G'day all, hope you are all ok.

I picked up a National 30 Watt valve PA amp at a recent hamfest.

I was hoping to use it as a bass guitar practice amp.

It has MIC1, MIC2, and Phono inputs and bass and treble controls.

However, the output valves have been replaced with 50C5s in push pull.

The amp sort of tries to work, in that the pre amp seems to work, the bass and treble controls work.

It has very low output power, less than 1 watt.

Would anyone have a circuit diagram for this little beastie?

For the life of me I cant work out what sort of valves would have been in the output stage originally.

The power transformer only has a 120 volt (HT) output so I assume from the 2, 2 way canned electro caps that it was a voltage doubler originally.
(2 x100 μF 350 volts, 2x50 μF 250 volts.)

The output transformer has 3.2k primary impedance written on it. It also has a separate feedback winding.

I COULD just bung say a couple of 6V6 s in it or if I have some, EL34 or 6L6s.

The power transformer has 6.3 volts at 800ma, 120 volts at 150 ma and a 25 volt output for bias voltage.

Someone has used a 100 volt primary tapping to power the heaters of the 50C5s in series.

Its doing my head in as I cant see how this ever even made CLOSE to 30 watts output!

The preamp part is on a printed circuit board and uses 3 12AX7s, 2 of which are branded "National"
and strangely 2 of the 12AX7s heaters are wired in series to the negative bias line which is at around 20 volts DC. One 12AX7 using the 12 volt heater, the other only half the heater at 6 volts, odd indeed.

Thanks,
Raff

How are you connecting that to speakers? 120V output says its a line amplifier & that cannot be connected directly to something like an 8 Ohm speaker.

It sounds a lot like it has been hacked.

So we need a bit more info. In such cases I have reverse engineered the circuit, t see where it went wrong. And you may be looking for valve with 25V bias

It may pay to download EL34 / 6CA7 sheets from Franks electron tube pages. To me on what little I see looks the most likely.

Marc

After studying several valve datasheets and looking in my old dusty valve collection, I decided to remove the crappy old 50C5's and put in a couple of 6V6GT's in push pull.

According to my RCA "tube databook" 2 6V6s in push pull are good for around 15 watts apiece, with 285 volts on the plates, and around -19 volts bias.

The existing output transformer seems right as well with a 3k2 ohm primary winding.

Maybe the amp originally had 6V6GTs in it?

So far Ive verified that the power transformer in spite of its labelling can indeed supply enough current to their heaters.

Ive replaced the sockets with used but good octal sockets, wired up the heaters, screens, plates and cathodes.

I just have to tinker with the plate B+ supply. I think I'll leave the 120 volt secondary alone and use a tapping from the primary at 200 volts to a full wave bridge to supply around 285 volts to the 6V6GT plates.

I think the bias supply might be ok as is.

Hopefully Ill muck about more in the next day or 2.

Wish me luck!

Raff

By tapping a primary you are tapping the direct mains. That will turn the set into a "live chassis" and if you have a death wish that is a sure way of achieving it.

Do you know what you are doing? Do you have you nomenclature clear? The primary is what the mains goes onto & the secondaries are what supply the tube elements. I am seeing a whole lot of confusion here that has a massive potential to end badly.

If you put maximum volts 315V on the plates of 6V6 then there has to be a divider to bring the screen back to 225V. I am still suspicious that it did not have 6V6's.

Marc

Marc,

Thanks for your concerns.

Yes I realise that tapping off the primary is dangerous. I was only going to do that as a test.

I was going to test the setup with an isolation transformer just to see if 200 odd volts on the secondary of a suitable transformer would give me the 285 odd volts I need for the plate supply for push pull 6V6s.

Yes I do know what I'm doing, (well most of the time), I've been a qualified tech since 1984, but suffer memory loss and other issues due to injury.

I perhaps didn't make it clear that the output transformer is not 120 volts, but the only high voltage, as such, output from the secondary of the mains transformer is 120 volts.

The output transformer has a 3k2 primary, and taps on the output for 4,8,16,110, and 170 ohms output impedances.

I see you've looked at the data sheet for the 6V6 but not the push pull part it mentions a plate voltage and screen voltage of 285 volts.

My concern was just how and with what valves in push pull, would supply the 30 watts as stated on the back of this amp with only 120 volts ac from the secondary, that's why I questioned that perhaps a voltage doubler was used.

The 50C5s were clearly a later add on by persons unknown, as the sockets are of a newer type and mounted on small squares of aluminium to be mounted where octal sockets once were.

That left me with looking at suitable octal, and perhaps loctal, output valves given the rest of the parts used which is why I guessed at push pull 6V6's.

When powered up with the 50C5s in circuit, the ht was around 175 to 180 volts.

The HT was derived from the 120 volt secondary with a full wave diode bridge to the can electros.

As someone in the past has clearly mucked about with this amp, and removed any models numbers and spray painted the cabinet blue, it makes it hard to find just what the original circuit was like, and what output valves were used.

I don't think 6CA7 / EL34s were used as the heater current is probably too high for the power transformer, same goes for 6L6s, which is why I tried the 6V6s.

Raff

Im going to try the 6V6s on the 180 odd volts from the "original" setup and see how that works out.

If I don't get much power out, then Ill try a voltage doubler from the 120 volt secondary.

I decided against the primary 200 volt tapping, as mentioned by Marc, its a dangerous thing to do, even as a test.

Raff

Update:

While mucking about with this amp, I found stamped into the chassis under where the squares of aluminium were, 50H-B26 for each output valve.

That explains the use of 50C5s as those 50H-B26s are a bit hard to find.

So I'm back to sorting out the push pull 6V6s.

Raff

Here are some pdf specs for the 50H-B26. The second pdf is quite large at about 45 meg:-

http://frank.pocnet.net/sheets/084/5/50H-B26.pdf

http://frank.pocnet.net/other/Matsushita/Matsushita_Receiving-Tubes_1971.pdf

Instead of 6V6GT's, consider using a pair of 50L6GT's. Then you can keep the original heater supply intact.

Instead of 6V6GT's, consider using a pair of 50L6GT's. Then you can keep the original heater supply intact.

Strongly seconded.