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Old 4th March 2008, 09:20 PM   #1
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Default output impendance

I have an ultralinear amp running from four EL34 with 8 and 16 impedance selectors and an 8 Ohm speaker. A technician cut off the cathodes from two tubes and told me that now the trafo sees same impendance as before, becouse the anodes are still wired from all four tubes, but the current is running only trough two of them.

How is it possible that the impendance is as before and the current changed?

Thanks,
Miha
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Old 4th March 2008, 09:58 PM   #2
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Default Re: output impendance

Quote:
Originally posted by overdrajv
I have an ultralinear amp running from four EL34 with 8 and 16 impedance selectors and an 8 Ohm speaker. A technician cut off the cathodes from two tubes and told me that now the trafo sees same impendance as before, becouse the anodes are still wired from all four tubes, but the current is running only trough two of them.

How is it possible that the impendance is as before and the current changed?

Thanks,
Miha
It's not. Find yourself another tech, that one is putting one over on you.

Remove two tubes and you have the same voltage, but half the current, so the load impedance doubles. Also, if two VTs are out, they don't have any rp, so that impedance doubles as well. Now the big question is: why did he cut out those two tubes in the first place?
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Old 4th March 2008, 10:08 PM   #3
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Default Re: output impendance

Quote:
Originally posted by overdrajv
A technician cut off the cathodes from two tubes and told me that now the trafo sees same impendance as before, becouse the anodes are still wired from all four tubes, but the current is running only trough two of them.


WHAT??

This technician was a technician repairing soccer goal-posts, or what?
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Old 5th March 2008, 09:07 AM   #4
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Default Re: Re: output impendance

I thought so..

Quote:
Originally posted by Miles Prower


Now the big question is: why did he cut out those two tubes in the first place?
I wanted to quiet it down
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Old 5th March 2008, 12:04 PM   #5
ilimzn is offline ilimzn  Croatia
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Default Re: Re: Re: output impendance

Quote:
Originally posted by overdrajv
I thought so..
I wanted to quiet it down
Your technician has no idea what he is doing.
What does it mean, quiet it down? Less volume?
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Old 5th March 2008, 12:33 PM   #6
jnb is offline jnb  Australia
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Someone on this forum can surely help you diagnose your noise problem. It's possible you may only need to describe it.
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Old 5th March 2008, 12:45 PM   #7
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Default Re: Re: Re: Re: output impendance

Quote:
Originally posted by ilimzn


Your technician has no idea what he is doing.
What does it mean, quiet it down? Less volume?
Yes. I'm using it for the guitar and i get the most desireable tone near the max output. Even though the impendances obviously don't match, the sound is better than before (it's softer, there are less highs).

Anyway, it is still too loud and I want to wire it in the triode operation by connecting g3 to the plate with 100 Ohm 2 Watt resistor. Do you think this is a good idea? I know that triodes have lower impedances than pentodes (not shure about triode connected pentodes) so the difference in impendance should be less than before (I'm driving an 8 Ohm load and the trafo wants to see 16 Ohm).

Pozdrav!
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Old 5th March 2008, 12:55 PM   #8
jnb is offline jnb  Australia
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It will sound different, YMMV.

You may need to rebias.
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Old 5th March 2008, 06:54 PM   #9
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Default Re: Re: Re: Re: Re: output impendance

Quote:
Originally posted by overdrajv
Anyway, it is still too loud and I want to wire it in the triode operation by connecting g3 to the plate with 100 Ohm 2 Watt resistor. Do you think this is a good idea? I know that triodes have lower impedances than pentodes (not shure about triode connected pentodes) so the difference in impendance should be less than before (I'm driving an 8 Ohm load and the trafo wants to see 16 Ohm).
If you convert to triode operation, you will cause a reduction in both the Zo and the Zl. They'll be operating into a higher impedance, which means less power, and less distortion. As to what that does to the sound, who knows? As for getting a different sound from these PPP types, pulling one set of finals is commonly done.

If the whole problem is too loud for close quarters, instead of doing all sorts of electronic surgery that may not have good results, why not add an attenuator between the amp and speaker? Lots of axemen use these for just that purpose. They even have a trade name: Power Soak, or something like that.
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