Hi Folks,
I built a 50w JCM800. Having issues finishing it, it is CRAZY dynamic.
The V1-V3 plate voltages are more reasonable if i have the preamp tubes in and the power tubes NOT in.
With just preamp tubes:
V1 pin1 - 295
V1 pin6 - 353
V2 pin1 - 216
V2 pin6 - 408
V3 pin1 - 286
V3 pin6 - 265
However, if i put in the power tubes, voltage at the V1 plate starts rising and rising as the power amp tubes heat up until it gets close to 500v and i freak out and turn off the amp.
what does this mean? do i have a blown power amp tube or is it something more serious? Can a bad power tube make the voltage in the preamp get super high?
Thanks for any help, I've gone over and over the wiring and I can't find anything wrong.
High voltage on 2204 V1-V3 Plates
Moderators: pompeiisneaks, Colossal
Re: High voltage on 2204 V1-V3 Plates
When you put the power tubes in, what load was on the amp, eg speaker cab, big beefy load resistor?
Point being that an unloaded amp can go nuts.
What were the control settings? It’s best to zero them all at this stage.
Verify that the shorting contacts of the input jack sockets are properly connected and functional.
A full voltage survey may be revealing, ie Vdc at all tube socket terminals, other than those for heaters.
And the Vdc along the HT, at each HT supply node, both with and without tubes in place.
It may be useful up take and upload some good quality photos to a hosting site, and provide links here.
Please ask if you don’t fully understand or are unclear about any of the above
Point being that an unloaded amp can go nuts.
What were the control settings? It’s best to zero them all at this stage.
Verify that the shorting contacts of the input jack sockets are properly connected and functional.
A full voltage survey may be revealing, ie Vdc at all tube socket terminals, other than those for heaters.
And the Vdc along the HT, at each HT supply node, both with and without tubes in place.
It may be useful up take and upload some good quality photos to a hosting site, and provide links here.
Please ask if you don’t fully understand or are unclear about any of the above
https://www.justgiving.com/page/5-in-5-for-charlie This is my step son and his family. He is running 5 marathons in 5 days to support the research into STXBP1, the genetic condition my grandson Charlie has. Please consider supporting him!
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Stevem
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Re: High voltage on 2204 V1-V3 Plates
With all the tubes in and the amp on hook up a voltmeter set to read AC across the speaker output, if you read anything over 1 volt then the amp is oscillating due to your build/ layout or you may need to swap the plate wires ( blue and green) feeding the OT.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: High voltage on 2204 V1-V3 Plates
Here are a bunch of pictures and the schematic/layout
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Re: High voltage on 2204 V1-V3 Plates
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Re: High voltage on 2204 V1-V3 Plates
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Re: High voltage on 2204 V1-V3 Plates
Some more detailed voltage info:
I'm getting 122v from the wall
I'm getting 360vac from the hv leads off the power transformer
i get 494 vdc after the rectifier diodes
bias voltage is -49
All my heaters seem to be functioning, i'm getting about 3.3vac on all 5 tubes
I'm using a 1x12 cabinet with a 200w EV in it as a load while testing.
preamp tube readings with no power tubes:
V1 pin 1 - 295
V1 pin 2 - signal
V1 pin 3 - 2.6
V1 pin 4 - 3.3
V1 pin 5 - 3.3
V1 pin 6 - 350
V1 pin 7 - signal
V1 pin 8 - 3.6
V1 pin 9 - 3.3
V2 pin 1 - 216
V2 pin 2 - signal
V2 pin 3 - 1.5
V2 pin 4 - 3.3
V2 pin 5 - 3.3
V2 pin 6 - 408
V2 pin 7 - 216
V2 pin 8 - 223
V2 pin 9 - 3.3
V3 pin 1 - 286
V3 pin 2 - signal
V3 pin 3 - 39.6
V3 pin 4 - 3.3
V3 pin 5 - 3.3
V3 pin 6 - 265
V3 pin 7 - signal
V3 pin 8 - 39
V3 pin 9 -3.3
I'm getting 122v from the wall
I'm getting 360vac from the hv leads off the power transformer
i get 494 vdc after the rectifier diodes
bias voltage is -49
All my heaters seem to be functioning, i'm getting about 3.3vac on all 5 tubes
I'm using a 1x12 cabinet with a 200w EV in it as a load while testing.
