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Old 10th October 2005, 01:32 PM   #1
mr mojo is offline mr mojo  United States
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Default 7591s flashing on start-up?

Howdy folks!

Recently finished my PP 7591 amp. Dual 5AR4s for B+. Replaced the ww dropping resistors last week to get my plate voltage on the 7591s down to 480. Before replacing them the B+ got as high as 540v.

It may've been doing this all along and I just now noticed, but I'm getting flashing in the 7591s on start-up. I should be getting a nice slow warm-up with the tube rectifiers.

Any ideas on where to look for the cause of this? Is this harmful to the tubes or the filter caps on the plates?

Any and all help would be most appreciated!

best,
mr mojo
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Old 10th October 2005, 03:29 PM   #2
kevinkr is offline kevinkr  United States
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Hi Mojo,
What exactly do you mean by flashing? Are you talking about the filaments lighting brightly at the application of power and then dimming down or are you talking about blue glow that appears abruptly around the plate structure as the tubes warm up or are you talking about arcing somewhere internally?

Give us a little more detail to go on..

Kevin
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Old 10th October 2005, 03:35 PM   #3
mr mojo is offline mr mojo  United States
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Hey Kevin,

Yup, it's the filaments flashing brightly as soon as I power up, dimming then gradually coming up to regular brightness.

No arcing-I found out the hard way what that looks like-ha!

Thanks,
mr mojo
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Old 10th October 2005, 11:18 PM   #4
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Hi mr mojo,

I think you'll find this fairly normal, your likely seeing the tail ends of the filament glow due the the inrush current and as the filament proper has more thermal mass and takes a little longer the reach tempreture and hence allow the current to go down to normal value, hence after several seconds all looks normal.

question filament volts is correct??

Robert
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Old 11th October 2005, 11:09 AM   #5
EC8010 is offline EC8010  United Kingdom
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There was a long thread on this topic quite recently. Do a search...
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Old 11th October 2005, 03:22 PM   #6
kevinkr is offline kevinkr  United States
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Hi Mojo,
Just check the filament voltage at your nominal line voltage and see whether it is within 5% of 6.3V and you should be just fine.



Kevin
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Old 11th October 2005, 03:32 PM   #7
mr mojo is offline mr mojo  United States
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Thanks for the help guys, much appreciated!

I read through the flashing thread, but I thought maybe there was a current problem inherent with the JJ 7591s.

Filament voltage measure 6.8(I massively over-specced the filament winding in case I wanted to experiment with other tubes with higher current draw, which in turn has led to slightly higher filament voltage. Didn't know better at the time-live and learn!)

Is the higher filament voltage a major problem?

Best,
mr mojo
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Old 11th October 2005, 05:38 PM   #8
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The filament voltage is still within spec, but adding a small resistor in series with the filament will bring it down to exactly right. Depends on the filament current, but a little coffin resistor of 0.47 to 3.3R should do the trick. A little under wont hurt.

Shoog
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Old 11th October 2005, 08:33 PM   #9
kevinkr is offline kevinkr  United States
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I think you proably want something around 0.1 ohms if all of your amplifier filaments are running on a single winding.

If you are using two windings - one per channel then you might try something like 0.22 ohms.

Note if have a grounded filament center tap divide the above values by 2 and put one in each filament lead to maintain balance.

Note that you will need to calculate the power dissipation in those resistors and then double that and choose the closest higher wattage rating. With a single winding you may need a 5W resistor or more.

You only need to drop 0.5V to be on specification, at 6.8V you currently don't have much margin for a somewhat high line condition
Kevin
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