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B+ powered before heaters. How bad was it?
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Author:  Danny [ 25 Dec 2017, 15:47 ]
Post subject:  B+ powered before heaters. How bad was it?

Last year I built an integrated amp using 6V6GT power tubes in a push pull config having separate switches for the heaters and B+. The idea being that to prolong the life of the tubes you apply power to the heaters, wait about 30 secs and then apply B+ power.
However, yesterday by mistake I applied the B+ power first for about 1minute. No input signal of course.
I have read that this is very bad. But, it seems that the amp is working fine.

Any comments?

Thanks.

Author:  gofar99 [ 25 Dec 2017, 16:08 ]
Post subject:  Re: B+ powered before heaters. How bad was it?

Hi, Not a good thing to do, but unlikely to cause any harm. As long as nothing arced and the voltages on the various capacitors were not exceeded you should be OK. It is not uncommon in amplifiers with solid state rectifiers to do this normally. They should however be designed accordingly.

Good listening
Bruce

Author:  Soundbrigade [ 25 Dec 2017, 16:50 ]
Post subject:  Re: B+ powered before heaters. How bad was it?

Is it from prolonged exposure of B+ w/o V-heater or with voltage applied to filaments w/o B+ that causes "cathode poisoning" or "cathode stripping"?

Anyway both variants are definitely not recommended, but I'd guess that a minute of applied B+ is not enough to cause harm. My guess.

Author:  M. Gregg [ 26 Dec 2017, 12:31 ]
Post subject:  Re: B+ powered before heaters. How bad was it?

One problem linked to B+ on before heaters,

is with direct coupling of tubes. This is where the voltage on grids exceeds the grid to cathode voltage.
This can be protected with diodes or a neon. If a neon is used the neon will light between grid and cathode until heaters cause current to be drawn through the tubes causing voltage drops across resistors and bring the grid/cathode to come into spec and the neon goes out.

In normal circuits the coupling caps block the DC onto the tube grids so this doesn't happen.
Years ago all TV's had heaters and B+ on together.
However in output sections with bypass caps in the cathode you don't want hot tubes with B+ applied because you get an initial inrush charge for the bypass caps the value of these has a direct effect on charge time/pulse on power up.

So the thoughts are:
B+ PSU cap voltage on start up will rise higher than with the amplifier drawing current, this can blow PSU caps if the working voltage is close to supply voltage.
However direct B+ on output stages with hot tubes causes a charge pulse in bypass caps in the cathode.
If the output section is current limited this can be reduced.
One of the problems in B+ delay is you need limited or ramped B+ if this is done via thermistors then fast cycle times cause charge pulses in the output stage, this in turn causes pops though speakers and speaker damage.

There is lots more but someone else will have their own Ideas.


Regards
M. Gregg

Author:  M. Gregg [ 26 Dec 2017, 13:00 ]
Post subject:  Re: B+ powered before heaters. How bad was it?

One other thing and it's a major problem.

Watch out B+ is never powered on with bias voltage low or charging or none at all.
It will damage the equipment and has been known to blow primary windings in output transformers.

Bias voltage should always be up and running before B+ is applied.
In very expensive equipment the B+ energise monitors all bias voltage where B+ shuts down should bias voltage fail.
Again in these type of circuits B+ shut down is direct power off so there are no capacitors still up supplying fault current.
Ie the DC is shut off and AC at the same time, PSU discharge is done separately.

This also links to charged B+ after power off. But that's a different issue.

Regards
M. Gregg

Author:  M. Gregg [ 26 Dec 2017, 14:15 ]
Post subject:  Re: B+ powered before heaters. How bad was it?

Danny wrote:
I applied the B+ power first for about 1minute. No input signal of course.
I have read that this is very bad. But, it seems that the amp is working


So in a nut shell because the heaters and tubes were cold the B+ capacitors would rise to the maximum voltage.
As long as the caps didn't blow and the amp did not flash over inside the tubes (you would have heard it a loud crack)

Then I wouldn't worry.
If you then switched the heaters on the B+ would have just slowly been pulled back to normal working.

So bottom line, its probably OK

Regards
M. Gregg

Author:  KochiyaYamato [ 28 Jan 2018, 19:31 ]
Post subject:  Re: B+ powered before heaters. How bad was it?

I think this is up to the design of the amp. A good amp should be designed to power up indefinityly without heaters on and recive no damage.

The preamps I design have a heater switch for using 12ax7 to Chinese 6n2 valves. I can turn my heaters off while listening halfway leave it there for half hour comeback no plorbem.
I can also powerup the amp without valves no plorbem.

Author:  gofar99 [ 29 Jan 2018, 16:37 ]
Post subject:  Re: B+ powered before heaters. How bad was it?

Hi, I'm not sure I buy that. A good design will be one where it is not possible to turn the heaters off when the high voltage is on.

Good listening
Bruce

Author:  KochiyaYamato [ 30 Jan 2018, 17:58 ]
Post subject:  Re: B+ powered before heaters. How bad was it?

I don't see any plorbem where a valve cannot be damaged with heaters turned off and high votlage on.

Author:  Gio [ 31 Jan 2018, 21:51 ]
Post subject:  Re: B+ powered before heaters. How bad was it?

Here is a good and entertaining read on the subject and why a standby switch is a good thing: https://peavey.com/monitor/pvpapers/Chapter6.pdf

The beer button made me laugh.

Quote:
Most musicians WRONGLY assume that the standby
switch is provided primarily to disable the amplifier in
such a way as to provide instantaneous operation by
switching on the “standby” switch. Some even refer to
this switch as a “beer button” to cut off the amp during
breaks in a performance.

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