Problem with Thunder 1 Amp

Hi,

I wonder if someone could make some recommendations regarding a Guild Thunder 1 amp I’m having problems with. (No reverb, just the tremolo)

Problem: after playing for about three hours I suddenly lose volume. The red power light goes out. If I wait a few minutes it’ll power up again. But generally if it’s gone down once, it will soon do a repeat.

History: I purchased the amp second hand in 1970. It’s gotten fairly regular basement band use since then: in other words about once a week for 4 hour sessions. I play it with the volume at about 1 o’clock. I’m using an old Hagstrom (pre Swede) played through an Ibanez Tube Screamer. I’ve used the same set up for years. I recently introduced a Dunlop volume pedal.

Service history: It’s had some tubes replaced over the years. I had it looked at a few years back for general servicing. The invoice states ‘Replaced cap in Cathode CCT’. Last fall the speaker (the original) blew. The shop replacing it with a Jenson JCH 12/70, 8 Ohm.

General comment: As you might be able to detect from my comments I am not greatly familiar with the inner workings of tube amps. What I’m hoping for are suggestions of basic detective work I might do before bringing it into my local shop. I don’t plan to pull anything apart (from the little reading I’ve done I’ve developed a healthy respect for capacitors) but just things I might try that could supply some diagnostic information. For example, I’ve turned it on and just let it run unplayed for 5 or 6 hours. There didn’t seem to be any problem.

I think it’s the intermittent aspect that’s got me worried. It probably makes it a lot harder for someone in a shop to replicate.

Any thought? Advice would be much appreciated. It’s a great little amp and I hope to get many more years of playing out of it: that is many more years without it cutting out right in the middle of my (substitute favourite song title here) solo.

Ted

Hi Tedd and welcome; as tempting as it is, troubleshooting electronics over the web turns out to be speculative and frustrating. Unless you know for a fact that the primary filter caps have been changed within the last 10 years, chances are it's time to replace them. They work for a while and then tire out. If they need to be changed, they will be challenging; the identical replacement parts are no longer in production so some work-arounds will be necessary. Stick with it, fun amp, welcome to LTG! cj

I would definitely get the filter caps replaced, especially if they are the originals. That's conducive to the amps health for sure.
Any usual noises, smells, runs hotter than normal? Tubes not glowing red?

It's something in the power supply maybe. Intermittent power failure, but it's not blowing a fuse, so it doesn't sound like a short in the tranny to me. I've had problems in the past with other equipment where it turned out to be a faulty power cord. There was a break in one of the conductors in the cable. Most times, the pieces were touching, but sometimes they would break contact. Does it have the original 2 prong plug? Turn the amp on and wiggle the cord around. If the power goes off, there's your culprit. Even if it isn't, get a 3 prong plug installed and have the tech remove the "death cap". He'll know what that is.

Hi Capnjuan and Default. Thanks for your ideas.

My lack of technical knowledge is going to show through quite quickly here. I’m assuming that the ‘cap in cathode CCT’ referred to in my repair invoice is not a ‘primary filter cap’. If it’s not then ‘no’, neither of these caps has been replaced in the last 38 years.

> Stick with it, fun amp

Yea, I love the amp. Great clean sound. Plenty of power for my purposes.

> Any unusual noises, smells, runs hotter than normal? Tubes not glowing red?

Noises?... Occasional popping sound. Nothing too distressing. Sometimes I’ll get radio free Europe or other far flung radio station. I’m learning to love it. I’m signing up for a Berlitz course.

I haven’t noticed a smell or it running hot. Also the tubes are glowing red.

I tried playing with the power cable. I can’t seem to replicate a power outage doing this. The times it’s happened the amp has been in a fairly stationary position. Not too likely the cable would have been disturbed when the loss of sound occurred.

Capnjuan mentions that it might be difficult finding a replacement primary filter cap. If my local shop says same, just what did you mean by a ‘work-around’?

