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Old 30th March 2009, 08:25 AM   #1
dwdw is offline dwdw  United States
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Default Nube challange12AX7 to drive 6C4

I'm trying to take the output of a 12AX7( first stage is a 12AY7 and then the 12AX7 as the second and third stage) to drive a 6C4 which in turn to drive 4 7868s in a push pull output. It s basically the Fender 5F6-C to a Bogen CHB 100
Should there be a capacitor between the treble pot wiper and the grid of the 6C4?
I also can't find a way to get negative feedback
The 5F6-C uses part of the out put from the audio transformer and the Bogen used the 70 volt output back to the original 12AX7. How can I do the same?
Here is a schematic of what I have putting the two together.
Thank You
For any help, being a real nube I'm drowning,
dwdw
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Old 30th March 2009, 12:56 PM   #2
dwdw is offline dwdw  United States
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Default Attachment12AX7 to drive 6C4

Here is the attachment had to reduce the size to fit tht rule of the attachment size.
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File Type: jpg fend bog e.jpg (93.7 KB, 222 views)
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Old 30th March 2009, 05:22 PM   #3
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Default Re: Nube challange12AX7 to drive 6C4

Quote:
Originally posted by dwdw
Should there be a capacitor between the treble pot wiper and the grid of the 6C4?
Looks like a 68pF capacitor to suppress RFI. There is so much gain ahead of the 6C4 that the original design may have had a problem with interferance.

Quote:
I also can't find a way to get negative feedback
The 5F6-C uses part of the out put from the audio transformer and the Bogen used the 70 volt output back to the original 12AX7. How can I do the same?
Here is a schematic of what I have putting the two together.
Thank You
For any help, being a real nube I'm drowning,
dwdw
Given that schemo, if you want gNFB, you can take it back to the first 12AX7 stage. Of course, that will require moving that tone stack outside the gNFB loop, otherwise, the gNFB will be trying to undo whatever it does. Probably best to leave that alone and just use it as designed without gNFB. It's a gee-tah amp after all, and distortion is a big part of the amp's tone.
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Old 30th March 2009, 09:26 PM   #4
dwdw is offline dwdw  United States
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Thank You for the reply Miles
Maybe I'm deeper in the water than expected. The gain didn't seem that much but I guess I'm wrong again!Here is what I have to start with all I changed was the input to the 6EU7 the upper had a 0.1mfd cap and after the was a 1meg to ground. The lower was running through a switch for the inputs.
The plate voltage was limited by 2 470K resistors in series to each plate of the 6EU7( they were used in a remote circuit ) I read to use a 100K resistor to each plate to be safe I added a 250K pot to lower the plate if needed. Maybe I'm dead in the water.
I tried to find the gain of all the tubes , found all but the 7868's
the 6EU7 is the same as a 12AX7 with 100 gain the 6C4 is about 16 gain. That would be 100, 100, 100 and 16 and what ever the 7868's are. How much gain is needed for a guitar amp? I can make the gain less by changing the 12AX7 to a 12AY7.
Thank You
dwdw
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Old 31st March 2009, 02:26 AM   #5
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I suggest you spend some time with the Radiotron Designer's Handbook before going any further.

"That would be 100, 100, 100 and 16 and what ever the 7868's are"

This just isn't true at all. Amplification factors are the maximum theoretical gain, and you'll only see gains close to that if you had CCS plate loads, and you don't. Furthermore, one half of that 12AX7 is a cathode follower, and that has a gain of less than 1.0. Same for the 6C4, which is being used here as a cathodyne phase splitter. You've really got just two gain stages in there, the 12AY7 and half the 12AX7. Still, both types can manage gains of about 20 and 50. That still leaves quite a bit of gain even considering the losses of the cathode follower and the cathodyne and the insertion losses of the tone stack. Guitar pick-ups probably don't produce much more than 10mV of output.
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Old 31st March 2009, 03:35 PM   #6
dwdw is offline dwdw  United States
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Thank You Miles for the reply.
I downloaded the manual and starting to read it. I think there is a huge amount of good information.
While I read I would like to do work on the amp.
The amp has a sensitivity of 2 mv is that sensitive enough to work as a guitar amp. The reason I'm trying to modify it is my grandson is helping me and we would like to have it working by the end of May for his graduation from grammer-school, a family get together.
Thank You for the Great link for learning and the help you have given me.
dwdw
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