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Pentode - Triode Question
Pentode - Triode Question
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Old 2nd February 2018, 06:08 AM   #1
jaidn is offline jaidn  Spain
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Default Pentode - Triode Question

Dear forum members,

I'm trying to add a pentode - triode switch to my amp (5F1 type, SE). In pentode mode sounds fantastic, but in triode mode I am getting a lot of hum. The problem is that once you remove the B+2, the filter goes off.

The triode mode is achieved using Robinette method (B+2 off, pins 3 & 4 together with a 470Ohm resistor). I like the slightly darker tone and the volume drop, tough the hum is very present. I tried different ways to solve it:

- Add 16uF cap to the first 33 uF cap -> no effect
- Add 16uF cap to pin 4 -> no sound
- running triode mode just removing B+2 from pin 4 -> no hum, but guitar sounds distorted.

I read in the other forum that someone with the same issue fixed it adding an extra first filter plus a 15H choke.

I have checked different schematics, for instance the vox ac15 heritage (which I own), and they just joint pins 3 and 4 after removing B+2. My AC15 is totally silent in triode mode.... Then, there must be a solution to the problem.

I would like to avoid chokes, since It can affect the tone of the amp (not clear to me; internet is full of a long discussion on the topic).

Any thoughts on the matter are very welcome.

Thanks people!
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Old 2nd February 2018, 08:18 AM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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In triode mode G2 is connected to the Anode.
The gain will go down slightly. No extra hum.
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Old 2nd February 2018, 09:11 AM   #3
Kay Pirinha is offline Kay Pirinha  Germany
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In case that there isn't any other reason, e.g. faulty wiring etc., increase of hum in triode mode is due to the lower plate resistance here. To cure it, you need either a choke (never heard of any impact on sound by this) or a regulated B+ supply.
Best regards!
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Old 2nd February 2018, 11:51 PM   #4
PRR is offline PRR  United States
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As pentode, 6V6 is about 50K internal plate resistance. With 5K load impedance, power rail crap applied to the load is divided about 10:1.

As triode, 6V6 is about 2.5K internal plate resistance. With 5K load impedance, power rail crap applied to the load is divided about 3:2 = 1.5:1

Yes, it hums/buzzes about 7X worse.

This accounts for the rush to power pentodes in the late 1930s. More than power or gain, the pennies saved in a cheaper power supply.

The 5F1 is already minimally filtered, even for the small (poor bass) stock speaker, because it was CHEAP amplifier. Using the data above, you would think to up-size the main filter cap about 7X (33u to 49u does not begin to get there), so triode is as clean as pentode. However that is severe abuse on the vacuum rectifier. And up-sizing the first cap is poor leverage.

If you are choke-adverse, at least add another R-C filter to the B+. You have a 16u cap. The 6V6 cathode cap is a suitable value for R; do you have another several-Watt 470r resistor? These values give about 5:1 reduction of 120Hz which will be a real difference. Power output is down some, but this Champ won't rock the stadium in any case.
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Old 3rd February 2018, 05:03 AM   #5
PRR is offline PRR  United States
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Old 3rd February 2018, 04:09 PM   #6
Enzo is offline Enzo  United States
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In the stock circuit, if you move the screen from the screen node B+ over to the plate, you lose the cleaner filtered DC of the screen B+ node. Since this is single ended, the amp has no natural hum cancellation like a push pull.


Note PRR suggested essentially moving the whole circuit down the B+ string one node. Now the plate and screen are both connected to what had been the screen node. SO it is better filtered, less hum to start with. Certainly should work.

When you tried adding a cap to pin 4 in triode mode, your new cap essentially filtered out your signal, because pins 3 and 4 would be connected together, so a big cap from plate to ground swamps the signal.


I find it interesting you want to avoid chokes because they might change the sound, yet your entire premise here is changing the thing to triode mode, which will certainly change the sound.

About chokes, they really more or less act like caps in a filter circuit. I mean in the sense that adding a choke does something similar to just adding more caps. In the PRR example above, his 470 ohm resistor could easily be a choke instead. And likewise, if you find a choke circuit, the choke can be replaced with a resistor. To me the important part of all that is increasing the filtration, not what component we use to do it.
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Old 4th February 2018, 09:32 AM   #7
jaidn is offline jaidn  Spain
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Quote:
Originally Posted by JonSnell Electronic View Post
In triode mode G2 is connected to the Anode.
The gain will go down slightly. No extra hum.
Then is it OK to connect pins 3&4 directly without detaching B+2? Every schematic I have checked disconnects B+2 from g2.

If it works, looks like a very easy mod.

I have also read that tubes can be wired as pseudo triodes, just connecting pins 3 and 8 through 1K resistor. People do it with EL34, but no idea with other octal tubes.

Thank you for your replies!
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Old 5th February 2018, 04:07 AM   #8
Enzo is offline Enzo  United States
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You should do it like PRR did in post #5.

If you simply wire pins 3 and 4 together, you are also shorting together the two B+ nodes.

And one result of that would be less filtration in the B+ there.
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