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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 14th June 2019, 02:46 PM   #341
Tubelab_com is offline Tubelab_com  United States
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What's C4 value, and B+?
I can see a 47uF cap and a meter reading 438 volts in the picture.
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Old 14th June 2019, 03:06 PM   #342
jerkbag is offline jerkbag
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I came across a used set of OTs and a PT. Does anyone know if these will work with 300b tubes?
EDCOR - CXSE25-3.75K (this seems right to me?)
EDCOR - XPWR008

The 300V on the PT is lower then recommended, but if I model it with PSUD it seems like it could work with a bit of tweaking? Also, the PT is only Center Tapped on the 300V - is that ok?

Thanks for the help!
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Last edited by jerkbag; 14th June 2019 at 03:11 PM.
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Old 14th June 2019, 03:50 PM   #343
elobilo is offline elobilo  Norway
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yes, 47uF cap and a meter reading 438 volts.

I changed to a 5U4 rectifier (I've got loads of these) and that helped a bit, with a 10uF, it reduced the voltage further down to 413v. Got no more lower value caps so will order more.

One quick other question, is my B- voltage OK at nearly 100v more negative?

Thanks for the help.

Last edited by elobilo; 14th June 2019 at 03:53 PM.
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Old 14th June 2019, 08:06 PM   #344
Tubelab_com is offline Tubelab_com  United States
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Quote:
I came across a used set of OTs and a PT. Does anyone know if these will work with 300b tubes?
They will work. I have a 300-0-300 volt power transformer in my test amp and I get 370 to 390 volts of B+ depending on line voltage ans how much current I run. Get a choke with low DC resistance to make the most B+. I use a Triad C17-X. It's ugly so you have to hide it under the deck.

The 6.3 volt CT is not needed with 300B's.
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Old 15th June 2019, 04:11 AM   #345
zman01 is offline zman01  Bangladesh
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After a 14 year run, the TSE must DIE!
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Originally Posted by Tubelab_com View Post
I use a Triad C17-X. It's ugly so you have to hide it under the deck.
George,

Is that a typo? Shouldn't it be Triad C-14X?
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Old 15th June 2019, 11:48 AM   #346
Tubelab_com is offline Tubelab_com  United States
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Is that a typo? Shouldn't it be Triad C-14X?
For most builders the C-14X is the best low cost choke. It does however have 200 ohms of DC resistance, which at about 200 mA total current, will drop about 40 volts of B+ voltage. With a 325-0-325 volt power transformer the B+ should be in the 375 to 400 volt range.

With a 300-0-300 volt power transformer the B+ will be in the 350 volt range, which works OK with 300B's and is close to WE's original 360 volt recommendation. Many builders prefer to run their 300B's near 400 volts, so swapping in a low DC resistance choke will boost the B+ voltage.

Compared to the C-14X , the C-17X has lower inductance which could increase hum, but it has far lower DCR at 40 ohms, which will increase the B+. 200 mA through the 40 ohm choke will only drop 8 volts, affording a 32 volt increase over the C-14X.

I have run my amp with a Hammond 372HX (same 300-0-300 ACV @200 mA) and a Triad C-17X. I get 375 volts with 2A3's cranked to 70 Ma each (current similar to 300B's). I hear no hum with headphones wired directly across the speaker leads.
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Old 15th June 2019, 12:40 PM   #347
zman01 is offline zman01  Bangladesh
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After a 14 year run, the TSE must DIE!
George,

Thank you for explaining!

Cheers,
Zia
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Old 15th June 2019, 12:53 PM   #348
jerkbag is offline jerkbag
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Quote:
Originally Posted by Tubelab_com View Post
They will work. I have a 300-0-300 volt power transformer in my test amp and I get 370 to 390 volts of B+ depending on line voltage ans how much current I run. Get a choke with low DC resistance to make the most B+. I use a Triad C17-X. It's ugly so you have to hide it under the deck.

The 6.3 volt CT is not needed with 300B's.
Thanks for the detailed response George -- much appreciated!
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Old 16th June 2019, 12:02 AM   #349
Mr_Zenith is offline Mr_Zenith  United States
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After a 14 year run, the TSE must DIE!
Default Another TSEII Up and Running...

Received my board in the mail yesterday, just in time for the weekend. Spent most of last evening soldering, checking, and re-checking. Today was all about the woodworking required to put it all together as a functioning 3-D mock-up of the final design.

Oh, boy is this a nice amp! Right now it's playing through an old pair of JBL62 bookshelf speakers (89 dB), and there's plenty of volume. Sounds crisp and clean, too - not exactly what I expected from small SET like this. I'll let the voltages stabilize before taking it upstairs and hooking it up to the main system, but I'm already expecting to be in for a real treat.

I'm running a pair of sino 2A3's (the cheap ones with the 300B-esque plate) at ~50 mA each. The OPTs are 10 W James 2.5 k / 3.5 k units; I'm currently using the 3.5 k taps. The B+ is a little hot at ~308 VDC as measured at the plates; this is likely because of the Hammond 159S choke, which puts only about 65 Ohms between the filter caps.

Thanks to George for such a good-sounding and elegant design. It's a keeper for sure. Woo-Hoo!

I'll post some more pictures when it's more properly attired...
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Old 16th June 2019, 01:30 AM   #350
zman01 is offline zman01  Bangladesh
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After a 14 year run, the TSE must DIE!
Mr_Zenith,

Congratulations on getting your TSE II up and running!
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