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Old 11th June 2005, 10:45 AM   #1
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Default EL84 push pull idea

Hi guys.
I'm trying to design a DC coupled el84 push pull.
I started from this:

http://www.wdehaan.demon.nl/mono/mono/el84dc.html

I would like to use a Gary Pimm's self bias ccs in the long tailed pair and a fed shunt regulator in the power supply ( to feed the long tailed pair). I've changed the working point of the driver stage, just to have much more current through the ecc88s. I think that, even if the power stage work in class AB, the grid current should not influence the driver stage in this way, however not so much like in the way it could if the current in every 88 would be only 1.8mA... At the start up, the self bias ccs doesn't start immediately to work. I think that I must delay the power for the el84s (or during power up the EL84's will have +255 volts applied to the grid they will be turned on hard).

I found a circuit that maybe should work here:
http://www.audiodesignguide.com/my/n...ew300bxls.html

The relay datasheet:
http://chdist35.distrelec.com/distre...E/~7559109.pdf

What do you think about this solution?

Some other thoughts: I'd like to replace the two res. on the cathode of the el84 with two current sources. In this manner the idle current will be set and stable even if the tubes are not matched. Changing the working point of the driver stage I'll be able to have a fine tunig of the bias. But what kind of ccs? Not a self biased ( too complicated for this purpose...the ccs will work only in dc)...not a sigle lm317( too high voltage).
Maybe a single mj340 or a cascode...

I'm thinking to try a different tube in the driver stage...maybe the 6h30.

Any comment or advice will be welcome

Mark
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Old 11th June 2005, 10:54 AM   #2
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Power supply:
Probably I will not use the -50volt regulator, because Gary Pimm has suggested to use a simple crc and a negative power between -40V and -500. Ripple and regulation aren't a problem with his ccs...
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Old 11th June 2005, 04:24 PM   #3
Sheldon is offline Sheldon  United States
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Default Re: EL84 push pull idea

Quote:
Originally posted by mark_titano
Some other thoughts: I'd like to replace the two res. on the cathode of the el84 with two current sources. In this manner the idle current will be set and stable even if the tubes are not matched. Changing the working point of the driver stage I'll be able to have a fine tunig of the bias. But what kind of ccs? Not a self biased ( too complicated for this purpose...the ccs will work only in dc)...not a sigle lm317( too high voltage).
Maybe a single mj340 or a cascode...

Mark
An uprated version of the one shown here?: http://www.diyaudio.com/forums/showt...393#post659393

Sheldon
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Old 11th June 2005, 04:46 PM   #4
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Yes, something like that one. Or maybe a single depletion mode mosfet like the supertex dn2540...
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Old 11th June 2005, 05:39 PM   #5
rdf is offline rdf  Canada
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Will bias stability be an issue with direct coupling? The EL84's are right up against the 12 watt (305 volts X ~38 ma) limit and slightly past the 300 volt max. I've only had the opportunity to play with modern production versions but my favourite is one:

http://www.jj-electronic.sk/tube_el84.htm

One third the measured distortion in-circuit of an Electoharmonix and far more robust.
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Old 11th June 2005, 06:45 PM   #6
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Hi rdf.

I think that the stability is not an issue when heaters, ccs and everything else are stable. The problems is the start up. Maybe I'm wrong but in the el84EH datasheet the maximum power is 14W. But probably you are right ...what do you suggest as good working point?
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Old 11th June 2005, 07:24 PM   #7
rdf is offline rdf  Canada
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That's interesting. The JJ sheet specs max dissipation at 12 watts, as does the 1966 RCA Receiving Tube manual.

My amp isn't directly comparable to yours. It's a direct coupled pentode-mode SE which uses the EL84's cathode voltage as feedback to the EF86 driver screen for DC stability. The outputs run ~255 VDC plate-to-cathode and around 40 ma cathode current, or just over 10 watts. The latter was chosen by monitoring THD and intermod while varying the cathode resistor. THD rose and intermod fell with decreasing standing current. I eyeballed a compromise between the two and haven't been tempted to revisit the choice, though I might in the future. For paranoia's sake the outputs are dressed in Pearl Coolers.

Simulations showed this circuit too has the potential to punish the '84s on cold start. Since the supply has B+ to burn I employed a 5U4G rectifier to slow things down. The bonus is a hypermegacool glowing bottle up front to create the impression of a really powerful 3 watts.
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Old 11th June 2005, 07:36 PM   #8
kevinkr is offline kevinkr  United States
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You might consider using a B+ delay relay or a slow warm up tube rectifier to allow the filaments in the ECC88 and 6BQ5's to warm up sufficiently before B+ is applied - this should prevent wildly high voltages from appearing at the grids of your output tubes.

I don't think the zeners are really necessary on the cathodes of the output tubes. Let the tubes find their optimum operating point.

Kevin
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Old 11th June 2005, 09:47 PM   #9
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rdf, is the topology of your amp something like this one ?

http://www.wdehaan.demon.nl/images/elb4cir2.jpg


If you look at the power supply schematics you can see the delay circuit. I don't like very much the reley on the HT but I don't really want to use two transformers for driver and power stage.
The great problem is the relay...please look at the datasheet link on first post and post your thoughts.

The zener on the cathode are there because "...when a class-AB cathode-biased amplifier sees a prolonged burst of signal, the output tubes' conduction can easily increase fivefold over the nominal idle current setting. This increase in cathode current will slowly charge up the bypass capacitor, which will effectively increase the negative bias voltage, as the grid will become more negative relative to the cathode. This new bias voltage will slowly collapse over time, but until then the amplifier will distort greatly as the output tubes might even be completely cut off after the large signal burst...The zenerís break voltage is purposely set higher than the bias voltage, so that when the amplifier leaves class-A, the zener limits the maximum voltage the capacitor can charge up to, which prevents under biasing the amplifier after a large current swing."

Look here:

http://www.tubecad.com/2005/April/blog0043.htm


I'm thinking to give a dual power supply to the driver (like +/-200V), then, finding a working point that guarantee the right negative potential for the ecc88 anode, I will be able to connect the 84s cathodes directly to gnd.

Mark
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Old 12th June 2005, 04:28 PM   #10
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by mark_titano
I'm thinking to give a dual power supply to the driver (like +/-200V), then, finding a working point that guarantee the right negative potential for the ecc88 anode, I will be able to connect the 84s cathodes directly to gnd.
Mark

If I understand correctly, to do what you propose you would need a bias voltage of minus 11 on the 84 grids. Which also means -11 at the driver plates, which means the driver grids are at -100 or more. To do that, you would have to cap couple the input and provide a high quality negative bias supply for the driver grids. Doable, but why do it that way? I would think it easier just to put coupling caps between the stages and have the negative bias supply on the output tubes if you want the cathodes grounded. Then you can put the operating voltage and bias of the drivers anywhere you want. But it's something completely different than the amp you started with. And you'd have to come up with a way to balance the output tubes, if you want that feature.

Sheldon
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