ST35 Tube Rectification Revisited

[ashok]

I am in the process of converting my diode-rectified ST35 (rev. C) to a tube-rectified one. Given my chassis layout, the rectifier and choke can only be mounted in a place that makes the existing, beautifully symmetric layout into an asymmetrical one.

So, what I would like to do is mount the tube rectifier on a small project box from Radio Shack. Then, wire the AC inputs (2 wires) , filament supply (2 wires) from the power transformer on the main chassis to this project box. Wire the DC output from the rectifier back to the main chassis. Thus, the filtering will be performed entirely in the main chassis. The choke will go on the underside of the top plate of the main chassis, and my symmetrical layout will not be disturbed.

Is this a workable approach?

Thank you,

Ashok

[Thermion]

Ashok,

I have seen many amps that do not have perfect symmetry. In audio, I believe in simplicity over complexity. IMHO you are better off to sacrifice a little symmetry than to add additional complexity. An outboard rectifier tube and umbilical cords add unwanted complexity and potential for problems.

One way to balance the rectifier tube and gain back some symmetry is to use a chassis mount capacitor. I did this with my first diytube ST35 amp. (see "Rev C completed" post for pictures) It also adds B+ filter capacitance that you may lose configuring your amp for tube rectification. The stock ST35 can use some additional power supply capacitance anyway.

JT

[DortoH]

Actually I believe it was Shannon, but someone here recommends a 32-40uF cap in place of C9 on the revD boards.

JJ has a 32x32uF can.

[ashok]

Good advice and point taken. I have ordered the JJ can capacitor and clamp from Triode Electronics. Thanks for the responses.

Ashok

[TerrySmith]

Since the PA774 don't have a 5v filament tap, I was thinking of trying two series strung damper tubes with filaments connected directly to the line. I'm Looking at the octal 25W4, or the 9 pin mini 25CT3. Both have 25 volt 300ma heaters, so 50v + dropping resistor of the correct size, and no extra strain on the power transformer.

[erichayes]

Hi All,

Terry, to dissipate the 70 volt drop when using a pair of 25 volt damper tubes, a resistor of between 220 and 250Ω and 40~50 watts will have to be employed. While not that expensive ($3~7.00 or so) the resistor will have to be mounted on a heatsink or other large chunk of metal.

I'm not discouraging experimentation; I just don't want people burning up good components before thinking the experiment through beforehand.

As far as symmetry goes, an engineer who used to convert my mechanical sketches into CAD drawings once told me "Form follows function" if you want to get optimum performance out of a device. That's why my amplifiers have their power supplies in New England and the input jacks and first audio tube in San Diego. It's also why they have -110~120dB hum and noise levels referenced to 1 watt. After I made this layout change, I took a look at the old McIntosh and Citation layouts. Even though the potted transformers make them appear to be symmetrical, they are electrically very similar to the wheel I reinvented. Those guys earned their paychecks.

[Tom Bavis]

Instead of a dropping resistor use a 7.5 uF AC rated cap. ZERO heat dissipated. You may find them as replacement parts for ceiling fans.

OR use a diode to drop the voltage to 85 (120 x .707), and a resistor for the rest.

If you need any 25AX4s, etc, I have plenty - yours for postage.

[TerrySmith]

Hey Tom,

I'm interested in the 25AX4's and any other damper tubes you want to get rid of. My E-mail is on the post above.