Super Twin Reverb... UL?

[dorrisant]

Does anyone know whether this amp's OT is UL or not? Fender PN: 012413. I know it is a 4 Ohm secondary. I don't know what the primary is... I can figure that out though.

I have one here. Just wondering how one would go about testing for UL. I figure more than likely it is not UL.

[brewdude]

I was under the impression that UL OT’s would have tapped primary leads for the PA screens.

[neskor]

Check here:

[Roe]

I don't remember the exact Z, its above 1k5 and below 2k2. It probably has 10% taps rather than 43% UL taps

[pdf64]

None of those Fenders are ultra linear, per se. The output valves operate pretty much in regular pentode mode. The primary taps act to take a bit of stress off the screen grids, rather than significantly modify the mode of operation.

[Stevem]

Any Fender silver face amp with a master volume is UL of some percentage or another!

That one is very interesting in that it has a driver and a phase inverter as 2 separate tubes to provide current drive to the outputs.

That amp must be a back breaker to move!

[martin manning]

Generically a power stage with screen taps on the OT primary is said to be working into a Distributed Load (DL). Various percentages can be used, depending on the desired power output vs. distortion.

Tony, if you make measurements, I'm interested to hear what the Raa and screen tap percentage is on that OT.

[dorrisant]

It is a backbreaker at 100 lbs. Maybe some neo magnet speakers would help a bit.

I was thinking of rebuilding as an SSS or similar... get rid of the compactron tube (nearly $100 last I checked), rearrange the TX placement too. Input is right in front of the PT ( ). These amps are very noisy and the wiring is certainly reminiscent of bird lodging. I am leaning toward keeping the active eq. It is one of the coolest features about the amp. Also, the knobs for the eq are not standard Fender, they range from -5 to +5. Maybe keep that in leu of the step filters.

My main question is how to arrange the power tube screen connections, as-is or ditch the screen taps. I would certainly test for what percentage the screen taps are, but I'm not sure how to proceed. Does anyone have a test procedure. My bench is pretty much loaded with test gear, so if anyone would care to advise. I'd gladly do it and report back. While at it, I'll verify the primary impedance as well.

[martin manning]

Put some signal into the secondary, say a couple of volts rms, measure that (Vsec), measure across the whole primary (Vaa), and across the screen taps (Vss). Calculate voltage ratio Vaa/Vsec and square that to get impedance ratio as usual: Raa = (Vaa/Vsec)^2 x Zsecondary. DL fraction is just Vss/Vaa.

[dorrisant]

Actually, I found this after a quick search...

"Yes, you can easily find the transformer tap ratio of an unknown transformer.

With the transformer disconnected from everything- take a low-voltage AC source (say, 5 volts or so) and put that into the secondary winding (the 8 ohm tap and ground tap is a good choice for this).

Keeping the voltage to the secondary constant (don't change it during the test), measure the voltage from the primary center (or B+) tap to a primary plate tap, and the voltage from the center tap to a primary screen/UL tap. The ratio of the measured screen tap voltage to the plate tap voltage is the UL ratio.

Say, for example, you had 150 volts on the plate, and 60 volts on the screen, during such a test- that would be a ratio of 60:150, or 4:10, or 40%...

Mind you, you will generate moderately high voltages during this test (probably in the range of 100-200v) on the primary taps, so don't grab any bare wires while the power is on. But, if you're reasonably careful, you can find what you need to know, in a jiffy.

In essence, the definition of UL ratio is the number of turns between the center tap and UL tap, compared to the number of turns between the canter tap and plate tap. Since the voltages in transformer windings are directly proportional to the turns ratios, this test is pretty much, by definition, the way to directly determine these ratios."

https://audiokarma.org/forums/index.php ... er.527748/

[pdf64]

As I recall, the OT in Fender 75s has a primary of about 4k4 with screen grid taps at about 12%.

[dorrisant]

2vac into the secondary gives 14.88vac across the primary. Looks like the screen taps are at 13.411% of the primary. Feel free to correct me.

10vac into the primary gives 0.496vac across the secondary. 20.16 ratio... with a 4Ω load, 1626Ω is reflected to the primary. So it looks like a 1k6 primary.

[pdf64]

Check the source waveform on your scope; some sig gens have trouble maintaining a decent sine wave into low impedance loads. If the waveform gets too far from sine then the readings may not be valid.

[martin manning]

2vac into the secondary gives 14.88vac across the primary. Looks like the screen taps are at 13.411% of the primary. Feel free to correct me.

10vac into the primary gives 0.496vac across the secondary. 20.16 ratio... with a 4Ω load, 1626Ω is reflected to the primary. So it looks like a 1k6 primary.

Your Zpri calculation looks ok, but I don't understand the other one. Vpri/Vsec should be the same as when you put the signal into the primary, but it's only 14.88/2.0 = 7.44??
With signal going into the secondary, measure across the plate leads and across the screen taps, and divide the screen measurement by the plate measurement to get the DL fraction.

[dorrisant]

2vac into the secondary gives 14.88vac across the primary and 1.87vac across the screen taps...

1.87/14.88 = 0.12567 = 12.57%

I don't know where I got the 13.411%

So not UL, but what does that difference mean?