C1, C4 and C5 cap voltage values

for Dynaco Mark II/III/IV and DIY PP monoblocks

C1, C4 and C5 cap voltage values

Postby junkwaffel » Mon Mar 03, 2008 5:24 pm

I just purchased a couple of Poseidon blank boards for my MK3 upgrades. Can I get by with 400v caps in these positions? Also, I have a 370VAC motor run oil cap that I plan on using as the 1st cap after the 5AR4. I have an SDS cap board installed (50uF, 50uF, 50uF, 50uF) and I plan on paralleling 2 50s, giving 40uF, 100uF, 50uF and 50uF for my power supply. Isn't the DC rating 1.6 times the AC rating?

Jerry
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Postby Shannon Parks » Tue Mar 11, 2008 6:30 am

I wouldn't recommend 400V caps as the B+ can soar to over 500V at turn on with no current drop across the plate resistors. You could gamble with 450V.

I don't know if I would use the motor run cap with a MKIII. Looks like it would be OK for a ST70, but those start up voltages would scare me for a 370VAC cap (with oil ready to spew everywhere instead of a fuse blowing).

Sorry for being a downer. Yellow_Light_Colorz_PDT_14
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Postby Ty_Bower » Tue Mar 11, 2008 5:43 pm


Grrr. Shannon, are you sure about this? I think you are probably correct, but I need to ask. I bought a set of Poseidon boards a while back and the seller bundled them with all the components. Here's an excerpt from the email conversation I had with him regarding C1:


> 3) I'm concerned the C1 caps supplied in the bag
> aren't rated for 600V. The manual says 600V minimum.
> In the bag of parts I received, C4 and C5 are large
> gray boxes rated 1000V but the C1 caps are small red
> boxes rated 250VAC/400VDC.

Actually they don't need to be 600 volt, they don't
see even 400 volts.


The good news (?) is that I have not yet run these boards in the amp. The bad news is that I still haven't gotten around to running these boards in an amp. Oh wait, that's the wrong bad news. The bad news is that I've already soldered in the 400V cap he supplied.

Assembled Poseidons

Do I really need to rip it out and put in the right part? Bah... :(
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Postby junkwaffel » Tue Mar 11, 2008 8:11 pm

Well, I am awaiting the parts I ordered from Mouser. I also have some Russian K40Y 400vdc oil caps on order as well. I do have some Vitiaman Qs for the .1 uF 600vdc coupling caps but I guess I should order some .22uF 630vdc caps as well. Looking at the voltage chart, plate voltage from the 12ax7 is 195v and plate voltage from the 12au7 is 219v; hence, my thinking that 400v caps would be OK. But I guess I am obviously missing something. From what I have been able to gleen from other web postings, a DC rating of a motor run cap is 1.6 - 1.8 times the AC rating, so a 370 VAC rating would equate to a 592 VDC rating, but I don't know if that is a safe assumption.

Jerry
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Postby Ty_Bower » Wed Mar 12, 2008 6:44 am

separks wrote:...B+ can soar to over 500V at turn on with no current drop across the plate resistors.


What Shannon is saying is that the whole power supply could be at very high voltages (over 500V) until the output tubes (KT88 or 6550) warm up enough to start conducting. When the output tubes are drawing 70 mA each (140 mA total for the pair) it pulls the whole supply down to "normal" voltages.

In other words, the startup voltages are a lot higher than the normal working voltages. The caps need to be rated to handle the maximum expected startup voltage.
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Postby Shannon Parks » Wed Mar 12, 2008 6:51 am

Actually, with a slow starting 5AR4 you can get away with 400V rated caps in the MKIIIs. I think you'll be fine for coupling caps.

But I would still be hesitant to use a 370VAC motor run at the input. ;)
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Postby Ty_Bower » Wed Mar 12, 2008 7:52 am

What if V1 (the 12AX7) stops conducting for some reason? Won't the voltage at C1 try to float up to whatever voltage is at point K? Under normal operating conditions, the voltage at K should be around 430V or so. That would be too much for a 400V rated C1 cap, I think.

In my opinion, the stock Mark III had a design flaw in that the amp wasn't safe to operate unless the output tubes were drawing normal current. Otherwise, the voltage rating of the can cap would be exceeded. I want to fix these guys up so they are better than new. I'm using the SDS cap board to fix the PSU problem. I don't want to recreate the same problem in the coupling caps by using under-rated parts, unless I know for certain they won't go over 400V.
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Postby Ty_Bower » Wed Mar 12, 2008 6:38 pm

Whaddaya know? The original Dynaco board used 400V caps between the AF amp and the phase splitter. (It had 600V caps between the phase splitter and the output tubes.) I can't say this is good, bad, or indifferent, but at least Dynaco thought 400V was enough.

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Postby junkwaffel » Wed Mar 12, 2008 7:19 pm

Yeah, I'll try the 400v .22uF K40Y Commie Oil caps when I get them in the C1 position and and the Sprague VitQ 600v .1 caps for the C4 and C5 positions. For the time being, I won't install the 370VAC oil cap and use the SDS board configuration as is for now. Shannon's uncertaincy about this cap has got me leaning to using them for my Stereo 70, which I have an unassembled SDS board for it. So much to do, so little time... When I get everything installed, I'll measure the B+ at startup to see how much voltage there is to the 1st cap in the power supply. Got my Mouser order, just waiting for the caps now.

I agree with you Ty on the fact that the original quad cap on the MK3 was undersized for the job, especially with today's line voltage. I guess that even with 115vac, the cap was running on the hariy edge upon startup, even with the 5AR4.

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Postby Ty_Bower » Thu Mar 13, 2008 6:40 am

While we are talking about the coupling caps, I feel it's worthwhile to link to this thread:
http://www.diytube.com/phpBB2/viewtopic.php?t=1779

Shannon says it is OK, perhaps even desirable, to use larger than specified (greater than 0.22uF) caps in place of C1.

The same is not true for C4 and C5. They should be 0.1uF.
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Postby Blair » Fri Apr 04, 2008 6:08 pm

I run 1% daytons rated for 250v there. It may be a poor choice, but over one year and no issues, and my poseidons are running a constant B+ of 589v :o .

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Postby audiowize » Tue Apr 22, 2008 6:35 pm

C1 voltage isn't super critical. With a slow-warmup power supply, you might even be able to get away with 250v rated caps. C4 and C5 "appear" to only have the B+ on them, but alas, you have negative bias voltage on the other side. In some regards, 600v caps are borderline there. Definately go with 630v caps if you can.
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