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Fender "75" mods

5/9/2004 7:37 PM
Kevin STP
Fender "75" mods
Well I just finished installing SoZO and silver mica caps throughout this amp,and to top it off I went through and trimmed all the excess wire length from grids and such, must taken at least a foot of wire out.Man it sounds great what a clear and punchy sound even the lead channel is nice and crunchy without being harsh.I set it up a while ago with EH EL34 power tubes-I have 515 vts on the plates and biased at 45mv using 1 ohm resistor method .I have been informed that is a bit high but on the low power setting it starts to breakup nicely at about 5 on the master volume.Any suggestion as to wether this is ok?
5/10/2004 2:49 AM
Chris ( CMW amps )

wayyyy too high ....... if you do use two 1 ohm resistors for measuring  
power-dis. at idle is 23.175 Watts and that's almost max. for an EL34 , try biasing below 17.5 Watts . Voltage will probably rise but if it's still 515 Volts try 0.033 across one 1 ohm resistor  
hope this helps ,  
5/11/2004 11:37 AM
TheElectricMoron Got a quick ? for you, Kevin.
If you still have that thing on the bench could you check the full power voltage on V3b grid? I'd be curious what is happening there in another amp.  
If you you might do that next time you have the chance?  
It seemed to me that the low power mode didn't make it much less loud. If you run it on that mode all the time you may want to consider biasing for that voltage and just check to see if it smokes the tubes on high power.  
many thanks  
5/11/2004 4:43 PM
Kevin STP

Charlie; I will check voltage and get back to you tonite,I'm in the middle of restoring an ES125T for a friend. Kevin
5/11/2004 8:21 PM
Kevin STP

Hey Charlie, I just checked the V3b grid--High power is 57.3--Low power is 56.8. I have 494v on the power tube plates and with the bias at 45mv there is no sign of glowing plates.I am still trying to figure out where to put an adjustable bias pot and keep the OT matching pot,would it be at the resistor after the diode? It connects to the brown wire from the PT. or at the OTM pot resistor that goes to ground.My amp with the main 8 and the master 5 the low power setting is louder than the high power setting and just starting to breakup any higher setting than thatI almost never get to use because it makes my wife nuts and thing start to fall of the wall,but it seems at higher settings high power just sounds more solid and clean not really louder.  
5/12/2004 10:39 AM
TheElectricMoron Baffles me!!
I simply cannot understand why the grid on V3b for me is twice what it is supposed to be. I mean, we're talking three wires and three resistors - WTF can go wrong there? AND those are now all new wires and resistors (and blocking cap too FWIW). AND prolly half a dozen different tubes.  
Check my update for news and thanks for the info - I appreciate your trouble to get me the reading.  
5/12/2004 4:33 PM
Kevin STP

I guess if it were me I would trace the supply to its point of origin and see where the problem starts.Either the voltage is high at the source or more voltage is leaking in from somewhere,unless you have somehow tapped into the DARK MATTER.  

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