Buck/boost? Yes or No?

[ronh]

I am looking to get some opinions about wiring a PT secondary for buck/boost. I am interested in the buck feature. Take an unused secondary winding and wire it in series
with the high voltage secondary to get a voltage reduction or a boost. This would be determined by the polarity of the smaller secondary. Lets say use a 5 Vac secondary.
It should reduce the voltage by 5 or boost by 5. What about if a 12 Vac secondary was used? What would be the stresses on the PT? With a full-wave rectifier the DC voltage should/would change by Vac X 1.414. Yes/No? Can it be made switchable? Forget this idea? Thanks.

[eschertron]

Without serious analysis, I'd assert that most heater windings are capable of much more current than the HV secondaries, and so would not add stress to the HV circuit.
But the idea begs a couple questions:
Do you need the heater winding for heaters?
Would a 5v zener dropper be easier and more flexible?
You mean of course a full-wave bridge rectifier, since the center tap would be off center?

[nickb]

What's your objective here? If it's power output control then you'll need much more than +/-5V due to the ear's logarithmic response.

The attraction of using the transformer is that is very efficient when compared to a zener or other voltage dropping device.

[ronh]

Thanks for the replies. The objective is to lower the B+ by a fixed amount. The PT has an unused 12 V winding. What I am thinking here is using a switch, one setting is normal and the other
switch setting is in buck mode. Lower B+. I am not sure how the center tap of the high voltage winding comes into play here. I need more reading. Another problem would be you cant switch
over "live". The first filter cap would have a higher voltage on it. Powering up in one mode only would work. The heater supply should be unaffected. I think the effect would be like "brown sound". Or it could be crap, voltage too low for operating point.

[R.G.]

This will need some thought on your part.

Transformer buck/boost is best done on the primary side of the PT. In this position, it raises and lowers all secondary voltages by the same amount, including especially the heater voltages. It's generally important that the heaters operate on 6.3V +/-10% for best tube life, so buck/boost setups are good for correcting older transformers made for lower AC lines (like the '50s and '60s 110VAC) to work with the modern voltages, which run to about 125.

If you only want to affect the B+, not the heaters, a buck arrangement will need two secondaries, one to remove the required AC voltage from each side of the B+ winding. You really can't do a buck/boost well on the CT. And you'll need more than 12V or so. You're probably going to need 24 to 75V difference on the B+ AC voltage to make a usable difference in the B+.

It is easy to add a B+ lowering device in the CT, using an "active zener"; people report this all the time on this forum. You can use a real zener as well as an active one. You can also add "sag" with a resistor in series with the rectifiers before the first filter cap. These are easy and cheap, and suffer only from generating a lot of heat as they burn off the excess voltage.

[nickb]

You would need to use a full-wave bridge on the extra winding to make it work. That bridge will lose a couple of volts across the diodes. 12V isn't going to make much of an audible difference.



Personally, I associate a 'brown' sound with cutting the highs together a very slightly overdriven and compressed sound. None of those attributes have anything directly to do with the supply voltage, except perhaps some sag on the supply. I suggest all you need is some series resistance in the supply to produce the sag.

[Malcolm Irving]

I seem to remember that 'brown sound' was a term coined by Eddie Van Halen, who used to run amps on a Variac, reducing all voltages in the amp (B+, Bias, heaters, even indicator lamps!) by a large degree. Was it 60V instead of 110V? The whole thing could be a myth of course. Not good for the life of the tubes running on such a low heater voltage!

I have sometimes played for a short while through an amp running on a light-bulb limiter, which must be something similar plus extra compression when the bulb lights up for the loud bits. It certainly gives a different sound and not unpleasant in a way. It also drastically reduces the power output, of course.

[eschertron]

I seem to remember that 'brown sound' was a term coined by Eddie Van Halen, who used to run amps on a Variac, reducing all voltages in the amp (B+, Bias, heaters, even indicator lamps!) by a large degree. Was it 60V instead of 110V? The whole thing could be a myth of course. Not good for the life of the tubes running on such a low heater voltage!

