Hi,
R6 and 7, replace the 47K resistors with 18K (2W)
R9, the common cathode R goes from 68K to 27K (3W)
The, to maintain the same B+ as before the series R in the PS R11 drops to 10K iso 20K 3W
Finally R14 and R15 are two 5K1 Rs in series which need to be replaced with a single 7K5 resistor.
Ciao,
R6 and 7, replace the 47K resistors with 18K (2W)
R9, the common cathode R goes from 68K to 27K (3W)
The, to maintain the same B+ as before the series R in the PS R11 drops to 10K iso 20K 3W
Finally R14 and R15 are two 5K1 Rs in series which need to be replaced with a single 7K5 resistor.
Ciao,
Hi,
R6 and 7, replace the 47K resistors with 18K (2W)
R9, the common cathode R goes from 68K to 27K (3W)
The, to maintain the same B+ as before the series R in the PS R11 drops to 10K iso 20K 3W
Finally R14 and R15 are two 5K1 Rs in series which need to be replaced with a single 7K5 resistor.
Ciao,
Wow! Thx a bunch! Is it your own calculation?
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I thought, haven't I seen that before and i checked my backup drive at home;
Reason why i never dear to try it out is that this changes is for the first version of mc100. Dated back to 2008-09. At this time pt was wrong (220v) and later (2012) yaqin made some changes for EU.
I just thought these changes applied for the "old" mc100 and maybe i could fry something in mine using these values.
For example at that time B+ was measured 430 something and now at same place around 485-510V.
Reason why i never dear to try it out is that this changes is for the first version of mc100. Dated back to 2008-09. At this time pt was wrong (220v) and later (2012) yaqin made some changes for EU.
I just thought these changes applied for the "old" mc100 and maybe i could fry something in mine using these values.
For example at that time B+ was measured 430 something and now at same place around 485-510V.
Attachments
Hi I have just got a yaqin mc100b from eBay cheap because it had a missing output valve kt88.
I have fitted all new KT120 valves but I was wondering what MA I should bais the valves at.
I have checked the transformers on he amp and they have 240v stamped on them.
When I checked the voltage at the B+ and the valves I got readings of 6.45v heaters and 475v B+.
I tested this with two meters and got same results.
I have made a buck transformer 240v to 230v and this gives me 6.25v heaters and 450v B+.
The transformer I have used is a 160VA 2 x 9v 2x 8.8A transformer.
Should I use the buck transformer or would it be better without the buck transformer.
I have also ordered a pair of Phillips holland ECC83 triangle symbol 2F2 valves.
I am also looking at changing the other valves any ideas of other valves that would go with KT120s and the Phillips I have ordered or any other mods for this amp.
I have biased the kt120s at 73MA is this too high or too low any help please.
I have also cut holes in the back of power supply covers and fitted grills to help keep transformers cool.
I've only had this amp 5 days and it's my first valve amp.
I've got 6 x pio k40y 400v capacitors from a previous project can I use these anywhere to improve my amp.
Any help or ideas please.
I have fitted all new KT120 valves but I was wondering what MA I should bais the valves at.
I have checked the transformers on he amp and they have 240v stamped on them.
When I checked the voltage at the B+ and the valves I got readings of 6.45v heaters and 475v B+.
I tested this with two meters and got same results.
I have made a buck transformer 240v to 230v and this gives me 6.25v heaters and 450v B+.
The transformer I have used is a 160VA 2 x 9v 2x 8.8A transformer.
Should I use the buck transformer or would it be better without the buck transformer.
I have also ordered a pair of Phillips holland ECC83 triangle symbol 2F2 valves.
I am also looking at changing the other valves any ideas of other valves that would go with KT120s and the Phillips I have ordered or any other mods for this amp.
I have biased the kt120s at 73MA is this too high or too low any help please.
I have also cut holes in the back of power supply covers and fitted grills to help keep transformers cool.
I've only had this amp 5 days and it's my first valve amp.
I've got 6 x pio k40y 400v capacitors from a previous project can I use these anywhere to improve my amp.
Any help or ideas please.
