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 Post subject: Re: Rockola 1468 help
PostPosted: Dec Sat 16, 2017 10:03 pm 
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Ok I believe I have it now. Except the vol dies right after the volume switch which I suspect the the mute function. Can anyone enlighten me on how to disable it for testing purposes?

Thanks,
Jason


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 Post subject: Re: Rockola 1468 help
PostPosted: Dec Mon 18, 2017 6:51 pm 
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Its a little hard to tell without seeing the whole schematic, but I think that if you leave the
mute socket unplugged you shouldn't have a problem there. The mute plug grounds the input of the volume control.

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 Post subject: Re: Rockola 1468 help
PostPosted: Dec Fri 29, 2017 9:10 pm 
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Ok we are passing signal now.
My DC voltages are as follows
12ax7 pin1 202 pin3 1.16 Pin6 191 pin8 1.16
6cy7 pin1 387 pin3 162 pin7 .02 pin8 2.35 pin9 191
6an8 pin1 255 pin2 102 pin3 112.5 pin7 76 pin8 .08 pin9 1.64
el34 1,8 24.59 pin2 235 pin3 376 pin4 378

Both el34 tested the same. Heaters look good for AC, but for some reason they also have 23.5dc on them also.
There is a fair amount of hum. Pulling the 12ax7 hum is still there. grounding pin3,7 on the 6cy7 halves the hum. I'm not sure where to go from here. If someone wants a larger schematic please pm me.

Thanks so much,
Jason


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 Post subject: Re: Rockola 1468 help
PostPosted: Dec Sat 30, 2017 8:34 pm 
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Ok I can ground pin2 on the volume connector which kills all hum. So it's all be fore that. Grounding pin7 on the 12ax7 has no effect on hum. Grounding pin3 on the 6cy7 only halves the hum.
So my best guess the hum is coming from the 6cy7 or a connection on it.

Jason


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 Post subject: Re: Rockola 1468 help
PostPosted: Jan Mon 01, 2018 3:29 am 
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reeves03 wrote:
Ok I can ground pin2 on the volume connector which kills all hum. So it's all be fore that. Grounding pin7 on the 12ax7 has no effect on hum. Grounding pin3 on the 6cy7 only halves the hum.
So my best guess the hum is coming from the 6cy7 or a connection on it.

Jason

The last section of the 6CY7 is a cathode follower stage that has no gain, it is used as a buffer so that the output has low impedance and can drive more current.
What is the AVC switch set to? Probably you should set it to off until you correct the hum.

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 Post subject: Re: Rockola 1468 help
PostPosted: Jan Mon 01, 2018 3:54 am 
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It is off. I have another 6cy7 tube on the way, to rule out the tube.

Thanks,
Jason


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 Post subject: Re: Rockola 1468 help
PostPosted: Jan Wed 10, 2018 3:24 am 
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Another tube displays same issue. What I find is I can clip a lead to ground and the other end to pin 2 of the vol connector kills hum completely.
Can clip it to pin3 of the 6cy7 and it halves the hum I am figurin here I still here hum as there is no cap to block dc from entering that point but this being cathode follower I should be grounding pin9?(I havenot done that yet awaiting confirmation).
Also I can ground pin 7 grid and the hum gets louder.
Included 2 pics one with annotations and the other clean so if I covered something with annotations you could read what it was I covered.

Any help would be most excellent.
Thanks,
Jason


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 Post subject: Re: Rockola 1468 help
PostPosted: Jan Wed 10, 2018 4:46 pm 
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reeves03 wrote:
Another tube displays same issue. What I find is I can clip a lead to ground and the other end to pin 2 of the vol connector kills hum completely.
Can clip it to pin3 of the 6cy7 and it halves the hum I am figurin here I still here hum as there is no cap to block dc from entering that point but this being cathode follower I should be grounding pin9?(I havenot done that yet awaiting confirmation).
Also I can ground pin 7 grid and the hum gets louder.
Included 2 pics one with annotations and the other clean so if I covered something with annotations you could read what it was I covered.

