RedBear MKX 50 Power Transformer

[Delta362]

Thank you, Helmholtz! I'll definitely be checking out that tool and learning along the way.

[Helmholtz]

Thank you, Helmholtz! I'll definitely be checking out that tool and learning along the way.

Here's a good start regarding rectifiers circuits:

http://www.valvewizard.co.uk/bridge.html

http://www.valvewizard.co.uk/psu.html

[Delta362]

Got the links bookmarked. Thanks!

[pdf64]

THe CT wiring doesn't matter for voltage calculation, it's only used for balancing the reservoir caps.
The term "voltage doubler" for this circuit is misleading.
...
For a 50W amp having a B+ around 400V I typically assume a full power DC current of 250mA.

I've seen a mischief maker on a forum describing the SLP type rectifier as a 'voltage doubler', so that may be the kind of thing you were referring to?

How to relate the AC current rating of the winding in a two phase rectifier arrangement, to the DC load current? eg can the effective AC current be doubled, as current only flows half the time; then the DC load current also flows via the winding CT; then there's the charging current for the reservoir cap. It's rather difficult for me to relate the AC rating to the currents in circuit.

Problem I'm having with some PT specs is that it's not clear if the secondary voltage is specified for no-load, some average current or max. current.

The Hammond 290 MAEX seems to acknowledge the issue, by providing no load voltage, voltage at 150mA, full load current, and winding resistances https://www.hammfg.com/files/parts/pdf/290MAEX.pdf

Would it have been better if I said 'full wave bridge' ? I was just going with the terminology that I normally see used when different types of rectifiers are discussed in regard to guitar amps..
While I am not saying I disagree, I have never seen the term 'two phase rectifier', it is usually just called 'full wave'. I have sometimes seen 'center-tapped full wave'.

I dunno if Merlin invented the two phase rectification terminology, or there's a historical precedent he dug out from an earlier source. Or perhaps he's fluent in German and he adapted their terminology
Whatever, it's surely beneficial to have a proper specific name for it, as the terminology around rectifier types causes a degree of confusion on forums.
Especially when analysing Marshall amps, as many 50watters with two phase rectification used a bridge rectifier package, and then the 100watters tend to use bridge rectification with a CT on the winding, and wire the CT to balance the VDC across the reservoir diodes. This allows it to be viewed either as a bridge, or as a dual / bipolar / split rail two phase arrangement. When viewed from the perspective of load current, the former seems to be a much better fit,

[Helmholtz]

I dunno if Merlin invented the two phase rectification terminology, or there's a historical precedent he dug out from an earlier source.

No, he didn't.

It can be found in standard literature, e.g. Terman's book.
Apart from that the term seems logical and self-explaining.
But no doubt, the two-phase rectifier is a full wave rectifier, just not the only one.

[g1]

Once you throw '3 phase rectifier' into the mix, you can see why '2 phase' can lead to confusion.
Unless you are on 2 phase AC service.

[Helmholtz]

Once you throw '3 phase rectifier' into the mix, you can see why '2 phase' can lead to confusion.
Unless you are on 2 phase AC service.

Well. maybe not so unambiguous with English terms if you mix in energy electronics.

In German we say two-WAY rectifier but 3-Phase rectifier (when 3 mains phases are meant).

Don't blame me for English terms.

[Helmholtz]

I've seen a mischief maker on a forum describing the SLP type rectifier as a 'voltage doubler', so that may be the kind of thing you were referring to?

Yes, that misconception seems to be quite common within the Marshall community.
I remember a discussion of that circuit on the MEF some time ago.

How to relate the AC current rating of the winding in a two phase rectifier arrangement, to the DC load current? eg can the effective AC current be doubled, as current only flows half the time; then the DC load current also flows via the winding CT; then there's the charging current for the reservoir cap. It's rather difficult for me to relate the AC rating to the currents in circuit.

There is no fixed ratio between RMS AC and DC currents in rectifier circuits. (Standard literature gives a ratio range for each rectifier type depending on conduction angle.)
That's why I find PSUD2 so useful.

The Hammond 290 MAEX seems to acknowledge the issue, by providing no load voltage, voltage at 150mA, full load current, and winding resistances https://www.hammfg.com/files/parts/pdf/290MAEX.pdf

Yes, that's a positive exception.

