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Old 6th February 2013, 10:35 PM   #611
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I have DAC and a pre amp with 6DJ8. There are no issues with microphonics or accuracy in sound. The 6SN7 I have no issues with in audio either. I have had and heard plenty of equipment using either tube. And they work well for audio. I do take issue with one tube in audio. That more often than not I do not like when present. And that is the 12AX7. I avoid that tube.
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Old 6th February 2013, 11:56 PM   #612
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Heya 1000volts,

As a thought, take Ralphs advice, his original OTL's used a combo of miniature tubes and he then migrated to the 6SN7, there would be good reason for that aside from availability.
Ralph is easy to talk to, give him a call and I'm pretty sure if he has the time he will explain all.
Seriously.

Cheers,

Drew.
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Old 7th February 2013, 03:33 PM   #613
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I avoid 12AX7s as well. Low distortion but bandwidth is a problem.
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Old 7th February 2013, 05:48 PM   #614
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Quote:
Originally Posted by atmasphere View Post
No. The 6DJ8/6922/6H30 family of tubes are not designed for audio and have troubles with microphonics. This is why the 6SN7 sounds better.
What does "designed for audio" mean? Just because audio isn't mentioned in the spec sheets doesn't mean they can't be used for that. The 6BQ7 spec sheet doesn't mention audio either, but I've put these to good use (as cascoded LTPs) and, at least in this particular application, sound great. Tube Lab also vouches for the 6BQ7 as a conventional triode LTP. Granted, it's a bit more work finding a good audio loadline, but it can be done: keep that Vpk up!

Types that mention audio as an after thought aren't necessarily any good for audio. One such type is the 12AV7: hideously non-linear. You can't find any good loadlines here. The 12AU7 is another, and both of these are RF types. Most of the circuits where 12AU7s appear are oscillator/buffers, PP frequency multipliers and drivers, product detectors/mixers, and quasi-digital and digital designs: multivibrators, Schmidt Triggers, R-S latches/flip-flops.

Other types that don't mention audio are the TV HD pents (probably since you can't get these into Class A without severe red plating) but they also offer some outstanding audio performance as Class AB1 amps, and the monster color TV HD types can put out some bigwatts without excessive voltages and drive requirements.

Quote:
Originally Posted by 1000Volts View Post
I have DAC and a pre amp with 6DJ8. There are no issues with microphonics or accuracy in sound. The 6SN7 I have no issues with in audio either. I have had and heard plenty of equipment using either tube. And they work well for audio. I do take issue with one tube in audio. That more often than not I do not like when present. And that is the 12AX7. I avoid that tube.
So far, I haven't required the gain that a 12AX7 can provide, but the less than satisfactory sonic results aren't a fault of the type, but rather the implementation. The 12AX7 has a very high Rp, (90K) and in all too many cases, the plate is loaded way too heavily. Like any other triode, the 12AX7 needs light loading. This is often hard to do with passive loads and low Vpp. You're better off with active plate loading if you don't have the voltage reserves to get that passive plate load up around 200K or better. Use them right, and sonic performance isn't a problem.

"I avoid 12AX7s as well. Low distortion but bandwidth is a problem".

Depends on the load. Types like this (and the 6SL7 -- which I have used) require friendly, Hi-Z, Lo-C loads, and likely buffering (cathode follower, source follower).
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Old 7th February 2013, 06:33 PM   #615
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The less microphonic tubes I ever saw, 6С17К-В, were never designed for audio, but for missile guidance heads in rockets. But they are still the best for microphone and phono inputs.

ГУ-50 were designed for radio frequency amplifiers. 6П15П were designed for video amplifiers. 6Н30П and 6Н6П were designed for computers.

And so on. Lots of best audio tubes were not designed for audio.
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Last edited by Wavebourn; 7th February 2013 at 06:36 PM.
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Old 7th February 2013, 07:13 PM   #616
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Quote:
Originally Posted by Miles Prower View Post
What does "designed for audio" mean?
It means that audio tubes have more structural re-inforcement to reduce microphonics. 6DJ8s are quite linear which makes them attractive, but they lack such structure. Finding examples that are low microphonics is harder. Almost any 6SN7 version made can be found in low microphonics, even the RCA coin base.

