Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Platefire on April 27, 2016, 10:09:17 pm
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Got this amp head in today that looks so far in original condition. The customer wants me to modify it install a Master Volume, three conductor cord and change operation of power tubes from 6550's to EL34's.
First I need to figure out what circuit it is and get a schematic. Will take some pics soon and post them. The serial # is SA 1273D Lead. Also on a tag on the chassis says"Lead 1459". I think he wants a post PPIMV. He provided a kit with a dual 250k pot, shielded wire, hook up wire, shrink wrap and two resistors. So right now I would appreciate some help determining what circuit I got. Thanks, Platefire
BTW-Looking at Doug's schmatics it looks like it may be 1987 Lead Mark II?
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Also here is a pix of what's written on the chassis plus my best guess on the correct schematic.
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You mean he wants to change output tubes to EL34's, right? :w2:
To change to EL84's is a whole different story. New sockets, double the number of output tubes, probably a different OT needed too.
If he really wants EL84s, I'd probably say he'd be better off selling this amp and buying the parts for you to build him what he really wants. :dontknow:
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No way I'd drill a hole in that amps chassis for a MV. :w2:
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pardon the directness: oh hell no! that thing is a classic & a jewel of amp as is. putting any holes in that amp would be like hacking the dash on a 64 1/2 mustang convertible to put a CD player in it. ugh! not only that, mods of that sort will substantially reduce the resale value.
sell it. build him one with a MV.
i think you mean EL34's right? EL84s would require a rework of the entire PS.
--pete
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Yes, sorry-----EL34's. As I understand it a re-biased and move the NFB from 4 Ohm to 8 Ohm tap will do it. He wants the MV in one of the input jack holes?? Never seen that before, if it would work, it wouldn't require any drilling. Platefire
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I'd say that's pretty much the right drawing for it. It may have shipped with EL34's. Wasn't there something though to do with the US distributor swapping them on arrival for 6550's as they were more reliable? i.e. Then the distributor got fewer warranty repairs. I guess Unicord was that distributor hence they are showing 6550's on their drawing.
Re-bias, not heard the requirement to move the feedback point though?
Fairly common to use one of the 4 inputs for MV. Looks weird but at least you don't have to drill. If it's a 250K and a couple of resistors it sounds like the LAR-MAR PPIMV mod. Replaces the 220K bias feed resistors (interestingly they're shown as 150K in that schematic, would be worth finding the equivalent UK EL34 drawing for that amp, plus of course opening up and seeing what's actually installed!).
If it were my amp I'd leave it all stock....
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Platefire,
It looks like you have 3 switches on the front: polarity, on/off, & standby. I think I would be inclined to see about using one toggle for the on/off & standby. IIRC, Sluckey has a schematic that shows how to do that.
http://vintageamps.com/plexiboard/viewtopic.php?f=1&t=78112 (http://vintageamps.com/plexiboard/viewtopic.php?f=1&t=78112)
Then you could put the PPIMV in the standby hole or even more the indicator light into the standby hole and use the empty indicator light hole for the PPIMV (if the indicator light hole is round).
OR you can lose the polarity switch (maybe that's not needed if you have a 3-prong cord) and then move the on/off and standby over ............. leaving an empty hole where the standby was.
With respect, Tubenit
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I'd say that's pretty much the right drawing for it. It may have shipped with EL34's. Wasn't there something though to do with the US distributor swapping them on arrival for 6550's as they were more reliable? i.e. Then the distributor got fewer warranty repairs. I guess Unicord was that distributor hence they are showing 6550's on their drawing.
If it were my amp I'd leave it all stock....
exactly.
perhaps a power soak may be an option?
--pete
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The usual place for the PPIMV is in the speaker extension jack, as it's less prone to P Oscilliation / closer to the EL34's / away from the sensitive input.
Check out google images
Rob
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Well I'm feeling kind of bad because I agreed to take this job on before I really realized what it is. It's a true classic in real good condition. The customer is very aware of it's value and it's place in amp history. He said he wanted me to save the two conductor cord I guess if it needed to be put back. He also wants me to examine the circuit to determine the originality of it. This guy is a musician that makes his living playing & dealing in new & vintage pedals on the net. So I'm not jumping into this with solder flying. I consider this a consultation period and anything I do will be after considering the best options. Thanks for all your input and suggestions. My next step will be to crack it open and examine the circuit for originality and being my experience it not in Marshalls, I Will need your help. I will post some good pics of the circuit once I open it up. Thanks, Platefire
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The usual place for the PPIMV is in the speaker extension jack, as it's less prone to P Oscilliation / closer to the EL34's / away from the sensitive input.
Yes, do this. I'd strongly advise against putting a master volume in an input jack hole; the circuit wiring would have to cross over itself a couple times, inviting oscillation.
Regarding the switch from 6550's to EL34's: Usually this involves moving the feedback tapping point on the OT; raising the bias-feed resistors to 220kΩ (if they're lower; some 6550 Marshalls are); checking the bias supply for acceptable range of bias voltage adjustment; and changing bias supply resistors to achieve bias voltage adjustment range, if needed.
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Well I'm feeling kind of bad because I agreed to take this job on before I really realized what it is. It's a true classic in real good condition. The customer is very aware of it's value and it's place in amp history
This is just my opinion ...................
I don't see any reason to "feel bad" about the job. I think it is reasonable to see "your responsibility" being to give him an "informed choice" so that he knows the value of the amp as is and possible de-valuing of the amp IF selling it if
one did irreversible mods. (It sounds like the mods considered are not irreversible if using an existing hole like the speaker extension.)
It's his amp! He can choose whatever he wishes to do with it. If he enjoys the amp more with the mods & gets more of the tone that he wants, why shouldn't he do so? You can choose to help accomplish his agenda or bow out.
I've modified an original amp and regretted it and lost $75 in value selling it after playing it for about two years. And I've modified another original amp and absolutely loved it & when I sold it, I made money!
Just my 2 cents worth. I think the key is that he is informed about his choice. :thumbsup:
With respect, Tubenit
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Well, there are a few things to consider. Yes this is a good example. However, switches look like plastic toggle replacements and pilot light is either broken or not original. Not original fuse holder in back. I would guess the inside probably has some fresh solder and maybe some new parts in it. Plate, I would go through the whole amp and see what has been done. Any solder point that is not showing red nail polish is new. A real high dollar collector would probably pass on this one just from what I see. I would think it would get average pricing
Plate there is a paper tag on the top front corner of the chassis. You may have to remove the chassis to get a good look. If its dusty/grimy carefully wipe it off. That will have the sign-offs by the women who assembled it. There will be dates.
