Mullard 5-20 in triode mode - diyAudio
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Old 5th October 2004, 10:12 PM   #1
icebear is offline icebear  Norway
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Default Mullard 5-20 in triode mode

Hi

What should the value of the resistor between the screen grid (4) and the anode (3) on the EL34 be to run it in triode-mode? Is there any other things I should be aware off? How about the OPT? Would it be suitable for the task? I'm thinking of just make a switch between pentode and triodemode.

Bjorn
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Old 6th October 2004, 12:40 AM   #2
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Hi,

Quote:
What should the value of the resistor between the screen grid (4) and the anode (3) on the EL34 be to run it in triode-mode?
You can use the same 1K (?) resistor. I'd recommend a 3W one.

Quote:
Is there any other things I should be aware off?
The most important thing to remember IMHO is that the amp was not designed for this kind of service so the gain will be different when in pseudo-triode mode and the OPT will be a mismatch as well as the impedance will not be optimized for the load either.
No damage should occur from this operation however.
It's just not really fair to compare the two modes of operation this way.

This could be of interest to you as general info in the context of the Mullard amp:

EL34

Cheers,
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Old 6th October 2004, 09:14 AM   #3
icebear is offline icebear  Norway
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Default OPT

I guess my OPT is ca: 3,2 kohm with a ultralinar tap. Can I use this for a lonly EL34 in SE triodeconnection?

Bjorn
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Old 6th October 2004, 12:32 PM   #4
SY is offline SY  United States
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3.2k is a pretty low load for a triode EL34. A better match would be 6.4K, which you could obtain by connecting a (nominal) 8 ohm load to the 4 ohm taps.
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Old 6th October 2004, 01:18 PM   #5
icebear is offline icebear  Norway
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Default ?

A resistor or my speaker?
b
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Old 6th October 2004, 01:28 PM   #6
SY is offline SY  United States
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The speaker. We like to pretend that an 8 ohm speaker is 8 ohms, but of course this is a fiction we use to sleep better at night. But whatever the impedance curve looks like, by connecting it to the next lower impedance tap on the secondary, we double to anode-anode load. And in this case, that's probably a good thing.

The other thing to watch out for here is stability and bandwidth. When you convert to triode, you impose a much more severe capacitive load on the driver. This newly-lowered pole can compromise stability, and if it's low enough, you can get noticeable high frequency rolloff. IIRC, the 5-20 uses a 12AX7 as a driver- that's a tube without much ability to charge and discharge a Miller capacitance very quickly.

Anyway, I'd try the connection using a resistor dummy load first, see if the bandwidth and stability are acceptable, then go from there.
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Old 6th October 2004, 08:43 PM   #7
icebear is offline icebear  Norway
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Default Opt

Checked my opt, and found that it was 6,6k. How should that affect it? I listened to both 4 and 8 ohm, but I think it sounds fuller and more natural on 8 ohm. The "S" sound is not as sharp as on the 4ohm tap.
Here it is: (http://www.demostenes.no/2.0/endr1be.asp?Kode=6009)

I want to connect the EF86 as a triode. Like its done in (http://www.lundahl.se/pdfs/claus_byr...fier_30wpp.pdf)

How will that affect the feedback when the output is connected in triode . Which values should I use?



Bjorn
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Old 6th October 2004, 10:53 PM   #8
SY is offline SY  United States
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6.6K is close to optimum.

If you change the input tube over to triode, you'll drop the open loop gain and reduce the feedback. It turns out, on closer analysis, that the difference is less than one would imagine, though. In "Valve Amplifiers", Morgan Jones does a very detailed analysis of the 5-20 circuit. If you're serious about experimenting here, it's a book you ought to have.
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Old 9th October 2004, 05:56 PM   #9
icebear is offline icebear  Norway
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Default It's done

So both the EL34's and the EF86's are in triode. I must say it sings like never before :-) I thougth about completing the mods described on the Lundahl pages, but for now I'm satisfied.

Here is a picture of the setup. The OTL in the rack is not mine. Just for loan.

http://www.memphis-highend.de/diy/ga...ag.htm?gbId=12

Thanks for the help (so far)

Cheers
Bjørn
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Old 9th October 2004, 06:03 PM   #10
icebear is offline icebear  Norway
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Quote:
Originally posted by fdegrove
Hi,



You can use the same 1K (?) resistor. I'd recommend a 3W one.





Came along this link: http://diyparadise.com/triode_mode.html

Any use for me to try this, or should I leve it with the 1k?

Bjørn
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