Power Transformer Load Test - 100 Watt Marshall

[flemingmras]

Has anyone ever done any load testing on 100W Marshall transformers? Would be nice if we knew what the current handling of the B+ winding was on the originals. It seems Hammond, Heyboer, and Mercury may well be overspec'ing the current handling, which reduces the sag on the power transformer, causing the power tubes to red plate on the high voltage units.

Classic Tone/Magnetic Components rated their 100W B+ secondary 175-0-175 @ 290mA. TAD rated theirs at 300mA. Mercury rates their MP100-475 at 186-0-186@390mA. Heyboer rates their 1203-80 Metro clone at 185-0-185@450mA. Metro's first gen kit transformer, the HTS-6010, is rated 188-0-188@450mA. Hammond rates theirs 185-0-185 @ 420mA.

[xtian]

which reduces the sag on the power transformer, causing the power tubes to red plate on the high voltage units.

Please explain, what does less sag have to do with red plating?

[flemingmras]

Please explain, what does less sag have to do with red plating?

The older Marshalls with their original transformers that ran around 500V don't have issues with red plating when running them hard under load. But many newer built clones that use transformers meeting the same voltage specs have had issues with red plating under load. The consensus on this is that the older transformers would sag more under heavy load, pulling the voltages down enough to not be an issue. But the ones made today are a lot tighter and don't sag nearly as much under load.

After learning what current many of these newer transformers are rated at, that makes me curious what the rated output current was of the originals.

[Stevem]

Tubes red plating is over wattage and wattage is made up of voltage and current.

On an original amp why don’t you place a meter set for amps in series with the B+ center tap and note the current when the voltage starts to sag?

Don’t forget that when the b+ voltage starts to sag so will the heater voltage and as that drops so does the level of signal driving the output tubes.

[flemingmras]

Tubes red plating is over wattage and wattage is made up of voltage and current.

On an original amp why don’t you place a meter set for amps in series with the B+ center tap and note the current when the voltage starts to sag?

Don’t forget that when the b+ voltage starts to sag so will the heater voltage and as that drops so does the level of signal driving the output tubes.

Yes I'm well aware of how it all works.

If I had an original amp I'd do the test myself.

[jbrew73]

What would it take to build a load resistor to test a power transformer?

[flemingmras]

What would it take to build a load resistor to test a power transformer?

They make AC load testers. I'm seriously considering buying an electronic 1kW load tester.

[Roe]

Some originals do indeed redplate on v4-v5 when you drive them hard

[Roe]

One of the most common issues with old 100W Super Lead's is tubes red plating. Modern EL34's can't handle over 450V on the screen grids. Usually what happens is the screen grids start glowing first and then everything else starts going haywire.

the problem is often screen overdissipation (beyond the 8w specs). And with high plate voltage and low Z, you risk redplating, esp. with an unbalanced phase inverter

[flemingmras]

I agree with the screens being the limiting factor. Most amps that run these tubes at 700-800 volts either have an Ultra Linear output transformer feeding them from 43% taps or have a separate supply running the screens at 40-50% B+ (dual rail). I actually have a 100W design I was going to do with 2x KT88s like that. 700V Plate/350V Screen with a 5K Zpl-pl OT primary.

Modern EL34's can't handle over 450V on the screen grids.

Even EL34 datasheets from the golden age of tubes limits them at 425V.

[Roe]

I agree with the screens being the limiting factor. Most amps that run these tubes at 700-800 volts either have an Ultra Linear output transformer feeding them from 43% taps or have a separate supply running the screens at 40-50% B+ (dual rail). I actually have a 100W design I was going to do with 2x KT88s like that. 700V Plate/350V Screen with a 5K Zpl-pl OT primary.

Modern EL34's can't handle over 450V on the screen grids.

Even EL34 datasheets from the golden age of tubes limits them at 425V.

yes, some datasheets say 425v, while others say 450v or 500v. These voltages can be exceeded at idle if you do not exceed the 8w screen dissipation, as pdf64 pionted out recently in another thread, based on a mullard datasheet

[flemingmras]

I was finally able to do some load tests on my custom wound power transformer. 69 Super Lead Plexi.

This transformer is a modified version of MetroAmp's "not quite" Drake 1203-80 (I say not quite because it was hardly a clone of that transformer, but I digress).

All tests done on a Variac set for 120VAC. Prior to internal measurements, the Variac was load compensated to ensure 120VAC at the mains fuse. Tests were done no load, quiescent, full clean power out, and full crank.

Warmed up and stable, it yielded a perfect 450VDC on the plate under quiescent conditions. 32mA per tube and zero crossover distortion. At full clean output, screen voltage drops to about 390VDC (1K 5W resistors on each screen) and the screens are dissipating 7.8 watts...just under their rated maximum of 8 watts. The AC only sags about 13VAC at full clean output power, and only 18VAC cranked with 100mVrms input.

115 watts clean power output! 180 watts fully cranked.

[jbrew73]

What were the b+ voltages at full and cranked power?

[flemingmras]

What were the b+ voltages at full and cranked power?

B+ dropped to about 422VDC at full clean output and 403VDC fully cranked.