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PostPosted: 29 Nov 2010, 10:45 
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Instead of a 6SN7, why not a CCS loaded pentode SV83/6p15p? It can swing your required voltage and provide lots of gain!


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PostPosted: 29 Nov 2010, 17:32 
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rock4016 wrote:
Instead of a 6SN7, why not a CCS loaded pentode SV83/6p15p? It can swing your required voltage and provide lots of gain!


Thanks for the suggestion, but I don't know. The datasheet says 300 volts on the anode. Apparently this is a Russian tube that's similar to an EL84.

I don't think you're going to get an adequate voltage swing on the output. I've tried several pentodes and none can tolerate a high enough B+ to output 240 Vp-p.

I'm going to breadboard the SRPP circuit I've designed. Here's the latest schematic using 600 volts on B+. The steady state bias voltages and currents are fine, and the output swing is over 250 volts. The output voltage will push the heater-cathode rating of the top tube to the limit, but it looks like it will be within spec. I'll need some additional voltage amplification prior to the SRPP if I want a lot of sensitivity but that's easy. I'll probably just use a single tube in an SRPP configuration.

Anybody care to speculate on the startup and shutdown of this circuit, or its operation? Have I missed something? Assume that the heater of the top tube is biased about 100 volts below the cathode voltage with no signal (about 200 volts) so the output swing will remain within the heater-cathode voltage "window". The cathode can swing 200 volts above the heater and 100 volts below according to the specs by Tung-Sol. There's also the possibility of using a small AC transformer to heat the top tube and tying it to the output so it floats with the output, leaving the heater-cathode voltage essentially zero. I don't know what the added load of the heater and the heater power supply would do, but I don't think it will be required..

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PostPosted: 29 Nov 2010, 22:34 
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There is nothing like sparks and arcs followed by smoke to make for an exciting project. I would use separate tubes for the upper and lower triodes. Then you could bias the top one so that it would not go up in flames. I know what Tungsol says, and it works for new tubes for a while..... If you are really after a lot of drive, why not parallel both sections in the 6SN7s and use them that way. just a thought.

Good listening
Bruce

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PostPosted: 30 Nov 2010, 10:13 
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Looking at the plate curves, it could swing down to 150V and up to 450V giving a Peak to Peak of 300V. I'd start with about 150V on g2.


sampleaccurate wrote:
rock4016 wrote:
Instead of a 6SN7, why not a CCS loaded pentode SV83/6p15p? It can swing your required voltage and provide lots of gain!


Thanks for the suggestion, but I don't know. The datasheet says 300 volts on the anode. Apparently this is a Russian tube that's similar to an EL84.

I don't think you're going to get an adequate voltage swing on the output. I've tried several pentodes and none can tolerate a high enough B+ to output 240 Vp-p.

I'm going to breadboard the SRPP circuit I've designed. Here's the latest schematic using 600 volts on B+. The steady state bias voltages and currents are fine, and the output swing is over 250 volts. The output voltage will push the heater-cathode rating of the top tube to the limit, but it looks like it will be within spec. I'll need some additional voltage amplification prior to the SRPP if I want a lot of sensitivity but that's easy. I'll probably just use a single tube in an SRPP configuration.

Anybody care to speculate on the startup and shutdown of this circuit, or its operation? Have I missed something? Assume that the heater of the top tube is biased about 100 volts below the cathode voltage with no signal (about 200 volts) so the output swing will remain within the heater-cathode voltage "window". The cathode can swing 200 volts above the heater and 100 volts below according to the specs by Tung-Sol. There's also the possibility of using a small AC transformer to heat the top tube and tying it to the output so it floats with the output, leaving the heater-cathode voltage essentially zero. I don't know what the added load of the heater and the heater power supply would do, but I don't think it will be required..

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PostPosted: 30 Nov 2010, 16:52 
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rock4016 wrote:
Looking at the plate curves, it could swing down to 150V and up to 450V giving a Peak to Peak of 300V. I'd start with about 150V on g2.


If the tube can handle 450V on B+ (I don't see that on the datasheet) then you may be right. What datasheet are you looking at that indicates that the anode can swing up to 450 volts?

