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Hello Everyone,
I apologize that my first post is a plea for help, but I really need it and a lot of my Google searches led me here, so I thought I'd ask the experts. I'm green on a lot of this stuff, but not brand new. I've run into a problem I can't figure out!!
The Problem:
It passes signal, it works, but not correctly and sounds like ass. It has ratty distortion at all volumes. It functions at very low volumes but even then has that terrible distortion to it. If I pick hard it cuts out but then will come back if I pick softly. The amp won’t get loud at all; it's VERY GATED sounding - similar to how an Octavia sounds (without the octave, there's no octave to the note). It sounds like I’m playing through a broken speaker, but worse. One thing of note is that if I strum a chord with the amp on standby and then turn the amp fully on, the sound is loud, big and wonderful for about 2 seconds and then decays into what I describe above, and stays there. I'm using a 12AY7 in V1 so it's not a 12AX7 causing it to distort early. I'm somewhat familiar with this type of amp (owned several Victorias of like model) and how the bass and treble controls can cause overdrive above noon on the dial, and it's not that type distortion. The distortion I'm hearing is unpleasant, muffled, ugly distortion even at the lowest of volumes. I kind of thought it might be leaky coupling caps, but I have no idea at this point.
Things I have tried: the speakers are good; they are in another functioning amp. I don’t think it’s the tubes; I’ve tried several different sets with the same result. I don’t think it’s the filtering/filter caps; I added more filtering just to check and it sounded the same (I removed the added filtering after checking). It’s not reversed output transformer leads that caused oscillation, I removed the negative feedback and have done all my testing with it disabled – same result. I've checked and rechecked the under board wiring, my connections to ground, the component values, etc. I'm not afraid to admit that it could still be any of these things. I'm just trying to lay out where I've looked so far.
Amp info:
The amp is basically a 5E7 with a bit larger transformers: Power: Magnetic Components Classictone 18073, 325-0-325, 650V centertap, 50V bias, 5V@3amp, 6.3V@5amp. Output: Allen Amps TO40MT, 4000 ohm primary to 4 or 8ohm. I wired the power tube sockets like later Fenders with a 1.5k from pin 1 to pin 5, and a 470ohm from pin 4 to pin 6. The NFB is connected to a Presence pot that has a switch when turned all the way down to defeat the NFB. I also moved the output transformer centertap upstream to the other side of the choke (Allen Amps TRCH 125C1A style, 3H, 100 ohms) between the plate and the screens; it’s connected to the bottom of the 1st filter cap.
Here are my voltage readings and such. All measurements are DC unless otherwise noted, all measurements take with amp on/tubes in/no signal/knobs at 0, all filament heaters read 2.9 to 3.0VAC, the voltage at the wall is 122VAC, GE 5U4GB rectifier.. The power transformer seems to be putting out more voltage than advertised. I don't know if that matters with the problem though.
V1
pin1 152
pin3 2.44
pin6 159
pin8 2.44
V2
pin1 209
pin2 1.54
pin6 319
pin7 209
pin8 212
V3
pin1 243
pin3 1.72
pin6 295
pin7 20.9
pin8 64.8
6L6GC
pin 3, read the same on both tubes 422
pin 4 , read the same on both tubes 422
pin 5, read the same on both tubes -39.4
pin 6, read the same on both tubes 423
5U4GB
Pin 2&8 to ground 427
Pin 4&6 no reading, number jumping around
Pin 2&8 to ground AC 939VAC (could this be right??)
Pin 4&6 to ground AC 359VAC
AC across pin 2&8 4.8VAC
AC across pin 4&6 724VAC
Filter caps
100uf -49.6
1st filter 426
2nd filter 425
3rd filter 363
I checked the top legs of the coupling caps for DC. All read zero except the .02uf phase inverter input cap which read 21V, and the two .1uf in the phase inverter read -39.5V. All the top legs of the coupling caps read zero for AC except the .02 phase inverter input cap which read 25VAC. The only other thing to mention is even though I pulled the NFB wire to the speaker jack, the Presence pot read 1.75VDC just as it reads at the junction of the NFB resistor and the 1.5k in the NFB loop.
Any help, advice, places to look would be greatly appreciated! I'll take any measurements and supply any additional info if you guys need it. Thanks!
