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Old 5th Jan 2007, 12:27 pm   #21
oldeurope
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Smile Re: Stereo amp upgrade

Thanks for the help, Jeffrey.I agree with you.
Hi Merlin, it is vise versa, the triode needs better filtering!
Connect the g2 to plate and listen!
In the pic you can see the output diagram of the EL84.
I calculated the equivalent resistor for the EL84.
Conclusion is 13% of the riples appears across the transformer.
But now comes the best:
The kathode of the EL84 is not blocked (!), this increases the output resistance of the EL84. Means Adis application and more than 95% of the riple voltage is suppressed by the pentode.

Darius
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Last edited by oldeurope; 5th Jan 2007 at 12:29 pm. Reason: added some text
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Old 5th Jan 2007, 12:37 pm   #22
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Default Re: Stereo amp upgrade

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Originally Posted by Merlin View Post
It is a bit counter intuitive yes. The actual ripple signal appears across both or course, but the ratio of the two impedances determines the amplitude of the ripple signal. When ra is high, so too is the ripple amplitude, which is fed into the OT.
It's counter intuitive because it's just not true. If almost all the ripple appears at the anode of the valve that's fine.

As an example let's say there's 50Vp-p ripple on the supply, the anode impedance is 100K and the reflected speaker impedance via the transformer is 2K. The ripple voltage will appear across this potential divider in proportion to the impedances. That is 49Vp-p ripple at the anode and 1Vp-p across the transformer.

I have treated impedance as resistance and ignored reactances in this example since it will have negligible effect on the result.

To explain it another way, the transformer is connected between the anode (lots of ripple) and HT+ (lots of ripple) so not much ripple across transformer. If the load was connected from the anode to 0V then the load would see lots of ripple.
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Old 5th Jan 2007, 1:56 pm   #23
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Default Re: Stereo amp upgrade

So you're saying that pentodes have better PSRR than triodes when transformer loaded, but worse PSRR when used as a resistor loaded RC coupled gain stage?

Darius, what do you mean "the kathode is not blocked"?
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Old 5th Jan 2007, 2:03 pm   #24
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Default Re: Stereo amp upgrade

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So you're saying that pentodes have better PSRR than triodes when transformer loaded, but worse PSRR when used as a resistor loaded RC coupled gain stage?
I'm pretty sure that's implied by what I've said. There is a fundamental difference between a transformer coupled and RC coupled stage. The transformer is connected between anode and HT+. In an RC coupled stage the output is taken between anode and 0V.

Beware of applying this theory to transistor amplifiers since these can use both NPN and PNP devices so the reference rail for any particular signal may not be immediately obvious.
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Old 5th Jan 2007, 2:18 pm   #25
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Default Re: Stereo amp upgrade

I see what you mean- getting my principles mixed up again!

Either way, I wouldn't liek to run a pair of SE EL84's right off the rectifier- the ripple will be ridiculously large.
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Old 5th Jan 2007, 3:23 pm   #26
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Default Re: Stereo amp upgrade

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Darius, what do you mean "the kathode is not blocked"?
No cap to ground or not directly grounded. How do you call this?

Darius
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Old 5th Jan 2007, 3:43 pm   #27
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Default Re: Stereo amp upgrade

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No cap to ground or not directly grounded. How do you call this?
This capacitor is called a decoupling capacitor. For a cathode connected without a decoupling capacitor you would say not decoupled or undecoupled. You might say with cathode feedback since the un-decoupled cathode resistor causes negative feedback.
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Old 5th Jan 2007, 4:22 pm   #28
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Default Re: Stereo amp upgrade

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This capacitor is called a decoupling capacitor. For a cathode connected without a decoupling capacitor you would say not decoupled or undecoupled. You might say with cathode feedback since the un-decoupled cathode resistor causes negative feedback.
Thanks Jeffrey.

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Old 6th Jan 2007, 1:49 am   #29
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Default Re: Stereo amp upgrade

Hmmm...interasting...

There is actually very little of any hum! The EL84's are being fed straight from the rectified cathode, but...no hum.

The EL84's seem to run abit on the hot side, but appart form that i havnt noticed anythign bad. I guess EL84's arnt the coldest runnign valves ever.

They are too hot to touch for any significant time, but thats about it. I think thats pretyt normal anyway?
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Old 6th Jan 2007, 9:07 am   #30
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Default Re: Stereo amp upgrade

Adi.

EL84's do run very hot...too hot to touch after some minutes. However if you are concerned, check the cathode current by measuring the voltage drop across the cathode resistor and apply ohms law. Otherwise just connect a meter in series with the cathode resistor and measure the current directly. If it is a little on the high side, you'll probably need to increase the cathode resistor slightly.

Don't have the characteristics of the EL84 to hand but I think around 45mA should be OK. Remember that measuring the total current through the cathode is a combination of anode AND screen grid current.


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Old 6th Jan 2007, 9:45 am   #31
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Default Re: Stereo amp upgrade

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Originally Posted by adibrook View Post
Hmmm...interasting...

There is actually very little of any hum! The EL84's are being fed straight from the rectified cathode, but...no hum.

The EL84's seem to run abit on the hot side, but appart form that i havnt noticed anythign bad. I guess EL84's arnt the coldest runnign valves ever.

They are too hot to touch for any significant time, but thats about it. I think thats pretyt normal anyway?
Good morning Adibrook,
what g2 voltage do you run the EL84s? I expected something between 250V - 300V DC. Did you test the ECC81? (see #14 #15)
What kind of speaker are you using?

Darius
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