Guild Model 66-J Guitar Amp

[CJ]

we get Gibson amps in here, once in a while a Gretch amp, but this is the first Guild amp we have had,

about a 1957 Model 66_J,

been worked over a bit but in pretty good shape,

 

[CJ]

Jensen P12Q !

some of the coupling caps have been replaced,  cathode resistor, some plate resistors,

 

[CJ]

microphone input, 2 guitar inputs, and a "Recording" input, which is a jack with a cap in series to the tube grid instead of a resistor, maybe this is to cut down on hum,

new handle put on,

 

[CJ]

this amp had some static when you hit certain notes, hard to track down, you could hit every component on the tag board with a chop stick but they all did the same thing which is make noise,

turned out they ground everything through a screw that holds the board down and the nut was loose, (blue arrow)

 

[CJ]

we took some AC and DC voltages and revised the schemo for this version which had a slightly different Trem circuit,

heaters are runnin a bit hi but that be normal in 120 land,

simple amp, one triode into the inverter and that's all you get, Baxendall tone circuit since no extra tubes for makeup gain,

ceramic tone caps and a big-azz mica probably from a surplus shop somewhere,

 

[CJ]

can't hack the OPT, too much vintage value, on consignment, un-ethical, immoral, dang it! 

really great freq response and sq wave,

 

[CJ]

here is an unmolested schematic with the weird plate resistor circuit on the trem/inverter tubes,

 

[PRR]

> Baxendall tone circuit

James.

 

[fazer]

Missed it by that much.  one cap difference .  Good eyes.  James vs Bax


I’m amazed at these old amps.  They look like a rats nest and still working.  Roll some tubes, clean some contacts.  Plug in guitar.  Play gig!   

 

[PRR]

Baxandall wraps-around a Gain Stage. It typically uses Linear pots and gives unity gain. You find equal-value caps for boost and cut (though some condense this to a single cap for each range).

James is a loss-only tone control. To get "20dB Boost" it typically uses 10% Audio-taper pots and has a flat loss of 10:1, which must be made-up somehow (not necessarily adjacent to the tone network, but somewhere). It typically has caps in 10:1 ratios, your 5000uuFd+500uuFd treble caps.

Guitar amp tone is not hi-fi tone so values may vary. Using LIN pots in James gives low and high boost which is an accepted default tone. The turnover freqs may not be similar boost/cut so you have 4000uuFd+0.001uFd bass caps.

 

[fazer]

I noticed in a modeler of James vs bax. How James turnover happens where the bass turnover is and how the bax had flat mids with a shelf bass and shelf treble.  I’m sure values of parts also have an affect .  I did not sub exact parts just used what came up.  Bax seem more hi if.

 

[abbey road d enfer]

I noticed in a modeler of James vs bax. How James turnover happens where the bass turnover is and how the bax had flat mids with a shelf bass and shelf treble.

This difference is not inevitable. Flat mids is a design choice, that can be implemented in both James and Bax. However, James is more difficult to be made as flat as Bax, unless choosing very distant bass and treble turnovers and buffering the input.

Bax seem more hi if.

Certainly more predictable, and the fact that the gain stage around which it is built is under heavy NFB contains distortion.