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tubecad el84 powersupply ps4 question

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Hi all. I´m thinking of building the Tubecad el84 pp, triode-strapped as in the stock schematic. Now, there is obviously something I don´t understand - using the PS4 powersupply as suggested;
I don´t understand how one can get 340vdc B+ from 250 - 0 - 250 secondaries with a GZ34. Please somebody, enlighten me.
 

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Peak voltage is (√2/2) X 500V (250+250) = 353.5V
But there are the 100R resistors in circuit, 2 of them, I suppose 70mA flows through, dropping about 14V across both. 340V should remain.
Now I have a question:
The PSU section has gone into the trouble of generating a super clean heater supply (voltage regulator etc.) but then it elevates this supply by 85V of potentially noisy line!
Why?
I would have used a high value/low current choke, and a capacitor to clean it up.
 
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Resistors in series with a rectifier may drop more or less voltage than you might expect from a DC calculation.

They could drop more voltage, due to the actual current through them being pulses with a peak value much higher than the average DC current. This is what you might see when a high DC current is being drawn.

They could drop less voltage, as at the end of the charging cycle (which is what actually determines the output DC voltage) they carry little current. This is what you might see when a low or middling DC current is being drawn.

KenTajalli said:
The PSU section has gone into the trouble of generating a super clean heater supply (voltage regulator etc.) but then it elevates this supply by 85V of potentially noisy line!
Where is this noisy line?
 
Where is this noisy line?
From the 100k/300K ladder.
(B+) is not as clean as the one from voltage regulator.
Frankly the tubes can handle 340V - the elevation for this amp is not needed, I would omit it. A simple HV fuse is sufficient.
Come to think of it, the DC heater supply is not needed in the first place.

BTW - the two 100R's are
1- at rectifier
2- between the two 47uF
if the last 47uF is made bigger, then the pulses in current draw are less of an issue, but generally - yes, you are correct.
 
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I can see that there is a lot I need to learn. The reason I wondered was that in other powersupplies I´ve seen, take morgan jones bevois valley as an example, there 300 - 0 - 300 thru a gz34 gives 330vdc. and I couldn´t really (still can´t) understand the huge differences.
 
KenTajilli said:
From the 100k/300K ladder.
(B+) is not as clean as the one from voltage regulator.
You may be confusing two separate issues:
1. the clean (probably unnecessarily so) heater supply from the regulator
2. the slightly noisy heater bias from the potential divider on the HT/B+ rail, but decoupled by the 0.1uF cap

Note that heaters were designed to run from AC, although I admit that modern valves have a reputation for poorer heater-cathode insulation than NOS ones.

Come to think of it, the DC heater supply is not needed in the first place.
Yes. DC heaters may have a place in mike or phono preamps, but not needed elsewhere.
 
You may be confusing two separate issues:
1. the clean (probably unnecessarily so) heater supply from the regulator
2. the slightly noisy heater bias from the potential divider on the HT/B+ rail, but decoupled by the 0.1uF cap

Note that heaters were designed to run from AC, although I admit that modern valves have a reputation for poorer heater-cathode insulation than NOS ones.


Yes. DC heaters may have a place in mike or phono preamps, but not needed elsewhere.
No confusion here. You call it "bias" I call it "elevation".
My point was that the heater supply is regulated, cleaned up for a purpose.
The heater bias(elevation) defeats it!
I would run the heaters on AC and forget about the "bias"!

As regards to Morgan Jones circuit, it all depends on power rating of the transformer.
Under "load" voltages are dependent on this.
BTW I like the Morgan Jones circuit better, amp included!
 
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KenTajilli said:
My point was that the heater supply is regulated, cleaned up for a purpose.
I am not entirely convinced of this. As a DC supply is not even a requirement, a clean DC supply might not be a requirement either. I suspect people use a regulator just because it is easy to do, and because everyone else seems to do it. The heaters don't need a regulated supply, just one which is sufficiently close to the right RMS value and without too much HF noise.

Let's assume that the coupling from heater to circuit (mainly the cathode) is capacitive. We know that 6.3V AC at 50Hz is good enough, so 15.75mV AC at 20kHz will be good enough - 0.25% ripple on a 6.3V DC supply. This is not too difficult a requirement, but using a regulator is an easy way to achieve it. In reality the coupling will not be purely capacitive so somewhat greater noise voltage may be tolerated.

Now consider the heater bias. 300k and 100k give a source impedance of 75k, with 0.1uF. This gives a corner frequency of about 20Hz. So provided the ripple on the main supply rail is less than 15.75V we are OK.
 
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