PT Filament current test?

[notabot]

Is there a way to safely test the max filament current on a PT? I have a garnet revolution 2 with 6L6's, wondering if it could handle kt66 or el34? PT #'s 6K2952 DGD7

[K O'Connor]

Hi Guys

When considering a power tube swap in an amp, or using a PT from a known amp in a new build, you have to begin with the amp the PT is/was in. What tube compliment did it support? You say the original amp had 6L6s, presumably two of those? How many preamp tubes and what types?

For example, say there are 2x 6L6 and 3x 12A_7.
each 6L6 heater is 900mA
each 12A_7 is 300mA at 6V
the total is (2x900mA) + (3x300mA) = 1800mA + 900mA = 2700mA.

A pair of EL34s would need 3200mA (1600mA each)

A pair of KT66s needs 2400mA (1200mA each)

In most cases the heater winding on a tube amp PT can handle up to 20-25% more load than the stock tube arrangement. However, it is always safest not to exceed what the stock tube set requires.

Were it the other way around and the stock tube set used EL34 or 6CA7, then it is no problem to sub the other types mentioned here as they all draw less current.

Apart from that, testing an unknown PT can be tricky.

The first question is what is the rated primary voltage? For North America it could be 115V, 117V, 120V - the latter is not common and would be for more recent PT designs. Ideally, you test the PT at its rated primary voltage and this may require using a variac or a small PT to buck excess mains voltage.

With the rated primary voltage, you can add load to the heater winding and see where it drops below 6v3 AC. At that point you know the load is getting excessive. You can use tubes as the load as this is probably easier than using power resistors. For example, a 6L6 heater dissipates just under 6W of heat, an EL34 is 10W; KT66 is about 8W. These are not outrageous heat levels but you need 7R, 4R and 5R3, respectively to represent these tubes. power resistors will get very hot and must be over-rated power-wise.

When not sure of adding higher heater loads, it is safest to add a small auxiliary PT to support the output tubes alone and leave the stock winding to support the small tubes. A 5A 6Vac PT is only 30VA which is not too large physically.

have fun

[notabot]

TY for the reply. I'll give that a try.