Guild Thunderbass tubes

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Our bass player has a Guild Thunderbass. Probably around a 1968 or 1969 model, does anyone have any tube info as for what to use as replacement tubes for this beast?

Thanks for any help.

Paul
 

fronobulax

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Welcome to LTG.

I don't know but there are several people who do and I'm sure they'll be along shortly.
 

capnjuan

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Hi Diplomat and welcome to LTG; which tubes depends on which amp you're talking about.

2 or 3 tube preamp Thunderbass; 6L6s. If original, black tolex.
4 tube preamp Thunderbass; could be 8417s or 6L6s. Can be either beige or black tolex.
Quantum Thunderbass; ID'd by the small tag on the front that says: 'Quantum'. These too can be either 8417s or 6L6s.

If it's an 8417 amp, you can put 6L6s in it although they will not do as well as they might and there's the chance of shortened tube life. Pictures? CJ
 
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Black tolex, has a visible 8417 tube inside, will have more details wednsday evening.
Thanks for all the info guys, its definately not a quantum.

--Paul--
 

capnjuan

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Hi Paul; the 2-,3-, and 4-tube preamp models all use a 7247/12DW7 as the driver; all the other preamp tubes are 12AX7s. If you aren't sure which is the driver, it's always the tube physically closest to the output tubes.

The 7247 has the same pinouts as a 12AX7; in an emergency, a 12AX7 can be subbed in. One half - the 'front' half - of the 7247 is the same as a 12AX7 but the other half is like a 12AU7 with a much lower gain factor and higher plate dissipation characteristics. In the Thunderbass designs, the driver 1/2 (the back half) of the tube has nearly 300 volts on the plate; the 7247 is there for a reason. CJ
 

RWood

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To add to what Cap'n said........if you are looking to replace the power tubes, do yourself a favor and make sure the screen voltage (pin 4 on each of the big octal sockets) is no more than 300VDC. This can be checked with the output tubes temporarily removed from their sockets. If it IS more, then look to see if it has a short, fat tube in the dead center, labeled 6GF7A. If it does, then there is also an adjustment pot on the chassis deck so that you can dial in 300 volts. Excessive voltage here is the leading cause of shortened tube life with the 8417.

It's also important to rebias the power tubes anytime you swap them around or get new 8417s. Fortunately, Guild made this very easy to do with a bias test socket on the back panel and two adjustable pots (which are a little difficult to reach). All can be done without removing the amp chassis from the cabinet. If you can't get the bias 'in range', take it to a tech or, if you are solder-friendly, we'll tell you more about what needs to be done.

If you need more help or explanation just ask. There are a half-dozen or so active members here who have become oh-so-familiar with the T-Bass amps. It is worth the effort, too, as they sound quite good when they are topped off and running on all cylinders.
 
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