Do you not have to take the mod one step further by also rebiasing the 12AX7 ??
Assuming a working amp and the correct iso trx mod, should the power supply hum (i.e filter caps and grounding) be the only issue after performimg the mod?
I have just recently performd the iso trx mod on an 801 chassis and now it oscillates when the volume is turn up approx half way. It is an oscillation at a higher frequency than the mains freq (60Hz and 120Hz)
Maybe the oscillation is related to the new gains introduced by the mod? i.e changing the cathode bias circuit grounding and increasing B+ by changing the rectifier circuit.
It seems that the trx mod is the main focus on the websites and forums but there is very little additional information on the effects of the mod on the amp circuits, especially this 801 chassis.
Can anyone help with this? An example of the changes made to the amp circuit after the iso trx mod would be really helpful. Thanks.
Assuming a working amp and the correct iso trx mod, should the power supply hum (i.e filter caps and grounding) be the only issue after performimg the mod?
I have just recently performd the iso trx mod on an 801 chassis and now it oscillates when the volume is turn up approx half way. It is an oscillation at a higher frequency than the mains freq (60Hz and 120Hz)
Maybe the oscillation is related to the new gains introduced by the mod? i.e changing the cathode bias circuit grounding and increasing B+ by changing the rectifier circuit.
It seems that the trx mod is the main focus on the websites and forums but there is very little additional information on the effects of the mod on the amp circuits, especially this 801 chassis.
Can anyone help with this? An example of the changes made to the amp circuit after the iso trx mod would be really helpful. Thanks.
Relocated to I & A once I figured out that this was a guitar amplifier.
Thanks.
I could not find anything related to this oscillation in that archive.
I have already installed the Triad N-68X transformer as per that archive.
The oscillation starts at approx half volume. The tone control changes the frequency of the oscillation but I think this would be expected. At max bass setting the freq of the osc is approx 220 Hz. If I turn the vol contol up further this frequency goes down to a min of approx 80Hz with the volume up full. At max treble setting the osc starts at 700Hz and goes down to approx 80 Hz at max volume.
I installed a full bridge rectifier to replace the existing half wave diode type rectifier. I would expect some hum around 120 Hz. Just to rule out power supply ripple, as an experiment, I installed a massive 2200uf cap across the B+ supply. I could not measure any ripple on B+ and the symptoms of the osc did not change.
The negative side of the bridge is tied to the metal chassis and I have connected all the components to the chassis with the shortest leads possible.
I have triple checked everthing and I cannot find any mistakes. New tubes makes no difference. I am confident that I have installed the bridge and iso trx correctly.
Any ideas?
I could not find anything related to this oscillation in that archive.
I have already installed the Triad N-68X transformer as per that archive.
The oscillation starts at approx half volume. The tone control changes the frequency of the oscillation but I think this would be expected. At max bass setting the freq of the osc is approx 220 Hz. If I turn the vol contol up further this frequency goes down to a min of approx 80Hz with the volume up full. At max treble setting the osc starts at 700Hz and goes down to approx 80 Hz at max volume.
I installed a full bridge rectifier to replace the existing half wave diode type rectifier. I would expect some hum around 120 Hz. Just to rule out power supply ripple, as an experiment, I installed a massive 2200uf cap across the B+ supply. I could not measure any ripple on B+ and the symptoms of the osc did not change.
The negative side of the bridge is tied to the metal chassis and I have connected all the components to the chassis with the shortest leads possible.
I have triple checked everthing and I cannot find any mistakes. New tubes makes no difference. I am confident that I have installed the bridge and iso trx correctly.
Any ideas?
Just out of the top of my head: originally line powered amps do not have the input and first stage circuit ground directly connected to chassis ground but separated from it by a high value resistor and a cap in parallel .... or the other way, the one which "floats" is the main PSU and power amp , for (poorly achieved) safety reasons, the point is that main supply ground and input stage ground are separated by said resistor and cap.
