Full Bridge Amp?

[Tony]

to the tube gurus here, please comment on this tube amp design.

[sagarverma]

we have an amplifier 'guru' out here.

he can show u the way.

[Tony]

hi,

actually i do not feel right about driving the upper tubes from the 12at7 plates as shown.

totem pole requires quadfilliar secondaries driving the tubes, but i post it anyway and see what they have to say.

[martinab2]

Interesting circuit! I think there is an easier way of driving the top tubes though:

If you remove the bypass caps on the top tube cathode resistors, and connect the plate of the bottom tube to the grid of the top tube (via the 1k stopper) you can then remove the drive to the top tube grids as you will then have an SRPP / SEPP arrangement where drive for the top tube is developed across the 180R resistor.

I'm not sure what advantage this circuit would have over either a conventional push/pull arrangement or even a single SRPP/SEPP with one side of the output transformer connected to ground. I guess you would have to build it and see how it sounds.

Good luck! I would be interested to know how you get on

[Tony]

hi,

my intention was to get max swing out of 4 tubes. the biggest andvantage for this arrangement is that the output transformer can be designed like that for the set but minus the airgap. and a capacitor in series with the output ensures no dc current that will cause decreased inductance will ever be present.

conventional push-pull output traffo have inherent unbalance due to nature of its construction, there is an inner and an outer winding whose dc resistances are not the same.

my intention is to get two diagonal tubes conducting at same phase.

i estimate an output of 9watts out of this, triode mode, classA.

[martinab2]

Don't know how well it would work in practice but... the output at the centre of each pair of tubes will be in reverse phase with the input to the grid of the lower tube. It might be possible to drive the right hand pair with the output of the left and avoid the need for a separate phase splitter..

[martinab2]

Lots of interesting stuff here The article is about preamp/driver circuits rather than power output stages but some of the same principles apply

this one is worth a look as well

[tubelab.com]

For the totem pole circuit to work right the drive for the upper tubes should be referenced to its cathode. Your circuit references the drive for the upper tubes to ground, which is at the same signal potential as the upper tube's plate. This causes the upper tubes to work like cathode followers, and the lower tubes to work like common cathode amplifiers. The gain and output impedance is unballanced between the upper and lower halves. There are amplifiers out there that do this, but they need a lot of feedback to work right.

I have been experimenting with this arangement, but using a driver transformer in the classic totem pole circuit. I have seen some circuits on the web that use active devices to reference the drive correctly, but I have not tried them.

The SRPP circuit (X2) with a phase splitter works OK also.

[Tony]

thanks fo the replies guys. i now have a better picture of what is to be done....