My first amp!

[bigwill]

Hello everyone. I am in the process of building my first valve amp. Attached is the circuit I have designed and I am building.

So far I have built the driver stage for one channel which seems to work (testing with temporary low current 300v supply) but currently has a 1.5k resistor instead of the CCS in the phase splitter and this seems to have caused a huge gain imbalance on each side. Touching the input gives me 108vrms of hum pick-up on one side and 45 on the other! The tubes are pulling roughly the same though. Hopefully adding the CCS will fix the imbalance issue.


I realise 400v is a bit on the high side for this kind of amp but unfortunately it is in the area of the secondary voltages on the transformers I have.

Pictures and listening impressions will follow soon. Tomorrow I shall build the power supply!

[Eli Duttman]

Will,

Change the topology slightly to use DC coupling between voltage gain tube and the splitter. Look here. I think part of the reason you get different hum level pickup lies in the splitter grids being at different potentials. The non-inverting triode's grid is "flapping in the breeze". FWIW, the customary value for R2 in the linked schematic is 1 MOhm. The amount of capacitance needed for C3 need not be particularly large. So, a high quality film and foil or, perhaps, a Soviet surplus PIO part is in order.

You can bring the B+ rail voltage down by using pseudo-choke I/P (cLC) filtration. An appreciable inductance, say 10 H., is appropriate. You tweak the value of the "fudge factor" cap., with the PSU loaded down, until the voltage is where you want it to be.