Would really appreciated some help. I have some Welborne 300B DRD monoblocks and want to add a 8/4 ohm impedance switch/selector on the secondaries of the output transformers. (Drill a hole and fit a toggle switch to the back)
In the manual there are instructions for changing the impedance rating by connecting the different coloured secondary wires together differently, to four posts on the internal amp board
2 ohm and 16 ohm settings are also possible but I think that might get too complicated with the wiring and switch. So I just want to do 4 and 8 ohms.
Would something like a 4 pole 2 way on/on switch be suitable? and what kind of current/voltage rating?
It’s not totally straightforward to wire up, as the output and ground that go to the speaker terminals are connected to different secondary wires for 8 and 4 ohms settings.
The instructions are to solder the configs to 4 posts on the board. I’d obviously be transferring this to a switch.
Pic from the manual is here: https://ibb.co/XS5GQR1
Any help/advice on the right kind of switch and how to wire would be really appreciated. I can solder well but am not so experienced. (I do however know how be safe, such as discharging caps and check with a voltmeter before doing any work)
In the manual there are instructions for changing the impedance rating by connecting the different coloured secondary wires together differently, to four posts on the internal amp board
2 ohm and 16 ohm settings are also possible but I think that might get too complicated with the wiring and switch. So I just want to do 4 and 8 ohms.
Would something like a 4 pole 2 way on/on switch be suitable? and what kind of current/voltage rating?
It’s not totally straightforward to wire up, as the output and ground that go to the speaker terminals are connected to different secondary wires for 8 and 4 ohms settings.
The instructions are to solder the configs to 4 posts on the board. I’d obviously be transferring this to a switch.
Pic from the manual is here: https://ibb.co/XS5GQR1
Any help/advice on the right kind of switch and how to wire would be really appreciated. I can solder well but am not so experienced. (I do however know how be safe, such as discharging caps and check with a voltmeter before doing any work)
Don't !
Most kind of switches will break for a while, and this may be fatal for a tube amp.
Why do you even consider this ?
Most kind of switches will break for a while, and this may be fatal for a tube amp.
Why do you even consider this ?
You switch it when the amp is turned off. They there's no issue with "break".
I used to have a switch on my KT88 for 4 or 8 ohm. The Hammond 1650N is a lot easier to wire though - just a DPDT switch is needed.
As long as the switch is rated for 5A or so, you're ok. Consider using a relay to switch, they contact faster - less "break" time - but I still wouldn't recommend switching the impedance when the amp is on.
I used to have a switch on my KT88 for 4 or 8 ohm. The Hammond 1650N is a lot easier to wire though - just a DPDT switch is needed.
As long as the switch is rated for 5A or so, you're ok. Consider using a relay to switch, they contact faster - less "break" time - but I still wouldn't recommend switching the impedance when the amp is on.
Most datasheets should tell you how many operations the switch is good for. I can’t see any failures unless you’re switching hundreds of times daily for years.
What is happening technically to damage the tube amp in the break condition? Is a make condition also an issue?
Thanks for the advice everyone. That’s what I thought. It’s ok to switch when off.
I know the output transformers can get fried if not connected to a load, but I’ve read before that is a bit exaggerated and it actually takes a lot longer than most people think.
I’ve accidentally not connected up one side quite a few times with different tube amps. I obviously then switched off when I realised and connected the wires and turned back on. Amps were many on for 1 minute each time. No damage done from what I can tell.
I know the output transformers can get fried if not connected to a load, but I’ve read before that is a bit exaggerated and it actually takes a lot longer than most people think.
I’ve accidentally not connected up one side quite a few times with different tube amps. I obviously then switched off when I realised and connected the wires and turned back on. Amps were many on for 1 minute each time. No damage done from what I can tell.
Thanks for the info. I agree these amps I’ve got look harder to wire than some. Is a 4 pole switch what I need?
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In the break condition, there will be no plate load on the outputs which can be destructive. Worse case, there's enough back EMF to cause an arc over which might burn out the OPT.
In the make condition, the load increases.
Thanks! I’m trying to understand the details a little more.
So no load case, naively I might expect voltages to rise because no current is being drawn. Maybe this might exceed rating of the output tubes?
The back EMF of the transformer, if the transformer is storing magnetic flux, this colapses when the load is taken away - this is flyback? This should be avoided!
