• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Moscode 300 Upgrade

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Jump to Latest
D

desperateaudio

Member
Joined 2004
Well the schematic and parts list does not appear to be completely correct. C1 in the 3 Moscode's I have had including the one I have now are 1uf@250vdc stock. I'm checking the rest now.

Also I am thinking from the schematic there must be a balancing process to bias current as when you pull the + rail fuse and place a current meter in the circuit you only read 110mA on both sides so I think you either need to have a load on it to set bias (as Adcom does) or you need to have a meter on both rails and set for the middle? Any thoughts on that?

Thx
 
D

desperateaudio

Member
Joined 2004
desperateaudio said:
Also I am thinking from the schematic there must be a balancing process to bias current as when you pull the + rail fuse and place a current meter in the circuit you only read 110mA on both sides so I think you either need to have a load on it to set bias (as Adcom does) or you need to have a meter on both rails and set for the middle? Any thoughts on that?

Thx
Click to expand...

Anyone got a thought on this? thx
 
K

kannan_s

Member
Joined 2009
You can try inserting low value resistor on the position ( I would use even as low as 0.1 to 0.33 Ohm) to measure the current without removing the fuse. Use a good DMM for accuracy- Fluke 867 in my case. This also would help adjust the bias. Fuse is a must to protect the amp.
 
D

desperateaudio

Member
Joined 2004
kannan_s said:
You can try inserting low value resistor on the position ( I would use even as low as 0.1 to 0.33 Ohm) to measure the current without removing the fuse. Use a good DMM for accuracy- Fluke 867 in my case. This also would help adjust the bias. Fuse is a must to protect the amp.
Click to expand...

No offense but I think you need to read my post again. I have the current reading but the actual process of how the amp is properly biased is not specifically known. The reading of current of the positive rail is 110mA at both channels and the amp works find and the supplied schematic from our friendly diyaudio member shows the bias network to be somewhat of a balancing network. So I was wondering if those who got a copy of the schematic might have an idea or thoughts on the process. I have my offset at +/- 3 mV

Thx again
 
D

desperateaudio

Member
Joined 2004
So finished the upgrade. I did the following:

1. All new caps both film and electrolytic on the board
2. Complete cleaning
3. Thermistor on power switch
4. All new power supply caps with and increase to 30000uf/rail with 22uf Solen's and 10K resistors across both rails
5. Removed power heat sensor relays as the Mosfet's are self limiting
6. Rewired the entire 117v power line plug assembly
7. Replaced several key resistors that were originally comp resistors with Mills
8. Rewired the entire speaker line set up and removed all fuses and replaced the lines with 12 gauge direct soldered units

Now I listen. For sure the unit sounds better. Cleaner, less distortion with deeper tighter bass.

Here is what is next.

1. Change cheap pots on board with new cermet multi-turn units. Still need someone's idea on how the unit is really set for current??? My previous posts give my idea so any help would be greatly appreciated.
2. I ordered a pair of 6CG7 to 7N7 adapter plugs which should be here shortly so I will be trying some 7N7's in place of the old 6CG7.
3. I would like to get rid of the speaker relay. GK states on the moscode site that he eliminates it by dropping the output of the tube front end to ground during warm up for 1 minute with a relay that once it flips is out of the circuit. Any thoughts on this by anyone?

Ill get pics up soon.

A
 
Last edited:
P

pedrocols

Member
Joined 2012
desperateaudio said:
No offense but I think you need to read my post again. I have the current reading but the actual process of how the amp is properly biased is not specifically known. The reading of current of the positive rail is 110mA at both channels and the amp works find and the supplied schematic from our friendly diyaudio member shows the bias network to be somewhat of a balancing network. So I was wondering if those who got a copy of the schematic might have an idea or thoughts on the process. I have my offset at +/- 3 mV

Thx again
Click to expand...
I know this is an old thread but I am interested to see pics of this project. Or I can give you my email so you can email them to me. Thanks.
 
J

JAMESP123

Member
Joined 2011
need transistor number next to the 2n3439 it's the small can one- Thanks James

llwhtt said:
Anthony,

The 300 schematic I have is the original factory version with parts list. I also have a hand drawn front end of the 600. Both are scanned and ready to send, where do what them sent? As far as the alignments go the left one R16 looks to be DC offset and right one R17 is bias. R16 is set for 0VDC at the speaker terminals. Since there are no resistors to measure across in the output stage you'll probably want to remove one of the rail fuses, install a current meter in it's place and set the bias that way. I don't know what to set it for though. Do you have a distortion analyzer? Audio generator and o'scope? Input 10KHz, set for 1 Watt output and check for a glitch at zero crossing on the scope. Set bias to just eliminate the glitch.

Craig

PS. The schematic you posted says 350ma, that's probably across one of the fuse holders as I said. I just did a BK ST-140 same output transistors but only one pair, it was set to 200ma. I guess 350ma for two pair wouldn't be that far off.
Click to expand...
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Top Bottom