# repair after blown tube



## guitarman2 (Aug 25, 2006)

My amp had a tube kind of explode on me taking out the Plate stopper resistor. I've been having alot of trouble with EH 6V6 tubes that came stock in the amp so I'm looking to dump them and not use EH anymore. I had inquired about NOS tubes and the guy that sells them said I should also have the cathode capacitor replaced as well. When a tube blows it compromises this cap. Just wondering what the amp techs here have to say about this. 
The builder of the amp was on the phone with the amp tech when he replaced the resistor so I'm just wondering if this were true why that wouldn't have been brought up. 
Right now the amp appears to be working perfectly. If there was a problem with the cathode capacitor would there be any symptoms? This amp is one of the quietest tube amps I've ever owned. I don't even hear the buzz from my single coil pickups anymore. So I'm thinking everything is ok. But just want to make sure that there aren't hidden problems that don't present themselves right away.


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## WCGill (Mar 27, 2009)

If the cap was shorted, then yes you'd have a lot of trouble keeping tubes in the amp for longer than 30 seconds. If it's open, there would be minor symptoms, a loss of volume and presence overall and you may not notice it at all. I've never seen one catastrophically fail, mind you most of the others have done many more repairs than I have. What's a plate stopper? Do you mean the grid-leak resistor? I've never seen one of those fail either. I guess I've led a sheltered life. I've had good luck with the Tung Sol 6V6's and the JJ's are bulletproof.


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## gtone (Nov 1, 2009)

I had an EL84 Polam/Telam die on me before in my Phaez Daisycutter, a push/pull dual EL84 high-gain circuit. Apparently, it burned out a grid leak resistor as it died. Musta been what I smelt burning before I could power it down... :sEm_oops:


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## nonreverb (Sep 19, 2006)

The cathode bypass cap is only conducting AC. A DC short across it will see it as an open. The cathode resistor though, will bear the full brunt of the excess current from the short making it smoke...
BTW, EH 6V6 are known to be susceptible to failure. I've seem my fair share go poof in Deluxe reverb reissues.


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## guitarman2 (Aug 25, 2006)

nonreverb said:


> The cathode bypass cap is only conducting AC. A DC short across it will see it as an open. The cathode resistor though, will bear the full brunt of the excess current from the short making it smoke...
> BTW, EH 6V6 are known to be susceptible to failure. I've seem my fair share go poof in Deluxe reverb reissues.


Yes I'm considering the JJ 6v6. I've heard they are more reliable. Although I also hear they are sonically very different. Everyone seems to think they are sonically like a 6L6 rather than a 6V6. That might not be a good thing for me. With the EH in there I like what I hear. But if reliability continues to be a problem I may give JJ's a try.

Mike from KCA NOS mentioned to me that as a precaution I should have the Cathode cap replaced as often it gets somewhat compromised when this happens. The builder of my amp agreed and will pay for me to take it to a tech. So for peace of mind I may as well.


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## nonreverb (Sep 19, 2006)

The only way I could see that cap getting a momentary overvoltage is if the plate voltage went directly to the cathode. The cathode resistor being around a couple hundred ohms, would instantly drop the entire DC B+ and the current across it would kill it. The cap would see an extremely high voltage for an instant. I suppose there's a possibility that that could compromise the dielectric and short the cap however, I've never in the 20+ years that I've been doing this gig, I've never seen it happen.
Having said that, I understand where they're coming from and for absolute reliability, it's a small price to pay.
Question: Does this amp have HT fuses as well?


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