Thoughts on trying the Van Halen Variac trick with a rectifier

[bandit2013]

Explore as you whish. I do not think it will pan out. I have not much to offer in this subject. I had thought about that long ago, the what if sort of questioning. Perhaps with the MWDR it may have some potential but not with the Roadster. It may depend on the PT and OT characteristics. Not something I would want to risk based on a rumor if it was actually true. The spongy mode runs the entire amp at a reduced voltage. this includes the bias. preamp tubes and everything else powered by the mains. No different than using a Variac except the Variac can adjust it lower or higher than the mains supply. Not sure how low of a voltage the amp can tolerate. I am not willing to sacrifice my gear to find out.

Reducing the bias voltage magnitude will get closer to the EL34 bias region. That is usually -39VDC where as the 6L6 bias is roughly -69V or at -51V in spongy mode. Then again, the plate voltage will also be dropping resulting in a change in load lines of each triode and power tube. To what extent will the reduction in AC voltage place the tubes in an unusable state? When will the heaters begin to drop resulting in less thermal energy to emit electrons?

I would recommend trying this with an amp that has no complex switching components or relays. Something made with peg board and hand wired as they made them that way in the early days. No sensitive electronic components such as JFETS, transistors and such, including any opto-couplers or similar devices. Things tend to fail or not work when the voltage is reduced too much. If you do it, use a meter to measure the voltage used for the relays and other electronics. There are limits on those devices.

 

[mark1406]

I though EVH did this purely to reduce the volume of the amp and in the studio he ran the amp stock?

I have heard he used a large resistor to shun the output stage to reduce volume and ran the amp on 6CA7’s as the EL-34’s didn’t like the abuse.

Personally, I think you are wasting your time chasing this mod.
I don’t know if anyone has mentioned that the filament voltage is affected by reducing the mains voltage too.

Regards

Mark

 

[jpistolas]

I always assumed the idea was to run the variac down to what the older amps/transformers were used to seeing. Used to be 110v or less. Now it's 115v & sometimes a tad more. The studio was probably wired up to more modern spec. even back then.

Edit: Mile High Effects said what I meant but explained it so much better. Ignore me & read that post.

 

[domct203]

Yes but is tye bias circuit running at a reduced voltage reducing the bias to be even colder or does it run at normal values as if not reduced? That’s my line of thinking.

Spongy mode or an external variac will lower the voltage to the entire amplifier in the exact same manner.

All voltages on the secondary side of the power transformer will be reduced.

As far as the bias voltage to the output section yes, it will be affected by the reduced voltage, but it’s not a linear effect.

Ohm’s Law will always apply.

Be careful if you go poking around inside the chassis of your amp. The power supply filter caps store over 400V for days, even with the amp unplugged from power. That is enough voltage to stop your heart, Spongy mode or not.

Dom