Jim
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Well taking everything in consideration, if the non original changes on the exterior is any indicator of what lies on the inside, it might not be so original. It does have the white tag with signatures in the upper front left hand corner of chassis. I will also take a close up of it. Thanks for all your insight on this subject and amp. I am a little excited getting to handle this ol Warrior. Hopefully I can get gut shots up soon! Thanks, Platefire
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Have you fired it up yet? Would be interesting to know how the 6550's and EL34's compare.
Jimi had some of his amps converted in the other direction - from EL34 to 6550, apparently sometimes with bigger PT for higher B+ and hence more headroom. Lots of info on this on the Metro amp forum from an engineer who worked on his kit.
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No I haven't fired it up because the customer sent a new set of JJEL34's in it in anticipation my re-bias. So I haven't got any 6550's, wish I did because I would love to hear it with those. I haven't had time to research what the change over from 6550's to EL34's requires, so I left it alone so far. I first read about a 47K resistor in the bias and the fix was to parallel another 47K beside it and then a lot of the post on this thread indicated something else. So it's a little fuzzy right now what to do.
When I got back from my vacation I found my computer crashed when I tried to boot it up. So I just got it back yesterday from repair and got it fired up. I've been trying to do all my internet stuff on my phone and you know how that is in dealing with schematics:>) So now I can actually print out a hard copy of the schematic and study it. First I've got to go to a funeral of an old friend that passed away. I'm still in recovery mode from my vacation playing catch up. So I will eventually get to the meat of the issue. Platefire
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Ok here are gut shots. Please explain about the nail polish because I'm not familiar with that. I do see several places with new solder and no nail polish. You Marshall experts, please let me know what you see!
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Gut shots continued.
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Gut shots continued:
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Last but not least are the tags and transformers.The little piece of red tag has an initial ending in m, you don't reckon that could be ?????? Platefire
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Please explain about the nail polish
We used it way-back as an enamel coat for corrosion, clean surface with 99% alcohol, add a dab. OEM's used it to *see* if someone made *unauthorized* repairs on warranty equipment.
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I think there is a pretty good chance that this amp originally had EL34's in it. It has a 220K dropping resistor in the bias circuit, I can't focus on any 1K/5W screen resistors, and the one original socket has pin 1 tied to pin 8 (although they may have always tied 1 and 8 no matter what tube was going in it). I would see what the bias range is and if it can do EL34's, no modifications are necessary.
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Thanks for cluing me in on the nail polish. It works pretty good because it easy to see what's changed. Only thing with my Mutli-Meter seems like it hard to make contact sometimes with nail polish acting as an insulator.
I tell you, I'm in the school of Marshall and it's somewhat different from Fender :BangHead:
But that's OK! A little different perspective.
2deaf, you are right! this thing appears to be biased for EL84's already!
I see some black burn marks on chassis around the replaced octal socket. Also the 50uF/50uf cap can associated with V1 and V2 power supply filtering has been replaced and they just run the new leads above the board(white wires) and just tagged on to the 10K/1W resistor leads. I think the other two filter cans are replacements too. Those are the main changes that caught my eye so far--still looking.
The 1987 Lead Mark II schematic had two 10K/1 W power resistors in series after the choke prior to PI and the latter 2204 has only one 10K/1 W after the choke prior to PI. Both for 6550 power tubes on schematics. The one I got has the two in series like the 1987 Lead Mark II----however---the nail polish is still on the bias circuit completely, so I guess it was originally intended for EL34's regardless of what the customer said because of the 220K bias feed resistor right? Platefire
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Amp appears to be an exact match for the EL34 version of the 1987 chassis (http://el34world.com/charts/Schematics/files/marshall/Marshall_jmp_lead_50w_1987.pdf). I presume the Yellow wire on the OT secondary is the 8Ω tap; the purple NFB wire is attached to it, as the schematic shows.
220kΩ dropping resistor in the bias supply, as well as 220kΩ bias feed resistors to the tube grids. All stock EL34-version items.
I see a changed coupling cap in the preamp, one changed output tube socket, a couple of new switches, and filter cap cans you note as being new. Everything else looks essentially bone-stock. And right for an EL34 amp.
It would be very nice if you had an ESR meter for the bias filter caps... They're the original Erie caps from the day the amp was made, but you'd like them to be in good operating condition. An ESR meter would tell you exactly how they're doing, as some Marshall fans get upset if they don't see 100% original parts.
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That red Erie (probably a .05) cap on the polarity switch needs to go. Put it in a bag and give it to your client.
Jim
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Thanks all for putting your eyes on this and pointing things out. I appreciate you sharing your experience and know-how! I think that just about covers everything replaced from original.
HBP---I'm not familiar with an ESR meter, please clue me in. I know two people who are Hospital Maintenance/Electricians who also know tube amps who may have one I could borrow.
Did anyone notice that metal plate on inside of chassis at strain relief where power cord enters chassis??? That don't look stock and wondering what the purpose is. The cord is a little loose in the hole. Does anybody know about this? here is the picture. Platefire
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I figured that the chassis was punched for the round British power receptacle and they used that plate to fill that hole and to mount the strain relief so that the cord wouldn't be pulling on the backplate. If it is a mod, it sure isn't a recent one.
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I found a gut shot online that has the same plate, so it is probably original. There are also a lot of backplates with the exact same setup as yours suggesting that Marshall sent out a large quantity with this format.
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Just do not cut the chassis. He probably sent you one of those Master Volume Kits Metro sells and adding a LarMar master only removes 2 resistors from the board. I have LarMar on my 68 Plexi in the speaker jack. I do not remove the parts and hand them to the owner. I print the schematic and cut the part where I made changes and roll those parts in the paper. Then I put the parts in a baggie and zip tie the baggie to the new power cord inside the amp.
With the larger parts like filter caps I baggie them and give them back.
Nothing wrong with Modding any amp that can be returned to stock if you do neat work. Now I do have an all original JTM45 and will hate the day I have to make the first new solder joint, but most collectors of Marshall amps expect some rework inside as they were made for road use.
It is the smaller Fender amps where most get picky.
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Thanks 2deaf and Ed
Well if this amp was mine, I think I would restore everything back to original save the filter caps.
I still don't understand why the person that replaced the filter can for the preamp stages didn't feed the leads through the holes that were already in the board to the turrets instead of letting it fly all above the board. Wouldn't have been much extra trouble and looked a whole lot neater. Guess it could still be done as a correction.
Can someone tell me which fuse holder is original. I think the one to the outside end but not sure. I'm getting ready to do a report to the customer and want to get all my info straight.
