• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

the grand return. 3d21A triode PP

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#41
interesting. i guess it's something you want to reserve for specifically high end transformers with good interleaving.

anyway i am still wondering about the plate resistors of the LTP pair. are they calculated how i am thinking, which is simply taking the 130v drop at 3mA per tube and dividing? i'm still a bit rusty with all this business.
 
#42
sorenj07 said:
are they calculated how i am thinking, which is simply taking the 130v drop at 3mA per tube and dividing? i'm still a bit rusty with all this business.
Click to expand...

Yes. But the loadline you used in your original diagram won't be flat. The CCS forces the combined current to be constant, the individual currents will still be moving up and down the loadline formed by the 43k resistor (in parallel with the following grid leak resistor).
 
#43
tikiroo said:
Yes. But the loadline you used in your original diagram won't be flat. The CCS forces the combined current to be constant, the individual currents will still be moving up and down the loadline formed by the 43k resistor (in parallel with the following grid leak resistor).
Click to expand...

thanks! then i think i'm all set with my design. it just bugs me that Mouser has a 50 euro order minimum, and then you get slapped with VAT on top of that. anyone happen to know a better place in the EU for small orders?
 
#46
did all the drafting, and i'm ready to cut some metal! following the age-old wisdom of building them one at a time, to not duplicate any mistakes! i'm also not bothering to mirror them, partly to avoid needless errors.

H0S73Ok.jpg
 
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#47
rsumperl said:
Good afternoon everyone,
I picked up 8 new 7403s and am going to try them in Triode P-P mode. With the Plate/Screen tied and at 500v, any suggestions on idle current?

Thanks
Ray
Click to expand...

those are nice sounding tubes.....https://frank.pocnet.net/sheets/131/7/7403.pdf
34
if MilesPrower is tuned in, can i request for a loading chart?

but my guess, the tube can do 43 watts plate dissipation,
80% of that is 34 watts, so at 500 volts plate, cathode current
is 68 mA......lesser if you want to run them classB..

knowing the tube mu in triode, grid voltage then is equal to (0.63 x 500)/mu
so assuming mu is 5 then grid1 voltage is around -63 volts,
you will have to adjust this to get the desired cathode current...
or if cathode resistor bias is desired, 1000 ohms 5watt resistor will do...
remember that these are all estimates, actual values in the actual build
will be different....
 
#49
at the chosen operating point, 7403 plate resistance is about 1000 ohms,
so that an OPT with 4.4k anode to anode loading is just about right...
AB1 is okey, for drivers, you need more than 126 volts peak to peak swing into the grids...
 
#50
I am throwing in the towel on using the 7403s I picked up. I have a quad set of Bendix and a quad set of Tung-Sol, all new. I will take the loss and sell these for actual shipping cost to the new owner by USPS. They can go out on Monday the 30th. If you want pictures, shoot me an E-Mail to compcar59@outlook.com

Ray
 
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#51
rsumperl said:
I am throwing in the towel on using the 7403s I picked up. I have a quad set of Bendix and a quad set of Tung-Sol, all new. I will take the loss and sell these for actual shipping cost to the new owner by USPS. They can go out on Monday the 30th. If you want pictures, shoot me an E-Mail to compcar59@outlook.com

Ray
Click to expand...
i'm sorry to hear that. i did send you a message though, if you're sure you want to get rid of them.
 
#55
i work with aluminum 1.5 to 3mm exclusively, very easy to drill,
for rectangular holes, i drill 10mm holes on the four corners and
then use a nibbling tool to cut, flat files to straighten up....
for round holes, i use hole saw cutters, i have them from 20mm to 70mm dia.
i also use step drill bits aside from the usual twist drills from 1 mm to 13 mm...
for deburring, i use angle grinders with flap sanding attachments,
i also use small rotary grinders for those hard to reach places....
 
#56
sorenj07 said:
thank you. i use a few unibits and a hand drill. an old knife and x-acto to deburr. and a little hand tap in m3 and m4. the boxes are standard hammond 12x8x2" aluminum.
Click to expand...

I've never owned or used a Unibit. I suppose that if you can manage to get it started on-center (always the trick), it would hold pretty well.. better than a series of progressively larger bits.

So do you start with a stubby center drill, then straight to the Unibit? Or do you size-up first?
 
#57
I imported several hydraulic punches from China can punch up to 5 mills, but I use 3 or 4 mill aluminum; I have square and round punches up to 104 mills only recently found how useful step drills are, also have sets of cross hole counter sink bits and centre drills from China. I have a Japanese hydraulic Punch; one die for this machine cost more than 10 complete sets of the Chinese Hydraulic punches.
 
#59
hey-Hey!!!,
The 3D21A, like some cousins like the 6146 are best run as pentodes with g2 set to tune the load line and g1=0V operating envelope. It is a fine tube when run within its limits. I consider it a cross between a 6V6 and a 20W TV sweep.
cheers,
Douglas
 
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