SSE checkout: so far so good, but have a B+ question - diyAudio
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Old 29th April 2011, 07:06 PM   #1
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Default SSE checkout: so far so good, but have a B+ question

Hi again,

I've just been through Ty_Bower's Photobucket pictures to do some basic tests on the SSE before throwing the switch for the first time. I have a problem with the volume circuitry shorting the pre-amp grid resistors somehow, but if I remove the volume from the circuit and just ground the PCB to the top plate, the problem goes away and all the resistances check out fine.

I powered it up for the first time with no valves and all the heater voltages were fine. I get 400V AC at the rectifier.

Then I stuck just the rectifier in and powered up. Again, all seems fine. No smoke, bangs, smells and the valve glows. I measured the B+ at R4 and get 530V, which exceeds at least one of the cap rated values so I turned off before cap got nasty with me.

My question really is can I expect this B+ to drop as I put the other valves in and load the circuit or will it be pretty fixed at this value? If it doesn't drop somewhat I'll have to have a look at the PT wiring since it doesn't seem sensible to run a 500V cap at 530V.

I'm using a 374BX, with about 250V coming out of the wall.

Thanks guys.
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Old 29th April 2011, 10:01 PM   #2
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Quote:
Originally Posted by Horsebox View Post
My question really is can I expect this B+ to drop as I put the other valves in and load the circuit?
That's exactly how it works. With no load, your voltages will be quite a bit higher. I'd expect you might see a 70 volt drop after you have both power tubes drawing current.

Last edited by Ty_Bower; 29th April 2011 at 10:09 PM.
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Old 29th April 2011, 10:56 PM   #3
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Thank you for the reply Ty_Bower, and thanks also for going to the effort of creating that PhotoBucket resource: material like that is invaluable to a novice like myself.

Anyway, I plugged the rest of the valves in and powered it up. Everything glows the right colours in the right places and nothing blew up. I had a probe on R4 again to measure the B+ and it came up nice and slowly (presumably due to the ICLs) to stabilise at 477V, so your 70V drop was spot on.

It's time to replace those dummy loads with real speakers, but I still need to sort out my annoying volume problem. Might have to wire it direct as a power amp to begin with and use the volume control on my iPod (I know....!) to control the level.

Thanks again for the advice. No music yet but thrilled that all appears fine so far.
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Old 30th April 2011, 12:16 AM   #4
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Quote:
I have a problem with the volume circuitry shorting the pre-amp grid resistors somehow,
If you are measuring the resistance from the 12AT7 grid to ground and finding that the resistance goes to zero as the volume control is turned down....That is completely normal.
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Old 30th April 2011, 01:09 AM   #5
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!!!!!!!!!!!!!!!!!!!!!!!!

Just plugged it into speakers.......Works!!!!!!........and works well! Am completely beside myself. Fleetwood mac Rumours sounds awesome!!

Can't believe it. Thank you everybody for help. And of course tubelab.com for producing something that even I didn't blow up.

Have been working up to this moment for about six months (gathering parts etc., working out what the hell is going on). Bloody thing works....just can't put it into words; the satisfaction.....thanks guys....

tubelab.com: Thanks for the reply. I'll look into it more later after sleep: I think I may have fried the volume pot earlier when testing it. (Stuck a 9V bat across terminals to see what would come out the other end; only smoke, bad smells, and small balls of fire!)

Off to listen to some Mark Knopfler......can't stop smiling!
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Old 30th April 2011, 04:31 AM   #6
rknize is offline rknize  United States
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Can't beat that DYI satisfaction. Glad it fired right up!

Great amp...still the most-used tube amp in my house despite being one of many. It "just works".
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Old 30th April 2011, 06:31 AM   #7
sureshm is offline sureshm  Australia
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Hi Ty, could you please post or pm the link to your Photobucket pictures. I will be starting my build soon. Thanks.
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Old 30th April 2011, 04:00 PM   #8
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Quote:
Originally Posted by sureshm View Post
Hi Ty, could you please post or pm the link to your Photobucket pictures?
Here's a good place to start:
Simple SE checkout for dummies
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Old 30th April 2011, 11:06 PM   #9
sureshm is offline sureshm  Australia
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Thanks....
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Old 4th May 2011, 05:57 PM   #10
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Quote:
Originally Posted by tubelab.com View Post
If you are measuring the resistance from the 12AT7 grid to ground and finding that the resistance goes to zero as the volume control is turned down....That is completely normal.
I've been at work for the past five days so no chance to tinker until today.

Yes, that is exactly what I found. Grid to ground for each channel measured around 100R to 70kR from zero to full volume; this is what I couldn't reconcile with Ty_Bower's checkout pages so I left the volume out. Anyway, I wired it in today and, as you said, it works perfectly!

This allowed me to do away with the iPod and put a mid-high-end CD player in its place at the input and noticed that the hum dropped to virtually zero and there is significantly more gain, even on 86dB speakers. I haven't had enough time yet to subjectively compare the sound; suffice to say I'm thrilled to bits with it.
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