Use of 6528A as power tube

[Loon]

Hi there

I would like to see if it would be possible to use a 6528 or 6528A tube.  It has a pin out the same as 6080 with a much, much higher transconductance of about 37,000.  http://frank.pocnet.net/sheets/077/6/6528A.pdf

The only potential problem I can see is that it draws about 5A of current, much greater than the 2.5A of the 6080.

To overcome this I am thinking of using a separate transformer to feed the heaters of the power tube.  I have found a Hammond transformer with two secondaries that output 9V @ 6.67A.  I would hook this up to a couple of regulated DC filament supplies, one feeding the input tube and the other the power tube.  These would output the correct voltage to each tube as a DC supply. 

Do you think my plan would work?  Expensive, I know, but an exercise in learning.

Thanks.

Naïfurne

[Paul Joppa]

If you rectify a transformer output, the rms current in the winding is roughly double the current into a resistor. So you need a 10-amp current rating.

Few sockets can deliver 5 amps to a heater for long. Google around for the sockets and precautions common with the Russian 6C33, which also has high heater current and transconductance. It's a serious undertaking - good luck!

[Loon]

Thanks Paul.

You guys are great!  I told you it would be an exercise in learning and I have learnt something within a couple of hours of coming up with the idea. 

Thank you.

[Loon]

OK.  Wow.  The 6C33 is one serious tube.

A quick read has suggested the following to me:

1. A tube with heat output that can melt solder needs some serious cooling
2. Active cooling might be necessary, perhaps in the form of a very quiet computer fan (thinking low noise Noctua, which can be silent for all intents and purposes, but will they be electrically silent)
3.  I will need to leave some space between the tube and other components.  Was going to custom build a case anyway.  May just have to make it a bit bigger.
4. The tube socket will need to be very robust.  I have ordered a teflon octal, but may need to go ceramic with very firm pin contact.

Does this sound right yo you?  Any feedback?  Need to find a suitable transformer in the meantime.

[Paul Birkeland]

Fans aren't really necessary for something this big, just give it some breathing room.

I don't know about octal sockets, but when I went searching for 6C33 sockets, I used Russian military surplus sockets designed for that tube in particular.  Teflon would be one of my last choices.

Rectifying the heater supply will add enormous cost, weight, heat, etc. to your project, and it isn't necessary. 

[Loon]

Thanks Paul.  For some reason I was under the impression that rectification would lower the noise floor.  Also had hum issues with my last Crack build when using a 12AU7- 6SN7 converter in the input position and thought rectification of heater supply would solve this problem.  Thought I may as we'll rectify both to see what happens.

You think a DC current to the heater of the CF won't make any impact on sound? 

Thanks!

[Loon]

I have done a bit more reading to try to work out any benefit of rectification in the output tube.

Please check what I have summarised below to ensure it is right.

1.  AC current feeding the heater of a tube can cause hum because the heater radiates an electromagnetic field
2. By implication, DC current is less likely to cause hum (will not cause hum?)
3. The power tube (eg 6528 in my case, 6080 normally) is a cathode follower.  Cathode followers always have a gain (slightly) less than 1.  Therefore, any hum arising at this point in the circuit will not be amplified.
4. In the event that hum does result from the use of AC power feeding this tube, it can be reduced by use of a transformer centre tap, an artificial centre tap or DC elevation.

Source:
http://www.valvewizard.co.uk/heater.html

http://www.valvewizard.co.uk/accf.html

In view of this, Paul's objection makes perfect sense as the transformer specs to supply the heater of the power tube can be greatly reduced by not converting AC to DC (the process of rectification)

This is what I think I should do:

1. Rectify the input tube (12AU7 et al)  using this from Peter Millett http://www.pmillett.com/LV_reg.html
2. Feed this rectifier circuit with a Triad VPS16-5000 transformer.  When the secondaries are wired in parallel it will output 8.0V @ 10A (I calculate that the rectifier requires at least 6.326V RMS @ 0.54A so, yes, I have over specified but I think it will work and be cooler as well)
3. Run the 6528 heater directly from the secondary windings (in series) of a Hammond 266PA6, which would supply 6.3 V @ 6A (the 6528 heater requires 6.3V @ 5A)

However......

