Members Gone_Acoustic Posted November 3, 2009 Members Share Posted November 3, 2009 I recently picked up a vintage Kay model 504 amp - it seems to be a copy of that time's Vibro Champ. 5Y3 rectifier tube, 12ax7 preamp, 12ax7 tremolo, 6V6 power tube (Class A). All tubes are Sylvania. Tremolo isn't working, but I will be checking if the tube is bad and also trying to turn on the circuit via a shorted guitar cable inserted in the effect footswitch jack. Mine didn't come with a schematic - I would like to upgrade the speaker, but the required impedance is not stamped anywhere and from what I gather can't easily be measured. If you own this amp and/or have a schematic, can you help out? Thanks! Link to comment Share on other sites More sharing options...
Members Killa J Posted November 3, 2009 Members Share Posted November 3, 2009 It should be fairly easy to measure (at least approximately). Hook up a multimeter and measure resistance of the speaker. Resistance and impedance aren't exactly the same thing, but the reading should be close enough. So if it measures close to 8 ohms, it's an 8 ohm speaker; if it's closer to 16, then use a 16 ohm speaker. Link to comment Share on other sites More sharing options...
Members Gone_Acoustic Posted November 4, 2009 Author Members Share Posted November 4, 2009 well, i tried that... set my multimeter to ohms, 200 range (lowest, above that it's kOhms) and got a reading of 1.1 - not exactly meaningful. That was with speaker connected to OT, amp off (unplugged). I was told by an engineer (with experience in amp repair) that you can't really measure this with a multimeter. Link to comment Share on other sites More sharing options...
Members Gone_Acoustic Posted November 8, 2009 Author Members Share Posted November 8, 2009 Does anybody have a schematic for this model? Link to comment Share on other sites More sharing options...
Members guitarcapo Posted November 8, 2009 Members Share Posted November 8, 2009 Pull the speaker and measure it unconnected to the amp. Buy a speaker closest to the impedance measured. The tech is right that the number won't be exactly 4 or 8 or 16. It has something to do with impedance not being exactly the same thing as "resistance". FWIW every schematic for Kay amps that are listed in Aspen Pittman's "The Tube Amp Book" lists "8 ohms" for the speaker...but the 504 isn't included. A schematic for a model listed as "500" and looking a lot like a Champ uses an 8 ohm speaker. Link to comment Share on other sites More sharing options...
Members Mxr20 Posted November 8, 2009 Members Share Posted November 8, 2009 Does anybody have a schematic for this model? I looked for a schematic, no dice. What output tubes and how many speakers? Link to comment Share on other sites More sharing options...
Members Mxr20 Posted November 8, 2009 Members Share Posted November 8, 2009 FWIW every schematic for Kay amps that are listed in Aspen Pittman's "The Tube Amp Book" lists "8 ohms" for the speaker...but the 504 isn't included. A schematic for a model listed as "500" and looking a lot like a Champ uses an 8 ohm speaker. Yeah, I looked at the 505 that's dual 6v6 and has two, 4ohm speakers. So my guess is 8ohm too if its a single speaker model Link to comment Share on other sites More sharing options...
Members guitarcapo Posted November 8, 2009 Members Share Posted November 8, 2009 The amp won't blow up if the impedance is off a step anyway. Just might sound different. Link to comment Share on other sites More sharing options...
Members Gone_Acoustic Posted November 8, 2009 Author Members Share Posted November 8, 2009 well, i think there's a bit more to it than that, if I remember correctly, tubes can wear out faster, OT can overload, etc. I still need the schematic for other potential stuff, like to try and fix the tremolo circuit. The amp won't blow up if the impedance is off a step anyway. Just might sound different. Link to comment Share on other sites More sharing options...
Members old man attic bandfiredog1 Posted December 23, 2020 Members Share Posted December 23, 2020 Just got one 1 hr. ago w/ schematic inside. Will post soon. Link to comment Share on other sites More sharing options...
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