A while ago I found some peculiar tubes that I bought for a decent price.
These E140 tubes, like the E60M, where manufactured by SFR and it was a reason for me to buy some.
They remained a while on a shelf till I decided to find some useful data's. Searching the web brought me very little information, not a surprise (all SFR production was intended for military use and records or data's are really unobtainable). But little was enough to catch my interest. I found that this tube was similar to a Philips transmitting triode, the TC04/10, for which I found data sheets.
I spent a lot of time plotting points to make a decent Excel Ia_Va/Vg graph that I could compare to the Philips data's.
The tubes are the same (almost) and at first sight not ideal candidates for audio use. High µ, high ρ, 10W power handling. I was a bit frustrated.
However these kind of transmitting triodes reminded me the mail I had with the late Nobu Shishido (I bought his book but did not understand a single japanese writing and he has been very helpful in translating some parts). In the last card I received from him he explained me how to use a line out transformer to drive tubes in A2 mode with low impedance and grid current.
At that time I was just wondering about the benefits using tubes with grid current while avoiding large NFB amount to temper HF ringing and keep good overall bandwidth. Moreover in the 90' I still could find plenty of good tubes I could use A1. It was simply not my way thinking HiFi.
I was wrong.
I have today a more open mind and I decided to give a try to Nobu's approach. It was challenging as I never used tubes that way. I was in "Terra incognita". I took some time to read more about A2 modulation and decided to start this project whatever the result.
A careful look at the E140 _ TC04/10 curves shows up the very good linearity with or without current grid.
A few maths later I realized that this project was feasible and I ordered parts to build an amplifier.
In this peculiar amp grid current do not flow all the time (unlike Shishido's amp) and I cannot take advantage of permanent current cancellation.
The E140 is biased such a way that the amp works A2 for a small portion of grid swing, then switch to A2/A1.
Many would think that this abrupt change will impart some sonic alteration.
Believe me it's not the case. I have breadboarded one unit and made a try with my cumbersome work, this amp sings very, very well even with the cheap parts I had on hand!
Encouraged by the results I am now waiting for high quality Hashimoto irons.
A small batch of triodes is a good idea when initiating such a project...
And to sort tubes to get matched pairs whenever possible is a good one too.
Next step, driver requirements. Stay tuned.