Fender SF Champ high voltages: dropping resistor on PT primary?

[CO_Hoya]

Hi, I'm new here.

I've come across a number of threads here regarding the high voltages measured on the B+ of BF/SF Champs. In my case, with mains voltage at 123 VAC that was 445VDC on the first B+ stage.

A couple of recent threads: Lower B+ in Champ with Resistor before B+, Silverface champ biasing hot, but there are many more

I tried the trick of installing a power resistor on the center tap of the power transformer B+ output. I used a 470 ohm, 6.5W and dropped B+ down to 390VDC, which seems reasonable. I guess the dropping resistor makes sense for a Class A amp since the idle current is at 100% and therefore avoids sag - I haven't worked through what happens when the amp has a signal applied.

However, I noticed that the 6.3V heater voltage was actually reading 7.2V, so out of spec per the Valve Wizard, and allowing the 12AX7 and 6V6 to draw about 30% more heater power than ideal.

It occurred to me that I could just put a dropping resistor on the input to the PT instead, so on the primary winding. This would be the simple/cheap way to emulate a bucking transformer.

And so I did, using a 33 ohm, 25W power resistor to drop from 124 to 113VAC. This knocked the heater voltage down to 6.55VAC (and the rectifier 5VAC down from 5.40 to 4.95VAC), and consumes about 3W. The B+ is about the same (395V) as with the PT B+ CT resistor, and so I didn't need to re-bias the 6V6 after making the swap.

Is there any reason that the PT primary dropping resistor is a bad idea here? The amp sounds good, but a bit clean. It doesn't want to start breaking up with a single coil pickup until above 7 on volume.

Pic of the wiring for critique attached.

[pdf64]

I think the idea is fine. Ideally a bucker transformer would be used, but with such a small amp, compared to your resistor, the cost etc drawbacks might reasonably be seen to outweigh the benefits.
I think it would be better to put it in the live, rather than the neutral feed. eg if the wire to it became dislodged, if it touched the chassis it would carry on working without blowing the fuse. Hopefully an RCD would trip, but there’s many regions where they’re not common.

[CO_Hoya]

I think the idea is fine. Ideally a bucker transformer would be used, but with such a small amp, compared to your resistor, the cost etc drawbacks might reasonably be seen to outweigh the benefits.

That's good to hear, thanks. I happened to have the 33 ohm resistor in hand, so the cost to me was essentially zero and that was a big motivator for trying it.

I think it would be better to put it in the live, rather than the neutral feed. eg if the wire to it became dislodged, if it touched the chassis it would carry on working without blowing the fuse.

I see your point that the fuse wouldn't blow since the PT would remain in circuit and would have adequate impedance to prevent it. But it's not clear why my wiring would be more susceptible to this failure state than the previous wiring, which joined the neutral and a flying lead off of the PT via insulated solder joint. Because of that previous flying connection, I reckoned that using the power resistor on the neutral side would be an improvement, safety-wise.

Looking at my picture, I see that I forgot to go back and add heat-shrink tubing to the connections on the power resistor - I had left those uncovered to check voltages. Would that be adequate for safety?

P.S. - No snark or sarcasm intended here - I'm truly just trying to learn how to do it correctly.

[PRR]

On most US wiring, and 2-pin plugs generally, the difference between White and Black line-wire is random chance. It is really hard to find a case where it makes a difference.

[pdf64]

On most US wiring, and 2-pin plugs generally, the difference between White and Black line-wire is random chance. It is really hard to find a case where it makes a difference.

But over the past five decades or so, haven’t polarised, grounded outlets and plugs become the norm?

… it's not clear why my wiring would be more susceptible to this failure state than the previous wiring, which joined the neutral and a flying lead off of the PT via insulated solder joint…

A wire joint should be fine if done properly. ie wires twisted together to form a solid mechanical joint, then soldered, then insulated.
Fault current can cause stuff to overheat, to the point of melting soldered joint, hence the benefit of making it mechanically secure first. As a general principle, a resistor and its connections are more likely to overheat than a wire joint. And things like fuses, NTC resistors, trips are fitted in the live, rather than neutral feed.

[CO_Hoya]

I thought I’d bring some closure to this thread.

I followed pdf64’s suggestion to relocate the dropping resistor onto the live leg of the power input, between the power switch and the primary of the PT. I remain quite pleased with the heater and B+ voltages with this design. I measured for change in B+ voltage with a signal applied on the bench, and saw maybe a half volt change (or just observed normal variation in wall voltage).

Regarding my comment that the amp is a touch cleaner than before adding the dropping resistor, I wonder if that could actually be from the grid stopper I added to the 6V6 at the same time? Otherwise, it sounds very good to my ear with little hum.

Here’s the final(?) wiring. Please do let me know if you see any issues.



Cheers!