Dad’s PA – Description
♦ History ♦ Description ♦ Schematics ♦ Performance ♦ Pictures ♦
Design
No numbers yet, as it has not been powered-up (nor will it be in this condition).
Input Stage
The input stage is 1/2 of a 6SN7GTB (section 2) twin triode, self biased at about -?V for ?mA of plate current. The plate voltage is about ?V and is directly coupled to the phase-splitter.
Phase Splitter
Like my Thomas PA, this amplifier uses the split load (or “concertina”) phase splitter using the other half of the above mentioned 6SN7GTB twin triode. The grid voltage is set to around ?V, resulting in about ?mA of plate current. The concertina, while well balanced when unloaded, has rather poor output impedance matching when faced with a difficult load to drive. In this case, it does not have to drive to output tubes directly because it is capacitively coupled to a driver stage via a pair of 250nF Sprague film capacitors of unknown composition.
Driver Stage
The driver stage uses both halves of a 6BL7GTA twin triode, one for each phase. They are self biased at about -?V for ?mA of plate current. The plate voltage is about ?V and is capacitively coupled to the output stage via a pair of 470nF film capacitors of unknown composition.
Output Stage
The output stage uses a pair of RCA 8417s with a fixed bias of about -15V(?). This results in a plate current of about 124mA when 560V of plate voltage is present. The 8417 has a reputation for instability when configured for fixed bias, which may explain the rather elaborate bias circuitry. They also have a problem with the screen overheating (particularly the GE version) and arcing over, which is why it has to be set to a much lower voltage than the plate (300V).
At this time, the 8417 tube is largely extinct. I have seen pairs of RCA tubes (made by Sylvania) selling for as much as $200. I have seen the GE version for about $50 each. The price hasn’t changed much in the past few years. Most people probably have either dumped the equipment or converted them for use with 6550s or KT88s. The 8417 was one of the last designs of the tube era. They were only made for a short time before RCA and GE stopped making tubes altogether. This would be a great tube for a company like New Sensor to bring back into production.
Power Supply
Tracing…
Original Configuration
Needs work.
Modifications
None yet.
