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Old 19th March 2014, 08:23 AM   #1
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Default 6SN7 at low plate currents

In the input stage of an amp I have designed a 6SN7 is used with both triodes paralleled in a common cathode gain stage. This has an unbypassed 270R cathode resistor and a constand current plate load, and then feeds a direct-coupled LTP phase splitter (also with a constant current source in the tail).

In very rough terms I have a gain of 20 in the common cathode stage and 10 in the LTP. Therefore to achieve the 120V p/p needed to drive the output stage I need 12V p/p into the LTP.

Having looked at the plate curves etc I started off running the common cathode stage at 10mA for a plate voltage of 100V. This simulated well and measured OK, and this plate current or higher is the 'conventional wisdom'.

With the rest of the amp built I then measured the THD open loop, using a commercial meter (NTi Audio XL2).

This is where it gets interesting ...

At 50% ouput from the LTP (about 24VRMS) I discovered distinctly lower THD at much lower plate currents in the first stage - I adjusted the constant current for lowest THD and ended up with 3.3mA plate current for a plate voltage of 42V on the 6SN7 triode gain stage plate!!

The low plate current also simulates well - what is going in here? My assumption is that the lower plate current in the first stage means that the second LTP stage is further from clipping, therefore at medium to high outputs the composite driver delivers lower distortion.

I had read that at low plate currents the sample variability of the tubes would become higher. I substituted a NOS RCA for the tung Sol I had done the testing on and got the same result (with an even lower plate voltage of 37V!!

I did not write down the readings and the amp is currently in pieces waiting final assembly, but the THD measurements were Z-Weighted at 1kHz, and the difference was something like -40dB THD at 10mA plate current to -50dB at 3.3mA.

The complete amp with GNFB in place measures very well to extremely high outputs, but that is another subject and another thread.

Is conventional wisdom just wrong?
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Old 19th March 2014, 08:46 AM   #2
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I would like to just comment your test procedure.
If you are designing an amplifier with GNFB and 120 Vp-p as required drive level.
Then you should ensure that your voltage amplifier and LTP combination is able to supply up to some 140 Vp-p with low THD.
It is not essential how low the THD is at 50 % voltage levels if the clipping begins at 90 % level.

It is not any surpise to me that very good linearity is achieved at low anode current when the load impedance is high like in your case.
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Old 19th March 2014, 11:09 AM   #3
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The 120V P/P is the minimum for full drive of the output stage, I do understand that there should be a lot of headroom in the driver. In fact at 3.3mA current in the first stage the driver overall clips at about 223V P/P so no problems there.

I was testing at 50% of output simply to get some idea of typical THD, and to optimise the driver operating conditions.

I've put this on the board because the linearity at low plate voltages is amazing, in the context of the 'accepted wisdom' for currents in 6SN7 gain stages. I had not really thought about it until over the last weekend I built the two 'production' driver PCBs from the cicuit diagrams and looked at the typical voltages on them.

A CCS giving as an infinite plate resistor seems to work wonders.
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Old 19th March 2014, 01:51 PM   #4
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Is there some 2nd harmonic cancellation going on here between the 6SN7 and the phase splitter/output ? - i.e. the low currents at the input are actually increasing the distortion of the 6SN7 but overall they cancel better with 2nd harmonic generated in later stages but in opposite phase.
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Old 19th March 2014, 02:11 PM   #5
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Its bad practice to design around harmonic cancellation because the cancellation will degrade as the triodes age differently. It is also fairly unpredictable over the whole frequency range and under real signals. Far better to operate the triodes in their most linear region and minimise the distortion this way. If that means running higher currents - then run higher currents.

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Old 19th March 2014, 04:26 PM   #6
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Quote:
Originally Posted by Xpersephone2 View Post
This simulated well and measured OK, and this plate current or higher is the 'conventional wisdom'... Is conventional wisdom just wrong?
Yes, it is wrong. Take a look at this loadline (attached). This is for the 6J5, a singleton triode very much like one section of the 6SN7. There is no reason why this type can't be used as a "current trickler" if that's what your application requires.
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File Type: png 6J5-Loadline-100K.png (45.1 KB, 240 views)
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Old 19th March 2014, 05:40 PM   #7
Bigun is offline Bigun  Canada
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Looks good - but I don't know how trustworthy the datasheet is down at the bottom edge ?
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Old 19th March 2014, 05:57 PM   #8
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^^

Quite trustworthy, I'd say. I designed against an RCA spec sheet, and used Westinghouse JAN 6J5s. The DC voltages and currents were spot on, as was the AC gain. If there is any problem with this, it's the very large rp and RP that compromise wideband performance. The -3.0dbv frequency was 45KHz measured. It's trivial to get a higher cutoff frequency with any transistor circuit, even if you assemble it on a solderless prototyping board without paying attention to lead dress (lots of Cstray) So I wouldn't recommend it if you're after wideband performance. However, for the intended application, it was NBD, as the OPTs cutoff at 30KHz (open loop) with a 35KHz cutoff under NFB (local and global).
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Old 19th March 2014, 06:15 PM   #9
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OK, so I will run up the driver board this weekend, measure the THD directly out of the driver board at various currents in the first stage. I was measuring the THD at the amplifier output... but I certainly was not designing for distortion cancellation, I'm not that clever!

On a lighter note the output KT88s glow blue on the glass for B+ above 500V. My science head has this down as harmless phosphorescence caused by impurities in the glass. Sadly KT120s did not glow when I tried them under the same conditions .. I am very disappointed.. I loved the glow ..
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Old 19th March 2014, 06:35 PM   #10
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Also to reply to Miles Power, I have no problem with bandwidth. the whole driver is measured-3dB at 150kHz with 3.3mA in the first stage. Increased currents will increase the first stage bandwidth but I am rolling it off anyway prior to the LTP to stabilise the GNFB loop and avoid slew rate problems as the rest of the driver has to push big volts (push pull KT120s with 520V B+).

I have looked at plate curves and never really understood the common statements about linearity at higher plate currents, more than anything I ended up thinking that an accurate simulation might show considerable differences in relatively small amounts but that unless huge amounts of distortion were at issue caused by very bad operating points then there was nothing to see for reasonably linear tubes.
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