How to make sense of power transformer current ratings?

[TheMagicEight]

The power transformer on my 1987 build is a Dagnall D2479, and I think it's a 345-0-345 setup (no clue on the current rating, can't find it on the web). Did some voltage measurements and found that (off 124 VAC mains) the B+ is around 468 when idle, and that drops to 410 when cranked.

I'd like to replace the power transformer with one that doesn't sag as much. Looking at PT spec sheets, the figure given comes in the form of secondary_voltage @ current_rating. I'll keep the secondary_voltage at 345 (don't want to blow my caps), which leaves the free parameter current_rating.

- Is there an obvious way for how to choose current_rating?
- If I know how much current my amp will draw, is there a way to calculate the expected voltage sag? Rules of thumb?
- I'd like the amp to idle just around 480 V on the B+, and sag to about 440 (numbers come from a reference amp I have). Possible to back-calculate the PT current rating?

Any insight appreciated; ideally I'm hoping to get this right on the first try, as trial and error will get expensive.

 

[playloud]

I'm not familiar with that particular PT, but there are other factors at play when it comes to voltage sag - notably, the resistance of the B+ winding and the capacitor values themselves. Randall Aiken has an article on this: https://www.aikenamps.com/index.php/what-is-sag

At first glance, the drop you describe sounds about right for a 50W Marshall. You should measure the (AC) secondary voltage directly if you are really curious (a variac is a safer way to do this).

Edit: if you have Windows, this could be useful too: http://www.duncanamps.com/psud2/

 

[2L man]

I'm not familiar with that particular PT, but there are other factors at play when it comes to voltage sag - notably, the resistance of the B+ winding and the capacitor values themselves. Randall Aiken has an article on this: https://www.aikenamps.com/index.php/what-is-sag

At first glance, the drop you describe sounds about right for a 50W Marshall. You should measure the (AC) secondary voltage directly if you are really curious (a variac is a safer way to do this).

Edit: if you have Windows, this could be useful too: http://www.duncanamps.com/psud2/

Where Aiken explain secondary coil resistance its effect to voltage drop is higher because current flow is pulses because of sine wave shape even it is rectified.

Center Tab transformer efficiency is lower than a single secondary coil transformer for Bridge Rectifier.

There is no standard which all transformer manufacturers use. One is where voltage and current are measured using resistive AC load so it can't be used direct for rectified and filtered DC. I have seen a graph/table where it is possible to interpret but I haven't stored it to this tablet I use now.

Hammond datasheet are best I know. Antekinc provide three voltages for three currents.

 

[TheMagicEight]

I'm not familiar with that particular PT, but there are other factors at play when it comes to voltage sag - notably, the resistance of the B+ winding and the capacitor values themselves. Randall Aiken has an article on this: https://www.aikenamps.com/index.php/what-is-sag

At first glance, the drop you describe sounds about right for a 50W Marshall. You should measure the (AC) secondary voltage directly if you are really curious (a variac is a safer way to do this).

Edit: if you have Windows, this could be useful too: http://www.duncanamps.com/psud2/

Thanks for the references, I'll check these out! Will be interesting to see how well my measured voltages align with the simulation software.

Regarding measuring the secondary, I did attempt to but I don't think my measurements are all that useful. At idle, I see a voltage of 362 VAC with a 2.72 VAC ripple on the B+. Cranked I see a drop to 346 VAC with 9.3 VAC ripple on the B+. The issue is that this is measuring RMS voltage, and I think what really matters for rectification is peak secondary voltage. Unfortunately I can't measure this with the tools I have (as far as I'm aware).

 

[TheMagicEight]

Where Aiken explain secondary coil resistance its effect to voltage drop is higher because current flow is pulses because of sine wave shape even it is rectified.

Center Tab transformer efficiency is lower than a single secondary coil transformer for Bridge Rectifier.

There is no standard which all transformer manufacturers use. One is where voltage and current are measured using resistive AC load so it can't be used direct for rectified and filtered DC. I have seen a graph/table where it is possible to interpret but I haven't stored it to this tablet I use now.

Hammond datasheet are best I know. Antekinc provide three voltages for three currents.

This makes sense to me. I can estimate current flow based on the ripple voltages and how much power I know is drawn at idle (around 72 mA at 468 VDC), but considering that's just average current flow, that it actually flows in pulses proportional to where in the sine wave the pulse is, I think I'd need a scope to actually see the max current flow / voltage drop.

Even then, connecting that info to the data that's available, it still seems like guesswork. I did notice the Hammond stuff has a little more info. Will have to have another look.

 

[william vogel]

The easiest way to accomplish the reduction in sag that you are after is to install a 100 watt Marshall power transformer in place of the 50 watt version. The idle B+ will be close to your wanting 480. It might be slightly higher and it’ll sag down to about 440. You’ll have to install a full wave bridge rectifier and you’ll have to wire the bias supply as like a 100 watt. All this is simple. The physical size of the transformer uses the same lamination steel. If it’s a lay down transformer it’s a simple bolt in. If it’s standup you might have to drill a few holes.

 

[2L man]

Nowadays Installing MosFet Regulator is option for HV which does not sag. However it can't prevent HV sag if power supply can not keep HV feed high enough for highest power.

 

[Ronquest]

Edit: if you have Windows, this could be useful too: http://www.duncanamps.com/psud2/

Can't seem to find the actual file to download now. I have a copy on another PC, but was needing this last week and no luck.

 

[Pete Farrington]

I think you now need to sign up and log in at https://groups.io/g/duncanampspsud