Stromberg-Carlson 433 : Add standby or not?

[Vespasian]

The ASR433 doesn't use a rectifier tube, the B+ comes up mighty quick - Would this amp benefit from a standby switch? Interested to hear the pro's and con's / and where the switch would best be located. (At the 130vac before the diodes?)
12ax7;6av6;6u8;EL84 - all using 6.3vac on the heaters.
TIA - Cheers, Ed

[Eli Duttman]

Ed,

That B+ PSU is a "full wave" voltage doubler. You could put a standby switch, with an arc suppressing ceramic cap. across its contacts, in the line between the grey wire of the rectifier winding and the center of the doubler cap. stack. While that will allow you to delay the application of B+ to the tubes, it does nothing about the significant B+ turn on surge that occurs.

An alternative to a standby switch is the insertion of a pair of CL150 inrush current limiters, as is done in "El Cheapo". The thermistors slow the start up surge down. That, in turn, safely allows an increase in total capacitance of the doubler stack, which is usually a good thing.

In any event, you should add the 10 nF. snubber cap. to the B+ PSU, as that reduces the amount of SS diode switching noise that gets into the power trafo and, ultimately (via the heaters) the signal tubes. Speaking of SS diodes, did you replace the OEM parts? Much quieter parts are available in the current inventory.

[Vespasian]

Originally posted by Eli Duttman
Ed,

That B+ PSU is a "full wave" voltage doubler. You could put a standby switch, with an arc suppressing ceramic cap. across its contacts, in the line between the grey wire of the rectifier winding and the center of the doubler cap. stack. While that will allow you to delay the application of B+ to the tubes, it does nothing about the significant B+ turn on surge that occurs.

Yeah, hehe, my eyes got real wide when I first saw the surge... Made me put my beer down for a second. Eli, what would the specs on the cap be?

An alternative to a standby switch is the insertion of a pair of CL150 inrush current limiters, as is done in "El Cheapo". The thermistors slow the start up surge down. That, in turn, safely allows an increase in total capacitance of the doubler stack, which is usually a good thing.[/B]

That sounds perfect - the CL150's, would you recommend a source?

In any event, you should add the 10 nF. snubber cap. to the B+ PSU, as that reduces the amount of SS diode switching noise that gets into the power trafo and, ultimately (via the heaters) the signal tubes. Speaking of SS diodes, did you replace the OEM parts? Much quieter parts are available in the current inventory. [/B]

The snubber is the .01 film cap you have just after the limiter in the El Cheapo? Sorry about all the questions...

I haven't replaced the diodes, but was looking at the hum the other day. Still replacing caps and what not so I thought I might get back to it later on. Scoping the opts, 5mv, 5ms, 0 volume, 0 input signal, 8 ohm speakers are connected:


Same with volume up halfway - Funny, the channel that is almost finished with is a worse than the other but perhaps it is just due to the increased gain I'm getting from it? It actually decreases as volume gets close to max which I think is a bit strange.


This is what the heaters are getting:


And for fun, push-pull of two EL84's (Rush Limelight):


Eli, thanks a lot for your help.
Cheers, Ed

[Eli Duttman]

Ed,

You can get a pair of UF5408s, 10 nF. snubber cap, and a pair of CL150s + PTFE tubing (as used in "El Cheapo") from Jim McShane.

It doesn't matter much if the 10 nF. snubber goes in front of or after the CL150 that feeds the diodes.

I don't like the heater current waveform, at all. In addition to better diodes and the rectifier snubber, I'd put ferrite beads on all 4 heater winding leads close to the power trafo. That waveform should be a perfect sine curve and the beads will kill HF trash. McShane stocks the beads you need.

If the B+ voltage comes in low after the CL150s are installed, wrap the CL150s in a layer of wool or acrylic yarn. The inrush current limiters are negative temperature coefficient (NTC) devices and their resistance drops as they heat up. The yarn holds the I^2R generated heat in. That will make the B+ voltage rise a little.

[Vespasian]

YIKES!

I have to fix my amp with Scrubbers, Metal Balls and Yarn?

Thanks again Eli - I'll check it out.

Cheers, Ed

[Brian Beck]

And for fun, push-pull of two EL84's (Rush Limelight)

Yeah, I recognized it.

[Vespasian]