AA764 with 6x4 or 6x5 rectifier instead of 5Y3 - (using a capacitor multiplier?)

[kagliostro]

I've some 6x4 and 6x5 tubes

I think that I can use it in AA764 or other 5C1 - 5F1 like circuits and you ?

5Y3 is rated 350v 125ma - first cap 10uF

6x4 is rated 325v 70ma - first cap 10uF

6x5 is rated 360v 80ma - first cap 32uF

I'm a bit confused because the indication for the first cap changes from one datasheet to other as an example

for the 6x4 here is 10uF
http://www.r-type.org/pdfs/6x4-1.pdf

and here is 40uF
http://www.mif.pg.gda.pl/homepages/frank/short/001/6/6X4.gif

as to bypass the low admitted value for the first capacitor I've think to cheat and use something like this



Using a capacitor multiplier with a value of the cap superior to the max value accepted by the tube will stress the tube more than that he can afford ?

Thanks

K

[tubenit]

What I did with several of the rectifiers that required 10uf on the first node was to do just that and put 20uf on the second node.

I will say I had a Bogen amp that came with 16uf on a rectifier tube that had 10uf specified as it's max. 

With respect, Tubenit

[kagliostro]

Hi Jeff

Many thanks for sharing your experience

I was looking on the net and someone was saying he uses (with 6x4 tubes) 20uf cap without problems

Something like the 33uF that Steve uses with the 5Y3 tubes

K

[HotBluePlates]

You will need to use a power transformer that has a secondary voltage less than ~300vac, or use a second 6vac winding.

The 6X4 is rated for 450v maximum between heater and cathode, which will be exceeded by the 320-0-320v secondary of the AA764 Champ, and that assumes wall voltage or a transformer with a 110v primary don't result in more than the rated voltage.

The second 6.3v winding would power only the 6X4 heater, and would be connected to the 6X4 cathode at the socket to minimize the heater-to-cathode voltage.

I'm a bit confused because the indication for the first cap changes from one datasheet to other as an example

I will say I had a Bogen amp that came with 16uf on a rectifier tube that had 10uf specified as it's max.

One day we'll get everyone to see those are not maximum ratings.

So if you do nothing else, the cap values listed might be considered "advisable values" for you to adhere to. They take into account the maximum cathode current rating for the rectifier and the peak charging current of the filter cap.

But power supply impedance can limit cap charging current and allow a bigger filter cap to be used. That impedance is often the resistance of the PT half-secondary plus the resistance of the PT primary, referred to the secondary. That is what Kagliostro's 40uF 6X4 sheet is showing; "Rt" is that transformer impedance, and is part of the assumption that allows the bigger filter.

Some equipment actually included small resistors in series with each rectifier plate, at the socket, to increase the supply impedance and limit the peak charging current.

There's a mess of charts and tables in RDH4 that can be used to work through all the steps of a power supply design and determine the needed supply impedance for a given rectifier and filter cap, but it's an involved process and few seem to actually make use of them. The charts simplify the mass of calculations you'd have to do to get the same answer.

[PRR]

The 10uFd or 40uFd are what the tube factory thought would be "typical" caps that radio/amp designers would use. Not MAX !!

When these tubes were new, caps over 8 or 10uFd were expensive, but field-coils allowed enough filtering.

A decade later, field coils were old but 40uFd caps were cheap-enough. Designers used 40uFd and some tube-sheets were re-issued with data for 40uFd use.

Vacuum rectifiers are pretty bad as Voltage Doublers. A rectifier has to pass as much current as your output tubes, but at much less voltage drop. That means a large heater. (6V6 takes 2.8 Watts heat, 5V4 takes 10 Watts heat.) In the Doubler each rectifier has to pass twice the current (more heat) but two voltage-drops add.

And of course separate heaters and heater windings, or heater-cathode construction (beware H-K voltage limits).

You sometimes find doublers on 117V (US) transformerless gear using a special big twin-diode. B+ rarely exceeds 200V (ideal over 300V).

You rarely find doublers in amateur radio transmitters using odd-voltage surplud PTs and surplus rectifiers and rect heater transformers.

Today a 1N4007 is a much better idea.

[catnine]

This rect and 1st or reservoir cap thing has always driven me nuts. I knew nothing about this at all , didn't care. I had a 73 SF champ and when I replaced the old original cap can it was a 40/20/20 @475 volts so that's what I replaced it with , the same make and the new one was a bit higher in voltage. The amp still had the original 5Y3 in it . Nothing even happened and I used that champ a lot.

 Now that awhile back I went seaching things out and found all this info of what cap goes with what rect tube. I put a 47uf in my SF champ build and was told to just place a 47 ohm resister between the 5Y3 pin 8 and the first cap so I did , it was to slow the inrush . Some fenders with the 5y3 use a 30 uf and the bronco like a vibro champ uses a 40 uf. I had an older 68 champ and it had the original cap can and it had 20/20/20/20 and 2 of the 20uf were wired together for 40uf or close or not so close since I don't have a way to measure the caps actual value.