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Old 20th January 2016, 01:54 AM   #651
phyciocc is offline phyciocc  United States
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Hello everyone:
I will have to make some measurements before I do anything, but here are my observations: my build is rather compact, but the tubes do stick out of the amp top plate well. The transformers run cool, initially, but after about 1-2 hours, the amp top plate gets hot and so everything, including the transformers, gets hot.

It seems as if the EL84 are really heating up the works.

Is this normal or (as the measurements I have to make will answer for sure) is the B+ a bit high? Or could be simply a result of tight spaces around the tubes? I have posted a picture of the build a few posts back. In the build I use 5ar4 but I have handy several other rectifier tubes (5u4gt and 5y3). I know that the 5u4 will lower the B+ by adding "sag". Is this advisable?
Thanks!
Marco
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Old 20th January 2016, 01:05 PM   #652
rknize is offline rknize  United States
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How hot is hot? Mine gets fairly toasty as well but works fine. How long can you hold your hand in the power transformer and top plate? R1 dissipates about 7W, so look out for that.

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Old 20th January 2016, 04:38 PM   #653
phyciocc is offline phyciocc  United States
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I actually think that the transformed are not getting hot by themselves. The are heated by the sheet on top of the amp, which in turn is heated by the tubes. I am not really familiar with tube amps, so I don't know what "Hot" is really hot. The transformers are actually cool initially and stay so for 1/2 hour easily. It is when the metal sheet gets hot, that everything gets toasty.

I have checked a few voltages.
Line voltage here is a bit high, at 125 Volts (Not much I can do, unless I add a variac)
The output of the power transformer are 315-0-315 (5% higher than spec, probably due to the high line voltage).

B+ with the Chinese brand 5AR4 is 343V.
B+ with the American made but vintage 5U4GB is much lower, at 303V.
B+ with a vintage American made 5y3GT is only 282V (not good, I think)!

Out of completeness I checked the filament voltages: 7.0 V for the 6.3 tap and 4.9 for the 5.0 ( bit low. Probably due to the large current drain of the rectifier).

All measurements were made with volume at the halfway point, no input and my speakers connected to the 8 Ohm taps of the OPTs.

I am going to try the 5U4gb for a while (inrush voltage problems and all) and see how I like the amp with lower B+.

Marco

Last edited by phyciocc; 20th January 2016 at 04:40 PM.
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Old 21st January 2016, 03:20 PM   #654
rknize is offline rknize  United States
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If you can keep your hand on the transformer for as long as you want, it's running at a safe temperature. If you can't hold it there for more than 5 or 6 seconds, it's something to be concerned about. The temperature of the plate is pro ably not that important as long as the chassis has some internal ventilation of some sort.
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Old 25th January 2016, 08:06 PM   #655
phyciocc is offline phyciocc  United States
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The amp is doing fine. It gets a bit toasty after about 2 hours, but I have come to the conclusion is because the footprint of my build is rather small, and I have got 7 tube heating up the works. No problems it looks like.

Today I had a bit of a "religious" experience:
I put on the "1812 overture" and cranked it up. I have fairly sensitive speakers (95db), and I had a blast (several, actually, if you know the piece... ). This little amp does really sing!

Thanks George!!!
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Old 28th January 2016, 12:21 AM   #656
jonwhitear is offline jonwhitear  Australia
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Hey guys,

I've had my SPP running for about a year now, and I'm really happy with it. It runs pretty much 12 hours a day at the weekends, so there can't be anything too far wrong, but after a few hours use, my PT and top late are hot to touch, i.e. I can't keep my hand on them for more than a few seconds. I am a bit worried about heat and the effect that might have on longevity. Also, my PT vibrates and one of the end bells has a quiet ring to it.

There's a picture of my amp in this post. I'm using the Edcor XPWR066 PT and CXPP25-MS-7.6K OPTs, Russian Reflektor 6P14P-ER power valves, Brimar CV4024s, and a Sovtek 5AR4.

I have ordered a choke to replace R1 in order to remove the heat that R1 currently dissipates into the top plate. I'm planning on drilling some ventilation holes in the top plate around the power valves (as well as the ones already in the base plate.) I'm also planning on mechanically isolating the PT from the top plate using some silicone grommets to hopefully fix the ringing. If I do so, what's the best way to ensure that the PT is grounded? I ground the paint off one of the PT's "feet" to ensure electrical continuity between the PT and the top plate, and I'm not sure of the best way to do that with silicone spacers in between - just grind it off the "top" of the foot and rely in the bolt?

Is there anything else I can do to cool down the PT? I've read that heat and vibration may be the result of DC in the mains, but I don't know how to test for that, or to fix it.

Cheers,

Jon
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Old 28th January 2016, 05:24 AM   #657
rmyauck is offline rmyauck  Canada
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phyciocc

You may want to try appropriate resistors to reduce your 7V heater filament voltage to even 6V for better tube life!



jonwhitear

I use a quiet fan (or you could use computer fans with lower voltage wall warts so they run slower and quietly) ) on my old Eico HF-12's in which the PS Trans runs very hot and is known to and it cools everything down nicely. I think the tubes may even last longer from what I've read!

Last edited by rmyauck; 28th January 2016 at 05:32 AM.
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Old 28th January 2016, 11:39 AM   #658
spendorite is offline spendorite  Canada
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Quote:
Originally Posted by jonwhitear View Post
If I do so, what's the best way to ensure that the PT is grounded? I ground the paint off one of the PT's "feet" to ensure electrical continuity between the PT and the top plate, and I'm not sure of the best way to do that with silicone spacers in between - just grind it off the "top" of the foot and rely in the bolt?

Jon
Use tooth washers. These will bite into the tabs. You can also scrape the paint just where the washer meets the tab.
After doing that you should use your multimeter to check for continuity with the IEC adaptor ground pin or your star
ground point.
If you cannot keep your hand on the power transformer for more than a few seconds then it is getting too hot and
calls for further investigation.
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Old 28th January 2016, 10:07 PM   #659
jonwhitear is offline jonwhitear  Australia
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Thanks guys. I'll implement my first round of mods and see how I'm going with the hot PT. Cheers, Jon.
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Old 8th February 2016, 04:52 AM   #660
jonwhitear is offline jonwhitear  Australia
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Hi guys,

I tried out a few different feedback caps on the weekend, measuring each with 5KHz and 10KHz square waves, with 10R/5W loads. I'll upload the screen captures later. The 270pF cap looked best to me, so I soldered it in.

Trouble is, when I connected the amp back up to he speakers and powered it on, it made a very loud sound like an oscillating square wave, until I quickly shut it down.

Do you know why that might have happened?

Cheers,

Jon
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