MasterOtono
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- Sep 19, 2015
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My friend asked me to help with his Triple Rectifier 150 W dual channel Amp.
recently he had an issue with a V3 tube.
Symptoms - volume drop on all channels. After replacing - am worked fine.
One month later he had another issue with a send-return loop switching.
Repaired - One dead optocoupler and one dead transistor in switching circuit.
But now there is another issue, clean channel distorting at peaks.
All knobs at noon, Orange gain at about 5-10%
When i'm playing slightly - it sounds like a regular clean channel, when i make hard picking or ply chords- it sounds distorted even at lower gain settings.
I connected a cable to a "Send" jack of this rectifier and connected it to a "Return" jack of another amp.
It sounds exactly the same, with distortion.
When i connected an external preamp into a "Return" jack - it sounded good without any artifacts.
So the power tube issue can be excluded.
Channel Style select switch is in the middle position, Orange channel gain is set to "Clean Rhytm"
All tubes have been changed to a new ones, i tried two different sets of 12AX7 tubes.
Heater voltages are OK - 6.2V
Then i tried to connect my sound-test probe to the each stage and to check the sound From V4 to V1 stage by stage. I found that the issue appears at the V3 stage.
After the coupling capacitor of the V2b stage the signal is fine, but on the V3a anode sound is being distorted.
I tested all optocouplers and they are looking pretty good.
about 100 Ohm open ant more than 2Meg closed.
Then I measured signal with Generator and Scope.
I applied 150mV peak-to-peak voltage at the amp input and checked voltages and gain stage-by stage.
Signal at he LDR8 and LDR9 is allredy distorted, I disconnected theese LDRs and it doesn't help - so the tone stack issues can be excluded.
With gain setting at about 50% I measured about 9V p-p signal at he V3a Grid, which is really big value, considering the -1.6V Grid Bias.
That means that this stage can accept maximum 3.2V p-p signal without distortion.
When the signal level is higher, Grid to Cathode voltage goes to positive value and the Grid - to - Cathode diode begins to distort sound.
In this amp input signal at the V3a stage is 3 times higher than this stage headroom.
My thought was that something went wrong in previous stages and it caused overall gain increase.
Maybe a shorted optocoupler could add the cathode bypassing capacitor to the one of these stages, which could cause gain increase.
I started to calculate and measure each stages gain.
and got the following results:
With 15-20% of gain setting I can see clean sine wave at the V3b Cathode,
With 50% of gain i have distorted sound after the V3a stage.
Calculated and measured gains have really close values, excepting the V3a and V3b stages.
My questions are:
1) Is it ok that with 150mV input signal and 50% Gain setting - we are geting these 9V at the V3a Input?
It looks like a bottleneck, but I heard Mesa DR clean sound and it was pretty fine without clipping till the 60-70% of gain.
Either the gain in V1a -V2a - V2a stages still is higher than it should be or V3 stage gone mad.
Could you please advice?
recently he had an issue with a V3 tube.
Symptoms - volume drop on all channels. After replacing - am worked fine.
One month later he had another issue with a send-return loop switching.
Repaired - One dead optocoupler and one dead transistor in switching circuit.
But now there is another issue, clean channel distorting at peaks.
All knobs at noon, Orange gain at about 5-10%
When i'm playing slightly - it sounds like a regular clean channel, when i make hard picking or ply chords- it sounds distorted even at lower gain settings.
I connected a cable to a "Send" jack of this rectifier and connected it to a "Return" jack of another amp.
It sounds exactly the same, with distortion.
When i connected an external preamp into a "Return" jack - it sounded good without any artifacts.
So the power tube issue can be excluded.
Channel Style select switch is in the middle position, Orange channel gain is set to "Clean Rhytm"
All tubes have been changed to a new ones, i tried two different sets of 12AX7 tubes.
Heater voltages are OK - 6.2V
Then i tried to connect my sound-test probe to the each stage and to check the sound From V4 to V1 stage by stage. I found that the issue appears at the V3 stage.
After the coupling capacitor of the V2b stage the signal is fine, but on the V3a anode sound is being distorted.
I tested all optocouplers and they are looking pretty good.
about 100 Ohm open ant more than 2Meg closed.
Then I measured signal with Generator and Scope.
I applied 150mV peak-to-peak voltage at the amp input and checked voltages and gain stage-by stage.
Signal at he LDR8 and LDR9 is allredy distorted, I disconnected theese LDRs and it doesn't help - so the tone stack issues can be excluded.
With gain setting at about 50% I measured about 9V p-p signal at he V3a Grid, which is really big value, considering the -1.6V Grid Bias.
That means that this stage can accept maximum 3.2V p-p signal without distortion.
When the signal level is higher, Grid to Cathode voltage goes to positive value and the Grid - to - Cathode diode begins to distort sound.
In this amp input signal at the V3a stage is 3 times higher than this stage headroom.
My thought was that something went wrong in previous stages and it caused overall gain increase.
Maybe a shorted optocoupler could add the cathode bypassing capacitor to the one of these stages, which could cause gain increase.
I started to calculate and measure each stages gain.
and got the following results:
With 15-20% of gain setting I can see clean sine wave at the V3b Cathode,
With 50% of gain i have distorted sound after the V3a stage.
Calculated and measured gains have really close values, excepting the V3a and V3b stages.
My questions are:
1) Is it ok that with 150mV input signal and 50% Gain setting - we are geting these 9V at the V3a Input?
It looks like a bottleneck, but I heard Mesa DR clean sound and it was pretty fine without clipping till the 60-70% of gain.
Either the gain in V1a -V2a - V2a stages still is higher than it should be or V3 stage gone mad.
Could you please advice?
