The 4-tube model (SFIII's schematic) indicates 3 X 12AX7s and 1 X 7247 driving the 6L6s; my earlier post suggested that all 4 were 12AX7s; my error. The 7247 now is more commonly known as the 12DW7:
Here
The 12AT7 has a lower gain factor than the 12AX7; same 'sound' just less of it assuming all things are equal...a 12AT7 could take some edge off and represent tweaking; same pinouts, same function; drop-in substitutes.
5751: you might be on to something Don: according to the folks at the Tube Depot:
"...If you want to reduce the gain of your amp so you can reach that creamy tone without blowing the windows out, then this is THE tube to get. Fully interchangeable with a 12AX7 tube, the US made JAN Philips 5751 will allow you to drive the power stage harder with less volume.".
According to this version of the Hot Rod Deville:
http://www.blueguitar.org/new/schem/fen ... deluxe.pdf This amp uses .1uf coupling caps and metal film resistors; more on both below.
ThunderCat posted some pics of his 2 X 12AX7 model in which Guild used metal oxide/film resistors, mylar caps, and relatively large value coupling caps ahead of the outputs; the Guild T1 RVT has .15ufs and the Thunderbass has .27ufs; larger value coupling caps plus metal film resistors is a recipe for 'clean'.
By contrast, my Gibson 6V6 amps have .02ufs in the same position even though both the Guild and Gibson driver tubes are operating at nearly the same voltage and, similar to the Thunderbass, the Fender 75 (2X6L6, 75 watts) has .022uf coupling caps.
Caps: re/ your suggestion John about 'clean' for bass and something closer to 'dirty' for lead in the twin-input Thunderbasses, I offer the following example of the effects of .15uf caps and larger-value caps in general between the driver and output tubes.
I have a working copy of a T1 RVT (.15uf caps), a top quality Gibson GA18 (.02uf caps - one seventh the value of the corresponding Guild cap in the same position), and a Marshall BluesBreaker II distortion pedal - and a really fine pedal it is. Both amps are 2 X 12AX7 and 15 watts nominal output.
Using the BB2 and the GA18, I get a sweetly gritty output. Putting the BB2 in front of the T1 RVT, I get clean; turn the BB2 pots, I get clean; turn the T1 volume up, I get clean; not Spam a la SNL, but 'Clean Clean Clean Clean...' Even though I am 'injecting' a disorted signal into the T1, I get clean out the speakers which are used Celestions in good condition; that is, any 'dirty' coming through the amp isn't being damped by sluggish speakers. The 'dirty' is being filtered out ahead of the output tubes; they never see the distorted signal due to the effects of large-value coupling caps.
Metal film resistors: the 'holy grail' of stereophiles; less noise, better resistance to heat and 'drift', and can be manufactured in tighter tolerances. The stereo crowd (I used to belong but lost my card...) likes these because the last thing they want is grit; okay if in the signal, not ok if generated locally.
Carbon composition resistors and lower-value coupling caps contribute to the the typical sound of the 1960s amp/era; raspy, gritty, twangy. Maybe Guild was after a particular sound the way Leo the Legend chased his or because Guild sold more jazz guitars and bass amps that needed 'clean', who knows but I wonder whether any of the mid/late 60s Guild amps can do 'dirty'.
I have to watch it here because Matsickma gets annoyed when I hit on Guild amps. My T1 does what it does very well; musical, versatile, decent tone, and a nice wide soundstage but if grind is the goal with a mid/late 60s Guild amp, better warm up the soldering iron...
Thanks for the tip on the 5751s DKL; just might buy a pair and do a little tube rollin'. Based on its schematic, I could see where the stock Hot Rod Deville might tend towards the 'dry' side.
Any news on the Thunderbird John?
cj