I have already ordered two of Peter Millett's rectifier PCB and two nice heat sinks that will dissipate 10-15W heat.

So this is what I think I will actually do.....

1. Feed the 12AU7 heater 6.3VDC as noted above
2. Provide the 6528 heater with 6.3VDC via Peter Millett's rectifier circuit
3. Feed the rectifier from a second Triad VPS16-5000 outputting 8.0V @ 10A.  To produce the required output voltage of 6.3 V @ 5A, the rectifier requires an input of at least 7.953V @ 9A (according to my calculations).  The Triad VPS16-5000 seems to come just within spec.

To have some voltage headroom, I might reduce the heater supply to 6.0 V.  This should also have the bonus of slightly extending tube life.  Will the resultant reduced emissions degrade sonic performance at all?

The voltage loss across the rectifier is only approx 1.65V, which my heat sink should be able to deal with (up to 15W).

The Triad transformer I have chosen is about $36 AUD per unit.

Do you think my plan will work.

Please give me feedback about the conclusions I have reached.  Am I on the right track or barking up the wrong tree?

[Loon]

I don't know about octal sockets, but when I went searching for 6C33 sockets, I used Russian military surplus sockets designed for that tube in particular.  Teflon would be one of my last choices.

Paul.  Teflon starts to deteriorate talk about 260 degrees Celcius.  What temperatures would you expect the pins to reach when a 6528 is in operation?  Is heat the only reason you wouldn't use teflon?

[Paul Birkeland]

Thanks Paul.  For some reason I was under the impression that rectification would lower the noise floor.  Also had hum issues with my last Crack build when using a 12AU7- 6SN7 converter in the input position and thought rectification of heater supply would solve this problem.  Thought I may as we'll rectify both to see what happens.

Providing a 6V supply for that tube may help if you plan to use adapters.  There's a big difference between making 6V for a tube that wants 600mA, and making 6V for a tube that wants 5A. 

[Paul Birkeland]

1. Rectify the input tube (12AU7 et al)  using this from Peter Millett http://www.pmillett.com/LV_reg.html
2. Feed this rectifier circuit with a Triad VPS16-5000 transformer.  When the secondaries are wired in parallel it will output 8.0V @ 10A (I calculate that the rectifier requires at least 6.326V RMS @ 0.54A so, yes, I have over specified but I think it will work and be cooler as well)

I would use a 9V at 1.5A transformer.  An 8V/10A transformer is well beyond overkill, and the transformer regulation will be poor, and the chassis space that such a transformer uses will be wasted.

3. Run the 6528 heater directly from the secondary windings (in series) of a Hammond 266PA6, which would supply 6.3 V @ 6A (the 6528 heater requires 6.3V @ 5A)

That should work. 

I have already ordered two of Peter Millett's rectifier PCB and two nice heat sinks that will dissipate 10-15W heat.

That's not quite how this works, other aspects have to be taken into consideration.

2. Provide the 6528 heater with 6.3VDC via Peter Millett's rectifier circuit

I would expect failure of the regulator in this situation.  I would check with Pete first. Part of the issue is that you want ~8V at your lowest line voltage, so if your line voltage creeps up, you may be cooking off 15 Watts on that heatsink rather quickly.

[Loon]

Thanks Paul

I will find a transformer for the 12AU7 heater matching your specs.  Have just contacted Pete to see if the Landfall heat sink will do the job. 

[Paul Birkeland]

Your summary of heater noise relative to cathode followers also mirrors my own experience. I would focus more on the real circuit adjustments that will need to be made to run that tube rather than going after a noise source that very likely will not exist.

[Loon]

Thanks Paul.  Good advice.  Sounds like my build will be much simpler if I supplied the Cathode follower heater with AC current and left it at that.

[Paul Joppa]

I took a class in design of experiments a couple decades ago, and learned the best, most useful advice: Budget 25% of your money (/time/effort/...) for the first experiment. The only thing you learn from the first experiment is how you should have done it. You need enough left over to do it right, and doing it right is always more work than you thought.

[Loon]

Wise words.  If at first you don't succeed....  Should be "when at first you don't succeed". Particularly in my case.  I am a life member of losers anonymous.