Alternative Tubes

[DaveP]

Attached is a copy of tests that Steve Bench made many years ago (I believe he is no longer with us).

Best
DaveP

 

[Tubetec]

Ive found it to take around half an hour before a tube properly settles down noisewise ,
Ive certainly heard those metalic ping sounds as the plate heats up . Electrolytic capacitors on the HT supply can also contribute some noise initially ,
it generally settles down with use ,
A foil final HT cap might be better for a test setup , just to rule out the possibility of the capacitor adding anything . I routinely bypass HT electrolitics with foils in any case , it wont make much difference to the measured noise level in dbu , but it most certainly changes the character of the underlying noise spectrum ,

 

[RuudNL]

For a microphone company I have tested hundreds of tubes for noise.
Mainly EF86 and EF806.
I was surprised to see that one brand started directly at a faily low noise level, while another brand started with a pretty high noise level that became lower after a number of minutes. Sometimes the difference was even 10 dB.
Could this mean that the tubes that started with a a steady low noise were already 'burned in' at the factory?
(Because in my experience the noise reduces especially during the first hour of use.)

 

[ruffrecords]

For a microphone company I have tested hundreds of tubes for noise.
Mainly EF86 and EF806.
I was surprised to see that one brand started directly at a faily low noise level, while another brand started with a pretty high noise level that became lower after a number of minutes. Sometimes the difference was even 10 dB.
Could this mean that the tubes that started with a a steady low noise were already 'burned in' at the factory?
(Because in my experience the noise reduces especially during the first hour of use.)

I no longer use NOS tubes so all the tubes I use are now 'current production'. For these tubes I have also noticed a reduction in noise in the first hour or so of use. Nothing as dranatic as 10dB but certainly on dB is common. After I complete a mixer I run it in for several days. I usually play the Stones Sticky Fingers though it.

Cheers

Ian

 

[Tubetec]

Maybe stuff is cooking off the cathode , or some of the chemical reaction from prodution still has to take place in the early hours of use .

 

[ruffrecords]

I just found my old note book from 2006. I tested 32 NOS 6AU6. I measured cathode and plate volts with them strapped as a triode, One with screen grid, suppressor and plate strapped and then with the screen grid as the plate and the other two grounded. I also measured gm, rp and mu. I separately measured the noise. This must have been before I got the Ferrograph because I was estimating rms noise voltages from the peak to peak value of the waveform seen on a scope..

Cheers

Ian

 

[Tubetec]

Be interesting to observe the prevalence of pops clicks and bangs of one sort or another over the first few hours of a tubes usage , as well as how the other parameters change during the 'cooking in' process .

Quite a spread of values across many parameters
what grid resistance value did you use for the test ?

There must be a few statistical noise analysis tools out there that could help catagorise tubes better .

 

[murrayatuptown]

Nice information!

I bought a box of NOS 6CB6 very inexpensively for pentode and triode experiments.

The shared tables are very helpful.

Murray

 

[ruffrecords]

Be interesting to observe the prevalence of pops clicks and bangs of one sort or another over the first few hours of a tubes usage , as well as how the other parameters change during the 'cooking in' process .

Generally speaking I have rarely heard pops and bangs. Pings are quite common and if you tap some tubes there is a definite resonance like maybe the grid vibrating.

Quite a spread of values across many parameters

Plus or minus 10% is normal for tubes but so called NOS tubes vary even more. Many NOS tubes, especially on eBay, are not new and unused and you really do need to thoroughly test each and every one before using it. That's why I only use current production tubes now.

what grid resistance value did you use for the test ?

Zero ohms. Noise in tubes is almost entirely voltage noise. Using anything other than zero just invites the pick up of external interference.

There must be a few statistical noise analysis tools out there that could help catagorise tubes better .

I am sure there is but I am am not sure it is worth it. There is probably some learned treatise about this in the RCA tube manual. When it comes to noise the duffers are easy to spot.

Cheers

ian

 

[DaveP]

Thanks Ian for your info, it was definately worth finding it.

best
DaveP

 

[ruffrecords]

Thanks Ian for your info, it was definately worth finding it.

best
DaveP

You are welcome. It is a little dated and I knew a lot less about tubes at the time but I think the info is still valuable.

Cheers

Ian

 

[murrayatuptown]

Wow, cool thread DaveP!

I've not delved really deep into the realm of "non-audio" tube types, but the few I've used have yielded some amazing results. The amplifier I use in my living room system is something I came up with off the top of my head, as an experiment in achieving the cleanest possible signal path and seeing what it sounded like. It's a triode-strapped EF86 DC-coupled to a 12B4A, which is a small (5 watt, IIRC) power triode that was commonly used as the vertical deflection amplifier in small CRTs back in the day. Back around 2005, I bought 8 "computer grade" NIB General Electric 12B4s that were branded Hewlett-Packard, for something like $35 with shipping.

The 12B4's characteristic curves are a beauty of linearity to behold, reminiscent of a 2A3 or 300B. I chose the EF86 driver for its low Gm and internal shield, with the hopeful intention of getting away with no grid stopper resistor. And, the 12B4's relatively low Miller capacitance allowed the triode-configured EF86's low current and not-so-low Ra to drive it satisfactorily into clipping at full bandwidth.

The direct signal path has zero caps or resistors, and is rock-solid stable and free from oscillations at any frequency that I could find. The sound is amazingly microdetailed and pure, as you'd expect. The caveat? It produces less than one watt per channel at its full, unclipped power, which isn't a problem at all with my moderate listening level and 99dB/1W/1m speakers.

The used 12B4_ rescues I saved came out of low bandwidth HP o'scopes (120B?). They are probably VERY old. I don't have very high expectations because some of the 6922's were really well-worn.

 

[murrayatuptown]

Some day I want to try pentodes as triodes, cascoded. I don't really have a specific goal. It is complicated for the sake of complication, when I have twin triodes readily available.

Deaf in one ear now, I think my interest leans closer to clean guitar preamp stages and no longer hifi.

Theoretical projects.

Murray

 

[GeorgeToledo]

The 6AU6 was commonly used in early tube mixers by the BBC wired as a triode. It has a handy pin out that allows you to ground the pins on either side of the heaters if you use the screen grid as a plate. I used these in my very earliest tube mic pre designs.

Cheers

Ian

Fwiw there are pictures of the earliest AKG C12, from the time it was produced, that appears to have 6AU6 /EF94 installed.

And of course Sony uses it.