In the make condition, position 1 and position 2 of the switch will be in parallel (no?) so the load drops as well. OP’s application of a 4/8 Ohm switch, a load of 2.7Ohms appears for the duration of the make.
This seems quite safe to me - well safer than no-load! Flyback might still ruin your day even with that drop? Maybe? I don’t know.
He said a DPDT = Double Pole Double Throw = 2P2T so only two poles are needed.
Take a look at NKK switches they have mini toggles with 5A ratings.
This is what I’ve got. The secondaries need to be wired quite a bit different in terms the way the wires needs to be connected together for 8 or 4 ohms.
Your transformer has four 2 Ohm secondary windings brought out, so a fairly complicated switching arrangement, but well worth it to get the optimum use of all of the windings, for best performance.
The issue of operating unloaded is one of voltage arcing in the output transformer primary windings. Unless the amplifier is being heavily overdriven, such as when oscillating, or used as a geetar amplifier, this isn't a problem. Some marginally designed amplifiers, especially with pentode outputs, could oscillate without load, because their open loop gain would increase enough.
Two good ways to tackle this: first, steering diodes from final plates/OPT primaries to ground, squashing the oscillation. Second and better option is Zobel networks on either the primary windings and/or the secondary windings. These provide loading at high frequencies to keep open loop gain from soaring. Zobels on the primary can be adjusted to optimize high frequency response, as observed with a square wave. Win-win.
All good fortune,
Chris
The issue of operating unloaded is one of voltage arcing in the output transformer primary windings. Unless the amplifier is being heavily overdriven, such as when oscillating, or used as a geetar amplifier, this isn't a problem. Some marginally designed amplifiers, especially with pentode outputs, could oscillate without load, because their open loop gain would increase enough.
Two good ways to tackle this: first, steering diodes from final plates/OPT primaries to ground, squashing the oscillation. Second and better option is Zobel networks on either the primary windings and/or the secondary windings. These provide loading at high frequencies to keep open loop gain from soaring. Zobels on the primary can be adjusted to optimize high frequency response, as observed with a square wave. Win-win.
All good fortune,
Chris
I think a 3 pole will do.
4 pole will be nice, you can use the extra pole for switching 4/8 ohm indicator lamp. 🤔
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Thanks a lot for all this info on how to protect the amps! . That’s great. Can just about understand most of that.
I might look to do that eventually, but for now I’ll just try to setup this switch and make sure I switch it when the amps are off to be on the safe side.
Wow, thanks so much for this sketch! 😁👍👍 That’s so helpful.
Good to know I could even go for a 3 pole.
I had been considering this 4 pole switch.
https://www.cricklewoodelectronics.com/Miniature-4-Pole-Double-Throw-Toggle-Switch.html
The 2 ohm setting gives extra flexibility, but I can’t really see myself ever using that in a rush. I once used some Audion push pull 25 watt 300B monoblocks with some Magnepans from their 2 ohm taps and within a limited volume range (and in quite a small room) the sound was incredible. 9 watt SET monoblocks like these Welbornes would be too far low in power though, particularly from the 2 ohm taps.
8/4 ohm switching would be very nice, but there are also quite a few 16 ohm speakers out there. 16/8/4 switching capability would be even better.
How difficult would it be to make some kind of 16/8/4 ohm switching with a suitable switch?. I see the 16 ohm tap needs even more different connections (5). Or would this pushing be it a bit too far?
If this is possible might some kind of triple toggle switch work or is a rotary switch the only option?
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I think you need a 6P3T rotary switch. In my experience, rotary switch give me more space for soldering.
If you want the 2 ohm as well, a 6P4T rotary switch is needed to parallel all four of the 16 ohm secondary windings together.
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I was going to suggest a rotary switch, but honestly, I can't wrap my head around the connections. That's really nice work. A while ago I was trying to work out an impedance switch for a Peerless OPT, and that's only six 4 ohm secondaries. ;-)
Correction:
If you want the 2 ohm as well, a 8P4T rotary switch is needed to parallel all four of the 8 ohm secondary windings together.
Wow thanks again so much for the sketch. I also can’t wrap my head around so many connections! Try but usually get stuck. I was having trouble with just 8/4 ohms!
I guess if I’m going to go for Rotary, maybe I should go all the way with a 8P4T and do 2 ohms as well.
I did read before on another forum someone saying that a Rotary switch might slightly degrade the sound compared to 2 position toggle switch. Might this be the case?
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