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Old 16th November 2012, 05:51 PM   #1
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Default input stage problem help please!

hello, nubie here!
i restore jukeboxes and usually have no problems rebuilding the amps, but this one has me stumped...i have attatched the schematic. basically the spec is 400mv for max output...[ceramic cartridge] but if i feed it anything over 100mv from my genny, i get severe clipping of the bottom half of the trace at the cathode of the follower [2nd section 12ax7] the input at the 2nd stage grid is clean.
i have never seen this tone control 1st tube layout before and am a bit baffled as to how it workes especially with the 13.5 volt from the power supply into the tone control at 6 on the schematic. all the HT voltages and the 15 volts on the cathode follower are spot on. the plate is 200v.
also the bass lift and cut is very slight although the treble is quite aggresive.
any help will be very appriciated as i have spent days trying to sort this!
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Old 16th November 2012, 06:15 PM   #2
kevinkr is offline kevinkr  United States
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Figure out or measure the source capacitance of the ceramic cartridge normally used with this amplifier and connect a capacitor of that value in series with the generator.

The capacitive reactance of the cartridge is part of the gain setting mechanism in this feedback amplifier. You are over driving the input circuitry, at best there should just be a couple of volts RMS at the output of the cathode follower. The power amplifier requires no more than a few hundred mVrms for full output.

BU2 incidentally sets the maximum volume of the system, by allowing you to jump out a pair of resistors in the attenuator circuit between the cathode follower and the input of the power stage.

The cathode follower is fixed biased, 15V is sufficient headroom given the low levels required for full output. (All of the attenuation following the CF hints at that.)
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Old 16th November 2012, 07:22 PM   #3
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thanks for the quick reply, i have found another schematic, it lists sensitivity at 530 mv@1khz for max output....i dont know the capacitance of the cartridge, but it is the correct elac KST 103 that the jukebox was made with. the problem is , apart from clipping with the genny as i said, it sounds distorted [on both channels] with a record playing. and the volume control at socket BU2 only needs to be up a fraction for it to really loud...infact it bleads through quite loudly with the volume off...the volume control was missing, it lists it at 20k log, the original was a tapped pot with a 560 ohm and 0.5 uf cap wired to ground from the tap, usually fitted to jukeboxes for munchousen effect correction. thse are impossible to find so i fitted a standard dual pot. this is not usually a problem on other jukeboxes. as can usually be compensated with the tone controls.
its as though, that 1st stage amplification is way too much, about 5v swing on the cathode folower grid , but it was designed to work, so something must be wrong, but its beating me what!
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Old 16th November 2012, 08:44 PM   #4
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clipping on both channels ? check parts / connections shared between channels ...

gain of the input tube is established thru negative feedback by a formula which basically is: gain= (R1+impedance(C1))/impedance(cartridge). Since the cartridge is a crystal pickup its impedance is mainly a capacitance. This is also known as a <charge amplifier> where the output voltage is a function of the input charge rather than the voltage: v(out)=-q(in)/C(feedback). If you just hooked up a signal generator with low impedance like near zero or 50 ohms the gain will be huge and depend on frequency. Connect 100pF or so in series with the input.

if only one channel distorts, check the R1/C1 feedback path components and interconnects.

If the pickup is old, it might just be gone. These crystals were made of potassium sodium tartrate (Seignette-Salz) and tend to decay over time in the presence of moisture and humidity.
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Old 16th November 2012, 11:33 PM   #5
DF96 is offline DF96  England
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The tone control stage appears to be something like a switched Sallen-Key filter, but combining high pass and low pass. It might give negative clipping if the standing current through the cathode resistor is too low for some reason, as the resistor obviously cannot go below zero current. However, you have measured 15V at the cathode so that can't be the problem.
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