6550 to EL34 - bias splitter change necessary? 2204

[Kapo_Polenton]

I am aware of the original spec differences between the european and USA/Canada 6550 loaded amps. I find my 2204 JCM800 to be stiff with 6550's and quite frankly, EL34's are cheaper with more variety. (or the KT77's I have kicking around) I am not Zakk Wylde so I don't need that huge low end. I would prefer that slight compression you get from 34's as this amp is stiff to start with. Just recapped it with all new F&T 50/50 filter caps, figured i'd do the swap while in there.Because getting to the components on the PCB can be a real bitch, I saw this mentioned over on the vintage amps bulletin board and apparently from Voodoo amps? website. Can anyone comment as to not changing the splitters? Affect on sound?

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A simple procedure that works is (copped from the Voodoo Amps website

MARSHAL 6550 to EL34
1.) Locate the 47K resistor above the bias pot and solder another 47K in parallel.
2.) Set the bias pot the max (around –38 volts).
3.) Locate the purple feedback wire (that is connected to the 4 ohm speaker tap) and solder it onto the 8-ohm speaker tap.
4.) Install the EL34’s and re-bias the amp.

 

[Anonymous]

That will work fine for 6550 to el34.
What you might try too is....Bias the 6550's as though they were EL34's

 

[Kapo_Polenton]

meaning hot as hell and cranked right up? Also confused, the additional 47k in parallel, would that not drop down the total resistance to somewhere around 23k? Or is that the point.

 

[LP Freak]

That will work fine for 6550 to el34.
What you might try too is....Bias the 6550's as though they were EL34's

Yes, try this first.

 

[Anonymous]

meaning hot as hell and cranked right up? Also confused, the additional 47k in parallel, would that not drop down the total resistance to somewhere around 23k? Or is that the point.

No..the opposite. 6550's like to in the low to mid 50's Ma's bias wise whereas..EL34's run in the 30's...all this of course depends on the plate voltage of the amp as to where the bias should be set at. The 6550's in that amp will put out more wattage than the amp was designed for if set at optimum bias and the amp is working hard.
Yes...thats the point.

 

[Kapo_Polenton]

Ok, changes made. I left the old 6550 tubes initially so I could get an accurate plate voltage reading off pin 3 of the power tubes. How in the hell can I be at 556 v?? Isn't that high as hell? The number kept climbing and climbing then backed off and settled in at 552-556. Yikes. Maybe the key is just using my probes and getting somewhere in the recommended range for EL34's and go from there? I can't imagine the two changes I made causing that... to my knowledge power tubes should have no effect on the plate voltage should they?(second reading went over 600..something has to be wrong)

 

[glip22]

Ok, changes made. I left the old 6550 tubes initially so I could get an accurate plate voltage reading off pin 3 of the power tubes. How in the hell can I be at 556 v?? Isn't that high as hell? The number kept climbing and climbing then backed off and settled in at 552-556. Yikes. Maybe the key is just using my probes and getting somewhere in the recommended range for EL34's and go from there? I can't imagine the two changes I made causing that... to my knowledge power tubes should have no effect on the plate voltage should they?

Something is not right with your plate voltages. Maybe you connected one of the filter caps incorrectly?The number should not have climbed or settled either. In case you go back to 6550. In a Marshall the output transformer is designed for el34. You should not bias the 6550's as high as you would in an amp with a transformer designed for 6550/kt88. You can go maybe up to 40-42 or so but any higher is diminishing returns. I stay at less than 40 depending on plate voltages. The OT is designed for EL34s so the primary impedances are usually around 1.7-3.5K. A quad of EL34's output will transfer power most efficiently when the primary impedance of the OT matches the combined out put tubes's impedance. In an amp designed for 6550/kt88 the primary impedance will be much higher. You will not operate 6550's in a MArshall with an OT designed for el34's at max performance. You will still only get about the same output power to the speakers as you will with el34's. With that said you should not bias them high. It will be very inefficient and you are not gaining much output power and you are stressing the OT some. Treat them similar to the EL34's as far as biasing in a Marshall. .

 

[Kapo_Polenton]

Well The filtering was fairly straight forward.. the old filter caps are colour coded a gold and red pin and a clear ground. The F & T caps are not colored but very clearly labelled +/+/and a - and of the same values.I'm really boggled by this. Changed the battery in my multimeter as well thinking that might have an effect. Readings on the filter caps themselves all are below 500 so well within their spec range. Frustrating. Any suggestions on how i might want to skin this cat?

 

[LP Freak]

What was the PV before you added the resistor?

 

[Kapo_Polenton]

To be honest, I had not biased that amp in ages I don't remember! I think it was high 400's though. I should have put it on a strip of hockey tape on the back of the amp so i would have known. That was my plan this time around. Wonder if i should just reverse my changes and see what i get. Rule out my filter cap job though it isn't rocket science. Old ones out, new ones in. How the hell could the amp be producing that voltage. I think first thing first is disconnecting the piggy back resistor and see what happens. One mod back at a time.

 

[LP Freak]

To be honest, I had not biased that amp in ages I don't remember! I think it was high 400's though. I should have put it on a strip of hockey tape on the back of the amp so i would have known. That was my plan this time around. Wonder if i should just reverse my changes and see what i get. Rule out my filter cap job though it isn't rocket science. Old ones out, new ones in. How the hell could the amp be producing that voltage. I think first thing first is disconnecting the piggy back resistor and see what happens. One mod back at a time.

