One post for DC, both posts for AC.
Dave
Dave
Scott LK-72 rebuild
- Thread starter juddboz80
- Start date
kvflyer
Super Member
And I didn't want to mention it but if you are careful and not ham fisted as some of us are (or not patient!), this will work.
Exactly.
juddboz80
Active Member
Where are u located? Can prob find an AK'er with a tester! I would test for you if you were close.....
I went ahead and dished out for a NOS Mullard with perfect lettering, wasn't cheap but it will live longer than me!
triode17
Super Member
I have and rebuilt an LK-72, Champaign face. It's neon started to flicker as well. Neons' don't like to run on DC. First off, try reversing the wires to it, that can often fix the problem. If not, Radio Shack sells one that retrofits in, P/N 272-0707.Got mine buttoned up enough to test. Still running a mixed bag of rubes. And had a bit of static pop until it fully warmed up. But it sounds so sweet and detailed. I should have new quads of 7951s and 12ax7s in a few days. Hopefully that will clear up and remaining issues. Also noticed the "pilot" light flickering slightly?
Also traded out all of those horrid hex head screws for stainless #6s with titanium anodized countersunk washers.
euromut
Active Member
Well, I received my new JJ/ Telefunken GZ34. As soon as I put it in, it makes my primary transformer buzz like crazy. I expect the transformer to make some noise. If I put my ear close to the amp, I can here a slight hum. But with the new Tube, its audible across the room. If I put the old Amperex 5AR4 back in, its very quiet.
On the surface, I would say bad tube, and move on. But I am slightly concerned that the new tube may be drawing more current, and exposing a fault in my power transformer.....
Any thoughts?
On the surface, I would say bad tube, and move on. But I am slightly concerned that the new tube may be drawing more current, and exposing a fault in my power transformer.....
Any thoughts?
Make sure that the socket pin receptors at both pins 4 and 6 are making good contact in the rectifier tube socket.
Dave
Dave
euromut
Active Member
Update - I received my new rectifier tube. This time a Tung-Sol 5AR4. And the buzzing is gone, we are back to the barely audible hum I had from the original Amperex. Looks like the JJ manufactured "Telefunken" GZ34 was the culprit. I have a friend trying it in his guitar amp as an additional test.
So, it wasn't a Telefunken finally...
euromut
Active Member
First off, I want to apologize for essentially jacking juddboz80's thread....
Yeah, I knew it wasn't a real Telefunken when I ordered it. Would be nice if they said who it was made by, when advertising the tube. Box says they are essentially cherry picked off the JJ assembly line and branded Telefunken "Black Diamond Series". In any event, we have tried the tube in a few guitar amps, and it performs as expected. It appears their is just an incompatibility with my particular LK-72.
I built a new Diode Rectifier for the Pre-Amp side. And replaced all the 18ohm resistors on the 4x75 cap with Allan Bradleys. I have -47.8 after the 4x75 cap. I know the schematic calls for -45, but I don't know if that's enough to worry about.
Yeah, I knew it wasn't a real Telefunken when I ordered it. Would be nice if they said who it was made by, when advertising the tube. Box says they are essentially cherry picked off the JJ assembly line and branded Telefunken "Black Diamond Series". In any event, we have tried the tube in a few guitar amps, and it performs as expected. It appears their is just an incompatibility with my particular LK-72.
I built a new Diode Rectifier for the Pre-Amp side. And replaced all the 18ohm resistors on the 4x75 cap with Allan Bradleys. I have -47.8 after the 4x75 cap. I know the schematic calls for -45, but I don't know if that's enough to worry about.
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Rectifier tubes don't have compatibility issues, unless the circuit is operating them beyond their published limits, which the Scott does not. I would again suggest that you carefully examine the pins 4 and 6 terminals within the rectifier tube socket. If one of them is making contact with the pin of the tube, while the other is not, it would cause the exact issue you are having when this tube is installed. As further evidence of this fact, the B+ voltage at pin 8 of the rectifier tube socket will also be notably lower when either one of these contacts is compromised.
Very, very nice work!!
Dave
Very, very nice work!!
Dave
euromut
Active Member
I love work.....
Well, after some fun with the Scope. It looks like using the Null Hum method of balancing the tubes works quite well, as they were almost dead on. I do not have a signal generator. So I don't know what other tests I can do.... Guess I'm done playing mad scientist for today.
Well, after some fun with the Scope. It looks like using the Null Hum method of balancing the tubes works quite well, as they were almost dead on. I do not have a signal generator. So I don't know what other tests I can do.... Guess I'm done playing mad scientist for today.
