Xformer to lower tube amp working voltage

[disorder]

Following RG Keen's Vintage voltage adapter guide... http://www.geofex.com/article_folders/vintvolt/vintvolt.htm

I'm going to build an external box housing a voltage lowering transformer to plug my Fender Champ silverface into. I'm thinking 15-24V secondary, somewhere around there. My champ is quite clean and loud for my apartment and I'm not looking to just get the amp sounding like it does on proper line voltage, but use this as a means of distortion. These amps have a 1A fuse, not sure what their nominal current draw is but I'm looking at transformers with max power of 100VA to 150VA. Is this overkill? Is there a better spec to look at for this design, like max current draw? Do max current draw and max power reflect the same spec essentially?

here is what I'm thinking so far...
https://www.digikey.com/product-detail/en/signal-transformer/241-8-24/595-1059-ND/952929

What do you think?

[R.G.]

The added voltage lowering transformer only needs its **secondary** rated for the full current load. In this case, with a 1A fuse in the Champ, you only need a 1A rated **secondary**, meaning that the added transformer needs only be rated for 15-24VA, not 100+ VA.

The added transformer only deals with a fraction of the power.

[PRR]

The fuse protects in disasters. Normal load better be much less.

Champ is only 57 Watts total.

The dropping tranny is, as R.G. said, the *part* of the load you are dropping.

10% is reasonable. 10% of 57 Watts is 6 Watts/VA. For 120V wall, 12V, 0.5A.

24V drop on my 125V wall is a BIG drop for tubes. You get barely 100V in a nominal 117V amp. Your "6.3V" tube heaters will be at 5.4V, way below spec. If the heater aint hot enough, the plate voltage "yanks" electrons from the cathode (rather than they just drifting off) and tube damage is possible. (In fact you may not notice. Many receiving tubes have huge reserve of emission.)

The 24V drop is NOT a big change of loudness. Simple math says 0.64 the power, so the mighty 5.7W Champ goes down to 3.6 Watts. If one is too much, the other aint enuff different to please your neighbors.

Use a variable B+ circuit to drop the Champ's +380V down to 150V, 100V, or so.

If you want a major project, rig two 6AU6 and a reverb transformer to make a Third Watt mini-champ. There is some hope of snot-beating 0.3 Watts in a small speaker without breaking your lease.

[disorder]

The added voltage lowering transformer only needs its **secondary** rated for the full current load. In this case, with a 1A fuse in the Champ, you only need a 1A rated **secondary**, meaning that the added transformer needs only be rated for 15-24VA, not 100+ VA.

The added transformer only deals with a fraction of the power.

Thank you RG, that makes sense!


Sent from my iPhone using Tapatalk

[R.G.]

Here's a Vintage Voltage adapter I built for a friend's Deluxe. Well, could be for any smaller amp. I boxed it with an IEC power cord input socket and an AC wall socket out, with an indicator that let you know it was live.

[GibsonGM]

How about a power Mosfet-based voltage scaler?  This allows the heaters to remain at their normal voltage while giving you the ability to dial back your B+.   They use these on small amps often (notably the Marshall 18W, where I use mine).   Should work slick.

[davent]

How about a power Mosfet-based voltage scaler?  This allows the heaters to remain at their normal voltage while giving you the ability to dial back your B+.   They use these on small amps often (notably the Marshall 18W, where I use mine).   Should work slick.

Did the same for my 18watt with O'Connor's scaling circuit.

Also used it in a couple SE 6AQ5 amps  and used 60¢ IRF830 MOSFETS without issues. The circuit works great (needs a master volume if there isn't one) and accomplishes what you want to do.

With the addition of resistor between the top of the pot and the MOSFET drain you can set the upper Voltge limit as i needed to do to accommodate  6AQ5's.

dave

[R.G.]

How about a power Mosfet-based voltage scaler?  This allows the heaters to remain at their normal voltage while giving you the ability to dial back your B+.

Every silver lining has a cloud. This does work great, although it's not a "voltage variable resistor". And it truly does not affect the heater voltages.

But that's also the down side. Bucking transformers lowering the incoming AC voltage can drop the heater voltages back down to where they should be in older amps which were designed, for instance, for 110Vac in, not 125Vac in as is common today.

A fairly complete setup for an older amp would include both an incoming AC voltage dropper to get the incoming AC -and hence the heater voltages - right, and a variable voltage regulator (that's a source follower, not a variable resistor) to lower B+ if needed.

An even better setup would somehow make the bias voltage track where it nominally should be for a different B+.

[davent]

I had to look back and check but Dana called the VVR circuit a Variable Voltage Regulator.

dave

[GibsonGM]

Yes, it's a "VVR". I picked it up on the 18Watt forum years ago (not the Electronics Workbench schematic, ha ha), and installed one in mine. It DID work great! 

Then I took it out.  I forget why.  I just run it loud now.   

But the circuit itself has solved a few problems for me over the intervening years!  Easy way to....make your voltage variable....

Yeah, I hear you, R.G. - the diff. between mains power 'then' and 'now'...CAN be of concern.   My tendency has been to use a series resistance etc. to accomplish the same with heaters in various circuits, but that's heating, and also you're then disturbing the original wiring, often a NO NO.  So the trafo way is certainly useful!!