Any help would be most excellent.
Thanks,
Jason

Well when you ground pin 2 of the vol connector you will kill all the sound. That does prove the problem is before that point. The schematic doesn't show which grid pin is which, but I guess the annotations sorts that out. I wouldn't ground any plate connections or cathode of the follower, that messes with the DC bias and can cause too much current to flow. Best to put a 1 to 0.1 uf cap in series with the jumper. I can't see why grounding a grid makes the hum louder unless you aren't tied to the right ground point. Maybe your jumper is long enough to pick up hum and add it in. Grounding the grid of the second triode unit (cathode follower) should have killed the hum.
Try disconnecting one end of the 1500 ohm resistor connected to the AVC switch and then connecting the second grid through a 1 to 0.1 uf to ground. That should kill the hum.

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 Post subject: Re: Rockola 1468 help
PostPosted: Jan Sun 21, 2018 1:20 am 
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Try disconnecting one end of the 1500 ohm resistor connected to the AVC switch and then connecting the second grid through a 1 to 0.1 uf to ground. That should kill the hum.

Are you suggesting doing both or one at a time?

Thanks,
Jason


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 Post subject: Re: Rockola 1468 help
PostPosted: Jan Sun 21, 2018 2:09 am 
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reeves03 wrote:
Try disconnecting one end of the 1500 ohm resistor connected to the AVC switch and then connecting the second grid through a 1 to 0.1 uf to ground. That should kill the hum.

Are you suggesting doing both or one at a time?

Thanks,
Jason

I believe what I was thinking was that maybe the hum comes through from the AVC switch, depending on the situation. By disconnecting the 1500 ohm from it you would stop that path. However the AVC circuit controls the gain so it might rise to a high level and magnify any hum from somewhere else. By grounding the cathode follower grid with a capacitor it should prevent any hum there without effecting the DC bias of that stage. So yes, both at the same time.

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 Post subject: Re: Rockola 1468 help
PostPosted: Jan Wed 24, 2018 2:25 am 
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ok it looks like the grids are tied at pins 3 and 6 I took a cap from pin6 to ground and disconnected the 1500 ohm.

No change in hum. So I reconnected the 1500ohm no real change in hum. and went back and disconnected the cap now that I'm back inside I think I used a .001 I will dig and see if I have a 1 or a .1 cap to try there.
Also was I correct in how I connected it?

Thanks,
Jason


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 Post subject: Re: Rockola 1468 help
PostPosted: Jan Wed 24, 2018 5:37 am 
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reeves03 wrote:
ok it looks like the grids are tied at pins 3 and 6 I took a cap from pin6 to ground and disconnected the 1500 ohm.

No change in hum. So I reconnected the 1500ohm no real change in hum. and went back and disconnected the cap now that I'm back inside I think I used a .001 I will dig and see if I have a 1 or a .1 cap to try there.
Also was I correct in how I connected it?

Thanks,
Jason

The .001 was probably too small.
Pins 3 and 7 are the grids of the two triode units in the 6CY7. One grid goes to a 47K and 270K resistor, you said the hum got louder when you grounded it. That isn't the way its supposed to work. The other grid is tied to the other triode plate and a 470K resistor, you said the hum was cut in half when you grounded that grid. Pins 1 and 6 are the plates.
EDIT: I went back to an earlier schematic that you posted and it has the tube pin numbers. Also, do you have a oscilloscope? You could just probe around and find where the hum comes from. However the first tube, 12AX7, has negative feedback circuit that can make things complicated to trace. By the way, you should either have a cartridge connected to the input or short out that connector. There is so much gain in the circuit it will pick up excessive hum if it is unconnected.

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 Post subject: Re: Rockola 1468 help
PostPosted: Jan Thu 25, 2018 2:57 am 
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Ok I will tie a cap from pin7 second grid tie to ground.


I wired the 6cy7 like this left side is 6plate 7 grid 8cathode, right side 1plate, 3grid, 9cathode. being that it's a dissimilar triode.
It looks like I should have wired it left side 1plate, 3grid, 9cathode, right side 6plate, 7grid, 8cathode looking at the schematic.

Thanks,
Jason


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 Post subject: Re: Rockola 1468 help
PostPosted: Jan Fri 26, 2018 8:25 pm 
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reeves03 wrote:

I wired the 6cy7 like this left side is 6plate 7 grid 8cathode, right side 1plate, 3grid, 9cathode. being that it's a dissimilar triode.
It looks like I should have wired it left side 1plate, 3grid, 9cathode, right side 6plate, 7grid, 8cathode looking at the schematic.

Thanks,
Jason

I'm not following you on this. From the schematic that has pin numbers, your first description is correct. We are talking about left and right on the schematic right? The signal enters the 6CY7 tube unit on the left first, via a 270K resistor, that would be the grid on pin 7.

