Guild Thunder 1 RVT : Fix-'Em-Up Part 1

capnjuan

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These are some pics of my work to repair and refurbish a 1965-1967 Guild Thunder 1 RVT. Some revisions were made to these amps while they were in production; cabinet details, chassis backpanel, and maybe electronics. 2X12AX7 pre-amp, 2X6GW8 outputs and 6CA4 rectifier. The stand-alone reverb amp has a 12AX7 and a 6BM8 output; and a pretty fine reverb it is; nicely variable although not as 'wet' as some others. When new, my copy looked more or less like this one with 'ears' on the upper front edges of the cabinet and, as matsickma described it, the 'fishscale' grillcloth.

Guild.jpg



I bought it on eBay as a 'project'; as some of us have learned :evil: , there are projects and then there are projects. I ranted before about it but, in short (pun intended) it was DOA as (1) one of the multi-section caps had leaked electrolytic paste on the chassis (2) the pins of the rectifier tube were 'corrosion-welded' to its socket, and (3) someone had tried to fix it by buiding a solid state rectifier with diodes on the pins of the dead tube.

I sent the amp to a friend who rebuilt the power and bias supplies and made some other improvements. The questions were could it be made to work again and, if so, would the quality of its tone justify a pretty extensive effort to rehab the particle board cabinet and fix some manufacturing problems along the way. The answers turned out to be yes. The following pics show the results of all this.

This is the stripped front of the amp cabinet showing the 'ears':

front01.jpg



Because they stick out and are made of particle board, they are subject to shipping and other casual 'injuries' - left and right - as shown below:

cornersold.jpg



The 'Fix' is to cut them back so they lie more or less in the same plane as the faceplate, as they do on later versions:

repairedcorner.jpg



The horizontal chassis supports provided both lateral weight-bearing and resistance to front-to-back movement. The left half of the following pic shows the original support (broken) through which a machine screw passed (the dark, banded area) up and through the corner of the chassis to give positive contact ... provided of course the piece of funky wood didn't break like this one did. The right half of the pic shows the 'Fix'; pairs of opposed angles with epoxied rubber tabs to provide load-bearing / cushion and yet another angle at the rear, machined and drilled, to provide an attachment point for the chassis:

leftsupport.jpg



The footswitch bracket was missing ... :evil: ... but another was fabricated from aluminum stock at the ShopzOfJuan: temporarily mounted inside left, now down low on the right:


FSbracket.jpg


Views of the chassis below: power supply including rectifier and output tubes on the left - pre-amp and reverb transformer on the right:

fullchassis.jpg



The 'bar' in the middle is, IMO, a Band-Aid. This chassis isn't stiff enough to support its own weight. In later versions, the chassis has a vertical steel, electrically-safe backpanel that provides the missing stiffness and protection. This is how it works on the early T1 RVTs: the left half shows the bar - it's 'anchored' on one end by pinching it between the faceplate and the volume pot (Yup!) and on the other with a screw into the back chassis lip. The right half, correct orientation, shows how the chassis weight is 'suspended' by a bolt through the top between the legs of the handle into a tapped hole in the strap...:

supportc.jpg



View of the tube deck, from left to right: T1 and 6CX4 rectifier behind, two Weber 3X40ufX450DC multi-section caps, the output transformer and two 6GW8 outputs, two 12AX7 pre-amp tubes, another Weber cap, and, what sets this amp apart from almost any other, the stand-alone 12AX7/6BM8 reverb amp with its own 8" speaker:

tubedeck.jpg



Partial reassembly showing the re-built chassis support on the left, the original but with new rear support angle on the right, new reverb can, 'Monster' shielded, audio-grade reverb return cable, #12 OFHC speaker drops, suspension 'bar' in place, and new baffleboard / grillcloth assembly through-bolted to the 'sculpted' replacment batten strips - originals were thin strips of particle board glued and stapled to the cabinet:

backgrill.jpg



The amp now has a 25 watt 12" Celestion G12M 'Greenback' and a 35 watt, 8" Celestion G8L reverb speaker in lieu of the CTSs that were original; they went to my friend to help pay for the power supply rehab not to mention that the 12" CTS had a 1" hole in it :evil: but who's complaining...

