Prew42
The objective is to build a safe and great sounding pre-war (WW2) guitar amp using as much technology from the day. This guitar amp will be based on a 1936 Aetna 19w66a tombstone radio. Only the loaded chassis was available. In another project, a complete Aetna radio was restored (https://stillampd.com/aetna-19a66w-tombstone-radio). The available chassis sported a great tube complement so it was thought that it might make for an excellent guitar amp. The design has evolved but in essence it will be a push pull amplifier using the original No42 pentode tubes. The Phase Inverter will use the original No75 single triode tube configured as a cathodyne phase inverter, The preamp will use the original 6D6 pentode. The original power transformer will be used along with the original No80 full wave rectifier tube. Most other parts of the radio are removed for use in other projects.
Prewar Tubeset
Surprisingly, the tubes available in the Aetna radio are quite similar to other tubes that are commonly used in guitar amplifiers which should make the circuit design much easier. The No42 pentode tube is very similar to the 6V6, the No75 cruise tube is similar to the 12AX7 (except for the dual diodes in the 75 and dual triodes in the 12AX7), and the 6D6 pentode is similar to the 6SJ7. Some of the numbers are noted below. In this application, the No75 the dual diodes are not planned for use.
A thorough tube comparison below.
* Assuming max plate current, but more likely around 90 in typical operation
Circuit Design
The Aetna radio usd the pair of 42 tubes in a parallel plate Single Ended (SE) design. It was initially thought to copy this same design so as to use the SE output transformer (OT) that was available in the radio. However, a single 42 tube wants to see a resistive load of 7000 ohms and a pair would need a load of 3500 ohms (half of a single 42). The measured impedance ratio for the OT was 1457 which meant that for a parallel plate 42 would need a speaker impedance of 2.4ohms. While this could be made with a couple of 4ohm speakers in parallel, it was decided to provide a bit more flexibility with a OT That had switchable speaker impedances. And since the OT was being replaced it was a great time to upgrade from single ended class A to push pull class AB. A company called Musical Power Supply makes a perfect OT that is 10k ohm primary (That two 42’s in push pull want to see) into a selectable 4/8/16ohm secondary, all at 18VA which is perfect for the 42 pair as they could produce about 19 watts.
Output Design
A great little prewar amp from the 1930’s was called the Volutone model 5. The power tube output stage will be based on this rather simple design. Shown in the schematic below. Two 42 tubes running push pull with a cathode biased approach. This design shows the screens running the same voltage as the plates. More information available at this site http://prewaramps.org/volutonemod.htm. However the Volutone 5 design has a reactive PI with the transformer center tap essentially tying the 42 grids to ground. Prew42 will have a resistive input to the 42 tubes and so it will need grid leak resistors tying the grids to ground. It is also decided to add grid stops in the style of the 5E3 (Fender Deluxe)
Phase Inverter Design
Typically Phase inverters use two triodes in long tail designs but finding a dual triode was difficult, the 6A6 was a possibility but it required a lot more heater and plate current that what might be available from the existing power transformer. Since there already was a single triode No75 tube available it was considered to design a cathodyne phase inverter that used a single triode. http://www.valvewizard.co.uk/cathodyne.html
The Fender AA1164 Blackface Princeton uses a 12AX7 as a Cathodyne that drives a pair of 6V6’s. Since the 75 and the 12AX7 are very similar (as well as the 42 and 6V6) it was considered to use the Cathodyne from the AA1164. This is shown in the schematic below. More information and discussion available at this site https://robrobinette.com/AA1164_Princeton_Reverb.htm
Second Channel
The radio chassis planned for use has an extra 6A7 tube that wasn't planned in the guitar amp. https://frank.pocnet.net/sheets/127/6/6A7.pdf
Since the 6A7 is a Heptode (7 element) or Pentagrid (5 grid) tube it would take some research to determine if this could be designed as a preamp tube. It was thought that maybe the 6A7 could be wired as a pentode. Often
Preamp Design
See this excellent writeup of using pentode as preamp stage http://www.valvewizard.co.uk/pentode.html
See post #6 at this like for 6D6 used as preamp https://el34world.com/Forum/index.php?topic=24476.msg263210#msg263210
Power Supply
The power supply will use the existing Aetna power transformer (PT) and full wave rectifier tube #80.
Layout
The layout of the amp follows the radio in that the existing tube sockets will be used as well as the power transformer. Since a new output transformer will be used, it will be mounted inside the chassis rather than on top since it is new and would contrast with the existing patina.
Lots of room after old radio components removed
The radio band selector switch will be removed and a guitar input jack installed in its place. The speaker jack will be mounted on the rear of the chassis along with the AC receptacle.
AC wired. All original heater wiring will be retained.
Power supply filter fabricated and installed. HV DC applied to preamp and phase inverter plates. Original radio cathode to chassis ground connections removed in preparation for cathode bias scheme of the guitar amp.
References
