AB763 stripped down?

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AB763 stripped down?

Postby cw » Sun Dec 27, 2009 11:16 am

has anybody here ever stripped down a Fender Showman or Twin, AB763 circuit, to get rid of the vibrato circuit? i am wanting to step up into a bit higher wattage amp (i'll still keep my 50 watter for backup thanks!) but i don't want to go so high as to have to respeaker my cab (a mid 60's dual cell blonde Bassman 212) which is presently loaded with G12t-75's parallel for a total of 150 watts, 8 ohms. 80 to 100 watts rms would be fine. if i would have to respeaker i can but i really like the tone. just want some more balls (short for ballistics). maybe another cab wiffem a 15 incher?

another thought would be to step up the OT in the same power out circuit to help prevent saturation and help keep it clean and strong with the bass response. presently the curcuit calls for a 022889 (new Fender number replacing the old 125A30A number).

the triode stock OT replacement or magnetics components number is 40-18013 for the 022889 tranny. i was thinking a Hammond 1650T might do the trick for about 100 to 120 watts. any suggestions or opinions? the idea is to get more clean bass headroom without a lot of overkill.

basically this is the idea:

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Postby EWBrown » Mon Dec 28, 2009 9:50 am

I used to have one of these amp a LONG time ago (got it through a poilce auction for $50 in 1974). I never used the vibrato, I just yanked out the 12AX7 and left everything else as original. I should have performed one more step (see next paragraphs for explanation):

If you just want to disable the vibrato, the easiest procedure would be to pull out the 12AX7 in the vibrato circuit, and disconnect the top end of the 50K-RA "intensity" pot.

This effectively removes the circiut from the rest of the amp, and still allows using the "vibrato" input stage as an extra "Normal" input stage, but with the addition of having the "middle" tone control. This also allows an easy restoration later, if you should need it. Unless you're playing 1960s surf or hot-rod rock, or emulating Duane Eddy or The Ventures, vibrato has gone the way of the dinosaur...

If you wanted a "middle" tone control on the "Normal" input, take out the 50K pot, install a 10K pot in its place, and then and use it to replace the 6.8K resistor in the bottom of the tone stack. This will make vibrato restoration a bit more difficult.

Re the OPT:

The Triode unit is probably a "step up" already:
http://www.magneticcomponents.net/40-18013_Stock.pdf

If you want better low end response, than any good 1.9 to 2.2K OPT with the appropriate (at least 100W RMS) power handling capability should be good to go. Though this could involve some extra chassis drilling to fit the new mounting holes pattern. The Hammond should be fine for this application. But make sure that it will fit on the chassis, before taking the plunge, it is big and heavy (around 14 pounds).

TRANSFORMER, OUTPUT, PUSH-PULL, HAMMOND, 120 WATT

P-T1650T


Designed for push-pull tube output circuits.
Enclosed (shielded), 4 slot, above chassis Type "X" mounting.
Frequency response 30 Hz to 30 Khz. at full rated power (+/- 1 db max. - ref. 1 Khz) minimum.
Insulated flexible leads 8" min.
Includes 40% screen taps for Ultra-Linear operation (if desired).
Typical applications - Push-Pull: triode, Ultra-Linear pentode, pentode and tetrode connected audio output.

Specifications
Audio Watts (RMS): 120
Primary Impedance (Ohms): 1,900 CT
Maximum DC per side (mA): 403
Secondary Impedance (Ohms): 4/8/16
Mounting centers: 3" x 3.31", vertical mount
Weight: 14.0 lbs.
Suggested tube types: 6L6GC, 5881, EL34, 6550B, KT88 (push-pull parallel, 4 or 6 tubes).


I'd also replace the old solid state rectifiers with something newer, like UF5408s would be good, and just use two in series instead of three - you could probably even get away with just using one per leg. The silicon rectifiers of the 1960s were not nearly as reliable and hefty as those available today. UF4007s might work, but I'd go with the heftier 5408s just to play it safe, at little extra cost.

/ed B
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Postby cw » Mon Dec 28, 2009 11:48 am

this is exactly the response (info) i need to make my choice for a beefier amp. i do want to use a four tube output for bass, as noted by Don H. but have been undecided about maybe using basically a Dynaco MK3 and trying to repop a Sunn 200S which is the Dynaco circuit via Conrads rendition. i was just shying away from a two tube KT88 output for so many watts and the fact i can use the tube stock i have already for my Bassman 10. (hugging my Raytheon 12AT7's now to get that warm fuzzy feeling) ;-).
i make no pretenses about not being an electronics tech, but i do know how to research and generally have a good idea of where to get answers from knowledgable people.

thanks for your input, your opinion is greatly valued.

PS: for Christmas my g/f gave me a 50's vintage Jensen P12Q her father had and it looks like new! one more and a couple of ceramics and i will have enviable speaker cab guts for a low watt guitar amp!

bill
you understand something when you can explain it to your grandmother ... einstein; faster horses, younger women, older whisky, more money ... tom t hall; batteries belong in flashlights...bill lawrence
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Postby EWBrown » Mon Dec 28, 2009 1:16 pm

Those 6L6GCs are pretty tough and reliable, and readily available I'd stick with them rather than trying to re-design for more "exotic" (and expensive) power tubes. Besides, in order to significantly increase the output power, the power supply would have to be increased in both voltage and current capability, and at that point, a whole new start would probably be better than trying an upgrade,

I'll leave the topic of which are the better guitar amp speakers to those who are far more qualified than I to make such decisions. That which makes for a good "audio" speaker isn't necessarily the same as what makes the best musical insttrument speaker

It has been said that the main reason that the Russian and Eastern Europe tube manufacturers are still in business, is because of the guitar players customer base,which is much larger than the number of audiophiles.

Just about all of the Russian made (Sovtek, E-H, Svetlana, etc) 6L6GCs are excellent, as are many (but not all) of the Chinese made ones (Ruby Tubes are probably at the top of the heap 6L6GCs from China). They have a better track record than most of the Chinese made EL34s, 6550s and KT88s, by far.

/ed B
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