I've never seen this before. Only with power tubes. The amp is a Chicago Bluesbox Roadhouse. Basically, a super reverb in a head. Has reverb and tremolo. Uses all 12AX7's, except for V2, which is a 12AT7. It's the 12AT7 that is redplating.
I only found this out because I got burnt when I touched the tube retainer. According to my temp gun the tube is about 380 degrees, while the other ones are about 120 degrees. The amp sounds great but I'm thinking this can't be normal.
Can this be an issue with the tube or would it have to be the amp causing this? I have a Mullard 12AT7 that I'd like to try if it's a possibility that it could just be the tube.
Thanks!
Preamp Redplating?
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gingertube
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Re: Preamp Redplating?
Schematics don't seem to be available on the web so I am truely guessing - anyone help with schematics?.
I'm guessing that the 12AT7 is the reverb driver and it runs parallel triode sections into a single ended output transformer like a Fender Twin Reverb.
If this is so:
Check that the cathodes are sitting at around +8V +/- 2 Volts with close to 0V on the grid and around 400V on the anodes.
If the cathodes read at 0V suspect a shorted bypass capacitor.
If tyhe grid reads positive voltage suspect the coupling capacitor from the previous stage.
The 12AT7 run quite hot, are you sure there is a problem?
Cheers,
Ian
I'm guessing that the 12AT7 is the reverb driver and it runs parallel triode sections into a single ended output transformer like a Fender Twin Reverb.
If this is so:
Check that the cathodes are sitting at around +8V +/- 2 Volts with close to 0V on the grid and around 400V on the anodes.
If the cathodes read at 0V suspect a shorted bypass capacitor.
If tyhe grid reads positive voltage suspect the coupling capacitor from the previous stage.
The 12AT7 run quite hot, are you sure there is a problem?
Cheers,
Ian
Re: Preamp Redplating?
Nor sure at all, thats why I was asking. I've never come across a preamp that was that hot and I've owned a lot of amps. I can't imagine it being ok that the tube plate is red hot, but I don't know.
I'll post some chassis photos when I get a chance, but in the meantime should a preamp ever run as hot as I mentioned? Redplating? 380 degrees?
Thanks for the info! This amp is pretty amazing btw. Third Chicago Bluesbox I've owned and they've all been stellar.
I'll post some chassis photos when I get a chance, but in the meantime should a preamp ever run as hot as I mentioned? Redplating? 380 degrees?
Thanks for the info! This amp is pretty amazing btw. Third Chicago Bluesbox I've owned and they've all been stellar.
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Stevem
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Re: Preamp Redplating?
Does the reverb work?
If not the primary of the reverb drive transformer could have s short.
If not the primary of the reverb drive transformer could have s short.
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Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Preamp Redplating?
The reverb driver will indeed run much hotter than the others, but it shouldn't be red plating! Either there is a leaky coupling cap or it has lost its bias somehow. (Or maybe the bias resistor was too small to being with...)
Re: Preamp Redplating?
Is the plate structure *actually* glowing red hot?lgehrig4 wrote:I can't imagine it being ok that the tube plate is red hot...
Are your temperatures in °C or °F? "Red hot" is something around 600°C / 1100°F
The cathode structure will probably be ~ 900°C / 1600°F
Re: Preamp Redplating?
I'm not technically savvy like you guys so I can't answer most of these questions.
Stevem - the reverb works and sounds great
Tillydog - the temp pf the tube was 380°F . The plate is actually glowing hot.
I'll have to take a photo and ask you guys to point out specifically what I should test.
My question in the meantime is whether it could be the tube or would it have to be caused by the amp? If it could possibly be the tube drawing too much current then I will try another just to test, but if the tube could not do this on its own I won't waste the time.
Stevem - the reverb works and sounds great
Tillydog - the temp pf the tube was 380°F . The plate is actually glowing hot.
I'll have to take a photo and ask you guys to point out specifically what I should test.
My question in the meantime is whether it could be the tube or would it have to be caused by the amp? If it could possibly be the tube drawing too much current then I will try another just to test, but if the tube could not do this on its own I won't waste the time.
Re: Preamp Redplating?
OK, that's bad!lgehrig4 wrote:The plate is actually glowing hot.
See Jelle's suggestion above - you're looking for a capacitor connected between pins 3 and/or 8 and ground. (I would guess that 3 and 8 might be tied together, but I haven't looked for a schematic.)
If you have a voltmeter you could check the voltage between pins 3/8 and ground - it should be a few volts.