power supply sag, selecting transformer

[Jerryz1963]

This has seemed rather hit or miss to me, even when dealing with the more simple audiophile amps. Selecting the proper power transformer. To me, it seems this can be complex for a couple of reasons: the voltage drop that occurs as more power is drawn from the power supply and how to predict what voltage one will end up with. Are there any rules of thumb or down and dirty cheats to get you in the ballpark?

When you have a dual-channel amp, clean and dirty, I'm guessing that sag isn't necessarily a desirable thing for the clean channel. Would it be optimal to use two power transformers?

My first client/customer said he isn't that wild about power supply sag, but the amp I made for him sounded better with a 5Y3 than with the 5U4. So, I left the 5Y3 in. for the prototype, I used an old Conn organ chassis and transformers and basically completely redid the circuitry, but did use the stock Conn power transformer, which is no doubt overkill. Wouldn't it be a good idea to get a smaller power transformer so that one would get more voltage drop (sag) when you turn it up? If this is already addressed somewhere, please show me the thread.
So, a transformer puts out say 360 volts. You rectify it and the voltage goes up. You start to draw off current, the voltage drops. A transformer rated for 200 ma, that would have less voltage drop as you turn it up than a XFMR rated at 100 ma, right? How do you predict your end results? How do you buy the right transformer without a bunch of trial and error?

[sound mind]

I think a lot of builders don't take the transformer current rating into account other than to make sure it is as high or higher than is needed.
I think the answer to get the sag of a smaller transformer without having to use one is to measure the resistances from CT to each end of primary on the smaller and larger transformers and make up the difference with concrete series resistors from both sides to the rectifier. You should then get the sag of the smaller transformer while using the larger one.
It also helps to current limit the surge to the rectifier at switch on.
The downside is finding the right spot for the resistors as they will get hot.
Charlie

[Jerryz1963]

I think a lot of builders don't take the transformer current rating into account other than to make sure it is as high or higher than is needed.
I think the answer to get the sag of a smaller transformer without having to use one is to measure the resistances from CT to each end of primary on the smaller and larger transformers and make up the difference with concrete series resistors from both sides to the rectifier. You should then get the sag of the smaller transformer while using the larger one.
It also helps to current limit the surge to the rectifier at switch on.
The downside is finding the right spot for the resistors as they will get hot.
Charlie

That's a great idea. Thanks. I have some big ceramic resistors that should be up to the task.

[ToneMerc]

My first client/customer said he isn't that wild about power supply sag, but the amp I made for him sounded better with a 5Y3 than with the 5U4. So, I left the 5Y3 in. for the prototype, I used an old Conn organ chassis and transformers and basically completely redid the circuitry, but did use the stock Conn power transformer, which is no doubt overkill.

My personal opinion is that often times folks confuse what's called "power supply sag" with simply lower rail voltages. For example; for that amp you built, did you document all the voltages while recording them with each rectifier type? Ask yourself this; why did the client prefer the 5Y3, maybe it was the fact that the 5Y3 resulted in less internal voltages?

Folks have different methods, but I rarely use an off the shelf PT. I like being able to choose my own ratings.

TM

[ToneMerc]

I think a lot of builders don't take the transformer current rating into account other than to make sure it is as high or higher than is needed.

Same goes for manufacturers; Tweed Bassman transformers are a great example, as most are flat wrong, typically around 30-50% too high.

TM

[sound mind]

I think a lot of builders don't take the transformer current rating into account other than to make sure it is as high or higher than is needed.

Same goes for manufacturers; Tweed Bassman transformers are a great example, as most are flat wrong, typically around 30-50% too high.

TM

Interesting to hear this as I had already suspected that even some big manufacturers may not have considered the effect on the sound. I hadn't gone as far as checking for myself so glad to hear from someone who has.
Charlie

[Jerryz1963]

My first client/customer said he isn't that wild about power supply sag, but the amp I made for him sounded better with a 5Y3 than with the 5U4. So, I left the 5Y3 in. for the prototype, I used an old Conn organ chassis and transformers and basically completely redid the circuitry, but did use the stock Conn power transformer, which is no doubt overkill.