Thanks again,

Ted

Here's the schematic for the Thunder ! with reverb. Underneath the amp, where the tubes are, there should be two small cans soldered or bolted to the chassis. These have several sections with a capacitor in each. These filter out the ripple in the high-voltage DC that feeds the plates on the tubes. Certain values of these caps aren't made anymore. If you have a tech worthy of the title, he can figure out how to provide the proper value at each point in the circuit.
The "cap in cathode cct" probably refers to low voltage capacitors that are attached to the preamp tube cathodes. They're not critical to the health of the amp, but perform a role in tone-shaping.

I'm just speculating at this point, like cj said, it's tough to troubleshoot over the web. :?

"....Space....the final frontier" Capt. James T. Kirk Hi Tedd; then as now, engineering and manufacturing amps necessarily involves trade offs including overcoming space limitations. In the upper right of the pics below in the magenta box; that is a 4-section 40uf/300VDC 'can capacitor' identical to what's in your amp. It has four isolated sections, with the aluminum housing as ground, in one package; a space-saving virtue of can caps in general ('Space .. the final frontier...'). The 'can' on the right is shown in the schematic on the left. The 4 little symbols trapped in the box are the cap sections.



The schematic is the T1 RVT showing yet another cap to the visual left in the green box. I don't know whether your amp has this cap or not but, if it does, the green box in the lower right indicates where in the T1 RVT this loose section was mounted and if present in your amp, where it would be mounted too. It was put there because there was no place else to put it ('Space .. the final frontier...')

Unfortunately, replacement high voltage / 4-section can caps, with one or two exceptions, are no longer made for this application. The closest matching 'cap' is shown in the lower left; the blue thingies.... These are Weber VST 'can caps' but they are limited to 3, not 4, sections ('Space,...the final frontier). Further, since the T1 RVT requires 5 sections (excluding the 3 sections for the reverb amp), a 2nd 3-section cap was installed by punching the chassis; this 2nd can cap base can be seen in the lower right pic just to the right of the magenta circle indicating the location of the factory / 1st replacement cap.

40uf/400VDC Sprague/Atom caps are available but they are 1" X 1 1/2" and in looking at the pic on the lower right, there isn't any place to put them ('Space...the final frontier'). Assuming your amp has a typical 4-section can cap; there are three choices that I know of. (1) The design calls for separate cap sections for each of the two preamp tubes. Provided the DC supplies to these sections were adjusted, they could be combined and you could use one Weber 3-section can cap and life would be good. (2) To duplicate your supply, assuming 4 cap sections, you'd need one Weber 3-section cap and a loose Sprague Atom sitting in the green box.

(3) "...and boldly go where no man (other than Leo Fender) has gone before...". Fender power supply and other HV caps are on the tube deck side of the chassis under a cover. The supply and return wiring are run through grommeted holes in the chassis. Fender amps were sold with individual cap sections - not can caps. Because cap wear is a function of both use and time, amps subject to heavy use need the caps changed more frequently and Fender amps accomodate this need. You can solve your problem by Fenderizing your power supply: making your own tag board, fabricating a custom cover, drilling/tapping holes in the chassis, and moving the high voltage electrolytics to more accessible location ('Space ... the final frontier').

This is the 'workaround' for the T1 and the T1 RVT. If you are not electronically handy, this isn't something you want to mess with on your dining room table on a rainy Saturday. Regardless of make or model, this problem of space limitations v. market resources is common to all vintage tube amps (except Fenders); what you see is a 'worked example' on a Guild T1 RVT. Except for a bad tube, nearly all 'I got no sound' tube amp problems are a function of the power supply.

Finally, agree w/ default that ‘cap in cathode CCT’ is a preamp cathode cap. The T1 doesn't have an output tube cathode cap in the conventional sense.

Good luck with your project! cj

Hi Guys,

I really appreciate the time you’ve taken to so thoroughly answer my questions and provide illustrations and schematics. The ‘caps’ tutorial was particularly helpful.

I’ll get back to you when I get this thing resolved.

Thanks again,
Ted

You're welcome Tedd and good luck with your project. cj