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Garnet mod repair info needed...

Repair Question 
9K views 133 replies 9 participants last post by  Frenchy99 
#1 ·
I got this Garnet 200 this week and a previous mod repair was done on it.


From what I could figure , this rig was a rental and all the work done was from the shop itself...

I know that a pot is missing for the stinger, it was broken off and the circuit is entirely there. The amp works flawlessly just no stinger at the moment.


Now for the repair... This transformer was added ontop of the chassis which is not stock.


At first, I thought it was a replacement choke but the original choke is still inside and connected...
 
#2 · (Edited)
The new transformer gets its power directly from the AC line and goes to a new added circuit . New transformer to circuit ( circuit is one cap and 2 diodes) , bleu and white wire.


This new circuit( circuit is one cap and 2 diodes) goes directly to tube 1 only on pin 5 and 6. These are the 2 green wires...


Anyone know what this might be ??? :confused:

Help !!! MJF$#
 
#3 ·
You're in luck! I have that exact same amp, but with different knobs on it. And it's totally virgin, never been touched. I'll take some pictures of the internals when I get home tonight.
The stinger needs a foot switch to operate (or short out the footswitch jack). You have some strange & unusual things going on in there for sure. I see several "wtf's".
That extra transformer with the blue & white twisted wires.......diodes, caps, and the light green twisted pair.......they don't belong. I'd almost swear mine doesn't have any orange drops either.
 
#5 ·
What tube(s) and pin(s) thereof is that transformer connected to?

My first thought is that the original PT had the 6.3V (or 5V if tube rectified) secondary developed an issue (short or open) and they replaced it with this small tx because it's cheaper than replacing the entire PT (like a lot cheaper), the rest of which still works.

You can confirm this by providing the info above but also :

- checking if there are any unused leads on the original PT (i.e. that were replaced by this new Tx)
- checking that there is nothing else connected to this PT other than the tube heaters

The orange drops don't necessarily have anything to do with a mod; in an amp this old you'd expect some caps to have been changed out.
 
#7 · (Edited)
What tube(s) and pin(s) thereof is that transformer connected to?
the 2 green wires are connected to pin 5 and 6 of the first power tube...

Nothing else is connected to this transformer.
No other wires is disconnected !


Since this seems to be a small power transformer and a cap and diodes were added before the tube . was thinking maybe the amp had a problem with rectifier circuit on one channel and this was his quick fix ??? :eek:
 
#6 ·
I can't tell exactly where the green wires go from that add on rectifier, but the resistor values look similar to what Garnet puts around the bias to output tubes. Also the way the diodes are wired off the transformer will give you a negative voltage. It's a full wave negative supply by the looks of it. Probably to replace the bias supply for some unknown reason.
The workmanship is very suspect as well.
I'd get this to a reliable shop ASAP.
 
#14 ·

Big thanks Lincoln!

Yours looks super clean compared to mine...

My mess is that badly installed power cord and the added transformer which I have no clue as to why its there...

I have to work outside of town for 2 weeks and will tackle this as soon as I get back... first by changing the power cord and wiring it up to the fuse and remove the death cap and then by disconnecting this transformer... see if it will work just as good as it is right now !

This thing is clean... not a hiss when on ! :confused:
 
#20 ·
It's definitely an attempt to modify the bias circuit. The question is why. Did the modder not know what he was doing, is it supposed to be an upgrade or has something else failed and the incompetent boob who fixed it didn't know any better.
What needs to be done is a step by step through the amp wiring. Are the original bias parts still there, are they damaged? All voltage need to be checked to see they fall in line with what should be there. Then I would put things right. I said this earlier, the work is crap, and this amp deserves to be done right.
 
#21 · (Edited)
It's definitely an attempt to modify the bias circuit. The question is why. Yep WHY ???
Did the modder not know what he was doing, Does not look like it !
is it supposed to be an upgrade or has something else failed and the incompetent boob who fixed it didn't know any better. No one seems to know... Boob is to polite...

What needs to be done is a step by step through the amp wiring. Are the original bias parts still there, are they damaged? The original rectifier is all there, intact and fully connected.

All voltage need to be checked to see they fall in line with what should be there. I will do that as soon as I have the time. Will follow the schematic.


