Determining Power Transformer Current Requirement

[Johnrcurry]

I considering building an amp based on Dave Hunter's original two stroke design. His original basic design is a single ended amp using a 5y3 rectifer tube, a 12AX7 preamp tube, and parallel 6v6 power tubes. He used a power transformer with a secondary rated at 310/310 VAC and 80ma. A 6.3 VAC filament supply rated at 4A, and a 5.0 VAC rectifier filament supply rated at 3A.

I have on hand a bunch of 6AQ5 power tubes and a power transformer rated at 240/240 VAC and 70ma. My transformer has a 6.3 VAC filament winding rated at 3A, and no 5.0 VAC winding. I am thinking about building a version of the two stroke with a 12AX7 preamp tube, parallel 6AQ5 power tubes, and either a 6X4 rectifier tube, or a diode rectifier. My most immediate question is how to determine whether the 70ma current rating of my transformer is sufficient for this proposed tube line up. Any help pointing me in the right direction will be most welcome.

[Chuck H]

A good start would be to look at the tube data for your line up. Assume the amp will spend some time at full tilt. Add at least 20% for safety and to avoid too much sag in the power supply.

I think your fine with the stuff you have. But for education sake you should look it up for yourself. Tube data is easy to find. Figure for the calss of operation and maximum output under those circumstances for every tube in the amp. the current draw under max and typical operation is listed in the tube data. Easy peezy.

In truth some amps use a smallish power supply and have a certain sound because of it. To duplicate the amp your cloning just figure that amps tube line up current max and see how it stacks up to the mA rating of it's power transformer. Then see if your stuff will give you close to the same ratio.

You can also check the current draw for all your heaters to determine if you need to go with a diode rectifier or if you have enough rating to use the 6x4.

Chuck

[Johnrcurry]

Thanks for your reply. I looked at the tube data sheets but I'm not quite sure which figures I should be looking at. Here's what I came up with:

6aq5, for class A operation shows:
"maximum signal plate current" of 47ma at 250 volts
"maximum signal screen current" of 7ma at 250 volts

12AX7 shows:
"plate current" of 1.2 per section at 250 volts

Now, for the 6AQ5 if the relevant number is the "maximum signal plate current" then it looks like my 70ma is not enough for two 6aq5's (2 x 47 = 94ma) and one 12ax7 (2 x 1.2 = 2.4ma).

As far as the amp I'm cloning, the data sheets show:

6v6, for class A operation:
"maximum signal plate current" of 35ma at 315 volts
"maximum signal screen current" of of 6ma at 315 volts

Looking at the maximum signal plate current, his 80ma transformer is very close to the maximum with 2 6V6's (70ma) and one 12AX7 (2.4)

Am I looking at the right figures from the data sheets?

P.S. it looks like my 3A on the 6.3 volt winding would be enough for 2 6AQ5's (2 x .45amps = .90), one 12AX7 (.15amps ) and one 6x4 (.6amps).

Thanks

[Robert M. Martinelli]

Hi,
Just a footnote, when it comes to true "class A" operation the plate current does not change significantly, because the output tube(s) will be conducting during the whole 360 degrees. In class "B" and "AB" the plate current swings significantly when the tubes move from their quiescent state to conduction.

This means your "maximum" current will be a "constant" one, and, also, remember to add the screen current(s) to your figures, this means your PT current figure will not be 96,4 mAmps ( 47+47+1,2+1,2 ) but rather 110,4 mAmps ( 47+7+47+7+1,2+1,2 ) ( in a single ended class A design the output transformer does not deal with screen grids current, but they need to be dealt with when sizing the PT ).

Chuck makes a good point about some amps having underrated power transformers and sounding "good" because of that, as the PT "sags" ( and voltage drops ) when the amp is fully cranked, but, as he wisely noted, the PT should be only "slightly" underrated to avoid an excessive voltage drop ( and, possibly, a PT failure ).

As to the heaters' current rating, be aware that the current figure you used for the 12AX7 ( 0,15 Amps ) is true only for a 12,6 ( series ) supply, while at 6,3 V ( parallel supply ) a 12AX7 draws 0,3 Amps ( you seem to still be within the limits for the 6,3VAC secondary nonetheless ).

Hope this helps

Best regards

Bob

[Johnrcurry]

Thanks Robert. Between Chuck's reply and yours I've learned exactly what I hoped to learn.

[Chuck H]

This is my bad. I recalled the 6aq5 as a lower output tube and thought it had a lower max current than the 6V6. I probably thought this because most amps that use them have a lowish output. But I guess that's more the amp than the tube.

Ok... So for best results you would want to go 100mA for the PT. You do still have the option of using the PT you have and see how hot it gets or if it under performs tone wise.

Chuck