Author Topic: 12AY7 design q's  (Read 118 times)

Offline pwatts

12AY7 design q's
« on: September 15, 2020, 09:07:51 AM »
Morning gents, first time posting in the valve section :)
In a new DAC project I'd like to use a 12AY7 valve buffer as output stage loosely based on a preamp design I built many moons ago and regret selling, even more so since I did not make proper notes back then.

The main design fun is in the rest of the circuit; the valve is just for fun and to add an integrated preamp of a design that I really liked. For the valve side there are a few things I'd like to shamelessly add in a single thread for those kind enough to share their wisdom on a field I have very little experience with:

- I recall some saying that starving the triode heater voltage can yield improvements, plus improving lifespan. I just blindly did it but did not do A-B testing between the usual 6.3V or 5.8V or whatever I settled on. Any comments?
- For a 'soft-start' circuit to similarly add series resistance to reduce thermal shock on turn-on, what is there a specific recommended amount? Once again, I remember using 30s delay but not the amount of current limit imposed with whatever series resistance I used.
- Is there any merit in super-regulating the heaters or will a standard LM317 or so suffice? Likewise with caps, several schematics explicitly reference all kinds of boutique electrolytics (Elna & Black Gate etc) on the heaters.
- I used a EZ81 rectifier: do all models sound the same or is there merit in splashing out on on a nice NOS model?
- The huge power supply (EZ81-10uF-10H-22uF-22H-33uF-12AY7) worked a treat but caused slow DC offsets (apparently called "motorboating") that slowly made the woofers visibly move back and forth. This was fixed with adding a series zener-follower before the 22uF and the 22H and I left it there, but ideas on improving this would be great.
- Original design used two different transformers, one for the triode power and EZ81 heater and another for the triode heaters (purely because of material availability). Is there advantage to this or can all the secondaries be combined on a single transformer without any performance penalty?

Offline handsome

Re: 12AY7 design q's
« Reply #1 on: September 15, 2020, 10:31:08 AM »
-Starving heaters can lead to longer valve lifespan but pre-amplifier valves last so long already - rather watch that you do not over-voltage
-Soft start circuits for heaters are not necessary for indirectly heated valves: they weren't designed with sof starting in mind and valves operate at much lower temperatures than light bulbs to which the comparisons are often made - 10 pairs of power valves on one amplifier is a different story but that is more an issue with current draw through your fuse/DB board at start up than any protection for the heaters. DHT valves are fragile. However an easy way to soft-start without any extra circuitry is to current regulate your heaters - use an LM317 and an appropriate resistor ~4.16R for 300mA - your valve then never sees more than the design current!
-For indirectly heated valves an LM317 (current or voltage operated)will do just fine, for DHTs you need to make a bit more effort. Boutique caps usually make the users feel better than the circuits and cap technology is much better today than it used to be but if it sounds good to you......
-In a preamp the rectifier shouldn't make any difference as its a constant current draw that's usually much larger than the signal current but see the boutique cap comment above....
-Your capacitors are quite small in that PSU - bump the ones after the choke to at least 100uF or more big caps are cheap today , they weren't cheap or easily available in the old days which is why they used small values - there is no magic there. Choke filters doresonate and using more than one in a PSU can invoke instability unless you carefully plan the time constants. Download PSUDesigner it makes PSU design very easy.
-Using a separate transformer for heaters is an excellent idea as diode switching spikes couple very easily from winding to winding via transformer capacitance

Offline fredeb

Re: 12AY7 design q's
« Reply #2 on: September 16, 2020, 02:59:53 AM »
Welcome to the valve section @pwatts ! I'm quite interested to see what you have planned . The 12AY7 looks like a very interesting tube . Low noise , decent gain with fairly low plate impedance .

In the Valve Wizard phonostage that a few of us built recently I used adjustable LT1083 DC regulated heaters - I just happened to have a cheap LT1083 kit . I found the stage quietest with filament ground tied about 60V above HT ground .

It's interesting to see the small changes in operating points with filament voltage changes , perhaps a good way to fine tune . See Klaus Narod's test curves for 6.3V and 5V filaments of 6N23P :

...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts