Mojotone British 800 Build

[neikeel]

Use the Larry grounding scheme on the Metro forum

It avoids the above issues.

 

[Chris-in-LA]

Use the Larry grounding scheme on the Metro forum

It avoids the above issues.

Ha ha. I don’t even know Larry but maybe I should.

 

[neikeel]

To keep AMS happy you might need to take a wire from each point to point 5 as a central star ground if using an aluminium alloy chassis!! (ps I don't, shhh!!)

 

[Chris-in-LA]

Yes, you are missing the 100nF (aka 0.1uF) cap on the phase inverter (between 1M grid and 10k PI tail resistor)

Ok, finally figured out what you were saying.



There, that’s better.

 

[neikeel]

 

[ampmadscientist]

I’m thinking about a ground wire on the back of the pots and running everything that I can to the bolts on the PT.

Any method that does not rely solely on the aluminum to ground the circuit.

We weld it / solder it / bolt it to the chassis so that the connection cannot corrode or break.

So w/ aluminum to make a solid connection, a bolt is not really enough be itself.

 

[FourT6and2]

To keep AMS happy you might need to take a wire from each point to point 5 as a central star ground if using an aluminium alloy chassis!! (ps I don't, shhh!!)

Hmmm... I don't personally like that. I would run a single ground bus either on the turret board or on the pots (NOT ON THE BACK OF THE POTS), by running a long bus wire through all the ground connection lugs on the pots that need it. Then run a wire from the end of that bus to the input jacks. Then from the input to V1 cathode. Then a bus wire under the turret board all the way down to the other end, connecting your ground points along the way. Then a single wire from the end of the turret board to a single star ground point (#5), where everything else connects. That way you only have one single connection to the chassis for all your grounds. Your mains ground connection still gets its own, though.

This isn't a Marshall, but you get the idea:

Or like this, with a bus bar on the board and flying leads to your pots and jacks:

 

[Chris-in-LA]

Hmmm... I don't personally like that. I would run a single ground bus either on the turret board or on the pots (NOT ON THE BACK OF THE POTS), by running a long bus wire through all the ground connection lugs on the pots that need it. Then run a wire from the end of that bus to the input jacks. Then from the input to V1 cathode. Then a bus wire under the turret board all the way down to the other end, connecting your ground points along the way. Then a single wire from the end of the turret board to a single star ground point (#5), where everything else connects. That way you only have one single connection to the chassis for all your grounds. Your mains ground connection still gets its own, though.

This isn't a Marshall, but you get the idea:

Or like this, with a bus bar on the board and flying leads to your pots and jacks:

That’s some good looking work.

 

[Chris-in-LA]

Almost done, just need to finish all these connections.

 

[Chris-in-LA]

Finished most of the connections except the main power supply and the input jacks. Thanks for the input and support from the forum members on this challenging build. Just need to double check everything before I fire it up.

 

[Chris-in-LA]

Just found a beat up old case for this bad boy.

 

[Chris-in-LA]

Ok, the amp is alive. Had a couple of glitches. The OT wires to the power tubes had to be switched due to that shrieking oscillation problem that can happen. Then, I discovered that the amp was silent because I had screwed up the placement of the resistor on V2.

I am having a problem with the bias. I put in a set of JJ EL34II’s and I can’t get the bias below 47, almost 90% plate dissipation. I plan to roll some other tubes in there but I may have to increase the 47k resistor to something else, any suggestions? 60k? 100k?

Also, the master volume pot has a strange taper to it. I have to turn the master up to 5 or 6 before the amp starts getting loud. A stock 2204 starts getting loud at 1. Mojotone supplies 1 meg audio taper pots but Marshall schematics show 1 meg log pots. Any ideas about how the different pots effect the overall sound of the amp?

 

[charveldan]

No standby switch eh ?

 

[FourT6and2]

Ok, the amp is alive. Had a couple of glitches. The OT wires to the power tubes had to be switched due to that shrieking oscillation problem that can happen. Then, I discovered that the amp was silent because I had screwed up the placement of the resistor on V2.

I am having a problem with the bias. I put in a set of JJ EL34II’s and I can’t get the bias below 47, almost 90% plate dissipation. I plan to roll some other tubes in there but I may have to increase the 47k resistor to something else, any suggestions? 60k? 100k?

Also, the master volume pot has a strange taper to it. I have to turn the master up to 5 or 6 before the amp starts getting loud. A stock 2204 starts getting loud at 1. Mojotone supplies 1 meg audio taper pots but Marshall schematics show 1 meg log pots. Any ideas about how the different pots effect the overall sound of the amp?

Audio and log taper are the same thing. And yes, sounds like you'll need to change a few values in the bias circuit to get the appropriate range (unless something else is amiss). Can't really see the color codes but make sure you're using 1ohm resistors on the tube sockets for reading bias. The ones you got there look rather large...

 

[Chris-in-LA]

Audio and log taper are the same thing. And yes, sounds like you'll need to change a few values in the bias circuit to get the appropriate range (unless something else is amiss). Can't really see the color codes but make sure you're using 1ohm resistors on the tube sockets for reading bias. The ones you got there look rather large...

Those 1 ohm resistors are 2 watts I think. I may be mistaken on the log pots, I guess the L stands for linear. I had some 68k resistors on hand and used one in place of the 47k resistor in the bias circuit and that dropped the bias from 65 down to 19 before adjustment. With the new resistor the bias range goes from 19 to 36, not a problem for now. I can fine tune later if I have to. Thanks for your input.

 

[Chris-in-LA]

No standby switch eh ?

It’s there, I had to shrink the picture to meet the size limits in order to post. The switch that you see there is the standby.

 

[FourT6and2]

Those 1 ohm resistors are 2 watts I think. I may be mistaken on the log pots, I guess the L stands for linear. I had some 68k resistors on hand and used one in place of the 47k resistor in the bias circuit and that dropped the bias from 65 down to 19 before adjustment. With the new resistor the bias range goes from 19 to 36, not a problem for now. I can fine tune later if I have to. Thanks for your input.

What is your plate voltage?

 

[neikeel]

Sounds like around 460v if 47mA is 90% or so

 

[Chris-in-LA]

Sounds like around 460v if 47mA is 90% or so

It wasn’t that high and I was going from my poor memory. I plugged the plate voltage and the reading in Mv into the Robinette bias calculator and it said I was running just over 85%, still higher than I wanted to be. I think my initial reading for one tube was about 75mv before I turned it down. Later, after letting the amp run for awhile, that 407v plate voltage settled down closer to 430v. For that I used this formula to compute the bias, 25/430 X .65 = 37.8mv. I haven’t had the time to do a sound test yet so we’ll see. I have a cabinet coming tomorrow and will be able to really test it then.

 

[FourT6and2]

It wasn’t that high and I was going from my poor memory. I plugged the plate voltage and the reading in Mv into the Robinette bias calculator and it said I was running just over 85%, still higher than I wanted to be. I think my initial reading for one tube was about 75mv before I turned it down. Later, after letting the amp run for awhile, that 407v plate voltage settled down closer to 430v. For that I used this formula to compute the bias, 25/430 X .65 = 37.8mv. I haven’t had the time to do a sound test yet so we’ll see. I have a cabinet coming tomorrow and will be able to really test it then.

If your Vp = 430v, and you can't get your bias higher than 36mA, then you still have work to do. You should be able to get your bias up to 70% even if you don't bias it that hot.