paulster wrote:They can work loose from vibration from the transformer.
In 50 years of playing guitar through tube amps, I've never had a transformer work loose.
I'm not saying they will likely work loose but it is a potential hazard situation and one of the reasons behind various safety bodies' specifications that the safety earth has to be on its own dedicated chassis connection.
Another is that during maintenance work (e.g. replacing the PT) the ground connection may be overlooked and not reconnected.
Granted, neither of these situations are likely in an amp but clearly these scenarios have been observed or are considered high enough risk to mandate a particular practice.
playonit wrote:
Yes with the guitar plugged in you can hear the motorboating not the guitar....
Ok, standard troubleshooting techniques now.
Start by grounding out the grid of each stage. Start with pin 2 of V1, next pin 7 of V2, then pin 2 of V2, do not ground pin 7 of V2 as there's voltage on that one.
If any of these stop the motorboating, you've located the stage when the issue is starting.
Ok... Finally Got the opportunity to ground the grids and lost the loud hum when I grounded pin seven at V3 I looked at the layout but afraid I am confused as to how to proceed.
Checked soldered connections and they appear fine? checked continuity between points and get that? what next??
BTW... I am really learning here and appreciate this.. my first 3 builds went without a hitch save for a couple of wires that needed dress attention.. thinking about this.. I probably needed this more than anything right now having these issues... learning more on this build than the 3 others combined!!!
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playonit wrote:Ok... Finally Got the opportunity to ground the grids and lost the loud hum when I grounded pin seven at V3 I looked at the layout but afraid I am confused as to how to proceed.
What this is telling you is that there's a signal coming from one of the earlier stages causing the noise. When you ground out the Phase Inverter input, pin 7 V3, you're preventing any signal from passing.
Follow the signal BACK through the circuit.
Pin 7 V3b connects to the 1M resistor, which has a red dashed line going to the .022uF coupling cap that is the output of V3a, the driver stage.
Start by checking that this connection is sound since you can't see it under the board.
Here's the schematic which might be easier to follow the signal path.
I was able to get rid of most of the hum as I was able to find some ground issues....
I was checking voltages and was very surprised by what I found on V6 rectifier. On pins 4 & 6 I found low negative volts( -5 )and on pins 2 & 8 I found high volts( 378v )should have been the other way around.. I followed Magnetic components drawing and have the wires where they should be color wise according to the drawing but reversed voltage?? Am I missing something here.. I also have a very high reading on pin 1 V1.. I have attached my readings (In pink) compared to published v readings I found for this amp. I also included the PT drawing...
Thanks
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playonit wrote:
Ok... Finally Got the opportunity to ground the grids and lost the loud hum when I grounded pin seven at V3 I looked at the layout but afraid I am confused as to how to proceed.
Checked soldered connections and they appear fine? checked continuity between points and get that? what next??
BTW... I am really learning here and appreciate this.. my first 3 builds went without a hitch save for a couple of wires that needed dress attention.. thinking about this.. I probably needed this more than anything right now having these issues... learning more on this build than the 3 others combined!!!
I question what's going on at pins 5 and 6 on the outputs. It seems the 1.5K grid stoppers aren't in circuit. Is the layout correct ? (I suspect not)
There may be other mistakes.
playonit wrote:
Ok... Finally Got the opportunity to ground the grids and lost the loud hum when I grounded pin seven at V3 I looked at the layout but afraid I am confused as to how to proceed.
Checked soldered connections and they appear fine? checked continuity between points and get that? what next??
BTW... I am really learning here and appreciate this.. my first 3 builds went without a hitch save for a couple of wires that needed dress attention.. thinking about this.. I probably needed this more than anything right now having these issues... learning more on this build than the 3 others combined!!!
I question what's going on at pins 5 and 6 on the outputs. It seems the 1.5K grid stoppers aren't in circuit. Is the layout correct ? (I suspect not)
There may be other mistakes.
rd
Here's a copy of the layout I used.... Thanks
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playonit wrote:I was able to get rid of most of the hum as I was able to find some ground issues....
I was checking voltages and was very surprised by what I found on V6 rectifier. On pins 4 & 6 I found low negative volts( -5 )and on pins 2 & 8 I found high volts( 378v )should have been the other way around..
4-6 are AC voltage as is 2-8. 8 to ground will be the DC high voltage.
rdjones wrote:
I question what's going on at pins 5 and 6 on the outputs. It seems the 1.5K grid stoppers aren't in circuit. Is the layout correct ? (I suspect not)
There may be other mistakes.
rd
Good catch, yes there appears to be a mistake on the layout. The red wire should connect to pin 6, not 5.
playonit wrote:I was able to get rid of most of the hum as I was able to find some ground issues....
