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Kofi Annan in: "Tube Amp for Multi-Way Speakers"
Kofi Annan in: "Tube Amp for Multi-Way Speakers"
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Old 20th October 2019, 05:24 PM   #131
kevinkr is offline kevinkr  United States
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Kofi Annan in: "Tube Amp for Multi-Way Speakers"
If you are 350V and under and no signs of red plating I would not worry about it, the supply voltage will likely sag a bit at higher powers. Otherwise I would recommend a power resistor in series with the HV secondary as already suggested.

Don't under any circumstances remove that last filter cap unless you want lots of cross-talk and generally poor performance.

Most of the antique 6BQ5 amps I have worked on were between 360 - 400V on the plates with a nominal 117VAC line (variac) and the only tubes that seem to have a problem with this are current production JJs IMVLE.. HH Scott stuff with 7189As were over 400V.
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Old 20th October 2019, 07:15 PM   #132
Kofi Annan is offline Kofi Annan  United States
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Thanks, guys. Appreciate the feedback on this.

Looks like the supply is delivering around 380VDC before the final resistor / cap, which is much higher than I expected. I'm guessing my line voltage is a little high.

It looks like the 100R (4 x 400R in parallel) dropping resistor I fitted is dropping 36VDC, so total current draw is 360mA. After the final resistor / cap in the supply, I'm getting 340VDC at the center tap of the OPT.

I'll need to find a way to add some turret boards to give me a few more solder points for the final resistor / cap combo.

Also-- I found that the 1A slo-blo fuse was blowing after a few minutes of operation. I replaced it with a 2A, but I usually try to keep it at 1A. I'm assuming the additional current draw may be heating up the fuse over time and causing it to blow.

Does that sound correct?

Kofi
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Old 20th October 2019, 07:32 PM   #133
scott17 is offline scott17
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You need a fuse larger than 1A.
360mA @ 340VDC is 122.4W, so you are drawing a little more than 1A on the primary side.
A 2A fuse is OK.
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Old 21st October 2019, 12:38 AM   #134
Kofi Annan is offline Kofi Annan  United States
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Quote:
Originally Posted by scott17 View Post
I'd leave the final cap in place. You could reduce the first capacitor to ~100uF to minimize the initial turn-on surge, and use something like 25R-10W in each leg of the PT secondary before the rectifier.
Hmm.. I believe someone mentioned this before but I blew by it.

So, I modeled another supply in PSUD, adding an additional 50R of source resistance to the power transformer to mimic the impact of the 2x25R resistors in each leg of the AC before the rectifier.

Does this look correct?

Kofi
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File Type: png EL84PPP SUpply with 2x25R Before Rectifier.PNG (216.6 KB, 112 views)
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Old 21st October 2019, 02:47 AM   #135
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Yes, but you should leave the last RC section in place as was mentioned. Ripple is significantly reduced.
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Old 21st October 2019, 03:10 AM   #136
Kofi Annan is offline Kofi Annan  United States
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Quote:
Originally Posted by scott17 View Post
Yes, but you should leave the last RC section in place as was mentioned. Ripple is significantly reduced.
Boom.
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Old 21st October 2019, 12:52 PM   #137
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Looks good. Also I see that you did not have to use as much series resistance on the PT secondary. Should work out great. If needed, the final R could be adjusted slightly to get right where you want to be.
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Old 21st October 2019, 02:09 PM   #138
Kofi Annan is offline Kofi Annan  United States
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Thanks. Yep-- I think I'll have to mess with the resistance a bit to get it dialed in.

Curious-- why would I only need a 10W before the rectifier when I would require over 20W for the final R?

Always learning here.

Kofi
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Old 21st October 2019, 05:19 PM   #139
scott17 is offline scott17
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I should not have posted late (for me) last night and I wasn't really thinking this morning.
You were correct with your model in post #134. If you take the whole supply into account there is plenty of filtering and all of the voltage adjustment can be done with the series resistors on the PT. And, you get rid of the big power soaking resistor. The power stage ripple should cancel in the OT.
I hope I didn't cause you too much trouble with my errant post.
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Old 21st October 2019, 06:39 PM   #140
Kofi Annan is offline Kofi Annan  United States
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No problem at all. This helps me quite a bit, actually, since I should now be able to mount the series resistors on the chassis and avoid cramming more components into the existing turrets.

Thanks so much for all your help!

Kofi
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