Hi
I have replaced the output transformer and all the tubes in this amp about 3 months back. It was working fine except for a very slight hum from the speakers. Now it is back with low volume and distorted sound .With the master volume at 5, low, mid and treble at 5, as I increase the channel volume it sounds OK until 3 and above 3, it does not seem to increase much but starts to distort badly. There is not much increase in volume from 3 to 10, but severe distortion. I have checked all the caps and resistors, and they are OK. One side of the plate winding has a DC resistance of 40 ohms, drops 2.7 volts across at a plate voltage of 470 volt. So dissipation is at 31.7 watts per pair. Dissipation the other side is 33.3 watts. It is biased low, but I think this should not be the cause for such low volume and bad distortion. One thing I noted is that when I plug the speaker into the Ext speaker socket, there is no sound. Schematic attached. Please assist.
Thanks
Sorry, this happens on both channels.
I have replaced the output transformer and all the tubes in this amp about 3 months back. It was working fine except for a very slight hum from the speakers. Now it is back with low volume and distorted sound .With the master volume at 5, low, mid and treble at 5, as I increase the channel volume it sounds OK until 3 and above 3, it does not seem to increase much but starts to distort badly. There is not much increase in volume from 3 to 10, but severe distortion. I have checked all the caps and resistors, and they are OK. One side of the plate winding has a DC resistance of 40 ohms, drops 2.7 volts across at a plate voltage of 470 volt. So dissipation is at 31.7 watts per pair. Dissipation the other side is 33.3 watts. It is biased low, but I think this should not be the cause for such low volume and bad distortion. One thing I noted is that when I plug the speaker into the Ext speaker socket, there is no sound. Schematic attached. Please assist.
Thanks
Sorry, this happens on both channels.
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No, problem with the crt, very blur image. Today I change the bias resistor from 33K to 27K. I got back my original volume level, but with master at 5, if I go above 3 on either channel volume, it distorts. There is a slight hum. The former OT had one of its primary shorted. The 470 ohms screen resistors and 1500 ohm resistors across the grid are OK. These are the readings I got today. The right primary winding reads 37 ohms DC and voltage drop of 2.69 volts at 505V plate voltage. The left winding reads 14 ohms DC and drops 1.3 volts at 505V plate voltage and that is the maximum voltage I can adjust on this side. I dont understand the large difference in DC resistance between the two primary windings. Can there be a partial short on the left winding. As an alternative, I soldered a 1 ohm resistor across the cathode and ground on both sides. I am getting 39mv on the right side and 60mv on the left side. The power dissipation calculations are different using the primary winding method and the cathode resistor method. They do not tally. Which is reliable. I am using 6L6EH Electro Harmonics tubes. Please assist.
Thanks
The difference between the DC resistance readings actually is weird. Are you sure that you didn't accidentally swap the plate and screen taps at one side? Otherwise we have to assume shorted turns again.
Best regards!
There is something I am not sure about. If you can spare some time and refer to the 2 attachments. On the amp schematic, the blue wire connects to the plate and the blue/yellow tap connects to the screen on the same side. On the other side, the brown connects to the plate and the brown/yellow connects to the screen on this side.
Referring to the OT schematic, note that the blue wire connects to the plate but the brown/yellow connects to the screen of thst side. On the other side, the brown connects to the plate but the blue/yellow connects to the screen of this side. I was not sure ( I thought an error on the OT schematic regarding the colours of the wires) so I followed the colour scheme as on the amp schematic, that is blue and blue/yellow on one side, brown and brown/yellow on the other side. This means that if the OT schematic (the colour of the wires) is correct then I have reversed the screen wires, but the plate wires are connected correctly. This was done about 3 months ago when I replaced the OT and the amp seem to have been working OK all this while until now. What's your opinion.
Thanks
The Hammond data sheet is clearly a typo.
Your amp was working before, why would we decide the wiring needs to be changed now?
One of my scopes long ago all of a sudden got to a real fuzzy image. I ordered up the manual, and read over the circuit, trying to decide what must be bad with the focus circuit. Ince I actually opened it up and looked, I saw the third anode cup had popped free from the side of the CRT. Snap that back on and clear picture returned. Worth a look.
Your amp was working before, why would we decide the wiring needs to be changed now?
One of my scopes long ago all of a sudden got to a real fuzzy image. I ordered up the manual, and read over the circuit, trying to decide what must be bad with the focus circuit. Ince I actually opened it up and looked, I saw the third anode cup had popped free from the side of the CRT. Snap that back on and clear picture returned. Worth a look.
Thanks for the hint. Now I cannot set the bias using the OT method as the winding resistances are way out and I cannot balance the bias as the voltage drop across the left side cannot get higher then 1.3V. Using the 1 ohm cathode resistor method, I can balance the voltage drop across each resistor. Is this 1 ohm resistor method really more reliable than the other.
Thanks
To set the bias current, fit a 10R 2W resistor in the cathode of each pair. Measure the voltage and that will give you the quiescent current accurately.
The DC resistance of the transformer looks suspect and definitely not to be trusted to work out the quiescent current.
There should be no hum if the valves are balanced properly.
The DC resistance of the transformer looks suspect and definitely not to be trusted to work out the quiescent current.
There should be no hum if the valves are balanced properly.
Not too long ago you opened another thread upon your Twin. Finally you confirmed that it was working flawlessly. And now you have another issue with it? What happened to it in the meantime?
Again: Follow my hint given there to confirm that the OT ist connected properly and that this aftermarket thang has the correct colour codes. As said before, I have the suspicion that the plate and screen leads of one side are swapped, most probably because of wrongly colour coded leads.
Best regards and good luck!
Again: Follow my hint given there to confirm that the OT ist connected properly and that this aftermarket thang has the correct colour codes. As said before, I have the suspicion that the plate and screen leads of one side are swapped, most probably because of wrongly colour coded leads.
Best regards and good luck!
It is the shorted turnings on one side. I manage to get a friend of mine who has the same amp to let me use the OT from his amp for testing. I put in the OT in exactly the same colour sequence. It works perfectly normal. I do not know what to do with this damaged transformer. I dont think the supplier will provide a replacement at no cost. By the way, thanks you all for all the assistance.
Rgds
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