Quote:
"I avoid 12AX7s as well. Low distortion but bandwidth is a problem".

Depends on the load. Types like this (and the 6SL7 -- which I have used) require friendly, Hi-Z, Lo-C loads, and likely buffering (cathode follower, source follower).
Agreed. Like I said- a problem. But not one that can't be solved.
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Old 7th February 2013, 08:27 PM   #617
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Originally Posted by atmasphere View Post
It means that audio tubes have more structural re-inforcement to reduce microphonics. 6DJ8s are quite linear which makes them attractive, but they lack such structure. Finding examples that are low microphonics is harder. Almost any 6SN7 version made can be found in low microphonics, even the RCA coin base.
Not so much of a "designed for audio" thing, but rather one of overall quality. There are a lot of cheap tubes out there. The same applies to 6BQ7s. The ones with the heaters connected in parallel are not microphonic to any annoying extent, but the cheap ones where the heaters are in the form of one continuous loop with a thin strand that flares up when you power up, ring like bells. (I avoid buying 6BQ7s sight unseen for this reason: I'm looking for parallel heater types.) These are just plain cheap tubes made by those who "thought" that ringing at 1500Hz was NBD for a tube that would most likely find itself in the first VHF pre in a TV tuner. Even that isn't right since the disturbances of microphony can cause detuning of RF circuits, and you definitely will notice that, in both sight and sound, in a TV set. Especially the ones with the old timy, click, click, click tuners that send lots of vibration into the tuner subchassis to make poor tubes ring.

As for 6SN7s, there are also a lot of cheap ones too, and they can be as microphonic as any poorly made VHF TV tuner VT.
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Old 8th February 2013, 03:50 PM   #618
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Yes, plenty of tubes not for audio that can work well. The elephant in the room is the output tubes 6AS7G not an audio tube. Same goes for 6C33B. But, both work well in an OTL and other tube designs. I have heard them used in......

What small signal tubes are used in the Novacron? People must really like this amp as it never comes up on the used market!!!
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Old 13th March 2013, 12:46 AM   #619
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Never mind. I see the Novacron is back. Will this also be in kit form? What is the retail price these days?
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Old 29th July 2013, 02:23 AM   #620
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I'm planning to build a circlotron, M-60 based, and this thread has been amazing for its wealth of information!

I have a few questions, not sure if they've been addressed earlier, but here goes.

1. Besides being in current production, is there any reason to prefer 6AS7G over NOS 6AS7GA/6080/WA/WB/WC variants? Aesthetically, the coke bottle envelope of the 6AS7G doesn't appeal to me, unless paired with similarly shaped driver tubes. Electrically, they seem to be equivalent per the data sheets, but I have a concern regarding control grid dissipation (radiator shapes and sizes are all over the place), and ruggedness and immunity to flash-over, internal cathode fuse blowing, etc.

2. I've been working on a scheme to use direct coupling from input to output, with suitable protection for the output tubes in case of driver or grid bias failure. My aim is to allow for dc feedback to be used all the way from the output cathodes to the input differential amplifier to automatically compensate for drift, reduce the frequency of bias/dc balance adjustments, and eliminate the coupling capacitors from the audio path. The dc stress in the loop could be used in conjunction with a window comparator to give the user an indication when rebiasing/rebalancing is needed. Is this a good idea (in terms of reliability, more things to go wrong, extra complexity, unintended consequences)? Have any of you tried this and abandoned it because it affected the sound or other secondary problems?

I was instructed very early on that a good active circuit design is tolerant of the characteristic variations of active elements (i.e. the circuit around the active device should control performance, not the other way around). Designing for worst case and repeatability is preferable to hand-tweaking. DC feedback seems like a good thing from this perspective, but I could be wrong especially where sonics are concerned.

3. I'm going to go ahead and place an order for output triodes (at least 16 for 2 channels). How many spares should I order above that number to ensure that I have a decent pair of octets?

Thanks for reading.
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