Hi I'm new here and recapping rt66's Thunderbird II

Great stuff here! I'm doing the recap on rt66's T-bird II amp and I'm taking a few pictures wihile I'm at it. I'll give the pictures to rt when I'm done and ask him to post 'em. It is a killer sounding amp as a lot of you know! I'm an old Fender amp cloner/tweaker who is not that easily impressed ( I've been a luthier, keyboard and amp tech full time on and off since 1968 so I've played through a lot of different amps I've been working on over the years ) but that bird really turns my crank. I've already got the two schems that capnjuan was kind enough to forward to rt66 but this one has notable differences.

If any of you have the schematic for this one I'd be willing to trade something for it. I've got alot of cool crap in my junkbox! This model has the FET controlled speed switch on the back panel and an extra tube in the preamp chassis that looks like it's in the reverb circuit (12ax7a) with the 6gw8. At least it's on the same pwb. It also sports a few smaller electrolytics probably for the fet supply ( one appears to be in the hum balance circuit on the preamp filaments) in the vibrato.

I made fiberglass boards for the larger recap caps and mounted them with long standoff screws in existing holes where possible. I found some nice CDE USA made 105 degree electrolytics and upped the values slightly for a little better low end. The amp sounds great and although the recap was a bit of hassel, it's nice to know this one will kicking the jams for quite a while longer.

I've got to go wind some pickups now;
thanks for the site, the pictures and the help

gb

Hi Bill and welcome to LTG! Send me a PM including your email address; I'll send/re-send whatever schematics I have. The people at FMIC were kind enough to send us what they had for our use but there's no guarantee they had every schematic for every design variant Guild ever produced. Good luck and welcome! CJ

Thanks capnjuan, as a follow up on the recap, I tested the amp late last night and while it sounded good I noticed that one of the 7591 output tubes was running too hot so I checked the bias on both. It was way off the recommended .35 volts dc on each pin 5 to ground. The two 6.8 ohm current limiters had changed to 18 ohms and 14k! Changing them out put it back to rights. The 6.8's were almost perfect looking but the one that had changed the most had a couple of small black spots on it upon closer inspection.

I'll encourage rt66 to post pics of the recap. It may help someone down the road.

cj thanks again for the schems.

gb

Awesome!

Thanks to cj I got the right schematic. Thanks to all you kind folks who replied.

If any of you need help with repairs I'd be glad to help----just pm me because I probably won't be on-line much.

Thanks again
gb

Hi guys
Here are some pics of the recap job Bill did for me.
Came out great and the amp sounds great too.
Had a couple of initial quirks that seem to have gone away.
One was after it warmed up a while there was a staticky kind of sound like an old LP record hiss.
Then one time fooling around with the foot pedal there was a couple of odd lound noises which went away and haven't heard since. Dirty pedal comtacts?
Its been real quiet since.
I do have to adjust the "hum balance" screw on the back when I change channels or sometimes even guitars. Is that normal for these amps?
The hum screw works real good though.
On recommendation I did get a couple extra G6W8 NOS tubes since they seem to be drying up with no replacements. Can't even find the military version C-something tube.
A canadian tube outfit The Tube Store had five in stock.
I got one made in Great Britain and one made in Holland. Both are RCA's so don't know the parent company except by reputation Holland and Britain made some great tubes.
They had at the time one more GB made tube and two made in Yugoslavia for 29.95ea if anybody is looking.
What does the G6W8 tube run anyway? Is the amp virtually useless without it?
They were used as power tubes in some models, Right?
All the other tubes seem to check out OK for now
Anyways Bill did a great job.
Don't have any pics of what went on under the circuit boards, but you can see there is a bit of modification to bypass the old canister caps.
Hope they will help if you need to do something similar.
Bill used a high temp sicone to set the caps on fiberboard platforms.
RT

RT 66 said:

Hi Ray; nice-looking work ... I know I sound like I'm preaching to the choir but there is just no substitute for rebuilding the power supply to put life in an amp; you can fiddle with tubes and speakers but nothing beats fresh power!