My personal opinion is that often times folks confuse what's called "power supply sag" with simply lower rail voltages. For example; for that amp you built, did you document all the voltages while recording them with each rectifier type? Ask yourself this; why did the client prefer the 5Y3, maybe it was the fact that the 5Y3 resulted in less internal voltages?

Folks have different methods, but I rarely use an off the shelf PT. I like being able to choose my own ratings.

TM

My understanding of power supply sag is the voltage drop associated with the amp (power section) consuming more current thus creating a voltage drop across the power supply elements. If my understanding is correct, it kinda seems one wouldn't want to use a lot of capacitance in the power supply. I preferred the 5Y3, client lives across the country and hasn't heard it yet it's hard to stop thinking audiophile and start thinking guitar, but I'm getting there. I popped in the 5Y3 because it has a higher voltage drop than the 5U4, so I figured it would make for more "sag." Maybe I don't understand sag at all?

[pops]

It's hard to think of a rectifier tube being important for the tonal character of the amp, but i find that i like the 5Y3 also as it drops more voltage, lowering the voltage makes for a warmer sound in most amps i have tried it in. More sag or looser sound, but warmer. A 12AY7 has the same effect, losing not as much power as you may think, allowing you to turn up higher and have a warmer sound. Turning to 3/4 volume is most likely the best place to be for tone, and what amp you bring to be able to do that is why no one amp can do it all. Sooooo we must build an amp for every circumstance we may encounter. Now is that a ramble, but a good excuse for lots of amps.

[thejaf]

I've always thought of sag as the power supply not keeping up with demand, principally the lower uF values of filter capacitors in most tube rectified power supplies. Results in squishy response when demand is too high, ala cranked JTM45 or Bassman.

As TM stated, I'd wager it was the lower B+ in the preamp that made the 5Y3 "sound" better.

I could be wrong, but I was always under the impression that secondary voltage ratings on PT was expected at or near the current rating (e.g., 350-0-350 at 100mA). Thus explaining the higher than rated voltage when the amp is unloaded (no tubes).

[vibratoking]

I don't find tube rectifiers necessary or desirable from a reliability POV. Where are you Miles?

I don't think of sag as the power supply not being able to "keep up". Yes, there is a voltage drop in the diode that contributes. The PS caps and "dropping" resistors also create a time constant in the power supply which impacts the timing of PS changes due to dynamic demands. Sag is impacted in this way. There is also an effect on the ripple present in the basic supply. The ripple couples into all the stages of the amp and may be audible as hum if large enough. So, a lower time constant results in more voltage variation, sag and ripple, and vice versa.

[ToneMerc]

Timing is everything, 60-65 years ago electrified instruments were still a novelty at best. Early amp pioneers only had access the smaller transformers designed for low watt audio and the smaller value capacitors. Musicians in a quest to be louder, cranked those early guitar amps up and the magic happened.

As music evolved with greater headroom and wattage demands, so did the power supplies.


TM

[Tone Lover]

Lets not forget what you guys hit upon earlier but didnt put it into a concrete idea.
The higher or lower B+ into the preamp needs to be considered as a tonal variation to tweek as much as the other parts in the amp to get the sound you are looking for.
You just stated that it was probably the lower B+ or the browner sound that was what made the 5Y3 rectifier sound best to your ears .
I have taken it one step further and will use 25k 2watt pot set as a variable resister. Both before the PI and after .
Because varying the voltage to PI and the preamp makes a huge differance in the sound of the amp. There is a sweet spot in both and once you find them you never go back.
Of course were not taking into consideration the stiffness or time constraints of the filtering because that opens up a whole differant discussion.
Bill

[talbany]