Another thing, on the impedance selector for Ohms speaker out, the selector handle with copper "U" is so loose that if you tilt the amp if falls out. Any good suggestions on correcting this?? Platefire
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... HBP---I'm not familiar with an ESR meter, please clue me in. I know two people who are Hospital Maintenance/Electricians who also know tube amps who may have one I could borrow. ...
I've gotten away with not having one for a long time. AnaTek (http://anatekinstruments.com/products/fully-assembled-anatek-blue-esr-meter-besr) sells an easy to use ESR meter which will give an indication of the health of electrolytic caps easily. Check Youtube for "Uncle Doug" videos, as he has at least one demonstrating that specific meter. IIRC, you can test the caps in-circuit.
The value would be verifying the health of electrolytics in old amps, which are either cumbersome to replace (like some multisection cans), expensive to find an exact replacement part (multisection cans, again) or where the owner wants to change as little as possible due to collector value placed on having original parts.
I mentioned it because to me, it is more critical to have well-functioning caps in the bias supply than perhaps even in the power supply. You'll get hum if either go bad, but you risk burning things up with a bias failure.
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I thought maybe the SS rectifier diodes were replaced, but the other picture shows the same type so maybe not. :dontknow:
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Can someone tell me which fuse holder is original. I think the one to the outside end but not sure.
Another thing, on the impedance selector for Ohms speaker out, the selector handle with copper "U" is so loose that if you tilt the amp if falls out. Any good suggestions on correcting this??
Probably neither one. I remember those things having the screwdriver slot holders. Frequently replaced because the slot would get trashed.
New impedance selectors are available for this model online. Apparently a lot of people lost those things because they were removable from the outside.
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Guess What??? I can't find any 1K screen resistors on this amp. Checking with my MM the yellow lead for screen goes under board directly to pin #4 on both power tubes.
See pix which shows the 10K/1Watt power resistor after choke. The inside yellow goes to filter cap can. The outside yellow that goes under board is screen voltage and the black is output from choke. This is a change we all missed!
Where are these screen resistors normally attached?? Platefire
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The EL34 version did not have any 1K/5W screen resistors. Just another indication that this is an EL34 amp rather than a 6550 amp.
http://el34world.com/charts/Schematics/files/marshall/Marshall_jmp_lead_50w_1987.pdf (http://el34world.com/charts/Schematics/files/marshall/Marshall_jmp_lead_50w_1987.pdf)
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Thanks sluckey, all my schematics had a 1K which wasn't exactly the right ones, so I copied that schematic you linked. Good, that's one thing less I'll have to straighten out:>)
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Can someone tell me which fuse holder is original. I think the one to the outside end but not sure. I'm getting ready to do a report to the customer and want to get all my info straight.
Another thing, on the impedance selector for Ohms speaker out, the selector handle with copper "U" is so loose that if you tilt the amp if falls out. Any good suggestions on correcting this?? Platefire
My '70 small box 50s had slotted fuse holders. I've seen '72 Majors with slotted fuse holders. My '73 has the tapered flower pot heads and it is original. Like 2deaf said, I don't think either one on your amp is original.
I would put that selector in a bag with the death cap and hand it to your client. A new plug is not going to fit any tighter unless you replace the entire assy and I still would not trust it. Those things rattling around in the receptacles were the cause of many unintended smoke effects on stage and a dead amp. Touring guys either soldered 8 or 16 direct to the jacks OR you can strip a 10 gage solid copper wire, bend it in a flat bottom "U" shape and it fits nice and tight. If you look at my pic below you can see mine. I didn't want to solder anything permanent because its all original, so I did the wire trick. I need a needle nose to pull it out.
Jim
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Well thanks everybody for helping me determine what I got regarding original parts verses replacements. He never indicated any interest in restoring it to original so I don't guess I will worry about that.
Next thing to do is fire it up and check voltages and bias. Voltage is no problem but I'm a little concerned about bias. As is the only way to check mA is the shunt method and you know how critical it is to get it right on MM hookup on that. Best I can tell the choke input and OT B+ hookup is on the .5 amp fuse holder. So I would have to put the MM red lead on the fuse terminal where the choke/OT is connected and the black MM lead on Pin #3 of each tube.
Also i have a friend that has a Eurotubes Octal Bias Probe. The type you plug into the tube socket and plug your tube into the probe and attach you MM leads to the probe leads(I think?)---haven't laid hands on it or its instructions but---I'm thinking this would be a safer way to go. Platefire
Jim---on the Ohms selector, I can't see the picture to clearly. Did you insert the new 10Ga "U" into the old twist handle or left it off?
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I would be really interested in the bias voltage range with the 1/2A fuse removed. Not only would it give you an idea of the ballpark and the ability to bias EL34's, but it would also let you know which way to turn the pot. to get the coldest bias if it's not obvious by looking at it.
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You don't have to use shunt method. I use this method from aiken site:
The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side. Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting. You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube. Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side.[/color]
https://www.aikenamps.com/the-last-word-on-biasing (https://www.aikenamps.com/the-last-word-on-biasing)
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As is the only way to check mA is the shunt method and you know how critical it is to get it right on MM hookup on that.
If the thrill of having two hands close to 500V has worn off, you can always use the OT DC resistance method. It can still be exciting if careless, but at least it's only one hand with no heart in the path.
Edit: VMS has posted the method I referred to.
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OK-thanks, I will give that a shot and see how it goes. I also ran across this this morning that is similar but shooting for 70% max plate dispensation. Whatever is safer and accurate.
http://www.marstran.com/50W%20Bias.htm (http://www.marstran.com/50W%20Bias.htm)
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How about Hoffman's bias checker? Works great! Easy to build and safe to use.
http://el34world.com/charts/BiasChecker3.htm (http://el34world.com/charts/BiasChecker3.htm)
With respect, Tubenit
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Jim---on the Ohms selector, I can't see the picture to clearly. Did you insert the new 10Ga "U" into the old twist handle or left it off?
I left the old twist handle off. Just make sure it fits tight!
Jim
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OK-thanks, I will give that a shot and see how it goes. I also ran across this this morning that is similar but shooting for 70% max plate dispensation. Whatever is safer and accurate.
http://www.marstran.com/50W%20Bias.htm (http://www.marstran.com/50W%20Bias.htm)
You could also get 1 ohm 1% resistors and put them in between the cathodes and Gnd.
Then you just measure the voltage drop across the resistor to tell you the current flow through the output tube.
This reading includes the screen current, so you just subtract the screen current out to get your bias reading measurement. :icon_biggrin:
I like being able to quickly and accurately check the bias measurements on fixed bias circuits. :icon_biggrin:
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tubenit Thanks, I really do need one of those!! Right now things are a little to busy for another project, so I think I will borrow my Friends for right now. I have a 50 watt Marshall project of my own in the works that I kind of laid aside temporarily to mess with this guys Marshall JMP 50. So that will go on a future project list.