spkers01.jpg



Except for tolex, the finished product from the front:

donefront.jpg



...and from the back including view of reverb supply 'Monster' cable:

doneback.jpg


The amp is warm, robust, and musical; the 12" Celestion and boosted power supply filtering provide excellent bass, a common shortcoming of amps of the period. The controls include treble and bass and pull-for-bright on the volume and a footswitch to operate the tremolo which responsive and vibrant and the deep, dry reverb. I think this amp is everything that our friend and fellow BB member Jay Pilzer said it is and, in this case, more; more bass and, thanks to the upsized power supply, more punch.

I like the amp but I'd probably like it more if I didn't feel like it beat me up. It was also intentionally built to be a 'clean' amp; I tried pumping the feed from my Marshall BluesBreaker II through it - came out clean anyway :shock: and I have a preference for the crunchy thing. As of the date of this post, the link to the SchematicHeaven schematic is 'Suspended'; if anyone wants a copy, let me know, free (the schematic that is) to a good home!

cj
 

john_kidder

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Very nice work, Cap'n. I'll study intensely as I put the cabinet back together on my Thunderbird. I think the Thunderbird has a slightly better support system for the chassis than the T1-RVT (especially with the three #12 machine screws as suspension mounts off the top of the cabinet), but I very much like your angle brackets with the rubber vibration dampers - I'll use that for sure.

There is really nothing left of the original particle board that's worth salvaging from mine - what do you think about using construction grade fir plywood, proper dimensions, screwed and glued with corner reinforcing rather than finger-jointing?

Again, it's a real treat to see such nice work and to hear about such a happy outcome to a "project". Thanks.
 

jp

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Great job capn!
I'm still trying to build my knowledge before I dive into any of my amp projects. It's been all Aspen Pittman lately. After the Gerald Weber book, I'm gonna take the plunge!

Like the flashy electric blue valve covers!
 

capnjuan

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john_kidder said:
Very nice work, Cap'n. Thanks John, my low-rent seller sold me as much trouble as he did amp.

Thunderbird ... the three #12 machine screws as suspension mounts off the top of the cabinet That's the way Fender did its later amps; lateral edge supports with top-down chassis bolts.

I very much like your angle brackets with the rubber vibration dampers - I'll use that for sure. My pleasure.

...construction grade fir plywood, proper dimensions, screwed and glued with corner reinforcing rather than finger-jointing? I think that would work fine provided the power transformer and reverb amp provide interior clearance; I think you'd want the corners to be almost as large as you could get them without encroaching. If not, you might consider long 4" to 6" angles screwed and epoxied.

If it was me, I wouldn't mess with fingerjoints unless I was working with material I wasn't going to cover. The lost strength of the extended gluing surface in fingerjoints is gained back with glue/screw/corners.
Thanks again for the compliments; except for the sound quality, if I had it to do over...

Regards,
John
 

capnjuan

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jp said:
Great job capn! I'm still trying to build my knowledge before I dive into any of my amp projects. It's been all Aspen Pittman lately. After the Gerald Weber book, I'm gonna take the plunge!

Like the flashy electric blue valve covers!

Thank you jp; they are Webers and he makes them in several colors; fire-engine red, cheese yellow... Weber limits the higher uf units to three sections; they are 'stuffed' tubes...like pasta with 3 individual 40uf/450VDC caps in them. If you needed to, you could open them and replace them not to mention walking away from $40-$50 leak-prone Spragues and Mallorys which well, IMO, you have to be looking for Museum-style restoration instead of mundane things like service life...

