Never seen this before:
Lower half of push-pull output has a grounded grid! https://www.radiomuseum.org/r/philco_g_1707s.html

Radical design seems to hold promise?

The German who uploaded the page,Hinrich Grensemann, states "15W output - quality unknown" (guess there are none of these in Germany?)

Have a friend restoring a pair of these (base unit + slave speaker/amp unit for stereo option.) He will evaluate its sound quality and I'll soon report the findings here.

Could be an interesting circuit study. They have a download thing but, the Malware program stops it in its tracks and locks it up.

They have a download thing but, the Malware program stops it in its tracks and locks it up.

Works fine for me and always has. Maybe you should white list that site.

Self-split (no phase inverter). Actually a pretty old idea, beloved of some guitar amp manufacturers.

https://dalmura.com.au/projects/Valve%20Info.html

Thanks GTC for interesting link.
The 1958 Philco circuit seems to have local feedback around transformer (as well as global feedback.)
Maybe that fixes any disadvantages of this design?

In my Radiotron Designers Handbook (Radio Designers Handbook 1967 7th imp) push pull amplifers, phase splitters et al are covered in excruciating detail.

Everbody seemed to be trying to use as few valves as possible to do what was so straightforward with a couple of 6SN7's! So there are all sorts of wheezes such as tapped cathode, plate or screen feedback and so on to drive the lower output valve. Each one worse than the last but touted as the "new" wonder circuit!

I always simply used a 6SN7 as a phase splitter and a 6SN7 (12AU7) as a cathode follower to drive the grids of parallel-push pull with voice coil feedback to the first triode and could wring the nuts off the output valves. Simple.

They all had an obsession with fancy circuits because they could easily fiddle small components with a soldering iron, the real problem was the output transformer and being hard to wind the "experts" treated that as a no-no area! I just kept winding onto grain oriented E/I lams, "C" cores and even bloody toroids trying to get inductance and plate matching without ringing and then transistors came along and I dropped valves like a shot!
Fred.

I did have a look at the Philips one with 6M5. typically Phililps with back bias & the bypass cap across it. However, the phase shift from the cathode of the other PP tube is the interesting bit.

Marc

Fred's point about driving the valves with cathode followers is a valid one. If you are building a guitar amplifier, you want max power without worrying too much about distortion. The cathode followers allow you to drive the output grids positive so they conduct harder and you avoid the pump-up (more negative) of the bias voltage that would normally occur when you drive to output towards clipping. My modelling shows nearly twice available power and the clipping is gradual, not messy like it is otherwise.

But Fred is right, there's no doubt about it, transistors are much better at driving speakers than valves.

Sometimes with transistors, that can be one step too far, if the polarisation is wrong. The MOSFETS really can handle a lot of power, whilst the rest of the circuit doesn't have to. By that comment, one notes the transistor is normally operated in series with the speaker (no transformer); I have seen a couple of examples where the VC tube of the speaker has been crushed when driven into the magnet.

Polarity where there is DC is important.

Well Marc you make a valid point re polarisation, There's no such thing as a PNP valve!!

The fact that you can have a pair of devices with complementary polarities is one of the many benefits of transistors! So much simpler and more efficient.

Polarity where there is DC is important

In fact crucial.

The point with a transformer couple with a valve like 6V6, is that there is no DC if its just driving a speaker.

With DC the speaker polarity is correct when the cone is driven outwards. The main reason the majority have a polarity marked on them.

Listened to it today with experienced audiophile familiar with amps of that era: It sounded very smooth, he rates it a strong '8-out-of-10', with "solid deep bass" despite its smaller output transformer.
In the picture, you see it is set alongside the next iteration (their1959 chassis) which he rates a '9' saying "6BQ5's have better clarity than older 6V6".

Photo uploaded to Post 13.

"There's no such thing as a PNP valve!!" Actually, if you made a valve out of antimatter, you could have a "PNP" valve; but it would be rather hard to connect up to a normal matter circuit. A large explosion would result!