Ritchie200---Does that copper "U" have to be attached for the amp to work??? or can I just stick it in a baggie with the handle? I may look around and see if I got some 10Ga copper wire but he hasn't requested it.
Paul1435---I don't usually do the bias resistors to an amp unless it is requested, especially a vintage amp.
Platefire
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Ritchie200---Does that copper "U" have to be attached for the amp to work???
Yes! :w2:
It's like a switch to choose the speaker load Z. That piece of U shaped copper is the switching element.
If it's not in place then the speaker jacks are disconnected from the OT secondary hot wire.
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Thanks, I didn't realize it was part of the switching circuit, I thought it just anchored in the switch to rotate it. So it intended to be pulled out and re-positioned to change Ohms? Again I not to familiar with Marshalls. Platefire
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So it intended to be pulled out and re-positioned to change Ohms?
Yes, it's the ohms 'hot' wire in between the the OT secondary and the speaker jacks. :icon_biggrin:
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Every amp seems to have something that's a weak poor design, this must have been Marshall's.
I planning to bias this thing prior to the weekend and if everything looks good, take it to Church and hook it to my Boogie 4-10 cab. I need to get a little taste of how this babe works as a preview of how my Marshall project might sound prior to installing the MV. If he insist, I guess I will go with the lar-mar type 2 PPIMV. Then I will probably need to re-bias again with the re-routing of the bias path. Platefire
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You should not have to readjust the bias just because of installing a MV correctly. Still a good idea to check the negative voltage on pin 5 of the output tubes before and after doing anything in the grid circuit.
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Well this is the plan I was intending to go by in attachment. Doing away with the 220K grid resistors and running the bias to the pots with the blue wire. I personally can't see how the bias grid flow can remain consistent running to each ground lug on the pots and coming out the output lug? It seems the change in volume setting on the MV would effect the amount of neg bias voltage going to power tube grids. Apparently I'm not understanding the circuit. Maybe you could explain it to me. Platefire
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There is no "bias grid flow" in a properly operating PP amp. Well, there can be some 'current flow' when the grid is overdriven hard, but you will be setting bias with no signal applied. No grid current flow means there can be no voltage dropped across those 220K resistors or that MV pot. Changing the MV volume setting will not change the grid bias voltage. If it does change, you have a problem. Just try it and see.
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Well it won't be the first thing I put together I didn't understand but still worked :icon_biggrin:
I just got through firing it up for the first time. Voltages are provided/recorded on a scan of that 1987 schematic attached below. Had a rough time at first because I had no sound with a guitar plugged in but could hear the amp driving the speaker. So I pulled out each preamp tube and tried a known good tube and when I got to V1 it started working fine. Boy is this thing loud!!!!
Anyway I'm not familiar with what Marshal voltages are suppose to be since they didn't put the expected voltages on the schematic like fender. It's pretty high on the power tube side(464), but that may be right for this one---you tell me? No red plating detected and the tubes don't feel abnormally hot.
So I need to get down to what the bias mA should be running that plate voltage. According to the Weber bias calculator running AB with EL34's at 464V plate voltage bias should be 37.7 mA. For those why know what the voltages should run on this amp, please check it out and let me know if they are OK. I'm not Marshall knowledgeable but they seem pretty OK, to me.
Also for added fun---I tried VMS's method he pointed out in post #40 for measuring resistance across the two OT primary sides, then measuring the voltage drop and dividing the voltage drop by the resistance---if I understood it right? here goes:
OT primary Ohms 44.4/45.5 Ohms, Voltage reading at center tap=464.7, voltage reading at plates 463.4/463.7, Voltage drop from CT to plates 1.3/1.0.
1.3 divided by 44.4=.029(29mA) and 1.0 divided by 45.5=.021(21mA)
If I did this method/calculation correct, looks like the bias is way cold!
For Even more added Pleasure: EL34=25 watts divided by 464 Plate voltage = .0538 or 54 milliamps X .7 = 37.8 mA bias---BTW that's pretty close to Webers calculation
Platefire
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Hmmm, that's a little high for that one. Are you connected to the 110v tap? If so, it needs to go to the 120v tap. MHO is the EL34 sweet spot is in the 390 range. The 110v tap is another trick to push the amp a little harder to keep up with the SVT armed bass player....
Jim
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Best I can tell there is a red, yellow, orange, brown and blue coming from the PT on primary side. The Blue is hooked directly to the polarity switch--see bottom pix. The brown appears to be hooked to the bottom of the little narrow PT connection board on the third turret from the end closest to chassis and has a black above board jumper to 2nd turret. Then the bottom of the 2nd turret appears to have a longer black jumper to both legs of bottom of power switch---see first pix. The examples of Marshall PT on Doug's site didn't show any brown primary leads? I hope someone else can tell what's going on with this hookup.
These two new pictures might help identify. Platefire
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Ok, I went ahead and hooked up a borrowed Eurotube bias probe to a socket. The mA reading was very low. I turned the bias pot to drop bias voltage as low as it would go(hotter) which only went to 14mA, the bias voltage changed to -48.8, plate voltage to 453.
So as the bias voltage was set a little bit hotter the plate voltage went down and mA went up. So I'm thinking there is probably no use proceeding until I replace those original 10uF/160v bias caps! Does anybody second this? Platefire
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The mA reading was very low. I turned it up with the bias pot as high as it would go which only went to 14mA
That's even lower than you measured using the OT method. You started with 29mA and 21mA. And all those readings are much lower than 70%. I don't trust one of those readings.
Pull the output tubes. Then measure the voltage on pin 5, first with the bias pot fully CW, then again with the bias pot fully CCW. What have you? You may need to change that 47K resistor in the bias circuit to get a good voltage range. Are you using 6550s or EL34s?
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Thanks, yeah I'm using EL34's. So you want to get an idea of the complete bias range in the circuit. Ok, will do as soon as I go take my walk:>) Platefire
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Bias range is from -49.54 to -66.6. A total coverage range of -17.8 volts. Platefire
BTW---I measured the bias resistors in place a 213.5K, 15.38K and 46.44K
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Bias range is from -49.54 to -66.6.
That's fine for 6550s but it's too high for EL34s. You need a range of about 35 to 50. Change that 47K to a 33K (maybe even 27K) and check the new range. You can do this without the EL34s plugged in.
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I installed a 27K first and got a range from-32 to -51. So that was right in the pocket for EL34 you indicated, so I didn't even try the 33K.
