Mojotone JCM800 Build

[Rotorcraft230]

Finished up the amp this morning. Got out the Variac and the current limiter (dim bulb tester),went through the normal first power up procedure, all went well. plugged in the preamp tubes, voltages were looking good at first glance. Moved over to the power tube sockets the voltages there looked good. popped in the power tubes, still on the Variac but did remove the current limiter, as I was bringing up the voltage, the bias pot on the tubes was all the way down, I was at 100VAC and the bias was at 38 ma. Had to stop there. I could only get -28, should be around -40. I'm thinking I have to lower the 220K resistor in the bias circuit to get more voltage in that circuit. Haven't run into this before, seems kinda odd that it would be that far off.. I did check the value of the 220K and the 15K where it feed to the pot for adjusting it, also the resistor on the pot measured 47K. Adjusting pot is a 10K linear

The 220K resistor is the one I need to swap to get the voltage where I want it correct?

I measured the HV supply with the power tubes out and voltage set to 120 VAC
HV 360 per leg

 

[william vogel]

The typical Marshall bias pot is 25k and it’s wired as a variable resistor. Yours is a 10k operating like a potentiometer which is fine but you might need to change the 47k to a 56k or get a 25k pot instead of the 10k.

 

[Chris-in-LA]

I think mine is set up with a 10k pot on the bias, no problem setting the bias. You’ll be able to tell if it’s ok once you fire it up.

 

[Rotorcraft230]

The typical Marshall bias pot is 25k and it’s wired as a variable resistor. Yours is a 10k operating like a potentiometer which is fine but you might need to change the 47k to a 56k or get a 25k pot instead of the 10k.

That was it thanks William. The 56k put the pot just about in the center of its travel.

I gotta get to work, I’ll post on the sound soon.

 

[neikeel]

I use a 50k pot and vary the ‘stop resistor’ to give widest range for tubes (so that I can use most spec EL34s, KT66 in many applications). There is more room for error of course!

 

[Rotorcraft230]

Yes

I use a 50k pot and vary the ‘stop resistor’ to give widest range for tubes (so that I can use most spec EL34s, KT66 in many applications). There is more room for error of course!

a bigger pot would give more spread and the 47k would have worked. I think I ended up with -37 to -42 I’m happy with that.

So here is something I want to ask, the low input is about 1/2 the volume as the high, I know there is an extra gain stage, but what can I do to get the 2 inputs closer together in volume, even if I loose some volume on the high, if you A/B them it’s unusable.
Help on this would be great, he wants to use the low for the cleans

Thanks Mark

 

[jchrisf]

Yes

a bigger pot would give more spread and the 47k would have worked. I think I ended up with -37 to -42 I’m happy with that.

So here is something I want to ask, the low input is about 1/2 the volume as the high, I know there is an extra gain stage, but what can I do to get the 2 inputs closer together in volume, even if I loose some volume on the high, if you A/B them it’s unusable.
Help on this would be great, he wants to use the low for the cleans

Thanks Mark

Is it his new favorite?

 

[Rotorcraft230]

Is it his new favorite?

We like it a lot, I think there is some tuning to do, it’s pretty bright. Already removed the bright cap on the preamp pot, which was better and not quite so bright. Need to get the two inputs closer together as he wants to wise both inputs at gigs and right now they are too far apart.

But it is an awesome sounding amp, it does VH and Megadeath better than the Plexi51. Definitely filled a gap that we have.

Here is a list of what we have

1988 Marshall 1959S (real Marshall) 100w
Ceriatone Silver Jubilee 100w
Ceriatone Plexi51 50w
Ceriatone dumble 50w
Trinity Amps Triwatt (HIWATT clone) 50w
Mojotone JCM800 50w
1962 Fender Brownface Pro 40w

Speakers
4-Celestion V30s
6-G12T-75s
4-G12T75s (vent back 1984)
2-Greenbacks
1- Jensen Blackbird 100w alnico

All the Celestion speakers are UK

So we have a great selection of speakers and amps to pretty much get the sound we are looking for.

