Simple PT rating question

[Malcolm Irving]

Also, it is difficult for a pentode output stage to provide enough voltage at low frequency to cause saturation. At low frequencies, the OT magnetizing reactance is proportionally reduced. The output pentode sees a low load impedance (compared to its own high internal impedance) and may not produce enough voltage across the OT primary to cause saturation.

EDIT: Ah sorry! This thread is about PTs and (above) I am referring to OTs.

The mains has such a low internal impedance that it is perfectly capable of saturating a transformer. Most PTs are designed so that they are 'just' saturating with the normal mains voltage and frequency. The level of saturation in a PT is not affected by the current drawn from the PT secondaries by the amp.

[nickb]

Mike & Malcolm - the OP was about a power transformer. The (valid) considerations you raise are not relevant.

[nsubulysses]

anyway researching more and comparing specs on Classictone site it seems all the 40-60W PTs that are Marshall styles have HV secondary ratings at 150mA while the Fender 40-60W PTs have HV secondary ratings in the 180-240mA range.

So it seems like in the case of a 55W amp that doesn't seem to keep pushing when you keep turning up the volume, this might be part of the issue, and maybe what pdf64 is talking about in post #32.

Is it safe to assume that a same power transformer, say rather than 690VCT at 150mA it was 690VCT at 240mA, would perform better at high volume, all else the same.

I don't think the OT is holding me up and I've built the same preamp, power amp, and power supply before. just with 4 power tubes and a hammond 278CX PT that has HV secondary of 800VCT at 535mA

I am trying to narrow it down but I think PT is the weak point.

It makes me think this PT would be better for a clean amp VS a high gain amp that is supposed to be powerful and bold sounding, especially at high volume.

[Malcolm Irving]

The current rating of a transformer winding is all about how much heat can be dissipated from the winding at a 'safe' operating temperature under given environmental conditions (air temperature and air flow). So a difference in mA rating between manufacturers might just reflect their different opinions on a maximum safe operating temperature and/or environmental assumptions.

[bob p]

Mike & Malcolm - the OP was about a power transformer. The (valid) considerations you raise are not relevant.

Nevermind.

[Malcolm Irving]

One of the (many) confusing things about transformers is they don’t really have a VA rating. They have a V rating and an A rating. We generally multiply these together to get a ‘VA rating’.

The primary and secondary currents determine the heating of the windings.

The primary voltage determines the heating of the iron core.

One of the tricks in transformer design is to get the right balance between the amount of copper and the amount of iron. A transformer is actually operating at its highest possible efficiency when the heat dissipated in the iron equals the heat dissipated in the copper.

With a high V rating, and a high A rating, we get a high value for VA which (neglecting power factor for now) means high power can be transmitted through the transformer.

[nickb]

I am curious how this works and how I should understand this spec in transformer ratings.

For instance, say a transformer has an 800V CT @ 535mA. I don't understand how this equates because if you have 4 power tubes that are being cranked full volume, it would seem that they would draw anywhere from 130-200mA each when driven hard. I understand it's not continuous but it does exceed 535mA. Is this ok because it is not continuous? Or is it ok beacuse the current is drawn from PS reservoir rather than directly through the high voltage secondary winding??

I am asking because I'm curious what would be necessary with 2, 4, and 6 power tube amps and how I should guage this when looking at different PT specs

I'm not sure you ever got a straight answer to this basic question. So I'll have a go...

Rather than looking a plate curves and working out average currents the easy rule-of-thumb method I use is based on efficiency. A class AB1 push pull tube amp is about 50% efficient by the time you have allowed for bias, preamp tubes and screen grid currents.

So, a 100W amp will need 200W of power. If the HT is 450V then the current is 200/450 = 444mA.
If you are driving the amp hard so it's fully clipped the efficiency increases so you don't need much more current to do it.

If you have a bridge rectifier then the RMS current for typical transformer resistance and filter capacitance will be about twice that, or 888mA. The Hammond design guide suggests a ratio of 1.61 for RMS/AVG. In my experience this figure to way too optimistic and 2x is nearer the mark.

So, a very conservative spec means you need the secondary rated at 888mA. In practice you are not going full power 100% of the time, so a more relaxed spec might be 50% of that i.e. back to 444mA. That is what I would go for most of the time.

For a full wave bridge (center tapped transformer) the RMS current will be 0.71 of the bridge version i.e. 630mA conservative or 315mA relaxed.

Is it safe to assume that a same power transformer, say rather than 690VCT at 150mA it was 690VCT at 240mA, would perform better at high volume, all else the same.

A higher VA rated transformer will typically have lower resistance and so output voltage will be higher and the regulation will be better (sag will be less) and that will translate to higher power output.

It makes me think this PT would be better for a clean amp VS a high gain amp that is supposed to be powerful and bold sounding, especially at high volume.

Implicitly a high gain amp has the breakup accomplished in the preamp and is followed by a clean power amp stage. So, by that way of looking at it, the power supply requirements are the same. The bigger VA transformer will give you more watts or more headroom depending on how you look at it. How clean it is, is really more to do with the preamp design.

[pdf64]

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?...

My guess is that both the MC and Hammond PTs will perform almost the same, with the 490V@150mA rating just being there to make customers reassured that they are close copies of the original PT used by Marshall (maybe the one they used was something from Radiospares that happened to have that rating?). So the real max current for both is likely to be over 200mA, and for both the voltage at that load would likely be somewhat lower than 490, ie similar regulation.
Perhaps the modern PTs here can legitimately have a higher current rating (yet still be very close copies of the original) due to better modern wire insulation?

I think it would be best to fully measure how your HT is holding up with the PT you've got, before getting another.

I like to measure the mains and PT HT winding Vac, and the HT Vdc at the plate (OT CT) and g2 supply nodes; all under the conditions of idle, max sine wave, and max square wave.

As if the g2 node is saggy, a PT with excellent regulation won't help much.