Hello forum friends
I'm confident that someone of You had previously stumble upon a similar gear
A friend of mine got this PA tube amplifier, that sports some unusual caracteristics.
It's a very powerful unit , intended use is to feed constant tension lines of 50, 75 and 100 volts, for sound reinforcement duty.
Now, we have had a listen to it through an adapter transformer , on a 8 ohm loudspeaker, and the sound was very interesting, the impressive bass behaviour in particular mode.
We do not have any circuit schematic , but I can describe it as follows:
Push pull output of two parallelled 6DQ6 driven through g2 (g1 is held to gnd)
Driver is a push pull single pair of 6BQ5's, whose output transformer secondary feeds the 6DQ6 screen grids
Input stage / phase inverter is by a 5AN8 pentode-triode, the pentode being the input stage, while the triode section do the split load inverter. NFB seems to be from the driver's transformer secondary, to the input stage cathode.
On the scope the unit puts out a clean sine wave of 100V rms on a 36 ohm dummy load.
Frecuency response starts to bend down at 34 and 17Khz
Now if someone out there knows of a circuit like this one, I'm talking the topology, not the exact tube arrangement. It can be interesting to drop an eye to the circuit, to better understanding. I already tried a search on g2 driven 6DQ6's, but wasn't able to find a similar unit.
Thanks
J.
I'm confident that someone of You had previously stumble upon a similar gear
A friend of mine got this PA tube amplifier, that sports some unusual caracteristics.
It's a very powerful unit , intended use is to feed constant tension lines of 50, 75 and 100 volts, for sound reinforcement duty.
Now, we have had a listen to it through an adapter transformer , on a 8 ohm loudspeaker, and the sound was very interesting, the impressive bass behaviour in particular mode.
We do not have any circuit schematic , but I can describe it as follows:
Push pull output of two parallelled 6DQ6 driven through g2 (g1 is held to gnd)
Driver is a push pull single pair of 6BQ5's, whose output transformer secondary feeds the 6DQ6 screen grids
Input stage / phase inverter is by a 5AN8 pentode-triode, the pentode being the input stage, while the triode section do the split load inverter. NFB seems to be from the driver's transformer secondary, to the input stage cathode.
On the scope the unit puts out a clean sine wave of 100V rms on a 36 ohm dummy load.
Frecuency response starts to bend down at 34 and 17Khz
Now if someone out there knows of a circuit like this one, I'm talking the topology, not the exact tube arrangement. It can be interesting to drop an eye to the circuit, to better understanding. I already tried a search on g2 driven 6DQ6's, but wasn't able to find a similar unit.
Thanks
J.
It will follow some pics, soon . Other details are a voltage doubler PSU in order to achieve something like 600V B+, the unit use fixed bias, (a very low 25mA for each 6BQ5 tube) and a circuit board placed over the 6DQ6 anode caps, that contains four short air cored coils, and a device to prevent sparks/arcing
(said "chispero" in argie tech service lingo)
(said "chispero" in argie tech service lingo)
Yep, screen drive with TV HD finals will give lotsawatts with good distortion performance. Not often you see it because of the complication of driving current into the screens. Today, we have MOSFETs that can get that job done easily, and with good sonic performance.
Not often you find a PA rig that also sounds good.
Not often you find a PA rig that also sounds good.
Yes, that surprised me
So, if I understand correctly, mosfet (source follower?) can replace the whole 6BQ5+ push pull driver transformer arrangement, for a fraction of the cost...
On the other hand, a diy all solid state 270 watt job would be, not only much more complex, but probably more expensive, and difficult to source components, draw and debug a good printed circuit, and so on
That's the idea. The reason you didn't often see Class *2 audio designs back in "the day" was due to the problem of current sourcing. This is why the PA rig described here is a power amp driving a power amp (PP 6BQ6s are good to 12W). For Class *2 you need current sourcing and a Lo-Z source.
It was more common to see Class *2 used for RF amps since there is no consideration for reducing harmonic distortion since RF amps use tuned LC circuits for plate loads that kill the gain at harmonic frequencies, and also bandpass filters, and antenna tuners to reject the garbage and keep it off the air and/or out of the signal chain.
It's on both counts where MOSFETs excel. The rd(on) of a MOSFET is an order or two in magnitude smaller than the rp of even the beefiest of audio triodes. The gm, even at low Q-point currents, is much higher, and that makes for the desirable Lo-Z source impedance. So a source follower has no problems slapping those grids around, not does it require an IST to get that source Z down.
Any 270W DiY job is going to be pri$ey regardless of implementation, starting with PTXs and ripple filters, and OPTs (for hollow state) or heat sinks (solid state). Good lay-outs and construction is equally important for either implementation.
Thanks Miles, very useful info. We'll explore the driver performance, to set the minimum specs for the mosfet follower replacement. Last time I forgot to measure the max unclipped swing at driver TX out . We'll scale down the O/S too, a single pair of 6DQ6 will be more than enough for any domestic audio task.
These Television Horizontal Output Tubes have lowish book value dissipation ratings (14 Watts) when used as Horizontal Output Tubes but were capable of nearly twice that in Class AB Audio Output.
Here is an article which came out of the Sydney Australia AWV Tube Factory where 6DQ6 were made. They say don't go over 25 Watts anode dissipation. The operating points shown are just as relevant for screen drive as normal g1 drive.
http://www.retrovox.com.au/6dq6aamp.pdf
Cheers,
Ian
Here is an article which came out of the Sydney Australia AWV Tube Factory where 6DQ6 were made. They say don't go over 25 Watts anode dissipation. The operating points shown are just as relevant for screen drive as normal g1 drive.
http://www.retrovox.com.au/6dq6aamp.pdf
Cheers,
Ian
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