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New plate transformers: Beavis and Butthead




 
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Author Topic: New plate transformers: Beavis and Butthead  (Read 399 times)
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Patrick J. / KD5OEI
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« on: October 04, 2018, 11:34:34 PM »

I was given this transformer because I already have one like it. Pairs are always 'enabling'  of increased devotion or interest in the hobby. I was told to name them Beavis and Butthead.. OK that's fun. This one's Butthead!

Just thinking on what tubes might work well with them in an AM situation. Could as well be a pair of hi-fi amps, but got enough of them things already!

So, what tubes work well on 645V@300mA? Pairs or other arrangements of tubes would be OK.
two 811s x two 811s? Other tubes? OK to use biased tubes or whatever -this is mainly an exploration.
The voltage and current above are from the calculations and assumptions below.

These transformers ought use choke input filters. Have some 300mA chokes, 20/4H swingers. Plenty of oil caps in the smaller sizes to go with that in a classical way but at the low voltages, electrolytics would be a good choice.

Running Linux on a second hand box, so I don't have any simulation tools right at the moment. PC still broke. Might try WINE and see.
Going by the classic estimating rituals, 1560CT=780V per side. 780*0.9 (choke input estimation) = 702VDC.
390VA/702VDC=550mA DC.
550VA/780VAC=705mA AC.
Hmm not sure about all that but 390W is 390W.

Anyone comment on primary and secondary VA ratings? I think the real wattage is linked to the primary's rating, and the higher secondary rating may account for the different secondary waveforms present when choke input filters are used - that is, with an adequate choke and @ high load, the secondary current waveform is almost a square wave.

transformer impedance seen by a load on the secondary:
r of pri = 0.6 Ohms
r of 1/2 sec = 51 Ohms
V ratio =780/115=6.782
Z ratio=6.782*6.782=46
secondary Z of 51 Ohms + primaryZ of (0.6*46) 27.6 Ohms = 79 Ohms transformer impedance

Voltage drop due the transformer Z, rectifier drop, and choke DCR
Considering the 300mA choke rating for what's on-hand, and going with the DCR of the choke which is 60 Ohms:
voltage drop at full load =0.3 * (79+60) =0.3*139=41.7V
Rectifier drop 15V assumed
702VDC-(41.7+15)= 702-56.7=645.7
So, 645VDC @ 300mA.

With a 550mA choke, voltage drop:
=0.55*139=76.45V
76.45V - 15V rectifier drop = 91.5V drop in DC voltage
702V - 92V = 610VDC@550mA -if I had the chokes

Looks like 650V from this with a 300mA load is in the stars.
My calculations could be off, anyone see errors?


Transformer looks rough but works well. I need to find the little ceramic cones that go over the existing cones. Those cones come up from the inside and are held up by the nuts on the tops of the outer cones. Not much chance of them falling in though, the thing is filled with pitch! Even so they weigh only about 20# each.

Good specs, low DC resistance too.
DC resistance Ohms: PRI 0.6 / SEC 101 (total)
Amertran Plate Transformer
Type PBNC
CYC 55/65
deg C rise 40
POL. ADD
RMS TEST 7.5KV
MAX DC Operating 1KV
KVA 0.390 PRI. / 0.55 SEC.
PRI 115VAC
SEC 1560 CT
SPEC NO. 31268
SER NO. 724915 - they made a lot of these? Military stuff?


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« Reply #1 on: October 04, 2018, 11:53:20 PM »

using full primary with a diode bridge and the beautiful good old 813?
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« Reply #2 on: October 05, 2018, 02:53:41 AM »

Potted transformers have a tendency to not have enough iron to increase there primary voltage from original. I see this one is 115vac. Another 6,7 or 8 volts will drive the transformer into saturation causing excessive current draw and will shorten the life of the transformer. Run it at 115 vac primary and use a doubler for the secondary. Works great.
Regards,
Gary
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« Reply #3 on: October 05, 2018, 08:35:53 AM »

Parallel 6146 would be my choice, my old ARC-38 runs 100 watts of AM with two 6159 as modulators and three in parallel as PA. The 6159 is a 24 volt 6146
The 6146 is small, cheap and plentiful and maybe itís time to move from the nineteen thirties technology of high voltage low current bottles like the 813 to nineteen fifties technology with medium voltages and medium currents?  800 volts is a little light for those old tubes but plenty good for 6146 stages.

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Patrick J. / KD5OEI
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« Reply #4 on: October 09, 2018, 08:18:08 PM »

If 6146 and their 24V brothers are really cheap.. I have a couple of 618S-1 rigs - very familiar with them, love the servos.

I also note the 1KV rating -these are oldie moldie and I would not like to risk them - so it's got to be ye olde full wave center tap with choke input, as the maker intended.

As for the primary voltage I do not doubt the need for low volts will have to run a graph on excitation volts vs current and see.  Shouldn't be a problem to buck the volts a little.
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