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simple method for changing plate/cathode resistors on PCB

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lordearl

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Joined 2007
I'm currently building a SE triode mode EL34 amp using a PCB and would like to have the ability to alternate between 12AX7 and 12AT7 valves for the gain stage.

Does anyone know of a good jumper or terminal block system that would allow for changing the plate and cathode resistors (ie. to avoid desoldering)?
 
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lordearl

Member
Joined 2007
Yes that could work - you mean bend the resistor leads at right angles and put them into the 2 way terminal block?

This approach might even work using a vertical style of terminal block:

Screenshot_2023-09-23-07-33-23-26_40deb401b9ffe8e1df2f1cc5ba480b12.png
 
rayma

rayma

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lordearl

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Joined 2007
Thanks @tonyp063. That's probably the simplest and best solution, can get resistors in and out in a jiffy!

Anyone have a suggestion for what resistors perform well and can be tightly matched when in a gain stage? I've tried a few fairly cheap metal film and carbon types but the actual readings of the resistance does vary a fair bit. The tubes themselves probably vary more but at least it makes sense to control the variables that are easily controllable!
 
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Mark Tillotson

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Joined 2018
lordearl said:
Thanks @tonyp063. That's probably the simplest and best solution, can get resistors in and out in a jiffy!

Anyone have a suggestion for what resistors perform well and can be tightly matched when in a gain stage? I've tried a few fairly cheap metal film and carbon types but the actual readings of the resistance does vary a fair bit. The tubes themselves probably vary more but at least it makes sense to control the variables that are easily controllable!
Click to expand...
1% metal film or if you really need it 0.1% metal film. 1% is pretty standard these days.
 
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lordearl

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Joined 2007
petertub said:
Off topic, but why do you want to change tube models ? If it is gain changing then a simple pot would
be a safer alternative to adjust gain then changing tubes.
Click to expand...

Good question. This isn't so much about gain but increasing sensitivity in triode mode and getting a good amount of current into the EL34 stage.

In the circuit, the first triode of the 12AT7 is used as a conventional common-cathode amplifier which is DC coupled to the grid of the second triode configured as a cathode follower. Because the grid of the cathode follower is DC coupled to the anode of the amplifier stage, its grid sits at about 120 Volts. That means its cathode must be sitting at about 122V, which in turn means the cathode is sitting about 122V above the grounded side of the 12V heater supply.

Cathode follower stages have no voltage gain but very high current gain, or putting it another way, a very high input impedance and a low output impedance.

In a conventional tube amplifier the anode of the input stage is capacitively coupled to the grid of the output stage - there is no cathode follower. This works well as long as the input impedance to the output stage is very high (which it usually is).

However, any low-power amplifier at reasonable speaker volume is very likely to be driven into overload on music peaks and transients, and when that happens the output stage input impedance falls as it is driven into Class A2 and the grid starts to draw grid current.

In a conventional amplifier circuit, the plate of the input amplifier stage will not be able to supply sufficient current when the output stage enters class-A2, and the progression into overload would be very noticeable.

In this circuit, the low output impedance cathode follower stage is able to supply the Class-A2 current into the grid of the output stage.

The input impedance to a cathode follower is very high, so there is no impedance loading of the input stage, which allows a higher gain closer to the published amplification factor of the tube.

Electro Harmonix have a 12AT7 with a heater-cathode breakdown rating of 200v, but I'm thinking of using a 12DW7 (half 12AX7, half 12AU7) which will increase sensitivity but also give excellent current drive. Ultimately that means a change of cathode and plate resistors. Sorry - long winded answer but the designer of the circuit has clearly given this a lot of thought!!
 
petertub

petertub

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Joined 2015
tubular-well.se
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lordearl said:
Good question. This isn't so much about gain but increasing sensitivity in triode mode and getting a good amount of current into the EL34 stage.

In the circuit, the first triode of the 12AT7 is used as a conventional common-cathode amplifier which is DC coupled to the grid of the second triode configured as a cathode follower. Because the grid of the cathode follower is DC coupled to the anode of the amplifier stage, its grid sits at about 120 Volts. That means its cathode must be sitting at about 122V, which in turn means the cathode is sitting about 122V above the grounded side of the 12V heater supply.

Cathode follower stages have no voltage gain but very high current gain, or putting it another way, a very high input impedance and a low output impedance.

In a conventional tube amplifier the anode of the input stage is capacitively coupled to the grid of the output stage - there is no cathode follower. This works well as long as the input impedance to the output stage is very high (which it usually is).

However, any low-power amplifier at reasonable speaker volume is very likely to be driven into overload on music peaks and transients, and when that happens the output stage input impedance falls as it is driven into Class A2 and the grid starts to draw grid current.

In a conventional amplifier circuit, the plate of the input amplifier stage will not be able to supply sufficient current when the output stage enters class-A2, and the progression into overload would be very noticeable.

In this circuit, the low output impedance cathode follower stage is able to supply the Class-A2 current into the grid of the output stage.

The input impedance to a cathode follower is very high, so there is no impedance loading of the input stage, which allows a higher gain closer to the published amplification factor of the tube.

Electro Harmonix have a 12AT7 with a heater-cathode breakdown rating of 200v, but I'm thinking of using a 12DW7 (half 12AX7, half 12AU7) which will increase sensitivity but also give excellent current drive. Ultimately that means a change of cathode and plate resistors. Sorry - long winded answer but the designer of the circuit has clearly given this a lot of thought!!
Click to expand...
That does not explain why changing resistors and tubes repeatedly is needed.
Btw, use a separate triode with it's own filament elevated to some level will keep the amp ticking
without problems, even if that means an extra tube. Trusting a "special" variant that is claimed to
be better is only a way to paint one in a corner. What to do when that special tube is out of production ? Or
if it don't work as claimed ?
 
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