[pdf64]

Once you throw '3 phase rectifier' into the mix, you can see why '2 phase' can lead to confusion.

I don’t see how? 2 is different to 3, it’s not as if 6 was 9

Unless you are on 2 phase AC service.

Aren’t the 240V domestic supplies for appliances in North America basically the same as a two phase HT winding, ie 2 lives feeds180 degrees apart, with a CT to circuit common?

[g1]

Well. maybe not so unambiguous with English terms if you mix in energy electronics.

Aren’t the 240V domestic supplies for appliances in North America basically the same as a two phase HT winding, ie 2 lives feeds180 degrees apart, with a CT to circuit common?

First off, I don't want you guys getting the impression I'm just here for an argument. I do think the terminology is poor and am trying to illustrate that.

240V in North America is considered single phase. It goes back to the distribution network before the transformer on your local power pole.
As Helmholtz stated, the english terms for electrical distribution phase throw a wrench in the works.
Also, if the 2 diode center-tapped arrangement is called a 2-phase rectifier, wouldn't a half-wave rectifier be properly called a single phase rectifier?

One thing I know, if you say bridge rectifier, everyone seems to be on the same page.

[Helmholtz]

Also, if the 2 diode center-tapped arrangement is called a 2-phase rectifier, wouldn't a half-wave rectifier be properly called a single phase rectifier?

If we agree that single-phase rectifier means that the diodes are fed from a single AC voltage and that 2-phase rectifier means that it uses 2 different-phase AC voltages,
It gets clear, that half-wave rectifiers and bridge rectifiers both are single-phase rectifiers by functional principle.

OTOH, both bridge and 2-phase rectifiers put out 2 rectified half-waves, so they are full-wave rectifier circuits.

When I made my comment in post #11, I wasn't aware that the term 2-phase rectifier is that uncommon or even unknown.
I didn't intend to cause confusion.

[Helmholtz]

How to relate the AC current rating of the winding in a two phase rectifier arrangement, to the DC load current? eg can the effective AC current be doubled, as current only flows half the time; then the DC load current also flows via the winding CT; then there's the charging current for the reservoir cap. It's rather difficult for me to relate the AC rating to the currents in circuit.

I misunderstood your question before.

Finding the correct AC current rating of the winding is a problem indeed.
The AC current as specified in the Hammond datasheet seems to refer to a resistive load, where voltage and current are sinusoidal and in phase.
So when you multiply the RMS values you get the real power.

But with the rectifier, winding currents are no longer sinusoidal and multiplying RMS current with RMS voltage gives apparent power which is always larger than real power because there is an unknown power factor <1.

PSUD2 gives the winding currents, but they are not comparable to the datasheet currents. In fact they are considerably larger.
But those are the currents which heat up the wire.

To find real PT power without a power meter it's best to add up all power portions delivered to the amp.
HT DC power is easy to measure. (It's typically around twice the output power with class AB.)
Once you know HT power, you can calculate the equivalent resistive AC current using the loaded secondary voltage.

In his book, Zollner gives an example for a 50W (actually 45W after the OT) amp using a GZ34.
Total secondary power adds up to roughly 140W at full power (without clipping!).
Assuming a PT efficiency of 90%, PT input power is 156W.

[Helmholtz]

I'm just doing some maintenance on?, of?, for?, with? (you tell me) a 1970 Fender Vibrolux Rev (2x6L6, GZ34).

So here are some real input power measurements (in brackets: apparent VA power).

@ Standby: 48W (65VA)

@ Idle: 93W (106VA)

@ Full output power: 145W (162VA)
Full output power is 35W at the onset of clipping. The small OT eats around 6W.

For these measurements I use a small and inexpensive "Energy Monitor" by Voltcraft (looks similar to a Kill-a-Watt), which is quite accurate.

PT "size" depends on apparent power (VA).

[Helmholtz]

Any native speaker willing to help me out with my language question above?
Always struggling with the English prepositions which don't have a direct correspondence to (with?) the German ones.

[pdf64]

I think ‘maintenance on’ works well, ‘maintenance to’ is an alternative, but the meaning would be apparent with any of the options, and probably a few more too