Problem w/ Thunder I Twin

adam79

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I'm having problems with my Thunder I Twin. The last time I played it, it was working fine. It takes a several seconds for it to power up. Once on, it emits a boomy hum, with the volume all the way down. When I turn the volume up, the pot is very scratchy and the noise increases. The tubes a glowing orange/red. Once I heard this, I was afraid to plug in my guitar. Any ideas one what the problem might be? I'm probably gonna bring it in to a tech for repair/tune-up.
The amp has the original Guild tubes from the 60s. If they are still full of life, I'll keep them in. If not, any recommendations on which tube brands make a good match for this amp?

Thanks,
-Adam
 

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You did the right thing taking it to a tech, the plates shouldn't be glowing that way. It's dropping too much current through the tubes.
IIRC, Coastie liked Mullards in his, but they're not in production. Tubesandmore has them for 30 bucks each, but hopefully your tech will straighten it out without the need for replacement tubes.
 

capnjuan

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adam79 said:
I'm having problems with my Thunder I Twin. The last time I played it, it was working fine. It takes a several seconds for it to power up. Once on, it emits a boomy hum, with the volume all the way down. When I turn the volume up, the pot is very scratchy and the noise increases. The tubes a glowing orange/red. Once I heard this, I was afraid to plug in my guitar. Any ideas one what the problem might be? I'm probably gonna bring it in to a tech for repair/tune-up. The amp has the original Guild tubes from the 60s. If they are still full of life, I'll keep them in. If not, any recommendations on which tube brands make a good match for this amp?
Hi Adam; just guessing of course but it sounds like one or more of the power supply filter caps is leaking. When they go bad, they leak low amounts of DC to the chassis. Since the audio circuit uses the chassis for ground, the DC on the chassis is picked up and amplified as hmmmm .... At a minimum, it needs new power supply caps, bias supply filter cap (possible source of your 'red-plating') and, so long as it's open, cathode resistor bypass caps, a 3-wire cord if it doesn't have one, and some spray cleaner in the pots. All this stuff is normal maintenance.

Too many good preamp tube brands to name but that amp uses 6GW8s a/k/a ECL86s for output. These tubes were widely used in Europe in table radios and the big mfrs competed for orders based on tone; Siemens, Mullard, Telefunken, Tungsram, Amperex Bugle Boys ... these are going to be the best-sounding 6GW8s you can find. There are US domestic NOS out there but they never earned the reputation for tone that the Euro tubes have. Default's right; our man Coastie is who to see on the subject of 6GW8s/ECL86s. Good luck with your amp. CJ
 
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