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Old 6th February 2012, 03:09 AM   #1211
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Rogon,
The diode is only there to protect the tube at power up, before it has heated and begins to conduct. During this time the cathode will try to go to the full negative rail. The diode will clamp this to one diode drop below zero. Once the 12AX7 is conducting current the cathode will sit a little above 0V and the diode will be reverse biased, effectively out of circuit, except as you stated for its capacitance which will tend to compromise balance of the diff amp at higher frequencies. I would definitely go for a small signal or low power diode rather than a 1N400X series.
Note that reverse voltage rating is NOT a problem. During startup, when its doing its protection of the 12AX7 heater/cathode job it is forward biased. After startup there will be only a couple of volts (reverse bias) across it.

Using resistor power dissipated = I squared x R, dissipation will be about 0.6W. A 2 Watt resistor will be ideal.

Cheers,
Ian
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Old 6th February 2012, 03:25 AM   #1212
rongon is offline rongon  United States
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Location: Across the river from Rip's big old tree...
Thanks again!

OK, here's what I got. Grid leak resistor is now 47k, -270V supply to the LTP tail, diode to clamp the cathodes to near ground potential while warming up. I hope it's workable now...

--

PS - Should the grid leak resistors for the EL84's be increased to 1M? Or is that hard on the EL84's?
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Last edited by rongon; 6th February 2012 at 03:29 AM.
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Old 6th February 2012, 03:33 AM   #1213
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There's no reason to make the input resistor so low. It doesn't affect frequency response.

All good fortune,
Chris
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Old 6th February 2012, 03:47 AM   #1214
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That looks like a "go'er".
Coupling caps could be 100nF (3.3Hz -3dB point) instead of 220nF (1.5Hz -3dB point). If you have 220nF use them but the smaller 100nF value would be fine.

Run without global feedback unless the speakers really need it (leave out the 12K to 22K resistor). I've generally found I like that local feedback set resistor (15K to 33K) at 15K to 18K but feel free to experiment - Set the local feedback first without global feedback connected. As you increase this (local feedback) resistor you put more demand upon the diffamp front end. You might like to get the diffamp current up marginally (to 1.4 to 1,5 mA) by using 180K tail resistor or used a -320V rail and a 220K.
All that can be tweeked once up and running.

I have to (respectfully) disagree with Chris - that input resistor is in parallel with the 12AX7 Miller Capacitance and WILL affect the top end response. The main top end frequency limit in this basic BH incarnation will be the 12AX7 driving the EL84 Miller Capacitance. Thats why the "full bore" incarnations use a MOSFET Source Follower buffer between the 12AX7 and the EL84s.
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Ian
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Old 6th February 2012, 04:01 AM   #1215
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Quote:
Originally Posted by gingertube View Post
I have to (respectfully) disagree with Chris - that input resistor is in parallel with the 12AX7 Miller Capacitance and WILL affect the top end response. The main top end frequency limit in this basic BH incarnation will be the 12AX7 driving the EL84 Miller Capacitance. Thats why the "full bore" incarnations use a MOSFET Source Follower buffer between the 12AX7 and the EL84s.
The driving stage's output impedance also appears in parallel at the amp's input. This input resistor only needs to be large for good compatability with other high impedance stuff (vacuum valve stuff) but it's easy enough to do.

Thanks,
Chris
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Old 6th February 2012, 04:09 AM   #1216
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OK Chris is quite correct.
I was taking a short cut to short circuit those who will see that resistor and substitute a same value pot. In that case the upper arm resistance of the pot will add to the driving stages output resistance and that is when we can get into problems. If using a pot stick to 47K (50K) maximum. For a single resistor you can use the higher values.
So my appologies to Chris.
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Ian
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Old 19th February 2012, 08:10 PM   #1217
vd1992 is offline vd1992  Hungary
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Hi Everybody!

Which is the better PP topology of EL84? (with only one preamp tube)
-a triode amplifier and a phase splitter
-a differential amplifier

Which has a better sound quality??


And what do I have to modify if I want to use the amplifier in class-A push-pull?

Thanks
VD
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Old 19th February 2012, 08:56 PM   #1218
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It doesn't seem like the 12AX7 should need grid stoppers at all, certainly not on the feedback side that is already fed with resistors (just keep those near the tube). In the current configuration resistance at the amp input combined with some added capacitance to ground for r.f. filtering can improve immunity to nearby a.m. broadcast, ham, and c.b. radio signals. The cap just needs to be small enough to not degrade audio bandwidth. Don't count on the miller effect for r.f. filtering because lower stage gain at radio frequencies (due to loading of next stage etc) lowers the effective miller capacitance. If the input pot is made smaller, the filtering effects of capacitance won't be so unpredictable (source and setting dependent). The idea is to avoid r.f. at the grid since the poor frequency response (at r.f.) in later stages won't remove stray audio that resulted from the r.f. being rectified (detected) at the input grid.

If one was willing to go with much lower input impedance and run in inverting mode, performance would be superior due to the lack of a common mode signal being handled by the input stage. Also, then the feedback would bring down any hum or microphonics from the input triode the major source of them, something it currently cannot do at all. Connected this way, the stress (and distortion) in the input stage is LOWER than open loop instead of worse, since there's only the smaller error signal being handled.

I know that some want really high input impedance to support preamps with weak output sections, but wouldn't it be better to fix those instead (adding cathode/source followers etc)? With a typical RCA cable having several hundred pF of capacitance rolling off high end, that alone should be enough reason to abandon working at high impedances for interconnecting.

Last edited by riccoryder; 19th February 2012 at 09:25 PM.
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Old 26th February 2012, 02:31 PM   #1219
vd1992 is offline vd1992  Hungary
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Is 330V much for 6P14P ? (8k ohm)

I know, the maximum allowed is 300V, but more 30V cause problem, or early die for tubes?

VD
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Old 1st March 2012, 12:15 PM   #1220
vgeorge is online now vgeorge  Greece
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Here is how my Huey looks finished completely. I have balanced inputs, regulated the -12V with LM337, 28K shunt feed back only (tried 15K, 18K and 22K) and at the end, Golden Lion ECC83 gave a really nice sound.
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