Stereo 70 B+ nodes

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Stereo 70 B+ nodes

Postby petercapo » Fri Jul 19, 2013 5:41 pm

Installed a Triode USA cap board in my Stereo 70. Referenced to the original schematic, B+ nodes "D" and "C" measure right on the nose. Node "B" (between the 6.8k and 22k resistors) does not measure very close to spec but is still within 10%, which is acceptable per the original Dynaco manual.

However, node A (other side of the 22k resistor) is 14% low, which is definitely out of spec. Any idea why this might be? Anyone else using the Triode cap board notice this? BTW this is not the classic SDS Labs cap board, it is the Triode USA "Upgrade" cap board.

Thanks.
Peter
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Re: Stereo 70 B+ nodes

Postby Geek » Sat Jul 20, 2013 2:40 am

14% is still good enough for most tube circuits.

Is the cap getting warm? Could be leaky...

Cheers!
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Re: Stereo 70 B+ nodes

Postby petercapo » Sat Jul 20, 2013 8:08 am

All the parts are new, and I brought the amp up slowly on a variac, although I am sure one or more of the electrolytics could still be leaky. If they are leaky, I have the resistor pairs in parallel with them. I lowered the values of the voltage-equalizing resistors across the series connected electrolytics. The resistor values Triode USA supplies is good for bleeding the power supply when the amp is shut off but seemed to me to not be much good for protecting the electrolytics. They supply two 330k for each pair of electrolytics in series.

I left the resistors at around 330k each for node D, but dropped them to 100k each for node C, and around 150k each for nodes B and A. Could that be the cause of the lower voltage readings on node B and especially node A whether the electrolytics are leaking or not? If so, why wasn’t the voltage at node C affected, as I used the lowest value resistors there (again, two 100k in series)?
Last edited by petercapo on Tue Dec 10, 2013 5:29 pm, edited 1 time in total.
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Re: Stereo 70 B+ nodes

Postby Geek » Sat Jul 20, 2013 4:22 pm

petercapo wrote:I lowered the values of the voltage-equalizing resistors across the series connected electrolytics.


Ah-ha! You made a voltage divider at the nodes w/o compensating the series resistors for the adjustment :))
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Re: Stereo 70 B+ nodes

Postby petercapo » Sat Jul 20, 2013 4:41 pm

Roight! I knew I came to the right place.

Thanks!

-Peter
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Re: Stereo 70 B+ nodes

Postby petercapo » Tue Jul 23, 2013 1:21 pm

I did a little experimenting and my initial estimates are that are that the 6.8k will have to be reduced to something like 3.9k. The 22k will have to be reduced to something like 12k to 15k. This should get the voltages back up.

I believe it will also shorten the time constant for charging and discharging the electrolytics, correct? Is this of any practical consequence?
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Re: Stereo 70 B+ nodes

Postby Geek » Tue Jul 23, 2013 3:33 pm

What's the value of the 'lytics on that board?
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Re: Stereo 70 B+ nodes

Postby petercapo » Wed Jul 24, 2013 8:48 am

First section between the rectifier and the choke has two 82uf in series with two 316k resistors in parallel.

Second section right after the choke and before the first resistor (originally 6.8k) has two 390uf in series with two 100k resistors in parallel.

Third section between the original 6.8k and 22k resistors has two 270uf in series with two 160k resistors in parallel.

Fourth section after original 22k resistor has two 270uf in series with two 154k resistors in parallel.

Again, for the voltages at the third and fourth nodes to be brought back up to the original specs, the 6.8k will need to be decreased to around 3.9k, and the 22k decreased to something like 13k, so I am wondering about the change to the time constants...

Thanks.
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Re: Stereo 70 B+ nodes

Postby Geek » Wed Jul 24, 2013 9:41 pm

Don't worry about it.

Cheers!
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Re: Stereo 70 B+ nodes

Postby petercapo » Thu Jul 25, 2013 7:27 am

Many thanks!
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