Newbie Question About B+ - diyAudio
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Old 28th October 2013, 02:10 AM   #1
ppinto1 is offline ppinto1  United States
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Default Newbie Question About B+

Hi everyone. I'm new to the forum - and new to amp building. I also have very little background in electronics - so please bear with me.

I'm trying to build a TSE. I've spent a lot of time reading on the site and in this forum. I'm building for 300B's w/400v B+ and 5K OPT's.

I'm using the Hammond 276X for PS.

I'm in the first stage of the checkout, and my B+ is very high (~445V).

I don't have a meter that can check the full output voltage of the 276X. But, the tap on the 640V measures ~334V (measured across T1-1 which has the tap connected to it, and T1-5 - which has the 640V output connected to it, along with T1-4). I'm supplying wall power to the 125V tap.

I've used all the parts specified in the "parts list" on the Tubelab site.

Out of the gate, I was getting ~450V for B+ (measured across R30).

To get down to 402V (my lowest yet), I put 550 ohms across R4 (resisters in series), removed C4, and adjusted R12 and R23 until B- is 202V (also bringing B+ down to the indicated level).

I tried measuring the 640V output on the 276X briefly (my meter only goes up to 500VAC), and I got a reading of 673V. But, of course, I'm not sure if this is accurate because of my meter.

My wall voltage is currently measured at 121.6V

Does this sound normal to anyone? I'd really appreciate some pointers.

Thanks,
Peter
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Old 28th October 2013, 03:21 PM   #2
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Quote:
Originally Posted by ppinto1 View Post
I'm in the first stage of the checkout, and my B+ is very high (~445V).
First stage, you mean without any tubes in place? Then B+ should show negative voltage.

On the second stage where rectifier tube is in place, it will show B+ as positive and it will be around 440 V. That's typical for TSE. Then when you put other tubes in (third and fourth stage of checkout), B+ voltage will go down.

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Old 28th October 2013, 04:35 PM   #3
ppinto1 is offline ppinto1  United States
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Evenharmonics,

Thank you so much! I had no idea I was on track The instructions are vague about test voltages (I suppose because of the multiple intended applications).

I currently only have a rectifier tube. I'm waiting on the rest from Hong Kong.

What a relief!

Peter
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Old 28th October 2013, 08:32 PM   #4
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ppinto1: Furthering Everharmonics comments above, your Hammond 276X is a center-tapped transformer that will give you 320-0-320 at rated load (172 ma). So it will measure 640V end to end or 320V from either end to the center tap (CT) when loaded.

With no current flowing ie, the transformer "unloaded", the voltage will rise to around 1.41 (sqrt 2) x transformer secondary voltage or 320 x 1.41 =450V. Keep in mind that with no current flowing the series R's in the power supply can't drop any voltage, so changing those won't do anything until you start drawing current from the supply.

When all of the tubes are installed and current is flowing, your B+ will drop....just be sure to adjust the bias adjustment pots to max negative voltage before installing the 300B's.

Measure the B+ DCV from the last PS cap positive to ground when bringing up the amp for the first time; as you adjust the bias voltage less negative, the output tubes will flow more current, and the B+ will fall. Assuming that you want to run the 300B's at 60-70ma or so, once you get the proper bias current (by measuring the voltage drop across the 10R anode R's), you will then see what your B+ is. You can then modify the B+ voltage by massaging cap and/or R values in the PS if required.

To do this efficiently, you'll need at least 2 multimeters, one measuring B+ voltage and another across each 10R 300B anode resistor. Ideally, 3 meters would be used to dial in both 300B's at the same time. Using clip leads makes the measurements safe, or else you must use the "one hand behind your back" method for safety.
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Old 3rd November 2013, 01:43 PM   #5
ppinto1 is offline ppinto1  United States
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Thanks for all the advice, Boywonder! I'm up to five multitesters. I would have had six - but one set of probes were DOA.

I'm at the point in the checkout where I've adjusted the plate voltage for the driver tubes. Everything is looking good.

I've been waiting - for what seems like an eternity - for a pair of 300B's from Hong Kong. I'm tempted to buy the Electro Harmonix platinum matched pair from Parts Express.
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Old 3rd November 2013, 04:03 PM   #6
rmyauck is offline rmyauck  Canada
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JJ from Eurotubes will be sonically better from my reading and they test their tubes well.

Plus they are quite affordable. Kevin Kennedy (moderator on this site) had great luck with them (many years of use) and probably they are still the same quality.

https://www.eurotubes.com/cart/index...category_id=24
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Old 3rd November 2013, 04:42 PM   #7
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Quote:
Originally Posted by rmyauck View Post
JJ from Eurotubes will be sonically better from my reading and they test their tubes well.
Whenever I read about tube sound comparison, I wonder how they compensated the bias per each test. Different tubes have different optimal bias points.
Quote:
(many years of use) and probably they are still the same quality.
That's more appreciable information.
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Old 3rd November 2013, 05:18 PM   #8
rmyauck is offline rmyauck  Canada
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Mostly from reading the SET forms over at AA , audioasylum and others.

Here's a nice review I just found.

6moons audio reviews: JJ/Tesla 2A3-40
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Old 4th November 2013, 04:26 PM   #9
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Originally Posted by rmyauck View Post
Mostly from reading the SET forms over at AA , audioasylum and others.

Here's a nice review I just found.

6moons audio reviews: JJ/Tesla 2A3-40
That review shows some useful info like reliability and bias stability. But whenever someone writes review on how they sound without clarifying how the amp levels were matched and how the switching was done, the red flag goes up. IOW, it's got to be done in double blind, level matched and instant switching. All others are just subjective observation which doesn't provide anyone with useful data.
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Old 4th November 2013, 06:28 PM   #10
rmyauck is offline rmyauck  Canada
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I agree on the sonic thing as like you say every amp treats the tubes differently and it might even be more hard to get right for 300B tubes from what I've read as they're hard to drive.

Plus maybe everyone hears a bit differently!
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