preamp tube readings with no power tubes:
V1 pin 1 - 295
V1 pin 2 - signal
V1 pin 3 - 2.6
V1 pin 4 - 3.3
V1 pin 5 - 3.3
V1 pin 6 - 350
V1 pin 7 - signal
V1 pin 8 - 3.6
V1 pin 9 - 3.3
V2 pin 1 - 216
V2 pin 2 - signal
V2 pin 3 - 1.5
V2 pin 4 - 3.3
V2 pin 5 - 3.3
V2 pin 6 - 408
V2 pin 7 - 216
V2 pin 8 - 223
V2 pin 9 - 3.3
V3 pin 1 - 286
V3 pin 2 - signal
V3 pin 3 - 39.6
V3 pin 4 - 3.3
V3 pin 5 - 3.3
V3 pin 6 - 265
V3 pin 7 - signal
V3 pin 8 - 39
V3 pin 9 -3.3
Re: High voltage on 2204 V1-V3 Plates
Readings with power tubes, immediately after powerup, all knobs at zero
V1 pin 1 - 281
V1 pin 2 - signal
V1 pin 3 - 2.34
V1 pin 4 - 3.3
V1 pin 5 - 3.3
V1 pin 6 - 331
V1 pin 7 - signal
V1 pin 8 - 3.3
V1 pin 9 -3.3
V2 pin 1 - 204
V2 pin 2 - signal
V2 pin 3 - 1.43
V2 pin 4 - 3.3
V2 pin 5 - 3.3
V2 pin 6 - 382
V2 pin 7 - 204
V2 pin 8 - 205
V2 pin 9 -3.3
V3 pin 1 - 269
V3 pin 2 - signal
V3 pin 3 - 35
V3 pin 4 - 3.3
V3 pin 5 - 3.3
V3 pin 6 - 252
V3 pin 7 - signal
V3 pin 8 - 35
V3 pin 9 -3.3
V4 pin 1 - gnd
V4 pin 2 - 3.3
V4 pin 3 - 465
V4 pin 4 - 458
V4 pin 5 - -49
V4 pin 6 - 467
V4 pin 7 - 3.3
V4 pin 8 - gnd
V5 pin 1 - gnd
V5 pin 2 - 3.3
V5 pin 3 - 465
V5 pin 4 - 458
V5 pin 5 - -49
V5 pin 6 - 467
V5 pin 7 - 3.3
V5 pin 8 - gnd
V1 pin 1 - 281
V1 pin 2 - signal
V1 pin 3 - 2.34
V1 pin 4 - 3.3
V1 pin 5 - 3.3
V1 pin 6 - 331
V1 pin 7 - signal
V1 pin 8 - 3.3
V1 pin 9 -3.3
V2 pin 1 - 204
V2 pin 2 - signal
V2 pin 3 - 1.43
V2 pin 4 - 3.3
V2 pin 5 - 3.3
V2 pin 6 - 382
V2 pin 7 - 204
V2 pin 8 - 205
V2 pin 9 -3.3
V3 pin 1 - 269
V3 pin 2 - signal
V3 pin 3 - 35
V3 pin 4 - 3.3
V3 pin 5 - 3.3
V3 pin 6 - 252
V3 pin 7 - signal
V3 pin 8 - 35
V3 pin 9 -3.3
V4 pin 1 - gnd
V4 pin 2 - 3.3
V4 pin 3 - 465
V4 pin 4 - 458
V4 pin 5 - -49
V4 pin 6 - 467
V4 pin 7 - 3.3
V4 pin 8 - gnd
V5 pin 1 - gnd
V5 pin 2 - 3.3
V5 pin 3 - 465
V5 pin 4 - 458
V5 pin 5 - -49
V5 pin 6 - 467
V5 pin 7 - 3.3
V5 pin 8 - gnd
Re: High voltage on 2204 V1-V3 Plates
After a few minutes the power tubes get very hot. the preamp tubes are still cool to the touch. The voltages in the preamp spike to this:
V1 pin 1 - 440
V1 pin 2 - signal
V1 pin 3 - 0
V1 pin 4 - 3.3
V1 pin 5 - 3.3
V1 pin 6 - 440
V1 pin 7 - signal
V1 pin 8 - 0
V1 pin 9 - 3.3
V2 pin 1 - 440
V2 pin 2 - signal
V2 pin 3 - 0
V2 pin 4 - 3.3
V2 pin 5 - 3.3
V2 pin 6 - 445
V2 pin 7 - 440
V2 pin 8 - 0
V2 pin 9 - 3.3
V3 pin 1 - 273
V3 pin 2 - signal
V3 pin 3 - 28
V3 pin 4 - 3.3
V3 pin 5 - 3.3
V3 pin 6 - 440
V3 pin 7 - signal
V3 pin 8 - 24
V3 pin 9 -3.3
V1 pin 1 - 440
V1 pin 2 - signal
V1 pin 3 - 0
V1 pin 4 - 3.3
V1 pin 5 - 3.3
V1 pin 6 - 440
V1 pin 7 - signal
V1 pin 8 - 0
V1 pin 9 - 3.3
V2 pin 1 - 440
V2 pin 2 - signal
V2 pin 3 - 0
V2 pin 4 - 3.3
V2 pin 5 - 3.3
V2 pin 6 - 445
V2 pin 7 - 440
V2 pin 8 - 0
V2 pin 9 - 3.3
V3 pin 1 - 273
V3 pin 2 - signal
V3 pin 3 - 28
V3 pin 4 - 3.3
V3 pin 5 - 3.3
V3 pin 6 - 440
V3 pin 7 - signal
V3 pin 8 - 24
V3 pin 9 -3.3
Re: High voltage on 2204 V1-V3 Plates
Using a 1x12 cab as a load.pdf64 wrote: ↑Sun Sep 02, 2018 8:32 am When you put the power tubes in, what load was on the amp, eg speaker cab, big beefy load resistor?