I went ahead and fired it up with power tubes using the bias probe. Set it at 40mA. At that setting plate voltage is 440 and bias voltage is -40. Weber bias calculator says 39.7mA.
Check the Voltage drop method which at OT CT measured 442 and at both plates were 440:
2V/44.4=.045----so it looks like we have 5mA difference in what the bias probe is showing and the voltage drop calculation. Tube is pretty hot to the feel but not red plating.
I do know that I can tell a marked difference/improvement in tone and response. It sounds good just dry signal. Platefire
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I installed a 27K first and got a range from -32 to -51... Set it at 40mA. At that setting plate voltage is 440 and bias voltage is -40. Weber bias calculator says 39.7mA.
You in the zone now! Nice work. Now write it on the wall! :icon_biggrin:
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Well thanks for your help. I appreciate it! Platefire
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I took the Marshall to the Church this morning and hooked it up to my 4-10 Boogie Cab and tweaked the amp and pedals for the new setup. I never played live on a Marshall before, so I can't let this opportunity pass me by. After all I need to test this new bias circuit change to make sure it's holding :icon_biggrin: I'm just really impressed with how good this thing sounds now that the bias is adjusted right---as different as night and day. The clean channel not only sounds good but is touch responsive also with nice harmonics. I usually don't like a dry amp but it even sounds good dry to my ears. I hope the Marshall 2204 I'm building sounds something like this. It's good that I heard this one and will give me something maybe to shoot for on my DIY one.
I mostly been playing though channel 1 for mostly a clean sound but have tried channel 2 and like the way is distorts as you crank it up. I can just imagine playing through this with a cranked in channel 2----some serious raunch. Platefire
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... The clean channel not only sounds good but is touch responsive also with nice harmonics. ...
I mostly been playing though channel 1 for mostly a clean sound but have tried channel 2 and like the way is distorts as you crank it up. I can just imagine playing through this with a cranked in channel 2----some serious raunch. ...
Both channels are the "clean channel" (unless there has been an unmentioned mod to cascade the two channels. I had this model, but one year later (when Marshall switched to their first p.c. board). I considered the channels "Icepick" & "Mud". Jumper the two channels with a patch cord connecting 1 jack from each channel, and use the two volumes to set your overall sound.
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Something is not making sense on this one. That plate voltage is way high. They changed from laydown to upright power trannies in 72. The laydown would run about 415-420 on the 120v tap with wall voltage in that range. The upright trannies would run in the 390-395 range on the 120v tap with wall voltage in that range. The trannie looks original, or is it? Is this an oddball that Drake sent them on the changeover? The 120v tap is usually brown, so we look good there. What is your heater voltage?
Jim :think1:
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HBP---no mods. I just plugged into channel 2 briefly and could swore it had more gain than channel 1 but I turned it up louder also. So both channels are clean? OK! I take it you were not to happy with your JMP 50?
For some reason, I don't know why, I've always resisted Marshalls. Back in the 70's when I was doing a lot of playing/traveling I always heard reports that they were UN-reliable. To top that off when I was living in Oklahoma City in 1979 I seen first hand Marshalls break downs on stage and the techs were going crazy trying to get it going again. I bought a Peavey Mace because in those days I was a big Skynyrd fan, it smoked and I had to send it back to the factory under warranty for a new board. So sometimes you get crazy ideas in your head.
Jim---You know when I had it opened up the other day, i forgot to check the heater voltage. I've got to take it down again and probably do a three conductor cord and maybe a MV mod, so I will do it then. You want to see if the heater voltage is high too huh? Well if somehow the PT is hooked up wrong, I sure would like to know myself. Maybe I need to check the numbers out more closely on the PT and see what I can find. It would be good to know if its not original. Platefire
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Here is a link to a discussion of Marshall voltages and it seems based on this with a stand up 1202 164 Drake with SS rectification 435-440 plate voltage is pretty normal. It kinda seems they know what they are talking about, check um out. The Plate voltage did come down off the 464 to 440 when I changed the 47K bias resistor to 27K that adjusted the bias range to properly bias the EL34. Here is the link:
http://forums.vintageamps.com/viewtopic.php?p=58287 (http://forums.vintageamps.com/viewtopic.php?p=58287)
I played it in my Church band this morning it was just a real positive experience. The volume was on 2:>)
I did use my Route 66 Compressor and OD and also a VS Route 808(set to clean boost). I even got some compliments on the sound. So it was just all right. Kept and eye on the tubes and touching them are hot but not overly hot--they don't burn your hand, no red plating even once since I set the bias. I think is very close to right. Platefire
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Wow Plate, that is interesting. It looks like the plate voltage was all over the place! From year to year even! I think those Drake employees were doing some chemicals! I was referencing my trusty Marshall book. It seems that real world is very different!
I would still check those heater voltages....! :icon_biggrin:
Jim
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Plate you also need to make a few adjustments next time you play it.....
Jim
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Well I tried to get him to come get it and try it now that it is biased right because it sounds so much better thinking he might forget the MV. He is determined to have the MV. So my plan is to put this version(see pix) in it that requires no more coupling caps. He brought me a kit with the dual 250K pots, two 2.2M resistors, a length of blue hook up wire and also shielded cable. The kit is from ValveStorm. So based on the parts I suppose that's what he wants even though he didn't specifically say the word "Lar-Mar", the parts he provided match the mod perfectly and he did say Post MV. Platefire
JIM-with that kind of adjustment my wife would have me kicked out on my ear. Shes always the one that tells me when I too loud :l2:
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OK, I'm proceeding on with first the three conductor cord and then the MV.
On the three conductor cord hookup my plan was to leave the polarity switch in place but more or less disabled. That way it would still be holding it's original part in place couldn't get lost being there bolted in. I have been reading previous post and looking at the situation. As shown in the picture the power cord white is tied directly to the polarity two top terms, the hot through the fuse comes by the yellow wire to the polarity switch two bottom terms and of course the two middle terms go through the cap to ground.
The PT primary blue tap is tied to bottom terms of polarity where the yellow hot from fuse/black power cord hot is also tied and PT primary brown tap is tied to the 120VAC tap of the multi tap AC power board and is tied to the bottom terms of the power switch by a black wire to the 120 tap connection board. This is somewhat shown in attached picture.
So I'm thinking I could accomplish what I'm desiring to do by just removing the cap from the center terms of the polarity switch and use the the ground lug that the cap was tied to on chassis to connect my green from the power cord and off course the white would go to polarity switch top terms and black would go to fuse.
Please check me and tell me if I'm thinking wrong! Platefire
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That's what I would do.
