Since the 6P7S SE gave way to the
6P7S-i SE Amplifier
the chassis of the former (lacking output transformers) was banned to the cellar.
6 years passed until recently I was chased by the DIY fever again and changed the output transformers of the Aikido driven 2A3 SE amplfier against new ones (but that is another story). I thought it would be a good idea to revive the original 6P7S amp using the TAMURA OTs. This is what it looks like now fitted with KT66:
The octal socket was rewired for 6L6 family tubes and the like. The output tube can be one out
of many 6L6 variants. I tried russian military NOS 6P3S, 6L6GA and 6L6GC from Sovtek, 6L6GC-STR
from TAD (a remake of the Philips/Sylvania tube) and a JJ KT66. Yes, even KT66. The power transformer
can easily manage the additional heater current required. The CCS assures the same bias current for
all variations and the cathode voltage settles somewhere between -20V and -21V.
The connection to the OTs is made in ultra linear mode and the capacitance in the power supply was increased. Plate voltage is about 258 V (to ground) and the current is set to 45mA. It was of course an experiment, a crazy idea to put a KT66 or even a 6L6 to work under conditions traditionally more fit for an EL84. And surprise surprise, the TAMURA OTs provide an extra winding for NFB and soon I catched myself considering what a bit of global feedback would do (huhh). After measuring the open loop gain, the feedback resitor was calculated targeting a minute 3.5 dB of feedback.