Simple PT rating question

[catalin gramada]

Based on the things I've learned on this thread I am now wondering why this Marshall 50W OT that has 690V CT @ 150mA is only 150mA if it's for two power tubes. Seems like this PT would not do good when you play the amp primarily cranked. But isn't that what people do with a Marshall JMP, JCM800 etc?

http://www.classictone.net/NOS16575.pdf

It has about 490V B+ so it does 55W at clipping. I use a 50W OT with it. compared to other same amps with different transformer sets (4 power tube versions), it seems like it has a poorer range on the volume knob. Meaning that once I get to about 4 it's full volume. Increasing the volume more does almost nothing. But also it doesnt' sound worse as you exceed max volume and keep turning it up like you would expect. You know how when you go past probably 6 or 7 in volume on an amp, it usually doesn't sound any louder, and usually just sounds a little worse? It makes me guess that in that case you are exceeding the OT power handling, or PS is maxed, or NFB loop is now ineffective because power amp is distorting and it just sounds worse and not louder.

How possible does it sound that this amp may be current limited by PT? This is just something I thought of as I was perusing PT spec sheets and thinking about my amp. So maybe I'm way off but.....????

edit: Or would the PT just supply the current demanded if the power supply can supply it, but get hotter ??

I just measured my last PT in hundred watts build. It draws from transformer 540 mA at 460v AC with two kt88. little bit over 100w (110-115w continuous clean sine before clipping) that means 250w arround for HT -say ca.300w including heaters. So can consider three times PT power rating for a AB class to be in safe area, meant 80* Celsius degree (176 Fahrenheit)- my suplier standard. This is hot, but regards the wire isolation not the overal transformer temperature which will remain acceptable even for max continuous operation. I asked to be built for 600mA-better a little more than little less, so have a 10% reserve
An undersized transformer is not necessary wrong when talk about MI as power supply is a big part of a tube amp tone and consider is more important to think right in this order: tone first, then durability. The problem is: can be designed an amp arround a undersized power supply to get a tone ?

[pdf64]

Based on the things I've learned on this thread I am now wondering why this Marshall 50W OT that has 690V CT @ 150mA is only 150mA if it's for two power tubes. Seems like this PT would not do good when you play the amp primarily cranked. But isn't that what people do with a Marshall JMP, JCM800 etc?
http://www.classictone.net/NOS16575.pdf
...Or would the PT just supply the current demanded if the power supply can supply it, but get hotter ??

I think that as the power tubes draw more current, the HT winding voltage will drop (ie below the noted 490V @ 150mA).
Hammond suggest that their Marshall PT has a max HT winding current of 213mA http://www.hammondmfg.com/pdf/EDB290GX.pdf
It's a self limiting thing; as the power tubes draw more current, the winding voltage and dc HT voltage are pulled down, which naturally reduces the current that the power tubes draw.
Hence even the solid state rectified 50W Marshalls tend to have a lot of overdrive compression; of course the EL34 also contribute to that character.

[catalin gramada]

Of course works like that. First, supliers operate with different standards and declare tranny performances from different points. Second the trannies are builded by different standards and that.s reflect direct in performances. Found old tech used less iron and more winding turns per volt reflecting in tranny internal resistance, adding cooper mass which absorb easy overloaded shocks. Eg: testing some "flashing" tubes with your hammond 278 and a "modern" one. same declared heaters winding 6.3V /6A. For the same tube the heaters flashes 3 times longer with Hammond than modern one which flashed for a fraction. I asked directly at Hammond factory they told me the heater winding is usable even for 10 Amps



I don.t know if software simulators reflect all of this consequences in signal reponse but what I know is a "modern" replacement will sound different in this kind of amps even with the same declared performances. If want to get the same reponse it need a "replica" replacement otherwise will sound worse or even better but never the same

[nickb]

Of course works like that. First, supliers operate with different standards and declare tranny performances from different points. Second the trannies are builded by different standards and that.s reflect direct in performances. Found old tech used less iron and more winding turns per volt reflecting in tranny internal resistance, adding cooper mass which absorb easy overloaded shocks. Eg: testing some "flashing" tubes with your hammond 278 and a "modern" one. same declared heaters winding 6.3V /6A. For the same tube the heaters flashes 3 times longer with Hammond than modern one which flashed for a fraction. I asked directly at Hammond factory they told me the heater winding is usable even for 10 Amps

[ATTACH=CONFIG]44949[/ATTACH]

I don.t know if software simulators reflect all of this consequences in signal reponse but what I know is a "modern" replacement will sound different in this kind of amps even with the same declared performances. If want to get the same reponse it need a "replica" replacement otherwise will sound worse or even better but never the same

I guess the approach if you want to match performance is to use a lower resistance version and then pad it with external resistance to meet your requirements.

Also, for anyone else reading this thread, never, never, never, exceed the specified transformer's ratings. You risk at the very least a dead transformer that will not be covered by the warranty, and maybe worse.

[catalin gramada]

[QUOTE=nickb;465597]I guess the approach if you want to match performance is to use a lower resistance version and then pad it with external resistance to meet your requirements.

