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HELP diagnose Twin Rev II prob!


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9/8/1997 9:09 AM
Mike
HELP diagnose Twin Rev II prob!
Please HELP!  
My Twin Reverb II (early 80's Paul Rivera design) is running REAL hot - if I wet my finger and touch the output tubes they sizzle! Also the power transformer gets hot. The amp seems to break up (distort) too easy, and after about an hour of use breaks up WAY too much. I checked the idling current through the 6L6's and it's about 32mA per tube. The plate voltage is about 460V, which is correct according to the schematic. I've matched it with the output balance control. The tubes are all new sovtek's and as far as I know the amp is stock. I put a different set of output tubes in and the problem is the same. The original tubes in an identical amp do not get as hot - I'd say I can put my finger on them for about one second before having to let go.
 
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9/8/1997 4:31 PM
Jim S.

I don't recommend touching hot output tubes - you can really burn yourself!!  
 
Even if you can avoid getting blisters on your fingers, using this technique to guage how your tubes are biased is very subjective and inaccurate. I believe that, in a properly biased amp - class AB or class A (especially class A!) you SHOULDN'T be able to keep your finger on a power tube for even one second, once the amp has been idling for a minute or two (unless the tubes are fan cooled -but that's different).  
 
32ma per 6L6 is a good idle current for that amp. Are you sure ALL FOUR 6L6s are idling at 32ma, give or take a few ma? (You didn't say how you are measuring bias - the transformer shunt method will tell you how much each push-pull pair is drawing, but not each individual tube). BTW, I highly recommend converting that output balance control to a true bias control - it's an easy mod. Anyway, if ALL the output tubes are biased OK then they probably are not responsible for any heat/distortion problems. I do seem to remember that the octal tube sockets on that amp are not the greatest - is it possible that, after there's enough heat and/or vibration, one or more of the output tubes does not make good contact any longer? This would cause a severely unbalanced output waveform - i.e., bad-sounding distortion. ( It's common for the Sovtek 5881s with the thin wafer base to slip out of the socket a ways, because the spring clips don't grab onto anything.)  
 
It's not uncommon for the power transformer to get quite hot after the amp has been running for a while, especially if it's being played at stage volume. It shouldn't get as hot as a power tube, but it's normal for it too get at least very warm. On the other hand, maybe there is some abnormal current draw occurring. Are the filter caps original? If so, maybe one or more are leaky, causing excessive current draw in the power supply.  
 
A possible cause of excessive break-up in that amp - the power supply has some high-wattage power resistors used for voltage dropping. I think one of them is a 20 watter - it get's quite hot. It's possible that thermal stress over time has caused the resistance to go way up (especially after it heats up), causing drastically reduced voltages in the preamp tubes. It's worth checking into, anyway.  
 
In any case, my main point is that the distortion problem you have may not have anything to due with the heat of the output tubes and power transormer. A good place to start might be to play the amp until the problem starts happening, and then check voltages all through the circuit, not just the output stage (and don't forget to make sure those 6L6s are well-seated.)
 
9/8/1997 6:18 PM
Mike

Thanks, Jim.  
 
You are right - I am using transformer shunt to measure each pair and got them matched at about 64mA per pair. I'll do the tests you suggested - but let me tell you - this amp gets HOT! Like you can barely touch the back panel by the AC outlet. I've never hed an amp where the tubes would sizzle a wet finger! I have an identical amp that does not get as hot.  
 
I'll let it cook for an hour and see if the voltages get out of whack... More tomorrow.
 
9/10/1997 9:40 AM
Mike

Ok - I traced the distortion to the input jack. When I press on it the distortion goes away, so I'm gonna re-solder that and clean it off. BUT there is still the question of why the amp gets so hot. I'm talkin HOT here! Like you can barely touch the back of the chassis by the ac outlet. The power transformer gave me a blister on my thumb when I was pulling the chassis out of the cab. I had it on all evening and it heated the whole room up to about 80 degrees!  
 
I measured 485v on the plates of the 6L6's (does this seem high?) and here's the funny part - 75v AC too - with no signal connected to the input. Does this sound like parasitic oscillation? The funny thing is that once the input jack is fixed the amp sounds great. But it's twice as hot as the other one I have so I know something is up.
 
9/8/1997 5:18 PM
charles

Mike,  
I'm gonna second just about everything that Jim said.. and add my 2 cents worth.  
 
Everybody has their favorite tubes, and although this is very subjective... the Sovtek 6L6's are usually not as nice as the Sovtek 5881WXT (which has the tall base that works better in the Fenders).  
 
The idle current sounds about right, maybe just a little low, but that's subjective too.  
 
It's not uncommon for an amp to take some time to warm up. I think there was a thread on thie recently. I've found that most amps take at least a half hour to get stabilized. Ditto for the heat to rise to a steady temperature.  
 
I suppose the key phrase is "breaks up WAY too much". I wonder if you could describe the breakup scenario a bit. Also, just for grins, are the speakers wired for 4ohms or 16ohms?  
 
Charles
 
9/8/1997 6:15 PM
Mike

16 ohms  
breaking up way too much means lots of distortion at only 3 on the dial, where the sound should be totally clean. This sucker gets HOT and the other one I have does not get hot at all...
 
9/8/1997 8:09 PM
charles

Well... not that it matters a whole bunch, but the speaks should be wired as 4 ohms.  
 
Wow, that's nasty breakup. I don't think it's related to the heat tho. Unless after a while, the tubes start to conduct a lot more current.  
 
You checked em after they ran for a while, yes?
 

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