Then I would put things right. I said this earlier, the work is crap, and this amp deserves to be done right. My goal is to bring this back to stock

Thanks for the help so far... just trying to figure out what kind of repair this is... after searching the net all night, still no answer...

What the /$%"& is this fix ???:confused: I keep asking myself Why ??? :confused:....
 
#24 · (Edited)

Thanks for the help so far... just trying to figure out what kind of repair this is... after searching the net all night, still no answer...

What the /$%"& is this fix ???:confused: I keep asking myself Why ??? :confused:....
you may never know. I see your point though. Why would someone go through all that trouble, work, and butchery to by-pass a circuit that's so simple and so easily fixed???? But you have access to schematics, you have access to a working identical amp for checking/comparing voltages. It just doesn't get any better than that. :)

The old girl is rocking some pretty good voltages. "A" is 520+ volts, "B" is 515+ volts. The drawing I'm looking at says -50 volts for the bias.
 
#23 ·
I'm with @dtsaudio: the first step is to see if the original bias circuit is there and if damaged and if not if it gives you the right voltage (schem says -44V, but obviously that's just nominal there will be some give/take on that).

I'd also measure the voltage coming out the new bias circuit (if it is drastically different then that's why).
 
#26 ·
Was also advise by a Garnet specialist that since Garnets are so different from the standard Fender and to sort, the tech might not have been able to figure out how to fix the bias... this was his solution... build a new bias adjustment...

Will be taking all the proper readings when back and change the Bias cap also... From what I was told, this should be an easy fix to bring back to stock and remove all this new $%/& in it...
 
#28 ·
One thing I see with some Garnet circuits is the suppressor grid (G3) is also connected to the negative bias voltage. This is totally different from the Fender, Marshall and most other amp's layout where G3 and the cathode are connected together. Maybe this had something to do with the way they modded it. The mod will have less ripple and be a quieter supply than the stock half wave rectifier.
FYI - this arrangement means you CANNOT use 6L6 tubes in this amp.
 
#38 · (Edited)
View attachment 173353

What I don't understand while looking at this picture is why the two legs (plate wires) of the output transformer are connected together with a good sized capacitor and a 12K 10W resistor.

I know Frenchy99's is a newer model than mine by the knobs. His plates are connected too, but he has a cap on each side of the resistor while mine has a cap on only one side. Can any of the techs clue me in as to what is going on with this connection between the plates? Fenders don't have it (at least not tweed, brown, & blackface)
If y'all think that weird, check out this 190 (think earlier Pro but could be wrong) schem (what I mentioned earlier):

Text Diagram Drawing Technical drawing Schematic


Going back to what @dtsaudio said about the original bias circuit having more ripple - in this version is Gar using a choke (half a Tx) to smooth it out? .... and then fed the bias not to the control grid but to screen (and that Choke looks like it's ALSO somehow involved in the phase splitter, cuz output from both the 6SN7 's plates are tied together to one end of it vs separate to each pwr tube). I'm no expert but it confuddles the snot outa me. Also this is the one where the bias circuit comes off the mains side of the PT so not isolated. Note also the 2 caps on either side of that resistor in the 'corrective circuit' @jb welder explained.
 
#30 · (Edited)
Text Technical drawing Drawing Diagram Plan


What I don't understand while looking at this picture is why the two legs (plate wires) of the output transformer are connected together with a good sized capacitor and a 12K 10W resistor.

I know Frenchy99's is a newer model than mine by the knobs. His plates are connected too, but he has a cap on each side of the resistor while mine has a cap on only one side. Can any of the techs clue me in as to what is going on with this connection between the plates? Fenders don't have it (at least not tweed, brown, & blackface)
 
#32 ·
What I don't understand while looking at this picture is why the two legs (plate wires) of the output transformer are connected together with a good sized capacitor and a 12K 10W resistor.
It's a conjunctive filter. It helps the power tubes see a more consistent impedance from the OT.
RCA tube manual calls it a 'corrective filter', and Gar probably got it from that book.
 
