coppatone GM70 /5

Time to get more concrete. Last time we ended with the general schematic and now there are values on everything:

I tried to write everything on the schematic, but nonetheless there are still some notes. Let’s start at the input and move to the right.

The input transformer is a 1:1 unit (you are free to experiment with other ratios to fit your needs). jc recommends lundahl ll1570 and, more luxurious, the ll7901, and so do I.

The input stage is convertible between 10Y and 205D. I aim for running either of them at 15mA, this means -30V grid bias for the 10Y and -34V for the 205D. Plate resistance is 5k for both when operated like this. Input stage biasing is done with batteries. Three ‘9V’ in series will get you very close to 30V (like 29.8), add four 1.5V AAs and you got an adjustable bias supply .

The second stage can use either 2A3 or 300B. I aim for running either of them at 25mA, the 300B needs a few volts more (negative) on the grid to dial in. For biasing this stage moving 5 Volts up or down goes a long way (like tens of mA). This is done with the 4k7 pot in the input stage. The DCR of the plate choke provides some of the bias (-9V) for the kathode follower, so watch out if you make big changes in this DCR.

Speaking of the kathode follower, the total DCR under it (choke + resistor) must add up to 1k, for the 6AV5 in triode (if you have not messed with the second stage choke).

The MOSFet biases the GM70. In this case moving 10 Volts up or down goes a long way (like tens of mA). The MOSFet has to be able to ‘swallow’ the 30mA peak grid current of the GM70, so I am running it at 40mA. Please no extra cap on the output (source) of the MOSfet bias supply, having this point cap-free is exactly the point: no time constant in this grid current path. The MOSFet dissipates around 7.6W, half of the 15 allowed (for the TO220 version). Better put it on a good heat sink.

A while ago we figured out how to ignite the VR tubes, reliably. That is what the two 330k resistors are for. But for the input/second stage things are a bit trickier, with two resistors feeding the stack of three VR tubes. The thing I wanted to know is: can this constellation lock up and burn the 2A3? No, the 8k resistor protects the 2A3 by rapidly reducing its plate voltage, when it wants to conduct (a lot) more current and the VR tubes  do not (yet) work as advertised. So no matter where the input stage puts the grid voltage of the 2A3 during startup and shutdown, it is protected. Also locking up cannot occur because that would put either a lot of volts over the VR150 tubes or the VR105 one, which would then strike and this quickly rectifies the matter.

Another step further. Next up: power supplies and the GM70 OPT.