12AU7 pre rebuild suggestions - diyAudio
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Old 7th January 2014, 02:40 AM   #1
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Default 12AU7 pre rebuild suggestions

I came across an old pre I did from a Bottlehead Foreplay II. This was the second rebuild of this pre. I used a stereo 23 step attenuator with 1% Dale resistors, added the Bottlehead Anticipation upgrade that uses active load boards that are voltage amplifiers (plate load) and cathode follower (cathode load) for each channel, some Gudeman 1.5uF output caps. and 220uF 200V DC filter caps.

What do you think about if I rebuild this for the third time and this time in a larger chassis to move the power supply far from the outputs? What do you think about going with larger filter caps? Any suggestions if you were modding this?
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Old 7th January 2014, 03:00 AM   #2
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I might get rid of the 12AU7 tubes in favor of the 6CG7/6FQ7 triodes. Better overall linearity. I don't think larger filter capacitors will buy you anything in terms of sonics. And is that RG179 cable I see? I've been using that for years. Great stuff. Silver/Teflon™ and low capacitance.
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Old 7th January 2014, 03:06 AM   #3
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Originally Posted by HollowState View Post
I might get rid of the 12AU7 tubes in favor of the 6CG7/6FQ7 triodes. Better overall linearity. I don't think larger filter capacitors will buy you anything in terms of sonics. And is that RG179 cable I see? I've been using that for years. Great stuff. Silver/Teflon™ and low capacitance.
I'm open to any change. I think winter has my need to solder going.

Yes, that is RG179. You have a good eye. I've used that along with RG316 for 20 years.
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Old 7th January 2014, 03:40 AM   #4
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Unfortunately, the Allied 6K27VF trafo, in a stock FP2, can only supply 600 mA. of heater current. So, energizing a pair of 6CG7/6FQ7 tubes is not feasible, without a power trafo change.

A Hammond 369BX has the filament winding "stones" needed and would permit a more favorable (IMO) operating point to be employed too. In addition to the heater winding limitations, the 6K27VF is rated for only 15 mA. of B+. Use 2X 1200 PIV Schottky diodes to rectify the B+ and completely eliminate SS diode switching noise.

Add "hash" filtration between the 1st filter cap. and the rest of the PSU filter. Large valued I/P caps., like that supplied by Doc B, yield a highly "triangular" ripple wave form. Per Fourier's Theorem, such a wave form contains lots of ripple freq. overtone energy. A "hash" filter blackens the background.
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Old 7th January 2014, 03:47 AM   #5
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Quote:
Originally Posted by HollowState View Post
I might get rid of the 12AU7 tubes in favor of the 6CG7/6FQ7 triodes. Better overall linearity. I don't think larger filter capacitors will buy you anything in terms of sonics. And is that RG179 cable I see? I've been using that for years. Great stuff. Silver/Teflon™ and low capacitance.
Most definitely. Ran across an article quite some time back wherein extensive testing was done of the various preamp tubes. The octal sockets bested the Novars with ONE notable exception, the 6CG7/6FQ7. They are considered the electrical equivalent of the 6SN7. I have done head-to-head comparisons of the two and the 6SN7 and 6CG7 do indeed sound better.
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Old 7th January 2014, 03:51 AM   #6
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+1 for the swap to 6CG7. Even the 6FQ7 sounds great (no shield in between the triodes). You can even buy adapters online so you don't need to re-wire the socket just yet (in case to want to experiment a little first).
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Old 7th January 2014, 04:26 AM   #7
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Originally Posted by Eli Duttman View Post
Unfortunately, the Allied 6K27VF trafo, in a stock FP2, can only supply 600 mA. of heater current. So, energizing a pair of 6CG7/6FQ7 tubes is not feasible, without a power trafo change.

A Hammond 369BX has the filament winding "stones" needed and would permit a more favorable (IMO) operating point to be employed too. In addition to the heater winding limitations, the 6K27VF is rated for only 15 mA. of B+. Use 2X 1200 PIV Schottky diodes to rectify the B+ and completely eliminate SS diode switching noise.

Add "hash" filtration between the 1st filter cap. and the rest of the PSU filter. Large valued I/P caps., like that supplied by Doc B, yield a highly "triangular" ripple wave form. Per Fourier's Theorem, such a wave form contains lots of ripple freq. overtone energy. A "hash" filter blackens the background.
Yes a better tranny is in order to get to the next level. So, instead of a rebuild, a new build may be in order. Do you know of a good circuit for a 6cg7/6fq7 pre? I'm really open to suggestions here.
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Old 7th January 2014, 01:20 PM   #8
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What do you think about the 6N6P on a rebuild version?

It looks like I may do a rebuild and a new build. I have ordered a new large preamp chassis for a new build.
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Old 8th January 2014, 12:18 AM   #9
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How about a "parafeed" setup? Excessive gain is a common complaint, when medium μ triodes are employed in line stages. MagneQuest offers suitable "iron". The step down turns gain into drive.

The B7-15K is right for 6SN7 family tubes, including the 6CG7. As both RP and μ are lower in the 6H6П (6n6p) than the 6CG7, the B7-10K seems to be the correct choice.
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Old 8th January 2014, 02:49 AM   #10
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Originally Posted by Eli Duttman View Post
How about a "parafeed" setup? Excessive gain is a common complaint, when medium μ triodes are employed in line stages. MagneQuest offers suitable "iron". The step down turns gain into drive.

The B7-15K is right for 6SN7 family tubes, including the 6CG7. As both RP and μ are lower in the 6H6П (6n6p) than the 6CG7, the B7-10K seems to be the correct choice.
Thank you Eli. You have forgotten more about design than I have ever known. I think this is what makes diyAudio such a great place; the sharing of the wealth of information. It helps make this hobby, passion, obsession, lunacy fun.
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