Mystery PRESIDENT amp into a 5E3 build

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andresound
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Mystery PRESIDENT amp into a 5E3 build

Post by andresound »

Hi Guys,

I purchased a MYSTERY President amp (NO info on their NETT), and have decided to use as a donor for a 5E3 build.
Being in Africa and not having too much access to components, I do not have a 12AY7 for v1. Can I use a 12AT7 (have lots of NOS lying around)? The 12 ay7 has a gain of 40 and the T7 a gain of about 60-70. What changes do I need to make to the circuit if I use this tube.??

I did notice that that the WEBER 5E3P "proluxe" uses a 12ax7 without any changes to the circuit!!! (Besides the use of higher filtering and a GZ34)

Help would be appreciated.
If it sounds good, it is good! Trust your ears
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xtian
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Re: Mystery PRESIDENT amp into a 5E3 build

Post by xtian »

No changes needed. If you decide you need to dump some gain, you can manage that in a number of ways. But I think you'll be happy with an AT. Or even an AX!
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Stevem
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Post by Stevem »

These numbers
12ax7 100
12at7 70
12ay7 40
Are the max gain that will be seen when the tubes are run at there max spec voltage and with a given correct plate load, cathode and grid reference resistor to do so.
in general you can play around gain wise by changing a plate or cathode resistor by 25 percent up or down and not have a concern of running the tube too hard .
It's quite possible to have a strong AT7 that out gains a weak AX7.
You can hook up your voltmeter set for AC volts on the out bound side of a plate coupling cap and note the voltage output differences from tube to tube that you test in circuit.
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andresound
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Re: Mystery PRESIDENT amp into a 5E3 build

Post by andresound »

Thanks.
So does decreased value on plate and increased cathode resistor reduce gain?
If it sounds good, it is good! Trust your ears
andresound
Posts: 701
Joined: Mon Dec 01, 2014 8:48 am
Location: Durban South Africa

Re: Mystery PRESIDENT amp into a 5E3 build

Post by andresound »

Thanks.
So does decreased value on plate and increased cathode resistor reduce gain?
If it sounds good, it is good! Trust your ears
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martin manning
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Re: Mystery PRESIDENT amp into a 5E3 build

Post by martin manning »

With the cathode resistor unbypassed, gain A is μ(Ra/(Ra + ra + Rk(μ+1)) so in that case increasing Rk reduces gain. If the Rk is bypassed (the most common arrangement), it's just μ(Ra/(Ra + ra)) and Rk doesn't affect the gain at all. Increasing the plate load Ra increases gain either way.

The amplification factor mu (μ) is what the stage gain would be if you had an infinitely large Ra. So ~100 for a 12AX7 and ~40 for a 12AY7. The internal plate resistances ra are different too, at ~62k and ~30k.

With typical 100k Ra, and 1k5 cathode resistor Rk (fully bypassed), the gain for a 12AX7 stage is ~60 and ~30 for a 12AY7. Bypassing the Rk is worth a factor of two for the 12AX7, so if you remove the cathode bypass cap the gain will be reduced to ~30, and it will behave pretty much like a 12AY7 with bypassed Rk. You could make that an option by putting a resistor between the bypass cap and ground (10x Rk), and using a switch to short it out for a high gain mode.
andresound
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Re: Mystery PRESIDENT amp into a 5E3 build

Post by andresound »

Wow!!! Thanks again Martin. Always a source of unbridled info and wisdom.
If it sounds good, it is good! Trust your ears
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