The resistance of the high-voltage secondary winding also creates sag. From Ohm's Law, the voltage drop across a resistance is equal to the resistance multiplied by the current flowing through it. This means that there is no voltage drop if there is no current, and the amount of voltage drop goes up linearly with increases in current draw. A typical power transformer B+ winding might have a resistance of 50 ohms - 300 ohms, depending upon the current rating and REGULATION of the power transformer. For example, if the current draw in a push-pull class AB output stage at idle is 70mA total, and it increases to 170mA at full power, there is a change of 100mA in the current drawn through the secondary windings. If the winding resistance of the secondary is 200 ohms, there is a voltage drop of 100mA*200 ohms = 20V in the plate voltage to the output tubes. Likewise, the resistance of the primary winding of an output transformer varies as well, typically 80 ohms - 200 ohms plate-to-plate, depending upon the primary inductance, the transformer power rating, and the rated impedance. This resistance also creates a voltage drop, but the amount of sag introduced is minimal in pentode mode, because the plate voltage doesn't have near as much effect on the plate current as does the screen voltage. In triode mode, there is more sag because the plate voltage has more of an effect on plate current in a triode. The supply sag created by the power transformer resistance lowers not only the plate voltage, but the screen voltage as well, since the screen is nearly always a filtered version of the supply going to the plate. The amount of sag induced by the power transformer winding can be offset if there is a large filter capacitor reservoir to hold the voltage constant during current peaks.
Rectifier sag is generated because of the internal resistance of the tube. The higher the internal resistance the larger the drop the more induced sag!!..We sometimes simulate this with a large 5-10w power resistor..
5Y3's can induce up to 60V drop...BUT is fairly limited in it's Max current rating and therefore pushing it in a 40watt amp!!..I would recommend something like a 5V4 instead..
Induced sag as well as plate/Screen voltages are IMO purely a matter of taste and subject to ones individual playing style and how they want the amp to feel and respond (at all volume levels)..Although most tube rectifiers can be substituted you should always check the rectifiers current rating and filtering specs to be on the safe side!!

Tony

[Jerryz1963]

I don't have any regulation anywhere, so that could very well be why the 5Y3 sounded better. I have a pretty high plate voltage on it. I know many are of the opinion that you should give it all the voltage it can take, even people with a lot of experience and some reputation in the community. I discovered quite by accident that lower voltage can sound very sweet indeed. I don't recall exactly what error I made, I think I only had one leg of the power supply functioning, but I had very low voltages on an amp I was building, 6n2p dual triodes and 6P3S power tubes, it had a triode sweetness to it with the severely curtailed B+. It sounded really good. I swear i have learned as much by experimenting as I have from reading and occasionally by accidents like this one.
I had intended to regulate the preamp and use the traditional methodology for the power tubes. I was biasing some triodes warm and some cold, kind of like that Marshall with the 10K cathode resistor and the 800 ohm cathode resistor in successive stages, and I'm using preamp tubes with big plates because I had read they give a "Fatter" tone and that they sound more like power tubes when driven into distortion. So, I'm experimenting with fat plate preamp tubes only and it sounds really cool. I'm not a guitar amp expert,so I forget which Marshall that is. I suspect it's what inspired Fisher (Fischer?) when he designed the Trainwreck.

[talbany]

I don't have any regulation anywhere

Yes you do!
If you have a power transformer you have regulation happening

I explained it in my previous post

Here it is again in simpler terms..

http://ecmweb.com/basics/basics-transfo ... regulation

Your Question?

the voltage drop that occurs as more power is drawn from the power supply and how to predict what voltage one will end up with. Are there any rules of thumb or down and dirty cheats to get you in the ballpark?

Answer?
A typical power transformer B+ winding might have a resistance of 50 ohms - 300 ohms, depending upon the current rating and REGULATION of the power transformer. For example, if the current draw in a push-pull class AB output stage at idle is 70mA total, and it increases to 170mA at full power, there is a change of 100mA in the current drawn through the secondary windings. If the winding resistance of the secondary is 200 ohms, there is a voltage drop of 100mA*200 ohms = 20V in the plate voltage to the output tubes. Likewise, the resistance of the primary winding of an output transformer varies as well, typically 80 ohms - 200 ohms plate-to-plate, depending upon the primary inductance, the transformer power rating, and the rated impedance. This resistance also creates a voltage drop,


Tony