GM70 Lower Voltage PP Question

[Mush]

Hi Everyone:
I have just inherited a few Webster WSA-230 monoblocks and am thinking of converting them to use the GM70 in PP. I happened to notice that Sakuma had run the 845 at unusually low voltages (http://cognitivevent.com/images/saku...eaned_1993.gif) and that another builder named Nick has a design for a lowered voltage GM70 se amp. (http://www.diyparadise.com/nikcgm70.html ).

These exampleshave made me consider using transmitter tubes such as the GM70 and 845 in the Websters at 650 vdc.
I have searched the diyaudio.co posts and noticed that a gentleman nmed Brett has made a two stage 12hg7-GM70 se amp. I would like to recreate something like that in PP form. I am curious if using a cathodyne splitter would be a good idea in front the 12hg7, either pentode like Pete Millett’s 813 driver or triode driver with a resistive load. Basically all it would be is a splitter placed before a mirror-imaged version of a circuit similar to this (http://www.diyparadise.com/nikcgm70.html) to act as a push pull amp.

The Webster WSA-230 monoblocks used a 7199 feeding a quad of 8417 to produce over 200 watts. The 8417 run about 650vdc on the plates. My Phillips data sheet states that the maximum signal plate current should be around 290 ma per pair producing 100 watts at 650vdc with a 4.2k plate-to-plate load. The quad should idle at around 250ma and operate at full power at around 600ma. The output transformer has at least seven 6 ohm taps that can permit a wide variety of loads. The power transformer has secondary windings of 464vac, 78 vac and 129 vac. I imagine that the larger secondary is in excess of 700ma.

Is it stupid to try converting the Webster’s to run either GM70 or 845 in push pull? Does anyone have any suggestions on a circuit? I don’t care about lowered power output from operating at 650 vdc. Actually I’m sort of cheap and the low cost of the GM70s on Ebay combined with lower voltages is very attractive to me. I’m terrified of anything operating at around a kilovolt.

I thank you all for your patience and help.

Ciao and be well -- Minto

[adam2a3]

Hello, I am designing a GM70 PP that will be driven directly through a 500 Ohm /10 K + 10K surplus interstage transformer by UTC, able to pass up to 4 Watts of power, provided that there will be no unbalanced dc. So this will do the phase reversal without a tube. I have seen Lundhall has similar transformers available. As for driving this, Philips in the 70's had designed a 2 tube 1 stage final amp for their radios, using the specialy built EL86 pentode. This amp was actually a simple SRPP stage, having a transformerless (OTL) output around 600 Ohms. Philips built special speakers with 600 Ohms voice coil in order to eliminate the use of output transformers. So this amp I think will do the job of driving the GM70's. The first stage can be any tube, like 27, 76, 6SN7, but I will start without a tube, using a 600 Ohms/50 K input transformer that I have.
Regarding GM70 biasing I will not use the standard cathode resistors/electrolytics, nor an adjustable bias supply. There is a circuit using one zener, one small resistor and a pnp darlington transistor that replaces the above. No electrolytics, no heated resistor! Filaments will be powered by a toroidal having 4 windings of 10-0-10 V @3A. I hope hum will not be audible.
This is the basic idea, regarding power, my Single Ended GM70 @1050 Volts gives 24 Watts per ch, at 800V it gave approx 13 Watts. So your low voltage PP will be ok.

[Shoog]

Originally Posted by adam2a3

Hello, I am designing a GM70 PP that will be driven directly through a 500 Ohm /10 K + 10K surplus interstage transformer by UTC, able to pass up to 4 Watts of power, provided that there will be no unbalanced dc. So this will do the phase reversal without a tube. I have seen Lundhall has similar transformers available. As for driving this, Philips in the 70's had designed a 2 tube 1 stage final amp for their radios, using the specialy built EL86 pentode. This amp was actually a simple SRPP stage, having a transformerless (OTL) output around 600 Ohms. Philips built special speakers with 600 Ohms voice coil in order to eliminate the use of output transformers. So this amp I think will do the job of driving the GM70's. The first stage can be any tube, like 27, 76, 6SN7, but I will start without a tube, using a 600 Ohms/50 K input transformer that I have.
Regarding GM70 biasing I will not use the standard cathode resistors/electrolytics, nor an adjustable bias supply. There is a circuit using one zener, one small resistor and a pnp darlington transistor that replaces the above. No electrolytics, no heated resistor! Filaments will be powered by a toroidal having 4 windings of 10-0-10 V @3A. I hope hum will not be audible.
This is the basic idea, regarding power, my Single Ended GM70 @1050 Volts gives 24 Watts per ch, at 800V it gave approx 13 Watts. So your low voltage PP will be ok.

Are you certain that such a step up will be possible using interstage transformers- especially old ones. Miller capacitance
will be amplified by the turns ratio, and there is likely to be considerable interwinding capacitance. Have you done any experiments ?
My experience suggests that a little old EL86 just wont have the balls to drive it.

Shoog

[j.michael droke]

Originally Posted by adam2a3

Hello, ... Philips in the 70's had designed a 2 tube 1 stage final amp for their radios, using the specialy built EL86 pentode. This amp was actually a simple SRPP stage, having a transformerless (OTL) output around 600 Ohms. Philips built special speakers with 600 Ohms voice coil in order to eliminate the use of output transformers. ...

Hi there: Does anyone have information on the Philips OTL power amplifier? ( or other Tube OTL power amplifiers )...regards, Michael