Rerurn to Romy the Cat's Site

I made experiments running HF filaments from dedicated Hammond 6.3V transformer vs. lifting ground on my 6.3V IDH filaments. I did not listen Sound but rather measure the noise – the result was identical. In the best cases I was able to get 0.63mV at output – way bayonet of what I need to worry even with 109dB sensitivity. In practical term I prefer the lifting ground on my large 6.3V IDH filaments secondary. What I drive my HT filaments from a separate desiccated transformer then I need to be very accurate with grounds as I have 1.2K resistor between amps ground and the HD filaments. Any problem as the cathode of DH tube accept ground potential and plate current of the tube run away to 100s mA. I have a couple of accidents like this running accidently the Sovtech 2A3 at 100s mA. The good part that the large 20W 1.8K PS resistor act as a fuse … unsoldering itself from the circuit. To my surprised the immune to sound Sovtech 2A3 is still operation after of few stresses like that.

With the floating the secondary of my 6.3V IDH filaments I can do whatever I want and there is no damage to DH bias, or a need to isolate anything. Thinking further I realized that if I use a dedicated DH transformer of floating the secondary of my IDHT then in both cases I use my cathode resistor on DHT as biasing resistor for my heaters. It might not sound kosher but it is what it is in all cases. I have a dedicated better made filament transformer (bile like audio transformer) coming next week as well as filaments isolated (6.3V to 4V) transformer coming. I will see what they do in terms of Sound but I think I do not have a fear anymore to use my cathode resistor as bias for IDHT filaments.

There are however more complication then that. I made some fine adjustments in values and now I have both tubes 2A3 and YO186 runs 41mA . Here is the new draft of the circuit:

The operation is very stable and there is no noise. The amp produce OK sound (to my little control monitor, not to my main playback) and I have to tell you that the idea to have an amp with both tube sockets was brilliant idea. It is so cool that I might just name it and I have no idea why others do not do it.  The plate impedances of those tubes are very close and the amp might have as well the sockets for 300B/45 or AD1. Just shut down the amp, plug a new tube, adjust lowest amount of AC mV on the OPT secondary via the filaments attenuator and here we go…

Now is the problem. My DH DSET measures like crap. I did not go deep into it yet, will do obviously. I was running the sine wave trying to determine the LF cut off and making sure that the amp’s filter kicks in before both of transformers clipping will come to the picture. It was fine. I am running the Innerstage transformer for now as 2:1 and I was measuring with my scope with AC voltage that driver stage delegate to DHT’s grid. It was fine as well – it sits deep in A1. Something made me then to switch the generator into square wave and to see what amp outputs. Holy cow it is something absolutely not identifiable – at all frequencies with all amplitude. At 10K the amp shell do very good square wave but in my case it was a nightmare.  I do not know at this point what is ringing in there: the input filter (highly unlikely), the Innerstage transformer, the output transformer, the secondary of the Innerstage need additional damping, or something else. I know that until I have the acceptable square wave from my DSET then I would not even make any attempt to listen the thing. This is over 3.2kHz DSET it shell be handling square wave very fine throughout it’s whole range.  I need to blow each stage and each transformer with squares individually to find out what is the problem with this amp so far…

Rerurn to Romy the Cat's Site