I was designing a delayed turn-on for my latest amp project and looked a several site for examples of such a schematic. One site actually suggested that there is no evidence that HT on the plates before the cathode comes up to temperature harms the valve. So, question one is: does anyone know whether this HT before warm-up is actually a problem
I then wondered if in fact the problem may in fact be over voltage on the plates before current is drawn. I know in my designs I like to use 6N1P valves as drivers and these have a peak rating for the plate of 250-300volts. With a simple RC network an over voltage on turn-on can easily exceed 300volts. I resolve this is in my designs by using MOSFET regulation.
So, two questions for comment. Is there any evidence that HT on turn-on is harmful? And is the problem more likely to be an over-voltage caused by lack of current flow through the RC networks. Interested in opinions.
I then wondered if in fact the problem may in fact be over voltage on the plates before current is drawn. I know in my designs I like to use 6N1P valves as drivers and these have a peak rating for the plate of 250-300volts. With a simple RC network an over voltage on turn-on can easily exceed 300volts. I resolve this is in my designs by using MOSFET regulation.
So, two questions for comment. Is there any evidence that HT on turn-on is harmful? And is the problem more likely to be an over-voltage caused by lack of current flow through the RC networks. Interested in opinions.
Just thinking off the top of my head...
Since you're already using a MOSFET to regulate the HT perhaps a simple modification to that regulator circuit could cause it to become a "switch" as well. Something like a capacitor that slowly charges through a resistor bringing the gate voltage up. That should cause the HT to rise slowly until it reaches its set voltage. Rise time dependent on the R/C combo. Just a thought.
Since you're already using a MOSFET to regulate the HT perhaps a simple modification to that regulator circuit could cause it to become a "switch" as well. Something like a capacitor that slowly charges through a resistor bringing the gate voltage up. That should cause the HT to rise slowly until it reaches its set voltage. Rise time dependent on the R/C combo. Just a thought.
I seem to remember you posting a (nice) circuit you did that brings up the B+ slowly as the filament/heater voltage charges a cap? Or am I imagining that?
Thanks, Yes, I have previously used 2M2ohm with a 10uf capacitor feeding the gate to achieve this end. I guess the question in my case was more in relation to the EL34 output tubes. (They are Feed from B+ without MOSFET regulation)
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Most delay timers switch the DC after rectifier. This timer switches the AC of secondary depends on limiting resistor, the HT can be controlled to as low as 50V to full output after time out (2-3mins). The bias voltage needs to be supplied with another separate winding so the bias will be on as soon as the amp switches on, thereby has full control over the tube current when HT is applies in full swing. I am sure you may have seen it before, just to share it again here.
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Thanks, I have not as yet finalised the design so no schematic yet. The voltage gain/phase spliter and driver stages are easy and can be addressed through MOSFET regulators.
BUT historically on my amps I have made no attempt in any design to delay the HT. Hence the underling question .....is it worth the effort. Is the use of delayed HT actually going to be a benefit to the output tubes and their lifespan....or will this be an unnecessary complexity. On reading Elliot Sound Product page on this matter he proposed a delay circuit but then he added a paragraph that there was no evidence that valves suffer from the early HT
The comment on post 2 (The Gimp) suggested that there is no issue at lower voltages
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HT without the delay timer is exactly like old radio, you will hear loud humming or even noise before it gone quiet after a while, I don't think HI-FI user can tolerate that, it is part of design to be able to mute the noise, as time (>11s) needed for filament to have full potential some even longer. So it is not solely whether it causes any harm or not to the tube.
It's worth the effort if you know there may be stress or damage, but as you haven't got a schematic with all parts identified then its not easy for others to say parts x, y z may be stressed.
The obvious concern is different time constants of part voltages, including all the preamp and driver stages if there are coupling caps between stages, and of course bias voltages (whether they be 'fixed' or cathode derived). Overlaid on that are thermal time-constants of all valves, and how that affects bias and coupling cap voltages. There can be many interactions, which can depend on the circuitry you have chosen.
