Autoformer vs Split Primary windings on Power Xfmrs

[nevetslab]

I just received another Hiwatt HLR "Little Rig" amp head in for service and after opening it up, having to deal with possible damage to the Standby Switch from the chassis mtg screw positioned directly below it, correcting the cattywampus orientation of the Pwr & SB switches to correct orientation, I powered this amp up, and.....having heard RF buzz from the previous three amps, one being another HLR model, I again heard that RF buzz out of the speaker, and saw it clearly visible on the output at idle.

After seeing the sketch of the Primary wiring diagram of it's power xfmr, I see it, and their other two models I've been inside on are all a tapped autoformer primary, rather than a split primary. for 120V mains, both primaries are in parallel (properly phased, of course).

I got to wondering if there's any relationship to this 120V Mains applied to the lower half of an autoformer-style primary rather than having two identical primaries in parallel, with regards to the clipped heater waveform that is providing that abrupt step that is the RF source that gets coupled thru the input preamp tube? I've attached a schematic of their Custom 50W Amp schematic, which is also an Autoformer style primary (uses a rotary switch to select AC Mains taps). Their Little Rig is hard-wired per the correct voltage tap, but still same configuration. With the Autoformer primary, you're running twice the primary current thru that lower half than if there were two primaries in parallel for 120VAC Mains.





For some reason, the client (Hiwatt) doesn't want this problem fixed.

[Helmholtz]

.....having heard RF buzz from the previous three amps, one being another HLR model, I again heard that RF buzz out of the speaker, and saw it clearly visible on the output at idle.

After seeing the sketch of the Primary wiring diagram of it's power xfmr, I see it, and their other two models I've been inside on are all a tapped autoformer primary, rather than a split primary. for 120V mains, both primaries are in parallel (properly phased, of course).

How can you hear RF buzz (assuming RF means radio frequency)? How does it sound and how does it look on the scope?

I don't see an autoformer in the schematic, just a primary with multiple taps - quite normal with many "Export" amps.
Paralleled 120V primaries are used when the primary wire is too thin for the increased primary current with 120V.

Neither primary configuration should have a direct influence on amp buzz.
Everything else being the same, a PT having less primary DCR should cause lower heater spikes.

Scope pics show the familiar heater hum spikes.

[catalin gramada]

Maybe the diodes are noisy as hell and filter caps don't decouple enough in high freq range. Couple of ceramic caps over diodes and filter caps may solve the problem, nothing related with PT...my2c

[nevetslab]

Obviously I can't hear into the RF range, but the HF content of that step from Clipped heater waveform down to the sine-shape at top and bottom....with this latest amp that's stock, I can definitely hear the HF Buzz,

I used the term Autoformer as the winding style of the multi-taped primary (0V, 100V-120V-220-240V). "Everything else being the same, a PT having less primary DCR should cause lower heater spikes." That's what I was wondering about. A number of other models just arrived early last month, but thus far, I have not been given permission to even look at them in the shop. This latest amp which came to me for a different reason, I went and fetched our identical model for A/B comparison. The modified one has no RF Buzz, while the stock one does. Hence the reason I want to have a look at the rest of what just arrived.

You've seen these scope photos before, and the inclusion of the 'humdinger' to null out the RF spikes is very effective. But, the client doesn't want to go down that road, as some of his experts are NOT in favor of adding that control.

[nevetslab]

Maybe the diodes are noisy as hell and filter caps don't decouple enough in high freq range. Couple of ceramic caps over diodes and filter caps may solve the problem, nothing related with PT...my2c

I was wondering about that too during the start of that "RF buzz" at the beginning back in November. I will have to order some HV ceramics for that purpose...none on hand at present. Merlin's and Helmholtz suggestion with the humdinger added seemed to solve it. But, that's not being allowed now. Sigh.............

[Helmholtz]

Obviously I can't hear into the RF range, but the HF content of that step from Clipped heater waveform down to the sine-shape at top and bottom....with this latest amp that's stock, I can definitely hear the HF Buzz,

I used the term Autoformer as the winding style of the multi-taped primary (0V, 100V-120V-220-240V). "[I]Everything else being the same, a PT having less primary DCR should cause lower heater spikes."

Sorry, but as your original post didn't show pics, I wasn't sure what kind of buzz you meant.

A PT with paralleled primaries may or may not have lower DCR than one with multi-taps. But DCR is easy to measure.

.. the inclusion of the 'humdinger' to null out the RF spikes is very effective. But, the client doesn't want to go down that road, as some of his experts are NOT in favor of adding that control.

Oh well...

[The Dude]

......You've seen these scope photos before, and the inclusion of the 'humdinger' to null out the RF spikes is very effective. But, the client doesn't want to go down that road, as some of his experts are NOT in favor of adding that control.

Sorry for this, but I just have to ask. Why are they not in favor of adding the humdinger? It doesn't make sense to me.

[catalin gramada]

Sorry for this, but I just have to ask. Why are they not in favor of adding the humdinger? It doesn't make sense to me.