Humming/instability/etc. take a secondary place comparing to not killing user (or not shocking him too much).
Search for the parts I mention and bypass them, so input and output grounds are directly connected .
ALL THIS SUPPOSING YOU PROPERLY INSULATED AMP FROM LIVE AND NEUTRAL LINE VOLTAGE PINS USING A PROPER ISOLATION TRANSFORMER AND USED A 3 WIRE POWER CABLE, WITH GROUND WIRE SOLIDLY CONNECTED TO METALLIC CHASSIS GROUND .
If you still have problems after that, then it can be tested and solved like on any normal amp, but not before.
I suggest you edit or redraw the Pepco amp schematic showing ONLY the proper grounded version and post it here for revision.
Humming/instability/etc. take a secondary place comparing to not killing user (or not shocking him too much).
Search for the parts I mention and bypass them, so input and output grounds are directly connected .
ALL THIS SUPPOSING YOU PROPERLY INSULATED AMP FROM LIVE AND NEUTRAL LINE VOLTAGE PINS USING A PROPER ISOLATION TRANSFORMER AND USED A 3 WIRE POWER CABLE, WITH GROUND WIRE SOLIDLY CONNECTED TO METALLIC CHASSIS GROUND .
If you still have problems after that, then it can be tested and solved like on any normal amp, but not before.
I suggest you edit or redraw the Pepco amp schematic showing ONLY the proper grounded version and post it here for revision.
Yes, I have bypassed the 150 ohm 5W resistor and the .047uf cap that was located between the chassis and the line voltage.
The 35W4 was repalced with a SS bridge rectifier and I used a triad N-68X isolation transformer. In order to prevent excessive voltges on the plate of the 50C5, I installed a 1k 10W resistor on the B+ side of the bridge. This provides the 134v B+ as per the attached schematic. The heaters are fed directly from the secondary side of the iso trx. The negative side of the bridge is tied to the chassis and the ground wire from the 3 prong line cord is also attached to the chassis.
I am confident that I have peformed this mod correctly. Therefore, I'm at the point where the issue lies within the amp circuit.
After further troubleshooting, I found a way to illimiante the oscillation but I don't know why it works. When I put my DVM acoss pin 7 of the 12AX7 and the chassis the oscillation will stop. So I added a grid leak resistor and a stopper resistor to pin7 but this did not work.
The osc starts at approx 25k of the 500k vol pot on the gate of the 12AX7. (pin7)
So, I replaced the pot with a 470k resistor in series with a 25k pot and no more oscillation. I can even change the 470k to a 100k resistor in series with the 25k pot and I get lots of output without the oscillation.
So, I replaced the 500k pot thinking it was defective. I get the oscillation again.
The 35W4 was repalced with a SS bridge rectifier and I used a triad N-68X isolation transformer. In order to prevent excessive voltges on the plate of the 50C5, I installed a 1k 10W resistor on the B+ side of the bridge. This provides the 134v B+ as per the attached schematic. The heaters are fed directly from the secondary side of the iso trx. The negative side of the bridge is tied to the chassis and the ground wire from the 3 prong line cord is also attached to the chassis.
I am confident that I have peformed this mod correctly. Therefore, I'm at the point where the issue lies within the amp circuit.
After further troubleshooting, I found a way to illimiante the oscillation but I don't know why it works. When I put my DVM acoss pin 7 of the 12AX7 and the chassis the oscillation will stop. So I added a grid leak resistor and a stopper resistor to pin7 but this did not work.
The osc starts at approx 25k of the 500k vol pot on the gate of the 12AX7. (pin7)
So, I replaced the pot with a 470k resistor in series with a 25k pot and no more oscillation. I can even change the 470k to a 100k resistor in series with the 25k pot and I get lots of output without the oscillation.
So, I replaced the 500k pot thinking it was defective. I get the oscillation again.
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