866A transient over-current capability?

angelfj · 22nd February 2007, 04:29 PM

I am beginning to assemble the B+ PSU for the AlumRocksClone. I would like to use the Caps and chokes I have on hand. problem is that with a choke input, there is a transient that exceeds the 1.0 amp maximum of my 866A's. See attached. Can these tubes take the abuse or must I select a smaller Cap size? Is there a way to soft-start the PSU to attenuate the peak of the transient?

Thanks in advance.

Frank

Dave Cigna · 22nd February 2007, 08:09 PM

Quote:

Originally posted by angelfj
Is there a way to soft-start the PSU to attenuate the peak of the transient?

Yeah, a variac.

angelfj · 22nd February 2007, 08:17 PM

Dave: Seriously? I can see doing this with a prototype, but what about the finished product? During a future senior moment I might forget to reset the variac.

sorenj07 · 22nd February 2007, 08:32 PM

It will probably be fine. Current doesn't start being drawn until tubes warm up, and besides, the "soft start" option doesn't model this slow increase of drawn current. Not to mention that it probably takes a lot longer than 1 second for the 866's to warm up...

tubelab.com · 22nd February 2007, 08:45 PM

866 type tubes must have the heater voltage applied and the mercury allowed to vaporize before anode voltage is applied. The data sheet specifies this warm up time as 30 seconds minimum. Violators are often rewarded with a tube arc which can really ruin your day if you use electrolytic caps.

If you can live without the cool blue glow use a 3B28 rectifier instead of the 866. No toxic mercury, no RF noise, no warm up time. Drop in replacement.

To soften the initial turn on surge, put an "inrush current limiter" device in series with the transformer primary. I used a CL70 but you may need a different size depending on total current. Search DigiKey for the specs.

Tweeker · 22nd February 2007, 09:11 PM

If this amp does not have a dead constant current draw, the PSU from that PSUD II pic has lots of resonances that will bite you. Try a stepped load and see.

EC8010 · 22nd February 2007, 09:38 PM

I can't believe the ESR of your capacitors is really as high as 5 Ohms. 10H is probably too small for such a high voltage supply. Check your choke current too.

Tweeker · 22nd February 2007, 10:11 PM

Capacitor ESR could be fairly high if its a string of electrolytics in series. Though 47uF makes no sense then.

This supply is 1250-0-1250? If so I get 90ma peak AC current with a 10H choke. VACRMS/(1386*H)= peak ac choke current @ 60hz.

agent.5 · 22nd February 2007, 10:21 PM

Quote:

Originally posted by tubelab.com
[B]866 type tubes must have the heater voltage applied and the mercury allowed to vaporize before anode voltage is applied. The data sheet specifies this warm up time as 30 seconds minimum.

Yes. Use a delay relay and separate filament supplies for the 866A. 866A has lower DCR than 3B28.

Bandersnatch · 23rd February 2007, 12:51 AM

Hey-Hey!!!,
My RCA data sheet for the 866A shows a 2A maximum current for up to 2500 PIV. You're very near that, so I would be careful about warming them up before applying the HV. that same sheet also gives a 500 mA max, averaged over a 30 second period( so a pair can deliver 1A of DC ).

I plan to try a pair of 866's with a filter that runs 1.3A peaks at SS conditions. They should glow very nicely...

cheers,
Douglas

22nd February 2007, 04:29 PM	#1
angelfj diyAudio Member Join Date: Sep 2004 Location: Philadelphia	866A transient over-current capability? I am beginning to assemble the B+ PSU for the AlumRocksClone. I would like to use the Caps and chokes I have on hand. problem is that with a choke input, there is a transient that exceeds the 1.0 amp maximum of my 866A's. See attached. Can these tubes take the abuse or must I select a smaller Cap size? Is there a way to soft-start the PSU to attenuate the peak of the transient? Thanks in advance. Frank

22nd February 2007, 08:45 PM	#5
tubelab.com diyAudio Member Join Date: Jul 2005 Location: South Florida	866 type tubes must have the heater voltage applied and the mercury allowed to vaporize before anode voltage is applied. The data sheet specifies this warm up time as 30 seconds minimum. Violators are often rewarded with a tube arc which can really ruin your day if you use electrolytic caps. If you can live without the cool blue glow use a 3B28 rectifier instead of the 866. No toxic mercury, no RF noise, no warm up time. Drop in replacement. To soften the initial turn on surge, put an "inrush current limiter" device in series with the transformer primary. I used a CL70 but you may need a different size depending on total current. Search DigiKey for the specs. __________________ Too much power is almost enough! Turn it up till it explodes - then back up just a little.

22nd February 2007, 09:11 PM	#6
Tweeker diyAudio Member Join Date: Dec 2004 Location: Cincinnati, Ohio	If this amp does not have a dead constant current draw, the PSU from that PSUD II pic has lots of resonances that will bite you. Try a stepped load and see. __________________ Be sure your foil hat has a good low impedance ground.

22nd February 2007, 09:38 PM	#7
EC8010 diyAudio Moderator Join Date: Jan 2003 Location: Near London. UK	I can't believe the ESR of your capacitors is really as high as 5 Ohms. 10H is probably too small for such a high voltage supply. Check your choke current too. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

22nd February 2007, 10:11 PM	#8
Tweeker diyAudio Member Join Date: Dec 2004 Location: Cincinnati, Ohio	Capacitor ESR could be fairly high if its a string of electrolytics in series. Though 47uF makes no sense then. This supply is 1250-0-1250? If so I get 90ma peak AC current with a 10H choke. VACRMS/(1386*H)= peak ac choke current @ 60hz. __________________ Be sure your foil hat has a good low impedance ground.

22nd February 2007, 08:17 PM	#3
angelfj diyAudio Member Join Date: Sep 2004 Location: Philadelphia	Dave: Seriously? I can see doing this with a prototype, but what about the finished product? During a future senior moment I might forget to reset the variac.

22nd February 2007, 08:32 PM	#4
sorenj07 diyAudio Member Join Date: May 2006	It will probably be fine. Current doesn't start being drawn until tubes warm up, and besides, the "soft start" option doesn't model this slow increase of drawn current. Not to mention that it probably takes a lot longer than 1 second for the 866's to warm up...

23rd February 2007, 12:51 AM	#10
Bandersnatch diyAudio Member Join Date: Jul 2003 Location: Ann Arbor, MI	Hey-Hey!!!, My RCA data sheet for the 866A shows a 2A maximum current for up to 2500 PIV. You're very near that, so I would be careful about warming them up before applying the HV. that same sheet also gives a 500 mA max, averaged over a 30 second period( so a pair can deliver 1A of DC ). I plan to try a pair of 866's with a filter that runs 1.3A peaks at SS conditions. They should glow very nicely... cheers, Douglas __________________ the Tnuctipun will return

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