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Old 17th April 2015, 05:29 PM   #1
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Default tubecad el84 powersupply ps4 question

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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Old 17th April 2015, 06:26 PM   #2
DF96 is offline DF96  England
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250V will be an RMS value. The peak value of the sine wave will be around 350V. A rectifier feeding directly to a capacitor will give you the peak value. Look for info on 'capacitor input power supply'.
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Old 17th April 2015, 10:36 PM   #3
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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.

Last edited by KenTajalli; 17th April 2015 at 10:55 PM.
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Old 18th April 2015, 11:15 AM   #4
DF96 is offline DF96  England
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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.

Quote:
Originally Posted by KenTajalli
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?
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Old 18th April 2015, 11:38 AM   #5
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Quote:
Originally Posted by DF96 View Post
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.

Last edited by KenTajalli; 18th April 2015 at 11:49 AM.
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Old 18th April 2015, 12:03 PM   #6
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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.
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Old 18th April 2015, 12:10 PM   #7
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can you dig up the circuit diagram for the morgan jones?
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Old 18th April 2015, 12:23 PM   #8
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Not entirely sure I´m supposed to do this, but if not, the moderator will have to remove the post ( copyright and stuff...)
But;
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Old 18th April 2015, 12:39 PM   #9
DF96 is offline DF96  England
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Quote:
Originally Posted by KenTajilli
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.

Quote:
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.
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Old 18th April 2015, 01:02 PM   #10
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Quote:
Originally Posted by DF96 View Post
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!

Last edited by KenTajalli; 18th April 2015 at 01:05 PM.
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