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Got the new power cord in but it was a headache :BangHead:
I found out what that inner plate at strain relief was for-----that's the only thing holding the cord. The hole in the chassis is about 1" in Dia so that's no use! You can't tell the hole is that big because it's covered with the face plate on the outside and on the inside has that extra plate---so the main hole in the chassis is covered up. With that plate not being anchored to anything made it really hard to feed the strain relief with cord through the plate. The only way you could anchor the inner plate permanently is drill holes through the chassis & face plate and install a couple of small bolts/nuts. I decided to install with inner plate loose just like it previously was. Had to file the strain relief down some with a round file and clamp the plate down to be able to put some pressure on it but finally got it through and it's in and pretty tight. Fired it up and seems to be working OK. Hardest cord I ever installed. Platefire
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Congrates Plate! You are now officially a Marshall Meister! :bravo1: You will be rockin' the Praise in no time, smashing guitars and blowing up your amps! It's a disease, there is no cure! :help:
Jim :icon_biggrin:
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I'm getting to old for this high drama in amplifiers :help: You would think installing a cord was brain surgery :l2:
Well got the post PIMV to go. I'm ready to get done with this job, too much research required to try to overcome the Marshall curve for me! At least this will give me a preview of my own 50 watt Marshall clone. Platefire
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I will be starting the Lar-Mar PPIMV today. One thing I was wondering considering preserving and a possible future reversal of this mod that does away with the two 220K bias resistors. My concern is the grid wires that run under the board to the power tubes. I'm wondering about disconnecting those wires from the power tubes, cap them and abandon them in place? or is that asking for trouble and would be best to remove?
I will be removing the extra speaker jack to install the MV pot in there. Should I preserve those old original leads from main speaker jack to the extra speaker jack? Never done this before, so being a classic I'm trying to be sure about each move for possible restoration to original. Platefire
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Got all the old parts removed, got new pot in, new wire cut and just need to solder
It up. Decided to take a nice break before I put my solder hat on. Platefire
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Those wires aren't reversed are they? If they are, it will change the NFB to PFB.
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Yeah thanks 2deaf for reminding me of that, definitely something to consider or will have big squeals. IOW to maintain the same connection I previously had I need to reverse the wires on #5 pins of power tubes sockets where black will be on the inside tube and red outside tube.
Also best I can tell on this one the OT secondaries is Gray 16 Ohm, Green 8 Ohm and yellow 4 Ohms. The NFB is connected to the yellow 4 Ohm tap---same a the 6550 version schematic shows. The only way I was able to determine this color coding is by the readout window on the impedance selector and noting the connections made from the inside. For EL34's on the 1987 schematic shows the NFB connected to the 8 Ohm tap. So if I'm understanding things right, I need to de-solder NFB from the yellow wire terminal and connect it to the green wire terminal.
I can't help but wonder if switching to the 8 Ohm tap for NFB will get me back in PFB territory again? May need to not solder up both pin 5's until I see what it behaves like powered up.
I hope some of you Marshall folks are looking in because I'm feeling my way on this one. Platefire
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Don't fret over a mole hill. Plug it in and turn it on. if it squeals, reverse the red and black on the board. Don't mess with the NFB wire or OT wires.
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Got the new power cord in but it was a headache :BangHead:
I found out what that inner plate at strain relief was for-----that's the only thing holding the cord. The hole in the chassis is about 1" in Dia so that's no use! You can't tell the hole is that big because it's covered with the face plate on the outside and on the inside has that extra plate---so the main hole in the chassis is covered up. With that plate not being anchored to anything made it really hard to feed the strain relief with cord through the plate. The only way you could anchor the inner plate permanently is drill holes through the chassis & face plate and install a couple of small bolts/nuts. I decided to install with inner plate loose just like it previously was. Had to file the strain relief down some with a round file and clamp the plate down to be able to put some pressure on it but finally got it through and it's in and pretty tight. Fired it up and seems to be working OK. Hardest cord I ever installed. Platefire
I try to connect the cord green/gnd wire under a PT lug nut if at all possible. :dontknow:
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Sluckey---my wife tells me I don't need to exercise because I get plenty making mountains out of molehills, flying off the handle and jumping to conclusions! I also suffer from another serious disease "The Paralysis of Analysis"
paul1453----I thought about those PT bolts but when I seen that little bolt where the death cap attached when I was removing it, I decided that's it! I did see a picture of an old Marshall with that round receptacle with the two prong/conductor socket. I suppose that might have been in it at one time, reason for the big hole? Platefire
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Plate,
That big hole was to accommodate the big Bulgin Euro mains plug. The cover plate you modified were fitted for the US exports.
Jim
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Sluckey---my wife tells me I don't need to exercise because I get plenty making mountains out of molehills, flying off the handle and jumping to conclusions! I also suffer from another serious disease "The Paralysis of Analysis"
:l2: :l2: :l2:
Jim
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Ok, Export to USA got the loose conversion plate :dontknow: I'm wondering about the origin, the customer said it had 6550's in it when he got it. The circuit is straight 1987 for EL34's with the exception of the NFB being on the 4 Ohm tap. The 1987 schematic shows it connected to the 8 Ohm tap. Can't help wonder if it originally had EL34's and at some point someone changed it to 6550's and now I'm changing it back to EL34's. Don't know, makes me wonder?
Also I copped me some 10Ga copper wire tonight. Going to make a "U" for that Ohms selector. I've been testing it with the old selector held on with Scotch tape---can't stand that! Platefire
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This is probably way over-kill but! This is the second or third Marshall I've had to build an Ohms "U" connector for and without the little window on the original handle your kind of flapping in the breeze to know which plug in connection to make to get the right Ohms? So I made a drawing to indicate this. I don't know if it would work for all selectors but for this JMP 50 will work. Platefire
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Plate, just a fyi once the ppimv is in. I usually find them dark sounding when turned down below 50% & usually need 100pf treble bleeds on both pots, ymmv?
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OK, thanks for sharing your experience. So your talking about a 100pf from input to output terms---right?
I will build it as is first, and if it's apparent it needs it, add them latter. Platefire
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I copped me some 10Ga copper wire tonight
Be real careful, at least in MI the cops are cracking down pretty hard on that sorta thing :icon_biggrin:
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Well, I think I will go ahead and turn myself in :sad2:
Life is rough on the run :happy1:
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Just soldered everything up, double checked everything and fired it up. Best I can tell so far everything is working good. The amp is operating nice and quiet. Only hums when I get too close to the chassis with my strat. Checked the bias voltage and it was holding around -40 or -41 same as it was set before the mod. I hooked a delay up to it and played around with it and I think the MV really helped it! You can really get some good sounds and response at lower volume now. I think the customer is going to be really happy with it.
