P.S. I have an e-mail I just re-discovered from a well know designer (whose name I shall keep anon here 
) who suggested the following:
1) Increase the 0.47uF 400v film cap to 2.2uF for better bass response.
2) Reduce the 100uF 16v cap on the input to 10uF (rather than remove it)
3) Make the 22k resistor on the NF 4K7.
4) Make the 330k resistor 100k.
Any comments on any of these mods? I'm going with yours Kevin as these have been tried and tested, but any up there which you care to comment on or might be useful implementing? (i.e. the 2.2uF cap?).
Many thanks (and Happy New Year folks )
- John
) who suggested the following:1) Increase the 0.47uF 400v film cap to 2.2uF for better bass response.
2) Reduce the 100uF 16v cap on the input to 10uF (rather than remove it)
3) Make the 22k resistor on the NF 4K7.
4) Make the 330k resistor 100k.
Any comments on any of these mods? I'm going with yours Kevin as these have been tried and tested, but any up there which you care to comment on or might be useful implementing? (i.e. the 2.2uF cap?).
Many thanks (and Happy New Year folks
)- John
#2 will definitely free you from the constraints of flat frequency response.
#1 will extend the feedback to even deeper subsonic frequencies, though why that should be a good thing in this case eludes me.
#3 will drop the gain considerably, but may turn the unit into a fine oscillator.
I don't know who the designer in question is, but I'd put my money on Kevin's suggestions.
#1 will extend the feedback to even deeper subsonic frequencies, though why that should be a good thing in this case eludes me.
#3 will drop the gain considerably, but may turn the unit into a fine oscillator.
I don't know who the designer in question is, but I'd put my money on Kevin's suggestions.
Many thanks for your comments, and - agreed - I'll be going with Kevin's suggestions at any rate
One question, Kevin said with the 330k that it doesn't need a wire link in place of the resistor once it's been snipped out, but surely this isn't the case? The 3.3uF cap needs to join to the point between the 1k and 2nd (on the circuit diagram) 12AX7?
Sorry if this appears obvious - I like to triple check everything before I begin though
Thanks,
- John
P.S. Just noticed on the circuit diagram there's a 0.022uF film cap between the 270k resistors - inside my Ming da those are 0.01uF. Does this make any difference here, and which would I be better off sticking with? Thank you.
One question, Kevin said with the 330k that it doesn't need a wire link in place of the resistor once it's been snipped out, but surely this isn't the case? The 3.3uF cap needs to join to the point between the 1k and 2nd (on the circuit diagram) 12AX7?
Sorry if this appears obvious - I like to triple check everything before I begin though
Thanks,
- John
P.S. Just noticed on the circuit diagram there's a 0.022uF film cap between the 270k resistors - inside my Ming da those are 0.01uF. Does this make any difference here, and which would I be better off sticking with? Thank you.
Gotcha - many thanks for that - they're now snipped out along with short lengths of associated wiring.
Two more questions if I may (this amp seems to throw them up every second!), again on the circuit diagram there is a 1k resistor, just to the left of the 22k NF resistor, and joined to the 3k3 resistor.
In my amp this is actually 22k instead of 1k. Again, should I change this to 1k - as listed - or is it OK at 22k?
Lastly (I hope!) just noticed that while both 12AX7 valve bases share indentical wiring layouts underneath, the 12AU7s seem to share an almost 'mirror image' wiring layout instead - the same components on both sides do not go to the same pins on each base, even though the heater wiring remains identical. If this normal, or does it indicate some huge mistake on the part of the manufacturer of this amp?
As an example, the output wiring to the 3.3uF cap from one 12AU7 is taken from pin 8. But on the other base the output signal to 3.3uF is taken from pin 3. ???
Thanks.
- John
Two more questions if I may (this amp seems to throw them up every second!), again on the circuit diagram there is a 1k resistor, just to the left of the 22k NF resistor, and joined to the 3k3 resistor.
In my amp this is actually 22k instead of 1k. Again, should I change this to 1k - as listed - or is it OK at 22k?
Lastly (I hope!) just noticed that while both 12AX7 valve bases share indentical wiring layouts underneath, the 12AU7s seem to share an almost 'mirror image' wiring layout instead - the same components on both sides do not go to the same pins on each base, even though the heater wiring remains identical. If this normal, or does it indicate some huge mistake on the part of the manufacturer of this amp?
As an example, the output wiring to the 3.3uF cap from one 12AU7 is taken from pin 8. But on the other base the output signal to 3.3uF is taken from pin 3. ???
Thanks.
- John
Well completed all the mods, step by step - must have triple checked everything a million times haha!
Result - it works - it's ALIVVVEEE!!!! I'd like to say a huge thank you to all who helped, particularly Kevin and Sy! And Hirafu_border for starting the thread of course
Volume is alot more manageable now, and the sound does indeed appear more open and life like now. Still has a thunderous bass as well which I was scared of losing, but is thankfully still there. Still need to try a set of 5751's when I have some spare cash to lower the gain still.
The only problem I have is a low level purr from BOTH speakers, and is totally independent of volume position. I take it this is an earth loop of some kind? Checked ALL the soldered earthing connections internally and all are sound. There's also a 10 ohm earth-lift resistor inside as well.
The CD player I use is earthed, as is the Ming da MC-7R. The amps power supply is earthed, but the amp itself runs from a DC only umbilical (Kingrex PSU powering an AMP6 Basic).
