Magnatone Replacement Power Transformer

[JZRepair]

Consider two extremes. A 60W amp on a 3W lamp, and a 60W amp on a 1,000W lamp.

60W amp on a 3W lamp, the lamp will get most of the voltage, maybe 95%, and the lamp will be at least 9/10 full brightness.

60W amp on a 1,000W lamp, the amp will get most of the voltage, maybe 95%, and the lamp will be at 5% full heat, which is NOT bright at all. (A trace of heat if you put your lips on it.)

If that does not make sense, yet, sleep on it.

Thank you for that explanation.  I'm trying to look at it as a resistive element.  You use a current limiting resister to dial down the amount of current running to a specific leg of a circuit.  The higher the resistance (wattage), the less current can flow to that leg.  The lower the resistance, the more current.  You're essentially choking off what the rest of the circuit can have delivered to it.  This is why I questioned the use of the 100R resistors to create the center tap for the 6V3 heater windings.  It's only 6V3, but 100R allows a lot more current to flow than a 10k.  From a design standpoint, that's a lot of wasted energy.  Granted, this is running from mains, but it's not efficient and will produce more heat than the 10k.

Essentially, it's the same concept with the light bulb limiter in that the light bulb is limiting what the amp (or whatever is in series) can receive.  It's the concept of power that throws me off, rather than the current.  With a resistor, you're limiting it with that resistance level and the resistance is given.  With a bulb, I have to calculate it out and that's where my mind goes blank.  It's not that I can't do it, it's just that math isn't second nature in my brain.  I'll get it though and I do appreciate the picture you painted.  That helped put it in perspective, as did uki's explanation that it's based off the power of the amp, as well.  I just have to think of both of them as resistive elements and not two separate entities that share nothing common, like my brain wants to perceive it.

[sluckey]

This is why I questioned the use of the 100R resistors to create the center tap for the 6V3 heater windings.  It's only 6V3, but 100R allows a lot more current to flow than a 10k.  From a design standpoint, that's a lot of wasted energy.  Granted, this is running from mains, but it's not efficient and will produce more heat than the 10k.

Don't even try to relate the artificial center tap to the current limiter bulb. They are two completely different things with two completely different purposes.

The sole purpose of a center tap on the filament winding is to be able to provide a ground reference to the filament circuit. There is no current flowing into this ground reference. But this ground reference will reduce the amount of undesirable 60Hz hum being coupled into the amplifier circuit. In the absence of a real center tap you can use two resistors to accomplish the same ground reference. Ideally, you would want this ground reference to have zero resistance, but that's impossible because doing so would short the filament winding. So, you must use some value of resistance. It has been found that 100Ω resistors provide an adequate ground reference without adding too much extra current to the filament winding. (Two 100Ω resistors across a 6.3VAC winding only draw 31.5mA from the winding. Barely noticeable.) Using 10K resistors would defeat the whole purpose of the artificial center tap because the ground resistance would be 10K and that will allow a lot (probably all) of the 60Hz hum to get into the amplifier circuit.

[uki]

Have you look into tube datasheets ? Lots of info on it !!

https://frank.pocnet.net/index.html

6V6 on this page
https://frank.pocnet.net/sheets66.html

Look at the current need for the tubes heaters.

[JZRepair]

This is why I questioned the use of the 100R resistors to create the center tap for the 6V3 heater windings.  It's only 6V3, but 100R allows a lot more current to flow than a 10k.  From a design standpoint, that's a lot of wasted energy.  Granted, this is running from mains, but it's not efficient and will produce more heat than the 10k.

Don't even try to relate the artificial center tap to the current limiter bulb. They are two completely different things with two completely different purposes.

The sole purpose of a center tap on the filament winding is to be able to provide a ground reference to the filament circuit. There is no current flowing into this ground reference. But this ground reference will reduce the amount of undesirable 60Hz hum being coupled into the amplifier circuit. In the absence of a real center tap you can use two resistors to accomplish the same ground reference. Ideally, you would want this ground reference to have zero resistance, but that's impossible because doing so would short the filament winding. So, you must use some value of resistance. It has been found that 100Ω resistors provide an adequate ground reference without adding too much extra current to the filament winding. (Two 100Ω resistors across a 6.3VAC winding only draw 31.5mA from the winding. Barely noticeable.) Using 10K resistors would defeat the whole purpose of the artificial center tap because the ground resistance would be 10K and that will allow a lot (probably all) of the 60Hz hum to get into the amplifier circuit.

So in essence, the resistors are creating a filter with the natural inductance and capacitance of the windings for the 6V3 portion of the transformer?

[JZRepair]

Have you look into tube datasheets ? Lots of info on it !!

https://frank.pocnet.net/index.html

6V6 on this page
https://frank.pocnet.net/sheets66.html

Look at the current need for the tubes heaters.

