Seeking advice re: 7 pin and unconventional 9 pin tubes for another junk build.

[Carlsoti]

I was recently given what seems like a regulated power supply for RF or some other such device. It's got a Thermador PT, p/n 5A-6086, rated at 300-0-300V/75mA. A 5Y3 rectifier feeds a Triad C-5X "filter reactor" 12H, 75mA, 390ohm choke, in a choke-input configuration. It originally had a pair of 9-pin 12B4A tubes, a 7 pin 6AU6, a 7 pin 5651 "glow discharge" diode tube, a 1M pot, and a fancy old 4 conductor connector for whatever it was supposed to attach to.

Initially, I was contemplating a TRUE class A, PSE 6005/6AQ5A build, not too unlike an old GA-8 Gibsonette, but it seems I can't get there from here, due to transformer/choke limitations. In choke input config, the PT should be good up to around 115mA, based solely on Hammond' s rectifier guide. I'm guessing that this 75mA choke won't play nice being abused like that.

On the note of the choke, can a "filter reactor" choke be used where a "Swinging choke" would normally be used? How do chokes react when pushed too hard? From what I've read, regulation goes out the window, and in this configuration, voltage spikes from VACx0.45 closer to the 1.414 factor, but how would an amp actually respond to that? Just increased noise and hum? Wild oscillations?

I could reconfigure the PS, but that puts my HT up in the 400V range and it seems most of the 7 pin tubes aren't cool with that.

If I truly CAN'T get there from here, can anyone suggest any 7 and/or 9 pin pentodes that might fit the bill?

FWIW, I'm not opposed to changing to a more conventional power supply, nor am I married to the mini-Gibsonette idea.

Any input would be greatly appreciated.

EDIT: I suppose I could go with EL84 output tubes and a couple 6AV6s, but that seems like the easy way out. I'd be happier completing something that uses cheap, mostly forgotten, NOS tubes.

[bmccowan]

I am not knowledgeable enough on chokes to help. PRR has helped me with chokes in the past.
But I like the mini-Gibsonette idea, as I have one and like it. The 6CM6 tube is like a mini 6V6. I built a single channel GA-40 with them, using Hammond AO-39 iron and a 5879 in V1. It punches above it's weight. A cool thing is that 6973 and 6CZ5 tubes can be subbed for the 6CM6 with no wiring change. RCA 6CZ5 are fine tubes and available at reasonable $$.
I'll be watching to see what I can learn about chokes.

[AlNewman]

I wonder what would happen if you were to run a resistor in parallel with the choke?
Where is PRR anyways?  Hopefully all is good...

[Carlsoti]

...I have one and like it. The 6CM6 tube is like a mini 6V6.

Where did your power tube bias end up at with those other tubes? I initially was considering using the 12B4As, but I think I read, or figured from the datasheet, that they end up with crazy high cathode voltage for such a small tube, so I kinda need to be wary of that, too. An "almost all clean" Gibsonette is kinda like having a really slow race car.

[kagliostro]

6P14P-EV

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7189-A

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With 9 pin there is also the 6P1P but plate/screen voltage and dissipation are higher than a 6AQ5 but only slightly

-

if it were my choice, I'll go for a 6P1P-EV

Franco

[bmccowan]

An "almost all clean" Gibsonette is kinda like having a really slow race car.

Ha! Yeah, a Gibsonette is more like a Dirt Track Date.
With the 6CM6 - about 12 watts with a 200 Ohm K resistor - about 300v on the plates - 18.5 on the cathodes.
Looking back on the datasheet - the 6CM6 specs are nearly identical to the the 6V6 - so it may not be weird enough for you goals.

[kagliostro]

I suggest to give a look to the 6CM6 datasheet (I don't remember the spec for those tubes) and verify the max voltage

Usually tubes that can be compared to the 6V6 but are in a smaller bottle can't afford the same B+ they work at a lower voltage (if compared to the 6V6 tube) and higher current (with different load)

Franco

[bmccowan]

On the datasheets I have looked at, both the 6V6GT and 6CM6 are rated for 315v. Of course - 6V6s are routinely run way beyond spec. Can the same be done with 6CM6 tubes? I do not know. Except for a Princeton Reverb, I never put more than 325V on 6V6s - that's where they sound best IMO and tubes have become too expensive, eh?

[tubeswell]

I wonder what would happen if you were to run a resistor in parallel with the choke?
Where is PRR anyways?  Hopefully all is good...

Changes peak resonance.


PRR maybe on vacation.

[bmccowan]

PRR maybe on vacation.

PRR lives in Maine - as do I. No need to go on vacation.
Regardless, I hope his is doing well.