Ya better get that soldering iron hot....

Thanks,
John
 

capnjuan

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Double Post? Edited out
 

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capn,

I like your project, particularly since I just got a thunder 1 rvt.
I guess mine is slightly newer in that it has the two-tone grille cloth and the beige tolex, plus there is a back edge to the chassis.

Mine has been gone through now by my amp guy and sounds pretty good.
He replaced some of the caps and a few resistors, the reverb section power tube
and fixed some issues in the rev/trem pedal.

The amp sounds a lot cleaner than it did. It's a lot of fun to play with
my P-90 ES-175.

I only wish it was a bit more powerful. It's almost loud enough to do a small gig with. Oh, well.
 

capnjuan

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gilded said:
I guess mine is slightly newer in that it has the two-tone grille cloth and the beige tolex, plus there is a back edge to the chassis. Matsickma is very strong on the evolution of this amp but, yes, yours is a later version and the back chassis panel is the tell.

...and fixed some issues in the rev/trem pedal. Mine had similar issues; the bias current is run from the power supply directly to the trem 'Strength' pot :shock: and then to the output tubes ... not ordinarily done that way.

I only wish it was a bit more powerful. It's almost loud enough to do a small gig with. Understood; it is what it is. I considered re-wiring the output sockets for EL84s. I'm sure it would spark up the tone but, short of a new transformer, it still wouldn't be any louder.

Thank you for the compliments.
cj
 

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Nice Work... Capn.Crunch. I like the cut corners, it looks good, smart thinking. My Guild project turned out to be alot more than I thought, and the amp is still the same! I am not familiar with 2X6GW8, are they El84's. Can you get a nice overdrive out of it or is it all clean? That greenback looks like it would rock.
 

capnjuan

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ThunderCat said:
Nice Work... Capn.Crunch. I like the cut corners, it looks good, smart thinking. Thanks TC; if patched and left alone, they'd just get broken again.

My Guild project turned out to be alot more than I thought, and the amp is still the same! Are you talking about your Thunderbass?

I am not familiar with 2X6GW8, are they El84's. No; the 6GW8 is most similar to a 6BM8. Two in A/B possibly 10-12 watts while travelling downhill. Best ones from overseas; Telefunken / Siemens / Amperex / Tungsram - which I'm running now, and Mullard; have 2 pair in reserve. If I were looking to update this amp, I'd rewire the output tube sockets for EL84s; requires lifting and moving 4 connections per socket, maybe splicing once or twice to compensate for length.

Can you get a nice overdrive out of it or is it all clean? Due to the circuit and high-uf output coupling caps, this one needs to be 'dimed' to distort but makes lovely sound while getting there.

That greenback looks like it would rock. I like it just fine; a little darker than Jensens / Webers but much better bass than my 6V6/Weber amps.
Thanks again; do you have any pics of your project?

Regards,
John
 

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Yes the project I mentioned was the Thunder Star Bass amp. The circuit is like your Guild RVT, it has to be cranked to get any distortion and the amp isint that loud for a 6L6 amp. Although it helps to have a guitar with humbuckers and an aux. speaker. It has a nice option of having a power amp in, and pre amp out. When I put the chasis back in something got caught on the copper foil and caused a short circuit.

 

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john_kidder said:
There is really nothing left of the original particle board that's worth salvaging from mine - what do you think about using construction grade fir plywood, proper dimensions, screwed and glued with corner reinforcing rather than finger-jointing?

Again, it's a real treat to see such nice work and to hear about such a happy outcome to a "project". Thanks.

John,

I'd probably look for a sheet of inexpensive hardwood plywood if I was bulding a long life cabinet.

They sell the stuff in places like Home Despot, imported from Asia, Mahogany like veneers.