One was after it warmed up a while there was a staticky kind of sound like an old LP record hiss. Crackle can be a function of dirty tube pins and sockets; particularly on the 6L6s; pull, squirt non-lubricating type cleaner in the sockets, push tube in/out several times, fire up, see what happens ... same for pots; did Bill get a chance to clean them (requires lubricating-type contact cleaner). Then one time fooling around with the foot pedal there was a couple of odd lound noises which went away and haven't heard since. Dirty pedal comtacts? Yes; open the switchbox, bathe the switch bodies in lubricating-type cleaner, push switches several times in succession ... go and sin no more.

I do have to adjust the "hum balance" screw on the back when I change channels or sometimes even guitars. Is that normal for these amps? No; sounds like one of the preamp tubes' heaters is pulling more current from the 6.3VAC heater winding than the other; suggest switching the positions of the two preamp tubes; if anomaly follows, then replace either/both 12AX7s and do - over. On recommendation I did get a couple extra G6W8 I got one made in Great Britain and one made in Holland. Both are RCA's so don't know the parent company except by reputation Holland and Britain made some great tubes. Likely Mullard and Amperex or possibly Phillips ... Yugoslavians if old more likely Tungsrams ... regarded by many as the best of the type. ... if anybody is looking.... If looking, the looker should look to our Coastie who has the finest stash of 6GW8s available.

What does the G6W8 tube run anyway? The 6GW8 is a cousin of the 6BM8; the tube is 1/2 triode and one 1/2 pentode. Explanation below. Is the amp virtually useless without it? No; but the reverb is. They were used as power tubes in some models, Right? Yes; on Guild's T1 and T1 RVT; the pentode halves are the power tubes; one of the triode halves is the final gain stage and the other half is the phase inverter / driver; this 'two tubes in one bottle' is the appeal of the tube in general; it saves another socket. ... there is a bit of modification to bypass the old canister caps. Bill used a high temp sicone to set the caps on fiberboard platforms. Seeing it; a very fine work-around for the problem of no availability of original or matching parts in the aftermarket.

Click to expand...

Reverb amp in the Tbird V2; signal is fed to the grid of the pentode half (coming down on the left) and amplified to drive the reverb tranformer hard enough to induce a matching signal but at a lower impedance/lower voltage on the secondary side. The signal is fed to the reverb can where the boingy boingy happens. The signal is recovered from the can and fed to the triode half of the 6GW8 where it's then amplied and returned to the audio circuit as shown below:



To get the best performance, you need to make sure the footswitch contacts and reverb pot (shown in blue circles) are clean, the 6GW8 socket and pins are clean, and the rca fittings at the in/out points of the reverb can are clean; same way, spray w/ non-lubricating type cleaner, shove cables on/off a few times.

On the hum balance, if tube-swapping doesn't cut it, you or Bill may need to experiment with adding a 100 ohm / half-watt resistor to one side of the hum balance pot to ground. Sometimes it's a function of the fact your wall voltage is closer to 120VAC than the expected 110V or uneven power consumption by the heaters but you can add the resistor to one side; if good, no problem. If no good, take it off and put it on the other side; if good, no problem. If the problem survives tube swapping and the resistor trick, it's something else; needing an end of line resistor? Who knows.

Bill did a good job with those caps; there aren't very many good solutions that don't require punching more holes for can caps; glad it's working, hope it sounds the way you want. Best, John

Hi John
Thanks for the info.
Will try your suggestions. Very good as always!
I missed Coasties offer. Got them after the fact. I don't think they were a bad price considering some clown was trying to get $100 for one somewhere.
I've got a small stash of NOS 12AX7 in reserve beyond what I retubed my 79 Fender Deluxe Reverb with. I got them a few years ago and already they are way more than I spent. I got some blackplate and grayplate RCAs from the 50's and 60's that I hope are holding up.
Good to know that the amp is still usefull without the 6GW8.
I use an echo delay pedal all the time anyway, but I still love it when a good boingy boingy happens.
Always amazed with you electron wizards. Bill may as well of been speaking Chinese for all that I was understanding what he was going on about.
Anyways I got another boat anchor that can crank out some heavy vibes.
The original speakers sound great to me. Will save that experimentation for another time.
Thanks again for the info and for helping Bill out with the schematics.