It is certainly worth the effort to appreciate what can be stressed - whether that ends up with a decision to add a HT delay is open-ended imho.
The obvious concern is different time constants of part voltages, including all the preamp and driver stages if there are coupling caps between stages, and of course bias voltages (whether they be 'fixed' or cathode derived). Overlaid on that are thermal time-constants of all valves, and how that affects bias and coupling cap voltages. There can be many interactions, which can depend on the circuitry you have chosen.
It is certainly worth the effort to appreciate what can be stressed - whether that ends up with a decision to add a HT delay is open-ended imho.
Yes, I did that experiment.
A voltage regulator with positive feedback by current. B+ goes up with the speed proportional to the current that is drawn from it. Softly starting, intelligent power supply.
I made it for a BC-348 military receiver. It needs +250 B+ and -12 for bias. I got both from the single rectifier, using 2 voltage regulators in series. The bias regulator was a shunt type, and it was used to sense the current, also it was providing a reference voltage to the B+ regulator that followed by bias.
You should find something else to worry about.
Huh?? EL34 are rated 800V *operating*, 2000V at cut-off. You will have trouble with sockets and B+ caps long before a good EL34 sparks-out. Considering the abuse that EL34s have been put to in transmitters and guitar amps, I don't believe any sane hi-fi amp is any threat.
http://www.mif.pg.gda.pl/homepages/frank/sheets/010/e/EL34.pdf
I would not be worried at all about cathode stripping for any indirectly heated tube if you will be well below 800V. EL34's can take a lot of punishment (as PRR says above).
I however do use slow start-up for my 45 and 2a3 amps. I have just found they seem to last a lot longer when you are kind to them. I allow their filaments time to heat up first before apply a slower rising B+ with damper diodes. There are not so many nice condition globe 45's left from the 1930's. WW-II 2a3's are getting kind of rare too it seems. The 'insurance' of soft start is not hard to implement for these old things.
Ian
I however do use slow start-up for my 45 and 2a3 amps. I have just found they seem to last a lot longer when you are kind to them. I allow their filaments time to heat up first before apply a slower rising B+ with damper diodes. There are not so many nice condition globe 45's left from the 1930's. WW-II 2a3's are getting kind of rare too it seems. The 'insurance' of soft start is not hard to implement for these old things.
Ian
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If you want a cheap and cheerful delay, here it is. I use this with a 12V 40A automotive relay to switch the 400+V B+. A 12V 40A relay won't interrupt 400V@40A, but it has no problem at 500ma...
NE555 DC 12V Delay Relay shield Timer Switch Adjustable Module 0 To 10 Second | eBay
NE555 DC 12V Delay Relay shield Timer Switch Adjustable Module 0 To 10 Second | eBay
Once again, it is not about anode voltage, it is about cathode current density. If say in my amp GK-71 on 850V consume 60W by filament, but only 130 mA anode current, what cathode stripping are you talking about? The current would ramp ump slowly from 0 to 130 mA while the cathode is being heated by 3A current, even more at start up!
However, there is a different story if say the tube on 1 KV of voltage consumes 1 KW of power on anode, and 100W on cathode! Anode and filament currents are comparable, so in this case it is possible that thinner parts of the filament get hotter, start emission earlier, and an anode current increases it's temperature even more!
Again, it is not about anode voltage, but about cathode current density of very powerful directly heated tubes, like for example magnetrons in radars.
However, there is a different story if say the tube on 1 KV of voltage consumes 1 KW of power on anode, and 100W on cathode! Anode and filament currents are comparable, so in this case it is possible that thinner parts of the filament get hotter, start emission earlier, and an anode current increases it's temperature even more!
Again, it is not about anode voltage, but about cathode current density of very powerful directly heated tubes, like for example magnetrons in radars.
I've bought similar boards from China and wondered how they can sell a 10A, 250V DPDT relay, much less an entire circuit, for a couple of bucks, shipped. The answer came when the relay contacts worked *most* of the time. I ended up paralleling poles to make it work *all* the time.
What does your MOSFET regulated PSU look like and is it more or less complex than a B+ switchon delay?
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