ODD TUBES, what to look for?

[aerius]

Loctals were already mentioned, but another cheap trick is to look for the same tube in a different voltage, a 12SN7 or 12SL7 instead of the 6 volt ones for instance. There's also the 6A3, and the 6AU7 instead of 12AU7.

[Wavebourn]

Compactrons.
Tubes and sockets are available on ePay.

[Miles Prower]

Originally posted by tubelab.com
In this audio world it would appear that a few people state the "facts" and they are blindly accepted by the majority. This happens a lot on one of the other audio forums. For instance, it is an accepted fact that the 12AU7 is not a good audio tube. This I haven't tested yet, but since I have about 1000 of them, I will. I have also heard the same about the 12AT7. I was searching for a good driver tube, when I discovered a large box full of 12AT7's. I wired one into a simple breadboarded SE amplifier using the common resistor loaded circuit, biased at the usual 2 - 3 mA. I tested the distortion spectra, and listened to the resulting (mono) amp. It would seem that the world was right, a thin sounding amplifier with a large amount of third and fifth harmonics. If I had given up there I could have said that the audio world was right. After a lot of experimentation, I discovered that this tube likes more current (5 to 10 mA) and a CCS load. Under these conditions a 12AT7 works really well.

I've found it helpful to take a good LQQK at the specs. The suggested plate currents for the 12AT7 are a good deal higher than what a lot of folks seem to be running with. That's where I'd certainly start looking for a good loadline. Of course, the main problem here will be less Av since the Rp's will have to be smaller with reasonable Vpp's. Still, you can always use an active load if you need high gains, or make up for it with a preceeding stage.

As for 12AU7s, these appear to work best as SRPP's and Mu Stages, or possibly as LTPs with a tail CCS.

[TomWaits]

Great feedback. I have a much bigger shopping list now.

Thanks,

Shawn.

[wa2ise]

There exist tubes like 8BQ5, 11BQ5, 8CG7 and so on. The same as their 6 volt cousins except for the oddball heater voltage. BTW, if you have a 12.6VAC supply avaliable, you can connect an 8V heater to it in series with a SS rectifier diode, no filter cap. This yields about the same amount of heater power that 8V AC would give directly. If all you have is 6.3VAC, run that thru a bridge rectifier and use a filter cap to get 8VDC to run the heater.

[Wavebourn]

Originally posted by wa2ise
There exist tubes like 8BQ5, 11BQ5, 8CG7 and so on. The same as their 6 volt cousins except for the oddball heater voltage. BTW, if you have a 12.6VAC supply avaliable, you can connect an 8V heater to it in series with a SS rectifier diode.

If to rectify 6.3V by Shottky diodes you'll get 8V DC. I use such trick to power compactron filaments. 8JZ8 is the example (one triode + power pentode in triode connection to drive power FET follower)

[Miles Prower]

Originally posted by wa2ise
BTW, if you have a 12.6VAC supply avaliable, you can connect an 8V heater to it in series with a SS rectifier diode, no filter cap. This yields about the same amount of heater power that 8V AC would give directly.

Mega-bad idea there. That will also yield lots of harmonics that'll add uncorrelated noise to the signal, and really do the sonics no good at all. If you're going to use DC for heating, it needs to be well filtered to get rid of those objectionable harmonics. Otherwise, DC heater power sounds worse than using AC. Better to drop the excess AC with a series resistor.