Power reduction

[lowell]

I had an idea about power reduction. I've been researching attenuation a bit lately and thought, what about placing a resistor from one side of the OT power tube plates to the other side. Depending on the resistance the out-of-phase signals would cancel right? I'm wondering if there will be issues with the OT maybe. Am I right in thinking we would not need a huge honkin' resistor because the voltage from plate to plate is nearly identical, just need to be concerned about current that'd flow between the plates and through said resistor. I am into this idea cause the power tubes grids will still see a large signal and the attenuation is happening "post" power tube grids.

[Jazz P Bass]

What is the connection point of the other end of the resistor?
0 Volts (ground)?
If so, your basically messing with your B+ voltage.
Are sure you want to go this route.
Typically power suckers are attached to the secondary of the OPT.
For this reason.

[lowell]

no ground connection. From one side of OT power tube plates to the other side. No voltage difference, just concerned about current, and maybe any other side effects I'm not aware of.

[BackwardsBoB]

So this is a push-pull class AB amp, and you want to tie the push and pull plates together with a resistor, right?

Are you thinking that when a tube sinks current, some of the current will be drawn from the other side of the transformer, cancelling the field in the core from some of the current through the usual path, reducing power?

You're wrong about not much voltage across the resistor, except at idle. with a 400V B+ and the tubes at full output, you'll have about 50V of saturation voltage on the pulling plate and something like 700V of flyback voltage on the "off" tube's plate (for a large value resistor), so you'll see something like +/- 650V on the resistor.

[balijukka]

To have any meaningful attenuation, the resistor should be so small compared to the reflected OT impedance, that it would seriously affect the tone of the amp.

jukka

[lowell]

mmmmhmmmm.... ok sounds like a dead end road.

[Bruce / Mission Amps]

Not sure if you mean this or not but if you mean substituting one leg of the OT from the center tap to the plate end, ....done it, been there, and have the Tee shirt from 7 to 8 years ago... etc etc etc...
I think some of us actually talked about here on AMPAGE years ago.

It does work to a degree, and all you need to do is use a metal Dale type 25-50 watt resistor equal to the 1/2 or the same OT primary zed.
It looks ugly on the scope but is still fun to mess with.
As long as you keep the drive level down it does work as a form of an attenuator.

[lowell]

actually Bruce I meant putting a resistor from one end of the OT to the other end on a p/p output.

however I'm interested in the purpose of what you describe. Is this for simulating a single ended output? I'd think only one half of the p/p output is making it to the secondary.

[Chuck H]

What I interpreted from this post was some sort of resistive load that would replace the primary impedance of the OT. Sort of the same way an attenuator substitutes the speaker load. You would have to do this without altering the B+ on the plates and without reflecting an innaproriate load to the power tubes. Basically it would be an attenuator on the primary side of the OT instead of the secondary side. Maybe a huge cap and a huge variable resistor acting as a variable shunt. Not sure about this. It's an idea I've thought about too though.

If what you meant was to simply place a resistance across the OT primary leads, I think this would mess with the primary impedance and therefore tube operating parameters too much by the time you got any usable effect. There is a mod that was supposedly used by Eddie VanHalen that is just a big resistor across the OT primary leads. It doesn't reduce the volume much but if your using a 100 watt amp every little bit helps I guess.

Chuck

[brucec59]

The resistor across OT primary idea was done several years ago by me and some other guys on the vintageamps forum. It was called the cerrem mod because that's the guy who described the mod, saying that the resistor is what EVH described as a stick of dynomite in his amp. Mr. Twistyneck and I and perhaps some others experimented with it. Warning, this thread is OLD and LONG: Vintage Amps Bulletin Board • View topic - First Foray Into Cerrem's Resistor Mod (I go by Marshall Maniac over there.)

[brucec59]

Okay, here's more:

Cerrem's Mod v2.0

[Old Tele man]

...everything OLD is NEW again...eventually.