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Old 17th October 2011, 05:18 PM   #1
Carlp is offline Carlp  United States
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Default To switch B+ or not?

I had read several places that it's a good idea to delay the turn-on of B+ to the plates until after heaters are on. Something about not hitting the anodes with high DC voltage until the heaters are working. But then more recently I've read the opposite, that you shouldn't delay B+, but rather should perhaps switch it on first. Not sure of the rationale given for this approach.

Can anyone (or lots of anyones) provide some thoughts on this? I just don't have enough experience or theoretical knowledge to make sense of these two options.

Thanks!
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Old 17th October 2011, 05:27 PM   #2
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This is true. A proper leignth of time is about 30 sec. Not doing so is quite hard on the tubes.
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Old 17th October 2011, 05:45 PM   #3
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by firechief View Post
This is true. A proper leignth of time is about 30 sec. Not doing so is quite hard on the tubes.
Hasn't really been my experience in 20 some odd years of designing tube kit, but it is not a bad idea nonetheless..

Cathode stripping is largely a myth at the voltages present in consumer gear, but there are certain types which are prone to this kind of damage including 6080, 6AS7G, 6336, 6C33, and derivatives - all low perveance types originally designed for voltage regulator service. They need to fully warm up before B+ is applied. (Some people use these in SE amps, and voltage regulators, but are more commonly found in OTL amps.)

Using a slow heating rectifier like the 5AR4/GZ34, 5V4, GZ32, GZ37, etc will usually suffice. Switching the raw B+ sometimes leads to problems with switch arcing, fried solid state rectifiers, and blown fuses.. (Switching the primary is usually fine)
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Old 17th October 2011, 05:46 PM   #4
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Well, there's several hundred million TV sets and radios over the last century that did not feature this +B standby or delayed switching....

Tube life was fine with them....

Never done it myself, and not had any issues....
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Old 17th October 2011, 05:52 PM   #5
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Quote:
Originally Posted by Alastair E View Post
Well, there's several hundred million TV sets and radios over the last century that did not feature this +B standby or delayed switching....

Tube life was fine with them....

Never done it myself, and not had any issues....


Good points.
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Old 17th October 2011, 05:59 PM   #6
DF96 is offline DF96  England
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Running valves with hot heaters but no HT and therefore no cathode current can cause cathode interface problems, except for special valves designed to avoid this (mainly SQ or computer versions).

Running valves with warm heaters and full HT can cause ion bombardment of the cathode. Suddenly applying full voltage to hot valves can create circuit surges. You choose which damage mechanism you prefer.

A valve rectifier avoids the problem, except in the rectifier itself. Standby switches may be needed for expensive transmitter valves, but not for ordinary audio. They may do more harm than good.
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Old 17th October 2011, 06:05 PM   #7
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Quote:
Originally Posted by Carlp View Post
I had read several places that it's a good idea to delay the turn-on of B+ to the plates until after heaters are on. Can anyone (or lots of anyones) provide some thoughts on this?
The root of this thinking incorrectly filters down from industrial and transmitting tubes and their use with voltages upwards of 1KV. Emission can be damaged and arc over can happen with some indstrial gas-filled rectifiers and thyratrons. Receiving tubes operating with potentials of around 500-600 volts are normally not subject to to these failures. This is old-school truth and still applies today.

However, because of the poor quality of todays foreign made power/output tubes, some limiting of applied voltage may be helpful in preserving them.
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Old 17th October 2011, 06:46 PM   #8
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I built an RH84 amplifier, originally using a 5Y3 tube as a rectifier.
Changed that out to a 6087 tube, that gives me around a 20 second warm up.

Steve
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Old 17th October 2011, 06:56 PM   #9
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Quote:
Originally Posted by kevinkr View Post
Cathode stripping is largely a myth at the voltages present in consumer gear, but there are certain types which are prone to this kind of damage including 6080, 6AS7G, 6336, 6C33, and derivatives
Many of these are $5 to $10 tubes. Assuming that instant-on B+ simply extends their life some, you would need to save quite a few of them to defray the cost of the slow start circuit.
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Old 17th October 2011, 07:17 PM   #10
Carlp is offline Carlp  United States
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My question came from my build of a SE Pentode (triode strapped) for which I have separate A and B iron (a Rat Shack 12.6v trafo for heaters, simple isolation iron with doubler for B+). I typically wait 10-15 seconds or so before throwing the B+ switch. I shouldn't have to worry about arcing b/c I'm switching the primaries. I included a switch for the B+ iron separate from the heater basically b/c that's what I had read about. But then I realized I really didn't know why I would do that, so my question.

Sounds a bit like it's not too important either way with the possible exception of certain tubes. Interestingly, I have a battery powered pre (Bottlehead Quickie) which has no HT switch (48v applied to anodes all the time). Power switch is for the 1.5v heaters only. No heaters, no conduction, so the B+ batteries don't wear out. But in this case, there's no sudden onset of full B+ (and it's only 48v), so no problems. I guess.

Thanks for all the great replies.
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