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Old 8th January 2013, 08:08 PM   #1
chip647 is offline chip647  United States
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Default Powerdrive for higher grid wattage A2 transmitter tubes

Hello,

I have successfully built 2 single ended tube amps using the Powerdrive MOSFET source follower into Eimac 35t tubes with positive bias. This works great. On the data sheet, it says that the max signal driving power for the 35t near my operating point is 7 watts.

I have been playing with the Eimac 100TH using the same circuit. While I can get the tube to play loudly, there is a bit of distortion. The 100TH max signal driving power rating is close to 20 watts. It seems that my IRF820 MOSFET biased at 7.5 watts is not cutting the mustard.

Is the solution to crank up the MOSFET bias to 35 watts (it is a 50 watt part) or do I need to use a larger MOSFET? Does this sound like the problem? The output transformer is the big single ended Hammond (6.5K) with an 8 ohm speaker on the 4 ohm tap.

Thanks
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Old 12th January 2013, 04:03 AM   #2
chip647 is offline chip647  United States
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So, I cranked up the current on the IRF820 and the distortion on the 100TH went away. I am running the IRF820 at 50mA and 19 watts dissipation. The fairly large heatsink gets pretty hot, hotter than you want to keep your hand on for more than 5 seconds. Is it safe to assume that the ability of the source follower to drive the grid is only a function of the mA the MOSFET is running, not the dissipation of the part?
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Old 12th January 2013, 08:58 AM   #3
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You should take SOA or Safe Operation Area into account, see IRF820 datasheet. Basically the SOA diagram describes drain current - drain:source resistance in ON state and pulse width as parameter. Next thing you should consider is thermal condition of mosfet operation i.e. pick proper radiator which ensures junction temperature is not exceeded.
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Old 12th January 2013, 10:30 AM   #4
AJT is offline AJT  Philippines
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any pics you can share? a big heatsink is good....
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Old 12th January 2013, 10:52 AM   #5
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Sorry if this sounds a bit obvious and possibly totally wrong, but couldn't you drop the voltage on the IRF820 and up the current to keep the heat down. even if you used a power resistor to take some of the load. After all, these components are capable of running at amps of current and they are generally more linear (not an issue here) the higher the bias current.

Just a thought.

Shoog
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Old 12th January 2013, 10:59 AM   #6
AJT is offline AJT  Philippines
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i was thinking of that one too....the drain voltage can be a tad higher the the peak grid voltage drive...
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Old 12th January 2013, 08:12 PM   #7
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Hi FET drivers,
I'm driving min 211 amp by IRF820, I run 30mA amps in the FET @ app 5W.
Remember to have 50V head room in both ends of the sine.

Best regards
Karsten
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Old 12th January 2013, 08:20 PM   #8
chip647 is offline chip647  United States
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Default Some pics

Yes, I will drop the drain supply from 350 to 100v. The pictures include my completed 35t amp. Thanks.

So as long as there is sufficient current and voltage headroom to cover the AC signal, increasing the dissipation does nothing but increase heat?
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Old 12th January 2013, 08:31 PM   #9
TheGimp is offline TheGimp  United States
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Nice looking amp.
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Old 12th January 2013, 08:38 PM   #10
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Quote:
Originally Posted by chip647 View Post
Yes, I will drop the drain supply from 350 to 100v. The pictures include my completed 35t amp. Thanks.

So as long as there is sufficient current and voltage headroom to cover the AC signal, increasing the dissipation does nothing but increase heat?
Correct.

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