Pre-amplifiers: Regulate and rule
I would be less happy about 500mV of ripple than 6.3V AC! The reason for this is that the act of rectification produces high frequencies that crawl easily into the audio, whereas the 50 or 60Hz has more trouble getting in. There's no question about it, regulate, and enjoy the slight noise improvement, plus the much greater life and stability improvement. A 317 regulator plus the (small) handful of components required to set 6.30V at the heater pins is dirt cheap.
Power amplifiers occassionally need regulated heater supplies, but it's very rare...
I would be less happy about 500mV of ripple than 6.3V AC! The reason for this is that the act of rectification produces high frequencies that crawl easily into the audio, whereas the 50 or 60Hz has more trouble getting in. There's no question about it, regulate, and enjoy the slight noise improvement, plus the much greater life and stability improvement. A 317 regulator plus the (small) handful of components required to set 6.30V at the heater pins is dirt cheap.
Power amplifiers occassionally need regulated heater supplies, but it's very rare...
EC/DC.
Hi,
It strikes me that you seem to insist on exact voltage for the heaters.
Any particular reason?
Most databooks agree on a +/- 10 % deviation and fron experience I can only sya that a - couple of % gives no problems whatsoever and saves on component count as well...
On a transcondustance tester I never noticed a difference wheter I heated at 6 or 6.3V.
Cheers,
Hi,
It strikes me that you seem to insist on exact voltage for the heaters.
Any particular reason?
Most databooks agree on a +/- 10 % deviation and fron experience I can only sya that a - couple of % gives no problems whatsoever and saves on component count as well...
On a transcondustance tester I never noticed a difference wheter I heated at 6 or 6.3V.
Cheers,
Perfection will do.
Damn right. We're talking about thermionic valves here. DVMs and regulators are cheap. Why get it wrong for the same price as getting it right?
Let's avoid the learning experience of having to find any problems caused by changing/incorrect heater voltage. The valve designers aimed for optimum operation at their stated heater voltage. Emission is critically dependent on cathode temperature, and although the heater has an element (ouch!) of self-regulation, it doesn't make sense to deliberately stretch a design.
fdegrove said:
Damn right. We're talking about thermionic valves here. DVMs and regulators are cheap. Why get it wrong for the same price as getting it right?
Let's avoid the learning experience of having to find any problems caused by changing/incorrect heater voltage. The valve designers aimed for optimum operation at their stated heater voltage. Emission is critically dependent on cathode temperature, and although the heater has an element (ouch!) of self-regulation, it doesn't make sense to deliberately stretch a design.
Datasheets State...
Hi,
+/- 10 for heater voltage.
With all due respect but...
I doubt that running a valve at 6V or 12V iso 6.3 or 12.6 heater voltage makes a difference.
At least, I haven't noticed any...the valves last longer, it's easier (avoids extra components when usings regs) and it makes them that little bit quieter...
O.K., I know I'm being me usual smart ***...
Hi,
+/- 10 for heater voltage.
With all due respect but...
I doubt that running a valve at 6V or 12V iso 6.3 or 12.6 heater voltage makes a difference.
At least, I haven't noticed any...the valves last longer, it's easier (avoids extra components when usings regs) and it makes them that little bit quieter...
O.K., I know I'm being me usual smart ***...
Ah, but you've split into two different issues here.
You've said that you're happy to run at 6V or 12V (rather than 6.3V or 12.6V) because it's easier on regulators. I don't have a problem with that, because you are bending operation within the manufacturers' limits for a known reason.
The second issue is that by regulating you have eliminated mains voltage variation from cathode temperature. This is very important.
The manufacturers' curves were produced using the nominal heater voltage, and if we want our designs to work as predicted from their curves, then we use exactly the right heater voltage - especially if it's trivial to do so.
Of course, if we are able to plot our own curves, then we can operate the valve completely differently. But I'm prepared to bet that you wouldn't like to operate a design optimised for 6V regulated DC with 6V AC complete with mains voltage variation?
You've said that you're happy to run at 6V or 12V (rather than 6.3V or 12.6V) because it's easier on regulators. I don't have a problem with that, because you are bending operation within the manufacturers' limits for a known reason.
The second issue is that by regulating you have eliminated mains voltage variation from cathode temperature. This is very important.
The manufacturers' curves were produced using the nominal heater voltage, and if we want our designs to work as predicted from their curves, then we use exactly the right heater voltage - especially if it's trivial to do so.
Of course, if we are able to plot our own curves, then we can operate the valve completely differently. But I'm prepared to bet that you wouldn't like to operate a design optimised for 6V regulated DC with 6V AC complete with mains voltage variation?
Heaters...
Hi,
That's not the complete picture though...
While I agree with the quote, with modern post 1960 valves the same trick can be applied with no penalty.
Neither measurably nor audibly..with the added benefit of relaxed regulator requirements and extended life.
