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Old 27th February 2003, 03:21 PM   #1
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Default Mullard 3-3

Oneday I will want to build a "classic" tube amp. The Mullard schematic posted here, is using the el84 in pentode right?

So I suppose I could ask for a UL tap as well when getting the output transformer wound? Then I could add a switch so I can switch between UL and triode mode.

When disconnecting the ul tab with a switch how should I wire the screen? I am still confused about triode or pentode mode. When I pseude triode mode I should just connect the screen to the anode via a small resistor?

[EDIT]As opposed to a pentode that has a seperate b+ connection?[END EDIT]

Thanks in advance.
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Old 27th February 2003, 04:02 PM   #2
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Default A LA MODE.

Hi,

Yes, the EL84 is run in penthode mode.

You could ask for an UL OPT, no obligation in actually using it though.

Quote:
When disconnecting the ul tab with a switch how should I wire the screen?
You could ground the screen or tie it to the cathode.

Quote:
When I pseude triode mode I should just connect the screen to the anode via a small resistor?
No, that is regular penthode mode.
Pseudo triode just means that you try to convert a penthode into a triode...since it is not a triode from the start they call it a pseudo-triode.
Confusing, isn't it.

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Old 27th February 2003, 04:09 PM   #3
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Thanks Frank,

It is especially confusing as certain diagrams say it is in pseudo triode mode but the screen is connected to the anode with a 150ohm resistor....fr instance in the little decware amps..

Cheers,
Bas
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Old 27th February 2003, 04:11 PM   #4
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Default Mullard 3-3

Yup,

The circuit using the EL84 is the original, the only variations I've come across remove the bass control from the feedback circuit and the top-cut treble from the input, and replace them with the passive bass/treble controls often seen with the 5-10 amp.

Because of the direct coupling between the EF86 and the EL84 I'd be very careful about possibly upsetting circuit conditions by converting the EL84 to triode operation. You could convert the coupling to traditional RC, but then you would risk greater distortion and less bass response. The 3-3 used direct coupling in an attempt to overcome these limitations in a small amp.

If I remember rightly even in the 60s the 3-3 came in for criticism, many purists felt that using grid bias on the EF86 rather than cathode bias could result in increased distortion.
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Old 27th February 2003, 04:18 PM   #5
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Bas, using a resistor is still "triode mode". The decware amp, among hundreds of others use the resistor to limit current flow surges - thus protecting the screen grid. In class A designs, they are not really needed.
So, in your amp, either the screen will be connected to the UL tap, or directly to the plate.

In no case would you ground it - I'm not sure why Frank suggested that. He is thinking of the "suppressor grid", G3, maybe? This is tied to the cathode internally.
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Old 27th February 2003, 04:19 PM   #6
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Thanks for the background on the Mullard!

I do not like the feedback etc...or the top cut treble, etc.. so I might just not include that..on the other hand that defeats the object a little ......as I wanted to build something verbatim..

but then I got doubts again and thought fr instance about the UL tap.


Maybe I should just build it stock first and then modify ...could be an interesting educational experience.
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Old 27th February 2003, 04:22 PM   #7
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Quote:
He is thinking of the "suppressor grid", G3, maybe? This is tied to the cathode internally.
Must be.

t..been thinking about that too...

Why do they always show that as a wire? When it is allready internally connected? Maybe because some were and some were'nt.?
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Old 27th February 2003, 04:34 PM   #8
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Default Mullard 3-3

Bas,

You've got it right - in some valves g3 was internally connected (like the EL84) and some it wasn't (lie the EF86).

I agree with you on the bass control in the negative feedback circuit - one of the advantages of using direct coupling in the 3-3 was that with less phase shift greater amounts of feedback could be used to improve linearity. By having a frequency selective system to reduce feedback at lower frequencies (allowing greater gain at low frequencies) it will therefore also allow more distortion at low frequencies, because of the lower feedback!
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Old 27th February 2003, 04:38 PM   #9
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Default TRIODE MODE.

Hi,

Quote:
In no case would you ground it - I'm not sure why Frank suggested that. He is thinking of the "suppressor grid", G3, maybe? This is tied to the cathode internally.
Isn't pseudo-triode mode operation achieved by connecting the grids together?

Which in this case will tie the grids to the cathode of the 6BQ5...

I could be wrong about this , but tieing the g2 to the anode doesn't turn the tube into a triode IMO.

Cheers,
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Old 27th February 2003, 04:43 PM   #10
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Default Re: TRIODE MODE.

Quote:
Originally posted by fdegrove
I could be wrong about this, but tieing the g2 to the anode doesn't turn the tube into a triode IMO.
Yes, you are wrong about that. It does turn it into a triode. A "pseudo triode" in fact. That is how you make one.

UL was also called "super triode" mode occasionally, back in the day.
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