It's mediocre quality but harder than fir, prices aren't much different. It machines much better. Nails & screws hold tighter in the stuff.
 

capnjuan

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ThunderCat said:
...The circuit is like your Guild RVT, it has to be cranked to get any distortion and the amp isint that loud for a 6L6 amp. Possibly might could use a bit more tinkering for volume...more on 'clean' below. It has a nice option of having a power amp in, and pre amp out. Yes, that's a good thing. When I put the chasis back in something got caught on the copper foil and caused a short circuit. Go to the Penalty Box, wait two minutes, then back on ice....
Most all amps connect the pre-amp to the power amp with coupling capacitors. These caps, aside from preventing DC from appearing at the input of the power tubes (you can listen to [the band] AC/DC but you really don't want to listen to DC) also have the property of filtering the AC (musical) signal.

The image below is from the schematic for a Guild Thunderbass (4-tube preamp) and shows the .27uf coupling caps:

Tbasscaps.jpg


This image is from the schematic for a Guild T1 RVT showing its .15uf coupling caps:

T1RVTcaps.jpg


This image is from the schematic for a Guild J100 showing its .047uf coupling caps:

J100caps.jpg


Each schematic has successively smaller value coupling caps and and are successively less 'clean' amps; the Fender Bassman, highly regarded as a guitar amp, has .1uf coupling caps Bassman Schematic here. The Thunderbass has 270% more and the T1 RVT 50% more 'filtering' than the Bassman; the J100 has 50% less making it, of this group, likely the easiest to distort and the Thunderbass the most dificult. To the extent that the pre-amp tubes are driven to 'clipping' or distortion, the coupling caps act to moderate some of the side effects and the output tubes never 'see' the sonic hash.

Assuming; you can get the volume up, you want this as a guitar - not bass - amp, and you want it to break up, you might want to consider replacing the coupling caps - .02/450VDC very common in 60s Fenders, Gibsons, others.

Are those can caps new? If not, can get here; fast, reliable: Antique Electronic Supply

Copper foil: looks like a shield which, if bridging any or all of the 'legs' on the control pots, will result in a short to ground. If you can't get it to behave, you can take it out and replace it with folded-up several thicknesses of aluminum foil, flattened / epoxied together, and epoxied in place...no point in spending more time in the Penalty box than necessary....

cj
 

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Hey its ok to take a penalty once and a awile, my team has Roberto Luongo in net!
Thats some great information Cap'n. The caps are original, 270% is alot of filtering. I bet it would sound more like a guitar amp with the caps changed but for now I'll play with the amp a bit more. Did you do any coupling changes on your RVT?
 

capnjuan

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ThunderCat said:
Did you do any coupling changes on your RVT?
Did not; I got it as a 'gesture' to the Guild brand and was impressed with Jay Pilzer's reviews of it. My copy was in pretty poor condition so, for me and this amp, it was 'first things first'; get it to design baseline and see what it can do; that's where it is now.

cj
 

capnjuan

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capnjuan said:
....and you want it to break up, you might want to consider replacing the coupling caps - .02/450VDC very common in 60s Fenders, Gibsons, others.
Sorry Colin; make that .02/600vdc not 450vdc ... heading to Penalty Box now....

cj
 

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ThunderCat wrote:
my team has Roberto Luongo in net!

Mine too. Vancouver? Where?

Yup I thought I;d rub it in that Vancouver Canucks have Luongo in net now.

capnjuan wrote:
.02/600vdc not 450vdc :

Ya.... I want the Thunderstar Bass to spit fire and brimstone, what ever it takes.
 

capnjuan

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ThunderCat said:
Yup I thought I'd rub it in that Vancouver Canucks have Luongo in net now. Our (Panther's) loss is your gain. He might have stuck around if the fans / mgmt / Panthers offense and defense had given a C___.

Ya.... I want the Thunderstar Bass to spit fire and brimstone, what ever it takes. In the next couple weeks, I'll try and find some tips to hot rod the amp. I think the Canadian expression is; 'able to kick the butt', no?

Regards, cj
 
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