Plots we did back in the mid-eighties showed the valve actually performed better when underheated...(within reason, say +/- 5%)
Current however was paramount to perfomance...admittedly most tests were done on what we call " small signal valves"..ECCs etc.
So, I'd say keep current constant or at least ample, relax the voltage a bit and all is bliss...
Surely, I'll be the last person to generalise on this.
Cheers,
EDIT: It often makes valves less prone to microphony too.
Hi,
That's not the complete picture though...
While I agree with the quote, with modern post 1960 valves the same trick can be applied with no penalty.
Neither measurably nor audibly..with the added benefit of relaxed regulator requirements and extended life.
Plots we did back in the mid-eighties showed the valve actually performed better when underheated...(within reason, say +/- 5%)
Current however was paramount to perfomance...admittedly most tests were done on what we call " small signal valves"..ECCs etc.
So, I'd say keep current constant or at least ample, relax the voltage a bit and all is bliss...
Surely, I'll be the last person to generalise on this.
Cheers,
EDIT: It often makes valves less prone to microphony too.
mig-ru said:
DC fils on DHTs are a different propostion to IHTs. Because the filament <i>is</i> the cathode, any noise (especially CM) added by the rectifier and caps etc is no different to injecting the noise at the grid. You can have a quite dirty DC supply for an IHT and see an improvement, but if you used the same on a DHT, it will very noticably degrade the performance of the amp.
In my GM70 I'm using a Schottky bridge, LCLC then a voltage reg, then a current reg and finally a CM choke and a small cap straight across the heater pins. The LCLC chokes aren't neccessary, I just had the appropriate chokes on hand. It's silent like a tomb, but not all bleached out sounding like most DC DHT fil supplies. For SE 2A3 LT1083's would be my preferred choice, but LM350's will work too if you want to save a few $. The designs are straight from the datasheets, and the CM chokes are about $3 ea from Digikey.
Frank and EC8010,
1. Spec sheets typically show +/-10%. That means that an average tube will enjoy a full and complete life at anywhere from 5.67V to 6.93V RMS heater supply. They can spec this because presumably, they did testing to prove it!
2. Frank noted there is no difference in Gm, although that was only a 5% deviation (6 vs. 6.3V).
3. Tubes are known to have reduced current and/or Gm (which can be summed up as emission) at lower heater voltage. However, a prime specimen should be good for 15 or 20% less; one test on my tube tester is a "life test" which involves watching for a drop in the reading as the tube cools, say from 6.3 to 5V. If the cathode can't keep up the emission at lower temps, it's probably not too much longer for this world.
I think there were other points I wanted to bring up but I forgot them already...
Ah well, that should be enough typing for now.
Tim
1. Spec sheets typically show +/-10%. That means that an average tube will enjoy a full and complete life at anywhere from 5.67V to 6.93V RMS heater supply. They can spec this because presumably, they did testing to prove it!
2. Frank noted there is no difference in Gm, although that was only a 5% deviation (6 vs. 6.3V).
3. Tubes are known to have reduced current and/or Gm (which can be summed up as emission) at lower heater voltage. However, a prime specimen should be good for 15 or 20% less; one test on my tube tester is a "life test" which involves watching for a drop in the reading as the tube cools, say from 6.3 to 5V. If the cathode can't keep up the emission at lower temps, it's probably not too much longer for this world.
I think there were other points I wanted to bring up but I forgot them already...
Ah well, that should be enough typing for now. Tim
AC / DC that is the question...
I have consistently found that supplying the heaters with dc affects the sound. In general I would describe it a 'bleaching out' the tone a little and, more importantly, adding some greyness even mild hash to the sound.
I suspect this is rectification and regulation noise - particularly up to say 200KHz - getting into the signal chain. I find that ac on the heaters is consistently nicer sounding provided that it is not picking up rectification or other rf noise...
Having said that I am using some very, very beefy and very well filtered dc supplies for my 845s without any sign of hash or greyness. I haven't tried it with ac so not sure about any bleaching. It is a nice neutral sounding amp but not as rich as the 2A3PSE but much more revealing without being Hi-Fi. It's actually more relaxed than the 2A3 PSE - always a good sign.
Any thoughts on the ac versus dc sonics???
ciao
James
I have consistently found that supplying the heaters with dc affects the sound. In general I would describe it a 'bleaching out' the tone a little and, more importantly, adding some greyness even mild hash to the sound.
I suspect this is rectification and regulation noise - particularly up to say 200KHz - getting into the signal chain. I find that ac on the heaters is consistently nicer sounding provided that it is not picking up rectification or other rf noise...
Having said that I am using some very, very beefy and very well filtered dc supplies for my 845s without any sign of hash or greyness. I haven't tried it with ac so not sure about any bleaching. It is a nice neutral sounding amp but not as rich as the 2A3PSE but much more revealing without being Hi-Fi. It's actually more relaxed than the 2A3 PSE - always a good sign.
