I'm replacing the original binding strip on my power amp with posts, but I only have room for one pair, leaving the 4 ohm and 16 ohm taps disconnected. Is there any reason that connecting the 4, 8, 16 ohm taps to a three position switch would be a bad idea? I know adding one more thing to the signal path has downsides, but beyond that?
I'll start the ball rolling!
Provided you don't operate the switch when the amp is powered up, and the switch has an appropriate current handling capability, I don't see why not.
Someone is sure to have done this, and will hopefully chime in with information on the practicalities involved.
Provided you don't operate the switch when the amp is powered up, and the switch has an appropriate current handling capability, I don't see why not.
Someone is sure to have done this, and will hopefully chime in with information on the practicalities involved.
https://www.schurter.com/en/datasheet/SWM1__Frontpl_#
Click on "Open complete table SWM1 (Frontpl)", scroll down to "0033.4032".
Don't switch while the amp is active, because operating a tube amp with no load (even while switching between taps) can damage the output transformers with high voltages (tubes, too).
Click on "Open complete table SWM1 (Frontpl)", scroll down to "0033.4032".
Don't switch while the amp is active, because operating a tube amp with no load (even while switching between taps) can damage the output transformers with high voltages (tubes, too).
Yeah. What happens when the output transformer is operated without a load is that a high inductive kick-back is generated. The OPT basically becomes a flyback transformer and the resulting voltage can be pretty high. High enough to arc over the transformer which may kill it and/or the output tube.Don't switch while the amp is active, because operating a tube amp with no load (even while switching between taps) can damage the output transformers with high voltages (tubes, too).
If you do add a switch to select between the transformer secondary taps, I recommend that you add a bidirectional Tranzorb or transient suppressor across the OPT primary. In a single-ended output transformer this suppressor should be able to withstand at least twice the B+ voltage. I'd probably go 10-20% higher than 2x B+ to make sure the transient suppressor never conducts during normal operation. In a push-pull amp I'd rate the transient suppressor for 10-20% higher than the highest signal voltage across the OPT primary.
The purpose of the transient suppressor is to protect the output transformer should you ever flip the output impedance switch with the amp turned on and signal applied.
Tom
Or you can add a "safety resistor": https://el34world.com/Forum/index.php?topic=18236.0If you do add a switch to select between the transformer secondary taps, I recommend that you add a bidirectional Tranzorb or transient suppressor across the OPT primary.
Guitar amps do it all the time.
Remember they are typically 50-100W, normally used way overdriven, connected to various speakers, moved and bumped around, etc.
And yet impedance selectors stand the grueling conditions.
This is the Marshall "official" type, relatively expensive because of Mojo (Jimi Hendrix used them 😉 but any sensible rated switch will do fine.
Think industrial rated 5A or higher, 10A being better.
It's coin operated, a chicken head knob being an irresistible temptation to idiots.
Remember they are typically 50-100W, normally used way overdriven, connected to various speakers, moved and bumped around, etc.
And yet impedance selectors stand the grueling conditions.
This is the Marshall "official" type, relatively expensive because of Mojo (Jimi Hendrix used them 😉 but any sensible rated switch will do fine.
Think industrial rated 5A or higher, 10A being better.
It's coin operated, a chicken head knob being an irresistible temptation to idiots.
This thread has taught me that I have done very bad things to my output transformer(s) 😬
They still work, so I guess I dogged a bullet. Is it possible I’ve damaged them in a way that isn’t apparent?
They still work, so I guess I dogged a bullet. Is it possible I’ve damaged them in a way that isn’t apparent?
The possiblity of damage to output valves/transformer only exists if you are playing signal through the amp when the speaker load is disconnected.
If no music is playing when you disconnect the speaker load then you'll get away with it!
If no music is playing when you disconnect the speaker load then you'll get away with it!
This one is about half the price of the Marshall one:
https://www.tubesandmore.com/products/switch-rotary-impedance-selector-3-position
https://www.tubesandmore.com/products/switch-rotary-impedance-selector-3-position
Also available from DigiKey https://www.digikey.com/en/products/detail/schurter-inc/0033-4032/641549
Good idea. Even something like 100 Ω across the 16 Ω winding will do wonders and won't impact the performance of the amp much.Or you can add a "safety resistor": https://el34world.com/Forum/index.php?topic=18236.0
Tom
100 ohm resistor across the 16 ohm speaker winding.
Its one of the ways to load a speaker output transformer/output section without a speaker.
But I prefer to use a 75 ohm or a 62 ohm Mills MRA12. 100 ohms works on some, but not very well on high voltage plate @ 90%+ from the max plate value
Its one of the ways to load a speaker output transformer/output section without a speaker.
But I prefer to use a 75 ohm or a 62 ohm Mills MRA12. 100 ohms works on some, but not very well on high voltage plate @ 90%+ from the max plate value
16 ohm tap and common on the secondary.
I do it to 50W heads when I convert to speak-on. and it doesn't effect the impedance selector.
I do it to 50W heads when I convert to speak-on. and it doesn't effect the impedance selector.
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From the 16 Ω tap to the ground tap on the secondary of the output transformer.Where do you connect the resistor though? Between what and where?
Tom
https://www.diyaudio.com/community/attachments/d70-mkii-schematic-dragged-pdf.1244432/
The ground tap on the secondary of the output transformer is on 4Ω
tap!
https://www.diyaudio.com/community/...w-low-output-please-help.406424/#post-7530372
The ground tap on the secondary of the output transformer is on 4Ω
tap!
https://www.diyaudio.com/community/...w-low-output-please-help.406424/#post-7530372
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