Guess is some related with original design . A noisy 100 percent clone may have more business opportunity than a quiet improved one...just thinking. They may claim : "Mr. Reeves never used..." If their sale strategy is based onto original designs is pretty understandable from a point but also a error from historical interpretation as may see on M Huss site they did a lot of changes in circuit in time.You cannot stuck with a "classic" because did not exist.

[nevetslab]

Sorry for this, but I just have to ask. Why are they not in favor of adding the humdinger? It doesn't make sense to me.

I think catalin may be onto the answer, but it also seems to be coming from whoever their director is speaking with. I'm sure whatever was communicated to his panel of experts didn't reveal what's really going on. If it wasn't known as being questionable years ago, then to remain 'classic', it won't be allowed, regardless of what buzzing you hear out of the new production units. Now, I've only seen/heard four of their new production amps. All four had this RF buzz. I'll happily restore it to have that, if that is what's going to make them happy. As a customer, I'd be less than thrilled to hear that out of a $3000+ amp head.

I sent the link to Merlin Blencowe's book, which opens to Pg 76 which was what g1 had added to that earlier post on the Hiwatt Custom 50W amp excessive rectifier buzz and suggested that be forwarded to those chaps that are saying no to. We'll see. It was very revealing once I had put the humdinger into place and dialed down the RF spikes, then swapping out input tubes and seeing a lot more of the coupling behavior that some tubes have that others are less sensitive to. That sure changed my view on the J/J ECC83S preamp tubes. Which is what all of the new inventory has in them.

[Helmholtz]

I forgot to mention that also low leakage inductance in the PT should reduce the spikes.
Also soft recovery rectifier diodes with snubber caps might help somewhat.

BTW, it's not necessary to drill an extra hole for the humdinger when using a small trimpot.

[The Dude]

......That sure changed my view on the J/J ECC83S preamp tubes. Which is what all of the new inventory has in them.

Can you clarify that? Does it mean your opinion is good or bad on the JJ's? I use JJ's almost exclusively and haven't noticed any problems.

[g1]

Can you clarify that? Does it mean your opinion is good or bad on the JJ's? I use JJ's almost exclusively and haven't noticed any problems.

I think he had found the JJ to be more problematic with regard to this particular buzz, here: https://music-electronics-forum.com/...250#post945250

[catalin gramada]

Hey, forgot to mention...Did you tried to wire the heater CT in a different spot ? Hook a bit longer wire and try on different point to ground it.( a "quiet" one as at the end of the bus or different points to chassis). I don't know if the real layout follows the schematic but if is so they choosed the worst point to hook it as ground reference. They should hook nothing between the first filter cap and CT (ok, there is a fuse in between but that will not do any difference), Also avoid any lugs at PT bolts if there are..and generally avoid any reference point to heavy currents ripple track.Then No between CT and first filter cap, even No between first filter cap and power returns. If they did it so they did it from convenience but the circuit present some vulnerability, so it is possible to be acceptable in some certain conditions but basically not.Hope it helps...

[J M Fahey]

After seeing the sketch of the Primary wiring diagram of it's power xfmr, I see it, and their other two models I've been inside on are all a tapped autoformer primary, rather than a split primary. for 120V mains, both primaries are in parallel (properly phased, of course).

120V Mains applied to the lower half of an autoformer-style primary rather than having two identical primaries in parallel,...... also an Autoformer style primary (uses a rotary switch to select AC Mains taps). ..... With the Autoformer primary, you're running twice the primary current thru that lower half than if there were two primaries in parallel for 120VAC Mains.

NOT an "autoformer" as "helmholtz" mentioned but a multi-tapped primary.

THE proper way used way back then, when cost was not *that* important, just look a thousands VOX/Marshall/Laney , in general British amps of thatb era.
Brits had a worldwide Empire way back then and in any case were Export oriented, so a "use anywhere" transformer was the *standard* configuration.

"Americans" on the other side focused on internal market, so basically 120V single primary, and dual 120V equal ones was their concession to that, and not on every amp.
With a couple exceptions only, such as Twin "export model" with their DANGEROUS exposed and easy to turn chickenhead red knob.



For some reason, the client (Hiwatt) doesn't want this problem fixed.[/QUOTE]
You canīt fix stupid.

Oh: and of course the 120V section used twice the section (1.4X the diameter) enamelled wire.

[catalin gramada]

NOT an "autoformer" as "helmholtz" mentioned but a multi-tapped primary.

THE proper way used way back then, when cost was not *that* important, just look a thousands VOX/Marshall/Laney , in general British amps of thatb era.
Brits had a worldwide Empire way back then and in any case were Export oriented, so a "use anywhere" transformer was the *standard* configuration.

"Americans" on the other side focused on internal market, so basically 120V single primary, and dual 120V equal ones was their concession to that, and not on every amp.
With a couple exceptions only, such as Twin "export model" with their DANGEROUS exposed and easy to turn chickenhead red knob.



For some reason, the client (Hiwatt) doesn't want this problem fixed.

You canīt fix stupid.

Oh: and of course the 120V section used twice the section (1.4X the diameter) enamelled wire.

[/QUOTE]

This looks like a joke, look ya serious business: 110v/220v selector with tap adjustments : -20,-10,-5, 0, +5,+10,+20 positions. There are 14 voltages you may select for primary use.