Ray

Hi Ray; glad you found Wild Bill; I guess anybody that far north who has technical amp knowhow has to be a little wild ... yes; try the flippy flippy with the preamp tubes ... I hope it works stopping the need for further intervention. It's nice to have all the pieces and parts working ... like the reverb ... whether you use it or not.

The 6GW8 ... speculating ... was chosen more for it's engineering consideration that sonic attributes; much in the way Gibson inexplicably fell in love - as it was collapsing in the amp market - with the equally misunderstood 6EU7. It isn't that the 6GW8 sounds bad, it is that the 6BM8, the tube used in the reverb amps of V1 Thunderbird and T1 RVT, used as a output tube, is considerably warmer and a little easier to overdrive resulting in a little more crunch at lower volumes. I mean I guess G engineers knew what they were after but the 6BM8 is still around today and the 6GW8 went the way of the 8417 and a few others.

Did your monster Guitar-Sign survive the winter up there? Best, J

Hey John
Yep it did.
This is about the 4th winter now and really looking like a pre war Martin thats been roadied up and down the Grand Canyon on a burro to serenade all them dudes sitting around their dude campfires.
Looks like I'll have an opportunity to do a face lift as the music store is moving around June or so to a new location.
Speaking of relocating, I got to go out and relocate about a foot of new snow we got last night.
You guys in Florida haven't seen any of this white stuff since when? The last ice age?

RT 66 said:

Yep it did. This is about the 4th winter now and really looking like a pre war Martin thats been roadied up and down the Grand Canyon on a burro to serenade all them dudes sitting around their dude campfires. Looks like I'll have an opportunity to do a face lift as the music store is moving around June or so to a new location.

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Hi Ray; I hope you don't mind but for the people who've signed on to LTG since you posted these, they might like to know just who is Alaska's most Mega-Luthier ... maybe in the western world too ... just an amazing thing ... :shock: :shock: :shock:

















I read recently in the newspaper that 1977 was the last time South FL got any real snow ... they get smatterings up in Orlando but down here ... not so much ... I feel for you ... going to get a fresh drink now ... will probably feel a little less though .... watch out for your back! :wink:

Thanks John!
Always nice to see it the way it use to be.
Going to wade out to my truck for a trip into town to see if my new 79 F50 has arrived from New Hope. I did a bit of trading with my JF 30-12 and Mark V.
Hope it don't look like Pete's guitar I just posted.
Will post a pic soon.
Unless it looks like Pete's guitar

I saw the pic of Pete's guitar ... speaking of axes ... :shock: good luck w/ your new guitar; yes, use caution when posting!

Hey Cap
Got the F50 yesterday and it don't look like Pete's guitar.
Man that sounds nice. Can see now why so many rave reviews.
Just a nice full clean sound that suits my Travis type fingerpicking style or any style.

I saw Bill yesterday and he concured with your assesment of the hum balance thing.
He also wants to replace some resistors that he put in with some better ones he just got. Higher value or something and decided to bypass the hum switch with a 100 ohm resistor same as you sujested. Apparently the hum thing is some archaic thing held over from the old days?
He was lapsing into Chinese again which started me thinking about picking up some Mongolian Beef for lunch.
Anyways, will keep you posted on the mods directly.

RT

RT 66 said:

I saw Bill yesterday and he concured with your assesment of the hum balance thing ... Apparently the hum thing is some archaic thing held over from the old days?

Click to expand...

Hi Ray; yes, that's about right. Guild put them on most of their mid-60s and later designs. Useful if/when they are needed but mostly they are a 'feature'. Congratulations on your F50 ... relieved to know that it's not a variant of Pete's! J