Please help me out. 1969 100w build

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no0sand1s
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Re: Please help me out. 1969 100w build

Post by no0sand1s » Tue Jun 12, 2018 5:49 pm

If they're OK, should I try firing it up w/ the power tubes installed?

Tazin
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Re: Please help me out. 1969 100w build

Post by Tazin » Tue Jun 12, 2018 6:20 pm

no0sand1s wrote:
Tue Jun 12, 2018 5:49 pm
If they're OK, should I try firing it up w/ the power tubes installed?
I don't think that would be a good idea since the voltages on pins 2,3,7, and 8 are way beyond the 10% variance.

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Re: Please help me out. 1969 100w build

Post by no0sand1s » Tue Jun 12, 2018 6:24 pm

The plate resistors are fine.

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Re: Please help me out. 1969 100w build

Post by Tazin » Tue Jun 12, 2018 6:26 pm

no0sand1s wrote:
Tue Jun 12, 2018 6:24 pm
The plate resistors are fine.
Go figure!...Shit, I'm running out of ideas!

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Re: Please help me out. 1969 100w build

Post by no0sand1s » Tue Jun 12, 2018 6:55 pm

:what:
Last edited by no0sand1s on Tue Jun 12, 2018 7:24 pm, edited 1 time in total.

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Re: Please help me out. 1969 100w build

Post by no0sand1s » Tue Jun 12, 2018 7:06 pm

I even checked the underside with a dental mirror for solder stalactites!

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Re: Please help me out. 1969 100w build

Post by Tazin » Tue Jun 12, 2018 7:41 pm

It's got to be something stupidly simple that were all over looking. I find it hard to believe that the increase in voltages on V3 is a resultant of that particular type of master volume setup. The 0.022uF PI decoupling caps should block any reverse dc voltage coming from the master volume pot.

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Re: Please help me out. 1969 100w build

Post by no0sand1s » Tue Jun 12, 2018 8:33 pm

You were right, twice! When I disconnected the filament and bias wires from the ground near F6 To place all PT wires on a terminal block, the ground from my presence wire was left disconnected. I thought it was soldered to the lug closest to the chassis, but it was just hanging. :palm: I soldered it up, and we are good to go! Thanks for sticking by me.
Last edited by no0sand1s on Tue Jun 12, 2018 9:35 pm, edited 1 time in total.

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Re: Please help me out. 1969 100w build

Post by Tazin » Tue Jun 12, 2018 8:52 pm

See!...I knew it would be something stupidly simple. And 'neikeel' even mentioned the ground for the Presence pot a little earlier. The good thing is that you figured it out and hopefully all will run smooth for awhile.

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Re: Please help me out. 1969 100w build

Post by no0sand1s » Tue Jun 12, 2018 9:33 pm

neikeel wrote:
Thu Jun 07, 2018 5:35 pm
It is impossible to tell from the close ups.
But as long as the wires match up it looks ok

What is your plate voltage (vs bias current)

Assuming 490v on plates 17.5/490 = 36mA so you would be a bit too hot.

If you have 460v on plates 17.5/460 = 38mA

Depends on how you are biasing (1 ohm resistors vs bias probe)

Sorry cant see voltage chart any more but personally I would back it off a bit whilst it beds in, maybe 30-32mA tops
I’m back in business-setting the bias to 32mv for break in, on your recommendation. Thank you for all your assistance.

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Re: Please help me out. 1969 100w build

Post by Tazin » Wed Jun 13, 2018 9:58 am

So what is the plate voltage with the bias set for 32mV?

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neikeel
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Re: Please help me out. 1969 100w build

Post by neikeel » Wed Jun 13, 2018 11:03 am

He was showing 493v which is pretty typical so I think if is still that at 32mA of draw that puts him a in the safe range at 60-65% if you allow for a few mA of screen current (I presume using 1ohm resistor on ground lug) which as you know (and now he knows!) is the sum of the screen and the bias currents. So if you use that method the number you measure will always be slightly higher than the actual bias current happening in the tube.
Neil

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Re: Please help me out. 1969 100w build

Post by Tazin » Wed Jun 13, 2018 11:12 am

So that should put him around 15.7 watts of dissipation providing that the 32mV reading was the highest out of the four output tubes. I usually shoot for 17 watts of dissipation at idle.

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Re: Please help me out. 1969 100w build

Post by neikeel » Wed Jun 13, 2018 1:23 pm

Tazin wrote:
Wed Jun 13, 2018 11:12 am
So that should put him around 15.7 watts of dissipation providing that the 32mV reading was the highest out of the four output tubes. I usually shoot for 17 watts of dissipation at idle.
I suggested that whilst he was running in and getting it all bedded down that he bias it conservatively.

When he is happy, has a few hours on it then he should ideally check the voltages and bias as the tubes settle.

IMO the 17.5watt at idle is a safety limit not a target, just my opinion tho'
Neil

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Re: Please help me out. 1969 100w build

Post by Tazin » Wed Jun 13, 2018 4:27 pm

neikeel wrote:
Wed Jun 13, 2018 1:23 pm
Tazin wrote:
Wed Jun 13, 2018 11:12 am
So that should put him around 15.7 watts of dissipation providing that the 32mV reading was the highest out of the four output tubes. I usually shoot for 17 watts of dissipation at idle.
I suggested that whilst he was running in and getting it all bedded down that he bias it conservatively.

When he is happy, has a few hours on it then he should ideally check the voltages and bias as the tubes settle.

IMO the 17.5watt at idle is a safety limit not a target, just my opinion tho'
Good advice as usual.

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