I get keys to my new house in about 2 hours, so I'll get back to you at some point in the future about the pentode driver. I can model something very similar in PSPICE to see if it works.

My only other concern is the high output impedance of pentodes. I don't know what the drive requirements are for the GM70 other than voltage swing. I know the SRPP has a low output impedance and a mu-follower is even lower than that, much lower than a pentode.

I really like the suggestion that Bruce made about paralleling the triodes in each of two tubes in an SRPP configuration. That will work. I also agree with Suncalc that an interstage tranny is probably not the best solution. I have my SRPP. If I can't find something better before build time I'll go with what I have (once I breadboard it and verify it works). I also bought some gas dicharge tubes at different voltages in case I find I need to put one or two in there to prevent tube failure on startup and shutdown.

Goodbye for a few days to everybody while I move all my stuff. I'll be back soon. :smoking:

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PostPosted: 01 Dec 2010, 11:06 
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I have 2 amplifiers that use the SV83 as the output tube. They both use the quiescent 300V plate to cathode volts. However the plates swing over a hundred volts AC to play music. When you draw your AC loadline you just have to look at the dissapation rating of the tube and make sure that the max dissapation doesn't cross your loadline and you'll be fine. This presumes the tube never goes over it's dissapation limit(or cuts off) it will last for years. Also you lose some of your plate dissapation into the load.

Take a look at the pic from SE Amp CAD. You'll see that I've got the 6BQ5 swinging to over 500 volts with no problems, (problem areas show up red) You're specific load line would be a lot flatter(and in pentode), but it gives you the general idea.


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PostPosted: 01 Dec 2010, 11:09 
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To quote Gary Pimm,
"Here is the schematic of a driver stage using a 6BQ5 Pentode and CCS combo. Highlights of the circuit performance are gain of 125, output impedance of 500 ohms and distortion of .25% at 90 volts RMS. Not many driver stages can do .25% at 90V RMS! The operating parameters and measurements are on the schematic. "

Here's the link.
http://www.pimmlabs.com/web/drivers.htm

I wouldn't worry about that having problems driving a GM70.


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PostPosted: 02 Dec 2010, 16:42 
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rock4016 wrote:
To quote Gary Pimm,
"Here is the schematic of a driver stage using a 6BQ5 Pentode and CCS combo. Highlights of the circuit performance are gain of 125, output impedance of 500 ohms and distortion of .25% at 90 volts RMS. Not many driver stages can do .25% at 90V RMS! The operating parameters and measurements are on the schematic. "

Here's the link.
http://www.pimmlabs.com/web/drivers.htm

I wouldn't worry about that having problems driving a GM70.


I've read Pimm's website. I even purchased some small, high voltage CCS chips as well as some high voltage FETs for the circuits he shows but I wanted to keep the GM70 free of any SS except the PS.

I think you're right about that working, but can I do it without a SS active load?

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PostPosted: 03 Dec 2010, 11:05 
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Yes, you don't need the active load, but the problem is then your gain goes down and distortion goes way up, but I think it's still worth breadboarding, it shouldn't take too long.

If you want to try just a resistor I'd start with a 10K load and go up from there.

Also, you've still got SS in the cathode of your output tubes, wich is still in the signal path. I havn't read any sound quality complaints about the LM317 used in all those odd watt amps.


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PostPosted: 07 Dec 2010, 17:25 
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rock4016 wrote:
Yes, you don't need the active load, but the problem is then your gain goes down and distortion goes way up, but I think it's still worth breadboarding, it shouldn't take too long.

If you want to try just a resistor I'd start with a 10K load and go up from there.

Also, you've still got SS in the cathode of your output tubes, wich is still in the signal path. I havn't read any sound quality complaints about the LM317 used in all those odd watt amps.

Good points. I bought the CCS chips for the purpose of building highly linear, low distortion preamps and drivers with wide voltage swings - why not use them? I think I might, but first I really want to see if this SRPP using 600volts on a couple of 6SN7s (6H8C) smokes or not. The tubes were only $10 so if it does blow up no big loss. That's when I go to "plan B".

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