I have sometimes played for a short while through an amp running on a light-bulb limiter, which must be something similar plus extra compression when the bulb lights up for the loud bits. It certainly gives a different sound and not unpleasant in a way. It also drastically reduces the power output, of course.

Holy Cow! A dynamically-variable brownout device!
But seriously, this makes me wonder if anyone's tried and/or discussed putting power resistors on the primary side of the PT, in series with the windings. A cheap and inefficient version of a bucking transformer, with the added 'sag' feature? It should behave in a manner similar to the light bulb limiter...

[trobbins]

You could use a doubler or double-doubler to generate a significant buck or boost isolated DC voltage to insert between say the first filter cap negative and ground.

[Mike Sulzer]

Holy Cow! A dynamically-variable brownout device!
But seriously, this makes me wonder if anyone's tried and/or discussed putting power resistors on the primary side of the PT, in series with the windings. A cheap and inefficient version of a bucking transformer, with the added 'sag' feature? It should behave in a manner similar to the light bulb limiter...

Light bulbs are limiters because the resistance rises a lot when they heat up. Resistors do not change that much.

[eschertron]

Light bulbs are limiters because the resistance rises a lot when they heat up. Resistors do not change that much.

Mike, I do understand that. That's why light bulbs are good at preventing fuses from blowing (and more expensive parts, too). My point was simply the increased voltage drop across the resistor as current demand increases - when the resistor is on the primary side - will affect all the secondary voltages they way a light bulb limiter does. Nothing more critical than that.

[Zozobra]

How many volts and at what kind of current draw are you trying to slip off the B+?

[tedmich]

I seem to remember that 'brown sound' was a term coined by Eddie Van Halen, who used to run amps on a Variac, reducing all voltages in the amp (B+, Bias, heaters, even indicator lamps!) by a large degree. Was it 60V instead of 110V? The whole thing could be a myth of course. Not good for the life of the tubes running on such a low heater voltage!.

WHY NOT EDWARD VAN HALEN'S MAGIC MARSHALL GUITAR AMPLIFIER OHMITE VT8-F VARIAC | eBay

[eschertron]

WHY NOT EDWARD VAN HALEN'S MAGIC MARSHALL GUITAR AMPLIFIER OHMITE VT8-F VARIAC | eBay

Doesn't look like Eddie's fingers in the picture, there, adjusting the dial

[J M Fahey]

I seem to remember that 'brown sound' was a term coined by Eddie Van Halen, who used to run amps on a Variac, reducing all voltages in the amp (B+, Bias, heaters, even indicator lamps!) by a large degree. Was it 60V instead of 110V? The whole thing could be a myth of course. Not good for the life of the tubes running on such a low heater voltage!

Eddie himself has debunked that, and asked to be forgiven for giving such useless and dangerous advice.
He took his time of course, that crap has been rolling all over the place for almost 30 years.

He claims "he had to answer *something* so he made that up"

In fact it was the other way round, I read that Guitar Player interview way back then and remember he said something like "I have to play fast and non stop because if I donīt, tubes start getting red holt and melt" which in my opinion applies to way too high applied voltage, and relying on supply drop at high volume to tame that a little bit.

I also remember reading (first hand in the paper magazine, not 1000th copy of copy in Internet) that "you pull one of those big caps in the input (I assume mains input) which looks like a stick of dynamite (that describes a waxed paper cover axial cap, not the blue plastic covered clamp mounted ones used by Marshall so that alone makes the claim iffy at best) and connect it "sideways" , it sucks up all the juice and all kinds of wonderful things start happening"

IF he partially shorted some winding to lower voltage (no mention of any Variac in that interview) , any load heavy enough to do that would also blow the mains fuse.
And in any case waxed brown paper large electrolytics do not belong inside a Marshall, maybe he once saw some Fender guts and thought "all caps are the same".
Not that many Guitar players would notice the discrepancy
FWIW I wasted a long time studying Marshall schematics and looking inside chassis until I was fully convinced he was not truthful (to put it mildly )

I think he was either making fun of gearheads or was simply drunk, of course once that appeared black on white on the most read Guitar magazine he couldnīt back down.

But a couple years ago he said "things were not that way" and apologized.