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Old 17th February 2006, 05:17 AM   #1
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Default 6SL7 Diffamp front end

I've completed a fixed bias 6V6 Ultralinear plus Cathode Feedback Push Pull Amp for a niece. It runs a current source biased 6SL7 diffamp front end with direct coupled MOSFET source followers to drive the 6V6 grids.

HT + to the diffamp is 270V with the current source tail returned to -20V.

I started with 47K Anode loads on the 6SL7 with 2mA per side.
- It sounded nice, warm, round BUT a bit covered (too much 2H distortion?)

I changed to 82 K anode loads with 1 mA per side
- This sounded less covered and more detailed with some additional top end.

This encouraged me to go to 150K anode loads with 600uA per side.
- This sounded even more detailed and brighter with maybe a little edge stating to creep in (too much 3H distortion?).

Is this just a function of the anode loads getting larger with respect to rp of the tube and therefore less overall distortion?

Is the edge in the 600uA per side due to running the 6SL7 where rp curve vs Ia is larger and hence I will have higher order distortion products?

Or do I not have a clue?

Anyone - can you explain what is happing here?
Anyone - what does TubeCad or other modelling say?

Thanks,
Ian
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Old 17th February 2006, 06:37 AM   #2
Jax is offline Jax  Sweden
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The higher current and the higher the anode load resistance the better the linearity. Best way to achieve this is to use a high supply voltage.

I like to run 6SL7 with 1.3mA and 220k from a 400V supply which is the operating point I have in my differential input stage in my EL34 amp.

In my opinon

Edit: I forgot the answer. You wish to place the load line in the IaVa curves as far away as possible from the curvier part. You can do this by increasing the current and/or lessen the slope of the load line (higher anode load resistance).
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Old 17th February 2006, 06:38 AM   #3
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This is interesting.

All three op points give about the same plate voltage of 170V. All three loadlines are highly linear (at least for the swing range required to drive the 6V6), as one can see below.

Gain rises with rising plate resistor values, Zout falls. Zout should not have much influence due to the high Zin of the follower.

What´s left? The gain changes. Did you listen at the same SPL? Did you change NFB (if any)?

Tom
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Old 17th February 2006, 06:40 AM   #4
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Hi Jan,

Quote:
The higher current and the higher the anode load resistance the better the linearity.
This point is moot in that case, have look at the load lines I supplied. All extremely linear for the swing required.

Tom
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Old 17th February 2006, 06:44 AM   #5
Jax is offline Jax  Sweden
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Quote:
Originally posted by Tubes4e4
Hi Jan,



This point is moot in that case, have look at the load lines I supplied. All extremely linear for the swing required.

Tom

I know but it doesn't hurt, does it?
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Old 17th February 2006, 09:56 AM   #6
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Ian, what did you have in the cathodes of the 6SL7? If you had a resistor (unbypassed) between each cathode and the tail (a CCS?) then that would effectively raise each triode's Rp by (mu+1) * valuoe of the resistor.

In addition, it's worth remembering that the 6SL7's Rp increases markedly as the plate current decreases. At 2mA it's the published 45k but at 600uA, it rises to around 60k.

I found my Sovtek 6SL7s perform very nicely with:

* 1.5k in each cathode and 4.7k across the cathodes;

* 120k plate load, shunted by 330k grid resistor of the following stage

* 220v plate-cathode voltage

* 700uA plate current (controlled by 6AU6 pentode CCS in the tail).
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Old 17th February 2006, 12:03 PM   #7
EC8010 is offline EC8010  United Kingdom
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Normally, as RL reduces, H2 rises, but the CCS will enforce balance and pretty well null that out. I would say, though, that 47k is a very low value of RL for 6SL7, and I would normally look for at least 100k, and preferably 150k. The problem then is that for any sensible current you need 400V of HT and I expect you only have 300V. You're in an unstable region of operation - you want to increase RL, but that means reducing Ia, and that increases ra, which is exactly the effect you don't need. I would expect that small changes will make a disproportionate difference to the sound and that it will change with the valve.
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Old 18th February 2006, 08:48 AM   #8
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Sorry, Ian - maybe my post was a bit of a red herring! I forgot to mention that my 6SL7 LTP is feeding a 6SN7 diff driver to EL34 triode-connected PP output stages using Thorsten Loesch's cross-coupled negative feedback (with 2.2Meg resistors from EL34 plates cross-coupled to 6SL7 plates). The raised 6SL7 Rp (caused by having resistors in the cathodes and low plate current ~ 0.7mA) serves the purpose of feeding the NFB loop. The 120k plate load resistors ensure that the 6SL7 can get the required plate voltage and current.
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Old 20th February 2006, 01:57 AM   #9
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Hi guys,
Thanks for the responses.
Ray - I have just 56R in each cathode for a little degeneration and hopefully better balance.
Current source is cascode BC547B with a reverse connected, floating base BC547B "low noise 7.6 Volt zener" as the reference.

I'm limited to that 270V rail - probably means that the 6SL7 was not really a good choice.

In addition the cathode feedback to the 6V6 increases drive voltage requirements from about 40V pk-pk to 56V pk-pk at full power (around 7 watts).

The whole thing is a bit of a compromise as I started off with a non-technical design requirement - I wanted something that looked "arty" to suit Kristy's (the niece) personal tastes. Hence some NOS 6V6G and the Octal 6SL7. The MOSFET source followers were added to cope with the 100K Rg1 for 6V6G in fixed bias and this then allowed me to increase the anode loads on the 6SL7.

Anyway, after the weekends work in trying to set anode to screen zobel networks on the 6V6s (they are ultralinear + cathode feedback) I found that the transformer resonances (cheap Hammond 1608) are way too low because I have separated the 2 off 4 Ohm secondaries to achieve the cathode feedback where they should be run in parallel. Best results were with 1K8 and 3n3 for the zobels but high frequency response is down 3dB at only 23kHz and a 10kHz square wave looks very "rounded". Soundwise this translates to an amp which has no real attack and is just generally a bit "bland".
For these reasons, I am now about to abandon cathode feedback and rewire these amps to Ultralinear plus shunt feedback as per the "Baby Huey" design BUT retaining the MOSFET source followers. This will allow me to wire the output transformer secondaries as per manufacturer instructions and hopefully get much better high frequency response.

They fullfill their main design requirement in that they look stunning - it is just getting them to sound stunning which is causing me some grief.

What I learned - For cathode feedback you need a good quality output tranny. I'll try it again when I build my next amp - 4 x KT88 Ultralinear + Cathode Feedback using Plitron VDV2100-CFB/H Toroidal Output Trannies. For cheaper transformer selections use the "Baby Huey" scheme.

Cheers,
Ian
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Old 22nd March 2006, 09:30 PM   #10
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hi

we can see a diagram?

thx
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