Valve Pulser stage

[Peterae]

I wonder what the costs involved are in putting a valve stage together.

SM gave some clues what he used as follows

I am using 6BQ7-A tube for the input and phase inverter because they are Vhf amplifier Triodes.
12.6V heater
But it does prove that my output transformer is capable of going up to 245kHz.

so because he was using a phase inverter he was using a push/pull stage, these would require a matching transformer, i think this is probably the hardest part to obtain, in the above does he mean that this transformer needs to be capable of up to 245kHz if so surely this is way above a conventional valve transformer and so i guess it would need to be specially made using a ferrite core.

If i can work this out then maybe i will try a valve stage.

I would imagine that our transformer needs to drive say a 1 Ohm coil up to 245kHz from the valve push pull impedance.

[Peterae]

I am seriously looking now at building 3 push pull valve stages, the stages will run @ 35kHz , 70kHz and 105kHz.

My first problem is making the matching transformer, i am going to use 500V-0-500V for the valve drive voltage and work on the basis of 250mA supply for now, this may yet change.

Can i design the transformer to have an impedance @ 35kHz of around 6000 Ohms for the valves to drive, or do i need to wind a transformer with a dc resistance of 6000 Ohms, then it should be a matter of winding a low impedance secondary to match the coil under test.

@ 35.705 kHz for a 6k Ohm impedance i need an inductance of 26.74 mH
@ 71.410 kHz 13.37 mH
@ 142.820 kHz 6.686 mH

**Edit i need to recalculate these for 35.075,70.150,105.225

Now there is some software by ferroxcube which having put in my parameters has selected my cores and the wire size and the number od turns.

@35.705 kHz
E25/13/7 E ferrite core
Core Type 3F3
Wire Size 0.224 mm
Turns 535
DC resistance 15 Ohms

Am i on the right track so far or am i wasting my time trying to attempt this.

[muDped]

Yes, you're on the right track.  For efficiency considerations
keep the DC Resistance of your transformer windings as low
as is practicable by selection of a suitable wire size.

As a matter of curiosity, what types of Valves (Tubes) are
you planning on using?

[Peterae]

Hello Dumped

Thanks for confirming my direction on the transformer, now that's interesting because there were choices of cores that gave me less turns and a much lower dc resistance.

Not sure on the valve's yet, just doing a dummy run of ruff parameters, my main problem was the transformer at these frequency's so wanted to confirm with you guys i am on the right track, then go over all the parameters and try building 1 stage.

I will now start looking for easy to get, triodes that fit the bill, and alter accordingly, as soon as i work it out i will post the info.

[Peterae]

I have been doing a bit of digging around, it looks like i am going to be in the range of about 25 Watts power stage and will hopefully use 6336A dual triode valves for the output push pull stage.

Am i missing something here, the data sheets i have seen do not mention anything about frequency, no charts and no data, i guess it's assumed you use them for audio range.

Anyone know how to find this for each valve, i need to be good for 256kHz if i am joining the SM club.

[muDped]

...

Anyone know how to find this for each valve, i need to be good for 256kHz if i am joining the SM club.

Try HERE.

Data Sheet links are at the bottom of the page.

Most vacuum tubes are capable of many MegaHertz without any problems.

Have you seen this page yet?

[Peterae]

Hi Dumped
yep seen both of those, the circuit diagram is finally the thing that made me decide of the 6336A along with ease of availability and cost and also being a dual triode in a single tube.

I had also seen the data link for the valve as well, but again i cannot see any frequency data for it, unless i am missing something.
Still SM only said his transformer was capable of 245kHz and that circuit goes to 130kHz and is probably limited by the output transformer, so i think i am almost in a position to start working on the construction parameter's.

So for this valve and low distortion 8K Ohms primary impedance transformer was used.
I think i will try using toroid's for the core, harder to wind but i will give it a go.

Not yet sure whether to use the vhf tubes SM mentions for the pre stages and what implications this will have with component values, i think i remember someone saying there was a valve simulation tool somewhere, maybe my best bet to try and sim the whole thing first.