I apologize that my first post is a plea for help, but I really need it and a lot of my Google searches led me here, so I thought I'd ask the experts. I'm green on a lot of this stuff, but not brand new. I've run into a problem I can't figure out!!
The Problem:
It passes signal, it works, but not correctly and sounds like ass. It has ratty distortion at all volumes. It functions at very low volumes but even then has that terrible distortion to it. If I pick hard it cuts out but then will come back if I pick softly. The amp won’t get loud at all; it's VERY GATED sounding - similar to how an Octavia sounds (without the octave, there's no octave to the note). It sounds like I’m playing through a broken speaker, but worse. One thing of note is that if I strum a chord with the amp on standby and then turn the amp fully on, the sound is loud, big and wonderful for about 2 seconds and then decays into what I describe above, and stays there. I'm using a 12AY7 in V1 so it's not a 12AX7 causing it to distort early. I'm somewhat familiar with this type of amp (owned several Victorias of like model) and how the bass and treble controls can cause overdrive above noon on the dial, and it's not that type distortion. The distortion I'm hearing is unpleasant, muffled, ugly distortion even at the lowest of volumes. I kind of thought it might be leaky coupling caps, but I have no idea at this point.
Things I have tried: the speakers are good; they are in another functioning amp. I don’t think it’s the tubes; I’ve tried several different sets with the same result. I don’t think it’s the filtering/filter caps; I added more filtering just to check and it sounded the same (I removed the added filtering after checking). It’s not reversed output transformer leads that caused oscillation, I removed the negative feedback and have done all my testing with it disabled – same result. I've checked and rechecked the under board wiring, my connections to ground, the component values, etc. I'm not afraid to admit that it could still be any of these things. I'm just trying to lay out where I've looked so far.
Amp info:
The amp is basically a 5E7 with a bit larger transformers: Power: Magnetic Components Classictone 18073, 325-0-325, 650V centertap, 50V bias, 5V@3amp, 6.3V@5amp. Output: Allen Amps TO40MT, 4000 ohm primary to 4 or 8ohm. I wired the power tube sockets like later Fenders with a 1.5k from pin 1 to pin 5, and a 470ohm from pin 4 to pin 6. The NFB is connected to a Presence pot that has a switch when turned all the way down to defeat the NFB. I also moved the output transformer centertap upstream to the other side of the choke (Allen Amps TRCH 125C1A style, 3H, 100 ohms) between the plate and the screens; it’s connected to the bottom of the 1st filter cap.
Here are my voltage readings and such. All measurements are DC unless otherwise noted, all measurements take with amp on/tubes in/no signal/knobs at 0, all filament heaters read 2.9 to 3.0VAC, the voltage at the wall is 122VAC, GE 5U4GB rectifier.. The power transformer seems to be putting out more voltage than advertised. I don't know if that matters with the problem though.
V1
pin1 152
pin3 2.44
pin6 159
pin8 2.44
V2
pin1 209
pin2 1.54
pin6 319
pin7 209
pin8 212
V3
pin1 243
pin3 1.72
pin6 295
pin7 20.9
pin8 64.8
6L6GC
pin 3, read the same on both tubes 422
pin 4 , read the same on both tubes 422
pin 5, read the same on both tubes -39.4
pin 6, read the same on both tubes 423
5U4GB
Pin 2&8 to ground 427
Pin 4&6 no reading, number jumping around
Pin 2&8 to ground AC 939VAC (could this be right??)
Pin 4&6 to ground AC 359VAC
AC across pin 2&8 4.8VAC
AC across pin 4&6 724VAC
Filter caps
100uf -49.6
1st filter 426
2nd filter 425
3rd filter 363
I checked the top legs of the coupling caps for DC. All read zero except the .02uf phase inverter input cap which read 21V, and the two .1uf in the phase inverter read -39.5V. All the top legs of the coupling caps read zero for AC except the .02 phase inverter input cap which read 25VAC. The only other thing to mention is even though I pulled the NFB wire to the speaker jack, the Presence pot read 1.75VDC just as it reads at the junction of the NFB resistor and the 1.5k in the NFB loop.
Any help, advice, places to look would be greatly appreciated! I'll take any measurements and supply any additional info if you guys need it. Thanks!
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