After a failed attempt of a cloned Maplin 4-20 amplifier I thought I would rescue the expensive components and try this Williamson design PCB.
I won't get the schematics until the PCB arrives but it appears to use the 4 x octal sockets for the EL34 output tubes and B9A sockets for the pre-amp and phase splitter tubes.
I'm guessing these will be EF86 and ECC83.
My OPTs are Hammond 1650H.
I won't get the schematics until the PCB arrives but it appears to use the 4 x octal sockets for the EL34 output tubes and B9A sockets for the pre-amp and phase splitter tubes.
I'm guessing these will be EF86 and ECC83.
My OPTs are Hammond 1650H.
Attachments
My rescued B+ transformers are probably a bit high at 500VDC but I'm aiming to drop B+ in a CRC configuration.
Initial C is 220uF then R at 200R with the final C at 600uF.
Initial C is 220uF then R at 200R with the final C at 600uF.
Might be OK for testing but the HT will droop with current changes on the PP if it goes into class B. May also cause LF instability. You would be better with the correct transformer.
I don't think U1 is an EF86 looking at the PCB.
Looks more like this:
I don't think U1 is an EF86 looking at the PCB.
Looks more like this:
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That schematic has no global NFB. That’s not going to oscillate unless you do something really stupid in the power supply to make unintentional feedback (the only mechanism left). All this bickering we’ve been doing applies only when global feed back is used.
Triode-wired outputs often sound very good and have sufficient damping without any global feedback. Is your board wire this way, and does it have any feedback from the secondary of the OPT to the cathode of the input tube?
Triode-wired outputs often sound very good and have sufficient damping without any global feedback. Is your board wire this way, and does it have any feedback from the secondary of the OPT to the cathode of the input tube?
I am not saying this is the actual schematic, but from the PCB the first stage and LTP do look like this.
Where the heck did that schematic come from? That's the schematic for my Altec 1569A amplifier that I drew 20 years ago!
I would do it differently today; it is from my Williamson obsession days.
I would do it differently today; it is from my Williamson obsession days.
The PCB is stereo so the valve count doesn't equate with that schematic.
The actual schematic will be coming with the PCB.
The actual schematic will be coming with the PCB.
Running the finals in UL, and both gain stages are low mu. There’s got to t be less than 10dB of feedback, lessening the chances of an instability. You may find it linear enough with the global feedback disconnected (and want the extra gain). Many of these amps that have instabilities use pentode mode outputs and high mu or even pentode front ends. Lots of open loop gain, and hence the stability problems.
On the HV supply, is ok to use 100uF / 10H / 100uF. Then, lighten up on the preamp caps...20 uF work just fine. Also, the first preamp rail resistor in the wrong place...
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It does its got a few volts across it and is dissipating a watt or two. Is got all the cathode current flowing through it. I guess in those days big trimmers were common. We would not do that today. The first rail resistor can come from the HT rather than the second rail depends on the filtering and HT you need. A LC filter is a good idea.
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