Cheers
Peter

[Peterae]

Just spent some time trying to work out how to use LTSPICE to simulate some valve circuits, it was going well until i wanted to add a 6BQ7A triode for which there was no model, i found this on the net and added it but Spice says there's a syntax error, anyone familiar see what the error is so i can continue, i just don't know enough about spice to work it out.

*
* model for one half of 6BQ7A
*
* heater not included
* parameters captured by curvecaptor curvetracer mode from spec sheet
* 23 April 2005
*
* 6BQ7A LTSpice model
.subckt 6BQ7A P G K
Bp P K I=(0.02394315169m)*uramp(V(P,K)*ln(1.0+(-0.03717325334)+exp((3.394790097)+(3.394790097)*((4 3.16466529)+
+ (-237.6116265m)*V(G,K))*V(G,K)/sqrt((34.2053511)**2+(V(P,K)-(8.721051942))**2)))/(3.394790097))**(1.400664452)
G2 G K VALUE = {IF(V(G,K)>0, 0.000918214796820349 * ((( 27.9774118982126 +V(P,K))/( 21.3125760081481 +V(P,K)))**4)
+ *V(G,K)**1.5,0)}
Ccg K G 2.6p
Cgp P G 1.2p
Ccp K P 1.2p
.ends 6BQ7A

[muDped]

Hi Dumped
yep seen both of those, the circuit diagram is finally the thing that made me decide of the 6336A along with ease of availability and cost and also being a dual triode in a single tube.

I had also seen the data link for the valve as well, but again i cannot see any frequency data for it, unless i am missing something.
Still SM only said his transformer was capable of 245kHz and that circuit goes to 130kHz and is probably limited by the output transformer, so i think i am almost in a position to start working on the construction parameter's.

...

Cheers
Peter

Yes, vacuum tubes aren't plagued by the frequency cutoff
characteristic which limits transistors.

Back in the day there was a Citizens Band transceiver kit
made by Allied Radio in Chicago which used the 6AQ5
Beam Power tube as the RF Final.  This tube was generally
used as the audio output stage in radios but was fully
capable of operation at 27 MHz and beyond.

The circuitry which surrounds the tube (as well as its mode
of operation) will limit its frequency.

[Peterae]

Cheers Dumped

wow right that explains why i cant find the data, so it would appear i am OK to use the 6336A without worrying about top end drop off.

So far i have not found a phase splitter circuit using the 6BQ7A triode, i figured i might as well follow SM's words to the full and use the same valves at least.

I am currently trying to get the Sim up and running, bit of a steep learning curve right now, never used the Sim before.
Life would be easier if i could find a model for the 6BQ7A that worked.

[Peterae]

I seem to have my sim up and running

Here's the splitter circuit and sim results @ 150kHz

The nearest western equivalent to the 6BQ7A is the 6N1P and i found a Spice model for it  

[Peterae]

First up was to get hold of a power supply transformer, i decided for the experience i would re-wind a microwave transformer.

I hacksawed through the 2KV winding to remove it and used layers of paper impregnated with varnish for insulation.

I found a website here http://ludens.cl/Electron/trafos/trafos.html

This was real handy it gave me a spreadsheet calculator for working out the turns i needed.

First i worked out that the transformer i had was using a flux density of 1.8249 Tesla this was worked out by using the area of the center steel lamination size and the existing secondary windings for the magnetron heater.

I wanted secondary taps of 310V,320V and 330V 320V being optimal for my build giving me my +440V HT Rail.
I allowed for 5% loading in the calculations.

So i had to wind 448 Turns for the 310V which was calculated to be 101 Meters of wire
320V 462T
330V 476T

I then wound a thick wire for my heaters.

My main problem with this transformer is noise and heat, the thermal transfer coefficient came out at 11.767 which means the manufacturer wound the primary to the maximun for air operation and was bordering on needing a fan for long periods of use, 12 is the point which a fan is recommended.

Here's a picture of the finished transformer along side with the 6BQ7A valves i picked up on eBay.