Diodes instead of diode-tube?

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johnheath

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Hi…

I have been fiddling around with an Altec 436C p2p home-brew for a while and the original schematic calls for tubes that require 6,3V for the heaters…

I would like to test with other tubes… preferably with ones that can take 12,6VDC for the heaters (for various reasons) but then the diode tube 6AL5 would need a significant voltage drop causing a lot of heat in the resistors…

And I came up with an idea to use diodes instead of the 6AL5, but I cannot seem to find any schematic where it is used and there might be a very good reason for that.

So my question is if it would be doable to use a pair of diodes instead of the 6AL5 in the side chain? Or am I swimming in deep water here?

ps. the manual is attached and the schematic is on the last page

Best regards

/John
 

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Ok - I did not know that there was one :) It would reduce the overall heat.

But I am still interested in the "potential" use of diodes since it also would recur less work-hours and space and money :)


Thanks for your answer sir


Best regards

/John
 
I don't think there's any good reason not to use silicon diodes. Unless you're planning to sell the compressor to someone. Then vacuum diodes will make it sound million times better.
 
SS versus tube diode for side chain rectification has in the past here been analyzed quite thoroughly, by members long since vanished much smarter than we. 
 
johnheath said:
So my question is if it would be doable to use a pair of diodes instead of the 6AL5 in the side chain?
It is certainly very doable, but the rectification characteristics would be different. For example, the diodes are usually biased with some DC voltage that they need in order to be at the threshold of conduction. That voltage would make Si diodes conduct all the time, so the bias voltage needs to be reduced, even suppressed. Also, the dynamic resistance of a vacuum diode is much higher than that of an Si diode, so the slope will be steeper. It is possible to add resistance in series but it is not excatly the same. It takes some tweaking but can be done.
 
I say just replace the vacuum diodes with silicon. The threshold control will work just the same with silicon except there's the added ~0.6V silicon diode voltage drop. Positive voltage on the diode cathode won't make it conduct all the time. Plate resistance from 6CG7 is some few kilo ohms and there's 33k in series with the release cap. Vacuum diode resistance living somewhere maybe between couple k and 200 ohms seems pretty insignificant to me. I wouldn't hesitate putting silicon diodes there.
 
Thank you all for your answers.

Abbey: Yes, That was my concern about the dc voltage on the tube and that it probably would change the threshold with ss diodes… I guess it will be examined on the work bench tomorrow. If it needs too much tweaking it might not be a win to change at all?

Heikki: You seem very confident about the use of si diodes and I like the idea, but you also mention that the sound would change … so much that it won't be a product for sale. How is that? I have no experience in the difference. :)

About the 12AL5 it seems that the 6AL5 is far more available than the 12AL5… at least in Europe?


Best regards

/John
 
The back-voltage is usually 5V to 30V. The Si won't conduct at idle. Any few-tenths difference seems small, and usually trimmable.

6/12-AL5 will drop 0.5V at 1mA, which is about where we peak in a limiter. That's like 500 Ohms. The Si diode will be 28 Ohms at the same 1mA. We may have much more than 500 Ohms of other resistances. In the 436C we come from ~~6K plates (+/-1K!)  through quite small capacitors, and with 20V back-bias. It is very hard to see how Si, maybe Si plus 390 Ohms, is any different.

Yes, if you see the bottled rectifier it must sound better.

6AL6 was dirt-common in industrial gear. (And FM ratio detectors, though the bulk moved to other types with added units.) I have never held a 12AL5, though I am sure they exist.

The waste power is "only" 2 Watts. If you use 6AL5 series with 6CG8, with a shunt resistor to bleed the extra 0.150A, it is only 1W.
 
Thank you sir

Much appreciated info there.

About the waste power "problem" it is all due to the fact that I am using a power transformer with a 12VAC secondary and using a 12AU7 instead of the 6CG7 that I don't have at home… it all would be solved with a power transformer with a 6,3VAC secondary… rectified and all for DC heating. That is one of the reasons I would like to experiment with tubes that take 12,6vdc for heaters.

Anyway I must ask… please don't get me wrong here (language barriers probably) but do I understand you correctly if you mean that the sound would not change using ss diodes instead of the cute glowing visible tube diode?


Best regards

/John
 
There is no point in worrying about the subtleties of changing from a tube to silicon rectifier when you are using the wrong tube for the power output.

A  12AU7 is not equivalent to the 6CG7, if it was they would have used it as it was a cheaper tube.  You need a 6SN7, a 7N7 or a pair of 6J5's to replace a 6CG7.

DaveP

 
Thank you sir

I am not worrying, just thinking and asking :)

I put the circuit together just to test it from what I had at home… I know the 12AU7 is not the same as the 6CG7, but not so far from it. It is not a "major" project just fiddling around with the circuit testing some ideas since the original circuit is rather simple I thought that it could be used for some testing… I am not trying to make a copy, just trying to understand how these limiters work and what I can change and not. :)

The idea about the ss diodes came from making a bit cheaper and lessen the work hours to make it…


Best regards

/John

 
A signal diode type 1n4148 comes to mind (do not use the usual power rectifier diodes here, their forward capacitance is too much and takes too long to turn off) but it looks like the signal could easily exceed the (as I recall) 100V max reverse rating, so you might need three in series, or a different signal diode with a higher reverse voltage rating.

I've held 6AL5's in my hand  (I recall the Heathkit VTVMs use it to rectify the AC range signals), and hadn't heard of the 12AL5, but not surprised to hear of it. Many tube types were made at several heater voltages.

A quick google shows 12AL5 available cheap enough (Ebay sellers in USA, $3 to $5), and as others implied, the extra little glass bottle with the two dull red glowing things inside makes the unit "More Desirable," "More Authentic" and thus "More Valuable." There's also no extra engineering involved.
 
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