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Will a 12AX7 sub. for 12DW7/7247?
Will a 12AX7 sub. for 12DW7/7247?
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Old 10th March 2015, 04:58 AM   #11
Miles Prower is offline Miles Prower  United States
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Originally Posted by VMUNIX View Post
But then possibly to reduce costs or to offer a simpler cheaper design (vibrato deleted) they've replaced the 6EU7 with a 6C4, which electrically can't be more dissimilar to half 6EU7/12AX7 and yet they've basically swapped tubes, only the 4K7 resistor is different.

Will a 12AX7 sub. for 12DW7/7247?


How is this supposed to work, and why they did this ?

I'm asking about V2 only and the values around it.
I have no idea how that's supposed to work. The 6C4 would make a good cathodyne, but you'd have to reduce the plate and tail resistors. 100K each is way too large for that type.
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Old 10th March 2015, 05:52 AM   #12
VMUNIX is offline VMUNIX  Argentina
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Originally Posted by Miles Prower View Post
I have no idea how that's supposed to work. The 6C4 would make a good cathodyne, but you'd have to reduce the plate and tail resistors. 100K each is way too large for that type.
That's what I thought, shouldn't be something like 33k ?
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Old 10th March 2015, 01:08 PM   #13
Tubelab_com is offline Tubelab_com  United States
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I had one of those amps a dozen or so years ago (6C4 version). My guess is that the voltage are real measured values on an amp built with 10 or 20% resistor tolerances. Either way it worked pretty good. It had a fairly clean tone at reasonable volume, and a good bluesy tone cranked.

I rescued mine from the trash. It had a 20 amp fuse in it and a fried power transformer. The 6X4 was toast, all because the filter cap had shorted. I stuck in a random power transformer I had, which came from an old TV sweep generator. It also had a 6X4 rectifier, so I tried it and wound up with 250 volts of B+.

The amp worked good so I never bothered to measure any of the other voltages or even pay attention to the resistor values.

The 12DW7 is half 12AX7, and half 12AU7. Rumor says it was something Ampeg wanted. At least one new production supplier is making them, but I can't remember which one, EH, maybe?

I have seen guitar amp heads stick 12AX7's or 12AT7's into 12AU7 sockets to get more gain and distortion. It does work, sort of in some amps, usually as the PI. It will not work as the reverb driver.

There is at least one guitar amp out there, Ampeg maybe, that uses a 12DW7 for reverb, the 12AU7 half for the driver, the 12AX7 half for the return amp.
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Old 10th March 2015, 01:27 PM   #14
VMUNIX is offline VMUNIX  Argentina
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Originally Posted by tubelab.com View Post
I had one of those amps a dozen or so years ago (6C4 version). My guess is that the voltage are real measured values on an amp built with 10 or 20% resistor tolerances. Either way it worked pretty good. It had a fairly clean tone at reasonable volume, and a good bluesy tone cranked.

I rescued mine from the trash. It had a 20 amp fuse in it and a fried power transformer. The 6X4 was toast, all because the filter cap had shorted. I stuck in a random power transformer I had, which came from an old TV sweep generator. It also had a 6X4 rectifier, so I tried it and wound up with 250 volts of B+.

The amp worked good so I never bothered to measure any of the other voltages or even pay attention to the resistor values.
That's right George, I will have to experiment that a bit. I think now we can leave this thread alone for another 7 years or so.
Thanks.
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Old 10th March 2015, 08:26 PM   #15
Miles Prower is offline Miles Prower  United States
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Originally Posted by VMUNIX View Post
That's what I thought, shouldn't be something like 33k ?
That would be more like it.
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Old 14th August 2019, 05:37 AM   #16
engel dela pena is offline engel dela pena  Philippines
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Originally Posted by blindbrett View Post
I recently bought a bogen pa amp and it uses both 12ax7's and 7247's in the preamp. My question is can I use a 12ax7 in place of the 7247 or do I need to change some resistors. I am asking simply to learn more about the tubes as I am learning how to repair tube amps and radios. Any help is greatly appreciated. Thanks folks
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Old 14th August 2019, 08:06 PM   #17
DAK808 is offline DAK808  United States
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Will a 12AX7 sub. for 12DW7/7247?
Quote:
Originally Posted by blindbrett View Post
I recently bought a bogen pa amp and it uses both 12ax7's and 7247's in the preamp. My question is can I use a 12ax7 in place of the 7247 or do I need to change some resistors. I am asking simply to learn more about the tubes as I am learning how to repair tube amps and radios. Any help is greatly appreciated. Thanks folks
Tube amp circuits have a greater tolerance built in. This will allow non specified tube substitutions, especially in the signal tubes, to "work", especially the 12**7 types. Subbing a 12ax7 for a 12dw7 (7247) will "work". Heater requirements, pin out is the same. Sometimes in a pinch you need to keep on going and making a change like this will not harm anything. I have subbed 12at7, 12av7, 12ax7, for that tube and had decent sound from the amp. That being said, the tube that is specified for the circuit should be the long term choice and will yield the best performance.

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Old 15th August 2019, 04:01 AM   #18
petertub is offline petertub  Sweden
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Will a 12AX7 sub. for 12DW7/7247?
Originally Posted by blindbrett Click the image to open in full size.
I recently bought a bogen pa amp and it uses both 12ax7's and 7247's in the preamp. My question is can I use a 12ax7 in place of the 7247 or do I need to change some resistors. I am asking simply to learn more about the tubes as I am learning how to repair tube amps and radios. Any help is greatly appreciated. Thanks folks


Short answer : NO


It won't hurt anything but it will not be accurate sound reproduction either.
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