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Author
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Topic: Fender Super reverb (1969) - Hot 5U4 tube
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Lefty
Member From: Grayson, Ga.
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posted 17 November 2006 01:40 PM
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I am restoring a 1969 Fender Super Reverb. This amp sounds superb, but after playing for 15 or 20 minutes the 5U4 rectifier tube seems to get pretty hot. Hotter than the power tubes, which stay fairly cool.
Can anyone offer advise to a possible problem?
thank you,
Lefty |
Eric Jaeger
Member From: Oakland, California, USA
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posted 17 November 2006 01:59 PM
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Have you tried replacing the tube? Does it glow oddly?Otherwise, in my experience what you're seeing is normal. The temperature of the power tubes in a class AB amp is driven by the signal being driven through them -- the higher the volume, the hotter the tubes. The rectifier is essentially running close to full all the time, so to an extent it should be hotter. OTOH, I don't know what "too hot" is in this context, and I could be totally missing something. -eric |
Chick Donner
Member From: North Ridgeville, OH USA
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posted 17 November 2006 02:16 PM
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Can you get an ammeter (OK - milliammeter) in the cathode circuit of the 5U4? Might have a bad filter, which would pull extra current right thru the bleeder. |
Mike Wheeler
Member From: Columbus, Ohio, USA
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posted 17 November 2006 03:09 PM
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If it's a bad cap, it should have a temperature...be very warm or even hot to the touch from the leakage current. |
Jim Sliff
Member From: Hermosa Beach California, USA
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posted 17 November 2006 05:26 PM
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What they said - although if your power tubes are cool even at idle something is amiss.Have you had the amp biased? It sounds like it may be biased WAY cold - not healthy for the amp and you're not getting optimum tone or output that way. |
Blake Hawkins
Member From: Land O'Lakes, Florida
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posted 17 November 2006 09:01 PM
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A hot 5U4 is normal. It will take the skin right off of your fingers.
How to tell if it is too hot:
The plates will be glowing red or will have a dull red spot in the middle of them.
That would indicate a short in the power supply and the amp would either not work or be working very poorly.
Neither the RCA Tube Manual nor the Sylvania Tube Manual give any data on maximum temperature ratings.
They just say to use "adequate ventilation. |
Lefty
Member From: Grayson, Ga.
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posted 18 November 2006 04:00 AM
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Thanks to all for the information.
Jim, I did add a new set of Swetlana GL6's and it needs to be biased. I bought some new old stock 5U4 (GE) tubes off Ebay.
I am going to take this one (69) in for a check up. I have the 68 in right now getting it returned to stock. It was modded by Wizard electronics at some point to turn the tremelo intensity into a master volume.
Jeff Bakos said he has returned many of these mods back to stock. These are my first Super Reverb amps and I have to say they make fine Tele amps.
Lefty |
Jim Sliff
Member From: Hermosa Beach California, USA
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posted 18 November 2006 06:38 AM
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Lefty, these are superb Tele amps. I played one that I returned to stock after some bad mods had been done to it, and it was a great match for a Tele.A hot 5U4 is normal - and you should always rebias when you put in new tubes; they're just not plug 'n play anymore...too many variations. If you don't have one, get yourself a Bias King or Bias Rite - fairly inexpensive tools that allow you to set the bias yourself in minutes. Pays for itself the first time you change tubes! Also, when you change rectifiers you need to rebias, as plate voltages can shift, causing your bias to go out of whack. I like the Svets at about 37ma in most SR's, assuming normal plate voltage. It's a tad on the hot side and for Tele lets you turn the amp up and go from clean to dirty using the guitar's volume control. You can push 40ma, but the tubes might not last real long. OTOH, if you like it clean keep the bias between 25-30ma. Weirdly, most of Fender's reissue amps show up biased at around 22ma. They play in the safe zone, but people wonder why they don't sound like the old ones...and a simple rebias does wonders. |
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