Thunder Star/Bass Amps : what's a 6GF7?

capnjuan

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megadan said:
Ah, I was wrong again.When I realized the tube in there was NOT a 12AX7, I left the old tube in. I identified it at one point, but now I've totally forgetten! It was all numbers and it was very rare, there weren't any avaliable directly anymore.
75-100 watts? Looks like a mean machine; if you look or can find the #s on the odd-ball, pass them along; growing # of tubistas here - plus places to look on the www. Just guessing but the 7591 could be either a voltage regulator or maybe it's the driver. Anyway, thing looks like a powerhouse... is this the one that was sold w/ 2 chasses or I am thinking of the Big Bird?
 

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The more I think about it, the more I think that tube was a 7247. Which is avaliable, all though not as common as 12AX types.

I'll have to pick one up next time I do a tube order.
 

capnjuan

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Hi Megadan; the 7247/12DW7 was used extensively by Guild as the output driver. It's most interesting characteristic is unusually high voltage on the plate. It is designed for higher voltage dissipation than a 12AX7 although a 12AX7 will work in the position. I have no schematic for your very rare 6550 amp; the 8417 Quantum uses a 6GF7 as a screen voltage regulator; I have the schematic for that amp. Your pic is marked '7591'. Guessing, it does the same thing the 6GF7 does in the 8417; voltage regulation.

Yes, the 7247 is easy to find. cj
 

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Heh-heh, war driving again!

I'm wondering if the high voltage on the low-mu side of the 7247 is to compensate for it's low amplification(20x) vs the high-mu(100x) side. 12au7's are cleaner than 12ax7's so to get more gain out of them, you'd have to drive them at a higher voltage.

With my little Maggie, if you swap out the 7247 with a 12ax7, clean goes right out the window.
 

capnjuan

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Default said:
I'm wondering if the high voltage on the low-mu side of the 7247 is to compensate for it's low amplification(20x) vs the high-mu(100x) side. 12au7's are cleaner than 12ax7's so to get more gain out of them, you'd have to drive them at a higher voltage. With my little Maggie, if you swap out the 7247 with a 12ax7, clean goes right out the window.
Not sure I could tell you. These amps; TBird 7591, TBass 8417, and TBass 6L6 all have 7247 drivers, 15K/2W plate resistors and, from the cathode-to-output coupling, a 14K/2W resistor to ground. If as you say the 12AX7 dirties things up, maybe any more gain destabilizes the output. Alternatively, intended to promote clean at the expense of grit/distortion; except for the Tbird, the other two are bass amps where fuzz isn't necessarily desireble but don't know where that would leave the Tbird. cj
 

megadan

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capnjuan said:
Your pic is marked '7591'. Guessing, it does the same thing the 6GF7 does in the 8417; voltage regulation.

That was our guess too. The tube is definitely a 7591, it was clearly marked (unlike the 7247 which was quite worn). In fact it was a 7591A, which according to R.G. Keen is very, very rare now! Whether that's part of Guild's original scheme, or whether this amp was modded somewhere in it's 37(!) year history, I couldn't say for sure!

Dan
 

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megadan said:
That was our guess too. The tube is definitely a 7591, it was clearly marked (unlike the 7247 which was quite worn). In fact it was a 7591A, which according to R.G. Keen is very, very rare now!

Dan

FYI, Dan, 7591s are available now and reasonable in price. One caveat, the Electro-Harmonix are made using a 6l6 bottle, but clearance doesn't look to be a problem on your amp.

I really wish he'd revise that page....
 

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Oh yeah, rereading this thread I realize I was a bit nebulous; I was talking about the wrong tube, as cj pointed out.

The tube referenced in thread title, the 6GF7, is similar in function to the 7591 in my amp.
 
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