Point being that an unloaded amp can go nuts.
What were the control settings? It’s best to zero them all at this stage.
Verify that the shorting contacts of the input jack sockets are properly connected and functional.
A full voltage survey may be revealing, ie Vdc at all tube socket terminals, other than those for heaters.
And the Vdc along the HT, at each HT supply node, both with and without tubes in place.
It may be useful up take and upload some good quality photos to a hosting site, and provide links here.
Please ask if you don’t fully understand or are unclear about any of the above
I zeroed all the controls to take the readings above.
hmm good idea i'll take a close look at the input jacks
what do you mean by HT supply node, you mean at the socket pins?
thanks for your help!
Re: High voltage on 2204 V1-V3 Plates
I didn't hear any oscillation last i had it running but i will try this tomorrow, thanks!Stevem wrote: ↑Sun Sep 02, 2018 11:14 am With all the tubes in and the amp on hook up a voltmeter set to read AC across the speaker output, if you read anything over 1 volt then the amp is oscillating due to your build/ layout or you may need to swap the plate wires ( blue and green) feeding the OT.
-
Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: High voltage on 2204 V1-V3 Plates
It maybe oscillating at a frequency far above what you can hear, or even what the speaker(s) can reproduce!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: High voltage on 2204 V1-V3 Plates
The HT supply nodes may be identified in your schematic as filter caps 1, 2, 3 and 4.
Generally filter cap 1, ie that connects to the rectifier, is referred to as the ‘reservoir cap’.
It would be highly beneficial to fit 1 ohm cathode resistors to the power tubes.
I can’t envisage why any modern designed or built amp wouldn’t have them, unless to sell bias probes.
It’s kinda essential to know (at least roughly) how much current the power tubes are drawing, and 1 ohm resistors facilitate that measurement fairly safely and with very little likelihood of affecting circuit operation.
As noted at TGP, when the power tubes get hot, V1 and V2 seem to stop drawing HT current; maybe a bad connection in their cathode connection to circuit common?
From the photos and the above symptoms, I wonder if there’s a dry joint on the black heater wires on terminals 4&5 of V3?
Generally filter cap 1, ie that connects to the rectifier, is referred to as the ‘reservoir cap’.
It would be highly beneficial to fit 1 ohm cathode resistors to the power tubes.
I can’t envisage why any modern designed or built amp wouldn’t have them, unless to sell bias probes.
It’s kinda essential to know (at least roughly) how much current the power tubes are drawing, and 1 ohm resistors facilitate that measurement fairly safely and with very little likelihood of affecting circuit operation.
As noted at TGP, when the power tubes get hot, V1 and V2 seem to stop drawing HT current; maybe a bad connection in their cathode connection to circuit common?
From the photos and the above symptoms, I wonder if there’s a dry joint on the black heater wires on terminals 4&5 of V3?
https://www.justgiving.com/page/5-in-5-for-charlie This is my step son and his family. He is running 5 marathons in 5 days to support the research into STXBP1, the genetic condition my grandson Charlie has. Please consider supporting him!
Re: High voltage on 2204 V1-V3 Plates
I checked every connection and reflowed everything but it did not help. So I tried taking a chopstick and poking all the connections and moving the wires around until I discovered the culprit. My V2 pin 8 connection was intermittently bad somewhere between the pin and turret. I could make it work or not by disturbing it with the chopstick. I took out the wire and made a new one and the amp is behaving now.
Flush with victory, I've also added a third 10k dropping resistor to get my preamp voltages even lower. Thanks everyone for all your help!
Flush with victory, I've also added a third 10k dropping resistor to get my preamp voltages even lower. Thanks everyone for all your help!
Re: High voltage on 2204 V1-V3 Plates
Well done!
Is the tube that was in V2 ok? Bear in mind that it seems to have had very high Vdc on its pin 7 grid.
Is the tube that was in V2 ok? Bear in mind that it seems to have had very high Vdc on its pin 7 grid.
https://www.justgiving.com/page/5-in-5-for-charlie This is my step son and his family. He is running 5 marathons in 5 days to support the research into STXBP1, the genetic condition my grandson Charlie has. Please consider supporting him!