Valve jr power transformer for push pull amp?

[steben]

Hi Guys,

I really want to transform my Valve Junior into a push pull amp.
Preferably EL84 again, but suggestions are welcome.
Can I use the power transformer? It is rated 260V at 0.09A (23.4 Watts)?
6.3V Filament power is not an issue. It's got plenty for that.

the issue is I need a power amp with voltage supply LOW enough to meet the current draw restrictions. This is 90mA, yet with the preamp in mind, I think this should be 85mA max.
The lower output as trade off is completely acceptable, since a coupe of Watts for such an amp makes no big difference.
+/-300 Volts supply seems ideal.
The 260V suppply becomes roughly 350V after the rectifier. A large (sag) resistor can fix this to any voltage but this will have a big power rating and heat!
Another option is using a large load output transformer (15k or so instead of classic 6k to 8k).
A cathode bias amp might reduce the max output and current draw on its own, but is this sound thinking?

[Manfred]

For the push pull power amplifier you need a different output transformer than for 5 Watt single end.
It must have the right primary impedance Raa = 8kOhms for optimal function.
Therefore you can not change the current consumption by changing the impedance of the output transformer.
Does the mains transformer have a continuous winding or with centre tap, 0-260V or 260V-0-260V?
In class AB operation the quiescent current Ia + Ig2 would be ~ 120 Milliamps for both output stage tubes, 90 milliamps would be too little.
Perhaps it would be possible with a different operating point at a lower anode voltage and lower output power
to achieve a lower quiescent current consumption and to get below 90 milliamperes.
However, a different primary impedance of the output transformer would also be required.
In my opinion, this would mean a lot of work and it would be easier to get a suitable power transformer.
BTW it is also the possibility to reduce the voltage with an HV voltage regulation using a MosFet, here the link to an example:

[bmxguitarsbmx]

Go for it. Class A amplifiers use more than 2x the current of Push Pull amplifiers. Change the output transformer, add another of the same output tube, and let it rip.

[steben]

For the push pull power amplifier you need a different output transformer than for 5 Watt single end.
It must have the right primary impedance Raa = 8kOhms for optimal function.
Therefore you can not change the current consumption by changing the impedance of the output transformer.
Does the mains transformer have a continuous winding or with centre tap, 0-260V or 260V-0-260V?
In class AB operation the quiescent current Ia + Ig2 would be ~ 120 Milliamps for both output stage tubes, 90 milliamps would be too little.
Perhaps it would be possible with a different operating point at a lower anode voltage and lower output power
to achieve a lower quiescent current consumption and to get below 90 milliamperes.
However, a different primary impedance of the output transformer would also be required.
In my opinion, this would mean a lot of work and it would be easier to get a suitable power transformer.
BTW it is also the possibility to reduce the voltage with an HV voltage regulation using a MosFet, here the link to an example:
[ Image ]

Definitely 0 - 260 V ... Adding an EL84 AND an ECC83/12AX7 means 1.1 extra Amps for the filament supply..... perhaps to be considered as well....
Well yes, the quiescent current and output impedance seems IMHO to be a choice above all.
Fact is I am considering to replace the power transformer with a multiple tap one with which you can select the B+. This gives the aray of voltage with which one can select output power.

[bmxguitarsbmx]

If you are planning on replacing the output transformer and the power transformer, you should just start a clean build.

[ppluis0]

If you are planning on replacing the output transformer and the power transformer, you should just start a clean build.

I'm agree !!!!!
In my opinion the best (and simpler) amplifier to build is a clone of a Matchless Spitfire.

Cheers,
Jose

[steben]

If you are planning on replacing the output transformer and the power transformer, you should just start a clean build.

I'm agree !!!!!
In my opinion the best (and simpler) amplifier to build is a clone of a Matchless Spitfire.

Cheers,
Jose

Very close to the Baby Will kit for Valve Jr.......

[steben]

Go for it. Class A amplifiers use more than 2x the current of Push Pull amplifiers. Change the output transformer, add another of the same output tube, and let it rip.

Cathode bias amps usually have large bias current keeping the tubes very much in class A at medium output.... Which means almost those 90 mAmps at idle.... With classic 8k output transformer at standard voltage you don't get the current low enough.

[bmxguitarsbmx]

Go for it. Class A amplifiers use more than 2x the current of Push Pull amplifiers. Change the output transformer, add another of the same output tube, and let it rip.

Cathode bias amps usually have large bias current keeping the tubes very much in class A at medium output.... Which means almost those 90 mAmps at idle.... With classic 8k output transformer at standard voltage you don't get the current low enough.

The Idle Current is up to you.

I was assuming that you wanted to do a Class AB push pull amplifier. If you are wanting to do a Class A Push Pull build, just start with a clean build. Sell the VJR.