Just lift one leg and check the PV.

 

[killertone]

Did you form the caps? That might have something to do with the drift in PV.

 

[Kapo_Polenton]

Is cap forming that necessary? I also don't have a variac to do that.. I've attached a pic of the filter caps and their readings as well. I don't know where to go from here. Checked the cap under the board as well to ensure a good connection.

Just lift one leg and check the PV

What do you mean? I've checked it now with both feet on the ground if that is what you mean. It is over 650 steadily on pin 3.

 

[Anonymous]

Ok, changes made. I left the old 6550 tubes initially so I could get an accurate plate voltage reading off pin 3 of the power tubes. How in the hell can I be at 556 v?? Isn't that high as hell? The number kept climbing and climbing then backed off and settled in at 552-556. Yikes. Maybe the key is just using my probes and getting somewhere in the recommended range for EL34's and go from there? I can't imagine the two changes I made causing that... to my knowledge power tubes should have no effect on the plate voltage should they?

Something is not right with your plate voltages. Maybe you connected one of the filter caps incorrectly?The number should not have climbed or settled either. In case you go back to 6550. In a Marshall the output transformer is designed for el34. You should not bias the 6550's as high as you would in an amp with a transformer designed for 6550/kt88. You can go maybe up to 40-42 or so but any higher is diminishing returns. I stay at less than 40 depending on plate voltages. The OT is designed for EL34s so the primary impedances are usually around 1.7-3.5K. A quad of EL34's output will transfer power most efficiently when the primary impedance of the OT matches the combined out put tubes's impedance. In an amp designed for 6550/kt88 the primary impedance will be much higher. You will not operate 6550's in a MArshall with an OT designed for el34's at max performance. You will still only get about the same output power to the speakes as you will with el34's. With that said you should not bias them high. It will be very inefficient and you are not gaining much output power and you are stressing the OT some. Treat them similar to the EL34's as far as biasing in a Marshall. .

This what Glip said!!
I bias mine like I would a 6L6...it's a happy medium between the EL34 and 6550..(I have 6550's in my marshall)

 

[Kapo_Polenton]

Not really what Glip said.. when you consider he is suggesting i might have wired or connected in the filter caps wrong, there is no wrong way these can go in is there?? + is +, negative is ground. Where am i going wrong here. F-cking 3 hours on this sack of $hit i'm about to take a hammer to it.

 

[LP Freak]

Not really what Glip said.. when you consider he is suggesting i might have wired or connected in the filter caps wrong, there is no wrong way these can go in is there?? + is +, negative is ground. Where am i going wrong here. F-cking 3 hours on this sack of $hit i'm about to take a hammer to it.

Fun aint it.

 

[LP Freak]

Is cap forming that necessary? I also don't have a variac to do that.. I've attached a pic of the filter caps and their readings as well. I don't know where to go from here. Checked the cap under the board as well to ensure a good connection.

Just lift one leg and check the PV

What do you mean? I've checked it now with both feet on the ground if that is what you mean. It is over 650 steadily on pin 3.

I hope that was a joke.

 

[Anonymous]

Not really what Glip said.. when you consider he is suggesting i might have wired or connected in the filter caps wrong, there is no wrong way these can go in is there?? + is +, negative is ground. Where am i going wrong here. F-cking 3 hours on this sack of $hit i'm about to take a hammer to it.

Well...I was referring to everything else except what "may" have been done incorrectly. Can't count how many times I've had pull, put back,check, re-pull circuit boards over and over and over again. I especially like it when you've done it so many times...traces start lifting ...Anyways....Hope you sort it out.

 

[Anonymous]

Is cap forming that necessary? I also don't have a variac to do that.. I've attached a pic of the filter caps and their readings as well. I don't know where to go from here. Checked the cap under the board as well to ensure a good connection.

Just lift one leg and check the PV

What do you mean? I've checked it now with both feet on the ground if that is what you mean. It is over 650 steadily on pin 3.

I hope that was a joke.

I'm extra careful...One leg lifted and one hand behind my back when working on amps....makes soldering a bitch but..I'm safe

 

[glip22]

Is cap forming that necessary? I also don't have a variac to do that.. I've attached a pic of the filter caps and their readings as well. I don't know where to go from here. Checked the cap under the board as well to ensure a good connection.

Just lift one leg and check the PV

What do you mean? I've checked it now with both feet on the ground if that is what you mean. It is over 650 steadily on pin 3.

Not being a tool but are the batteries in your meter good? When batteries in your meter get low you can read some insane voltages. One day a few years back I plugged my meter in an outlet in my house and read 260 volts ac. I freaked out and shut the power down in my house thinking some carzy surge was happening. After that I learned about my meter batteries. It happened two more times in an amp where I was reading 700+ DC on the plates. It was the meter batteries again. Not saying you wired a filter wrong.They look correct. I bet your meter needs batteries. I hope so as its an easy fix. What is your house AC? There is no way a standard 2204 PT is putting out over 600vdc. If you are reading 465 on your mains cap their is no way you should have 600 on your screens. Did you check tube bias as suggested? First thing I would have done. But even if a tube was running away you wouldn't have those voltages. PV would actually be lower. Take a reading also from the mains fuse tip as that is usually the same as pin 3 and your mains cap positive