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 Post subject: Cold startup B+ surging too high
PostPosted: Aug Mon 06, 2018 4:01 pm 
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Location: Lexington, SC
Working on Fisher 50A audio amp. The nominal B+ is 460V when everything is warmed up. The amp plays well.

Problem is arcing is heard inside the audio output transformer during a cold startup. Monitoring the B+ shows it going to 600V before settling in.
This is hard on the 525V filter caps as well.

Thought about various solutions such as thermistors, nominal load resistor or zeners on the B+, time delay circuits, etc. But none seem all that clean.
The high voltage xformer is 550VAC using 5R4 rectifier tubes with 1614 output tubes. Its a bit of a beast! Any suggestions? Thanks.

Richard


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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Mon 06, 2018 4:43 pm 
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Is the 5R4 the original type rectifier for the 50A?


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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Mon 06, 2018 5:30 pm 
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Hi Johnnysan,

No.
It was missing all the high power tubes. Label on the chassis says 5AW4. Uses a rectifier on each leg of the HV winding for a total of 2.

Richard


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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Mon 06, 2018 5:58 pm 
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The 5AW4 has a greater voltage drop , and current capability . I also also draws 3.7 to 4 amps filament current each , so using a 5R4 it would be drawing about 4 amps less filament current allowing the B+ to ride up even higher .

http://frank.pocnet.net/sheets/049/5/5AW4.pdf

http://frank.pocnet.net/sheets/191/5/5R4GB.pdf

You may have a bad output transfomer if it is arcing .

I have a couple NOS 5AW4's if you need a couple .

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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Mon 06, 2018 6:14 pm 
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Hi battradio,
You bring up an excellent point about higher filament current helping to hold down B+ in all phases of operation.
Fingercrossed about any damage to output transformer. Using it on a variac for now.

Richard


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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Tue 07, 2018 3:56 am 
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I've had a few output transformers arc during power up under normal voltage conditions, even with the surge using directly heated rectifiers.

they should not do that.

they ended up being defective and the arcing proved it.

it is like the day 11 years ago at 37 when my blood pressure was 210/135. if I would have had an aneurysm, it would have popped that day, believe me.

steve

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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Tue 07, 2018 6:08 pm 
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Replace the rectifier with an indirectly heated type. These have to warm up and take longer so the other tubes can get heated and load the supply down. Also don't forget that equipment was designed for a lower line voltage and you may be hitting it too hard and it is recommended that you use a bucking or auto transformer to lower the line voltage.


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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Tue 07, 2018 7:18 pm 
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http://frank.pocnet.net/sheets/035/5/5AR4.pdf

http://frank.pocnet.net/sheets/049/5/5AW4.pdf

I would rather see a timed relay that had a 2 to 3k ohm 20 watt load switched in for about 45 seconds till all the other tubes warmed up on this amp and a couple of movs in the ac line .

Two 5AR4's would take 10 watt's load off the power transfomer in filament current and have 30 volts less drop across the rectifier tubes !

Mark

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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Tue 07, 2018 8:20 pm 
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Audioman wrote:
Replace the rectifier with an indirectly heated type. These have to warm up and take longer so the other tubes can get heated and load the supply down. .


true, but be careful, though because this is not a good practice and it alters circuit design.

although it would work, you do not want to use another rectifier that has a filament current rating higher than the original.

for example, i had an RCA that used a 5W4 that gave a nice surge at turn-on until it heated up. a solution was to use a 5Z4 that had nice controlled warm up time, but the B+ was a bit higher and it drew an extra half-amp on the transformer for the filament.

i'm not loading up an 80 year old transformer with even 500 mA these days.

i don't recommend changing the rectifier, but opinions may vary.

steve

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Last edited by Dutch Rabbit on Aug Tue 07, 2018 11:27 pm, edited 1 time in total.

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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Tue 07, 2018 9:18 pm 
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Location: Albuquerque, NM 87123
I'm not a fan of changing circuit design unless it is proven the original design was flawed. Fisher usually didn't make mistakes; if this were a Heathkit I would say change anything to make it work as their cost-cutting measures resulted in some pretty questionable products.

As stated before the arcing inside the output transformer is a bad sign; even if voltages are permanently kept low you may have trouble in the future. If that were my amplifier and I had no intention of selling it I might consider the time-delay relay route.