Personally, I don't consider building PUSH PULL Class A. With Push Pull, you get all odd order harmonics. With Class A, you don't get Sag based compression, so it's like the worst of both worlds. I have converted many class A amplifiers over to Class AB push Pull though simply by adding a tube and changing the OPT. It is very reckless and only genuine Cowboys should try this, but I have always had great luck with it. There is a limit to how much you want to try to modify any of these crappy modern PCB amplifiers. If it is a "Get in, get out" scenario, go for it. Once it get's too involved, just sell your VJR and use the money to buy the ideal transformers and a nice chassis.

hope that is helpful

[steben]

Go for it. Class A amplifiers use more than 2x the current of Push Pull amplifiers. Change the output transformer, add another of the same output tube, and let it rip.

Cathode bias amps usually have large bias current keeping the tubes very much in class A at medium output.... Which means almost those 90 mAmps at idle.... With classic 8k output transformer at standard voltage you don't get the current low enough.

The Idle Current is up to you.

I was assuming that you wanted to do a Class AB push pull amplifier. If you are wanting to do a Class A Push Pull build, just start with a clean build. Sell the VJR.

Personally, I don't consider building PUSH PULL Class A. With Push Pull, you get all odd order harmonics. With Class A, you don't get Sag based compression, so it's like the worst of both worlds. I have converted many class A amplifiers over to Class AB push Pull though simply by adding a tube and changing the OPT. It is very reckless and only genuine Cowboys should try this, but I have always had great luck with it. There is a limit to how much you want to try to modify any of these crappy modern PCB amplifiers. If it is a "Get in, get out" scenario, go for it. Once it get's too involved, just sell your VJR and use the money to buy the ideal transformers and a nice chassis.

hope that is helpful

I understand your points.
yet...This how I see it:
It is interesting to mention most cathode baised push pull amps are class AB anyway, because AB means: at least some part class A, at least some part class B. In ACXX amps the parts are about 50/50. that's what I ment with medium power sticks to class A. In full power, they are in class B territory, drawing some more current and giving just a "tad" of power sag. VOXes simply do not have prominent power sag because they are hot biased towards class A. They do have cathode bias shift however. That's what makes the VOXes VOXes (along with the lack of feedback). I never heard many complains about the qualities of those amps. The Spitfire or 1974x is about the same concept but slightly colder biased. And they have more power sag along with the bias shift. On the other hand this lowers "headroom"... always a trade off.
Even order harmonics can be achieved with the preamp stage or booster circuits. Personally I think even order is overrated. It boosts IM distortion a lot. And practice learns "shimmering" clean guitar tones usually come from linear amps with no massive even order distortion. Any fixed bias Push Pull AB amp needs just as much preamp circuitry for an amount of even order harmonics.
Music Man hybrid amps are opamp based solid state preamp and push pull tube power amp. Yet it is not rumoured they are horrible amps either.

I think I learned a lot in the last couple of days.
First of all: stock Vjr PT only is usable with higher load output transformer, taming the max output current, setting the amp on lets say output max of 10Watts.
Second: if I want practical options (variable power) I loose the option of maximum EL84 power (15 to 18 Watts) when sticking to the stock PT.
Yet... I love the enclosure (it is a head) which comes with the cabinet. And, Baby Will/ Spitfire circuits are relatively simple and easy to put in the given space... The Baby Will PCB is designed for the enclosure.

[bmxguitarsbmx]

Ok. Just as long as you know you can bias a Class AB el84 much colder than 90mA, which is totally out of range for the VJr PT.

I've built a lot of amps in my day and while the ac30 is a legend, I don't prefer hot bias. I rarely ever think that biasing hot sounds better. The AC30 power section is overhyped. Rebias it and it will sound better. Not only that, but it sounds and feels better with some negative feedback. Class A is also over hyped, though sometimes it has a place. Those are based on tones I find inspiring though.

I think you should do a clean build with the parts you want. 100% don't mess with the VJr. It's tempting to turn that little chassis into a rats nest, but my advice would be to sell it off.

[steben]

Ok. Just as long as you know you can bias a Class AB el84 much colder than 90mA, which is totally out of range for the VJr PT.

I've built a lot of amps in my day and while the ac30 is a legend, I don't prefer hot bias. I rarely ever think that biasing hot sounds better. The AC30 power section is overhyped. Rebias it and it will sound better. Not only that, but it sounds and feels better with some negative feedback. Class A is also over hyped, though sometimes it has a place. Those are based on tones I find inspiring though.

I think you should do a clean build with the parts you want. 100% don't mess with the VJr. It's tempting to turn that little chassis into a rats nest, but my advice would be to sell it off.

As I said, your words are noted.

Perhaps it wasnt clear I never ment 90mA as bias, but 2 x 45mA, since both tubes are "ON" at idle.
And I agree the AC design is a somewhat "popular pleasant sounding accident". And way too hot biased, hence the wear....
A description which applies to all early tube amps.
Feedback should give more clean headroom - aka LESS harmonic distortion build up - to the amp. Non-feedback amps are "mushy". At least that is the theory.
Class A... all classic preamp stages are. But for power amp "tone" I agree class A is not the ultimate goal.