Solid state output valve

[Ed_Dinning]

Hi Folks, as well as the Fetrons I think there was also a solid state "valve " used for video output for TV's in the 80's.
It was never an OEM fitting but to replace a valve that had very short life. It also had to accommodate a power resistor to preserve heater chain continuity.

Ed

[ppppenguin]

What I was wondering is to modify the circuit above, would it be as simple as having 2x approx. 3R resistors across the filament pins on the socket & connecting the cathode side of the FET circuit and omitting the G1 connection or is there more to it?

Using resistors to replace the heater/filament is only required in a series heater chain. I can't imagine a PX4 ever being used like this. Just ignore the filament pins, except that one must be used as the cathode.

[ekjdm14]

Mark, wouldn't be me if I wasn't scanning the main text & utterly missing the "small print"! Oh well, I do plan to become more active on this forum also since we have plenty of time currently!

Jeffrey, that is a very good point & one I'd not thought of. Mainly because I'm not super-experienced with how valve radios function & I was merely going from another FET circuit kindly devised by someone on the other forum which did include the "filament" resistors. To be honest though, I may carry on and use the pair of resistors just to keep the loading on the filament winding similar to if it had a valve in place because it might help stabilise the voltage and avoid the other valves being over-run (we have fairly strong line voltage here, mid-high 240's usually)

That aside though, just ignore the G1 part of the circuit & include a 75 or 150mA fuse?

Thanks again, Dan.

[ppppenguin]

I still wouldn't bother with a dummy load for the filament. Of course you can measure the LT and HT voltages, both with and without a dummy load. I doubt you'll see much difference. If the mains transformer is run close to its limits then not having a heater load will be a help. In any case the set will run cooler.

The real hazard with high mains is saturating the core of the mains transformer. This is independent of load. The primary current will rise rapidly as you increase mains voltage and the core starts to saturate. you can measure this with a DVM on current range and a variac to adjust the voltage. Take care as you'll have a temporary rig with live mains.

If the mains is high, saturating the core or giving unacceptably high LT and HT, then the answer is a buck transformer. It's just a small mains transformer with a secondary voltage of the amount you want to subtract. The secondary is wired in series with the incoming mains so as to reduce the voltage. You have a 50:50 chance of getting a boost rather than buck first time you try this

I'm using a buck transformer in a small TV monitor to reduce the dissipation in a series regulator transistor: http://golbornevintageradio.co.uk/forum/...p?tid=7789

[Raditechman]

The PL802T was the solid state replacement for the PL802 luminace output valve,  I still have one!.
http://www.r-type.org/exhib/acl0076.htm
Johhn

[jjl]

I still wouldn't bother with a dummy load for the filament.

Dan

Things are complicated a bit by the fact that the AC044 / PX4 is directly heated so effectively the electrical centre of the heater is the valve's cathode. The SAC35 has a centre tapped heater supply with the centre tap tied to chassis. Also, the SAC35 is one of those sets where valve bias is derived from resistors in the negative HT line and fixed bias is applied to the AC044 grid.

I think you'll be OK to tie the degeneration resistor in a MOSFET source ("cathode") to chassis, but watch the voltage that is applied to the AC044 grid - this is likely to be greater than the maximum Vgs of your MOSFET which is typically +/-  20V. You'll have to perform a bit of maths to determine the value of "cathode" resistor that reduces the gain of the MOSFET sufficiently while keeping within a safe gate to source voltage range. I haven't done the maths, but it may be that this isn't practical without changing the existing bias arrangements for the output valve.

John

[jjl]

Dan

Thinking about this a bit more, if you add a capacitor between the AC044 grid connection and yourMOSFET gate, you'll block the DC bias applied to the valve's grid and can arrange the MOSFET bias to suit. You can't directly use Al's arrangement though as there is no screen grid supply - the AC044 triode doesn't have a screen grid.

John

[ekjdm14]

Hi, I have an IRFBF20 and 5 off 7N80, first has Vgs of 10v whereas the 7N80s have Vgs of 30v so likely more suited. The blocking cap might be a good solution as you say, then I could be a "Trimpot Trev" and adjust the bias, or the other potential lazy way I was thinking of finding out the resistances needed was to assemble the basic circuit from the other forum with ballpark resistances but using the 7N80 & employing Al's source-gate Zener. Then using a solid state rectifier bring the set up slowly and monitor the circuit & adjust resistances as required.

I definitely have a lot to learn as far as making silicon work where there ought to be vacuum! No huge rush though, I've a resurrected RG73 with different rectifier and output valve to be playing with in the meantime It's starting to work rather well now but was a lot simpler as it's still all-valve!

EDIT- and Jeffrey, I'll have a look at that & see how it's behaving. Be interesting to see how it's handling it.

Thanks again, Dan

[Alistair D]

I found this on the Internet. By the look of it it knocks spots of my attempt.


  ValveSubstitute.pdf (Size: 217.33 KB / Downloads: 24)

Al

[jjl]

Dan

I think now that a blocking capacitor is essential. The original circuit applies something around -35V grid bias to the AC044 with the notional centre tap of its cathode at 0V. But your MOSFET is an enhancement mode N channel device which needs its gate to be more positive than its source to start to switch it on. A large negative voltage is therefore the opposite of what you need.
You need the FET's gate to be about 4V more positive than its source in order to start to bias it into the most linear part of its curve. Unfortunately, this characteristic of FETs has huge sample to sample variation, so it's not possible to give a precise number.
The other thing you need to do is ensure that the FET passes roughly the same quiescent current as the valve did i.e. about 50mA with no audio signal applied. Too low and the HT voltage will rise which may upset the rest of the radio's circuitry, too high and you risk damage to the output transformer, rectifier valve, speaker field and mains transformer.
I wouldn't try to set this up with a trimmer - there's every chance that a tiny touch will cause an enormous change in drain current and consequent destruction.
It would be worthwhile breadboarding to find the ideal Vgs to give a reasonably linear Vgs to Id curve. You can choose a suitable bias voltage and source degeneration resistor combination to give an approximation to the valve's characteristics.
Another thing to bear in mind is that, if you approximate the valve's characteristics then the MOSFET will be dissipating as much power as the valve did i.e. about 15W - it will need a heatsink and you won't want this heatsink to be at drain / anode potential.

Good luck, this might be a bit of a project.

John