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Old 29th September 2009, 03:22 AM   #231
Tubelab_com is offline Tubelab_com  United States
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but this B+ measurement is supposed to be taken with the system up and running with full bias current, right?.......so that there's zero bias current, then B+ goes up to ~420V. Is this to be expected?
Yes, it is normal for any unregulated power supply to drop under load. Tube rectified supplies drop more than SS rectified supplies.

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I do notice that my B- (bias supply, at R7) is around -295V, this seems extreme, given that the schematic says -150V... Is this also to be expected?
Yes, again. The -150 volt readings were taken on a 45 based amp with a smaller power transformer.

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the 56uF cap, and guess how much difference it made? Zero.
I have not had the time to post here recently but I need to explain that the value of C4 will affect the B+ voltage up to a point. Small values (2 to 20 uF) can be used to reduce the B+ voltage if your transformer creates too much B+. Once you reach the limit larger values will only make your rectifier tube unhappy. I put a 100uF in my Lexan amp in an attempt to squeeze a few more volts out of a small power transformer. It did not make any difference over a 47uF. That amp runs the 300B's at about 295 volts and it sounds very good, but only makes about 4.5 watts per channel. I usually run 45's in that amp where I get about 320 volts of B+ due to the much lower bias current (30mA).

I have another amp using 300B's and a Hammond 272JX. The B+ voltage used to be in the 380 to 390 volt range but now I get about 345 to 355 volts. What changed? The biggest change happened 2 years ago when the power company replaced the 30 year old large metal cased pole transformer (lightning zapped it) with a new one that is plastic cased and much smaller. Now the Hammond emits a faint buzz where it didn't before, and the B+ voltage is lower. The B+ voltage took another dive over the past 6 months as everyone got new flat screen TV sets. The 60 Hz sine wave coming from my wall outlet now has visible flat topping and measures between 8 and 12 percent distortion depending on the time of day.

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Or is ~345V of B+ sufficient and I should leave it as-is? I'm trying to make a decision here but there doesn't seem to be a straightforward answer (at least not that anyone's willing to put in writing ). George mentions in numerous places that the B+ should be higher, so it seems to me that's more important.
If you are happy with the sound of your amp, then by all means leave it alone and enjoy it. One of the big "secrets" about the 300B tube is that it has a rather large sweep spot (it sounds good over a large range of operating conditions). I have not noticed any change in the sound of my amp as the B+ dropped over the past 2 years. The output power probably went down 1/2 to 1 watt, but only the measuring equipment will find it. I only noticed the change because I had the unfinished amp on the bench when the pole transformer blew and I noticed the loss in B+ the next day. My Simple SE lost a few volts too. The amp is still unfinished and I have taken it appart several times to work on the cabinet only to put back on the plywood breadboard for listening. It is currently apart again. I really will finish it soon, but I have some Simple P-P's to build.
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Old 29th September 2009, 12:14 PM   #232
PaulyT is offline PaulyT  United States
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Thanks George, I appreciate your time and detailed response. So, it sounds like I have nothing to worry about. I'll probably put the 47 C4 back in and just go with it. Anyone have a feeling for whether it's better to use a higher bias current but lower B+, vs the other way around?

I don't know how the amp sounds yet... still hooked up to crappy test speakers. I've been afraid to put in the good speakers because I wasn't sure the amp was behaving right. (And how do I know it's behaving right? George says, because it sounds good. Chicken or egg...) But yes ultimately I understand the final measure of the amp is the sound, it's just that the scientist in me wants an objective quality assurance measurement first.

How would one increase B+ then in a more general, put in a higher-voltage power supply? Or are there limits to the voltages that the rectifier and other existing components can handle, such that a larger PS would require more substantial redesign? Just wondering... not planning to do this myself!
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Old 29th September 2009, 04:01 PM   #233
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The near-term practical limit is going to be the voltage rating of the caps in the B+ and B- supplies. Then you need to start adjusting the dropping resistors in the bias circuits for both B- and the FETs. Next up would be the rectifier, but that is well out of the range of the output tubes anyway.

If you want to eek out another 10 volts or so, you can try a different choke. Another trick would be to unload the 6.3V winding on the Edcor and use a separate transformer. That would free-up some capacity on the primary and the core which should give the other secondaries a little boost. Of course, that would sort of negate the point of that transformer. You could also go to a silicon rectifier.

BTW, how does your 5V look?

Like George implied, it's probably not going to make a noticeable difference to your ears with the 300Bs.
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Old 29th September 2009, 04:22 PM   #234
PaulyT is offline PaulyT  United States
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Originally Posted by rknize View Post
BTW, how does your 5V look?
You mean the 5V tap of the PT (AOT the 5V F+)? That seems to be ok - at least it measures right around 5V at the PT leads (T1-2,3) when the system is running. Guess I could also measure the current coming from the various PT taps, make sure I'm not somehow over-stressing the PT beyond the specs.
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Old 29th September 2009, 04:53 PM   #235
rknize is offline rknize  United States
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OK..just checking that it was in spec. If it was low, it might cause lower emissions in the rectifier. Sounds like you are fine.
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Old 1st October 2009, 11:03 PM   #236
PaulyT is offline PaulyT  United States
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Ok, looking around for a possible diferent choke, 6-10H range, 200mA, <100ohm DCR... anybody know of a relatively inexpensive source?

What's the actual current coming out of the B+ supply? Is a 150mA choke sufficient? I guess I could measure this for myself... may do that tonight as I start tinkering around, hopefully starting work on the enclosure that I've been too lazy to get to. Sigh...
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Old 2nd October 2009, 01:16 AM   #237
Evenharmonics is offline Evenharmonics  United States
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Pauly, my TSE initially had around 340V for B+ which I was going to use as is but only increased it over 350V after hearing about Valve Art 300B sounding better above that voltage. If your EH 300B sound good as is, I wouldn't change a thing.
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Old 5th June 2018, 09:01 PM   #238
belyitigr is offline belyitigr
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Hello guys,
My name is Pavel, I`m from Columbus Ohio.

I have some experience in DIY, I built speakers and amp, not tubeamp, just stereo double mono (spent more than 400$ its not Chinese kit)). But I never heard how sound tubeAmp in the real, only a lot of good reviews and words. And I wanna try build SSE, but before spend another hundredths dollars will be cool hear amp in real.
Maybe we have member in Columbus area and we will able compare my amp and SSE?
Thanks.
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