What can I make with this transformer?

[jimi]

I have an old RS power transformer its 300-0-300v 120ma. 6.3v 3A. 5v 2A. 6.3v 1A. What could I use it for? I wanted to make a 50W 2xEL34 amp but I don't think it's powerful enough is it? What would i need for a 50W amp? Thanks

[Tom Phillips]

I'd say that this PT would work best for an amp in the 15 to 20 Watt range like a 2x6V6 or 2xEL84 design.
Tom

[Bruce / Mission Amps]

You can get peak blast of power close to that but probably can't get more then about 25-28 watts continuous power out of that PT no matter what you do.
It takes around 180ma to 200ma to get close to the 45-50 watt range.

But, it would be perfect for an 18w to 20w version of a TMB plexi type Marshall amp with three 12AX7s and a pair of fixed bias JJ6V6.
Don't use a tube rectifier, but do use a reissue Deluxe Reverb OT from AES for great sound.

Bruce

[jimi]

Thanks for the replies. I might shelve that one for now. I have two things in mind, a 50watt amp I can use in a band and another small amp under 10 watts. I have a SE output transformer for the small amp 5k primary impedance I'm not sure what power it can handle. Maybe I could power that one from the RS transformer or would that be overkill for a little amp?

[Bruce / Mission Amps]

OK you can hardly be too stiff for a class a SE amp.
You could use the PT for a single ended 6L6 or EL34 but you'll need to buy a 150ma 5Hy choke too.
Forget the rectifier tube thing and instead put a solid state FW rectifier in it.
Take the cathode lead (+) from the FW rectifier right to the choke, then after the choke put a 16uF to 33uF 450v cap.
This is where the amp starts... feed the OT from the point after the choke and bias the power tube up pretty high, around 80%-90% of its rating.
That should be pretty good for a 9-10 watt output Class A 6L6 amp, or a 5-6 watt class A SE 6V6 amp.

[jimi]

Thanks for the advice I think I will go with that. I was going to use solid state recifiers to keep things simple. Why don't you like tube rectifiers?
Will this output transformer be ok with all those different tube types at 5k ohm primary?

[Bruce / Mission Amps]

I do like tube recitifers but with a class A amp like this max current will be with the amp at idle and using a choke input filter will only net you a B+ equal to about 90% of the applied hi voltage DC before filtering.
So rectifier tube will lower that even more.
In other words 300vac into a SS rectifier will give you almost 300vdc x .9 or 270vdc.
A 6L6, at say 25 watts in idle, will draw about 95ma of current.
So you'll have decent power supply headroom with a 120ma PT.
Plus, the choke input filter will allow the B+ rail to deliver a little more current then a capacitor filter B+ rail, so it works good.

[Andy]

you know.... if it's a 300-0-300VAC transformer, it should make about 420VDC with a solid state full wave rectifier, and 120mA is more than enough for a 50W class AB amp. I'd think it would work just fine for his original application. correct me if I'm wrong here please.

[Tom Phillips]

Andy,
A lot of the power pulled from the PT is lost to inefficiencies before it gets to the amp output. The largest factor is that a typical AB2 amp is less than 50% efficient. Therefore a 20W power amp will draw more than 40W from the power supply. Looking at the 300V/120ma PT very simply (without considering sag, peak power, conservative design etc.), it can only supply 300V X 120ma = 36W. Note that when you rectify and filter the 300VAC you get a higher voltage but you do not create more power. The continuous current available is less than 120ma at 420V. Additionally, when you draw full power on the B+ line the 420V will sag down even with a SS rectifier.
Bottom line is that this PT is about right for a 20 Watt amp design.
Regards,
Tom

[jimi]

Just a thought......If this transformer says on it 300-0-300v at 120 ma could that mean 600v at 120ma and 300v at 240ma? I don't get this multiplying the ac voltage by 1.414 thing. I measured in an orange amp 376vac and 504vdc it calculates at 531vdc even though it had doubled up diodes in a bridge rectifier surely it wouldnt drop that much? What happened to the other 27volts

[R ski]

Good point Tom. Many years ago when I used industrial 120/440 150 VA
control transformer with class AB configuration, it wimped out using a 10 ohm power resistor.

[Tom Phillips]

Just a thought......If this transformer says on it 300-0-300v at 120 ma could that mean 600v at 120ma and 300v at 240ma? I don't get this multiplying the ac voltage by 1.414 thing. I measured in an orange amp 376vac and 504vdc it calculates at 531vdc even though it had doubled up diodes in a bridge rectifier surely it wouldnt drop that much? What happened to the other 27volts

Typically the transformer designer would size the wire and the core assuming that the 300-0-300V winding would be used with a full wave rectifier circuit with the center tap grounded. Therefore, each half of the winding would supply current for 50% of the time. If you hook across the whole 600V winding with a full wave bridge you will be asking for 100% duty cycle. I suspect that the transformer will overheat or the core will saturate (depending on the power drawn from the other windings) or both. That is, unless the transformer is overly designed or very conservatively rated.
That’s why you shouldn’t expect 600V at 120ma. That’s twice the power.

You can’t wire the two halves of a 300-0-300V secondary in parallel to get 300V at 240ma. To do that you would need two independent 0-300V coils with all four leads brought out so they could be paralleled in phase. In addition, the core and wire size would need to be larger to allow twice the power. (Again unless the beast was over designed to start with)

At first your idea sounds logical but there aint no free lunch.



As to the 1.414 multiplyer. That calculation is the ideal case with no load. You always get a little less. In your example, 5% less sounds about right.


Cheers,
Tom

[Andy]

Andy,
A lot of the power pulled from the PT is lost to inefficiencies before it gets to the amp output. The largest factor is that a typical AB2 amp is less than 50% efficient. Therefore a 20W power amp will draw more than 40W from the power supply. Looking at the 300V/120ma PT very simply (without considering sag, peak power, conservative design etc.), it can only supply 300V X 120ma = 36W. Note that when you rectify and filter the 300VAC you get a higher voltage but you do not create more power. The continuous current available is less than 120ma at 420V. Additionally, when you draw full power on the B+ line the 420V will sag down even with a SS rectifier.
Bottom line is that this PT is about right for a 20 Watt amp design.
Regards,
Tom

I understand where you're coming from, but isn't the 300-0-300 supposed to be at its rated load of 120mA in most transformers? at loads less than that, it would be somewhat higher. a 2xEL34 AB amp will idle around 75mA total bias current with a B+ of 420V, which leaves plenty of room for additional current draw.

[Tom Phillips]

Andy,
My thinking is:
We have 36 Watts of power available from the HV winding. Granted, this could be a little more or less depending on the exact transformer design. After rectification and filtering we will have a B+ of ~420V as you stated. I submit that the average power available is still the same 36 Watts since we can’t create power. You could draw more power for a short time not the rated rms power. Now we calculate that 36 W at 420 V is 86ma. That’s only 11ma (15%) more than your 75ma idle current.

Another interesting point: I recently measured the current draw at the OT center tap of a Fender Deluxe reverb as it was putting out 19 Watts into the dummy load. The B+ voltage had sagged to just a little over 400V. That tells me that the power amp was consuming over 40 Watts of power to put out 19W.

Well – That’s my thinking anyway. Lunch time has ended & I must get back to work.
Additional thoughts welcome,
Tom

[jimi]

Output tubes get pretty hot that must make up some of the wasted power or maybe most of it?