Any suggestions on how to best this humming beast?
Many thanks again!
- John
Result - it works - it's ALIVVVEEE!!!! I'd like to say a huge thank you to all who helped, particularly Kevin and Sy! And Hirafu_border for starting the thread of course
Volume is alot more manageable now, and the sound does indeed appear more open and life like now. Still has a thunderous bass as well which I was scared of losing, but is thankfully still there. Still need to try a set of 5751's when I have some spare cash to lower the gain still.
The only problem I have is a low level purr from BOTH speakers, and is totally independent of volume position. I take it this is an earth loop of some kind? Checked ALL the soldered earthing connections internally and all are sound. There's also a 10 ohm earth-lift resistor inside as well.
The CD player I use is earthed, as is the Ming da MC-7R. The amps power supply is earthed, but the amp itself runs from a DC only umbilical (Kingrex PSU powering an AMP6 Basic).
Any suggestions on how to best this humming beast?
Many thanks again!
- John
Glad to hear that it is working and sounding so much better.
It would be good to identify which of the components is the culprit.
I'd start by disconnecting the source and seeing if that helps..
Next temporarily lift the ground on the power amplifier using a cheater (or probably make a 2 wire line cord since you are in the UK) - this should not be the long term solution as you have defeated the safety ground.
Once you identify the source fixes can be applied as and where needed.
It would be good to identify which of the components is the culprit.
I'd start by disconnecting the source and seeing if that helps..
Next temporarily lift the ground on the power amplifier using a cheater (or probably make a 2 wire line cord since you are in the UK) - this should not be the long term solution as you have defeated the safety ground.
Once you identify the source fixes can be applied as and where needed.
OK just tried the MC-7R with a cheater plug and still get the hum, so the problem is with the MC-7R then I guess.
Could it be an impedence mis-match as I am using it with a solid state (for the time being until my tube power amps are finished) amplifier? Or perhaps an earth loop within the MC-7R itself?
Many thanks,
- John
Could it be an impedence mis-match as I am using it with a solid state (for the time being until my tube power amps are finished) amplifier? Or perhaps an earth loop within the MC-7R itself?
Many thanks,
- John
Reason I said that was a few years back when I was using my Ming da with a totally re-built Quad 303 the hum/gain problem was solved for me (by some kind soul on the World Audio Design forums I believe) by the suggestion of using a potential divider network soldered into each phono output plug on the interconnect between pre and power amp. This worked like a treat - no hum at all. Such a while ago I cannot remember the divider specs for the resistors though alas...
Another strange thing, when turning my Amp6's KingRex PSU on the fuse has blown twice today... seems like I need to let the MC-7R totally warm up before turning on the power amp or the odd noise while the valves warm up seems to make the power amp blow it's fuse. Odd!
- John
Another strange thing, when turning my Amp6's KingRex PSU on the fuse has blown twice today... seems like I need to let the MC-7R totally warm up before turning on the power amp or the odd noise while the valves warm up seems to make the power amp blow it's fuse. Odd!
- John
Not a big problem if the filament supplies are regulated, otherwise use a small value resistor to reduce the voltage to acceptable values. The transformer itself is probably just fine. 230V/240V mains is well within the 10% margin that any reasonably good 220V transformer should be designed to. If the transformer has a dual primary (international version power tranny) it is possible that it is 120V/240V despite what the rear panel says.
Measure the voltage at a couple of the tube sockets and let me know what it is.
Measure the voltage at a couple of the tube sockets and let me know what it is.
johnm said:
No, this isn't odd - it is a very common problem when using tube pre-amps and solid state amplifiers. I have encountered it repeatedly with modern and vintage kit. The solution is to allow the pre-amplifier to warm up for at least several minutes prior to turning on the SS amplifier. Be sure to turn off the amp FIRST and then the pre-amp to prevent similar misbehavior. (The issue is very large dc transients during warm up and shutdown at the output of the pre-amplifier.)
You can add a timed relay to the output of the pre-amp that does not unmute (outputs shorted) until the power has been on for about 2 minutes. A 555 timer IC, a few passives and a dpdt relay with audio outs wired to the NC contacts works very well. (Other side of relay contacts are grounded - this way no additional contacts are in the signal path when operating.) Use a relay expressly designed to switch low level signals. Coto, Panasonic and a couple of others make suitable ones.
Incidentally if you are going to be listening a lot during the course of the day it does not make sense to turn the pre-amplifier on and off frequently, on at start of day and off at end - exception is if you are not going to be around for awhile. You are greatly increasing the likelihood of tube and other component failure, etc. In addition tube gear usually sounds best after several hours of warm up/use.
A tube amplifier would both sound better and eliminate this concern.
Valves rule and all that rot!
Before you can do a lot of additional trouble shooting please let us now what kind of test equipment you have at your disposal. First and foremost do you have a scope? Indispensable for anyone building or modding gear, imo this comes right after a good dvm and solder iron. And before anything else.
Your noise issue could be an internal ground loop, noisy supply, wiring error when doing the mods (unlikely I think) or an external loop. It could be a variety of other things like an undetected vhf oscillation in the pre-amplifier for example.
Your noise issue could be an internal ground loop, noisy supply, wiring error when doing the mods (unlikely I think) or an external loop. It could be a variety of other things like an undetected vhf oscillation in the pre-amplifier for example.