Lower B+ options

[basschops1528]

I've read so many threads today about VVR power scaling, using Zeners, and MOSFETs with/without Zeners; all to lower the B+. Has anyone successfully attempted to remove portions of the HT secondary winding as a means to lower B+? Obviously this sounds like a simple mod-- you're cutting a wire shorter and rewinding the rest (of course easier said than done). If it were done, how would the rest of the amp be affected, mainly the bias setting. I would imagine that lowering B+, thus lowering screens would need a re-bias and would therefore change the whole tonal characteristics. George Metropolous does this on one of his amps where the standby can switch between regular, standby, and lower B+ which I would guess keeps the bias the same but reducing the headroom and getting more power amp drive.

[HotBluePlates]

... Has anyone successfully attempted to remove portions of the HT secondary winding as a means to lower B+? Obviously this sounds like a simple mod-- you're cutting a wire shorter and rewinding the rest (of course easier said than done). ...

I'm sure some have used high-voltage windings with more than 1 tap to change supply voltage... Which is then the same as you have a tapping X-turns in from either end of the high voltage winding, with fewer total turns.

The approach you proposed is only practical if you are able to wind your own transformers.

... If it were done, how would the rest of the amp be affected, mainly the bias setting. I would imagine that lowering B+, thus lowering screens would need a re-bias and would therefore change the whole tonal characteristics. ...

If plate voltage, screen voltage and bias voltage all change in proportion, the overall sound change may not be that big (other than power output).

But yes, if you're lowering output tube plate & screen voltage far enough, the bias will need to be changed (shifted a little closer to zero volts). That's why the O'Connor's powerscaling circuit has a tracking bias supply to automatically shift as supply voltage is changed.

[Fresh_Start]

There's an approach I've thought about using a power transformer with two pairs of high voltage taps. Use solid state rectification and fixed bias with the higher voltage secondaries, then tube rectifier and cathode bias with the lower voltage secondaries. My thoughts are to have 1) a relatively large difference between the B+ levels, 2) a stiff, higher voltage option with more sag with the lower voltage option from both cathode bias and tube rectification, and 3) ability to adjust bias for higher voltage and simply let cathode resistor(s) do the work for the lower voltage alternative.

I've drawn, but not completed, a schematic for this power supply. Switching is a bit tricky.

Consensus here and elsewhere seemed to be "just build the amp the way it's supposed to be."

Chip

[eleventeen]

[HBP] "The approach you proposed is only practical if you are able to wind your own transformers.


Or if you are considering removing turns from an existing tranny, that the HV winding is the outermost winding. I have no idea if that is the case or not but it should be fairly observable.


Me? I wouldn't do it, or perhaps I should say, the idea doesn't strike me as attractive. IMO you could damage some portion of the insulation on an existing winding. If the HV winding was underneath other windings, you would really, really not want to create a gap without filling it with something or other. But there are people who delight in winding their own and I suppose if you have done 2 or 3 or 10 then it's no big deal at all.

[HotBluePlates]

[HBP] "The approach you proposed is only practical if you are able to wind your own transformers."

Or if you are considering removing turns from an existing tranny, that the HV winding is the outermost winding. I have no idea if that is the case or not but it should be fairly observable. ...

I considered that, as I've repaired an open non-center-tapped high voltage winding in a VTVM by opening the PT and peeling back a few winds until I found the break. I got lucky, as the open was near an end of the winding, so only a few turns had to be removed & the voltage output of the repaired transformer was negligibly-impacted.

But the OP is asking about a transformer which is center-tapped. Likely, the winding is made by winding half the secondary turns, adding the tap, and winding the other half of the secondary turns. So you can unwind some turns from the outer-most layer and that side of the CT, but you can't get to the other side of the secondary at the Start of the winding, as it's buried.

I'm aware of bi-filar-wound transformers, but since most guitar amp transformers seem to have winding halves with different d.c. resistances, bi-filar winding seems to be the exception.

Earlier I said I was lucky the break in my VTVM's PT was near the end of the winding. I was actually doubly-lucky that it was near the Finish end of the winding rather than near the Start end... (because then I'd have to cut off & replace the entire secondary winding)

Anyway, Manipulating power transformers is a brute-force method of changing supply voltage. It's do-able, and easy to understand, but electronic supply voltage reduction (a la power scale, VVR, etc) is a better choice.

I hope to never have to repair one of my HP variable supplies (0-320vdc). They use a very elegant method of lowering the voltage feeding the regulator which essentially reduces the duty cycle of the rectifier feeding the initial power supply filters, which lowers voltage at those caps before the unregulated d.c. is presented to the regulator. Bottom line, it keeps the regulator cool when you're working at low output voltage (where there would normally be high voltage drop & high power dissipation at the regulator device).