Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Voxbox on November 30, 2022, 06:07:15 am
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Hello,
I'm needing some advice.....
I am considering changing out the Standby switch in my Tweed Deluxe build so I could easily change between a resistor and choke in the power supply as I like the difference in response but dont necessarily prefer one over the other all of the time.
I was thinking of using a Carling 2GM51-73 3 way switch: with positions Choke/Standby/Resistor so that switching between resistor and choke is always done by going through standby - I wonder if there are any issues to consider by doing this and the extra wiring/layout?
Cheers, VB
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Surely, unless something else is also changed, the 6V6 overdissipate if the 4k7 HT dropper to the screen grid node is swapped for a (low resistance) choke?
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That will work. But you need to add two 470Ω/1W screen resistors. Kinda like the AB763 Deluxe Reverb.
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Thanks guys, I did wonder about the low resistance of the choke.
However my Tweed Deluxe has operated with a choke and no screen resistors for 15 years without issue.....
It was the switching of the HT and the additional wiring I was concerned about.
Can you kindly explain in more detail about the need for the screen resistors - I dont know a huge amount about valves though I'm an electronics engineer......
Cheers, VB
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Read this...
https://el34world.com/Forum/index.php?topic=21489.0
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Can you kindly explain in more detail about the need for the screen resistors ...
The "screen resistor need" is heavily dependent on average screen current draw when you're blasting the amp.
However, that average is not easily predicted from idle conditions or data sheets, except that beam power tubes like 6V6 oughta be "lower than other tubes."
Kuehnel shows (https://ampbooks.com/mobile/books/deluxe/) it is low except for brief moments near max plate current, when screen current spikes up.
If you're really curious, you'll need to place an ammeter in-series with your screen, set for an Average reading, and play the amp as loud as you'll play it. Then choose a resistance that drops enough voltage to keep the screen at/under its dissipation rating (2w for 6V6) at that "max power average screen current."
Or just slap on a 470Ω resistor & be done with it. :laugh:
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… If you're really curious, you'll need to place an ammeter in-series with your screen, set for an Average reading, and play the amp as loud as you'll play it. Then choose a resistance that drops enough voltage to keep the screen at/under its dissipation rating (2w for 6V6) at that "max power average screen current."
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How to measure screen grid dissipation with reasonable accuracy though, what meter settings to use?
Assuming worst case condition, eg output stage fully overdriven to square wave output into a resistive load.
The reason I ask is that it’s been pointed out elsewhere that DC readings will be ‘average’, rather than the rms that is required for the power calculation.
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… If you're really curious, you'll need to place an ammeter in-series with your screen, set for an Average reading, and play the amp as loud as you'll play it. ...
How to measure screen grid dissipation with reasonable accuracy though, what meter settings to use? ...
I have not done it, but Page 10 of my meter's manual (https://assets.fluke.com/manuals/87______umeng0800.pdf) talks about its capability to make "Min Max Average" readings over time.
I haven't tried recording Average Screen Current in this manner, but I have used it in the past to observe Min/Max Voltage to catch peaks when a scope wasn't available.
And personally, I'd wind the amp up as loud as it went, and bash a guitar as hard as I can to generate my "Average Screen Current." Real life conditions oughta be less severe than that.
How to measure screen grid dissipation with reasonable accuracy though, what meter settings to use?
... DC readings will be ‘average’, rather than the rms that is required for the power calculation.
Perhaps you were more focused on "Voltage" and "Dissipation."
I think that's a recursive problem: I envisioned someone starting by dividing Screen Dissipation Rating by Average Current. Then start with Idle Volts and determine the Voltage Drop needed to land on the output of the last step. Voltage Drop / Average Max Power Screen Current gives a resistor value.
But Screen Voltage at the filter cap likely will drop some amount, and reduce Screen Volts below what is needed for protection. Now one might use that Min/Max/Average function again to identify Minimum Screen Volts with the new screen resistor in place & bashing at max volume.
The steps above will probably tell us we went too far with the initial screen resistor value, and now we might do a back & forth dance dialing it in by repeating all of the above steps.
Or we copy a resistor value off some amp until tube failures prove we need a higher resistance.
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Read this...
https://el34world.com/Forum/index.php?topic=21489.0 (https://el34world.com/Forum/index.php?topic=21489.0)
Thats a great thread HBP, many thanks sir!
Cheers, VB