 

[StingRay85]

So here is something I want to ask, the low input is about 1/2 the volume as the high, I know there is an extra gain stage, but what can I do to get the 2 inputs closer together in volume, even if I loose some volume on the high, if you A/B them it’s unusable.
Help on this would be great, he wants to use the low for the cleans

Thanks Mark

That's not possible. It's the nature of the beast. High is the "normal" channel, low is for pedal platform or bass guitar. It's a single channel amp where high V1A (bypassed clean gain stage) cascades into low V1B.

 

[neikeel]

The low input will be lower volume - you lose a whole triode gain stage.
You can compensate by using a higher master setting or make the cascaded stage quieter by putting a voltage divider to shunt some signal to ground but then you lose the essence of the 2204 gain

 

[neikeel]

I cannot see how the B+ dropper resistor is wired (the wiring diagram will not load for me). Normally two 10k resistors there (is one mounted on the cap under the board?)

 

[Rotorcraft230]

The low input will be lower volume - you lose a whole triode gain stage.
You can compensate by using a higher master setting or make the cascaded stage quieter by putting a voltage divider to shunt some signal to ground but then you lose the essence of the 2204 gain

Well dang, we want the 800 gain, ah it will be alright. He usually uses two amp when he gigs anyway.

I cannot see how the B+ dropper resistor is wired (the wiring diagram will not load for me). Normally two 10k resistors there (is one mounted on the cap under the board?)

There are 2 on the board (blue ones 10k) and one is mounted on one of the 50-50 cap by the PT. Do these pics help?

 

[Rotorcraft230]

 

[jchrisf]

Well dang, we want the 800 gain, ah it will be alright. He usually uses two amp when he gigs anyway.

The 2204 takes pedals extremely well on the low sensitivity channel. I sometimes like it better with the right pedal than the high input.. and you don't need an effects loop using it this way. Have him try some pedals in front and see if that don't change his mind if he wants clean and drive together. I was blown away when I tried this input this way. Plus he can use his volume knob on guitar on the high and it cleans up really nice. But you're a genius with these things so you'll probably figure out a way to do it like you are wanting too.

 

[william vogel]

Have you tried a boost on the low input to kick it up a little?

 

[Rotorcraft230]

Have you tried a boost on the low input to kick it up a little?

Have not yet, but will. We did some experimenting With the cold clipper resistor V1b. Tried it with a trim pot clipped in and had a range of 5k-32k. Found the higher value made the amp darker. Liked the stock value of 10k to be the best one. Pulled one leg of the bright cap, that tamed the highs just a little.

 

[william vogel]

Have not yet, but will. We did some experimenting With the cold clipper resistor V1b. Tried it with a trim pot clipped in and had a range of 5k-32k. Found the higher value made the amp darker. Liked the stock value of 10k to be the best one. Pulled one leg of the bright cap, that tamed the highs just a little.

Putting a 80-120pF cap across pins 1-3, the plate and cathode terminals of the tube socket, will shave some top end also.

 

[AndyD]

Have not yet, but will. We did some experimenting With the cold clipper resistor V1b. Tried it with a trim pot clipped in and had a range of 5k-32k. Found the higher value made the amp darker. Liked the stock value of 10k to be the best one. Pulled one leg of the bright cap, that tamed the highs just a little.

What speakers and cab are being used? These amps are particularly bright with open backed cabs. I also keep the presence and treble down and roll a little tone of the guitar. Plenty of options to fine tune.

 

[Rotorcraft230]

Putting a 80-120pF cap across pins 1-3, the plate and cathode terminals of the tube socket, will shave some top end also.

Kit came with a 10pf silver mic, we liked it in vs out

What speakers and cab are being used? These amps are particularly bright with open backed cabs. I also keep the presence and treble down and roll a little tone of the guitar. Plenty of options to fine tune.

right now we are playing through Mix ov V30s and Greenbacks. Little darker than 75s. Haven’t tried a full set of 75s yet.

 

[AndyD]

I forgot to mention that the mid control on these amps are quite responsive. If you dial the mids back, it also soften the edges a bit. Have fun!