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Author Topic: Toby GTR amp  (Read 3134 times)

bobhill

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Toby GTR amp
« on: July 17, 2012, 06:07:50 PM »

I guess I'm being a typical newbie with my first post asking for help/info... ::)

For a little background, I have been playing / working on my (and others) gear for going on 5 decades now, I first got into electronics so I would not have to pay someone else to repair my amps. Spent better than 4 decades working for phone company and a major US railroad as an electronics tech, retiring three years ago. Got into pedal building a couple years back and do them from the ground up, etch my own boards, populate and troubleshoot them until working. Have and use a variety of test equipment including DVM, o-scope, sig generator and a Tracker 2000 for in circuit testing. Have a pretty good idea which end is the hot end on my soldering stations. :cheesy: I have no problem reading schemes and tracing signal through just about any kind of gear. Lately I've been reading through Teemu's excellent book, and enjoying the read, if that tells you what kind of geek I might be. Thanks Teemu.

I have a few amps in the collection currently, including a pair of old Fender BF Bandmasters that I've rebuilt, but also have and enjoy three Fender SS amps, a pair of Champion 30's and a Deluxe 90. How come Fender puts B taper pots on their volume controls in the SS amps? Doesn't really matter, log taper pots are going in the 90 (just picked it up a week ago) and are already in the Champs. I changed the caps in the tone stack of the Champs to reduce their icepick tendencies.

But to get back to the subject of this post, I am helping a local shop repair an old 60's vintage SS Toby GTR amp. Everything in it works except the tremolo. From what little I've been able to find about these, they were made in Ft Worth TX in the mid 60's to early 70's by a jazz bass player who now has a speaker company that makes high end speaker setups. (tobyspeaker.com) Gabor Szabo played through one of the guitar amps  for the last decade of his career back in the day. There were less than 100 made total, so info is a bit scarce, and most of what little I've found is on the bass amps. From contact with the company, the electronics were made by Ramco, and the enclosures and speakers by Toby. There are no schemes that I have been able to locate through google,  I am just starting to trace the board, I figure that will keep me busy for the next day or three, but I just thought I'd check here to see if anyone had heard of these or had any info they could pass on. Most of the trannies on the board appear to be obsolete germanium, of course, but I do have a smallish stash of germs I can probably match as far as Hfe etc. to substitute if needed. I've got a pretty good stash of passives from all the pedal building (40+ pedals so far) I just need to make or get a decent schematic and I think I can bring this beauty back to life. Have not done any board pics yet, but if anyone wants I can get some camera shots done.

Thanks, and howdy y'all.
Bob
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J M Fahey

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Re: Toby GTR amp
« Reply #1 on: July 18, 2012, 01:01:30 AM »

Congratulations, welcome to the Forum, and good luck.
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Roly

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Re: Toby GTR amp
« Reply #2 on: July 18, 2012, 06:37:04 AM »

Hi Bob, welcome - and a very helpful intro with your background and test gear.

Quote from: bobhill
How come Fender puts B taper pots on their volume controls in the SS amps?

One opinion is that it makes the level leap off the stop, giving the impression of a "powerful" amplifier.  You won't be the first to swap in a log pot to get a better and more controllable spread on the volume control.


You say the tremolo on the Toby GTR "doesn't work" - could you be a bit more specific, i.e. does the LFO oscillate but you get no signal modulation, or is there no sign of the LFO actually oscillating?

These are almost always phase-shift oscillators with series caps and shunt resistances.  If the LFO transistor checks okay, suspect any emitter bypass electro fitted.

Check that it isn't one of those circuits that won't work unless a footswitch is plugged in.  Most default to "on", but some don't, and  if the footswitch uses a switch-jack check that the switch is operating correctly and hasn't got dirty contacts.

What method of modulation does it seem to use, a lamp and LDR, or a transistor in the emitter bypass of a preamp stage, or...?

If you seem to have LFOscillation but no modulation it is likely that the small pea lamp in the lamp/LDR "roach" has gone open circuit, and the fix is obvious.

Keep us in touch with how you go.

HTH

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bobhill

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Re: Toby GTR amp
« Reply #3 on: July 18, 2012, 10:07:27 AM »

The LFO setup appears to be one of the oddball aspects of this amp. From my first cursory examination of the circuitry, the power indicator lamp has a ldr mounted next to it, so my presumption is that it will flash at the LFO rate. The lamp, which is good, does not light up when the amp is powered on. If it gets it's drive from the LFO, that isn't happening. Until I get this thing traced and identify the LFO section I won't be able to say for sure. The foot switch jack seems to only control the reverb on/off as it is wired directly to the reverb pot with the tremolo activated by toggle switch labeled "Psychedelic Sound", although I have never heard trem referred to as that, and I lived / played through that era. It may be that switch activates some form of clipping, although I don't see any diodes on board I would expect to handle that so if it is it would be via the Fuzz Face mis-biasing method of generating distortion. I will have to reassemble it to see just what the switch does, if anything.

All 26 electros  ::) have been replaced just due to the age of this thing. I don't trust them to be not dried out after 40+ years, so replacing them is kind of an automatic first step for me on anything this age.

Well, it's time to go shoot some board pics, with backlighting so I have traces and components and start building a scheme. It is a good thing that this is not a need it tomorrow project, I have a feeling this could keep me amused for a while. :)
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Roly

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Re: Toby GTR amp
« Reply #4 on: July 18, 2012, 12:42:59 PM »

"Psychedelic Sound" - ha, that's a good one!  I'll bet that's just the trem and their wishful thinking.

I'll also bet that it's no more complicated than a single PSN LFO driving a buffer transistor.  I've seen secondary indicators of the front panel but this is the first where they have been too cheap to use two globes.
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Tobyguitaramps

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Re: Toby GTR amp
« Reply #5 on: December 25, 2013, 06:29:52 PM »

Does any one have the schematic for it? I have the same problem?
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