6.45V for heaters is not that bad. When you install all Kt120 it will get lower due to higher filament current of these tubes and thus higher loss in power transformer. B+ is low enough already, so you do not want any lower here. So I vote against backing transformer.
With Kt120 tubes I recommend bias set at 100-105 mA.
If you change capacitors to k40y, check this amp with square wave signal for excessive ringing. These capacitors have higher internal inductance and can alter phase enough to make amplifier unstable.
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Thanks for your reply and help.
6.45v is with the tubes in place 475v is also with the tubes in place.
When I had the KT88 tubes I was getting 6.7v at heaters but I had two valves missing.
It sounds great with the kt120 verry clear and detailed with a big soundstage.
I would like it to have a bit more warmth in the mid without losing any detail perhaps a bit more bass as well but this is just being picky.
6.45v is with the tubes in place 475v is also with the tubes in place.
When I had the KT88 tubes I was getting 6.7v at heaters but I had two valves missing.
It sounds great with the kt120 verry clear and detailed with a big soundstage.
I would like it to have a bit more warmth in the mid without losing any detail perhaps a bit more bass as well but this is just being picky.
Hi AJT I will measure the transformer tomorrow when my kids are at school and check the B+ voltages now I have increased the B+ 100MA.
I was thinking of adding two new transformers and mounting them inside the amp.
Thinking to use these transformers for 6.3v and make the 6.3v DC.
This might take some strain off the main transformer with the bigger KT120s.
I was also thinking of adding chokes and bigger caps to the 475 power supply would these mods help.
I'm new to valve amps but made a valve Dac a per lampizator site.
My dac uses DC heaters and chokes that's why I thought of using them on this amp.
I was thinking of adding two new transformers and mounting them inside the amp.
Thinking to use these transformers for 6.3v and make the 6.3v DC.
This might take some strain off the main transformer with the bigger KT120s.
I was also thinking of adding chokes and bigger caps to the 475 power supply would these mods help.
I'm new to valve amps but made a valve Dac a per lampizator site.
My dac uses DC heaters and chokes that's why I thought of using them on this amp.
Thank you.
My output transformers are 90mm wide x 110mm high,
100mm deep at the coils x 70mm wide 80mm high at coils.
Power transformer is 90mm wide x 110mm high,
110mm deep at coils 70mm wide at coils 80mm high at coils.
Basically the same sizes but power transformer is 10mm thicker at the coils.
The power transformer has 241v going onto transformer 366v coming out of transformer and 475v DC at B+ but I get 1.6v AC at B+.
The amp weighs 57lbs but I think most of the weight is the transformers.
My output transformers are 90mm wide x 110mm high,
100mm deep at the coils x 70mm wide 80mm high at coils.
Power transformer is 90mm wide x 110mm high,
110mm deep at coils 70mm wide at coils 80mm high at coils.
Basically the same sizes but power transformer is 10mm thicker at the coils.
The power transformer has 241v going onto transformer 366v coming out of transformer and 475v DC at B+ but I get 1.6v AC at B+.
The amp weighs 57lbs but I think most of the weight is the transformers.
Ive got some new valves coming from eBay 2 x Phillips ECC83 o getter triangle sign 2F2 miniwatt one tube tests 52-44 and the other 54-48 on a TV7Hickok tester what ever that means Cost £26 + £10 postage.
Two matched 1940s sylvania 6sn7gt black plate Bad boys tested on Hickok tv-7D 98-98 and 98-96 the other tube for £28 + £4.40 postage.
4 x new old stock Russian 6sn7 gt nos silver grid 6H8C on valves OTK and a diamond with a O in the middle.
£23 + £8 postage.
I'm not sure if I've got carried away and got rubbish or duff valves but I was itching to try some different valves.
Two matched 1940s sylvania 6sn7gt black plate Bad boys tested on Hickok tv-7D 98-98 and 98-96 the other tube for £28 + £4.40 postage.
4 x new old stock Russian 6sn7 gt nos silver grid 6H8C on valves OTK and a diamond with a O in the middle.
£23 + £8 postage.
I'm not sure if I've got carried away and got rubbish or duff valves but I was itching to try some different valves.
