QB2/250 amplifier

[Eli Duttman]

Send a PM to "Bandersnatch". Doug did work on PP 813 circuitry and his experience should be of benefit.

[Fuling]

Thanks for reminding me, I have several boxes full of stuff that I´ve paid good money for with the good intention of building an 813/845/211/GM70 SE amp. Lundahl OPTs and ITs, various HV PIO caps, a bunch of power transformers and chokes. I´m leaning towards 813 as I have a few pairs already, what´s stopping me is those darn 10V 5A filaments. AC heating is not an option and any kind of non-switching DC PSU will be massive. Though, I really should build this kilovolt, top capped beast while I´m still a single man without kids or pets...

[flatheadmurre]

The kids and wife are HV trained, if it glows stay clear !!!

Have to start checking the supplys for serious HV parts.

On top of everything else a Scania truck descided to take a bite out of my pickup today so new car is on the shopping list .

[Eli Duttman]

I´m leaning towards 813 as I have a few pairs already, what´s stopping me is those darn 10V 5A filaments. AC heating is not an option and any kind of non-switching DC PSU will be massive.

AC heating might not be as bad as you think. Between cancellation in a PP O/P trafo and individual hum balance pots., you could easily have an acceptable residual hum level.

[revintage]

AC heating is not an option and any kind of non-switching DC PSU will be massive.

Me and hoktuna let Rod Coleman build us custom CCS/Gyrator heater circuits. Hoktunas for GM70 and mine for 813. Should be no problem using switched supply to drive them. Fuling, I remember yours got hickups at upstart.

The SE topology below can be doubled and used for PP if driven by an 1:1+1 input transformer. Pentode effiency and triode characteristics.

[flatheadmurre]

The breadbord is coming together slowly.



Managed to wind a filament transformer with dual secondary and electrostatic shield.

Shottky rectifier, small commonmode choke.

6x3300uF 50V Sprague, Filament choke another 6x3300uF.

15.3v raw dc 75mV ripple.

Rods filamentreg.

I probably tried 10 different chokes i found in storage non did the job, had to wind one.



Time to decide what HT rectifiertube to use.

Found some nice ones in the storage but 21A heating ruled them out.

6 pcs of GEC U-19 is perhaps a choice or the 3B28 i first thought of.

[flatheadmurre]

813 up and running 930V 80mA catode bias triode.



Until my OPT´s arrive a 5K has to do.

Next is driver D3a C3g C3m E80L ??

Suggestions ?

Perhaps first D3a into 813 triode then i have to try Revintage schematic.

And it´s time to clean the bench before something happens .

[Fuling]

Nice work. I recognize some stuff from Bhiab

[flatheadmurre]

Thanks Fuling.

Bhiab is a treasure chest .

D3a triode up and running.

Not a good bypass cap or DN2540 in the boxes .



At 85mA it comes alive, little scared to turn it up more with the small OPT.

Found some nice supercaps to try out for the D3a.

Why the ...isn´t there anyone selling IXUS or SUPERTEX around here.

[v4lve lover]

dc heating whit a LM396 or a LM317 whit two mj2955 use a surplus Power toroid as a choke if you have a too high voltage .

lm350t at €0,90 each you could paralel four of them . filter caps will be hefty at least 10,000 U

or AC
if you could get 2*12v you could earth the centertap and use two rheostats as hum potentiometers

have a good one dont fry yourself