Intermittant Red Plate

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Voxbox
Senior tube assistant
Posts: 165
(5/22/05 3:16 pm)
Reply  Intermittant Red Plate
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 Hi,

Is it possible to have a tube that goes red but only intermittantly?

The amp is an Ampeg V4B. I've moved the tube to two other sockets and intermittantly goes red in two out of the three sockets. Running the amp on load makes no difference.

As a set of these 7027As (well, NOS RCAs) cost an arm and a leg I need to be sure before changing them.

Any ideas?

Cheers
 
tubenit
Hey get your own solder
Posts: 256
(5/22/05 3:57 pm)
Reply  Re: Intermittant Red Plate
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 Yes, I've had that happen with two different amps using two different tubes. Both cases only one tube did that and it was intermittent. (Electro-Harmonix 6V6 and a Magnavox black plate 6V6). I switched to JJ 6V6's and have had no more problems.
 
Voxbox
Senior tube assistant
Posts: 166
(5/23/05 3:45 am)
Reply  Intermittant Red Plate
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 Thanks Tubenit.

BTW, I've tried some JJs recently, E34L, EL84s and found them to be good.

The EL84s withstood a very high plate voltage for some time without going red, and the sound of the E34Ls compared to Svetlana EL34s is much preferable, deeper, tighter bass and smoother upper mids to my ears.

Guess my customer is going to have to buy the RCAs for his Ampeg.

Cheers
 
HotBluePlates
I only work on Fender's
Posts: 1182
(5/23/05 11:49 am)
Reply  intermittent red-plating
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 The tube could also be drawing excessive current for other reasons, and only intermittently.

For example, if the bias pot wiper was making intermittent contact, then the loss of bias voltage would cause the tubes to red-plate.

I know your case is really related to only one tube, but I thought the info might be helpful for others.
 
Voxbox
Senior tube assistant
Posts: 167
(5/24/05 5:38 am)
Reply  Re: Intermittant Red Plate
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 Thanks HBP,

I did check all of the fixed resistors in the bias circuit for bad joints. (there is no bias pot). And applied a technical tap or two to the board around them. Dont like intermittant faults, they can come back to bite you, not what I want when the customer is shelling out about £155 ($280) for four new RCAs....

As the fault followed the tube when I moved it to another position, I assumed that the fault is in the tube itself. But I thought I'd ask for advice as my exposure to tubes is minimal. They stopped teaching about tubes the year I stared college! Bummer! So while I have loads of electronics repair experience, not a lot of it is on tubes. So I tend to work from first principals rather than "tube experience".

Cheers
 
HotBluePlates
I only work on Fender's
Posts: 1186
(5/24/05 7:19 am)
Reply  Re: Intermittant Red Plate
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 Well, I think you're on the right track about the fault being with the tube, as shown be the fault following the tube.

I can only suggest having known good spares on hand of most common tubes to sort out troubles like this quickly.
 
 
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