homebrew amp schematic

[Gito]

Hi

This is a schematic of my simple min bias supply.(A)

Look at B ,in this circuit like WB6IYH wrote wont work ,because when one of the VR tube has a slight difference on triggering ,the one that has a higher voltage when triggered . Triggered first and the voltage drops a across the 3000 ohm resistor  at the junction of R3 and the 100 ohms drops to a low level,that makes the voltage to low to trigger the other VR tube.
At least at this circuit  because A VR tube needs at least 40 t0 50 VDC higher
than it's operating condition ,like VR105 it needs 150 VDC to trigger it.

At (C) the VR tubes are directly drop from the power supply ,which each dropping resistor,
So the voltage drop across each dropping resistor won't affect the operating condition of each VR tubes.

Gito.N

[AB2EZ]

Jim

Yes... that is exactly what I have in mind... based on your previous comment about the need to provide a path to ground for the average grid current when grid excitation is applied.

Thanks!

Stu

[W8ACR]

Hi all,

Thanks for all the comments. I think I'll just try the bias circuit as I have drawn it and see how it works. I am attaching two schematics, one is my hand drawn rendering of the bias supply circuit from the original Globe King 500, the other is from K9ACT's schematic of his 8000 tube amp. Both bias supply circuits are essentially the same as mine. If I have problems, I'll add the diode as suggested by Jim and Stu. If it still doesn't work, I'll ask for more help.

One more thing. Will an LF parasitic act the same as a VHF parasitic? Can I expect runaway plate current with both types of parasitics? If an LF parasitic is present, can it be cured by changing the value of the bypass cap which is adjacent to the RF choke? Or should I just remove the RF choke and replace it with a resistor?

Thanks again, Ron W8ACR

[Gito]

hi.

From QST.

Regulator circuit for loads that may vary as much as 70 ma.the two VR tubes are indentical.R1 and R2 are chosen so that 40 ma is drawn by each under minimum load conditions.
That means R1 and R2 is in kilo ohms value. not 100 ohm and directly taken from the B+.

But I see that You have succeed in building you  min bias supply.
                                                                                                                  When using 100 ohm equalizing  resistor ,it means to equalize the current flowing trough the VR tubes not the triggering voltage of those tubes.And the same VR tubes because the manufacturing variations,can have different triggering voltage.

From GE application ......operating of glow tubes in parallel is not recommended

Why still used Choke filters,in the old days they used choke filter because it's hard to find high value Capacitors ,in now days we can find 220uf 450VDC easily,so without a choke ,the DC voltage is well filtered
And it is used to feed a regulator tube that ads to the filtering of the supply? so I believed it well filtered min bias.

Gito.N

[WD5JKO]

Brian,

   Using grid leak bias on a class C amplifier is ideal. The correct operating point (conduction angle) is then functions of the grid leak resistance, and the grid current. This is a time tested no compromise solution.

   Where the debate comes in, as it seems, is to protect the Class C tube in the absence of excitation. Here we can use an overload relay. I think the BC-610 does this. We can also provide a means of protective bias to keep the tube below ratings without excitation. Playing with the ratio of fixed bias to grid leak bias can compromise class C operation.

   I just don't see any need to regulate anything here with zener diodes, or VR tubes. Doing so complicates the circuit, and also may compromise the class C tube performance.

   My vote is the overload relay with grid leak bias. This approach will also give you the benefit of protecting the tube from high off-resonance plate current as well as plate current that goes high for any other reason such as a parasitic.

   If the overload relay is undesirable for some reason, then grid leak bias with diode isolated fixed bias has already been proposed. Here when the drive is at the proper level, the bias is 100% grid leak -> ideal. Then with low drive, or NO drive we have fixed bias to protect the tube.

  Now, if this class C tube was also going to run class B as a linear amplifier, then regulating the grid bias (Low Z source impedance) is something to look at. Here we want load regulation, but NOT voltage regulation.

Just one mans opinion.

Jim
WD5JKO

[WD5JKO]

By the way, how would using a single 0B2 tube or a zener diode and then one dropping resistor complicate the grid circuit?

Good question. Let me take a stab at it. The big Eimac tube wants -240v @ 40ma grid current (1st post this thread). So if we use a bias supply along with fixed bias resistor, as Stu pointed out, the supply must be in series with the grid leak resistor. Doing this we lose our path to ground because the direction of current is opposite what the diodes allow (in power supply). We fix this with a zener diode, or a VR tube as a shunt regulator where we connect this device between ground, and the junction of the grid leak and the bias supply. The shunt device must have a series resistor back to the bias supply to limit current. For a VR tube we need to maintain at least 5 ma minimum. Now when we fire up the grid drive at 40ma, this is additive to the VR current from the bias supply, so maybe we have 40+5=45ma when an OB2 is good for 30ma. Also the grid leak resistor needs to provide 240-105=135 volts to get -240. Using a zener instead of a VR has to do with the thermal issues of a zener, i.e. a 5W zener means 5W when the leads are attached to an infinite heatsink at 25 Celsius. In real life, a 5W zener is really like a 1w zener. We can deal with this by adding many LV zeners in series...

Maybe I'm full of it, but this scheme seems to complicate things..

Jim
WD5JKO

[WD5JKO]

Brian,

   The neat thing about ham radio, and hobbies, is that we don't have to design in a way to pacify the bean counters. Making a 'one-of' prototype can use outrageous strategies because we want to experiment. So there is no need for us to always follow the KISS principles. (KISS = keep it simple stupid)

   As an example, I took a Sherwood 30+30W stereo amp with FM, and applied the 'regulate everything' approach. The results were a fantastic boost in unit performance even though the concepts and approach might be ridiculous.

  So here is an example of one of my anti-KISS FOS designs:

http://pages.prodigy.net/jcandela/Sherwood_S8000/HV_REG/S8000_Mods2.htm

   Been going 7 years now....almost daily usage.

Jim
WD5JKO

[WD5JKO]

But where are the vacuum tubes

  Well, the tubes still do all the audio amplification as before. I just modernized the power supply with SS since it would all fit on a 2"X2" PCB using an old CPU heatsink with fan. Doing the same thing with tubes would result in a chassis the size and weight of a Hammerlund SP-600...

   So as a tube jock, do you think that is cheating? If so, do the two diodes in the bias supply (original post) need to be replaced with something like a 6X4? 

Jim
WD5JKO