I really can't tell any loss of highs myself. In fact when I started jamming on it I turned the treble, middle and presence down. The bass was already up about 75%. I was able to get the kind of tone and response I like out of it. I couldn't turn it up like I would like, the wife is home:>) Platefire
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Come on Plate, crank it up! That's rock and roll - taking chances, living on the edge, having women scream at you! :laugh:
Jim
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Yeah, now that it's got a MV, Rock n Roll at a lot lower level :icon_biggrin: and still get dinner! :l2:
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:laugh:
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I decided to double check the bias now that I got the mods done. Guys I need some help, things are not adding up on the bias!
First I've been calculating the bias by the measuring the resistance across the OT primaries and measuring the voltage drop which the resistance is to V4=43.9 Ohms and V5=45.2 Ohms.
The calculations I did with his tubes he sent in the amp shows a vast difference in mA between both tubes. He claims his tubes are supposed to be matched tubes and shouldn't be that far off. The last calculation I did with his tubes were way off. From my past experience usually a big miss match in the tubes had been due to the tubes. So I have a new matched set of EH EL-34's for my Marshall clone project and decided to try those to see if it helped the situation.
One of the problems I'm having using the method is in the amp loaded voltage at the OT center tap is fluctuating by a volt or two continually. So by the time you've made three voltage readings and recorded them, the voltage has changed. Here are my last readings and calculations just to show you what going on as the amp warms up as follows:
1-(A)OT center tap=442.6V/V4 plate=441.2V/V5 plate=441V
(B) V4 442.5V-441.2V=1.4 Voltage Drop & V5 442.6V-441.0V=1.6 Voltage Drop
(C) V4 1.4 Voltage drop divided by 43.9 Ohms=.031 & V5 1.6 voltage Drop divided by 45.2 Ohms=.035
(D) Bias is V4=31mA & V5=35mA
2-(A)OT center tap=440.5V/V4 plate=439.1V/V5 plate=438.6V
(B) V4 440.5V-439.1V=1.4 Voltage Drop V5 440.5V-438.6V=1.9 Voltage Drop
(C) V4 1.4 Voltage drop divided by 43.9 Ohms=.031 & V5 1.9 voltage Drop divided by 45.2 Ohms=.042
(D) Bias is V4=31mA & V5=42mA
3-(A)OT center tap=437.6V/V4 plate=436.5V/V5 plate=436.2V
(B) V4 437.6V-436.5V=1.1 Voltage Drop V5 437.6V-336.2V=1.4 Voltage Drop
(C) V4 1.1 Voltage drop divided by 43.9 Ohms=.025 & V5 1.4 voltage Drop divided by 45.2 Ohms=.030
(D) Bias is V4=25mA & V5=30mA
4-(A)OT center tap=442.4V/V4 plate=441.2/V5 plate=440.2V
(B) V4 442.5V-441.2V=1.3 Voltage Drop & V5 442.5V-440.7V=1.8 Voltage Drop
(C) V4 1.3 Voltage drop divided by 43.9 Ohms=.029 & V5 1.8 voltage Drop divided by 45.2 Ohms=.039
(D) Bias is V4=29mA & V5=39mA
5-(A)OT center tap=440.9V/V4 plate=439.5V/V5 plate=440.1V
(B) V4 440.9V-439.5V=1.4 Voltage Drop & V5 440.9V-440.1V=.8 Voltage Drop
(C) V4 1.4 Voltage drop divided by 43.9 Ohms=.031 & V5 .8 voltage Drop divided by 45.2 Ohms=.017
(D) Bias is V4=31mA & V5=17mA
So 5 separate checks and you see the progression as the amp warms up! It's apparently not the tubes with a new matched set in there. Bias voltage is pretty constant at about -40V. So what could be causing the unbalance to this degree and variation in voltages? I'm not going to be able to properly bias this until I can get to the bottom of this. Kind of at a loss on how to proceed to correct this. Help please :w2: Platefire
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Buy a bag full of 1Ω resistors. You will get much more consistent readings.
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Using the Shunt method with my Fluke 115:
His tubes-V4=42mA V5=24mA (These 18 mA apart)
My tubes-V4=26mA V5=34mA (These are 8mA apart)
The plate voltage is fluctuating between 445 to 437 range. It seems to start off high and drift down as the amp warms up. Never seen an amp fluctuate in plate voltage that much?
My tubes are new. 8mA is a pretty big divide for new tubes. My inclination using my tube is to set V5 at about 40mA and that may bring V5 up to 32mA. Platefire
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What? Don't anybody have any idea of the cause from variation/drifting in voltage from 445 to 437 in plate voltage? Even after I tried new tubes the drifting voltage continued. Is the original rectifier failing or could failing filter caps cause the drifting---the amp is pretty quiet. I tired it without the power tubes in and the voltage drifted from 468 to 460.
Also the great divided in mA of biasing even with new matched set(8mA dif)? Is it something in the amp causing the divide in the amp? or is all in the tubes? It's about 1.3 Ohms difference in OT CT resistance to plates (43.9/45.2) but I don't think that would make that much difference.
I am wanting to get the bottom of possible issues with this amp before I return it to the customer so he may have some trouble free operation. I have no explanation from my experience for the amp behavior and I was hoping some of you more experienced hands would chime in regarding these issue above. :help: Platefire
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What? Don't anybody have any idea of the cause from variation/drifting in voltage from 445 to 437 in plate voltage?
I don't consider that a drift. Certainly not a problem. The power supply is not regulated. Even if it was regulated, consider this... Suppose the voltage was supposed to be 440VDC. Your voltage only increased by 1.12% and only decreased by 0.68%. That's would be considered pretty good regulation.
Quit climbing the mole hill! Get a bag of 1Ω resistors so you can get some consistent and reliable bias current measurements. Once you try it and see how easy it is you'll never use that klutzy OT voltage drop method again. IMO, a service tech needs to consider these 1Ω resistors as standard bench items, just like solder, or a meter.
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OK, thanks! Good to know the voltage changes is not a problem.
Honestly I have avoided the 1 Ohm resistors because I was avoiding any additional mods. I have the 1 Ohms bias resistors on hand. I did contact the customer and he said yeah. I won't drill holes for outside contacts and ground but just remove the existing cathode ground wire and replace it with the 1 Ohms.
It may only confirm my previous readings regarding un-matched tubes, but I will then know what they are for sure and it will then be set for easy bias readings from now on. Platefire
BTW-Just for basic information, this type of MV with bias running through the pots does effect the bias voltage 1 negative volt from off to full up. Actual readings are: Off=-40.92, 50%=-40.70 and 100%=-39.93
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I installed the 1 Ohm bias resistors. Results is it pretty much confirmed what I had already determined by the other methods.
