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Vox AC-30...odd redplating problems

11/28/2004 3:45 PM
Vox AC-30...odd redplating problems
I have a Korg AC-30TBX that i'm having some problems with.  
Here's the pre-amp too:  
I had a quad of Ei EL-84's selected for low draw that were in the amp for a year and a half with no problems.  
But I recently went to retube it and i'm having problems with the new tubes redplating. I've tried 3 seperate sets of new EL-84's. With every set, one or 2 of them redplates slightly in the amp after a few minutes at idle. They're all run of the mill EL-84's, Groove tubes or Sovteks from the local music store.  
I started pointing the finger at the amp. But...  
I'm getting 50.7 ohms across my cathode R's. Which seems fine, as they're supposed to be 100 ohms each.  
All the screen grid R's seem fine, measuring anywhere from 96 ohms to 99 ohms. The input grid R's are all between 1K4 and 1K5.  
The 2 sides of the OT primary are measuring 56.2 and 57.6 ohms, respectively (to the CT).  
Do these amps really just run so hot that I need a set selected for low current draw? I'm not talking bright redplating...just a low level glow. For all I know, the EI's could've been running like that all along anD I never noticed it. Though I doubt it.  
Any ideas or suggestions? Should I try another low draw set from the Lord, or is there's something else going on here?  
Thanks guys...  
11/28/2004 5:50 PM
The Rk being 50 Ohms is fine.. but what is the actual current they are drawing and how does that work out as a dissipation value.. eg: if the tubes are pulling 14W at idle, they are going to redplate.. The screen R's only factor in when the amp is running, ditto the input grid R's. At idle what matters most is your B+, your bias setting and the individual tube's behavior. You need a volt reading at the top of the cathode resistor at idle, should be around 10V.  
How is the B+ in your amp? If it's much over 340V, look into a different rectifier tube, a zener, a dropping resistor or perhaps something like RG's bucking transformer to knock the B+ down. Alternately you might want to go to a bigger cathode resistor value (to cool the bias).. maybe 60 ohms or 75 ohms.  
What can happen sometimes is you short a turn on the PT primary and the secondary volts can get bumped up. I doubt this is your issue bc you would have seen it with the old set of tubes (probably) not just when you went to swap in new ones.
11/29/2004 2:39 AM
Dave H

Iíd use a 68 ohm cathode resistor. The data sheet value is 135 ohms for 2 x EL84s so itís 67.5 ohms for 4 x EL84s. I donít know why Vox used 50 ohms. It runs cooler and sounds fine with 68 ohms.  
11/29/2004 9:10 AM

Thanks for the replies guys...  
The plate voltage on each tube is roughly 338V. On each cathode i've got about 9.8V  
Embarassingly enough...i've actually never worked on a cathode biased amp. Guess I should learn, seeing as I own one. ;)  
How do I figure out current draw across my cathode R's?  
If i've got 9.8V from cathode to ground, and my cathode R's are measuring 50.7 ohms in parallel...and 338V on the plates, can I just use ohm's law and figure this out?  
V = I x R  
9.85V = I x 50.7 ohms  
I = 0.1943A  
0.1943A x 338V = 65.7W  
But is that per all 4 tubes? Which would make it 16.5W per tube?  
That's a little on the hot side, no? Isn't the max plate dissipation for an EL-84 like 12W.  
Correct me if i'm wrong on this stuff...and thank again for the replies.  
11/29/2004 9:51 AM
Dave H

If you measured the plate voltage wrt ground subtract the cathode voltage to get the voltage across the tube.  
0.1943A x (338-9.85)/4 is about 16W per tube.  
Thatís plate + screen dissipation. The screens are ~1W so the plates are running at 15W.  
11/29/2004 9:55 AM

Thanks Dave...  
Very hot for EL84's, no?  
I can't find any voltage data for Korg AC-30's...and the schematics don't even have a B+ marked on them.  
I've got about 120V at my wall, which shouldn't be driving my B+ above normal.  
Can I assume that the amp is functioning normal...and it actually runs that hot.  
11/29/2004 10:17 AM
Dave H

Itís too hot unless you fit a fan to blow some air across the tubes. Iíve run EL84s at 14W like that for years. I think itís running normally. Iíd use a 68ohm cathode resistor to cool it down a bit. It will still sound great.  
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