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Leak TL10 cathode bias
Leak TL10 cathode bias
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Old 14th November 2017, 11:45 AM   #1
Matt Rowland is offline Matt Rowland  United Kingdom
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Default Leak TL10 cathode bias

Hi folks, having installed 6L6s in my TL10 with the leak recommended modifications, I have been looking at the on load frequency behaviour with my scope. The leak mod suggests omitting the cathode bypass capacitor (I've installed the single/common bias resistor config). Applying a sine wave and sweeping the frequency from about 100Hz to 18kHz, I find that the wave shape suffers visible distortion up to about 10kHz, then the shape improves until 18kHz. If I bypass the cathode resistor with a 25uF cap, the waveform improves below about 10kHz and worsens above. So I seem to have a choice of either high (with cap) or low (without cap) frequency distortion. Any observations or advice welcome. Cheers. Matt
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Old 14th November 2017, 06:28 PM   #2
bigwill is offline bigwill  United Kingdom
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Do you know if the 6L6s you have are fairly well matched? A bad mismatch could well contribute to ugly high frequency waveforms - there's less feedback up there to clean things up.

If you have a single bias resistor, any mismatch between the output valves could become exaggerated further - one might be drawing significantly more quiescent current. Not only does this cause a magnetic offset in the output transformer, the valve drawing more current will likely have more Gm at its operating point than the other - compounding the issue if it was the one with more gain in the first place!

If this is the issue or not I would still recommend independent cathode resistors / capacitors as per the original schematic, with a suitable value for 6L6s. A single resistor introduces more problems than it allegedly solves.

Last edited by bigwill; 14th November 2017 at 06:32 PM.
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Old 14th November 2017, 06:49 PM   #3
Matt Rowland is offline Matt Rowland  United Kingdom
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Thanks for your reply. The amp came as standard with the single cathode resistor. I have measured the anode currents and they are within a couple of milliamps- they were JJs bought as a matched pair. I have subsequently removed the load and looked at the open circuit waveform and the effect disappears. Maybe it's just the way the amp is when delivering power, maybe the o/p transformer is showing its age. I will try the separate cathode resistor config, just to see if things change. Was the single resistor method employed just for cheapness?
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Old 14th November 2017, 06:59 PM   #4
john_tracy is offline john_tracy  United States
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Obviously matched pairs are a necessity. That aside, when you run a PP output pair from a single cathode resistor without bypassing, you have created an LTP with a low impedance "tail". In my experience with my own PPP KT-120 mono-blocks, switching from a low impedance resistor to a high impedance CCS results in a significant improvement in SQ. As with all LTPs, a high impedance cathode "tail" acts to enforce AC balance. Doing this does restrict the output to class A operation. If you can live with the reduced power output, not a bad thing in my opinion.
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Old 14th November 2017, 07:24 PM   #5
Matt Rowland is offline Matt Rowland  United Kingdom
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Thanks John. SQ? Have you got a ccs circuit you can share? Best. Matt
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Old 14th November 2017, 07:39 PM   #6
john_tracy is offline john_tracy  United States
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SQ = sound quality.

How much current do you need to sink? In my case it's 300mA. I tried running various cascoded CCSs in parallel and had issues with oscillation. I do not recommend paralleling CCSs. If you current requirements are such that you can use a single cascoded depletion mode MOSFET current source then that should work. In my case I ended up using a TL-431 variant to control the current in a large power MOSFET. Kevin Carter at K&K Audio was kind enough to humor me by designing the PC board for this and supplying the boards. I believe he now will sell this type of current source as a kit. He also sells kits for cascode MOSFET CCSs. If you ever do use a CCS like this on the output stage of an amp be conservative with heat sinking and preferably have the fins external to the amp.

As an aside, I once had an output tube arc during a crescendo. There was no damage and the amp continued to play after the arc cleared.
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Last edited by john_tracy; 14th November 2017 at 07:44 PM.
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Old 14th November 2017, 08:01 PM   #7
Matt Rowland is offline Matt Rowland  United Kingdom
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Doh, SQ. I was thinking too technical!

I need about 70mA total. Now you've pointed me in this direction I'll do a bit of research. Thanks again. M
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Old 14th November 2017, 09:54 PM   #8
john_tracy is offline john_tracy  United States
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At 70mA and I'm guessing about 50V or so you'd be looking at ~3.5W of dissipation. A simple cascode MOSFET should work fine. The IXYS MOSFETs that Kevin uses in his kits perform better in listening tests than the usual DN2540s.
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Old 15th November 2017, 09:00 AM   #9
Matt Rowland is offline Matt Rowland  United Kingdom
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Thanks. I found a good tube cad article on ccs and cathode bias. I'm gonna try using a 317 with their diode/capacitor bypass circuit first- mainly because I have those parts to hand. Best. M
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