push pull 45

[jmillerdoc]

Anybody know of a push/pull design based on the 45 tube?
Thanks,
Jeff

[Mosquito]

Dr. Google doesn't give nothing about, but some time ago, while playing with the Broskie software, it comes out(if memory doesn't cheat) that a push pull of 45's, cathode biased, 275V B+ and around 34mA each tube, will give an optimistic 4W on a 8ohm load .Optimum AA load seemed to be 4K . Who knows? Certainly I'll never spend the money they ask for a quartet of 45's.

[jmillerdoc]

Guess I have to be the first to build it and post it in such a way you could Google it.
Jeff

[Mosquito]

Ooops! much more 2.5W than 4, I would say, and 41V peak on grids for full output. I can imagine that to remain coherent with the output tubes class, the chosen driver tube will be other than a mere noval double triode

[danFrank]

The correct P to P load for a pair of 45's is around 8K ohms, not 4K. 4K is better suited for 2A3 or 300B PP.
Daniel

[kevinkr]

I have an old budgetary design for PP 45 that makes about 5Wrms per channel with cathode bias. Works well with the same sorts of OPTs commonly found in old 6BQ5 amps. (7K - 10K PP)

Front end half of a 12AX7A per channel, half of a 12AU7A as split load phase inverter - the design originally used a single 7247 per channel, and the filaments of both were heated by the combined cathode current from the output stage - the downside of this is that the tubes had to be well matched as the cathode currents summed, a small additional current from the plate supply was summed with the cathode current in order to provide the 150mA/24V required for the driver tube filaments - there was obviously another series resistor to drop the balance of the bias voltage, and the whole thing was bypassed by a single good cathode bypass cap.. This approach was also used in the Scott 99 and worked well enough to inspire me to purloin the idea..

Fixed bias or independent cathode bias could also be used along with a conventional filament supply..

I can post the design if anyone is curious.. It dates from a time when 45s were a bit cheaper than they are now. It sounded reasonably good in fact..

[alexmoose]

Whats the draw behind using a 45? Why not opt for a 2A3? I'm not criticizing your decision to make a 45 amp, or anyones for that matter. I just want to know the motivation behind it.

[atmasphere]

I have a P-P 45 based amp that uses an OPT from a 6V6 based amplifier. It makes about 4.5 watts. I run it self-bias.

The driver is a simple 6SN7 circuit, using a single gain stage direct-coupled into the balanced driver. Very simple, no feedback.

For fun I run it with a balanced input. Pin 2 of the XLR is the input grid, pin 3 is the cathode via the cathode bypass cap, which is tied to pin 3 of the XLR rather than ground. If running single ended I put a jumper in the XLR that ties pin 3 to ground (pin 1). It works beautifully but you need a preamp that does not care about the low impedance of the grounded grid input.

[zobsky]

Why not use push pull 6V6 triode mode. Same amount of power as 45, and , if I recall, pretty much as linear. No DHT bragging rights though ...

[tubelab.com]

Why 45's? Because they are the best sounding tube ever made! I have a Tubelab SE with 45's running slightly over spec 320 volts and 30 mA into a 5K Electra Print OPT for a big 2 WPC. Best sounding amp I have, but not very loud on my 87db speakers. Sounds excellent and painfully loud when it went to visit some 106db Lowther based horns.

45's in push pull? No problem. How much power? Oh, only 20 WPC! 20 watts from P-P 45's? Yes and no extreme spec busting is involved, just the same 320 volt B+ used in the Tubelab SE. Idle current is 15 mA per tube, but full crank current is 150 mA for the P-P pair. I used a 3300 ohm load for this test but I think I will use a 3000 ohm pair of UTC's if I ever build the amp. Where did I come uo with this set of operating conditions? They come directly from the RCA data sheet! They claim 18 watts from a 275 volt B+. Yes, it is AB2. And no it doesn't sound like $*#@.

The test amp used the driver board developed in the 6L6GC in AB2 thread except for 45 output tubes and a lower B+ voltage.