Reviving my older Joplin amp. Went through all soldering connections and started the break-in. B+1 to ground = 311 VDC (all OK).
ECC 81 pin #1 = 60VDC
ECC 81 pin #6 = 55VDC. Manual states 70 VDC.
Pin #1 of right 5687 = 207 VDC.
Pin #1 of left 5687 = 200 VDC. Manual states 170 VDC.
Not sure where to go from here.
Thought / advice appreciated.
ECC 81 pin #1 = 60VDC
ECC 81 pin #6 = 55VDC. Manual states 70 VDC.
Pin #1 of right 5687 = 207 VDC.
Pin #1 of left 5687 = 200 VDC. Manual states 170 VDC.
Not sure where to go from here.
Thought / advice appreciated.
Seems like ECC81 is running stronger than it should. 5687 are weaker than it should. But either of these should cause DOA (no sound). Your issues may be else where. What's wrong with the amp?
I lent this to a friend a few months back and he just stated it did not work -- to your comment DOA. With the weather frigid, decided to open it up and start checking. Maybe I should have the tubes checked out?
This is a monoblock? i.e. one of a pair? Or a stereo amp?
Is the 'other amp' working?
Do you have other test equipment besides the multimeter?
Sometimes just touching the meter probe to the circuit will produce a hum in the speaker - did you notice this?
It is a stereo amp -- the schematic is one side. Have not inserted 2A3 tubes yet so no speakers connected at this point, just going through the preamp stage first. I do not have other test equipment.
1. With the 12AT7 plate voltage only at 55V instead of at 70V, the voltage at the grids of the 5687 are low.
With low grid voltage, versus the 15K self bias resistor (and LTP to make the stage a phase splitter), there will be less current on the 5687, and their plate voltages will be high.
But the DC voltages are only different than normal by 15V on the 12AT7, and 30V on the 5687. Compare that to the fact that with signal, they will swing plus and minus from the normal voltages by much more than +/- 15V on the 12AT7, and +/- 30V on the 5687. You should still be able to pass signal through the amplifier.
Many amplifiers will work with 10%, or even 15% different DC voltages.
Look for some other cause.
What is the voltage across the 750 Ohm or 820 Ohm resistor?
Which one are you using?
Are the grid voltages of the 2A3 zero volts?
What are those grid voltages?
Are C3 or C9 leaky, causing positive voltage on one or both 2A3 grids?
Is there a chance the output transformer became shorted (the amplifier was operated with maximum signal in, volume control turned up all the way, and no speaker connected to the output tap).
How can that happen . . . when we do not hear anything from our stereo, we tend to turn the volume control up, even though the real problem is the preamp input selector is turned to phono (and no record playing), or the tuner is turned off, or the loudspeaker is disconnected. When we fix that problem, the amp blasts us out of the room (I have experienced that). I was lucky, the amp output transformer was not destroyed, even if the loudspeaker was disconnected.
2. Yes, do have the tubes tested.
With low grid voltage, versus the 15K self bias resistor (and LTP to make the stage a phase splitter), there will be less current on the 5687, and their plate voltages will be high.
But the DC voltages are only different than normal by 15V on the 12AT7, and 30V on the 5687. Compare that to the fact that with signal, they will swing plus and minus from the normal voltages by much more than +/- 15V on the 12AT7, and +/- 30V on the 5687. You should still be able to pass signal through the amplifier.
Many amplifiers will work with 10%, or even 15% different DC voltages.
Look for some other cause.
What is the voltage across the 750 Ohm or 820 Ohm resistor?
Which one are you using?
Are the grid voltages of the 2A3 zero volts?
What are those grid voltages?
Are C3 or C9 leaky, causing positive voltage on one or both 2A3 grids?
Is there a chance the output transformer became shorted (the amplifier was operated with maximum signal in, volume control turned up all the way, and no speaker connected to the output tap).
How can that happen . . . when we do not hear anything from our stereo, we tend to turn the volume control up, even though the real problem is the preamp input selector is turned to phono (and no record playing), or the tuner is turned off, or the loudspeaker is disconnected. When we fix that problem, the amp blasts us out of the room (I have experienced that). I was lucky, the amp output transformer was not destroyed, even if the loudspeaker was disconnected.
2. Yes, do have the tubes tested.
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As said on the the previous post, a common mistake with folks not use to tube amp is thinking that it is Ok not to hook up speakers. This could cause all kinds of issues which lead to the amp getting destroyed. When that happen, it usually happened on the output end (ie: 2A3 and its surrounding). Hopefully it is not the output transformers.
It is easiest to test the 2A3s on a tube tester (finding one is another story). That should be tested first. Next is hook up the amp to a variac and begin testing. A Variac is a device that reduce the wall AC voltage so that if thing are bad, less smoke and fire. Some folks built this with a light blub.
Look at the bright side, good thing your friend did not hook up an audio cable to the two RCA plugs on top of the chassis which is connected to "high voltages" via a 1M resistor.
It is easiest to test the 2A3s on a tube tester (finding one is another story). That should be tested first. Next is hook up the amp to a variac and begin testing. A Variac is a device that reduce the wall AC voltage so that if thing are bad, less smoke and fire. Some folks built this with a light blub.
Look at the bright side, good thing your friend did not hook up an audio cable to the two RCA plugs on top of the chassis which is connected to "high voltages" via a 1M resistor.
I fully agree but I checked with him and he did not do that. (also told him to leave the top RCA plugs ALONE!!!) I have a local source that can test the tubes, but I'm not at that point yet in my analysis. Have not installed them yet. Still working through the voltage readings on the preamp section.
http://www.diyhifisupply.com/files/6DJ8JoplinV2.1.pdf
I would begin with do the filaments light up?
Check / trace voltages in Power Supply.
There were apparently also 2 or perhaps 3 versions of this amplifier. If the link is not for the one you have it should still be quite useful.
Andrew
I would begin with do the filaments light up?
Check / trace voltages in Power Supply.
There were apparently also 2 or perhaps 3 versions of this amplifier. If the link is not for the one you have it should still be quite useful.
Andrew
Exact amp (2A3 version) and manual I have. I'm on the 4th bullet point, page 26. Preamp tube filaments light up just fine.
But I am quite puzzled here.
Why are you not supplying a circuit diagram with
ALL the relevant voltages you are measuring at both channels?
Why are you not supplying a circuit diagram with
ALL the relevant voltages you are measuring at both channels?
Reviving Joplin Amp
Found something interesting last night. Attached a picture. Across R5, R8 (left channel), R10 & R12 (right channel) = 105 VDC. Across C3 - 0 VDC, C7, C8 & C9 are 195 VDC. To ground at R5 = 205, R8 = 211, C3 - 0 VDC. On the right channel R10, R12 and C8 = 205VDC. It would appear I have an open on the other side of the circuit board from C3 to R5 connection.
Thoughts?
Found something interesting last night. Attached a picture. Across R5, R8 (left channel), R10 & R12 (right channel) = 105 VDC. Across C3 - 0 VDC, C7, C8 & C9 are 195 VDC. To ground at R5 = 205, R8 = 211, C3 - 0 VDC. On the right channel R10, R12 and C8 = 205VDC. It would appear I have an open on the other side of the circuit board from C3 to R5 connection.
Thoughts?
Forgot earlier. 2A3 grids are zero volts - no tubes installed yet. Using the 820 ohm resistor and zero across it.
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