I have this guitar amp that blows the main fuse when the standby switch is turned on.I found a shorted part that looks like a diode but numbers dont match anything when Googled or searched on this forum.
Its black with a silver ring on one end just like a diode but its about 1/2"long and about the size of a 1 watt resistor.
The markings are VARO VG2 8610
It connects to a pin 3 on a GT-6550 tube and has a 470p cap across it.There is another one in the circuit on 3 GT-6550's paralleled that tests open in both directions but this one reads 7 ohms.
Since I cant find anything about it Im not even sure its a diode.
Does anybody know what this thing is or a suitable replacement?
Its black with a silver ring on one end just like a diode but its about 1/2"long and about the size of a 1 watt resistor.
The markings are VARO VG2 8610
It connects to a pin 3 on a GT-6550 tube and has a 470p cap across it.There is another one in the circuit on 3 GT-6550's paralleled that tests open in both directions but this one reads 7 ohms.
Since I cant find anything about it Im not even sure its a diode.
Does anybody know what this thing is or a suitable replacement?
It is probably a high voltage diode used for protection. They sometimes blow if the amp is cranked flat out into an unsuitable load. You can just remove them for test purposes: the old Fenders and Marshalls never had them.
A suitable replacement would be the SF1600 from Vishay. (Assuming the other end went to ground.)
A suitable replacement would be the SF1600 from Vishay. (Assuming the other end went to ground.)
Wrong forum, moving to Instruments & Amps..
Yup, in theory it could be a TVS diode or "thyrector". However, I've only ever seen regular high voltage diodes in this position, with the anodes to ground.
Thanks for your help guy's.
I found it. Its made by VARO,the part # is VG2,the 8610 is a date code.
Its a 2kv 100ma diode
This KMD head was made in England and the are mostly garbage from Ive read but this one was their flag ship model and is pretty decent.I just got a guitar(after30 years of not playing) to try and teach my daughter how to play and needed an amp.Someone brought this to me to repair years ago and abandoned it.Now all I need is speaker cabnet.
Thanks again,
Randy.
I found it. Its made by VARO,the part # is VG2,the 8610 is a date code.
Its a 2kv 100ma diode
This KMD head was made in England and the are mostly garbage from Ive read but this one was their flag ship model and is pretty decent.I just got a guitar(after30 years of not playing) to try and teach my daughter how to play and needed an amp.Someone brought this to me to repair years ago and abandoned it.Now all I need is speaker cabnet.
Thanks again,
Randy.
It's main function is to protect your output transformer in the case no speaker is connected, or when the speaker wiring or connections fail. A lot of us refer to them as flyback diodes - rightly or wrongly.
Any 2-3kv low current diode will work. Fender uses R3000 these days, Peavey uses another I forget. SOme makers just string two or three 1N4007 in series.
The amp will work fine without them, you just will no longer have that protection.
Any 2-3kv low current diode will work. Fender uses R3000 these days, Peavey uses another I forget. SOme makers just string two or three 1N4007 in series.
The amp will work fine without them, you just will no longer have that protection.
Ground.
I also found a TL072 opamp that had 12volts Dc on the output pin.I took it out of the socket only to find its not the opamp its the socket with the DC.So now I have to take that board out to track that down.Figures,its got 13 pots and,8 1/4" jacks and 3 switches connected to it.I figure the DC on pin 3 of the TL072 is what caused the diode failure.
Ive never worked on tube amps before.Back when I played the only thing we had to do was take the tubes to a place with a tube tester.I hope the music shops still have them because I will have to test these tubes.5 12au7's and 4 6550's.
Its so ironic.I can repair 3k watt class d mosfet amps everyday and this 100watt tube head is giving me problems
I also found a TL072 opamp that had 12volts Dc on the output pin.I took it out of the socket only to find its not the opamp its the socket with the DC.So now I have to take that board out to track that down.Figures,its got 13 pots and,8 1/4" jacks and 3 switches connected to it.I figure the DC on pin 3 of the TL072 is what caused the diode failure.
Ive never worked on tube amps before.Back when I played the only thing we had to do was take the tubes to a place with a tube tester.I hope the music shops still have them because I will have to test these tubes.5 12au7's and 4 6550's.
Its so ironic.I can repair 3k watt class d mosfet amps everyday and this 100watt tube head is giving me problems
Just curious, please post voltages on all 8 pins, without the Op Amp.
*If* it's used single supply, having +12/16V on some pins might be normal.
In that case, you should have 0V on pin 4 and some +24 to 30V on pin 8.
EDIT: anyway whatever happens to a preamp Op Amp is unrelated to a shorted protection diode in a power Tube plate.
You will have to draw and post at least a partial schematic of that tube stage:
AT LEAST the power tubes, how they connect to the OT, voltages to ground on all pins, biasing, coupling caps into their grids, screen resistors, protection diodes if any, because bthose are the parts that would be directly involved in blowing fuses.
