Tin Man, a 45B5 SE Stereo Amplifier

(Bread Pan Amplifier)

Front View

The 45B5 SE amplifier was a fun project inspired by the homebrew golden age habit of building on bread pans and 'bread boards' with the "Tin Man" name derived from old bread pans being commonly made of tin. But, in keeping with the tradition of using whatever was available, the modern bread pan here is teflon coated "who knows what" and the 'bread board' was a spare plank of "who knows what."

The design is reminiscent of a typical 1960's line powered 'phonograph stereo' using 45B5 power pentodes and a 20EZ7 dual triode, which is a 20V heater version of the 12AX7. As it turns out, and despite the completely different numbering, the 45B5 is a 45V heater version of the 6CW5 (used in the 6CW5 guitar amplifier). The design is modernized with a safety 1-1 isolation transformer and DC powered heaters. Phono cartridge gain is exchanged for negative feedback to improve performance. Input is, then, standard consumer line level with output power 3+ Watts per channel, putting this amplifier in the 'PC Speaker' class.

 

Schematic

There is nothing particularly unusual in this typical Single Ended (SE) design. Input stage gain is maximized by C3 bypassing R10 with R6 into R3 providing negative feedback. The dashed lines indicate the 22.1k and 33uF preamp filter is part of the power supply and common to both left and right channels. Not shown is the input jack consisting of two jacks in parallel. This allows both the PC sound card and a set of solid state PC Speakers to be simultaneously plugged in so the cheap SS speakers can be turned on and used for Windows 'blip and beep' sounds to conserve tube life when not playing music.

rear

The dual input jacks can be seen here under the speaker output RCA jacks. The bolt head is chassis ground

PS Schematic

As previously mentioned, the entire amplifier is line isolated by T1, a Triad N-68X 50 Watt 1-1 isolation transformer. Screen power comes from a MOSFET filter simulating a choke. Note that the MOSFET chosen includes on die diode protection so no external protection is needed but would be recommended with other types. Preamp power is then further filtered by R6 and C5.

DC for the series heater string comes from R2 into C3. Note that D4 isolates the higher amplifier DC on C1 from back feeding into R2; a necessary feature to reduce heater power and, hence, total power through the isolation transformer.

 

Inside

Inside Tin Man

The inside wall mounted OPTs come from Radiodaze. The perfboard holds the power supply rectifier, input filter, and heater DC generation. The remaining point to point wiring components use terminal strips secured by the socket mounting screws, plus two extra around the 20EZ7.

 

 

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