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Old 23rd November 2014, 11:06 PM   #1
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Default Marshall 1974X

I got this 18 watt combo that I am obviously having a hard time understanding. It is my buddies and he dropped it off and asked me to look at it because it doesn't sound right lately. First things is that it doesn't put out rated output power, so I figured I would go through it with a fine tooth comb for him because this is his favorite amp. He only uses the tremolo channel so I started there, I am measuring a gain of 34 for the first voltage gain stage The amp is wired correctly according to the schematic, the Vpp is 300 and the Va is 110. I replaced the bypass capacitor with a new one and it still has low gain

I calculated that the gain should be around 77 for this circuit, is that correct?

100 * 220000 / (220000+62500) = 77

I am using 220k for the load, which would be the volume pot in parallel with the 470k resistor to ground......maybe this is my error?

-bird
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File Type: pdf 18watt_Schematic_08-28-03.pdf (216.8 KB, 81 views)

Last edited by famousmockingbird; 23rd November 2014 at 11:57 PM.
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Old 24th November 2014, 05:27 AM   #2
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I simulated the circuit and it matches up. Must have had the load calculated wrong.
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Old 24th November 2014, 10:59 PM   #3
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Swap V2 and V3 - any change?
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Old 25th November 2014, 12:42 AM   #4
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Quote:
Originally Posted by djgibson51 View Post
Swap V2 and V3 - any change?

Good idea, and I was thinking it was a weak tube issue but I tested the tubes in my Hickok 539b and they tested good. I haven't had the time to look at it again but when I do I will probably try all known good valves from my stash just to be sure.
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Old 25th November 2014, 12:51 PM   #5
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sorry if this is alittle off topic after looking at the linked schematic can you clear me up on how the HI & LO tremelo input jacks provide a gain difference to the input of V3a?
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Old 25th November 2014, 06:47 PM   #6
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Originally Posted by turk 182 View Post
sorry if this is alittle off topic after looking at the linked schematic can you clear me up on how the HI & LO tremelo input jacks provide a gain difference to the input of V3a?

By loading the high output impedance of the guitars passive pickups, "Hi" has a 1 Meg grid leak resistor and the "lo" uses the "Hi" channels 68k grid stop resistor as it's grid leak resistor. The gain in the channel doesn't change but the gain of the passive pickups does.
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Old 25th November 2014, 07:32 PM   #7
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ok thanks (brain fart moment) i was overlooking the fact that one end of R28 see's ground through the switched contacts in the HI jack
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Old 26th November 2014, 12:34 AM   #8
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I got it narrowed down to something in the phase inverter, one signal going to a power tube has a less amplitude and the waveform looks distorted, everything before the PI is fine.
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Old 26th November 2014, 06:49 AM   #9
Tesla88 is offline Tesla88  Italy
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Test V2 or swap V1 and V2 and measure both outputs from PI. If still unbalanced and distorted try removing EL84s and see if problem goes away, other try swap EL84 and see if problem moves from side to side...
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Old 26th November 2014, 10:41 AM   #10
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leaky coupling cap perhaps?
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