Any thoughts on the ac versus dc sonics???
ciao
James
See what you think.
When the valve manufacturers specified heater voltage limits, they did not seek to imply that there was perfect operation within those limits and performance fell off catastrophically outside. More, they meant that the centre value was optimum, and that operation within the limits was acceptable.
I've just tested an ECC88/6DJ8 with 100V on the anode and -2V on the grid:
6.615V: 6.8mA, 9.7mA/V
6.300V: 6.1mA, 9.1mA/V
5.985V: 5.8mA, 8.7mA/V
5.670V: 5.4mA, 8.2mA/V
The heater voltages correspond to +5%, nominal, -5%, and -10%.
(Sadly, the forum resented my attempts at generating a table using the space bar.)
When the valve manufacturers specified heater voltage limits, they did not seek to imply that there was perfect operation within those limits and performance fell off catastrophically outside. More, they meant that the centre value was optimum, and that operation within the limits was acceptable.
I've just tested an ECC88/6DJ8 with 100V on the anode and -2V on the grid:
6.615V: 6.8mA, 9.7mA/V
6.300V: 6.1mA, 9.1mA/V
5.985V: 5.8mA, 8.7mA/V
5.670V: 5.4mA, 8.2mA/V
The heater voltages correspond to +5%, nominal, -5%, and -10%.
(Sadly, the forum resented my attempts at generating a table using the space bar.)
BTW... hit the # button for monospace text, ala ASCII drawings or text tables. 
Lesse, for comparative purposes I'll change those to percentages of the 6.3V values:
Looks like your tube has a *lot* to go for, as far as current goes, at higher voltages. Would you be willing to do an average of 5 or 10 random tubes?
Tim
P.S. The interesting part of this is, for -10% of heater voltage, and thus an ideal drop of 20%(??) power, you only lose 13% of your current; thus, this tube is, in fact, more efficient at lower temperatures!
Lesse, for comparative purposes I'll change those to percentages of the 6.3V values:
Code:
Heater voltage | Current | Gm (rounded to the nearest .1% since I'm lazy)
+5% | +11.5% | +6.6%
0% - N/A
-5% | -5.2% | -4.6%
-10% | -13% | -11%Looks like your tube has a *lot* to go for, as far as current goes, at higher voltages. Would you be willing to do an average of 5 or 10 random tubes?
Tim
P.S. The interesting part of this is, for -10% of heater voltage, and thus an ideal drop of 20%(??) power, you only lose 13% of your current; thus, this tube is, in fact, more efficient at lower temperatures!
Sch3mat1c said:
Perhaps. But not today.
Unfortunately, you can't apply Ohm's law to heaters and lightbulbs because of the non-constant temperature. And before you ask, I wasn't monitoring heater current simultaneously.
The test certainly shows that you don't want excessive heater voltage, or anode current will go through the roof. Otherwise, it all depends on how critical your application is.
It is much easier to get a ripple free DC supply thanks to the low price of 10V electrolytics: you can buy a 10000uF 10V electro for dirt cheap. This combined with a 50 cent regulator will give DC so flat you could cut yourself with it.
For small signal tubes, especially in high sensitivity areas like guitar or phono input stages, there is no reason not to go DC.
With power tubes, (excluding DHTs) the heater makes virtually no difference to the sound, so AC is fine. It should be operated at the precise design voltage, generally 6.30V
I can understand the purist argument though- if you are going to use voltage regulators, why not just use MJE340s and 350s for the preamp section? Why not just build a big honking solid state amp?
For small signal tubes, especially in high sensitivity areas like guitar or phono input stages, there is no reason not to go DC.
With power tubes, (excluding DHTs) the heater makes virtually no difference to the sound, so AC is fine. It should be operated at the precise design voltage, generally 6.30V
I can understand the purist argument though- if you are going to use voltage regulators, why not just use MJE340s and 350s for the preamp section? Why not just build a big honking solid state amp
?
Re: AC / DC that is the question...
I have to make DC work well, as 3mV of hum on my speaks is very loud. The V/I regs I'm using was based on the experiences of some diehard AC users on other fora.
Another thing that intrigues me is HF filaments (AC from a Wein or similar), but it's complex, and I'd want a specan to track down any birdies, and check for IM artefacts. There's been some talk about it elsewhere recently (A2 and Joe)
James D. said:
I have to make DC work well, as 3mV of hum on my speaks is very loud. The V/I regs I'm using was based on the experiences of some diehard AC users on other fora.
Another thing that intrigues me is HF filaments (AC from a Wein or similar), but it's complex, and I'd want a specan to track down any birdies, and check for IM artefacts. There's been some talk about it elsewhere recently (A2 and Joe)
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.