London Power's "PA"-series Tube Power Amplifier Kits

[makinrose]

Have you tried one of these? Post your questions or comments!


These octal, push-pull power amp kits use London Power's full-audio-bandwidth OT-10, OT-25 or OT-50 toroidal output transformers. Performance is so good they can be used for hi-fi. The power amp can be used with any of the common octal-based output tubes that use the standard pin-out. Tubes can be mixed as well!

Switching between fixed-bias and cathode-bias can be "global" (both tubes) or independent.

Lots more info at:
10W Power Amp Kit (PA66-10)
25W Power Amp Kit (PA66-25)
50W Power Amp (PA66-50)

Hi Maikonrose

That's a good question

Frankly, I cannot find it, although I did discuss it on the PowerScaling forum back when that was active. Gingertube then took it up in his amps and reported about it on other forums.

Basically, the resistors to ground from each end of the OT are a benign form of energy limiting and absorption in the case of a discontinuous load. The resistors also help stabilise the amp against oscillation by working against parasitic capacitance and inductance.

The modern use of reverse-wired diodes across the tubes is a bit brutal. Two 1N4007s provide 2kV of reverse voltage before the diodes themselves are damaged, but it is the diodes on the opposite side that perform the protection when they are forward biased and clamp the negative peak to about a volt or so below ground, in turn clamping the reversed-biased side to a volt or so above 2 x Va. This seems a little improbable because the conducting tube cannot pull its plate any closer to ground (cathode) than the limits of Vsat dictate even at zero current.

Of course, in the above scenario if the tubes stop conducting because the reflected load has risen to infinity (theoretical but impossible), then the transformer voltage can rise similarly. The diodes shunt the energy to ground.

Using the resistors instead of diodes provides a continuous path for control of flyback energy, with no discontinuous or abrupt conduction events, so no impact on performance of the amp.

It goes without saying that any tube amp designed without a "minimum load" tied to its secondary is poorly designed. So I won't say it All the TUT 3&5 project amps have such a resistor in place. This is much better in my view than having a dead short across the secondary ala Fender and their clones for when the load is disconnected. In Fender's case, the disconnection has to be at the amp end of the speaker cable for the scheme to offer any facade of protection - but a short reflects as a short and the tubes are strained even if the OT is protected. It's a choice.

I don't have one these kits so I also don't have a schematic but from what I can see the 150K flameproof resistors are tied in series from the plates (pin 3) to pin 1 which connects to the bias monitoring resistor and then to  the ground? 

In speaking to a Mesa Boogie Tech one time trying to determine the type of diodes on wired as you described (one set had blown) he mentioned that at high volumes the diode had some effect on the tone of the amp.  Do you find the resistors have any effect on the tone?  

On the secondary the 270 ohm 5W like in TUT it sure seems like just a few cents save a lot of potential problems.