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Old 9th March 2014, 08:44 PM   #1
pboard is offline pboard  United States
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Default Tubelab SSE - coupling caps and run capacitor

Hi all, thanks first for a great forum. I've lurked a long time, learned a lot, and just finished my first build, a Tubelab SSE. It sounds fantastic and many thanks to George for a great design and instructions. (I'll try to attach a picture of my amp just to show off!)

I don't know much about electronics, so I have two questions... first, what sort of difference would higher quality coupling capacitors make? I see some of these special caps (Auricaps etc) for $10-15 each. I just bought really cheap ones, maybe $1-2 each from Mouser, before spending more. How much difference does it make? Also, I bought fairly small output transformers (Edcor GXSE 10-8-5) so would this be more of a limiter and not worth buying better coupling caps?

Second, I have an optional run capacitor, but for aesthetic reasons didn't want to use an ugly mouting bracket so I superglued it to my chassis top (yes, I know, the advanced assembly technique of superglue). But this means it's not grounded. I can test a few volts between the capacitor housing and ground, and I know for safety I should ground this, so I have some copper wire wrapped around it temporarily until I think of something else. But my questions is whether that small amount of voltage seems normal or some problem with the capacitor?

Also, I measure B+ at around 479 volts. Does that sound right/safe?

Thanks in advance.
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Old 9th March 2014, 10:45 PM   #2
zman01 is online now zman01  Bangladesh
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Tubelab SSE - coupling caps and run capacitor
pboard,

Your tubes look like EL34 from the picture in your post, so 479 volts would be on the high side for those. If you don't have a choke, you might consider adding one - it will help with improving the power supply section and dropping the voltage to around 450v, which should be ideal. A small choke like the Triad c-14x can be hidden under the chassis.

The Edcor GXSE has been used with success by quite a few builders.
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Old 9th March 2014, 10:51 PM   #3
pboard is offline pboard  United States
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Thanks. Yes, they are EL34 tubes. I do have a choke but it's small--the Triad C-24x. I wasn't sure if I could squeeze the c-14x inside my chassis. I might check the dimensions to see if it might fit.


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Old 10th March 2014, 12:55 AM   #4
JP is offline JP  United States
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You can put the mounting bracket for the motor run cap under the chassis. The cap will sit quite a bit lower, but the bracket won't be visible. A heck of a lot safer too, both for safety grounding and hot-glue-breaking-loose standpoint.
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Old 10th March 2014, 02:24 AM   #5
pboard is offline pboard  United States
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Jpjones, good idea. I hadn't thought about mounting with the bracket inverse. I actually might like it sitting lower. Just would need to cut the hole precisely to get a nice fit or maybe dress up the joint with some sort of simple wire or rubber ring.

Another question for anyone... How hot is ok for the amp? After about 30 minutes of playing, my top plate around the tubes gets hot enough that I can only touch it for 2-3 seconds. Sound normal? The transformers are fine. Opts get warm due to proximity but not terrible.


Perry
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Old 10th March 2014, 10:58 AM   #6
moggi1964 is offline moggi1964  England
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Sounds a bit hot to me.

You might consider putting ion some venting holes around the tubes. You could raise it up on some slightly higher feet as well to get more air flowing underneath.
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Old 10th March 2014, 11:25 AM   #7
spendorite is offline spendorite  Canada
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Quote:
Originally Posted by pboard View Post

Another question for anyone... How hot is ok for the amp? After about 30 minutes of playing, my top plate around the tubes gets hot enough that I can only touch it for 2-3 seconds. Sound normal? The transformers are fine. Opts get warm due to proximity but not terrible.


Perry

I think this is normal for an SE amp as the tubes are operating at their full potential
ALL of the time unlike a push pull amp.
Nice neat build by the way
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Old 11th March 2014, 01:35 PM   #8
pboard is offline pboard  United States
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I measured my available space. Just barely 2.25 inches that might fit the larger choke.

Ok, here's my attempt at understanding some of the circuit...I see the larger choke has 150 ohms resistance (as is the R1 optional resistor) but my choke has only 50 ohms. If the larger choke doesn't fit and I were to keep the small c-24x, would it work to add a 100 ohm resistor somewhere in the circuit to lower B+ voltage a bit? If so, where? In series with the choke?


Perry
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Old 11th March 2014, 02:23 PM   #9
Sodacose is offline Sodacose  United States
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Perry-

Yep, in series with the choke. Make sure it's a high wattage resistor (5W+). 100 ohms will only eat a few volts though. I actually did just this on a SSE build to keep things closer to George's spec'd choke but I've since removed it because I wanted a little extra B+ and I figured it was just generating heat.

You might also consider other rectifier tubes that will drop more voltage than the GZ34. Depends on how much current you have on the 5V tap and potentially your value for C1. A 5U4 will drop 30V more than the 5AR4/GZ34, but it needs a 3A heater and it probably wouldn't very much appreciate the spec'd 47uf C1 value. Tougher to change the heater current, but the cap would be easy to swap. It would put you right in the 450V sweet spot.
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Old 13th March 2014, 07:51 PM   #10
dmb5245 is offline dmb5245  United States
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Perry, 480v is a bit higher than most, but not too high. What size cathode resistors are you using (R17/R27)? The tubelab website has a table that shows the predicted dissipation for different tube types for a given B+ and cathode resistance. An increase in cathode resistance decreases the dissipation and makes life easier on the output tubes. Pushing them too hard is probably not "unsafe", but it will decrease their life expectancy.

The heat generated on the top plate is a little concerning, however. My SSE runs a high B+ too, but it gets nowhere near that hot. Is the heat coming directly from the tubes or from underneath the top plate? Is the pcb right up against the plate? Are you certain that all of your resistors are rated high enough (R17/27, R18/28 in particular)? They could produce heat if they aren't.

Good job though, just take care of that motor run cap and it will look great!
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