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Old 22nd March 2004, 12:47 AM   #1
arnoldc is offline arnoldc  Philippines
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Default Question about PP EL34 power supply behaviour

Hi all,

I made a PP EL34 amp and I have a question on its B+ behaviour.

The simple power supply is

350V -> bridge (1N4007) -> 470uF -> 5H -> 470uF

It takes 17.5 seconds for the amp to spit out any sound. Is my heater supply "slow" and the EL34 conducts longer? The B+ goes to a high of 460V first before settling down to 416V (420V target) after 20 seconds.

Is there a danger to this behaviour?

Cheers!
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Old 22nd March 2004, 12:57 AM   #2
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Hi,

Quote:
Is there a danger to this behaviour?
That will depend on the insulation of the cap behind the bridge.

At worst you'll see 350*1.4 = 490VDC provided the mains is stable.

If you want to reduce it use some resistance in both legs of the primary or use a NTC.

Cheers,
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Old 22nd March 2004, 01:54 AM   #3
arnoldc is offline arnoldc  Philippines
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hi frank, thanks for the info. so does that mean the 17.5 second "delay" is ok?
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Old 22nd March 2004, 02:17 AM   #4
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Hi,

Quote:
If you want to reduce it use some resistance in both legs of the primary or use a NTC.
Dang....I meant to write secondary, the winding you use for your B+, not the mains primary, that would bring all voltages down.

Quote:
so does that mean the 17.5 second "delay" is ok?
I don't know what tube(s) are causing this but don't expect a tube amp to play full blast after 20 secs of warm up.
If you should do this expect damage to the output tubes' cathodes (stripping away emissive materials).

17+ seconds for any audible sound to come through seems rather long, OTOH some people go to great lengths to achieve just such a slow start....

IOW, without seeing the actual circuit it's hard to tell what causes the slow ramp up.

Cheers,
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Old 22nd March 2004, 03:44 AM   #5
arnoldc is offline arnoldc  Philippines
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Quote:
Originally posted by fdegrove
Hi,



Dang....I meant to write secondary, the winding you use for your B+, not the mains primary, that would bring all voltages down.

I thought I read secondary

I have a pair of 12R 50W resistors here which I can try. But with 500V on the caps, is that something I will be worried about?
Quote:
IOW, without seeing the actual circuit it's hard to tell what causes the slow ramp up.

Cheers,
frank, it is this one - I want to make a simple UL PP amp
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Old 22nd March 2004, 04:07 AM   #6
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Hi,

Quote:
I have a pair of 12R 50W resistors here which I can try. But with 500V on the caps, is that something I will be worried about?
550 V would be safer if you can lay your hands on one....as a figure of speech I mean.

I had a look at your circuit, it doesn't show the power supply so I'm still in the dark.

If you used big caps in the PS it's going to take some time for them to charge but other than that I wouldn't worry about the warm up delay if I were you.

Leaving a tube amp to warm up for half an hour is common practice after all.

Cheers,

EDIT: just saw it when submitting the post:

Quote:
350V -> bridge (1N4007) -> 470uF -> 5H -> 470uF
That's big enough to explain the delay.
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Old 22nd March 2004, 05:42 AM   #7
arnoldc is offline arnoldc  Philippines
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thanks frank!

Quote:
550 V would be safer if you can lay your hands on one....as a figure of speech I mean.
yeah, not even when the power is off
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Old 22nd March 2004, 05:53 AM   #8
SHiFTY is offline SHiFTY  New Zealand
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EL34s have huge cathodes that take a long time (about 20 secs!) to warm up and start conducting. But they have great transient response as a result. More electrons = more peak current!

I would install a B+ (standby) switch after the diodes but before the first filter cap. This allows you to warm up all the tubes before you slam the high voltage on them. You could rig a timer circuit with a 555 chip and a relay if you only want one switch.

Also I find 1n4007 a little weedy for my liking. You should be using 1N5408 at the very least!
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Old 22nd March 2004, 06:54 AM   #9
arnoldc is offline arnoldc  Philippines
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hi shifty,

all my set amps have standby switches. i thought about that for this amp but left it out in the process. so in it will go

i'll replace the diodes with your recommendation.

thanks!
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Old 22nd March 2004, 03:19 PM   #10
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Hi there......in my service racks I always replace discrete PSU HV diodes with encapsul bridge types.......Diodes on same Silicon die make less noise.....

Soft start posibilities is large......on all my t'amp inrush circuits ..I allow nearly half- minute for KT88/6550' amp to warm-up....I use 22R 25W WW res in series with prim winding 650VA tranny. This W.W res is S/C by relay after 15 secs. ......after this (soft-start relay) has closed, allow another 10 secs for interstage B+ to reach spec volts .....total before sound gets through = 25secs approx. Nothing unusual.....

General rule; electrolytic caps are usually run at between 0.8-0.9 of stated working voltage. Don't go higher unless surge value is stated.

I'm not sure about the effects on tube life with B+ already ramped up at peak 1.414 AC= DC volts before tube anodes + g2 are conducting..... my trade notes mentions this was bad practice on output tubes. Can anyone illustrate why this is detrimental to tube life.......?

rich
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