RedBear MKX 50 Power Transformer

[Delta362]

I'm working on a RedBear MKX 50 that doesn't have the PT with it. Originally these units came shipped with a 220V primary winding on the transformer. Customer wants me to install a Mercury Magnetics PT, but I'm having trouble determining the appropriate secondary winding voltage for the B+ generation so that I can then determine which PT is appropriate.

From the second sheet of the schematic in the tube pinout voltage table, it shows the plate voltage on the power tubes as 429. What should I be looking for in a transformer secondary voltage that I can drop in? I can make the bias circuit work with a dedicated 50 to 60V winding. The heater circuit would be driving two 5881 (or 6L6), and four of the 6N2P preamp tubes.

Thanks!

[Helmholtz]

What is the rated output of the amp?

PT secondary voltage varies with load current.
To use PSUD2, I need to estimate the DC current at full output.

[Delta362]

Thanks, Helmholtz! The amp is rated 50W. Two power tubes are 5881. The four preamp tubes are 6N2P.

Understood on the load current impacting secondary voltages. From looking at various schematics, I see some amps using anywhere from 315/0/315 windings to 380/0/380; and others using 175/0/175 and having similar B+ voltages.

The Marshall SLP reissue 100W 1959 schematic looks to have a very similar design for B+ generation, though it would be sized for four EL34 tubes needing more current. I can't find any voltages on that schematic to get me in the right ballpark for the high voltage windings.

[Helmholtz]

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.
So what matters for calculation of B+ is the full voltage between the ends of the PT secondary.

For a 50W amp having a B+ around 400V I typically assume a full power DC current of 250mA.

[Delta362]

Thanks for the explanation on the CT! The 250mA for full power makes sense.

I'm not seeing voltage doubler on anything. Did you intend to upload something?

For the heaters, it looks like I'll have 2 tubes at 0.9 [5881], and 4 tubes at 375mA [6N2P] so I'd need a 6.3V heater winding with at least 3.3A capability.

I did find a Magnetic Components transformer 40-18046 that shows a similar schematic of B+ generation on it utilizing 175/0/175 (350VAC Center Tap) as a reference for the voltage and circuit design. I realize the rest of the transformer doesn't meet my application, but am looking to understand how this one has a 350VAC center tap vs others that I see 660VAC center tap achieving similar B+ voltages between both of them.

I guess where I'm stuck is understand the circuit design of the MKX 50. I did find and download the PSUD2 that you mentioned above. Am messing around with it trying to learn how to use it.

Ultimately my biggest confusion would be - am I looking for something with a 200VAC winding, a 350VAC winding, or a 660VAC winding, as examples of three vastly different voltages, for the given amp's topology.

[Helmholtz]

According to PSUD2 a PT having a secondary voltage of 300V@400mArms would give a B+ of around 430VDC at idle (80mA).
AC current is at least 1.6 times DC current.
No-load secondary voltage is 340V (= 2x170V).

I assumed a typical PT secondary equivanent source resistance of 100R.

PT should be rated at around 150W.

This said, many amp manufacturers seem to assume a max. DC current of 200mA or even less for a 50W amp, probably arguing that full current is needed only temporarily.

[Delta362]

Great information and guidance as always, Helmholtz!

I check with MM and they suggested the MP100-68 for this application. It's listed as a PT for a 100W amp so for a 50W amp application, I expect it to run fairly cool. Does this look good to you?

I'm asking about a few other ones in the MP100 series, namely the -425, -450, and -460 as I'm realizing the number at the end is roughly B+. Just checking the other windings to see what they have, too. There is no cost differential between this and the other ones.

[Helmholtz]

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.
But I assume a PT for a 100W Marshall should be strong enough.

[Delta362]

I've seen some datasheets specify the voltages as loaded, but more times than not, I don't see them specified at all.

Makes sense for this application that it should be strong enough.

The other three I asked about are all stand up versions, but the main difference is their windings of 2x156 (-400), 2x166 (-425), 2x 175 (-460). They didn't provide the -450, but it's probably around 2x172.

Thanks again for your help!

[g1]

am looking to understand how this one has a 350VAC center tap vs others that I see 660VAC center tap achieving similar B+ voltages between both of them.

I haven't looked at the circuits yet, but usually when you see a secondary VAC difference of roughly double, one circuit will be full wave rectifier, the other will be bridge rectifier.

[Helmholtz]

That^^!

Sorry for nitpicking, but a bridge rectifier is a full wave rectifier as well.
For better distinction the circuit with the 2 diodes and grounded CT should be called a two-phase rectifier.

[Delta362]

Thanks, g1 and Helmholtz! That makes sense as I'm reading up more on the two topologies to get a better understanding of them.

The transformer is ordered, and it's pricey, but the customer is ok with it. I'll let you know how it goes.

[g1]

That^^!

Sorry for nitpicking, but a bridge rectifier is a full wave rectifier as well.

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'.

[Helmholtz]

Merlin says two-phase rectifier is the technically correct term.
Our German term would translate into two-way rectifier.

[g1]

Merlin says two-phase rectifier is the technically correct term.

From what I can find he seems to be alone on this. And it does beg the question, how many phase is a full wave bridge? Googling 'single phase full wave rectifier' will give you the same 2 types of rectification as 'full wave' (2 diode CT and 4 diode bridge), but if I search 'two phase recitifier' I get lots of hits with full wave bridges.
Pretty poor terminology for such long standing common circuits.