Fender Ultralinear Power Supply Questions

[lonote]

I ended up with two basket case UL Pro Reverbs a year or so ago. I finished rebuilding one, keeping it as designed, but I am thinking of repurposing the second transformer set for a small bass amp & rebuild the cab/chassis down the road into something more BlackFace-ish

The same power supply design of that era was used for most all the big amps; Pro, Super, Twin, etc. & while I know it works as drawn, I would like to have a better understanding of things, particularly the PT center tap configuration.

The PT s are all 182V-182V (ish) with a FWB, landing at about 510V, then there is a hi-watt voltage divider to drop B+2 down about 90V. The PT CT was landed between the two balancing resistors of the 2 series reservoir caps, instead of being taped off.

I am not clear on what exactly this is adding to the operation on the PSU?

No issues re-using the PT in the same design configuration, but I would like to have a better grasp on what is going on. I am also wondering about including a choke at the front end of the filter section, or does this design offer better filtering to begin with?

Sections of two schematics attached, the terrible Fender issued version & a redrawn (not by me) much more user-friendly version.


Thanks in advance for any insights.

UL Fender Power Rail-B+.jpg

Fender UL PSU 2.JPG

[sluckey]

The CT connected to the junction of the series reservoir caps forces the B+ to divide equally across each cap. A choke may not be worth the effort.

[lonote]

The CT connected to the junction of the series reservoir caps forces the B+ to divide equally across each cap. A choke may not be worth the effort.

Makes sense, I guess best to use it (CT) if it is there.


Thanks

[pdf64]

The HT winding CT node was used in some models to provide a half voltage = 1/4 power mode.
https://el34world.com/charts/Schematics ... _schem.pdf

I think the screen grid taps on the OT primary allow such a high stiff HT to be used without causing them to risk overdissipation.

Changing to a regular pentode output stage, with just a choke between OT CT and screen grid HT supply nodes will act to circumvent this mechanism, ie at high power outputs, the screen grids may overdissipate.

[lonote]

Changing to a regular pentode output stage, with just a choke between OT CT and screen grid HT supply nodes will act to circumvent this mechanism, ie at high power outputs, the screen grids may overdissipate.

I had meant to add in a choke to the otherwise unchanged B+ rail design (just in front of the 2.7K/10W, on that same node. Reservoir>Standby>Choke>2.7K). Essentially for improved filtering.

Poorly posed question, my apologies.

[sluckey]

I had meant to add in a choke to the otherwise unchanged B+ rail design (just in front of the 2.7K/10W, on that same node. Reservoir>Standby>Choke>2.7K). Essentially for improved filtering.

Don't do that. That 2.7K/30K voltage divider draws a lot of current and that current would have to flow through the added choke. This would require a sizable choke. I suggest putting the choke between the 2.7K and Node B. Now the current passing through the choke is much smaller and you can use a physically smaller choke. I would probably use the Fender 125C1A choke or even the smaller 125C3A choke.

[Roe]

instead of the big voltage divider, I'd consider a thermistor that limits the startup voltage and current that the filter caps sees. Also, the filter cap for the phase inverter and reverb driver should be at least 500+v and you may then increase the 2k7 to get the right voltage

[lonote]

Don't do that. That 2.7K/30K voltage divider draws a lot of current and that current would have to flow through the added choke. This would require a sizable choke. I suggest putting the choke between the 2.7K and Node B. Now the current passing through the choke is much smaller and you can use a physically smaller choke. I would probably use the Fender 125C1A choke or even the smaller 125C3A choke.

Thanks for the tip. I had wondered about the current being an issue & now recall reading that the order of the choke/resistor didn't matter. A smaller, more available & less expensive unit make sense to try.

instead of the big voltage divider, I'd consider a thermistor that limits the startup voltage and current that the filter caps sees. Also, the filter cap for the phase inverter and reverb driver should be at least 500+v and you may then increase the 2k7 to get the right voltage

Even to me (still figuring much of this stuff out), that seems like a clunky implementation, but in service it is a proven design & I will likely keep it as drawn (save testing a choke in there). I had thought that design might be part of the Ultralinear thing but digging around I see that the same power supply design was used on the big amps previous to the Ultralinear models.

