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Old 11th March 2003, 05:46 PM   #1
G is offline G  United States
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Default 7189 in pentode mode?

I currently have a 150 ohm resistor running from the plate to the screen in my SE 7189 amp. I would like to run it in pentode mode and eek out a little more power. To do this I gather that I should connect the screen directly to B+? Any help is appreciated

G
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Old 11th March 2003, 06:05 PM   #2
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Default Pentode

G,

Did you check out the discussion under "Mullard 3-3" Bas Horneman Threadstarter? It is on page 2 of Tubes forum.

I think you are correct but check out the discussion--all the usual commenters posted--I learned a lot!

Rick
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Old 11th March 2003, 06:14 PM   #3
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Default Re: 7189 in pentode mode?

Quote:
Originally posted by G
I currently have a 150 ohm resistor running from the plate to the screen in my SE 7189 amp. I would like to run it in pentode mode and eek out a little more power. To do this I gather that I should connect the screen directly to B+? Any help is appreciated

G
No, that's not the way to do it.

To make exact recommendations we would need to know your HT voltage and the current you are running through the 7189s.

Reference to the curves will enable you to select a suitable operating voltage for g2. The curves will also tell you how much current the screen will draw. Having decided on a screen operating voltage and knowing the current, you can then calculate the value of the screen supply resistors (i.e. one for each 7189). You must then by-pass this supply to ground; a typical capacitor would be about 3.3uF. Note that the behaviour of a pentode is almost entirely determined by the voltages at g1 and g2 - anode voltage has virtually no effect on the amount of current that the valve passes - which is why the curves are horizontal.

You will also have to adjust your bias accordingly.

Unfortunately, connecting as a pentode will a) raise the anode resistance of the 7189 from perhaps 2k to about 40k and b) will cause the valve to produce a considerable amount of odd-order harmonic distortion. The effect of (a) will be that the reflected impedance as seen at the speaker terminals will change dramatically. You may find gaps in the frequency response and probably rather strange bass - it is difficult to predict.

This is why pentode amplifiers are usually run with about 20dB of negative feedback; this largely cancels the distortion, improves bandwidth and lowers the output resistance of the amplifier so that it can drive loudspeakers properly.

Many people believe that feedback destroys the sound of an amplifier (this despite the popularity of "vintage" designs such as Leak, Radford and the Mullard 5-20, all of which used plenty of negative feedback).

Unfortunately for satisfactory operation it is necessary with pentodes. And this is one reason why triodes, or triode-strapped pentodes are so popular.

Best of luck

7N7
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Old 11th March 2003, 06:14 PM   #4
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Thanks Frag. I just looked up the thread. I think I will try it.

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Old 11th March 2003, 06:20 PM   #5
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G,

When you convert to pentode mode the HT current will increase - will your power supply be able to handle the extra current?

Also, the optimum output impedence of the 7189 will change, and there will be a mismatch with the output transformer, although it would still probably work. However, because of the extra anode current drawn by a tube in pentode mode you need to check that the tranny can handle the extra current.
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Old 11th March 2003, 06:21 PM   #6
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Gosh,

Two posts put up while I was writing my last one...sorry for any duplication!
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Old 11th March 2003, 06:32 PM   #7
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G,

Having just seen the pictures of your amp on the other thread I don't think there would be any problem in their power handling capability for running the 7189s in pentode mode. Also, with a primary impedence of 4800 ohms they are probably a better match for pentode mode rather than triode mode.

If you are not using feedback then a small cap, about 0.001uf rated for 1000vdc across the primary of the output transformer will help reduce high frequency transients.
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Old 11th March 2003, 06:51 PM   #8
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Default ... and also...

In addition to all the good advice above, can I stress that in pentode mode the screen grid needs a low impedance dc source - relative to it's current requirement i.e. make sure that the screen voltage does not vary according to amplifier load. This is the quickest way of introducing psu modulation into a pentode - this is particularly true of SE designs.

Gary Pimm's schematic for his P-P DHP amp (see the thread with this title) shows a neat way of introducing voltage feedback to a pentode via a resistor between plate and control grid.

Finally I would use an anode stopper when running in pentode mode with a modern pentode as they will oscillate under certain conditions - a 150R carbon comp. is usually sufficent.

hope this helps

ciao

James
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Old 11th March 2003, 07:54 PM   #9
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Yes I agree with James, that a g2 supply should have a low output resistance.

My experimental "Instock" project, which was the subject of a thread about a month ago, uses pentodes (D3As) as cathode followers.

On these, g2 is supplied by a relarively simple emitter follower circuit. This actually consists of two resistors a transistor (MPSA92) and a 470nF capacitor. The two resistors form a potential divider which sets the base voltage of the transistor. The lower end of the potential divider is connected to the cathode of the pentode to g2 so that it stays in "step" with what's going on around it!

7N7
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Old 11th March 2003, 09:01 PM   #10
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G,

Do your output transformers have UL taps? (if they are Hammond they do). Connect your screen to those. You will get good sound plus more power.

As for how to connect the 7189 screens, all the spec sheets I have seen for them (EL84/6BQ5/7189) says 300 volts...anode voltage... or basically as you surmised, B+.

As for how 7189's sound in pentode, they sound very nice. I have heard an old SE stereo using them in a portable phono. Very nice. You won't be disappointed. At close to full power and clipping is where the distortion may eek.

AAMOF, I helped a young friend of mine who also built a 7189 based SE amp for a bi-amped system. We were both pleasantly surprised how good it sounded... after I helped tweak it for more output, similar to your problem.

BTW, NFB with pentodes is more for stability since pentodes have a very high amplification factor. Triodes can be as high as 100, where pentodes can be easily 10 times higher (this is because the transconductance is much higher... whatever that is ). Yes, this makes for more distortion, so both reasons are true. But they can be run without NFB as long as you don't use too much gain in your preamp/driver.

Hope this helps,
Gabe
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