Yes, looked at them very briefly, but never paid any attention to the current on the heaters.  450mA isn't too bad.  So about 1A for the outputs.  I'm going to bookmark these pages.  I usually just Google them every time, but this is much handier!  Thank you.

[JZRepair]

That's good. Now you can put the limiter under the bench because it's done it's job. Time to plug the amp directly into the wall. Set your voltmeter to measure DCV and connect red probe to pin 2 of the 5Y3. Black probe connects to chassis. Turn the power switch on but keep your hand on the switch and be ready to quickly turn it off if things go badly. It will probably take a few seconds for the 5Y3 to warm up and put out some B+.

Yep, all good.  I read about 340V, so it's a bit low, but my outlet is only putting out 115, so I'm not surprised.  I need to figure out what's going on with this outlet, because the other one is on a different circuit and I get 123V.  Might be the surge protector I have on it, but not sure.  I feel comfortable with the 115V on the mains for right now, though I do want to plug it into the other outlet to verify it's still going to be ok, and for my own knowledge of seeing what I would normally get.

My bias current is only about 10mA with no signal, and that seems low in my head, but I haven't done the biasing yet, so we shall see.

**EDIT**
Ok, plugged it into the outlet with the 123V.  Tubes are all glowing, no red plating.  Output plate current is about .03A.  B+ is ~365V.  Everything appears to be running smoothly.  Haven't prodded around with the probe, since I had it clipped on to pin 2 and always makes me nervous disconnecting it on a live circuit, because I don't want to be jittery and short something.  Will check around different points to verify all the other voltages are good.

[JZRepair]

Look at the current need for the tubes heaters.

One thing that I find confusing with the datasheet is on page 2.

https://frank.pocnet.net/sheets/093/6/6V6GT.pdf

There's 3 columns, but nothing is labeled.  Also, I've been told that the 6V6 are usually pushed with regards to the plate voltage of 400+V.  Yet, the datasheet shows the three values of 180, 250, and 315.  Looking at page 1, I can see that a class A amp is max rated for 315, but is this a class A?  I've always been confused with the classes, even when in school.  Oddly enough, when I studied electrical engineering, I had no desire to work on audio and because of that, I didn't pay as much attention as I should have.  It wasn't until I worked with hearing loops that I got interested in audio, but those are constant current and most of what I worked on was class D, but it still never made sense.

**EDIT**
This is simply a guess, but on page 2, is the left column minimum ratings, middle is max, and right is ideal/nominal?

[sluckey]

So in essence, the resistors are creating a filter with the natural inductance and capacitance of the windings for the 6V3 portion of the transformer?

Everything you need to know about heaters...

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

[PRR]

> minimum ratings, middle is max, and right is ideal/nominal?

There's no "minimum". Except in the commercial sense: you don't buy a 2-ton truck to haul a six of beer.

Irving's Intercoms wants a box to speak. Power but not a lot of it. 2 Watts? There are smaller tubes, but frequently the 6V6 has been the most available and often within pennies of smaller tubes. Hey, here's an old ad! 6V6GT is in the 45c bin, along with 6F6 6K6 (similars), 6AU6 (maybe 1/3W), 35/50*5 (110V radio 1W outputs). Yes, TAB was a bottom-barrel supplier, 6V6 "should" cost $0.95-$1.10 in those days. But Irving doesn't expect a lot of tube-abuse. Or a lot of sales. And if he has no sales at all, he can always turn and dump 6V6 at not too much of a loss.

The 250V condition is classic in good parlor radios. 4+ Watts is fine. All the numbers are there.

Fender wants the MOST he can get for his buck. The 5.5 Watt numbers are there, and if that eats tubes GE/RCA won't blame Leo too much, will replace his early failures.

Any ONE tube audio stage MUST run class A, tube conducts for the ENTIRE audio cycle. If it ever cuts off, that's gross distortion.

[PRR]

Irving's Intercoms wants a box to speak. Power but not a lot of it. .... Fender wants the MOST he can get for his buck....

Here's another market survey. Olson sold many amplifiers, here are two.

That Olson LoudMouth sure looks like a Fender Champ!! 8" PM speaker. Oh, it has Tone, the first Champs didn't. 6V6GT power. Claims 6 watts, which is probably the 5.5W condition on the datasheet, rounded-up.

The intercom head for $9.99.... they can't be using one-buck 6V6 in that. In fact by the tubes they did use we know this is a transformerless "hot chassis" amplifier; don't put your lips on it! (Note "plastic cabinet".) However at a higher class of product (Bogen?) a designer might pick 6V6 (a solid choice) and run it at the 2 watt conditions for long life and good filtering.