Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on March 06, 2012, 08:10:30 pm
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Hi,
A GUILD 66J just came in last night to get serviced. One of the issue is the tremolo. I tried for one hour to fix it ( I don't even know what the circuit is all about hey hey :w2: ) What is the most frequent tremolo issue ? ( tube is good ).
Thanks
Colas
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That is the most illogically laid out schematic I've ever seen! Roll over Spock. I'm coming! :l2:
It's actually a simple, straight forward amp once you get past the stupidity of the layout. The trem circuit is a typical phase shift oscillator. The trem signal goes thru a cathode follower to the strength control and then wiggles the grids of the output tubes. The PI is a typical LTP.
Start by checking plate voltage on the trem oscillator. It will be bouncing around quite a bit if the oscillator is working. If it ain't oscillating, check the cathode bypass cap, the 3 caps in the oscillator feedback loop, plate resistor, etc. Oh, wait a minute. I see the problem. There ain't no plate load resistor!
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Bonjour !
Wouldn't it be simpler to re-do the trem circuit like a vibrolux or other simple design ( that works hey hey ) ?
Colas
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The circuit is simple. The drawing is a nightmare! I've redrawn everything except the preamp for you. Should make troubleshooting easy. Check the plate voltage of the trem oscillator. In fact, check voltages for all pins on the trem tube.
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Wow, really neat job! Like you said, it will make it easier for me. I just started servicing amplifiers and sometimes I think I should have kept my promises to my parents to do an architect. Trying to be a R & R star didn't work out either :laugh:
Thanks, you are a bro........and a pro.
Colas
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At least you know how to keep the ladies happy! :wink:
Here's the entire schematic. If you don't see a component value listed, it's because it wasn't listed on the original.
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Well, with my new hobby ( amp. servicing ), my lady won't be happy too much........her man down to his atelier 23 hours a day. I plan to change my motto to: ''MAKE AN AMP, NOT WAR'' .
You can now add this GUILD schem to the library, I guess, so beautifully re-drawn !
Best regards
Colas
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Well, with my new hobby ( amp. servicing ), my lady won't be happy too much........her man down to his atelier 23 hours a day. I plan to change my motto to: ''MAKE AN AMP, NOT WAR'' .
You can now add this GUILD schem to the library, I guess, so beautifully re-drawn !
Best regards
Colas
Just Curious, I was wondering how the repair/restore is going.
I just got one of these and sadly the tremolo is not functional. The amp itself does function otherwise and sounds absolutely fantastic.
I replaced the obvious suspects (.01 & .001 caps) in the trem circuit but still no luck. I must be missing something though, I know with 100% accuracy all of the tubes are good as I have several complete sets.
I also replaced all of the electrolytic caps including the can cap & cathode bypass cap.
The pots measure ok but somewhere I have something obviously off spec, the trem controls do absolutely nothing at this point.
I hope I can get this baby working, it is really an amazing sounding amp otherwise.
If I could just get the trem to do something? It does not function at all LOL
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in my case, it was a bad solder, under the board. Did you try ( you prob, did...) replacing the tube hey hey ?
good luck
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in my case, it was a bad solder, under the board. Did you try ( you prob, did...) replacing the tube hey hey ?
good luck
Hey, Thanks for the reply!!
Yes, all tubes are known to be 100%.
I have the amp on the bench right now, I loosened the board nuts to look underneath while changing caps to be certain nothing under the board was loose.
One thing certain, this board & chassis has a really weird layout.
The amp functions perfectly. Just no trem, I can not find an off spec part yet, I wonder if it ever worked at this point LOL
Edit,
I was just looking more closely at the schematic posted and noticed a few components that are of different values.
Power amp section;
6V6 grid resistors are 1K
The (2) 470K resistors are 220K
Tremolo section
On the speed pot, instead of 68K it has 100K
All of these parts appear to be original and undisturbed.
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try with a chopstick.tapping on all solders and components may reveal a bad connection/part
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try with a chopstick.tapping on all solders and components may reveal a bad connection/part
Been there, done that long ago. I have built no less than 50 amps in the past 35 years, fixed many more so the typical/common stuff is never over-looked.
Something is not jiving as many parts are not to the schematic values. I know Guild offered this model for a good number of years and most certainly it appears there were a few tweaks along the way.
I talked to the guy I purchased it from today, he said the Tremolo never worked and the guy he bought it from said the same thing. That is the sum of history on the amp.
Like I said, I actually question if it ever worked properly because it appears to have a few possible errors when comparing the layout to the schematics.
I'll be hammering away on this one a couple days more I guess, worse case I will have to do a total re-build which I had hoped to avoid.
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I am sorry, buddy, I did not want to hurt you with the chopstick story. I don't have any advice to give you then, since you've got that much experience ( much more than me !!! ).
Good luck :icon_biggrin:
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an engineer pushes his pencil, a tech. pushes his chopstick, pretty much the same :laugh:
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Got it!
After filtering through another couple hours of testing I figured it out. Actually, the re-drawn schematic posted above helped a bunch as well.
Turned out to be a badly drifted resistor, in the schematic it is listed as 1 meg. This amp had a 1.2 meg, but it drifted upward to around 1.8meg.
I was taking voltage measurements and had left the volume up, as soon as my meter probe touched the 1.2meg resistor I could hear the tremolo start to pulse.
I replaced the resistor, BINGO, it works and sounds wonderful!!
I put in a 1meg since that is what the schematic called for originally.
Go figure, el-cheapo 1meg meter saves the day.
On this amp, all of the coupling caps are astron .047 rather than the .022 in the schematic, they must have just used whatever was in stock. Kind of reminds me of another more famous company.
Oh well, I am very pleased.
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Good ! ( I knew you'd get it , hey hey ! ).
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Curly, his name AFTER.