Protecting OT when switching OT.

[Alan0354]

Hi guys. This is a continuation of the diode protection of the OT in the other post. This is what I am really after.

I am trying to compare the Classic Tone Vibrolux OT to the Classic Tone Marshall Plexi 50W. My ears are not good, I need to have a way to switch instantly between them to A/B comparison. If I have to switch the amp off, then switch, then turn on, I don't think I can compare. So, I need to find a way to switch the OT hot!!!

This is the circuit I come up after confirming from the other post. But the mechanism of disconnect the primary of the OT is different. If I disconnect both wires of the primary at the same time, current get cut off on both side at the same time. So both sides want to swing up at the same time. Does that mean there will be no HV spike on the terminals if the switch is switch off EXACTLY the same time? I don't know how this will work. So I put all the protection diodes AND on top, I short the secondary of the OT to the ground for the one that got switch out to load the primary down. Here is the circuit, please comment.




The switch is a 4P2T switch so it switch at the same time( close). Please tell me whether this will work switching hot. I don't want to burn the OT!!!!

[gui_tarzan]

This is just a thought... what if you made a high quality recording of each and listened to them back to back instead?

[Dave H]

I wouldn't switch the primaries. Just connect them in parallel. Put a resistor of a few hundred ohms across each secondary and use a single pole switch to swap the speaker between them.

[Alan0354]

I wouldn't switch the primaries. Just connect them in parallel. Put a resistor of a few hundred ohms across each secondary and use a single pole switch to swap the speaker between them.

I think I can get way with something like a 50 ohm. But can the OT truly reflect high enough impedance back to the primary without starting to get inductive? That would really change the sound. OT is designed to work with low impedance to 16 ohm only, it might not have enough turns to keep it behaving like a real transformer.

Do you see anything wrong with my circuit?

[Dave H]

I think I can get way with something like a 50 ohm. But can the OT truly reflect high enough impedance back to the primary without starting to get inductive? That would really change the sound. OT is designed to work with low impedance to 16 ohm only, it might not have enough turns to keep it behaving like a real transformer.

Do you see anything wrong with my circuit?

I have actually tried what I suggested above on my 18W EL84 amp and nothing smoked but I only played it clean. I had 470 ohms permanently wired across each OT's 16 ohm tap for spike suppression. I couldn’t detect any tone change when adding the unloaded OT in parallel but there was definitely a tone difference when switching between the OTs.

I think your circuit will work but I’d be worried about arcing across the switch when hot switching the primary side of the OTs.

[Alan0354]

I have actually tried what I suggested above on my 18W EL84 amp and nothing smoked but I only played it clean. I had 470 ohms permanently wired across each OT's 16 ohm tap for spike suppression. I couldn’t detect any tone change when adding the unloaded OT in parallel but there was definitely a tone difference when switching between the OTs.

I think your circuit will work but I’d be worried about arcing across the switch when hot switching the primary side of the OTs.

I guess it's not hurt of trying, I have not received the 4P2T switch.

Thanks

[Alan0354]

I have actually tried what I suggested above on my 18W EL84 amp and nothing smoked but I only played it clean. I had 470 ohms permanently wired across each OT's 16 ohm tap for spike suppression. I couldn’t detect any tone change when adding the unloaded OT in parallel but there was definitely a tone difference when switching between the OTs.

I think your circuit will work but I’d be worried about arcing across the switch when hot switching the primary side of the OTs.

I just wired up as you suggested. I use 75ohm instead. I figure that will be 10 times the speaker impedance and the effect is not significant, also low enough to prevent spikes. I had experience that I got good original sound even if I substitute one of the 8 ohm speaker in a 2X12 with 10 ohm resistor. This is going to be a lot better.

Its very late, I powered up and test the sound at low volume, I can only hear very very minor difference between the Classic Tone Vibrolux vs Classic Tone Marshall 50W. I have to try at high volume tomorrow again. The Marshall OT seems to give an ever so slightly heavier sound of a Marshall. But it could be psychosomatic on my part. Hope the difference is more distinct when I crank it up.

[Alan0354]

I have actually tried what I suggested above on my 18W EL84 amp and nothing smoked but I only played it clean. I had 470 ohms permanently wired across each OT's 16 ohm tap for spike suppression. I couldn’t detect any tone change when adding the unloaded OT in parallel but there was definitely a tone difference when switching between the OTs.

I think your circuit will work but I’d be worried about arcing across the switch when hot switching the primary side of the OTs.

I tested out at higher volume. The parallel transformer works very good, the very minor difference comparing with the other amp with normal OT is not very noticeable.

Back to the OT comparison, I cannot hear any difference between the Classic Tone Marshall and Classic Tone Vibrolux. Maybe my ears are not that good. But I am sure you can get the same result with either OT by minor tweaks. This is NOTHING like changing speakers. Why people are so picky on a particular brand and model? My experience is you can make major change of the sound of the amp by tweaking the values of resistors and caps in the signal chain. You can literally make the amp sounds completely different by one or two resistor and cap in the signal chain. Why worry about what brand!!!!

MM is trying to be so deceiving when they compare there OT with stock using Fender Deluxe Reverb. They forgot to mention they used Weber Speaker on theirs. One that looked very expensive with big magnet!!!

Thanks, That saved me so much trouble.