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When strapping an output pentode to triode....

24.20.10.236
Posted on August 4, 2020 at 19:55:42
viridian
 ...by shorting out the screen, does the overall gain of the power amp decrease? Sorry, I am not able to navigate the maths that would take me to the correct answer so I defer to the more knowledgable. Thanks in advance.

 

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RE: When strapping an output pentode to triode...., posted on August 4, 2020 at 20:17:59
Chip647
Audiophile

Posts: 2652
Location: The South
Joined: December 24, 2012
 Well, here is the thing: A triode / pentode switch is a blah idea unless you also have a feedback adjusting switch. You change the feedback and the gain will change. Flipping a pentode to triode switch will not reduce the overall gain much if at all. Running triode with massive feedback defeats the purpose IMO.

 

RE: When strapping an output pentode to triode...., posted on August 5, 2020 at 00:02:34
trobbins
Audiophile

Posts: 189
Location: Melbourne
Joined: March 26, 2012
 Sure does change the open loop gain, so if you are after a specific feedback dB level, then you may need to adjust the feedback network, depending on how large the open loop gain is. Pentode mode has noticeably more gain than triode mode for the output stage itself.

 

Thanks so much gents..., posted on August 5, 2020 at 09:00:18
viridian
 It was a practical question, in that I already have the amp. So the egg is already broken. I am having an issue with excessive gain architecture in the system and steady state noise is accentuated in ultra linear, as opposed to faux triode. Just waned to get a feel for the issue, before trying to formulate a way forward. Thanks again.

 

RE: Thanks so much gents..., posted on August 5, 2020 at 10:32:42
kobasa@xs4all.nl
 normally the gain of a penthode or a triode depends on the loadline resistor (Anode resistor)

 

RE: Thanks so much gents..., posted on August 5, 2020 at 10:41:30
kobasa@xs4all.nl
 So without a schematic nothing special to say about the gain

 

Seek and ye shall find...., posted on August 5, 2020 at 13:40:41
viridian

Schematic:

 

RE: When strapping an output pentode to triode...., posted on August 5, 2020 at 19:37:07
Lee of Omaha
Dealer

Posts: 1800
Location: Omaha NE
Joined: September 8, 2006
 One of our signature rebuild/mods is of the Dynaco ST-70, and we install ultralinear/triode switches. The gain seems to be about the same, but the sound quality is ridiculously better in triode.

By the way, using only passive components, we're able to get the ripple down to 7mV on the 480V line.

 

RE: When strapping an output pentode to triode...., posted on August 5, 2020 at 22:16:52
trobbins
Audiophile

Posts: 189
Location: Melbourne
Joined: March 26, 2012
 In practice, the open-loop gain will depend on the output stage configuration. The voltage gain from valve grid to anode is simplistically the mu multiplied by the anode resistance (ra), and although the mu doesn't change much, a tetrode has a much higher ra than if it was triode strapped. But the output stage may be presented with a different loading impedance, and hence turns ratio, so that may not be held the same when triode strapping.

 

Just to be clear..., posted on August 6, 2020 at 17:12:29
Lee of Omaha
Dealer

Posts: 1800
Location: Omaha NE
Joined: September 8, 2006
 You don't "short out" the screen, you tie it to the plate through a 100 Ohm resistor (recommend 2 watt non-carbon composition).

 

RE: When strapping an output pentode to triode...., posted on August 6, 2020 at 19:45:43
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 The gain of a pentode is higher than the gain of a pentode wired as a triode.

Because the triode wired pentode has less gain, there will be less feedback so the gain of the amplifier as a whole will be, more or less, the same.

If there was no global feedback to start with then the amp as a whole would end up having less gain.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

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