/QUOTE]

Agree from a point but is really identical? just asking. A low resistance tranny tend to keep the current more constant till a limit than drop more faster than a stone in a lake. On the other side we have a tranny which drop more but in a more progressive way. That.s my thought . a low resistance winding wil drop more severe after a limit if try to exceed no mater if have a series resistor or not. but I think it will simulate well the same condition into working limits. I.m pretty interested by subject too so happy to learn

late:I have not a Marshall 50w suplied at 490v today but someone which have one can do easy a quick checkin to find if draw from HT tranny winding more than 150mA when dimmed as is declared

[nickb]

Agree from a point but is really identical? just asking. A low resistance tranny tend to keep the current more constant till a limit than drop more faster than a stone in a lake. On the other side we have a tranny which drop more but in a more progressive way. That.s my thought . a low resistance winding wil drop more severe after a limit if try to exceed no mater if have a series resistor or not. but I think it will simulate well the same condition into working limits. I.m pretty interested by subject too so happy to learn

There is no sudden voltage drop like you describe. It doesn't matter that the resistance is distributed throughout the primary and secondary windings or as a lumped external one. The result is the same. V= IR, it's the law

[nsubulysses]

I think that as the power tubes draw more current, the HT winding voltage will drop (ie below the noted 490V @ 150mA).
Hammond suggest that their Marshall PT has a max HT winding current of 213mA http://www.hammondmfg.com/pdf/EDB290GX.pdf
It's a self limiting thing; as the power tubes draw more current, the winding voltage and dc HT voltage are pulled down, which naturally reduces the current that the power tubes draw.
Hence even the solid state rectified 50W Marshalls tend to have a lot of overdrive compression; of course the EL34 also contribute to that character.

Am I reading into this wrong to think that based on what you're saying, the Hammond PT rated at 213mA vs the equivalent Magnetic Components same PT but HT winding rated at 150mA, would have less compression, and less sagging B+ at high volume because the transformer is same B+ but more robust for current handling? so its a transformer that has better voltage regulation in comparison?

I guess the real thing I'm wondering is which transformer to try to swap on this 2 6CA7 amp with 490V B+ that does 55W at clipping to try to make it perform as good as the 4 power tube versions with different transformers that seem to have a better usable range on the volume knob, and also seem to actually get more good sounding at 4-7 on master volume where as this 50W amp has not much change on volume 4-7. It goes up to 4 and sounds good and then as you keep going not much else happens.

Is it my PT with 150mA HT winding that cant keep up when I crank the amp up loud (beyond 4-5 on volume)

or my 50W OT? If it does 55W at clipping it must do 70-80W when amp is cranked and power amp is starting to distort too, and output resembles more of a square wave.

I understand Im asking you to speculate, but I really think one of these transformers needs a swap for better performance!

[catalin gramada]

There is no sudden voltage drop like you describe. It doesn't matter that the resistance is distributed throughout the primary and secondary windings or as a lumped external one. The result is the same. V= IR, it's the law

I just can compare two trannies with similar specs and found it is: was a Hammond378cx 450v/ 540mA and a custom low resistance 460v/600mA pretty close but huge diferences. Trying to supply 4kt88 the big ones drops a lot more than hammond did. So what is the clue ?

late edit: maybe I.m rushed with my conclusion as Hammond have an iron dimensioned for 465VA and custom tranny just 320VA ? so there is a big difference in between...

[nickb]

I just can compare two trannies with similar specs and found it is: was a Hammond378cx 450v/ 540mA and a custom low resistance 460v/600mA pretty close but huge diferences. Trying to supply 4kt88 the big ones drops a lot more than hammond did. So what is the clue ?

I think we'd need to see the data sheets for both to give an answer. Can you post them?

[catalin gramada]

378cx.pdf

the other one is in post no.21

I have not measured results as I found both unusable to can supply the circuit but to drive 100w kt88 pair are allright

As I mentioned in previous post the irons are dimensioned for 465VA for Hammond and 320VA for custom so are not quite similar as I mentioned above

[nickb]

[ATTACH]44952[/ATTACH]

the other one is in post no.21 [ATTACH=CONFIG]44953[/ATTACH]

I have not measured results as I found both unusable to can supply the circuit but to drive 100w kt88 pair are allright

As I mentioned in previous post the irons are dimensioned for 465VA for Hammond and 320VA for custom so are not quite similar as I mentioned above

In that case go grab your meter and measure the primary and secondary resistances for the toroid.

[catalin gramada]

In that case go grab your meter and measure the primary and secondary resistances for the toroid.

I.ll do it next day, is not in my basement. I can also ask my suplier to provide technical details. Thanks for help

[catalin gramada]

How much a smaller iron (or cheap one) can do a difference when exceed the VA parameters, please ? In other words can be simply predicted the limit when transformer goes into unlinear region judging by core size?

[Merlinb]

In other words can be simply predicted the limit when transformer goes into unlinear region judging by core size?

No, the core size has nothing to do with 'unlinear'. A small VA transformer will have more resistance and therefore more 'droop' under load than a high VA transformer, but it is a linear effect. The transformer will go 'unlinear' (I presume you mean magnetic saturation) if you apply a primary voltage that is too high.

[Mike Sulzer]

No, the core size has nothing to do with 'unlinear'. A small VA transformer will have more resistance and therefore more 'droop' under load than a high VA transformer, but it is a linear effect. The transformer will go 'unlinear' (I presume you mean magnetic saturation) if you apply a primary voltage that is too high.

But also remember that the voltage allowed before saturation rises with frequency. If you think you have saturation when playing an open low E, there should be none when playing higher. This applies to push pull amps. For single ended, the T has to take the dc current, and so is significantly larger for the same output power.