how to undestand Power transformer specs

[ToneMerc]

Hey Guys,Being on the topic of trannies,I found an old brand new in the box RCA PT.I would like to know if this thing would be useful on any kind of amp build.Here are the specs_Pri. 117 V., 60 cy. Plate 290.-0-292 V. 72 Ma DC Fil. #1 5 V. 2 amp Fil #2 6.35V 2.75 amp Thanks Guys,Mikey

It's 290@72mA, 5V@2A, 6.35@ 2.75A correct? If so , it would make a good PT for a SE 6V6 tube rectifier build.

TM

[Phil_S]

Hey Guys,Being on the topic of trannies,I found an old brand new in the box RCA PT.I would like to know if this thing would be useful on any kind of amp build.Here are the specs_Pri. 117 V., 60 cy. Plate 290.-0-292 V. 72 Ma DC Fil. #1 5 V. 2 amp Fil #2 6.35V 2.75 amp Thanks Guys,Mikey

Looks more than adequate just about any variety of a Champ type amp and many of the low power AX84 type projects. It will probably work for a 5E3 or similar, as well, though you might be pushing it at 72mA. For the 5E3, you need about 70mA for the power tubes. If the 72mA is a conservative rating and you can squeeze a bit more from it, you'd be OK.

You'll need to keep an eye on the 2.75A rating for the filament winding. That probably will be the limiting factor. It's certainly good for a pair of 6V6 and a 12AX7 or 2.

See above...get the tube data sheets and see if you can confirm what I'm saying. It is good to know how.

[martin manning]

OK, Not trying to contradict anyone, but thankful for the thread and replies, it helps me to try to get a grasp of this too. But, doesn't the OT primary impedance play into this? I was under the impression that it did, that the current draw would be different between say, a pair of 6L6's with a 4K primary and the same tubes with a 6K5 secondary.
I know that for efficiency's sake that a person would match the tubes impedance with the output's, but do these play into the power supply selection?

I suppose the OT primary plays into the load of the power tubes. I would treat it as a fixed load and it is not very much of a load compared to the internal plate resistance. The load of the tube itself varies with the amount of heat and the signal being passed through it. I'd say that the tube itself, in respect of the OT primary is not modified very much by the OT for this particular determination. As I see it, this particular discussion concerns how to get a grip on the B+ supplied to the plate.

I think DoctorD's question is regarding the power amp current draw and how it is affected by the choice of the load impedance, and yes it will be affected because the current at which the tube saturates will be affected, and this also depends upon the screen supply and how well it is regulated.

The values on the tube data sheets are listed as "plate current at maximum signal," which looks to be the rms current along a load line that passes through the knee assuming a fixed plate and screen supply voltage. This seems reasonable- a little conservative because there will be some sag, maybe a lot of sag, and a little non-conservative because it is possible to drive a guitar amp to square wave.

[Phil_S]

I suppose...

OK, OK. Thank you, Martin, for the "clarification", and your good humor. I should not pique,
Phil

[ampmike]

Thanks for the input on the RCA,Ill hold on to it for a small build,Thanks again,you guys are the best!!!Mikey

[tubeswell]

The load resistance, bias voltage, HT voltage, ra and gm affect the current. In an output tube pushing an OT, the idle current is about the mid-point of the peak theoretical current draw. (I say theoretical, because you usually never get all the way to zero voltage in reality).

For Class A SE, where the current is 'on' for the 360 degrees of the cycle, you can figure on the peak DC current draw of an output tube being about 1.2 x the DC idle current draw (due to inefficiencies), 1.5 x would put you in 'safe' territory. (e.g.: you would want to allow for 120mA for 2 x 6V6 in parallel SE).

With a PP amp, one side is up while the other side is down, and in Class AB1, each tube is conducting for less than the full cycle, hence for 2 x 6V6 in Class AB1 PP, you only need 70mA-80mA. (maybe 120mA for a BFPR)

[rdjones]

With a PP amp, one side is up while the other side is down, and in Class AB1, each tube is conducting for less than the full cycle, hence for 2 x 6V6 in Class AB1 PP, you only need 70mA-80mA. (maybe 120mA for a BFPR)

Note that the Princetons use the same power transformer as the Champ rated at 70 mA I believe.

rd

[azatplayer]

I thoroughly enjoyed that thread! Good read, and a bit of a laugh