#44 · (Edited)
I just ran a spice simulation on that output stage, and as weird as it looks the center tapped choke does indeed give a phase split for the two output tubes. The trick here is the capacitor coupled choke and the center tap being referenced to the bias. I've never seen this before, but the simulation works. I'd like to actually see one and confirm this is what he did.
His wiring of the output tubes is, I think, just Gar being sloppy. He's connected the G3 grid to high voltage. That would effectively cut the tube off. I think. Also the bias diode is just drawn wrong. It should be on the other side of the transformer. That would be a nasty shock hazard. Every other Garnet schematic I have shows the bias diode on the secondary.
Yeah, well we know from 'transformer-balanced' outputs (e.g. on pro rec gear), that that a TX can be used to invert phase. I suspected the reason for using the centre tap for the bias voltage is so that tubes get the equally filterred voltage. That would not be necessary if the signal wasn't being inverted via that same choke (end to end - effectively direct, bypassing the choke, to the top pwr tube, and inverted, through the whole choke to the bottom one. I do wonder if the difference in inductance between the signal to each tube is an issue or consideration at all. Anyway, lovely example of 2 for 1 part usage (just like smaller amps that use the OT primary as a plate voltage filter instead of a dedicated choke). ... It also looks like the transformer is being used to mix in the tremelo signal - so that's a 3 for 1!

So if T3 is the PI then I don't understand why use both halves of the 6SN7 (essentially a driver not phase splitter in this circuit, right?) - a single half of a 12A_7 would work for that .... thought there was an unused half somewhere but no; there is an unused (triode) half of the 6AU8 (pentode used for the trem). The only thing I can think of is that he is using the 6SN7 as both a mixer and make up gain (not 1:1; one is lossier than the other) after the treble and bass boost switches that feed it (trem comes in the PI centre tap post driver as mentioned above and Fuzz is mixed in before the boost switches at the 2nd half of V1) - see points S (bass boost out) and T.T. (treble boost out).

Am I on the right track? This is fascinating and mind blowing.
 
#45 ·
That early Pro schematic Granny posted is only for the 6SN7 type power amp. The center-tapped choke is used to get the phase inversion.
It's a similar concept to early solid state power amps that used an interstage transformer as a phase inverter.
Gar ditched the CT choke (probably due to availability) and started converting the 6SN7's to a more common PI circuit whenever he got them in for repair. There's a schematic out there somewhere showing the conversion.
Granny, I think the 2 halves of 6SN7 paralleled are just to get extra current drive.

His wiring of the output tubes is, I think, just Gar being sloppy. He's connected the G3 grid to high voltage. That would effectively cut the tube off. I think. Also the bias diode is just drawn wrong. It should be on the other side of the transformer. That would be a nasty shock hazard. Every other Garnet schematic I have shows the bias diode on the secondary.
Agree, they are just typos. The resistor values in the bias circuit would not work out for 120VAC. He did not label the various grids like he did in the other schematic and just drew the tube elements in the wrong places.

@Frenchy99 You don't have the 6SN7 circuit so you can ignore all that discussion about the PI circuits.
If you can, just for interest sake, remove output tubes and measure bias DC voltage at pin5 of each power tube socket before you do anything.
Removing the mod will be easy, one socket has not been touched. On the other one, you just have to remove the green wires and reconnect that 1K5 (standing up) back to pin 5 like on the other socket. Then disconnect the blue & white wires from the rectifier board.
Power it up without the power tubes and verify the bias voltage at pin5 of each power tube socket. If there is around -50VDC at both sockets you are good to go.
 
#49 ·
@Frenchy99 You don't have the 6SN7 circuit so you can ignore all that discussion about the PI circuits.
If you can, just for interest sake, remove output tubes and measure bias DC voltage at pin5 of each power tube socket before you do anything.
Removing the mod will be easy, one socket has not been touched. On the other one, you just have to remove the green wires and reconnect that 1K5 (standing up) back to pin 5 like on the other socket. Then disconnect the blue & white wires from the rectifier board.
Power it up without the power tubes and verify the bias voltage at pin5 of each power tube socket. If there is around -50VDC at both sockets you are good to go.
Thanks for the input and info.

Once back home (might be a week or two) will be taking all the readings and posting them... curious to find out the difference. Wish I had a bias meter like some have that you plug the tubes in...
 
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