Thank you so much for this video! I have a 1938 tube radio and a 1940 radio/phonograph and I was concerned about operating them on our average voltage of 122. Problem solved - I will order the transformer after I submit this comment. Update 03/02/2021: I received the transformer and have the circuit working this morning - 107.6 volts out. My old gear thanks you too.
Excellent! Your description of the internal bucking trans is exactly what I needed to hear. I have a huge hunk of iron from an old Lowry organ that had about a million tubes in it. It has a secondary winding that once powered a bank of tubes and is no longer needed, so I'm hoping to drop my PT output by about 13vac by wiring that in series. Thank you!
Great! That was excellent. I'm just in the process of making one for my restored Fisher 500c, I use the Antek transformers so I have several around. I use thermistors in my restores. The 47D-15 will drop about a volt when hot and my line is ~121volts, using a 6V Antek I can drop it down to ~114V out of the thermistor. Fisher says 105-117 volts for the 500c so I'm pretty good with a 5 or 6 volt xformer and I still have a little headroom if the line goes up a volt or two.
I built one of these a few months ago for my 65 Fender Deluxe Reverb amp using the Robinette design. Works great. Now I know how and why it works - thanks! And yes, the cost of everything has gone up. But still a third of the price of the Brown Box and was fun to design and build.
Another great video Mark. Have built one of these based on the website you mentioned. They really are easy to do and a heck of a lot cheaper than the Brown Box. Thank you again!
Excellent video Mark. I’ve been converting a magi 6302 and was concerned about the high filament voltage and B+. This seems like a simple solution. Thanks for all your videos.
One of the most important topics in the transformer field. Thank you for the clarity and multiple levels of detail. I am a linear PSU expert and it would have taken me a full day to do what you did in minutes. There is a RU-vid channel run by a major audio gear manufacturer that flat out misinforms viewers on this topic as part of a sales campaign targeting audiophools. They would save many thousands of dollars by watching this video.
Note: By using a bucking scheme as described, You lose the protection and isolation provided by the transformer. Could be dangerous in some applications.
Thank you for making these videos, real solid info here. I'm a beginner amp builder and wondering what is the reason that we don't use resistors on our primary or immediately after the secondaries? (Line voltage = 123v)
Nicely done Mark, thanks. I have kind of the reverse situation, that is, where I live I get about 103V AC and I want to raise that to something closer to 115V AC. Can this DIY bucking transformer do that if I were to wire it in the same bucking configuration but put the source voltage in the secondary and take the output off the primary? Also, assuming I use the same 4 amp transformer shown in Rob Robinette's design, could I expect the same output current in my proposed step-up configuration?
Looks like a bucking auto transformer works great for a small voltage reduction, 120vac to 110vac. Wondering how it would work stepping 120vac down to 12vac, i simulated a bucking transformer in a circuit simulator stepping 120 down to 12vac with a 0.9 ratio and there are a lot of amps on a long secondary. On the other hand a step down nonbucking auto transformer with a .1 ratio has a much shorter primary with high amps.
If one doesn't have a 10V winding but only a 6.3V could this be used to mitigate at least some of the excess voltage whilst still providing the heater supply?
Will that work if it is the heater tap windings, just tie one leg of the heater winding to the primary and take the other side as the outlet for the buck voltage?
I like the bucking transformer solution, bot if it's just the filament voltage that is the issue and a slightly higher B+ is acceptable, how about using a power resistor in series with the heaters. For example, to drop from about 7V to about 6V @ 5A wastes about 5W, which can be achieved by bolting an aluminium clad resistor to the amp chassis. The thing is with using an existing amp transformer with the primary connected to the a secondary winding, don't we lose electrical isolation from the mains supply side or did I misunderstand?
Can anyone think of any scenarios where bucking the common lead instead of the hot lead would be a bad idea? In other words, when you measure hot to ground, it would show full mains voltage, but if you measured from hot to common, you'd see decreased voltage. I have a reason why I do this, but I'm curious if any of you brilliant minds could think of a reason why it is not good.
Can you please explain how putting DC voltage on a transformer winding will eat the transformer up? I only ask, because most tube amplifiers have hundreds of volts DC on the primary winding of the output transformer.
I was saying if you fed the primary with only DC, its basically a short circuit minus the resistance of the windings. Will quickly heat up (remember resistors drop voltage via converting it to heat) and burn up the windings. That's different than having DC riding on top of a winding that doesn't connect directly to ground.
Yes, absolutely. You don't want direct current flowing through the winding, but a non-alternating voltage is absolutely fine as long as it doesn't close the loop on itself by shorting elements inside the tube, etc. I'm with ya. Love your videos.
Okay, how many of us grabbed a meter to check the mains voltage at an outlet at the start of this video? I admit nothing, but as I type this mine is 122.5 VAC.
That first buck transformer setup drawing must be 10 years old, that Hammond transformer is now $40+ if you include shipping. You can make an ok setup for around $60-$140 depending on case, transformer and outlets/plug etc but it will not look very nice with your high end tube amps..That nice gold accent with the meter and voltage adjustment seems very reasonable. If you spend $1000 on a monoblock amp and $80-100 per tube that $256 ebay buck transformer seems cheap. Not to be too pedantic but you misspoke several times explaining the volt amperage values/ratings on the transformer examples and some other terms. I know it is hard to catch all the verbal errors, but at least the diagrams and illustrations were correct.
