1978 jmp 2203 biasing problems

[jlw001]

Hello. I thought I would see if anyone had any insight into my problems with my 78 jmp master volume amp.

First, the amp started to hum. So, I decided to replace the tubes. New 12ax7s and 6550s. Well after replacing the tubes I check the bias with my bias rite probe and I get 20ma. Hmmm. Way too low for the 6500s. So, I try to adjust the bias pot and I can only get about 25ma. The amp was made for the American market, so I know it wasn't meant for el34s...el34s would draw even less current.

I notice that the hum hasn't gone away and I see that the filter caps have never been changed...still the blue Daly ones... I order some JJ replacements and install them. The hum is still there. Arrgh.

Anyway, I decide to try some KT88s that I have and the result is even lower bias current. Ok, maybe my bias probe is bad. I buy a new probe from EuroTubes and a new multimeter. Same result.

I have a new bias pot on order and new power and standby switches. I ordered the switches because when I plug the amp in the power switch glows dimly even in the off position. When I flip the power switch it glows brightly. I didn't know if this was normal behavior or not.

Anyway, I suspect something foul is going on in my amp and I suspect the dimly glowing power switch (when the switch is off) is a clue to something wrong with my power transformer. However, my plate voltage is at a normal 470volts for the 6550s.

I'm at the end of my abilities and I'll probably take it to a tech, but I thought maybe some TGP experts might be able to spot the problem immediately.

Thanks for your patience and advice.

[phsyconoodler]

Well first off,you simply need to change the bias range resistor to get the bias in range,not the bias pot itself.As far as hum goes,it could be a great many things causing it.The heater circuit needs to be looked at and see if it has 100 ohm virtual centre tap resistors or uses a heater center tap.
Measure the negative voltage on pin 5 of the power tubes.To get more current it needs to be closer to zero.Not zero obviously,but more towards the positive.It's likely around -35v now.

[jlw001]

Ok...so I need to decrease the bias range resistor that is in series with the bias pot.

OK, I'll check pin 5 voltage.

Thanks for your reply.

[kevzep]

What is the Bias voltage?
If its around 37 VDC it should have EL34's in it, for 6550's it should be around 55-60VDC.
Another thing to check is on the tube sockets, for 6550's pins 1 and pins 8 must be linked.
If they are not and the bias voltage is around 37VDC then it should have EL34's in it...............

[Blue Strat]

If the amp is humming regardless of what tubes you're using have a cap job done. It's at least 10 years over due. It's quite possible that the bias supply cap is the problem.

If the amp was set up for EL34s you'd be drawing WAY too much current, not too little.

Bias range resistor probably needs to be changed.

[mike shaw]

Did you bias etc. with the head out of the cabinet? That could be the problem. The cabinet has a sheet of aluminum inside to block out some of interference in the air.

[phsyconoodler]

EL34's require pins 1 and 8 to be linked also.and if he's only getting 25ma of current he needs more like -30v not more.More negative voltage = less current.

Mike,he already changed the filter caps.

Quote: "Did you bias etc. with the head out of the cabinet? That could be the problem. The cabinet has a sheet of aluminum inside to block out some of interference in the air."

What? Are you serious? That has NOTHING to do with this.

[Blue Strat]

Mike,he already changed the filter caps.



D'oh!

Ok, be sure that the bias supply cap is installed correctly...with the + side to ground.

Also, try pulling the preamp tubes to see if it still hums. If not, the problem is somewhere in the preamp (possibly a tube). What new 12AX7s did you get?

[jlw001]

I used EH 12ax7s and and EH gold KT88s.

I'll check the bias supply cap, but I made certain to do one cap at a time and to follow the orientation and connections.....I could have made a mistake, so I'll check. ...wait...Is the bias supply cap one of the 50muF 50muF electrolytics? Those are all I replaced.

The bias was low and the amp was humming even before I replaced the caps and retubed.

I did some rough calcs for piggybacking a resistor on to the 47K bias range resistor and according to the amperage I was getting and what I want (about 52ma at my 470volts) I'm going to put a 20K ohm 3 watt resistor in parallel with the existing 47K ohm.

I think this should fix my bias problem....the hum I believe to be a separate issue...thanks for all your help.

[TELECOLLECTOR]

Uh-oh.............

The mysterious High Intensity Radiated Field (HIRF) effect rears its ugly head.

J/K

[kevzep]

EL34's require pins 1 and 8 to be linked also.and if he's only getting 25ma of current he needs more like -30v not more.More negative voltage = less current.

Mike,he already changed the filter caps.

Quote: "Did you bias etc. with the head out of the cabinet? That could be the problem. The cabinet has a sheet of aluminum inside to block out some of interference in the air."

What? Are you serious? That has NOTHING to do with this.

My apologies, its the other way around, 6550's have 1 and 8 linked internally inside the tube, whereas EL 34's dont. So a 34 amp should have 1 and 8 linked. (just checked data sheets)

The bias circuit for an EL34 amp is -37vdc according to the schematic, and the 6550 versions are -63vdc according to the schematic.

I hear what your saying about the OP's current, or lack of current...............somethings not adding up there, the bias voltage on my amp with 6550's is sitting at -46vdc................just checked it now......

All I was trying to do was to establish whether the right tubes were in his amp..........

I have my "home only" marshall out of its box, just sits on top of the quad with its pants off..........it does not hum ,never has.............
So that will not have anything to do with it, you cant adjust the bias unless its out of the box anyway...........

[kevzep]

The bias caps are the two smaller 10uf's or 8uf's on the circuit board.........