Oh, and the power supply diagram, complete with voltages.
It's not much, we are talking some 20 parts, all interconnected in some way and living close to each other in the PCB.
One doubt: those power tubes, are they driven by a tube PI, an SS PI or cathode driven á la Musicman, with 2 medium power transistors?
*If* it's used single supply, having +12/16V on some pins might be normal.
In that case, you should have 0V on pin 4 and some +24 to 30V on pin 8.
EDIT: anyway whatever happens to a preamp Op Amp is unrelated to a shorted protection diode in a power Tube plate.
You will have to draw and post at least a partial schematic of that tube stage:
AT LEAST the power tubes, how they connect to the OT, voltages to ground on all pins, biasing, coupling caps into their grids, screen resistors, protection diodes if any, because bthose are the parts that would be directly involved in blowing fuses.
Oh, and the power supply diagram, complete with voltages.
It's not much, we are talking some 20 parts, all interconnected in some way and living close to each other in the PCB.
One doubt: those power tubes, are they driven by a tube PI, an SS PI or cathode driven á la Musicman, with 2 medium power transistors?
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You might find a few places with them still?, ours was destroyed in a fire many years ago - and one we came across the other year was sent off for scrap as no one wanted it.
But there's no need for a valve tester, simple standard fault finding techniques are all that's needed.
When our valve tester was working I don't remember ever using it for a repair, only for testing valves customers bought in the shop.
You just need to understand how valve amps work, they are crude and simple, so generally nice and easy to repair.
Unfortunately modern engineers don't have experience (or training) on valves, I was at college in the early 70's and valves were mentioned just once, one afternoon (when the usual lecturer was off for some reason).
However, I already had extensive valve experience before then 😀
Well, I started studying Engineering in 1969, tubes were not "officially" taught any more, nothing beyond a very basic description, but Professors of course knew them very well, they were still in wide use (amplifiers and BW TV sets) and I started buiklding Fender amp clones, straight from Jack Darr's book, obviously tube type.
And Faculty Library was chock full with tube books, so the resources and the teachers were there, for anybody interested.
So,of course, I took advantage of that. 🙂
And Faculty Library was chock full with tube books, so the resources and the teachers were there, for anybody interested.
So,of course, I took advantage of that. 🙂
I think it is probably VERY unlikely your TL072 is connected directly to the 6550 circuit, and in any event was not the cause of your failed flyback diode.
There is a +/-15 vdc that is powering the TL072.But I was looking at it wrong.The DC is on pin 6 which is the inverted input.I am very familiar with this opamp as I repair car amps for a living so If I see a DC on that pin when the opamp is configured as a filter or buffer Its a UREEKA moment.But being unfamiliar with Guitar amps I was wondering if maybe they bias it for the distortion of the overdrive circuit then block the DC with a cap.
Another thing.When I got this amp someone had removed the fuse holder labeled HT(2.5A).The other fuse is next to it is labeled MAINS(5A) (the one that keeps blowing) looks like it was jumped to the HT fuse holder.There is a cut green wire that goes to the power transformer with pairs of orange,yellow,red a white,blue and brown wire.Since its the only wire with no termination I have to assume it went to the fuse.Plus I did find pictures of this amps little brother and there is a green wire that goes to a 2A fuse on the board next to 2 other fuses for the +/-15 volts which this amp also has.My thinking is I should get the opamp and diode problem fixed before connecting the the green wire.
There are also (4) 820 ohm 5w resistors(one on each 6550) and 1 of them is reading 900 ohms.They have got so hot they have burnt the board brown so I will replace those also and space them about 1/2 inch from the board.
I havent messed with it since my last post because I have a lot of customer units backing up I need to take care of.Im not computer savvy so I will have to have someone show me how to post pics or drawings if it comes to that.
I appreciate all of your help.
Another thing.When I got this amp someone had removed the fuse holder labeled HT(2.5A).The other fuse is next to it is labeled MAINS(5A) (the one that keeps blowing) looks like it was jumped to the HT fuse holder.There is a cut green wire that goes to the power transformer with pairs of orange,yellow,red a white,blue and brown wire.Since its the only wire with no termination I have to assume it went to the fuse.Plus I did find pictures of this amps little brother and there is a green wire that goes to a 2A fuse on the board next to 2 other fuses for the +/-15 volts which this amp also has.My thinking is I should get the opamp and diode problem fixed before connecting the the green wire.
There are also (4) 820 ohm 5w resistors(one on each 6550) and 1 of them is reading 900 ohms.They have got so hot they have burnt the board brown so I will replace those also and space them about 1/2 inch from the board.
I havent messed with it since my last post because I have a lot of customer units backing up I need to take care of.Im not computer savvy so I will have to have someone show me how to post pics or drawings if it comes to that.
I appreciate all of your help.
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