Hi Everyone, thought i would share my experience with this amplifier. I got it off Ebay delivered including taxes for £500. Ordered it thursday midnight and it was at my front door midday the following monday. Wow was quick.
Anyway i connected it up and had a listen. It sounded terrible to be honest. I reached for my soldering iron and got a circuit diagram. I removed the feedback loop completely. Got rid of the volume control and selector switch from the circuit. I then wired the front set of phono sockets to the grids and added a 47k grid stopper in series. I then noticed that the grid leak resistor was actually 510k and not 39k as per the drawing. I fitted 47k instead. I then removed the capacitor across the input stage cathode and made the two cathode resistors one value.
I threw away the ecc83s and 6sn7s that came with the amplifier and fitted mullard and sylvania respectively. I switched on and biased the output stage to 65ma having measured an anode voltage of approx 420Vdc.
Time to listen. I was gobsmacked by the sound I heard coming from my B&W683s. Sublime is the word. Amazing imaging and depth, with massive space around all the vocals and instruments. I played disc after disc and the hours flew by. I didn't want to switch it off. The sound continued to improve as the amplifier burnt in. Sounding good on CD and Vinyl.
6 months on and i'm still grinning. I have since replaced the ECC83s with Russian 6n2ps and modifed the PCB to take into account the different heater arrangement. I split the track between pins 4 and 5 with a junior hacksaw blade and moved the heater supply wire from pin 9 to the now spare pin 5. I've also replaced the 6 0.22uf caps with wima MKP 10s. This has added another dimension again. The amplifier sounds so fluid and natural and makes listening a real pleasure.
I use it with a Conrad Johnson PV10a and a heavily modded Mission PCM4000 and Technics 1210 with SME V.
My only wish is that the amplifier had more power, but for the money I can't complain. I punches well above it's weight.
I will post any further mods I make. Next mod is to use a bunch of Elna Cerafines leftover from another project in the power supply. Should spice things up a bit more.
Bye for now
Hi Aleks1 how did you remove ground loop does it make a lot of diferance ?
I was also thinking of changing the 6 blue 220nf capacitor for jantzen z silver oil 1.uf capacitors ?
What was the input capacitors you removed ?
My amp is showing 6.4v at heaters and 475v at B+.
The only mods I have done so far is change valves KT88 are now KT120s
Two of my 6N8p have been replaced with sylvania 6sn7gt 1940 bad boy and the 12AX7 have been replaced with 1960s Phillips miniwatts.
I like the sound best on triode and my speakers are sensitive 96db so verry loud at about 10 o,clock on my amp.
I got my amp cheap from eBay because one of the kt88 was missing and the onother was faulty (flashing)
The mains socket was also missing on my amp the last owner has fitted the power cable directly to the mains switch.
It's my first valve amp but it sounds much better than my old Cyrus two amp by miles.
Feedback loop removal
Hi Dave,
I assume you meant feedback loop removal ?
You need to break it somewhere between it's source at the output transformer secondary and it's termination at the cathode resistors R4 and R2. I obviously went further and simplified the first stage so that both cathode resistors were of the same value and removed the switch that governs the feedback ratio. Removing the cathode bypass cap reduces the overall gain of the amplifier which was desirable in my own case. I also added a grid stopper as the amplifier was more prone to noise after the mod.
Removing the feedback loop means the amplifier reverts back to higher gain open loop operation and also means the overal damping factor is reduced. Frequency response is also reduced but often this isn't problem for most users. Some people report that the bass can sound a bit bloated, I didn't notice this with my own set up. I guess subjectively the amplifier sounded more open and I prefer the sound of open loop or low feedback designs over designs with a lot of feedback. All you can do is give it a try and see if you like it, if not keep it closed loop. It also depends what speakers you have and the complex load they present to the amplifier. I also prefer this amplifier in triode mode but again you might not. Same for goes with the six caps you mentioned, I've used cheap WIMA MKPs in mine and prefer the sound comapared to those it came with. This is a DIY forum so experimentation is encouraged.
I changed the input tube from ecc83s to the russian 6n2p as it is cheap, plentiful and sounds good. Getting good ecc83s is becoming expensive.
I'm considering removing the first stage all together as my next mod but that will have to wait until next year.
I do like the sound of kt120s in this amp, they have a much better bass than the supplied kt88s. My transformers haven't got any hotter as a result.
Hope this helps
Aleks
Hi Dave,
I assume you meant feedback loop removal ?
You need to break it somewhere between it's source at the output transformer secondary and it's termination at the cathode resistors R4 and R2. I obviously went further and simplified the first stage so that both cathode resistors were of the same value and removed the switch that governs the feedback ratio. Removing the cathode bypass cap reduces the overall gain of the amplifier which was desirable in my own case. I also added a grid stopper as the amplifier was more prone to noise after the mod.
Removing the feedback loop means the amplifier reverts back to higher gain open loop operation and also means the overal damping factor is reduced. Frequency response is also reduced but often this isn't problem for most users. Some people report that the bass can sound a bit bloated, I didn't notice this with my own set up. I guess subjectively the amplifier sounded more open and I prefer the sound of open loop or low feedback designs over designs with a lot of feedback. All you can do is give it a try and see if you like it, if not keep it closed loop. It also depends what speakers you have and the complex load they present to the amplifier. I also prefer this amplifier in triode mode but again you might not. Same for goes with the six caps you mentioned, I've used cheap WIMA MKPs in mine and prefer the sound comapared to those it came with. This is a DIY forum so experimentation is encouraged.
I changed the input tube from ecc83s to the russian 6n2p as it is cheap, plentiful and sounds good. Getting good ecc83s is becoming expensive.
I'm considering removing the first stage all together as my next mod but that will have to wait until next year.
I do like the sound of kt120s in this amp, they have a much better bass than the supplied kt88s. My transformers haven't got any hotter as a result.
Hope this helps
Aleks
Hi Alek thanks for your help and reply yes I did mean feedback loop.
I removed the feedback loop on my amp it made the sound more open but it didn't suit my speakers.
My speakers are monitor audio studio 1200es and are already forward open sounding on the bright side of neutral.
It made male voices a bit lean and girly I noticed when playing dire straits private investigations the bass lost its clarity and impact I also got humm.
When funds allow I will try changing capacitors to jentzan z silver caps.
I would like to try monorf silver/gold caps but they are out of my price range,Jantzen caps are only £10 for .22uf and £12 for 1uf caps from hifi collective still a lot when I need six.
I agree the KT120s sound great I put grills on the back of the transformer housings.
Bass is amazing from this amp clean and deep.
I suppose what I would like to change about the sound is a bit more body in the voices mids it still sounds a bit lean clinical to me but without loosing the open sound or clarity would be nice.
I removed the feedback loop on my amp it made the sound more open but it didn't suit my speakers.
My speakers are monitor audio studio 1200es and are already forward open sounding on the bright side of neutral.
It made male voices a bit lean and girly I noticed when playing dire straits private investigations the bass lost its clarity and impact I also got humm.
When funds allow I will try changing capacitors to jentzan z silver caps.
I would like to try monorf silver/gold caps but they are out of my price range,Jantzen caps are only £10 for .22uf and £12 for 1uf caps from hifi collective still a lot when I need six.
I agree the KT120s sound great I put grills on the back of the transformer housings.
Bass is amazing from this amp clean and deep.
I suppose what I would like to change about the sound is a bit more body in the voices mids it still sounds a bit lean clinical to me but without loosing the open sound or clarity would be nice.
When you make the amp open loop the output impedance drops, because the feedback was (once it went around the houses) presenting the transformer primary with a low impedance drive. This means that as speaker impedance varies (by frequency) the sound energy also varies - for instance losing bass as you noted. You can stabilise speaker impedance to some extent but at some point a lower impedance drive will be useful. Try the 4ohm taps too - they are half the impedance.
You did notice the sound opening up though - which is something I have noticed too when removing feedback loops.
You can however have the best of both worlds with 'partial feedback', there is a great article about it on tubelab you can google. What I like is to extend partial feedback over the final two stages to give a good dose of feedback between the (very fast) tubes.
There is a recent discussion here that may interest you: http://www.diyaudio.com/forums/tubes-valves/256512-strange-motorboating-issue-6.html
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