I can get some clipping. Not sure how else to do that without running a CC stage.
I have a spare triode, so CC option looked good, but I'm open to other suggestions.

[Leo_Gnardo]

Well, I need gain (at least 20 - more is probably better) so I can get some clipping. Not sure how else to do that without running a CC stage.
I have a spare triode, so CC option looked good, but I'm open to other suggestions.

Gain of 20? You're looking at a mic/line transformer. Mouser used to have a selection of handy cheap little trafos but not for the last couple years - someone must have them. Mouser now carries a shorter list of slightly larger transformers, some Japan brand, Tamura IIRC, but good quality for the price, at say $12-20 still affordable. You could block DC from the primary with a cap and boost the signal with the transformer. Happy experimenting!

[uneumann]

Might also have a peek at the old "Gibsonette" I think GA-5. If my memory serves correctly this amp that's otherwise Champ-like used a fingertip-size IT to deliver signal to the output tube. I had to fix one & my best guess was a tiny IT from Antique/CE. It did work well, and the trans was rated for a 3:1 stepup ratio. Don't have the part # hand OTOH Antique/CE don't have very many IT's of this sort, I think maybe three. It was @ $15. You might be able to do something with that one.

Thanks - another good pointer... I found this schematic (see below). This is what you probably mean. It uses the IT as a PI.
So far, I'm trying to use a simple Hammond 148Q - they're pretty cheap and small. I've had one in my parts box for a while waiting for a purpose. I'm not wed to it, but can't see why it shouldn't work or why something else would be better.

[Chuck H]

The spare triode would certainly be the easiest and cleanest way to flip the phase. That won't give you any transformer mojo. But thinking about, using a transformer that early in a dirty channel is going to require careful location and grounding to avoid hum.

With a spare triode that's the way I'd go. If you don't want to use the triode you could always do the stacked RC load thing typical of oscillator circuits. Since the total load for such a circuit might be about 250k to 333k I don't think you'll lose much gain.

[uneumann]

Gain of 20? You're looking at a mic/line transformer. Mouser used to have a selection of handy cheap little trafos but not for the last couple years - someone must have them. Mouser now carries a shorter list of slightly larger transformers, some Japan brand but good quality, at say $12-20 still affordable. You could block DC with a cap and boost the signal with the transformer. Happy experimenting!

Right - A line xformer will give some voltage gain, but it's not going to clip like a tube stage would.
But - your point hits on something I had not considered... why not put the IT inversion *before* the CF? I'll have to think on that too... amazing how many options and opportunities there are for even just one stage in a preamp.

[uneumann]

The spare triode would certainly be the easiest and cleanest way to flip the phase. That won't give you any transformer mojo. But thinking about, using a transformer that early in a dirty channel is going to require careful location and grounding to avoid hum.

With a spare triode that's the way I'd go. If you don't want to use the triode you could always do the stacked RC load thing typical of oscillator circuits. Since the total load for such a circuit might be about 250k to 333k I don't think you'll lose much gain.

Thanks again - so many good points...
You're right about the hum and coupling issues. I've run into that before. I'll have to see how bad it is once I do a proto build. I've noted that keeping primary and secondary impedances low really helps reduce noise pickup by the transformer. It's like a low impedance 600 ohm mixer input transformer. The key to reducing noise pickup in the line and in the transformer itself is the low impedance.

[Chuck H]

There are also quiet spots that can be found moving the transformer around and listening before you mount it. Different phase alignments of the existing and induced fields and such. It's surprising but sometimes you can find a dead quiet spot.