Will this circuit work? Also, how would I calculate the ohms rating for the primary and secondary of the 1st audio transformer?

You may want to check out this site for ideas.

http://vintageham.com/projects.htm

Dave

Look in a RCA or other tube manual for the data , you also need the proper cathode resistors and and bypass caps , not to mention filament voltages

http://www.pmillett.com/6B4G_push-pull_amp.htm
http://www.the-planet.org/Shishido.html

this output transformer would work

http://www.edcorusa.com/p/356/xpp15-8-5k.

An interstge transformer that would have good audio chacteristics would cost a lot more .
P-T124E
Transformer - Hammond, Audio Interstage, 15K PRI, 33.8/135K S

http://www.tubesandmore.com/


http://www.edcorusa.com/p/449/cxpp60-8-4_2k

.
http://www.tubesandmore.com/

Would work but probably not sound outstanding with no grid bias on the 45's, I would think. Sort of pure class B amplifier? The interstage transformer would just be high impedance all the way around. Look at some old catalogs that spec. tube "interstage" transformers. I assume one side of the filiment supply is at ground potential? Well actually in the old schematic I have, they have a center tap on the filiment trans. that is ground potential.

Here's a 2A3 amp from RCA's RC13 tube manual, what's needed for a 45 is similar...

http://amradio.freeiz.com/tubestuff/2a3amp.jpg

Tom

A Philco 96 uses 45's in push-pull and looks close to what he is looking for, exept it uses a 27 instead of a 26, driver.

From what's shown, no, it won't work because there's no bias for any of the tubes.

First rule of engineering: plagiarize And a good place to start is the tube datasheet, which often includes 'typical' operating specs.

Golden tube web site: http://www.mif.pg.gda.pl/homepages/frank/vs.html

Put 45 in the search box and, having already looked, pick the 'undefined' 45 datasheet, which happens to be RCA (so who knows why it's listed as undefined).

Conveniently they give you 3 Class AB1 and 3 Class AB2 operating points, in both fixed and self bias, complete with OPT plate to plate impedance and suggested drivers and interstasge transformer ratios.

I cleary have "90VDC" typed for the bias of the 26 tube and "200VDC" typed for the bias of the 45 tubes.

You have B+ listed, which is also very necessary... Bias is the negative voltage applied to a grid or developed across a cathode resistor connected to ground(known as self bias)...

Tom

The 90VDC and 200VDC is not bias but plate volts. "Bias" is the grid to cathode voltage, which can be set by either fixing a negative voltage to the grid or putting a resistor under the cathode so tube current through the resistor raises cathode above ground. Cathode positive is equivalent to grid negative as bias is 'relative' to each other.

Let's take one of the datasheet operating points as an example: 250V B+ fixed bias Class AB1 producing 4.85 Watt. It says grid volts should be -56.5 and a convenient way to accomplish that would be to take the interstage CT to -56.5V instead of ground.

The datasheet also suggests "cathode loading" the driver. I.E. Cathode bias: a resistor under the cathode for self bias. That makes sense for the single ended input stage because it'll be operating Class A and cathode bias is not only simpler to do there but more stable. Because of degenerative feedback the cathode resistor reduces gain, though, so you'd probably want to bypass it with a cap so it's 'out of circuit' for audio frequencies.

As a side note, have you considered the cost? I ask because 45s aren't cheap and if cost matters there are lots of other ways to make a 5 Watt tube amplifier.

For the tubes to operate Class B, they would need to be biased at cut-off so no plate current is drawn unless the top half of the waveform is present. This would require even more negative grid voltage than needed for class A operation.

Dave

How about this?

I have a whole bunch of tested 45 tubes laying around so, I don't have to buy any for this amplifier.

Bias for the 26 tube will work.

For 45's add a resistor from center tap of the filament to ground. Since tubes don't draw grid current a resistor there won't generate necessary voltage.

Should I keep the capacitor on the secondary's center tap or should it go strait to ground?

The transformer CT can go straight to ground. That will save a resistor and cap.

Also how would I add the resistor from center tap of 45 filaments to ground if there is no center tap on the winding of the power trans. ?

Attach a small value resistor to each filament connection. Connect other ends of these resistors together. This gives you a center tap. 10 or 20 ohm resistors will work. Be sure both resistors are the same value to make a center tap.

You'd be a lot better off to put negative bias on the interstage CT because cathode bias under the 45s is ging to burn off a huge portion of your B+ for nothing more than generating bias.

That's true but where will he get negative bias? Need another power supply or batteries for bias. The tubes will generate bias by drawing current through a resistor.