t-bird amp questions

Hello all,i am new to this site and to Guild amps.I just picked up a 66 or 67 thunderbird amp,12",8"speaker combo w/7591a output section. I'm am looking for a schematic for it and also some direction as where to find so nice 4 section main filter caps for this unit.Any help and/or comments would be greatly appriciated!Also not to keen on using the stock speakers,don't want to damage them....any ideas for a good replacement

Welcome to LTG. I suspect one of the amp gurus will be along presently.

Hi rockinadaddy and welcome to LTG; use the PM button below my post ... send me a PM with your email address and I'll send you a schematic; yours has the banded side panels and two-tone grill cloth ... like this one? If not, post a pic or describe it.




The 4X40uf can cap has been out of production for some time. Vibroworld has a CE Mfrg 30/30/30/10 but it's 1.5" dia and 3.5" high ... not sure the diameter matches what you have. http://www.vibroworld.com/parts/tech15.html#cans Mojo had 3X40uf cans last year but apparently not now.

Are you speakers original; might be CTS (137XXX) in early models and Jensen 'EM' series in the later? Don't have one so I couldn't tell you what might sound good in it but if your speakers are original, you can probably get them re-coned and a new vc if needed for what new might cost. If you don't want to mess with what you have, you can't go wrong with a Celestion G8L for the 8" ... used in the $35 range.

Send me your email addie, I'll send you the schematic. CJ

Email w/ schematic sent. CJ

I was browsing and found this item from CD Distribution. Don't know the voltage rating of the original from the Thunderbird, but I may have to do some measurements to see if this would fit.

C-EC40X4-350
2009 Catalog
page 37
CAPACITOR, ELECTROLYTIC, 40/40/40/40 UF @350 VDC


Made in the USA, to the same specs as the Original Mallory, on Mallory's original machinery.

* 4 sections - all 40 µF @ 350 VDC
* -10%, + 50% tolerance
* 85° temperature rating
* 1 3/8" diameter x 2 1/2" tall

Hi Talisman; not completely sure that cap will work in the amp. Part of the version 1 Thunderbird schematic (2X7591s) below with the red caps being the primary B+ filters connected in series followed by the next two circled in blue. That choke is only dropping 5V so the B+ appearing at the next section - Tap 2/screens - is 355VDC which is more than the rating of the can cap you are suggesting. The next cap in blue sees something less than 355VDC but only to the extent dropped across the 2.7K R.




Tube Depot has a CE Manufacturing 2X40uf/2X20uf@525VDC (PN CP-CE-40X2-20X2-525V). Granted it doesn't supply the design uf but it's either than or a cap that won't withstand the voltage ... everybody has their own sense of how much risk to take but I don't think the amp's tone would suffer all that much. The tech for another BBer who owns one of these said that he'd found a 4X40ufX525V can cap ... in Alaska where it had to have been sitting on a shelf for a long time ... it's been a while since the 4X40uf has been available. CJ

just to play devils advocate,i thought an old cap,i.e. sitting for many years,would cause the electrolyte to dry out also. Or is it when only used and then sitting for years it drys out?!?

Hi Rock: both ... if used and then followed by long period of no use, the electrolytic paste inside dries out. Same can happen if they sit on shelves for an extended period ... I'm talking about 15-20 years. The electrolytic paste gives them some of the same attributes as batteries although they typically don't leak like AA or flashlight / alkaline batteries do. Good practice with amps that haven't been on in a while is to slowly power them up with a Variac; a variable transformer that can adjust wall voltage ... to 're-form' them. Sometimes it works but sometimes the caps just let go when the voltage gets up anyway. Making any progress? CJ

oh yeah,it works and sounds great,i did use a variac to power up,replaced the 6.8ohm cathode resistors,one was burnt and getting some speakers so i don't destroy the originals. Funny thing, i opened up some old Kasino PA cabs i have laying around,they were loaded with 8" CTS speakers,identical to the one Guild used,except they're 16ohm instead of 8ohm.Thanks again for the schematic

It's up to you of course but at South Florida Speakers in Lake Worth FL, for $40-$45 you could get the voice coil changed to 8 ohm and a fresh cone - any good speaker shop near you could do the same. If you leave it 16 ohms and wire it in in parallel as it was originally, you'd raise the load to 12 ohms which would still be okay under the 'Half or Double' rule; can safely cut the load to no less than half or increase it to no more than twice the rating. Your original configuration of two 8 ohm speakers in parallel produced a 4 ohm load; your amp was already running safely under Half/Double and letting the amp work a little easier but leaving that speaker at 16 ohms would triple the load originally seen by the output tubes from 4 to 12 ohms. CJ

oh no i wasn't gonna use the 16 ohm,just stating a fact that they were'nt 8ohms with would have been perfect!

Ok R-daddy ... was mostly thinking out loud. :wink: