Hello,
Recently I have acquired a complete Papworth setup consisting of a preamp and 2 M200 mono's. The pre was pretty much okay, the Mono's were mistreated by not changing tubes for ages.
By now I have cleaned them out and replaced some broken resistors.
It looks like these amps are fixed bias. Looking for a schematic or values for the correct bias settings.
All help is welcome.
Greetings from rainy The Netherlands
J.
Recently I have acquired a complete Papworth setup consisting of a preamp and 2 M200 mono's. The pre was pretty much okay, the Mono's were mistreated by not changing tubes for ages.
By now I have cleaned them out and replaced some broken resistors.
It looks like these amps are fixed bias. Looking for a schematic or values for the correct bias settings.
All help is welcome.
Greetings from rainy The Netherlands
J.
Papworth website is not updated for nearly 20 years and the email address is no longer valid.
The messy schematic of model M 100 may be similar :
https://www.dropbox.com/s/dd7xpj6w5d1eqcl/M100HR.jpg?dl=0
The messy schematic of model M 100 may be similar :
https://www.dropbox.com/s/dd7xpj6w5d1eqcl/M100HR.jpg?dl=0
Wow! Many thanks as_audio.
From what I do see inside of the M200's, there is a big similarity in the op stage with what's on the M100 diagram. Just double the amount of EL34's and a 12BH7 as a driver.
The bias setting makes sense too. 25mA @ 500V plate is about 60% of max for an EL34. I think I can try this and will post my results once I have tried.
From what I do see inside of the M200's, there is a big similarity in the op stage with what's on the M100 diagram. Just double the amount of EL34's and a 12BH7 as a driver.
The bias setting makes sense too. 25mA @ 500V plate is about 60% of max for an EL34. I think I can try this and will post my results once I have tried.
So I continued. After carefully checking all Voltages and inserting 8 new JJ's, Mono number 1 biassed up correctly. That is, I started with g1 as negative as possible and then slowly up. No issues: flawless and it seems it holds the bias well on all tubes.
Number 2 is a different beast. After doing exactly the same, it starts making really loud ticks. (Usually only one or 2 every minute) starting after a minute of warm up.
And currently the tubes in mono number 2 are only biassed for around 5mA or so. I stopped there because of the loud ticks coming out of my test speaker.
So I went on and tested all tubes on my CT160. Tapping the glass envelope lightly to see if none of them is misbehaving. Unfortunately to no avail so far.
What could this be? I have a spare set of EH EL34's that I may try tomorrow and see if those keep silent.
To be continued.
Number 2 is a different beast. After doing exactly the same, it starts making really loud ticks. (Usually only one or 2 every minute) starting after a minute of warm up.
And currently the tubes in mono number 2 are only biassed for around 5mA or so. I stopped there because of the loud ticks coming out of my test speaker.
So I went on and tested all tubes on my CT160. Tapping the glass envelope lightly to see if none of them is misbehaving. Unfortunately to no avail so far.
What could this be? I have a spare set of EH EL34's that I may try tomorrow and see if those keep silent.
To be continued.
Let it run for some time, under-biased, no speaker but resistive load. Monitor the voltages.
It may get better with time or you will see the failing component later ..
It may get better with time or you will see the failing component later ..
Since one monoblock is ok,maybe you could swap all tubes, including the other stages.
Also a table of all voltages on valve pins could help flag up a difference.
I updated the electrolytics in my TVA10, which looks similar. There are electrolytics in the PSU and also in the crosslinked OPT windings, and they could need to be replaced by now.
Also a table of all voltages on valve pins could help flag up a difference.
I updated the electrolytics in my TVA10, which looks similar. There are electrolytics in the PSU and also in the crosslinked OPT windings, and they could need to be replaced by now.
It would be at guess and highly speculative to answer the question, so
I will leave this to others. Why no close up picture of the open bottom ?
I will leave this to others. Why no close up picture of the open bottom ?
Those electrolytics in the crosslinked OPT windings sound like a real possibility to me.
I have done a complete Voltage comparison and they both carry almost exactly the same Voltages. I guess I will be swapping the tubes tomorrow.
You can see (close to the RCA input) where I found a fried (exploded) cathode resistor. Obviously the old tubes were cooked.
I do have a variac but it isn't strong enough for this kind of amp. Are you suggesting that it may suffer from deformed electrolytics?
It will do no harm if you run it at full voltage, but some defective non-essential part may eventually blow.
I think I will not take that chance and try to be a tad more methodical.
Plans for tonight:
Swap the tube set
Locate the cross winding capacitors and change those if needed.
Eventually (if nothing helps) Imay run it at half mains Voltage for some time.
Plans for tonight:
Swap the tube set
Locate the cross winding capacitors and change those if needed.
Eventually (if nothing helps) Imay run it at half mains Voltage for some time.
