Do you need some kind of capacitor for isolation and stability between the SSHV2 and Ale's gyrator?
No need at all. With good filtering the gyrator provides an additional 100dB attenuation at 100Hz. With a good ground layout and if the DHT is in good shape, then no hum at all. You can't get it better than that. My system is extremely quiet and DHT end to end. I can't hear any hum when listening even at very low levels or very close to the speaker
Yes I know but if your system can't stand the gain then what other choice do you have if you like these tubes? I have a 4 way horn system that is over 100db efficient and several amps that can't handle that kind of gain. I like the tube between my source and amp to add what tubes do, especially since my 2 direct coupled 2A3 is collecting dust these days and I'm using a First Watt M2 that I built and love.
Jim
I have some "old style" prototype gyrator board, which is consist space for 400nF capacitor on B+, but if you have proper umbilical (I usually use two -PSU and amplifier- box solutions), it is unnecessary.
Each +/- pair wire -with sufficient thickness and insulation- twisted (common mode rejection).
If circuit sensitive to hum -the preamp don't use CCS or gyrator-, each B+ pair individually shielded. Each shield connected to ground point of PSU box.
If necessary, the whole umbilical shielded.
each B+ pair individually shielded......do you mean each B+/- pair??
Would you think a shielded Cat-5 could make a proper umbilical ??
Or what cabling are you using please??
Would you think a shielded Cat-5 could make a proper umbilical ??
Or what cabling are you using please??
Yes.
No.
I exclusively use 600V teflon insulated cables.
Twist them, and -if necessary- shield with copper braid (2-3 layer, over 85% covered).
Not needed in most of the cases. If you're using a simple HT supply with no regulation, then it won't be required, however it doesn't harm leaving it in. I included it on my first circuits but then I removed it.
If you have a regulated PSU (even shunt) you may require a bypass as some designs doesn't like the high impedance presented by the MOSFET drain of the gyrator which makes them unstable.
When I test high-bandwidth stages (e.g. 4P1L with gyrator) with my bench power supply, I have to add a 30uF cap to bypass it to make it stable.
In most of the cases, this isn't needed.
Here's the 26 preamp I implemented:
http://www.bartola.co.uk/valves/dht-pre-amplifier/26-dht-preamp-gen3-gyrator-version/
Cheers
Ale
"...shield with copper braid..." what copper braid ?? and where to get them please??
What is the recommended value for a grid leak resistor on the 26? I plan to use a 100k volumpot, but would rather like to have a grid leak resistor in there as well.
#26 grid current is -almost- immeasurable, so you can use even large grid leak resistor.
Although the usual use is line level input, large resistor's -almost thermal- noise is be considered.
For example 1MOhm resistor's thermal noise (20 Celsius grade, 20kHz bandwith) about 19uV (-95 dBV).
100k is optimal compromise.
Although the usual use is line level input, large resistor's -almost thermal- noise is be considered.
For example 1MOhm resistor's thermal noise (20 Celsius grade, 20kHz bandwith) about 19uV (-95 dBV).
100k is optimal compromise.
Do you guys know if there is some current "lost" in Ales gyrator or is the current just whatever the 26 tube draws at whatever voltage set on the gyrator?
There is 500uA to 800uA used by the CCS (LND150) pair to set the reference voltage. I tend to round it to 1mA when sizing the HT supply. In practice you can ignore if anode current isn't small.