Hi all, I really didn't know whether to post this new thread or post this in the original thread I started from 9 years ago https://www.diyaudio.com/community/threads/gibson-ga-8t-tremolo-fault.239662/ (mods, if you feel it should be moved, please do so).
This will be lengthy and I sincerely apologize........
I built this circuit with parts I had on hand, As I didn't have a 5Y3 rectifier and PT at that time I installed a 6V4 rectifier & PT I pulled from an old radiogram.
Two nights ago, I pulled the amp out of long term storage and fired it up to discover no sound/nothing except for the heaters which were working fine, I checked the voltages at the rectifier B+ and discovered the first dropped resistor 250ohm 5watt had blown, I replaced the resistor, powered back on and all went well for a few minutes, I then noticed the 6V4 was glowing an unusual red colour and then I heard a small pop, The resistor I had replaced also blew.
So I made a decision to replace the 6V4 for a SS rectifier, 250ohm resistor changed to 330ohm 5watt (knowing I would slightly have higher B+ with the SS rectifier)
A careful voltage check after installing an SS rectifier is as follows,
B+ at rectifier surges to 374v then settles to 297v.
B+ after first dropper resistor surges to around 360v then settles to 270v
V1 pin 1 - 204v
2 - 0
3 - 2.6v
6 - 140v
7 - 0
8 - 1v
9 - 0
V2 6BM8 pin 1 - 31v
2 - 18v
3 - 0
6 - 262v
7 - 255v
8 - 51v
9 - 162v
V3 6BM8 pin 1 - 0
2- 18v
3 - 0
6 - 260v
7 - 257v
8 - 1.5v
9 - 108v
The voltages seem in the ball park, compared to the original circuit values here https://schematicheaven.net/m-gibson.html However, should be worried?
I also mentioned the 6BM8 in V2 socket/position was red plating, So I swapped that into V3 position and the red plating followed, I'm thinking this is possibly a bad tube?
The PT secondary HT for the circuit is 260-0-260vac.
I do appreciate the time taken to read this and hoping for advice.
cheers Ron
This will be lengthy and I sincerely apologize........
I built this circuit with parts I had on hand, As I didn't have a 5Y3 rectifier and PT at that time I installed a 6V4 rectifier & PT I pulled from an old radiogram.
Two nights ago, I pulled the amp out of long term storage and fired it up to discover no sound/nothing except for the heaters which were working fine, I checked the voltages at the rectifier B+ and discovered the first dropped resistor 250ohm 5watt had blown, I replaced the resistor, powered back on and all went well for a few minutes, I then noticed the 6V4 was glowing an unusual red colour and then I heard a small pop, The resistor I had replaced also blew.
So I made a decision to replace the 6V4 for a SS rectifier, 250ohm resistor changed to 330ohm 5watt (knowing I would slightly have higher B+ with the SS rectifier)
A careful voltage check after installing an SS rectifier is as follows,
B+ at rectifier surges to 374v then settles to 297v.
B+ after first dropper resistor surges to around 360v then settles to 270v
V1 pin 1 - 204v
2 - 0
3 - 2.6v
6 - 140v
7 - 0
8 - 1v
9 - 0
V2 6BM8 pin 1 - 31v
2 - 18v
3 - 0
6 - 262v
7 - 255v
8 - 51v
9 - 162v
V3 6BM8 pin 1 - 0
2- 18v
3 - 0
6 - 260v
7 - 257v
8 - 1.5v
9 - 108v
The voltages seem in the ball park, compared to the original circuit values here https://schematicheaven.net/m-gibson.html However, should be worried?
I also mentioned the 6BM8 in V2 socket/position was red plating, So I swapped that into V3 position and the red plating followed, I'm thinking this is possibly a bad tube?
The PT secondary HT for the circuit is 260-0-260vac.
I do appreciate the time taken to read this and hoping for advice.
cheers Ron
Last edited:
Capacitor from V3 triode cathode to power tube grid has shorted.
And let's have the schematic in-thread.
Thanks heaps for your reply.
For the life of me I cannot see the cap you are describing? The only capacitor I see in the above circuit diagram is the 20uf capacitor going to ground from pin8 of V3A?
There is also the 20uf capacitor from pin2 of V2B to ground.
I can't see in the circuit what your describing?
cheers Ron
This is a joke or what ? Many voltages are very different indicating a mistake \ fault in one or both chanels , like 0V vs 31V ...
Last edited:
The first part of your reply seems rather sarcastic, I'm pretty I did not state I was seeking sarcasm in my first post.....Some members might appreciate it but I'm sure most don't, its a good way to destroy a forum such as this.
cheers.
Thanks Kay, I really appreciate your advice. I don't have any .02's on hand, It'll be a couple of days before I post back on any progress.
cheers.
Hi Kay, An update for you, I've managed to overcome the problems I was having with this circuit, namely the red plating of the 6BM's.
I swapped out the solid state diode rectifier for an 285-0-285 PT and installed the 5Y3 as per the original circuit, These came from an old non-functioning test instrument I had in my collection. This made a difference almost straight away.
The only component values I then needed to changed was resistors V1 pin3 & V3 pin8 (as per original circuit diagram) I was getting a lot of distortion at low volume which has now been eliminated.
I was also getting a large amount of interference and microphonics which seemed to be coming through/from the input jacks, So I rearranged the wiring, shortened the leads as much as possible, put in better shielded wire for the 6.3v heaters, Its now running quite nicely, much much cooler than it was.
Once again, thanks heaps for your input
Congrats! This means vanishing of the red plating even without replacing the coupling capacitors?
Your PSU modification is nothing else than just increasing it's internal resistance, which indeed may be desirable for a geetaar amplifier, but doesn't per se cure red plating in an amp designed with SS rectification.
Best regards!
Your PSU modification is nothing else than just increasing it's internal resistance, which indeed may be desirable for a geetaar amplifier, but doesn't per se cure red plating in an amp designed with SS rectification.
Best regards!