Advice on Building a SE guitar amp from radio

Discussion in 'Amp Tech Center' started by rosa, May 29, 2020.

  1. rosa

    rosa TDPRI Member

    Jun 10, 2011
    washington dc
    I was given a Radio with some 9 pin tube sockets. The power transformer on the secondary is giving 323-0-323Vac and 7v for use with an ez80 rectifier and el84 output tube. The output transformer is 7000ohm primary for 8ohm, or 3500 for 4. The speaker in then radio has dc resistance of 4.5ohms, so im thinking it is a 6ohm speaker.

    Assuming i can use an EZ81 to rectify, can someone with more experience tell me if this is a good scenario for a single el84 output with a 12ax7 as a preamp/driver? Would I be better off using a 4ohm or 8ohm speaker?

    With this limited information is it possible to suggest values for power supply voltage drop resistors?

    Id like to have one +-300 ohm resistor and a 50uf filter capacitor before B+1, and then 22uf, 22uf and 10uf.

  2. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

    Dec 30, 2019
    Do you know the specs of the PT?
    When the old radio/tape recorder PTs are not identified, I will only tax them with what was used in the original circuit. I will count up the tubes and the current draw. On the 7vac line you are suggesting using a EZ81 which draws .4A more than a EZ80. Can you get away with that?

    A 50uF is fine as a first filter cap with either EZ80/81 rectifier.

    Seems like the PT is similar to f51 and f52 type designs so similar dropping resistors/filtering can be used. 22uf down to 8uF caps are typical.

    Find a R/C calculator on the internet and plug in the resistor/cap values to determine what values you would like to use.
    Last edited: May 29, 2020
    Wally likes this.
  3. Jon Snell

    Jon Snell Tele-Meister

    Aug 31, 2015
    Jurassic Coast. UK.
    A single EL84 will not draw enough current to get anywhere near maximum draw on an EZ80 so there is no point in over loading the heater winding on the mains transformer.
    The voltage drop is similar to the 81.
    Older loudspeaker impedances were 3.75 Ohm, 7.5 Ohm and 15 Ohm. Now 4 Ohm, 8 Ohm and 16 Ohm. There has never been, until more recently the Sony and Yamaha people wanted them, been a 6 Ohm speaker.
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