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 Post subject: Re: Rockola 1468 help
PostPosted: Jan Fri 26, 2018 8:31 pm 
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I think my confusion comes in from looking at a tube data sheet pinout and trying to correlate that to the schematic.

I put a .1uf cap on ground(at the 6cy7 tube socket where it mounts to the chassis and pin7. not much noticable hum drop.

Tonight I will scope the grids and try to take a picture of whats on the scope.

Thanks so much for your help.

jason


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 Post subject: Re: Rockola 1468 help
PostPosted: Jan Sat 27, 2018 4:52 pm 
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reeves03 wrote:

I put a .1uf cap on ground(at the 6cy7 tube socket where it mounts to the chassis and pin7. not much noticable hum drop.

jason

This may be a good clue. It suggests that the hum is not coming from the 12AX7 tube circuit. If you disconnected the 270K resistor that connects to the 6CY7 pin 7 and keep the 0.1uf cap on the same pin to ground and there is still hum, that will confirm it. Maybe that 6CY7 tube has an internal short between the cathode and the filament.
However I don't know how much hum you are hearing, most tube audio has some hum if you listen hard enough. High end audio often uses DC to run the filaments, etc. BTW, have you tried adjusting the hum control pot connected to the filament circuit?
Usually if you have a scope that is one of the first tool to use, rather than the technique of AC grounding each stage to trace the problem. Be sure to use the ground clip connection on the scope probe.

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 Post subject: Re: Rockola 1468 help
PostPosted: Jan Sat 27, 2018 7:59 pm 
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Disconnected the 270 ohm resistor, no change in hum. hum balance doesn't change anything.
Previously I did disconnect the heater to the 6cy7 and connected a regulated dc power supply to power the tube and it didn't change hum either.
I did purchase a nos tube from someone on ARF. I will check the 2 6cy7's I have on the jackson for shorts.
It seems these tubes are getting harder to find.
jason

Is there possibly another tube I can use, even if having to rewire the circuit for that section?


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 Post subject: Re: Rockola 1468 help
PostPosted: Jan Sun 28, 2018 12:03 am 
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They check ok in the tube tester. For kicks I took them and clipped a multimeter lead on pin4 touched each pin and got OL on all except pin5 on both tubes. So I think that rules out Heater Cathode shorts but would it rule out hum?
Following your lead I can scope any relevant section you want, but mind you I'm still novice.

Thanks so much

Jason


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 Post subject: Re: Rockola 1468 help
PostPosted: Jan Sun 28, 2018 5:12 am 
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reeves03 wrote:
Disconnected the 270 ohm resistor, no change in hum. hum balance doesn't change anything.
Previously I did disconnect the heater to the 6cy7 and connected a regulated dc power supply to power the tube and it didn't change hum either.
I did purchase a nos tube from someone on ARF. I will check the 2 6cy7's I have on the jackson for shorts.
It seems these tubes are getting harder to find.
jason

Is there possibly another tube I can use, even if having to rewire the circuit for that section?


If you disconnected the 270K resistor on pin 7 then that rules out the 12AX7, that's kind of good since the first stage has a lot of gain and can be sensitive to noise and the like. You shouldn't have to change the 6CY7 for something else. Leave the 270K unconnected.
You can just follow the signal path in reverse and see where the 60 Hz hum shows up. Look at the signal at the input to the volume control, pin 9, pin 3 (pin 6 should be the same), then pin 7. There should be no hum on pin 1. You might see some on pin 8. Ideally there shouldn't be hum at any of those places. Because of the amplifier gain, any signal at pin 7 could be up to 10 times larger by the time it gets to pin 9. You did check the DC voltages against the schematic I hope. Set the scope on AC coupling. I think that either there is a bad component, bad connection or something is mis-wired.
make note of how much hum you find at each spot.

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"Excellent!" I cried. "Elementary," said he. - Sherlock Holmes


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 Post subject: Re: Rockola 1468 help
PostPosted: Jan Mon 29, 2018 12:45 am 
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Here's what I have pin 9 199vdc pic is 5v/div

Pin3 171vdc 5v/div
Pin7 .012 vdc .2v/div
pin1 398vdc 5v/div
pin8 2.39vdc .2v /div

Pics are labeled as pin number. What I think I'm seeing is poor filtering coming into pin1 because of the sawtooth form.
Thanks,
Jason


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