Hi, I'm new to this forum so please bear with my ignorance! I have a Mengyue mini that has four multi-turn presets (I assume for bias adjustment), what I would like to know is how to set up / adjust bias settings. The reason I ask is that it had problems with hum and popping intermittently on the left channel so I have gone through and replaced all the electrolytic caps, the hum has gone but the popping is still there, it seems to follow the output valves so a valve problem is suspected? I have just ordered a new set of valves but I'm not sure if the bias is out of adjustment or will need changing with new / different valves?
Any help would be greatly appreciated! 🙂
Any help would be greatly appreciated! 🙂
Thanks for your quick response! Part of the problem is finding an "accurate" schematic diagram, I have found a few on the net but they all seem to be different and comprise different value components too. Some of the images clearly show the pre-sets, others don't appear to have them?
I rebuilt my Mengyue mini, and as I recall the presets were not for bias (it was a cathode biased amp). They were actually a humdinger and PI balance pot for each channel.
If it is a 6P1 tubed Mini, then look on this forum for a long thread on a big rebuild that a couple of us did. Mine is long sold, so I can't be of much more help.
An externally hosted image should be here but it was not working when we last tested it.
I found this image on the net showing the pre-set trimmers, not sure if this helps at all?
Yep, that's how mine looked. The ones mounted practically on the valve sockets are the humdingers.
Great! Do you know what anode current they are meant to run at? That is do you have a specific voltage across the anode resistors or a specific measurement point?
The PI is a paraphase, so one of the triodes (the "slave") receives its signal from a potential divider controlled by the trimmer, something like this:
You therefore need to hook a scope up to the grids of the power valves and adjust the trimmer until you see equal signals. It doesn't affect the DC voltages.
An externally hosted image should be here but it was not working when we last tested it.
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You therefore need to hook a scope up to the grids of the power valves and adjust the trimmer until you see equal signals. It doesn't affect the DC voltages.
On checking the grid wave forms with a sig' gen' & scope I found that one of the valves was way out in comparison to the others but jumped on adjusting, looks like the pot needed a little "exercising" .Another thing, having replaced all the electrolytic caps I notice that they are still getting pretty darned hot, any thoughts there? Putting music through it it sounds great, no more "popping" either, I think it may have been the dodgy pot!
ps, this forum has been a real boon to me, thanks for all who have taken the trouble to respond to my questions!
ps, this forum has been a real boon to me, thanks for all who have taken the trouble to respond to my questions!
Mine had the output tubes biased really hot, like 15W each or something, so the transformer also got very hot, so I guess the air temp inside the case could be pretty high which might explain your hot caps (unless you've connected them backwards or something!).
When I rebuilt mine I biased them a lot cooler. Less output power, but I'd rather have a mini amp than one with a burnt out power transformer...
When I rebuilt mine I biased them a lot cooler. Less output power, but I'd rather have a mini amp than one with a burnt out power transformer...
Hi, I'll try and check the biasing, I'm running the unit upside down at the moment to get to the pre-sets, it's only some of the caps getting hot & I was careful in checking polarity on replacing them, it's been on long enough to have popped the cases if they were round the wrong way I think? I'll try and check across the anode resistors & figure out what sort of power I'm running at. Thanks for your help 🙂
Hi, I've just ordered a couple of 50K pots to replace the existing ones, one of them is definitely suspect and is going open cct! There are two other pre-sets (200ohm) which are across the 6.3Vac heater supplies to ground, am I right in thinking that these should be adjusted to a mid point between the 6.3 Vac?
Getting there by degrees! 🙂
Getting there by degrees! 🙂
The heater trimmers ("humdingers") should be adjusted for minimum hum, which may or may not be anywhere near the middle.
IIRC, with mine there wasn't much difference where they were set, except for one thing: one of the heater wires was shorted to ground inside the transformer (Chinese, lol!)! Therefore, I had no choice but to remove that humdinger. There wasn't a hum problem in any case.
IIRC, with mine there wasn't much difference where they were set, except for one thing: one of the heater wires was shorted to ground inside the transformer (Chinese, lol!)! Therefore, I had no choice but to remove that humdinger. There wasn't a hum problem in any case.
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