Couple of Super questions.

[AcornHouse]

I'm thinking about doing a tweed Super build and have an idea to throw out for perusal. The only difference between the 5e4a and 5f4 versions, as far as I can tell, is one resistor. So, I'm thinking about putting both on a SPDT switch and be able to use either 6V6s or 6L6s.
Anyone see any problems with that?

Also, in the 5e4a schematic, the inputs are "Instrument" and "Microphone". But in the 5f4 schematic they are labelled "Normal" and "Bright", yet there's no difference in the input circuits. Am I missing something, or did Fender just relabel them?

[Firestorm]

There must be an OT difference, too. Fender preamps are essentially identical within each generation. Voltage and Zout change though.

[AcornHouse]

There must be an OT difference, too. Fender preamps are essentially identical within each generation. Voltage and Zout change though.

No, they used the same OTs (and pushed the 6V6s beyond their specs).

[Firestorm]

Well, you made me look it up and, yes, there seem to be examples with both tube complements running a Triad 1848 OT (and sometimes the same PT, too, but there's a hint that some 6V6 amps had 5Y3s.) Definitely a crazy period in Fullerton.

I wonder if anyone has measured the primary impedance of the 1848? If it's high (6Kish), that would be cold for 6L6s. If it's low (4Kish) that would be hot for the 6V6s, especially if they got the 8087 PT.

I don't know how much stock I'd put in the schematic, given all this.

[Stevem]

The mic input is the one that everyone goes for , so skip the Instrument input from the get go!

[pdf64]

...I wonder if anyone has measured the primary impedance of the 1848? If it's high (6Kish), that would be cold for 6L6s. If it's low (4Kish) that would be hot for the 6V6s, especially if they got the 8087 PT...

The Classic Tone 40-18088 http://www.classictone.net/40-18088.html seems to be their equivalent of the 1848; it's a 6k primary.

I can't see a reason why a 6k primary should cause 6L6 to run cold?
For a class AB amp, I think that increasing the p-p load may effectively move operation closer to class A (less % of the cycle spent in cut off), or at least should allow hotter bias without exceeding the limiting plate dissipation.
In a regular guitar amp circuit, my finding is that 6L6 seem to put out most power into a load a little under 5k but it doesn't drop off much until below 4k or above 8k.

[Firestorm]

Good to know. It explains why Fender could be so "creative" in that period. I'll just add this one point: everything Fender did seems to have been to squeeze more gain out of the amps. This is different. Is this the period when they were having problems with the Strat? Maybe preoccupied? There weren't many of these amps made and modern players seem to like them. 6V6s get creamy by themselves, but keeping 6L6s in Class A longer should increase second order harmonic distortion, right?

[pdf64]

keeping 6L6s in Class A longer should increase second order harmonic distortion, right?

Not particularly in a push-pull amp. 2nd harmonics are common mode, and common mode signals introduced into a balanced system will tend to be cancelled out.

[Firestorm]

Ah, yes. That explains why I am so fond of SE amps.

[martin manning]

keeping 6L6s in Class A longer should increase second order harmonic distortion, right?

Not particularly in a push-pull amp. 2nd harmonics are common mode, and common mode signals introduced into a balanced system will tend to be cancelled out.

My understanding is that a Class AB p-p power amp will not introduce 2nd-order HD, but it will reproduce it if it is contained in the input signal. However if the input signal is low enough that the amp remains in Class A, then it will introduce some 2nd-order HD of its own.

[Structo]

There is a bright cap on the top volume pot.

[galtjunk]

I built a 4 hole 6v6 5e4a as a head using bassman iron.
It turned out great.
You can hear it on the latest Barrence Whitfield and the Savages album.

Peter Greenberg played my 56 5e4a and loved it but wanted more power.
He didn't want another 6l6 amp so I built him a 4 6v6 5e4a.

[didit]

[...] if the input signal is low enough that the amp remains in Class A, then it will introduce some 2nd-order HD of its own.

That's my understanding as well.

On PP Zout, my various books/datasheets for 6L6 types range from 3.8K up to 9K with related variation on bias and B+ voltages. See http://tdsl.duncanamps.com/show.php?des=6L6G for a few data points. I have 6L6 running in what started as 5E3 clone from Mission Amp. Still has Bruce's stock OPT 8K primary and it sounds great. Class AB with lots of A range. I didn't document it and don't recall specifics. With a 5AR4 rectifier I recall finding ~18W of useful output. A long way from crunch & 50-60W but sweet 50's era tone.

Best .. Ian

[JTU]

Isn't 2nd order distortion the kind where the top and bottom of waveform are different? So its the transformer that gets rid of it and not the class of operation of the power tubes. Any 2nd order created by the preamp gets amplified normally.

In theory perfect class A amp would not create 2nd order distortion, because the top and bottom would clip in perfect symmetry. Tubes cannot be perfect (in clipping) because grid limiting and cut off are different.

Sorry if its of topic...

[martin manning]

Even without clipping, plate voltage vs. grid voltage is nonlinear. In a Class-A amp the top and bottom halves of the input waveform are reproduced by different parts of the power tube characteristic curves, so second-order harmonic distortion is produced. In a Class AB amp the top and bottom halves of the waveform are reproduced by the same part of the characteristic curves (to the extent that two devices can be "the same"), so there is no second order HD produced.