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PostPosted: 09 Nov 2017, 03:08 
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poty wrote:
There is also a simple mod that almost eliminating the need of the 25Ohm pot (use its own CCS for each tube and connect the power tube cathodes together by a small resistor of 10-15Ohm value). The mod also helps with heat problems and allows the usage of lower current devices.

I haven't seen that but it sounds interesting. Do you have a link to that post or a schematic?

Thanks.

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PostPosted: 09 Nov 2017, 03:20 
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Greg2109 wrote:
Do you have a link to that post or a schematic?
Unfortunately not on the forum, but it is easy to draw.


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PostPosted: 09 Nov 2017, 06:08 
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poty wrote:
You can also use a DN2540 or other depleted mode MOSFETs in pairs. The main problem here is the need for some biasing to allow several volts across CCS, but it's doable.

I thought only one depleted mode MOSFET and a resistor from gate to source would suffice for a CCS. Do you mind explaining better the solution with a pair of MOSFETS? Also, the DN2540 is good for 500mA and 400V, limited to 15W dissipation. Wouldn't it self bias like the LM317HVT CCS?


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PostPosted: 09 Nov 2017, 07:07 
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victorzwk wrote:
I thought only one depleted mode MOSFET and a resistor from gate to source would suffice for a CCS.
Yes, you are right in general. There is a parameter in datasheets - Idss - which means the saturated current @ Vgs=0. For example, for DN2540 it is 150mA @ 25V. It means that self-biased DN2540 without a resistor from gate to source (short-circuited) will output no more than 150mA of current. In reality, the current is around 100mA for practical Vds voltages in the range of 5V. (Compare to the Oddwatt needs for KT-88 = 180mA). Dynamic impedance in this case varies from batch to batch from 30-70kOhm (enough for the application, but much less than for LM317HV).
Adding a gate-source resistors increases the impedance, but lowers the saturated drain current (which is already less than needed). Plus it adds some voltage to the abovementioned minimal Vds for the MOSFET to work and bias properly.
Second approach - biasing the D-MOSFET itself to make more drain current and to add gate-source resistor big enough to improve the dynamic impedance. The circuit becomes complex and the bias adds to the compliance voltage (that is why I mentioned the connection of the CCS to some negative voltage relative to the ground).
Certainly there are several D-MOSFETs that has enough Idss to work in the Oddwatt, but I do not like them as much as DN2540 due to some high frequency limiting. I agree that there can be unknown to me models which suits all our need.
victorzwk wrote:
Do you mind explaining better the solution with a pair of MOSFETS? Also, the DN2540 is good for 500mA and 400V, limited to 15W dissipation. Wouldn't it self bias like the LM317HVT CCS?
From the considerations above it is feasible to use a trick used by Bruce in his KT120 Oddblock: paralleling the CCSs. Having 90mA for each tube we have easy DN2540 layout with a gate-source resistor and bearable impedance. The only limitation left is the 5-6V compliance across the CCS. It could be eliminated either by a simple negative supply or just some increasing of B+ and putting positive bias on the power tubes' grids (mentioned earlier by Bruce).
If we have headroom in the voltage we can use cascoded version of Depleting mode MOSFETs (two in the totem-pole config for each CCS) and have excellent frequency response and much higher impedance. One of my friend measured a considerable improvement in the THD for such layout.


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PostPosted: 09 Nov 2017, 08:49 
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You mean something like that?


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PostPosted: 09 Nov 2017, 08:55 
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The CCS part is OK, but you won't be able to adjust it for 180 mA, so you need two of them. You can connect them directly to each cathode and put one 10Ohm resistor between them.


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PostPosted: 09 Nov 2017, 09:04 
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poty wrote:
The CCS part is OK, but you won't be able to adjust it for 180 mA, so you need two of them. You can connect them directly to each cathode and put one 10Ohm resistor between them.


Why can't it reach 180mA? According to the DN2540 curves, a Vgs around -0.3 V should make it conduct 180mA for Vdc from near 0 to 400V.


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PostPosted: 09 Nov 2017, 09:13 
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Don't know what datasheet you use, but in practice even connecting gate to source directly rarely gives 120mA (one example - SSHV on diyaudio forum). You can try. DN2540 is now not Supertex, but Microchip, so it is possible that they changed something.


Last edited by poty on 09 Nov 2017, 09:49, edited 1 time in total.

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PostPosted: 09 Nov 2017, 09:45 
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You are right. I have misinterpreted the datasheet.
Any disadvantages (sonically) on using two CCSs?


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PostPosted: 09 Nov 2017, 09:49 
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From my experience - no, usually there are only advantages in using MOSFETs.


Last edited by poty on 09 Nov 2017, 09:50, edited 1 time in total.

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