[Merlin]

I was recently given what seems like a regulated power supply for RF or some other such device. It's got a Thermador PT, p/n 5A-6086, rated at 300-0-300V/75mA. A 5Y3 rectifier feeds a Triad C-5X "filter reactor" 12H, 75mA, 390ohm choke, in a choke-input configuration.

So we're looking at a choke input PSU delivering maybe 220Vdc @ 75mA. Personally I'd keep it that way, it's nice to see a choke input for a change. Sounds ideal for any EL84/6V6-alike output tube in SE if you ask me.
Why not a single 6AQ5A? That would run the PSU comfortably low and put that old iron into an easy retirement.

Nevertheless, it would be fine to use the choke as a conventional smoothing choke if you really want to. (Newb note: you can't do it the other way around, i.e. don't use a smoothing choke as a choke input. Smoothing chokes are not built for it and may overheat)

[Carlsoti]

Why not a single 6AQ5A?

Thank you for chiming in, Merlin. FWIW, I picked up two of your books a few years ago. One or the other gets opened at least once a week, even if only as a springboard to the more technical maths in the ancient texts. I even elicited a chuckle from my girlfriend last night when I read one of the footnotes to her. (Something about nobody remembering what we learned in school.)

In regards to the single output tube: I often try to do things that are out of the ordinary, just because, only to find that there are various unspoken reasons others haven't done it before. The Parallel SE powered GA-8 Gibsonette is one of the designs that spans the gap. The deeper I dig into this, the more it seems like a single output tube or a tiny PP output will be the direction I have to go.

  My other option would be to blatantly disregard the max plate voltage of the output tubes, rewiring the power supply for a capacitor input load, and trying to bring plate dissipation down by way of large cathode resistors. My readings tell me this would likely result in gnarly crossover distortion at high signal inputs, if they don't arc internally trying to get there.

   I don't know why I'm driven to insist on doing things the hard way. I feel like it may an extension of my artistic side that was systematically beaten out of me in my mid to late teen-aged years. 

[bmccowan]

I don't know why I'm driven to insist on doing things the hard way.

"We choose to do these things not because they are easy, but because they are hard." John F. Kennedy, 1962
I think it admirable that you are trying to make use of what you have. Too many people gut old equipment and make a Fender clone.
I'm wanting to learn more about chokes - Merlin's post is a good start.

[Merlin]

I'm wanting to learn more about chokes - Merlin's post is a good start.

Chokes are just transformers that haven't had a baby secondary coil yet. Either way, you can only shove so much current into them before they're full up with magnetism and overflow. At that point they lose their inductance (they suddenly look like an air-core), which is likely to make the current spike even higher. The copper then heats up and the magic smoke starts to escape. An input choke has to handle much bigger peak ripple current than a normal smoothing choke.

[kagliostro]

Does there is a rule of thumb to use a filter choke as input choke ?

I mean something like a 150mA filter choke if used as input choke must be used for a max load of 100mA

 

Thanks

Franco

[AlNewman]

https://www.aikenamps.com/index.php/chokes-explained

Here's a good write up at Aiken's site.  Searched input vs filter choke.

[kagliostro]

Thanks

Franco

[Merlin]

Does there is a rule of thumb to use a filter choke as input choke ?

I mean something like a 150mA filter choke if used as input choke must be used for a max load of 100mA

Good question, let's give it a try:

The peak current in a choke input is approximately:
I = [(0.1 * V)/fL] + [(0.9 * V)/R]
Where V is the rms transformer voltage (across one half of the transformer winding if using a centre tapped rectifier), and f is twice wall frequency.

If we have a smoothing choke we can re-arrange the equation to find the load resistance that would be acceptable:
R = (0.9VfL)/(fLI - 0.1V)

Let's try a 10H smoothing choke, rated for 150mAdc, with a 300-0-300V transformer:
R = (0.9*300*100*10)/(100*10*0.15 - 0.1*300)
R =120000 /120 = 2250 ohms

The DC voltage from a choke input is theoretically 0.9*V, but probably more like 0.8*V in practice, or 240Vdc in this case. The max DC current is therefore:
Idc = 240 / 2250 = 106mA

So our 150mAdc smoothing choke is good for 106mAdc as a choke input at these voltages.
The problem is, choke inductance can vary depending on the AC and DC currents in use. We assumed our choke had 10 henry inductance, but is that still true when handling a larger-than-designed-for AC current component? I don't know, I've never tried it! If anyone does try it, do post your results.

EDIT: I just noticed I used the European wall frequency rather than US, but you get the idea.

[kagliostro]

Many Thanks

Franco