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Old 4th December 2011, 02:26 PM   #511
cnpope is offline cnpope  United States
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Quote:
Originally Posted by M Gregg View Post
I agree something is not right,

Just looking at your wave on the scope..

I will have a look there is something strange...

If power is that low...it does not sound low...

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M. Gregg
Can you put a DVM on the output, and measure the rms voltages?

Chris
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Old 4th December 2011, 03:16 PM   #512
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by cnpope View Post
Can you put a DVM on the output, and measure the rms voltages?

Chris
Something is wrong the ouput RMS voltage is 4.6V...That can't be correct...Don't know if I have a fault...

Alastair can you check your RMS voltage out?

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M. Gregg
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Old 4th December 2011, 03:40 PM   #513
cnpope is offline cnpope  United States
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Quote:
Originally Posted by M Gregg View Post
Something is wrong the ouput RMS voltage is 4.6V...That can't be correct...Don't know if I have a fault...

Alastair can you check your RMS voltage out?

Regards
M. Gregg
Well 4.6V rms is certainly consistent with your scope trace showing 13V peak to peak.

I believe autobias could be expected to give a lower output power than fixed bias, but I'd be surprised if it should be as low as you are finding.

Chris
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Old 4th December 2011, 03:52 PM   #514
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by cnpope View Post
Well 4.6V rms is certainly consistent with your scope trace showing 13V peak to peak.

I believe autobias could be expected to give a lower output power than fixed bias, but I'd be surprised if it should be as low as you are finding.

Chris
I agree,

I am surprised..I find it hard to believe its this low by comparison..
I can get 10W out of EL34..auto bias..
I am going to look closer at the driver stage...
Also the distortion on the scope trace (on one side) is mystery at the moment...

Regards
M. Gregg
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What is the sound of one hand clapping?

Last edited by M Gregg; 4th December 2011 at 03:55 PM.
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Old 4th December 2011, 05:51 PM   #515
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Have to check in and enjoy the progress.

Think itīs time for Alistair to fire up the scope and check if he gets the same reading.

M Gregg can you cut the feedback and measure the different stages to see whats going on ?

Lifeīs slowly returning to normal so soon i have time to finish my amp and can participate in the measuring.

Looking forward to more readings of driver and PI.
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Old 4th December 2011, 07:00 PM   #516
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OK Right--Here goes....

Yup--I Boobed, I'm an idiot, a total fool--Go on, you might as well all larf at me.....

Today I did some semi-proper testing using my Tektronix 454 Scope and a pukka sig-gen, and some much,-- now obvious-- Tuning Up!.
(This is why its taken me a while to do this, Setting all this gear up when I have the place to myself....)

I found that the PI cannot currently swing more than around 20V p-p before flat-topping/distortion, similar to what M-Gregg was having.

I have increased the values of the two PI resistors to 47K and now have 60V p-p before any mishaping/distortions, just a nice sine-wave.

PI +B Supply is 320V...

I've also dropped the Grid resistor to PI to 100K.

Running PL509, a single pair, the amp will drive 11V p-p of CLEAN, Perfect sine wave @1KHz with No sign of distortions or flat-topping, into 8 ohms.

Increasing the O/P to apprx 11.5V, we get the usual flat-topping (Symetrical) And Crossover Distortion, Caused by the Cathode-Caps charging and altering the bias of O/P valves.

This was done into an 8 Ohm non-inductive wire-wound resistor.

So, The Peak Watts value I currently have from a single channel with only a single pair of PL509 is 15.125W --Undistorted.

This equates to--If my maths is good-- to 10.6 Watts, RMS.

--Which means the O/P valves are supplying 1.375 Amps of Current.

--So, Although the O/P pair COULD make More Power if Fixed-Bias, the Self-Bias is effectively limiting the Current Passed to just a Little Under the Max Pulse value in the data-sheet (1.4A for the '509), which cant be bad.

I left it running just before the point of clip like this to see what happened to the O/P valves.

--The answer was, Nothing! They got fairly warm but no red-plates or glowing screens, just worked...

I did a Full Power Frequency test (sine-wave). I was unable to determine the LF rolloff as my sig-gen only goes to 3Hz, so its below that.....
The -3dB upper freq limit was 200KHz, Good enough even for the Batmen among you...

The Only slight issue I have which I have not yet addressed is a Slight Overshoot on Square-waves but happening at 30KHz.....

So--Go on, have a good chuckle, Yup--I boobed....
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Old 4th December 2011, 07:17 PM   #517
cnpope is offline cnpope  United States
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Quote:
Originally Posted by Alastair E View Post
OK Right--Here goes....

[...]
Running PL509, a single pair, the amp will drive 11V p-p of CLEAN, Perfect sine wave @1KHz with No sign of distortions or flat-topping, into 8 ohms.

Increasing the O/P to apprx 11.5V, we get the usual flat-topping (Symetrical) And Crossover Distortion, Caused by the Cathode-Caps charging and altering the bias of O/P valves.

This was done into an 8 Ohm non-inductive wire-wound resistor.

So, The Peak Watts value I currently have from a single channel with only a single pair of PL509 is 15.125W --Undistorted.
[...]

This equates to--If my maths is good-- to 10.6 Watts, RMS.
....
Hang on, there's something wrong here. 11V peak-to-peak means 11/(2 sqrt(2)) V RMS, i.e. about 3.9V RMS. This means the continuous power (RMS power) is about (3.9)^2/8 which is about 1.9W.

Chris
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Old 4th December 2011, 07:19 PM   #518
M Gregg is offline M Gregg  United Kingdom
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Alastair,

I aint laughing at you...

I think its great you manage to get more answers each time...
Well done!

Got to try it yet though...LOL
If it cleans up the distortion thats a good start...


Regards
M. Gregg
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What is the sound of one hand clapping?

Last edited by M Gregg; 4th December 2011 at 07:23 PM.
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Old 4th December 2011, 07:24 PM   #519
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Err--Summit wrong somewhere then.

2.8W Hmm....

How On EARTH is it Just So Damned LOUD then! Defo comparable with a 25W sand-amp i have here running the Same Speakers!
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Old 4th December 2011, 07:52 PM   #520
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Alastair try dropping the gridstopper to 10k or lower when your at it.

It sounds like something is odd if you should have around 3w and smokes a good 25w amp

Next weekend itīs OTL time and iīll put it to test on my HP equipment, then we have a spectrum and distorsion reading.
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