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Old 26th October 2010, 02:54 PM   #11
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Could I have a look at the schematics in single ended? There are not many GU-50 amps schematics out there and even if there are, many are in P-P mode.


Quote:
Originally Posted by Ian444 View Post
Not many people use GU-50 which is a great thing as it keeps the price low on ebay. Maybe due to the high voltage needed to make them sing, and high power output, and the sockets aren't cheap either.

The schematics I have found for GU50 single-ended have a 250V regulator on the screen, so they are running pentode.

Try valvewizard for theory that does not turn your brain inside out. Its very practical-orientated.
Do a search on "valvewizard"

Edit: Its easy to build a design that is known to work, but when you want to do your own design you need to know the basics behind it, which isn't that difficult to learn but it requires effort.
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Old 26th October 2010, 03:06 PM   #12
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Just a further point: 700VDC + newbie = very dangerous situation

Take note from those who have been zapped, melted clip leads and scope probes, and generally made all the common mistakes at 400V: death is still permanent and one little mistake at 700V will kill you.
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Old 26th October 2010, 04:19 PM   #13
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Quote:
Originally Posted by quikie22 View Post
Hi Wavebourn,

You are one of my most respected forumers :-) The reason I am trying the GU-50 amp in preference to the other more "common" tubes is because of reading about all your experience with it. It seems like a "super tube", very well made and works extremely well.
Yes, if to judge fairly one GU-50 must cost at least 5 of KT-88, but both tubes were made for high power and high voltages that is dangerous.
Let's discuss it theoretically; I am not suggesting you to play with high voltages until you have enough of experience.

Here is a picture from Erik's link:


Click the image to open in full size.

Curves on it represent dependence of anode current on voltage between anode and cathode. Each curve was taken by measurement, when negative voltage on control grid was fixed. Different voltages on control grid resulted in different curves.

Blue vertical and horizontal lines represent selected idle point. As you may see, it is 350V and 100 mA. 350Vx100mA=35W dissipation by anode. It is below max rate 40W (actually, 100W shortly is Ok for this tube; in transmitters anodes sometimes were red when antenna detuned), so in such mode the tube will live long life. As you may see, the point where this lines cross each other is right in the middle between -40V and -60V curves. That means, it corresponds to -50V bias voltage. -50V/100mA = 500 Ohm, so 510 Ohm resistor will be fine. Power dissipated on the resistor will be 500x100=5W, so 7.5 - 10 W resistor would be fine. I use in such cases Caddock film resistors, they look like power transistors in TO-220 case.

Pink, yellow and orange lines are load lines for different load resistances (you can understand it thinking of Ohm law: how will look on the graph dependence of current on voltage when resistance is fixed) for 5K, 3.5K, and 2.5K loads. For each load you can draw a table, of dependence of anode current and voltage on control grid voltage.
For example, for 2k5 when input voltage is zero max current and min voltage on anode corresponds to 150V and 175 mA.
Max voltage and min current would mean max swing to the opposite side. 350V (idle) - 150V (max current) = 200V swing, so an opposite swing will be 350V (idle) +200v (swing) = 550V (max voltage on anode).
This max voltage as you may see occurs when voltage applied to the control grid is something like -120V according to graphs.
Now, to calculate output RMS power we need to know RMS voltage from peak one. 200/1.414=141V approximately. 141 squared is 20,000 approximately. 20.2000 divided by 2k5=8W output power. It is quite a big power for SE amp!

What can you conclude? B+? Cathode bias resistor? Output transformer? Voltage swing needed to drive the stage?

You have all data in a nutshell, thanks to Erik!
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Last edited by Wavebourn; 26th October 2010 at 04:23 PM.
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Old 26th October 2010, 05:13 PM   #14
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Waveburn I think 8W is rather optimistic. The Pout for 350V/100mA and 2K5 load (without considering the insertion loss of the output transformer) is a bit less than 6W. For 100V peak-to-peak swing you get (from the curves) approx. 360V PP and 130 mA PP which in turn are 127 V RMS and 46 mA RMS. So Pout = 127x0.046 = 5.8W+

@Erik I have had a look at your blog and I can see the same thing about the 45. 1.3W with 202V anode supply and -33V bias is impossible if you don't drive the tube into positive grid. The Pout for this operative conditions is around 0.7 W. I am also sure about this because of direct experience on new/NOS 45 tubes (which test/measure perfectly).

45

Last edited by 45; 26th October 2010 at 05:16 PM.
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Old 26th October 2010, 05:21 PM   #15
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Thank you 45;
however real life is a bit more complex than all models. Non - linearity, grid currents, losses in transformer, etc..., all plays own role, but for now let's our new friend understand basics.

10K load will give much less power, but with high sensitive speakers sound will be crisp and smooth, like God's kissing.
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Last edited by Wavebourn; 26th October 2010 at 05:24 PM.
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Old 26th October 2010, 06:09 PM   #16
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Hi Wavebourn,

In post #5 you say: "it is better to use cathode bias since the amp is SE".
Please explain (unless you presume a high value grid resistor).

Pieter
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Old 26th October 2010, 06:33 PM   #17
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Quote:
Originally Posted by Wavebourn View Post
Thank you 45;
however real life is a bit more complex than all models. Non - linearity, grid currents, losses in transformer, etc..., all plays own role, but for now let's our new friend understand basics.
Of course it's more complex and usually is worse. However that estimate is rather good starting point.

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Originally Posted by Wavebourn View Post
10K load will give much less power, but with high sensitive speakers sound will be crisp and smooth, like God's kissing.
Well, I am not sure. More likely you will end up with 2.5W! I cannot see the advantage of this.
Better to go with a 2A3 (a nice chinese Golden Dragon or a vintage 6C4C are just fine) and 5K load then.
With this GU50 I would rather go for lower current at approx. -60V to -65V and use feedback and have more power. Actually I would use it as pentode and take advantage of its higher efficiency. IMHO, of course.

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Old 26th October 2010, 06:55 PM   #18
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Quote:
Originally Posted by pieter t View Post
Hi Wavebourn,

In post #5 you say: "it is better to use cathode bias since the amp is SE".
Please explain (unless you presume a high value grid resistor).

I presume variations of mains voltage and tube tube emission. Cathode bias is a negative feedback by DC current that keeps the tube in the regime. Fixed bias is a workaround for P-P stages that draw current modulated by signal's envelope. If the amp works in class A it is not needed, so you can take an advantage of cathode bias that you loose when have to apply fixed bias to class AB amps.
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Old 26th October 2010, 07:14 PM   #19
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Quote:
Originally Posted by Wavebourn View Post
If the amp works in class A it is not needed, so you can take an advantage of cathode bias that you loose when have to apply fixed bias to class AB amps.
Sorry Wavebourn to quote you again but I cannot resist to comment this!
If the amp works in class A a fixed bias is not needed only for a textbook example. You yourself have already mentioned a series of issues that actually make fixed bias more desirable in most situations....
If then we talk about the sound is even more desirable.
Cathode bias is just simpler and more suitable for a novice.

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Old 26th October 2010, 07:15 PM   #20
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What I meant to say is that SE does not include the use of cathode bias.
Think of interstage or cathode follower driven SE class A2 power stages (845 and many transmitting tubes) where the use of fixed bias is normal.
Variations of main voltage is not so much a problem in well designed fixed bias power stages; when main voltage rises so will the negative bias, resulting in higher plate voltage and lower plate current at the same time so the tube will stay in the regime.

Pieter
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