[jlw001]

ok, gotcha...the two small can caps on the circuit board? Thanks kevzep

[phsyconoodler]

You need to make sure your filter caps are installed correctly and check all grounds before proceeding.
I'm wondering if the power transformer is big enough to handle 6550's in the first place.There may be a different transformer for the 6550 versions because they draw so much heater current and plate current.I'm just guessing,having only worked on EL34 versions.
Also make sure about current measurements.
I like to do it Hasserl's way.Measure the ohms from the OT center tap to pin 3 of each power tube and then power up and measure the voltage at those same two points.Then simply divide the voltage x the ohms to get the current at idle.
So say you get 1v and 70 ohms.(guess) 1/80=.0125mv or 125ma and then divide by two tubes=62.5ma per tube.

[kevzep]

You need to make sure your filter caps are installed correctly and check all grounds before proceeding.
I'm wondering if the power transformer is big enough to handle 6550's in the first place.There may be a different transformer for the 6550 versions because they draw so much heater current and plate current.I'm just guessing,having only worked on EL34 versions.
Also make sure about current measurements.
I like to do it Hasserl's way.Measure the ohms from the OT center tap to pin 3 of each power tube and then power up and measure the voltage at those same two points.Then simply divide the voltage x the ohms to get the current at idle.
So say you get 1v and 70 ohms.(guess) 1/80=.0125mv or 125ma and then divide by two tubes=62.5ma per tube.

I own two 2203's which I converted to 6550's, they have been working fine for the last 6 years..............
Sounds like to OP has one which was an American 2203 with 6550's anyway..........
I was trying to establish whether it was a conversion...............

I set my bias with the scope, start off cold and bring it in until the crossover distortion is resolved.............
Its been working well for my amps....................

[Blue Strat]

The bias cap(s) is/are on the board. The big can caps should be replaced if you haven't done so. They're a prime suspect if the amp is humming.

Do any of the controls affect the level of hum? How about pulling V3?

[phsyconoodler]

an underbiased(too hot) amp will hum too.He needs to determine if the measurements are correct first.The 6550's may draw a lot of current especially if they should be at -55v or so.

[slider313]

Grid wire leads are a common problem with Marshalls and hum. Working on a live amp can be dangerous, so be VERY careful. With the amp on and speakers plugged in, use a chopstick or sharpie and move the grid wire coming off V1 first and then V2 until the hum subsides.

[jlw001]

Thanks guys for all the input.

First the amp is one of the American 6550 models. It came factory with 6550s. I have checked the installation of the the large can filter capacitors once already and I will do so again. The technique I used was I changed one at a time and clipped the tabs off the old ones. Then I soldered the tabs to the tabs of the new cap. I kept the directional orientation of the new caps the same as the old.

I've decided I'm going to take the amp to a tech to have it gone over. I'm just frustrated with it and I want to play it so bad. I'll have him check my filter cap install.

Thanks for everyone's input.

[walterw]

I'm wondering if the power transformer is big enough to handle 6550's in the first place.There may be a different transformer for the 6550 versions because they draw so much heater current and plate current.I'm just guessing,having only worked on EL34 versions.

i'm not an amp tech, but even i know this one; it's the same iron on both versions, the american distributor just converted them to 6550s thinking they were more "reliable" that way.

[Roe]

lower the bias splitters to 150k (max) if you use 6550s/kt88s and decrease negative feedback by moving the purple wire from 4 ohms secondary to 8 ohms secondary

[jlw001]

Well, I took it to a tech today and he told me I did the cap job correctly and he put it on a signal generator and a scope. He's said he would get it biased no problem and we determined that the hum is coming from the preamp because the hum changed with the tone stack controls and we pulled the phase inverter and there was no hum. That means my trannies are ok so I am happy.

Thanks again for everyone's input. These amps were changed by the distributors to take the 6550s as walterw said. Maybe the bias circuit wasn't re-ranged properly in this one and its been cold from its the beginning. Anyway, I should have my Marshall roaring back to good health soon......I love my amp and was getting very discouraged with the problems...I feel better now.

[kevzep]

Well, I took it to a tech today and he told me I did the cap job correctly and he put it on a signal generator and a scope. He's said he would get it biased no problem and we determined that the hum is coming from the preamp because the hum changed with the tone stack controls and we pulled the phase inverter and there was no hum. That means my trannies are ok so I am happy.

Thanks again for everyone's input. These amps were changed by the distributors to take the 6550s as walterw said. Maybe the bias circuit wasn't re-ranged properly in this one and its been cold from its the beginning. Anyway, I should have my Marshall roaring back to good health soon......I love my amp and was getting very discouraged with the problems...I feel better now.

There you go!!
That is correct about what the distributors did, a change for the better I think...........
There is a change to the bias circuit also, I think from memory its the 47K in series with the trimmer but I cant remember what value, it was 6 years ago I did mine.........
I bet that thing sounds great, I share your enthusiasm for this amp......

[mike shaw]

Can you find out what specifically in the preamp circuit caused the problem?

[jlw001]

I will find out the details about the preamp when I get it back next week. I'll update the thread so maybe someone else might benefit from the suggestions from everyone and the outcome.

[mike shaw]

Sounds good. Thanks!

[jlw001]

Turns out that the hum was not in the preamp. The hum was from the extremely low bias current. The tubes are now biased and the amp sounds fantastic.

After I got the amp home I clipped the treble bleed cap from the preamp volume pot (easily re-soldered so as to keep the amp original) and the clean (low preamp/high master) is now as big and bold as the distortion. It takes pedals like nothing I've ever used now. Wow!