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In Reply to: RE: Heathkit w-5M questions posted by daseman on June 24, 2015 at 15:22:11
The only job I will use U7s for is cathode-followers where they are blameless. ? The 12AU7 has the highest distortion of all the 12A_7 family. And, I don't know of any noval twin-triode audio valve that is worse.
As drivers for power tubes they might be slightly ahead of the prevalent use of 12AX7s as drivers, which IMO don't have enough gm (grunt) for that job.
So, IF the PTs have sufficient filament/heater-current reserves replace them with 12BH7s. IIRC these are drop-ins, so you may not need to change any of the surrounding components. But, lowering the NFB around the input and driver stages is worth considering.
The originalists might bitch but they are your amps. Vintage PP valve amps with decent iron offer the opportunity of high-end sound for not much money, if you give them a lot more PSU storage in the PSU, decent driver tubes, and less total/loop NFB.
Increasing the storage will require; replacing the Valve Rectifier diode, or paralleling it with soft recovery HV SS diodes. That idea will keep the amps looking original. There are a few threads at 'Tubes' here on this latter idea with Eli Duttman contributing. I used the socket hole for another big capacitor.
Chokes in PSUs have advantages.
Bypassing the last two caps in the PSU with 10% styrene or polypropylene film caps can't hurt.
You should be able to get a few dbw more power / headroom for peaks as your B+ voltage will be a bit higher. In any case the power bandwidth can be a bit wider. NFB bandwidth components will need to be adjusted.
LBNL double the wattage of all the small-signal Rs, if it uses cathode-bias use the best low inductance Rs that you can afford. Less noise.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Follow Ups:
There does not seem to be a lot love for the 12AU7 around here, but it does seem to be used a lot in vintage euipment. I have never used it, but I have used the similar 6SN7, and thought highly of the equipment it was in. Why is the 12AU7 so popular in commercial gear?
Dave
Just a guess as to why the 12AU7 was so popular back then was that it was similar enough to the octal 6SN7 that it could be adapted to previous 6SN7 circuit designs with little if any change.....at least for audio apps. Unfortunately, the 12AU7 is really a dual 6C4, not very linear and more at home in RF apps. A little after the intro of the 12--7 series of tubes came the 6CG7, a near exact duplicate of the 6SN7 but in the more compact 9 pin noval format. By this time momentum was probably with the 12AU7.
A common "upgrade" for the W5 and Mac amps using the 12AU7 is to convert them to the 6CG7/6FQ7. Requires some wiring changes at the sockets because the pinouts are different but usually no other changes necessary.
Of course Marantz 8/8B doesn't need to be modified because he used the 6CG7 for the long tailed pair.
Dave
Timbo in Oz:
Thanks for the detailed message, but I don't have the technical ability/interest/time to modify my gear.
Would appreciate any 12AU7 suggestions from current or former Heathkit w5m users.
Cheers,
Blake
Follow up: I ended up with a quad of Amperex (made in Holland) 12AU7s and they sound mighty fine to me.
Rest of the tubes are (reissue) Gold Lion KT66s and dual bottom getter RCA 5R4GYs.
Very pleased with the sound of these W5Ms in my system.
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