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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Wed 08, 2018 12:31 am 
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Hmmmm.......I had the slightly newer model 50-AZ and the output transformer went south. I wonder if it’s a weakness on these.

-David


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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Wed 08, 2018 12:46 am 
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there certainly are some "trends" in this business. whether or not this is one of them, i have no experience to comment.

OTOH, as for crosley power transformers and late 30s RCA output transformers, i have lots if experience with those "trends" :( .

steve

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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Wed 08, 2018 1:04 am 
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There is a .0022 cap in the transfomer can in some versions that may be breaking down .

Image

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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Mon 13, 2018 2:47 pm 
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Settled on a pair of NOS 5AR4 rectifiers. No surge, about 10V higher B+ than before.
JJ brand 5AR4s I tried were arcing internally from PIV exceeded and blow fuses.

The 5AW4s are the original spec but they would surge to 600V and settle back to 455V.
Didn't like the surge on the 500V power supply caps.

The 5R4s first used were going well past 600V on cold startup.

Thanks for everyone's insights and special thanks to Mark.

Richard


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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Wed 15, 2018 1:08 am 
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I always install a CL-90 surge limiter (thermistor) on old tube-type electronics. It saves wear and tear on the rectifier, etc. And compensates, a little, for higher modern a.c. line voltages.

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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Wed 15, 2018 2:42 am 
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i am working on a crosley right now. the second filter surges at 350 and then idles at 188.

the turn on surge can be almost twice the value of the "normal" voltage once all settles in.

steve

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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Sun 26, 2018 1:54 am 
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You have two choices I see,
1 - Replace the speaker transformer,
if it is arcing at 600v it won't be long before it's arcing at 460v
it's on the edge.

2 - instal a time delay circuit so that the HT is only conectted
after it has charged up the caps and has settled.
But I think this is just delaying the inevetable.
replace the transformer !


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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Sun 26, 2018 4:40 am 
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here is an interesting first hand experience from days gone by that i love to tell:

i had a large late 1950s bogen 50 watt amplifier on the bench playing away right after it was fully restored with capacitors, resistors, and tubes.

at the end of my bench, i had my trusty little 13" tube black-and-white tv with the two built in whip antennas watching the price is right from KDKA Ch 2 in Pittsburgh. the picture had some snow, but was totally watchable up here at our elevation in central pa.

when a good song came on and i turned up the restored amplifier's volume that morning, i noticed that the tv picture would get some interference/noise type horizontal lines that appeared on the tv picture when a bass drum note was hit in the music playing thru the bogen amplifier.

also, these horizontal noise lines increased in quantity and thickness when a bass note was hit in the music with respect to the volume control on the bogen amp.

my 15" three-way bench speakers are right under the bench so it was [what i believed to be] a no-brainer that a tube or something inside the tv did not like the vibrations. Realistic Mach Ones (original series) speakers will move some air and easily vibrate the entire bench even when they are not given some power.

i popped off the cover to the tv and began tapping the tubes and areas all around the chassis looking for a loose connection or a defective tube inside the tv.

while i was "debugging" the tv, the amplifier was still pounding away some hard-hitting rock as it was all morning and at this point, it was bellowing "my sharona" by the knack.

all of a sudden the music quit and when i looked over at the amp, a nice stream of smoke was emanating from the windings of the output transformer.

also, there was no more interference on the tv.

there was internal arcing in the output transformer during loud bass notes that caused interference on the nearby tv with OTA reception.

with a poof of smoke, Sharona broke another heart and killed my amplifier :cry: .

---

don't mask off or band aid the problem. fix this set properly because the transformer must go, period.

it will only set somthing/someone up for a bigger disaster down the road.

opinions might vary.

steve

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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Thu 30, 2018 1:54 am 
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Note to myself: Should pick up that 13" TV for my diagnostic instrument shelf. :lol:

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 Post subject: Re: Cold startup B+ surging too high
PostPosted: Aug Thu 30, 2018 3:25 am 
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it is long gone.

the flyback eventually started to arc internally--showing lines on the screen EXACTLY like the amp did, but it worked for a long while like that.

it gave the shop a nice whiff of ozone for a long time before it died.

stev

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