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kevin gilmore

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On 10/6/2020 at 11:18 AM, matthew-levi said:

I could not adjust the bias down to 230V (for my normal bias Lambda). The lowest I could adjust is 333V.  How should I modify the voltage divider circuit to all the voltage to go down further. 


At the power supply schematic – shouldn’t D12 anode be connected to the cathode of D1?

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What should be the upper limit of the off-set  ( between O+ and ground). It fluctuates from a couple of voltages to 100 mV range. This is after the amp has been running for about half an hour. When its so unstable, does it mean something is not quite right. One of my friends told me that the off-set should be in the 10s of mV range. 

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how is the voltage O- to ground does it behave and vary similarly to the O+? 

are both channels approximately the same?


Unless there is a servo to automatically adjust the offset then you will see some variation with time as the device heats up and reaches a stable temperature. My T2 takes more than 3 hours for the offsets to become mostly stable. Even with a servo there is a tendency for some low frequency oscillation since the servo will alter the current flow and the change in current current flow causes a change in the temperature of the components and temperature change effects the gain and therefore makes the correction over or undershoot - a servo is always chasing its own tail.... Plus unless your valves have regulated heaters, mains voltage varies with time and the variations in the ac heater voltages will affect the valve temperatures and therefore offsets too.

You need to put the offset into context.  The stax headphones are built to withstand 100s of volts of input. The music signal is going to be far far higher than this offset. For example the T2 is capable of more than 600V RMS output into pro bias headphones... even at very moderate listening levels of say 50Vrms output, 0.1V offset is a tiny fraction of the signal...

It's also normal for the offsets to be fairly high (10s of volts) on initial switch on, fairly rapidly decreasing while the valves initially warm up, especially if you do not have a delayed high voltage switch on circuit. 

Edited by jamesmking
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11 hours ago, jamesmking said:

how is the voltage O- to ground does it behave and vary similarly to the O+? 

are both channels approximately the same?

Yes, both O+ and O- behave similarly, and both channels behave similarly too. For the balance i manage to get it down to around 100mv or less. 

I will do more testing and run it longer to see how they go. Right now I feel better knowing its should be ok. 

Thanks for the guidance. 

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  • 3 weeks later...

After almost three and a half months, the KGST is finally in operation, and is sounding great. Thanks to all who have responded to my questions and given guidance. I am really surprised at  how good the 40 year old headphone sounds with this amp. 

Special thanks also to Bigir and Dr. Gilmore for such wonder contributions to the community. 



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  • 2 months later...

Hi CrowDaddy,

Thank you for the encouragements. I will send you the BOM I used. 

I think its a personal choice on whether to use an EI or toroid transformer. It's a matter of availability for me. I have since replaced the transformer with one that can also supply the 15-0-15V and removed the smaller LV transformer.



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  • 1 year later...

Based on your first question you should probably stick with the original design.

The 24V zener diode protects the MOSFET from excessive Vgs during power up, when the output capacitor is charged up from zero.

Since you recognized the CCS, I probably don’t have to elaborate the benefit of a CCS vs a resistor. The simple answer is yes, you can replace the CCS with a resistor with reduced performance. The 10M90S is in short supply, at least from the US distributors. You can replace it with IXTP01N100D, or based on your rectified DC voltage, try a DN2450 or DN2470. The 1.2k resistor would need to be adjusted to obtain the same current through the zener diode chain as with the 10M90S.

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  • 2 months later...

I took the challenge again to squeeze a DIY eStat amp into a small enclosure. This time is a KGST in a Breeze Audio 2409.


I was using an Antek AS-1T350 with a separate low-profile quasi-toriod for the LV supply. The Antek was well shielded with both copper and steel bands under the wrapping, I still need to move it all the way back to reduce the hum. I'm glad I pulled the trigger and ordered the custom R-core transformer. I asked the vendor to build two identical bobbins to balance the flux, each with a 115V primary, a 350V, a 17V and a 6.3V secondaries. With the copper band on the outside, the flux leakage is so low that I couldn't pick up anything significant at 60Hz or 120Hz on the FFT.

I have more than twenty 6S4 and 6S4As, many of which are NOS. However I had a hard time matching a second pair, and had to decide between a well matched pair that is slight noisy and one that's less well matched. The curves look all similar below 400V, but could go all over the place on the higher voltage region. The below picture is kind of an extreme case, an example that two tubes both testing good on a tube tester at 150V on the plate, may be quite different when you need them to swing up to 800V:


<To be continued..>

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Although the KGST uses the same front-end as BH and Carbon, its simplicity comes at a cost. In the 'stock' form, the open loop gain is about 78dB and closed loop gain about 59dB. Having only moderate global NFB means that the circuit has to have a wider open loop bandwidth and good linearity to begin with. It also suggests that the measured performance would depend a lot on the matching and inherent linearity of the tubes.

Below is the THD+N vs amplitude plot I did earlier on the 'stock' circuit. The max output is about 200V. It kind of falls short for an amp with ±400V supply.


With the experience from the pentode experiment on Carbon, I tried a few pentodes and tetrodes with the 'stock' circuit. The first one I tried was the 6973. Why 6973? Because it's pinout is very close to the 6S4A, such that I didn't even need to cut any PCB trace in order to use it.


The result was quite ok. The max output went up to 500V and the lowest THD reduced by about a half. Knowing how much the 6973 costs on ebay I think I should have a better use for them 😉


Then I tried a few others, including the 832A. It has one of the cutest bottles and yet is not that expensive. I was eyeing on the Breeze audio 832A SE amp enclosure. The box may look hideous to some people, it would be really cool to have a unique eStat amp like that - if still offers reasonably good performance. The 832A didn't pan out very well. The THD flattened out at 0.01%-ish, mostly 2nd harmonic, due to the mismatch between the two tetrode sections. Some people may appreciate the 'tuby' sound, I decided to move on.

I looked for a triode with amplification factor of 30 (twice that of the 6S4A), which also allows 400V DC on the plate (800V peak). It's not that easy to find real triodes like that, unless I go with the exotics such as the 801-A/10Y. I heard about the 12HL7/12HG7/12GN7 family. The video output pentodes designed for the color TVs in the late 1960's. They allow 400V on the plate, have a mu of 30 to 50 when triode-strapped, and said to have great linearity. Below curve was captured with one of them at X=100V/div, Y=5mA/Div, Vg1= 2V/step


It turns out (again) it's not easy to find well-matched pairs because the curves in the high voltage region vary a lot from tube to tube, and the g2 power would be over spec when triode strapped idling at about 400V.

I did get some nice distortion plots when they are wired in pentode mode (below FFT with 12HL7 at 100Vrms output). Unfortunately not all of them are happy with 800V peak on the plate, although 400VDC is okay on paper.


So I came back to the drawing board, trying to make it work with the 6S4A. In the 'stock' circuit, the limiting factor to the output swing is the signal amplitude at the 6S4A grids. Adjusting the ratio between the 200K/6.8K resistors on the active loading NPN pair can increase the idling voltage and improve the max swing into the 6S4A. The downside is that the output impedance of the driver stage will increase, and the open loop bandwidth drops a bit due to the Miller caps on the 6S4A. Also I don't need the full 60dB gain. '200mV sensitivity for rated output' was established from the analog-era when tuners and tape decks are the most popular sources, not to mention 100mV for rated output. Doubling the resistors at the source of the input JFET aught to do it.

The end result is extended output swing:


And a not super clean, but still okay output FFT spectrum at 100Vrms. 


Well, that seems to be the best I can do. I'd live with(in) its limitations, close the lid and enjoy the music!

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