Determine transformer specs

[EFK]

Can someone explain to a layperson (me) how to determine the specs of the PT and OT in an existing amp? Is there an easy or semi-easy way to do this?

[Old Tele man]

PT is spec'd by its' secondary windings, each one providing a VOLTAGE & CURRENT rating...and, altogether, they determine (add-up) its' physical SIZE/WEIGHT and WATTAGE rating.

OT is spec'd by its' TURNS-RATIO (TR = N.pri/N.sec) which matches OT-output Z to OT-input load [the tube(s) you use] and how much output power it must handle determines its' physical SIZE/WEIGHT.

[tubeswell]

For the us lay-peeps, in determining the HT winding voltage, pull all the tubes out (and unhook the rectifier if it is a diode rectifier), and carefully measure the VAC across each end of the HT secondary. This will give you the unloaded VAC (which you will need to divide by two if you are working with a centre-tapped secondary, to get the VAC from each side to ground).

Then make an assumption about the sum total and types of tubes it has in it, as to the current rating. e.g. 2 x 6L6CG plus 3 12AX7 will draw about (50mA + 50mA + 2mA + 2mA + 2ma) = 112mA x say a factor of 1.5 or 2 for safety margin (not a hard & fast rule - because some HT windings are seemingly way underrated, like the 70mA for the 5E3), means the HT should be rated at about 150mA - 200mA to work without overheating.

For the current on heater winding, add up the current draw of all the tubes in the amp (from the data-sheets). In the above example we would get .9A + .9A = .3A + .3A + .3A (plus .3A for the lamp, if that is driven from the heater winding) and you have something that is at least 3A - which is probably maybe 3.5A or 4A, if the manufacturer has allowed a safety margin.

For the current for the rectifier winding, go by the data-sheet for the rectifier tube - usually 1.9A for a 5Y3GT, 2A for a 5AR4, or 3A for a 5U4G.

For the OT, you probably should carefully (and without shorting anything) hook the secondary up to a 12VAV-20VAC source variAC and measure the resultant VAC across the primary (with the CT grounded if it is a PP OT). The load resistance will be the square of the ratio of primary VAC to secondary VAC - so for a voltage ratio of (say) 344VAC:12VAC - which is 28.7:1; the impedance ratio is 825:1 - which 6k6 with an 8R speaker.

Or, an even lazier way for some of the popular typical Fender/Marshall etc trannies, the attached thingy from one of the suppliers might be handy...

[naciketas]

Hi,
in determining how many mA are needed on the secondary winding, where can I find the value for other kind of tubes (6ca7, 6bq5, 6v6, ef86 and so on ...), I'm asking because usually on the datasheet for a given tube the heater current draw is specified, but it seems I can't find any useful value for the PT secondary rating.

As I understand tubes draw current from the PT only on the heater side, just to warm up the cathodes: do tubes draw current "to keep" their plates and screen at that specific potential or Maybe this current rating is in function of the filter caps along the PSU?

Thanks

[tubeswell]

Hi,
in determining how many mA are needed on the secondary winding, where can I find the value for other kind of tubes (6ca7, 6bq5, 6v6, ef86 and so on ...), I'm asking because usually on the datasheet for a given tube the heater current draw is specified, but it seems I can't find any useful value for the PT secondary rating.

As a rule of thumb for longevity, you should allow about 1.5x to 2 x the total current draw of all the plates and screens in the amp under idle conditions. (This doesn't always hold true because the HT secondary for the classic 5E3 PT is only 70mA). You find the quiescent (sometimes stated as 'zero-signal') plate and screen current specs on the datasheets.

Datasheet ratings are conservative because the manufacturers wanted people who were using their tubes in various applications not to abuse them. Most of these tubes were being built to use in radios, TVs and Hi Fi equipment. Guitar amps sound better when the tubes are abused a bit more

As I understand tubes draw current from the PT only on the heater side, just to warm up the cathodes: do tubes draw current "to keep" their plates and screen at that specific potential or Maybe this current rating is in function of the filter caps along the PSU?
Thanks

Current, voltage and resistance are inextricably intertwined. So wherever you have voltage and resistance you will have current. (So yes, plates and screens draw current from the High-Tension (HT) winding, and heaters draw current from the heater winding, and so on...)