Exo-metal
Well-known member
You have two variacs and read all the stories but do you have any of your own? Let's hear about it. Try to refrain from anything except your direct experience.
7704A
Well-known member
Almost sounds like the prompt for a text-adventure game.
> Walk left
> Grab variac
> Walk left
> Grab variac
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harddriver
Well-known member
Are you asking about the Chris Merren theory of using two variacs with a Marshall plexi/SL?
One variac used to variac the power supply of the amp and then variac two used between the head and the 4x12 speaker cabinet as a type of attenuator.
One variac used to variac the power supply of the amp and then variac two used between the head and the 4x12 speaker cabinet as a type of attenuator.
thegame
Well-known member
Two variacs, 1 cup holder. The true secret to Billy Blades tone aka the brown and green sound
Exo-metal
Well-known member
Just asking for direct experience using two with a plexi. That is all. I have heard of that and seen a clip and know it works and I have heard about a direct coupling to the OT. I just want direct experience for the discussion.
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harddriver
Well-known member
I have done the two Ohmite Variacs exactly the way Chris Merren described it and laid it out. Interesting enough the second attenuator variac is set at 90VAC and it does provide some volume reduction and the amp does react and sound different with some different aggressive artifacts to the tone and how the amp feeds back. The amp tends to feedback in a more immediate interesting way compared to not having the variac in the signal chain. I found it to be cool and different enough when I used it. You may find it interesting you may not, I don't think it's anything uber magical but it is different. I guess I would describe the tone a bit more sizzlely in the attack and how the amp crunched and would easlily break into really great feedback on held notes.
Essentially the attenuator variac sounded best with the variac set to 90 VAC which was what Merren concluded as well, basically you have two electrical cords one male and one female with 1/4" male plugs on the ends. Plug the 1/4" male into the speaker jack of the amp with the female three prong plug connected to the main power plug of the Ohmite Variac(always must be turned on) then the output signal goes through the variac's rheostat windings set to around 90VAC(not to be confused with the variac set to 90VAC on the mains power supply) then you plug in the second electrical cord with the male end in the output outlet of the variac and then plug the 1/4" male plug into your 4x12 speaker cabinet. I dug out my setup and took a quick pic
There was another method that sounds like what you may be describing and that was called the Cerrem mod which had various resistors wired directly to the output transformer and the speaker jacks but don't quote me on that setup, I never tired that one. I find that one the more dangerous method but Mr Twistyneck tried it.
Here is the Cerrem Mod illustrated by Mr. twistyneck.
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harddriver
Well-known member
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harddriver
Well-known member
It's been quite a while since I ran this setup so I am quoting from the best of my memory on how it was set up.
Exo-metal
Well-known member
This was the original clip 10yrs ago who ran it after the head into the cab. I can't find his clip explaining it, there was one. Again, this isn't about tone chasing but theoretical discussion much like the two methods of how to wire a 412. This ties into the NFG and OT and plays a huge part in your final tones without question.
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novosibir
Well-known member
I wouldn't try this to not risk a smoked OT in my amp.
A variac is a transformer and a transformer not only transforms voltages, but also impedances.
If you have the variac half way down, then half the voltage & half the current goes to the cab, means a quarter of the power the amp puts out.
But your cab's impedance (of 16 ohms) then also is transformed to the amp what means, that the amp is seeing four times the cab's impedance, 64 ohms.
What might happen?
Voltage spikes up to 5 kV inside the OT what will damage the insulation between the OT's secondary & primary. Better stay away from such experiments!
A variac is a transformer and a transformer not only transforms voltages, but also impedances.
If you have the variac half way down, then half the voltage & half the current goes to the cab, means a quarter of the power the amp puts out.
But your cab's impedance (of 16 ohms) then also is transformed to the amp what means, that the amp is seeing four times the cab's impedance, 64 ohms.
What might happen?
Voltage spikes up to 5 kV inside the OT what will damage the insulation between the OT's secondary & primary. Better stay away from such experiments!
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novosibir
Well-known member
If it were so easy and risk-free, why haven't all manufacturers of attenuators taken up this idea 
TheGreatGreen
Well-known member
Thanks for the clarification. It sounds like an insane idea for sure.
novosibir
Well-known member
I've just edited my post, because in the described case the amp even would see 4 times the cab's impedance.
Using a 16 ohms cab reflects 64 ohms to the amp with the variac half way down in between.
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harddriver
Well-known member
YUP, that's the tone I remember.
If you like what you hear you may want to try it even though Larry advises against it...........sometimes you just have to hear things for yourself.
A cheap way to get close to that tone if you like it.....is to install a Jose Master with 20V/20V( down to 5V if you want to experiment) Zener diodes on a push/pull switch. You will not use the Jose Master as a Master volume( kept 100% turned up) but it does affect the tone when you engage the diodes.
I didn't damage my output transformer doing the variac attenuator setup (but I also pulled two tubes) but Larry knows his technical details without question.
I would add the fact of early chronicling of the 78 tour that a certain someones amp went through alot of output transformers......Was this the reason for that??????????????????
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novosibir
Well-known member
Then when set the impedance selector to 16 ohms you've increased the real output impedance of the amp to 32 ohms
Definitely!
Cless357
Active member
Didn't Marshall try this with the Power Brake, which uses an autotransformer ?If it were so easy and risk-free, why haven't all manufacturers of attenuators taken up this idea
thegame
Well-known member
Isn’t this one of those instances where the transformer may incur non catastrophic yet accumulative damage over time, even if it doesn’t blow immediately? Which may lead the user into a false sense of security. Same thing with pushing a greenback cab too loud for too long.
Exo-metal
Well-known member
If what was said implies internal arcing. I know this works but to what extreme causes failure I'm not sure. There must be a solution to protect the condition if implemented.
novosibir
Well-known member
Have an OT wound for you, whose isolation resists 10 kV...
... only then the screen grids of the output tubes will melt due to overload.
You can't outsmart physics
... only then the screen grids of the output tubes will melt due to overload.
You can't outsmart physics
Exo-metal
Well-known member
Besides not doing this, it will be interesting to hear your solution.Have an OT wound for you, whose isolation resists 10 kV...
... only then the screen grids of the output tubes will melt due to overload.
You can't outsmart physics
Testla already has his...wireless coupling!
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