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Old 22nd March 2004, 04:39 PM   #11
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Hi,

Quote:
my trade notes mentions this was bad practice on output tubes. Can anyone illustrate why this is detrimental to tube life.......?
Actually it isn't bad practice provided you don't try to draw current before the cathodes have fully warmed up.

I wouldn't implement it in a commercial offering but leaving the caps charged keeps them formed. Well electrolytic ones anyway.

Quote:
I use 22R 25W WW res in series with prim winding 650VA tranny.
Primary as in on the mains side of the xformer?

Cheers,
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Old 22nd March 2004, 11:53 PM   #12
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Quote:
Originally posted by fdegrove


Primary as in on the mains side of the xformer?

Yes...my power demands are somewhat different...That triple B+ switchable amp alot want to see....(photo here); B+'s can be selected for tube longevity (and different types) to select Pout's of 25W+25W; 130W+130W using parall-paired 6550's; or full wacker 200W+200W (>800W bridged) at 600V B+ with 8x JJ KT88's. That's the reserve headroom for my trumpet.... Note with boost mode switchmode technology the comparative size of mains tranny compared with each o/p transformer. Total chassis weight ca, 65kg.
For power transformers above 150VA, I'm against fitting soft start resistors in H.V secondaries due to risk of serious arcing assisted by winding parasitics resonating by failing fuse..

With constricted chassis space I wound the E&I beast (in centre) with a higher B max (1 to 1.35Twith a higher temp rise than orig) ....Drawback is that primary inrush current is higher and without prim start ballast resistor would annoyingly knock out AC mains currrent limit breaker and blow primary fuse. It behaves somewhat like a <soggy toroid>... . Trick is to select resistor rating, so combination of transformer primary winding resistance and of B+ cap(s) can charge up over several AC cycles. I can use a F3.15A fuse without popping on a 650VA transformer.
An advantage with start up ballast in primary is a lower fuse rating can be used, so long the soft-start relay can "knock out" fast enough on a sudden supply interruption, i.e before the supply kicks back on the peak of the following cycle.

For those interested in designing powerful tube amps with large 2:1 current swings, protection watchdogs are essential to save the tubes and output transformers in failure conditions. Unnecessary complication....No.....not with todays electronics. Sequencing logic between neg bias and B+ is easily done......with inhibiting lockouts and hot switching.

For some years I've been using discharge resistors in series with 30V zener diodes across each set of hi C value electrolytics esp in stepped cap arrangements to boost voltage ratings, ......as you rightly mentioned to keep the electroyte film healthy & encourages cap memory effect....it also greatly reduces on-surge start-up currents.......Others take note...

Viva..........800W at 0.25% THD...... the ultimate in gobsmacked trumpet power.......or deepest pedal bass.....

rich
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Old 23rd March 2004, 12:03 AM   #13
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Hi,

Quote:
it also greatly reduces on-surge start-up currents.......Others take note...
Good idea, I'll try to remember that.

Mighty big amps you have there...

Cheers,
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Old 23rd March 2004, 10:21 AM   #14
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Yes Frank.......with brilliant spec...that will be the last of mammouth designs.....only a handful of well made tubes will take that slaughtering B+ voltage...... I will concentrate on preamps...and more humble things..enclosed small glimpse THD graph (if this comes through). I will ztry and get some soft start arrangemnts on this for others to get ideas.

rich
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Old 23rd March 2004, 12:40 PM   #15
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Default 10nF snubber cap

Hi arnoldc

Maybe adding a 0.01uF 2kV ceramic cap across the 350V secondary before the sand bridge to reduce rectifier swithching noise. I know it worked miracles in my SS rectified tube preamp. Couldn't hurt. And no I aslo don't think 17 sec is too long. I would also change the first cap (470uF) to a lower value, say 220uF. Less chance of power supply ringing and your power transformer would run cooler. I'm guessing you take your power for the output section after the 5H choke, correct?

Cheers
Wayne
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Old 23rd March 2004, 01:01 PM   #16
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Quote:
For some years I've been using discharge resistors in series with 30V zener diodes across each set of hi C value electrolytics esp in stepped cap arrangements to boost voltage ratings, ......as you rightly mentioned to keep the electroyte film healthy & encourages cap memory effect....it also greatly reduces on-surge start-up currents.......Others take note...
Good idea, get the ole soldering iron heated up!
Also having a fairly large cap say around 68nF - 100nF across the mains switch also allows a small current to flow although not nearly enough, 5V or so in my amp's PS IIRC.

Wayne
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Old 24th March 2004, 12:15 AM   #17
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Default Re: 10nF snubber cap

Quote:
Originally posted by cogsncogs
[B]Hi arnoldc

Maybe adding a 0.01uF 2kV ceramic cap across the 350V secondary before the sand bridge to reduce rectifier swithching noise.
hi wayne, i've seen on some schematic that each diode is shunted with these ceramic caps. will it have the same effect as you described? or i can do both?
Quote:
I know it worked miracles in my SS rectified tube preamp. Couldn't hurt. And no I aslo don't think 17 sec is too long. I would also change the first cap (470uF) to a lower value, say 220uF. Less chance of power supply ringing and your power transformer would run cooler. I'm guessing you take your power for the output section after the 5H choke, correct?
i used PSUD to model the power supply and i will surely try the 220uF. yes, i do take the power for the EL34s after the choke. i was planning to have another choke for the driver section sometime soon.

thanks wayne!
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Old 24th March 2004, 12:37 PM   #18
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Smile Re: Re: snubber caps

Hi arnoldc

Quote:
hi wayne, i've seen on some schematic that each diode is shunted with these ceramic caps. will it have the same effect as you described? or i can do both?
You can do either or both or all the way around! Here's where a good dual trace 20mHz or better scope comes in handy. If you don't have one or can't borrow one use your ears. Try different combinations, it's different for each Pwr xformer, grounding scheme etc. I've attached a generic schematic. All caps are 2kV.

Wayne
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Old 24th March 2004, 01:11 PM   #19
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Default Re: Driver section

Quote:
i was planning to have another choke for the driver section sometime soon.
You could try a 1N4007 in series with a dropping resistor in place of the choke. What I've heard called a poor man's regulator. That's what I have in my amp. Say the output section draws enough current say on a signal peak or a sustained 19Hz organ pedal note!, and the voltage where the output section taps from drops below the voltage where the driver section taps from. The diode prevents the driver section bypass cap from discharging back into the output section bypass cap. At least that's the theory behined it.
I hope I explained that clearly.

Also I'd like to add that if you're using your ears to check for rectifier switching noise, it sounds like buzzing at your pwr line freq. Altough the actual frequency is in the hundreds of kilo hertz to mega hertz range. You may not be able to actually here it at all, that's where a scope comes in handy. I do know from my experience, once you get rid of it your amp/preamp sounds much cleaner. Opens everything up!

Try it!
Wayne
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Old 25th March 2004, 01:16 AM   #20
arnoldc is offline arnoldc  Philippines
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hi wayne,

you made a mention about the low bass...

the amp i prototyped got a very good midrange in which i was surprised myself. however, it definitely is lacking in the bass department, more so low bass. it is like it's there but it isn't. like running out of steam if i may say.

i am trying to pinpoint the cause and am looking at the output transformer which is custom wound 5K p-p 80mA per side, or the power supply.

the PSU if i may add uses Lundahl LL1648, which i believe is rated at 600mA but only 60% of that is available at how i use it.

a) i thought the OPT might be saturating, so i asked for a 6k6 p-p 100mA per side to me made for me

b) change the Lundahl power transformer

i'd appreciate if you could share your experience on this having built that magnificent PP amp of yours.
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