Fender Twin Reverb Ultra Linear-Master Volume

there is some debate over the sound of the ultra linear Twin Reverb,

what does ultra linear mean?

just that the OPT has screen taps, and supposedly 135 watts of power,

schematics here>    http://schems.com/manu/fender/fender.htm

this is a great sounding amp, nice reverb, speakers sound fine,

chassis>

this amp does weigh a ton, bigger pwr trans and bigger OPT,

but this particular amp sounds fantastic,

there was a complaint that the reverb was cutting out,

noticed a cheezy reverb cable jack had been put on the return cable,

so we snagged some original Switchcraft plugs off evilbay, four for $2.99

more available>

http://www.ebay.com/itm/181410527049?_trksid=p2060778.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

these plugs will keep the jack from hanging out of the back of the chassis where they can get damaged,

there was a half baked solder joint on the RCA socket inside the chassis,

somebody tried to fix this by replacing the cable jack,

you can spread the coax shields with needle nose pliers in order to expand it for the RCA jacks, fun to do old school cable prep,

510 V-dc on plates and screens, -57V-dc bias,

RCA black plates will red plate at this voltage, JJ tubes work great here,

here is the UL OPT diagram>

we rewound the Reverb XFMR just to make sure this thing stays working,

good thing as the CBS 022921 has a collapsed core tube and cheap wire,

5200 / 99 turns on a 21 EI, short stack,

when the transformer was installed, amp tone seemed off,

had to investigate, all voltages checked good, hmmm.

why would the reverb xfmr change tone even with the reverb knob turned down?

had to redraw master vol circuit, too hard to understand the Fender CAD jockeys,

after the redraw we found that phase is important,

MV circuit uses neg feedback with switch in, no feedback with switch out,

MVS=Master Vol Sw, two sections, one changes tone, one inserts neg feedback,

normally reverb xfmr phase does not matter as the springs mix everything up and delay signal anyway, but with this amp, you have to get the polarity right,

tomorrow we will flip the xfmr leads and hope that it fixes the tone,

this will change the positive feedback into negative feedback,
as it is, there is very little difference with the MV switch in or out,
interesting circuit, never seen it before,

here is the MV circuit redraw>

I never cared for the sound of this circuit.  Maybe I never got the phase right when I tried it.  It never did that marshall thing and had a sick quality to it.  It wasn't used to my knowledge after this era of amps.  Maybe the phase was a problem.  I don't see why its a feedback design.  Just looks like they are plucking extra gain out of the reverb driver.  I do see how reverse phase could create the nasal sound these things had.

ok this is weird but you get a boost with the reverb transformer  installed either way,

in phase gives more boost but sounds real nasty,

out of phase gives less boost and sounds much better, hmmm, 

tightened loose bass control that was causing the bad sound,

re capped Mallory 220/285 with Nichicon 220/350 to bump voltage from 570 to 700

series caps into OPT gives 110 uf, Blackface was two 70/350 for 35 uf into  OPT,

over filtering can steal good sound sometimes, but this guy uses the amp to play country music where more capacitance is sometimes better,

soldering reverb jacks to ground for max connection,

this amp has a cool tuber matching pot that allows you to dial out hum,

but it could use a bias adj as new tubes seem to want a different set point,

I will come back later, but ultra linear is that topology with a tap ~75% IIRC for the 2nd grid, which lowers the distortion of the output stage without needing any other feedback, (it is a kind of feedback it self) There are other approaches without the taps to emulate it, using resistive dividers, AC and DC coupled IIRC... Used in some hi-fi tube output stages as well, with 7189 (military EL84) for example in dynaco around 50's stereo amplifier, I'm playing from memory, as I said I will get another read and edit this post correcting wrong info and adding if I found something else.

JS

here is some stuff>

"Ultra-linear electronic circuits are those used to couple a tetrode or pentode vacuum-tube (also called "electron-valve") to a load (e.g. to a loudspeaker).

'Ultra-linear' is a special case of 'distributed loading'; a circuit technique patented by Alan Blumlein in 1937 (Patent No. 496,883), although the name 'distributed loading' is probably due to Mullard. In 1938 he applied for the US patent 2218902. The particular advantages of ultra-linear operation, and the name itself, were published by David Hafler and Herbert Keroes in the early 1950s through articles in the magazine "Audio Engineering" from the USA. The special case of 'ultra linear' operation is sometimes confused with the more general principle of distributed loading.

Operation

A pentode or tetrode vacuum-tube (valve) configured as a common-cathode amplifier (where the output signal appears on the plate) may be operated as:

    a pentode or tetrode, in which the screen-grid is connected to a stable DC voltage so there are no signal variations on the screen-grid (i.e. the screen-grid has 0% of the plate's output signal impressed on it), or
    a triode, in which the screen-grid is connected to the plate (i.e. the screen-grid has 100% of the plate's output signal voltage impressed on it), or
    a blend of triode and pentode, in which the screen-grid has a percentage (between 0% and 100%) of the plate's output signal impressed on it. This is the basis of the distributed load circuit, and is usually achieved by incorporating a suitable "tap" on the primary winding of the output transformer that the vacuum-tube (valve) is connected to.

The impression of any portion of the output signal onto the screen-grid can be seen as a form of feedback, which alters the behaviour of the electron stream passing from cathode to anode.
Advantages

By judicious choice of the screen-grid percentage-tap, the benefits of both triode and pentode vacuum-tubes can be realised. Over a very narrow range of percentage-tapping, distortion is found to fall to an unusually low value—sometimes less than for either triode or pentode operation[2]—while power efficiency is only slightly reduced compared with full pentode operation. The optimum percentage-tap to achieve ultra-linear operation depends mainly on the type of valve used; a commonly seen percentage is 43% (of the number of transformer primary turns on the plate-circuit) which applies to the KT88, although many other valve types have optimum values close to this. A value of 20% was recommended for 6V6GTs. Mullard circuits such as the 5-20 also used 20% distributed loading (but did not achieve ultra-linear operation), while LEAK amplifiers used 50%).

The characteristics of the circuit which make distributed loading suitable for audio power amplifiers, when compared to a tetrode or pentode amplifier, are:

    The output impedance is lowered to be about two to three time that achieved with a triode.
    Distortion is lowered to approach that achieved with a triode tube, but may be even less for ultra-linear operation.
    The power output is higher than from a triode, approaching that delivered by a pentode.
    The power output is more constant as distributed loading is a combination of a transconductance amplifier and a voltage amplifier.

The distributed load circuit may be applied to either push-pull or single-ended amplifier circuits.

Note that the term 'ultra linear' was expressly reserved only for the condition of optimum tapping point. As Hafler and Keroes wrote: "Our patent claims cover the use of any primary tap in this circuit arrangement. However, we have restricted the use of the term "Ultra Linear" to the conditions where the dynamic plate characteristic curves are most linear".
Related circuits

The "QUAD II" amplifier from QUAD uses a circuit in which the cathode has a portion of the output signal applied to it, and was referred to as "distributed load" by Peter Walker of QUAD. In the United States, McIntosh Laboratories used this technique extensively in their vacuum-tube power amplifiers. Audio Research Corp have also used a similar circuit.

This is one of the Fenders that uses the Reverb Driver as a distortion stage also.

So reverb transformer phase will matter to the distortion tone.