I was checking voltages and was very surprised by what I found on V6 rectifier. On pins 4 & 6 I found low negative volts( -5 )and on pins 2 & 8 I found high volts( 378v )should have been the other way around..
4-6 are AC voltage as is 2-8. 8 to ground will be the DC high voltage.
Sorry Bob... I am a little confused... the voltage chart I have says the high voltage should be on 4 & 6 where I have the red wires that show as 355v on the PT drawing but am getting the high v where the yellow wires hook up at 2 & 8 where it shows as 5v..... Also, I have to be honest the schematic stuff is still a little over my head but I am learning I just can't connect all the points yet The red wires going to the wrong pins 5 instead of 6 will that cause the voltages to flip flop on v6 and cause the high voltage at pin1 V1???? I can't thank you enough for taking the time to work me through this ...I learning here and it's a great experience.....
playonit wrote:Sorry Bob... I am a little confused... the voltage chart I have says the high voltage should be on 4 & 6 where I have the red wires that show as 355v on the PT drawing but am getting the high v where the yellow wires hook up at 2 & 8 where it shows as 5v..... Also, I have to be honest the schematic stuff is still a little over my head but I am learning I just can't connect all the points yet The red wires going to the wrong pins 5 instead of 6 will that cause the voltages to flip flop on v6 and cause the high voltage at pin1 V1???? I can't thank you enough for taking the time to work me through this ...I learning here and it's a great experience.....
Actually, the pin 5/6 mistake on the outputs is a minor issue. The amp "should" still work with this wiring, lots of amps do without the grid "stoppers".
HOWEVER - if there's an parasitic oscillation happening the stoppers will (hopefully) help eliminate it.
No, the pin 5/6 issue will not cause the other effects being observed, unless they are the result of a monster parasitic.
Note that in this wiring layout pin#6 is only being used as a tie point, it does not connect to anything in the tube and in fact there's most likely not even a pin in that location on the tube. Grid stoppers are most effective when placed physically very near the grids.
Should I disconnect the yellow 5v and red 355v wires at V6 and check voltage on them to make sure that the PT does not have an issue from the manufacturer by having accidentally crossed those wires?? I don't want to switch them on the pins until I am sure whats going on there.... Thanks
Your preamp voltages lead me to believe that you don't have your voltage dropping resistors connected correctly.
Check that your B+ goes from the 10K on the 16uF filter cap to the junction of the 56K and 100K for V3, then to the 10K for V2, then from the other side of the 10K to the two 100K plate resistors for V1.
playonit wrote:
Sorry Bob... I am a little confused... the voltage chart I have says the high voltage should be on 4 & 6 where I have the red wires that show as 355v on the PT drawing
The 340v from the PT is AC volts. The 5U4G rectifies this AC into DC. The red wire supply the 5VAC for the heater in the 5U4G, then the DC voltage comes out of pin 8, reference to ground.
Again, that AX84 theory doc will help you out with the schematics.
playonit wrote:
Sorry Bob... I am a little confused... the voltage chart I have says the high voltage should be on 4 & 6 where I have the red wires that show as 355v on the PT drawing
The 340v from the PT is AC volts. The 5U4G rectifies this AC into DC. The red wire supply the 5VAC for the heater in the 5U4G, then the DC voltage comes out of pin 8, reference to ground.
Again, that AX84 theory doc will help you out with the schematics.
Thanks for the link.. just what I needed something that breaks the schematic down into sections for me ... I briefly looked at it today and will really get into it tomorrow during a flight I have.
I understand now what you were saying about AC converting through the rectifier to DC. The voltage looks good my only issue there now is at v1 where voltage is all over the place. My voltage chart says volts on 1 & 6 should be in the neighborhood of 140v but are around 285v? and on pins 3&8 chart says 2.3v but I'm getting 10.4 on each pin.. I checked the path Tom suggested ( "Check that your B+ goes from the 10K on the 16uF filter cap to the junction of the 56K and 100K for V3, then to the 10K for V2, then from the other side of the 10K to the two 100K plate resistors for V1").. and all points check out.. right now I am looking for something that tells me if voltages should drop along that path? I see high voltages all the way along with very little drop? I was wondering since one of the choke wires hooks at the beginning of the path at the 16uf cap if the choke could be the problem?? Thanks!
Two things I would check would be to measure the actual resistance of the 820 ohm cathode resistor that is connected to pins 3 and 8 of V1. If that checks out then I would test to make sure I had a good ground at the negative end of the 820 resistor and the 25uF cap. Did you possibly forget the ground jumper under the board? No way your cathode voltage should be that high. Might try a new tube in V1 also. If you get those cathode voltages on V1 down where they belong, you will probably be getting close. Sounds like V1 tube is drawing no current even in a state of quiescence.
