Hey guys, I recently picked up a ‘75 OR120 and it sounded absolutely amazing when I crank it up and dime it out but it was getting some pretty heavy red plating on the 2 left power tubes. I opened it up and reduced the bias and it helped a lot with the red plating but now it doesn’t sound nearly as mean! I was wondering if there’s any tubes that can handle the higher bias to get the extreme power amp breakup sound without red plating.
The amp is currently running EHX EL34’s, and has been recapped with all new sprague and F&T caps.
Tubes that can handle a maxed out OR120 without red plating?
Moderator: bclaire
Re: Tubes that can handle a maxed out OR120 without red plating?
Are the tubes new and biased propeely to begin with? EHX have been good in my experience, although I’ve never had sn OR120.
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Re: Tubes that can handle a maxed out OR120 without red plating?
Welcome to what happens when you run an amp designed to see 115 volts out of the wall in the USA that's getting 120+...
I had to try several diferrent tubes until we found some that wouldn't melt down. Not sure what they were but it was quite a while ago. I put a DIVO in there...
I had to try several diferrent tubes until we found some that wouldn't melt down. Not sure what they were but it was quite a while ago. I put a DIVO in there...
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Re: Tubes that can handle a maxed out OR120 without red plating?
Shuguang tubes are supposed to be the shizz in vintage OR120s. I have a set and haven’t installed them in mine yet. I’ve been hearing about them for years.
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Re: Tubes that can handle a maxed out OR120 without red plating?
Also, if your tubes are red-plating, you might have a high frequency oscillation in your amp that is messing with the bias on those tubes. This is a thing that happens to some 70s OR120s.
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Re: Tubes that can handle a maxed out OR120 without red plating?
I run Svetlana EL-34's in my 1971 head, which has about 515V on the plates, and 510V or so on the screens. The Svetlana EL-34's are rated for 500V on the screens, which is why I chose them. Most EL-34's are well within spec on the plate voltage, but it's the screen voltage rating that can be problematic. EH EL34s are rated for 425V on the screens.DrRiff316 wrote: ↑Sun Mar 18, 2018 7:11 pmHey guys, I recently picked up a ‘75 OR120 and it sounded absolutely amazing when I crank it up and dime it out but it was getting some pretty heavy red plating on the 2 left power tubes. I opened it up and reduced the bias and it helped a lot with the red plating but now it doesn’t sound nearly as mean! I was wondering if there’s any tubes that can handle the higher bias to get the extreme power amp breakup sound without red plating.
The amp is currently running EHX EL34’s, and has been recapped with all new sprague and F&T caps.
That being said, if only 2 of them are red plating, this sounds like an issue with the amp and not the tubes. What are the voltages you are getting on each socket, and what is the plate dissipation for each tube? Also, have you swapped the tubes to see if the red plating follows the tubes or stays with the sockets?
As Gladmarr mentioned, some of these amps have HF oscillation issues due to poor lead dress. This is usually the output transformer primary wiring, which is easy to clean up. Make sure the leads are twisted and tucked in the corner of the chassis when they pass by the impedance selector. When they make the turn to the tube sockets, make sure they're up against the chassis edge right until they need to make a turn to the sockets. And keep that negative feedback loop wire away from the sockets as well. There's plenty of free space to keep it routed on its own, from the selector to the resistor on the board.
I'd also make sure no-one removed the 1000pf cap that's supposed to bypass the 3rd stage plate resistor. That cap is there to bleed off top end so the amp is stable. It was a popular mod for a while, and some people removed it. Leave it in, it's there for a reason.
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