As Kevinkr points put if thAT 10R cath ressy is correct then you have 50mA per tube which is correct. All 4 EL34's should be similiar.
I think the ECC88 is a bomb shell for a phasesplitter unless one really knows how to use it. Try the ECC83 inlieu (with heater wiring surgury) and have a peep at the values given in the Philips data sheet (Franks pages).(I can't make out the values on that schematic)
The plate volts (280) is highish, and I suspect that something else is going on. The ECC88 is really an RF triode and will nicely oscillate into the vhf with poor layout. The decoy is, it will pass audio too.
I noticed this in my preamp line output stage.....the use of a screening can actually solved the problem......
Don't alter the output stage.....there are loads exactly the same around which work. A trick one can do is to disconnect the coupling caps (or remove the ECC88) and see if the problems you have disappear.
Do things sequentially.
richj
I think the ECC88 is a bomb shell for a phasesplitter unless one really knows how to use it. Try the ECC83 inlieu (with heater wiring surgury) and have a peep at the values given in the Philips data sheet (Franks pages).(I can't make out the values on that schematic)
The plate volts (280) is highish, and I suspect that something else is going on. The ECC88 is really an RF triode and will nicely oscillate into the vhf with poor layout. The decoy is, it will pass audio too.
I noticed this in my preamp line output stage.....the use of a screening can actually solved the problem......
Don't alter the output stage.....there are loads exactly the same around which work. A trick one can do is to disconnect the coupling caps (or remove the ECC88) and see if the problems you have disappear.
Do things sequentially.
richj
The only other coupling capacitor I see in the driver circuitry would be the one on the grid of the ECC85, if this one was leaky it would cause the operating point of the phase splitter to change, and could result in large amounts of distortion.
I think the best way to trouble shoot this would be to drag the amplifier down to your lab, fire it up and do a series of dc measurements on the plates, and cathodes of the ECC83/12AX7A and the ECC85/6DJ8. You can measure the voltage at the rc network on the ECC85 - bear in mind that your meter's input resistance will probably load down the voltage on this node. Measure also the plate supply and the voltage across the 150uF driver stage decoupling capacitor. Record them and remeasure about 20 minutes later.
Check the 150uF driver stage decoupling capacitor as well as the others in the supply, you should not see much audio on any of these caps. The first cap after the rectifier may have several (or more say 10Vpk-pk) Vpk-pk ripple on it, this is normal. 100V of ripple would not be normal for example.
You should not add any decoupling capacitors from the cathodes to ground in any stage of this amplifier, doing so will defeat the feedback connections in the first stage and render the phase splitter inoperative in the second. Component values look reasonable.
Have your recently replaced any of the tubes? Try substituting other ones if available for the 12AX7 and 6DJ8.
Hard to read the schematic so if I have got a tube type wrong in one of the stages my apologies.
Kevin
I think the best way to trouble shoot this would be to drag the amplifier down to your lab, fire it up and do a series of dc measurements on the plates, and cathodes of the ECC83/12AX7A and the ECC85/6DJ8. You can measure the voltage at the rc network on the ECC85 - bear in mind that your meter's input resistance will probably load down the voltage on this node. Measure also the plate supply and the voltage across the 150uF driver stage decoupling capacitor. Record them and remeasure about 20 minutes later.
Check the 150uF driver stage decoupling capacitor as well as the others in the supply, you should not see much audio on any of these caps. The first cap after the rectifier may have several (or more say 10Vpk-pk) Vpk-pk ripple on it, this is normal. 100V of ripple would not be normal for example.
You should not add any decoupling capacitors from the cathodes to ground in any stage of this amplifier, doing so will defeat the feedback connections in the first stage and render the phase splitter inoperative in the second. Component values look reasonable.
Have your recently replaced any of the tubes? Try substituting other ones if available for the 12AX7 and 6DJ8.
Hard to read the schematic so if I have got a tube type wrong in one of the stages my apologies.
Kevin
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