[B Ingram]

I ended up with two basket case UL Pro Reverbs a year or so ago. ... I am thinking of repurposing the second transformer set for a small bass amp & rebuild the cab/chassis down the road into something more BlackFace-ish ...

Some hard-earned Wisdom: The transformers in these amps are great for building an amp with an ultralinear power section, using the same tube types, and essentially the existing power supply configuration. An attempt to use them for anything much-different that that will end in heartache.

The power transformer high-voltage winding was previously described as "182v-0-182v" but that is wrong-thinking. Instead, they are "364v winding" (which is 182v x 2) that just happens to have a center-tap.

• The full-wave bridge rectifier used here makes use of the entire secondary winding, all the time. A full-wave (non-bridge) rectifier as found in Fender's blackface amps uses only half the secondary at any moment. The bridge configuration reduces the amount of copper used in the transformer windings, at the expense of a more complicated rectifier setup (which is a non-issue in the solid-state era).

• This bridge-rectified winding happens to have a center-tap. The voltage at the center-tap must always be 1/2 the voltage from end-to-end of the secondary, so it's a handy way to force equal-voltage across the filter caps as Sluckey said. There are still 100kΩ resistors across these caps to drain voltage when the power is shut off.

• Ultimately, you still have 364vac being rectified, and that yields about 515vdc minus a few diode drops. No blackface Fender amp runs voltage that high, so now you need to make some choices about how to handle this.

A quality power transformer for a ~40w blackface Fender amp is around $180 or less. To me, there's much more than $200 worth of hassle trying to make this amp's power section do something other than "500v DC." That means my solution would be to buy "a transformer that gives the voltages I want."

Some folks will try to "blackface the phase inverter resistor values" but I also think that misses the point.

• This amp's power section (phase inverter, power tubes, output transformer, power supply) was built for high output power from a pair of 6L6GCs.

• The first move is crank up the supply voltage. This increases the possible plate voltage-swing (increases power one way), and raises the possible peak plate current (increases power another way).

• The screen grids would normally risk overheating when pushed to high power-output, but the Ultralinear arrangement transfers much of the screen-power to the speaker as audio output. That reduces screen dissipation.

• Ultralinear changes how the output uses respond, and opens up the possibility of driving the grids positive for even more power output, if the driver/phase inverter allows.

• Fender halved the plate load resistors in the phase inverter, probably to gracefully handle the 6L6GC control grids being pushed slightly positive. The output impedance of the inverter still isn't low enough to really allow "Class AB2" operation, but the inverter won't have its output clamped as-quickly as with 82kΩ & 100kΩ.

Now you can swap the power transformer & get a lower B+ voltage. You can modify the bias supply circuit to allow some adjustment & not just "bias balance." You can modify the phase inverter circuit, while leaving the output tubes connected ultra-linear fashion. You can even tape off the output transformer's screen-taps, and wire up the power tubes "blackface style." I just winder after having done all that, what was the value in starting with this assemblage of parts instead of a blank slate?


The "Wisdom" part of the above (to the extent there really is any wisdom present) is that I've gone through the pain of trying to convert some amp into something it isn't, only to find it was more expensive and worked less-well that I would have hoped. These days, any conversion I attempt better be "90% there" before I start, and I will retain the power supply and output section (and usually the phase inverter, too) as-is (aside form replacing failed components). I limit modifications to "changing the preamp."

[martin manning]

I ended up with two basket case UL Pro Reverbs a year or so ago. ... I am thinking of repurposing the second transformer set for a small bass amp & rebuild the cab/chassis down the road into something more BlackFace-ish ...

Some hard-earned Wisdom: The transformers in these amps are great for building an amp with an ultralinear power section, using the same tube types, and essentially the existing power supply configuration. An attempt to use them for anything much-different that that will end in heartache.

I'm with you there. Band-Aid B+ reductions are often problematic.

To me, there's much more than $200 worth of hassle trying to make this amp's power section do something other than "500v DC." That means my solution would be to buy "a transformer that gives the voltages I want."

Sound advice. Alternatively, use it as designed, i.e. a clean power amp, and adapt to that premise. For me, this suggests an excellent power section for something Dumble, where the preamp is used to generate the distortion.

• The screen grids would normally risk overheating when pushed to high power-output, but the Ultralinear arrangement transfers much of the screen-power to the speaker as audio output. That reduces screen dissipation.

The screen voltage is pulled down along with the plate voltage, which compresses the plate curves. The load line, which must be chosen to stay within plate dissipation limits, will then pass through the lowered Vg1=0V knee, and all is good.

PS Welcome to the forum!

[Bergheim]

PS Welcome to the forum!

Somehow I think B Ingram is the same person as HotBluePlates. Am I wrong?

[lonote]

Some hard-earned Wisdom: The transformers in these amps are great for building an amp with an ultralinear power section, using the same tube types, and essentially the existing power supply configuration.

An attempt to use them for anything much-different that that will end in heartache.

I limit modifications to "changing the preamp."

Thanks for your very insightful comments, I appreciate the time.

To your point (& Martin's), I believe we are all on the same page.

When I got the first Pro from one friend, I was initially of the mind to "convert" it to a more desirable circuit, but as I researched & learned about what Ultralinear was, I came to realize that it was a very different beast & that I should just embrace it for what is was. To that end, I rebuilt the first, keeping it stock, other than removing the distortion circuit.

The second one turned up several months later from another friend, in similar water-damaged condition. These both sort of appeared, nothing I was really looking for.

It is sad that these amps got the short end of the stick in many ways, as it is a nice sounding amp in & of itself, but unfortunately plagued with many less-than-desirable issues/features; the nearly unclean-able waxy boards, the spaghetti wiring, plastic D-shaft pots, the not-great distortion circuit, the weight, etc. not to mention the undeserved reputation for not sounding good. Both of the amps I got were refugees from leaky-roofed storage sheds.

So while there are some pangs of guilt parting one out, I am now aware of the extent of the work required to bring one back to working condition & I feel I have somewhat served the cause by already resurrecting one of the pair.

The second amp has suffered some internal water damage, another very waxy board, along with a crumbling baffle, crumbling grille & missing back panels. But from some initial testing, I know that the transformers are still OK.

So with my new-found familiarity with what I am actually looking at, I landed on the idea of using the transformer set (keeping the design as-is, as you both suggest) as a power section for a 2-6L6, Ultralinear 50W-ish bass amp, built as a head, as I actually play bass more than guitar.

I can see this output section with a Fender TMB &/or a B-15 Pre being a nice mid-power bass amp.

Repurposing the UL transformer set to a bass amp project leaves the Pro chassis (which needs a new board in any event) & the cabinet orphaned, which is where a BlackFace circuit might come into play, using a more suitable PT & OT, if that were to even happen.


My initial post was trying to better understand the power supply, & also determine if there were any improvements to be made in filtering when it was rebuilt into another amp.

[B Ingram]

When I got the first Pro from one friend, I was initially of the mind to "convert" it to a more desirable circuit, but as I researched & learned about what Ultralinear was, I came to realize that it was a very different beast & that I should just embrace it for what is was. ...
So with my new-found familiarity with what I am actually looking at, I landed on the idea of using the transformer set (keeping the design as-is, as you both suggest) as a power section for a 2-6L6, Ultralinear 50W-ish bass amp, built as a head, as I actually play bass more than guitar. ...

Excellent! It sounds like I misjudged where you were at with these. Re-purposing as an ultralinear bass amp is probably one of the best choices that can be made with these amps.

[Smitty]

Agree. BTW, those ultralinear output transformers have more primary inductance (compared to the earlier transformers) which will extend the low frequency response.

I've got a few bass amps including a, Blackface Dual Showman, a Bassman 100 and an SVT. My Bassman 135 Ultralinear sounds just as good as the others. I'd look at the Bassman 70 schematic for inspiration.

https://schematicheaven.net/fenderamps/ ... sman70.pdf

It shares transformers with the UL Pro Reverb.

https://schematicheaven.net/fenderamps/ ... ro_rev.pdf

Skip the second preamp channel since the accordion player bought his own amp back in '62. Along with nixing V3 in the schematic you may be able to get by without V2 and even the master volume. Two preamp tubes. Boom! Done!!

[lonote]

I'd look at the Bassman 70 schematic for inspiration.

Never thought to look, but makes sense there is already such a thing.

Thanks for that.