With his tubes last reading after good long warm up: V4 Plate 439.5V/41.8mA, V5 Plate 440.1V/24.5mA, -41.06 Bias voltage (These tubes are 17.3mA out of balance)
Using my tubes last reading was: V4 Plate 444.4V/33.4mA, V5 Plate 444.6V/43.4mA, -40.35 bias voltage (These tubes are 10mA out of balance)
Platefire
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Very good. That's easy and safe. So, what are you gonna do about his tubes?
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Did you flip/flop the tubes? Left tube in right tube socket, right tube in left tube socket? Then take readings again. You have a 50/50 chance they will be closer in mA's.
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He is going to bring by some more tube sets for testing. I offered to sell him my new set but it even checked 10mA off. I have tried swapping and it may have helped a hair but the divide pretty much followed the tube. Platefire
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I see a possible match...
his V4 (Plate 439.5V/41.8mA) and your V5 (Plate 444.6V/43.4mA).
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Well you have to realize, I wasn't using the same bias voltage on his set as mine. I was biasing each set to get the mA about 2 to 3 mA above the 39.5mA setting for the plate voltage in order to bring the low side up as high as possible without getting too far above safe on the high side. I would say probably just a volt and a half higher his than mine. That is probably enough to be 7.3mA dif than your recommended match at 1.6mA dif.
So I did try your recommended match. It came in at V4=438.4/42mA V5=439.1/34.7mA @-40.93 bias voltage
Anyway, this is the closest match yet with what tubes I've got on hand. Platefire
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We all probably obsess a bit too much about perfect current matching. I know I did a few years back. I've relaxed quite a bit since I installed a dual bias circuit in my Sunn just to get a 'perfect' match. But what does that really mean? And who's to say how well that perfect match will be a month from now after the tubes have had some time to settle in?
The last fixed bias amp I worked on was my AO-63 conversion. The "matched set" of 7591s were running at 24.4mA and 18.2mA with plates at 470V. I called that good enough.
Don't let this matched tubes craze get to you. If you do, then prepare to keep a bucketful of tubes on hand. Just get it close. Then let your ears tell you when it's done.
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sluckey, I hear you on not being obsessed by a perfect match. Hardly any of my PP amps are any closer than 5 mA. Believe me it's the customer that's obsessed in this case. If it sounds good and not to far out of balance, it's good with me. I'm trying to be patient until this job is finished but I'm way ready to be done with this :BangHead:
Platefire
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Well the customer brought me several sets of EL34's and I tested 4 sets and picked the closest match(5mA dif) and biased them. So this is done. I also made an announcement that I won't be repairing other folks amps anymore. If I talk to myself just right, I might still repair my own :l2: Platefire
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sluckey, I hear you on not being obsessed by a perfect match. Hardly any of my PP amps are any closer than 5 mA. Believe me it's the customer that's obsessed in this case. If it sounds good and not to far out of balance, it's good with me. ...
RDH4 addressed this topic, at least with regards to output triodes (and then noting the results with pentodes were similar).
An push-pull output stage was investigated which had two different tube types in the 2 sockets, with one triode having half the transconductance of the other side. This should equate to a 50% mismatch in 2 tubes of the same type, or maybe one new and one "dead" tube out of a pair of the same type.
The end result was ~5% THD (and IIRC, slightly lowered output power). The conclusion was tight matching was not critical unless the maximum clean output power is required.
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Thanks HBP
What I've been doing and don't know if it's correct but at the same time I haven't had any major burn outs yet, is in balancing the mismatch. Just for instance say two EL34's running plate voltage of 420V and your goal is to bias at 41mA. When you bias your hottest tube up to 41mA and lower one is running 32mA.
What I do is run the high side up a couple of mA above the safe zone to bring it up to 43mA that brings the lower up to around 34mA. I don't know if that good practice or not?
How do you handle adjusting the mismatch? Platefire
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I did go read RDH4. It did say not over bias more than the tube recommended dispensation. So if you're already at a pretty hot 70%, I guess I shouldn't raise the high side above that in order get the low side up. According to them the divide is not a big deal in the overall sound other than being a little less clean. So I may discontinue my balancing method mentioned in previous post :embarrassed:
So I would say within 5mA apart is OK and to shoot not to be more than 10mA if possible. Platefire
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In the end, it's all a judgment call.
If you bias a bit over 70%, but play the amp full-tilt and see no redplating (may take a darkened room to see the onset), then you're fine.
I just re-looked the triode and pentode comments on mismatching in RDH4. It did note that "the effects of mismatching are more serious in pentodes than triodes". But if you know first that push-pull operation with perfectly balanced tubes, circuit & OT cancels all even harmonic distortion generated in the output stage, and that RDH4 is written from a perspective of engineering to high-fidelity standards, well... Then mismatched output tubes means you'll likely get more distortion in your output section.
Severe mismatching (60mA in one tube, 10mA in the other) could temporarily erode the OT's ability to transfer power from the primary to the secondary. That's because the mismatched idle and presumably signal) currents will tend to magnetize one half of the primary more than the other, and looks like d.c. through the OT in the direction of the higher-current tube. Push-pull OT's use smaller cores for the same power through-put than single-ended amps precisely because equally-balanced idle currents looks like 0mA d.c. to the transformer. The push-pull OT core then is only big enough to transfer the full power of the output stage, rather than full power plus some extra core material to support the magnetization due to idle direct current.
So even severe mismatching idle currents might only lead to OT core saturation, and another form of distortion (as well as some shaving of bass response at max power).
Mismatching could be a problem if it's bas enough to let hum through the OT to the speaker. I don't know if you've had a tube try to burn up while you were biasing, but other than the red plate, what usually happens is you hear hum in the speaker as the tube current is getting really out-of-hand (I heard it once when a tube lost bias, right about the point it hit 110mA or so). Hum at the 1st filter cap is usually cancelled by the balanced nature of the push-pull OT and because hum from the B+ is applied common-mode to an OT which transfers differential power from primary to secondary, but greatly imbalanced tube currents seem to overcome this somehow.
Assuming the amp's power supply/heaters would support it, your customer could put a KT88 on one side and an EL84 on the other side (keeping each tube biased within its limits). The amp wouldn't be able to develop clean output power in excess of using a pair of EL84's, but neither the amp nor tubes would suffer as a result.
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BTW-Just for basic information, this type of MV with bias running through the pots does effect the bias voltage 1 negative volt from off to full up. Actual readings are: Off=-40.92, 50%=-40.70 and 100%=-39.93
The bias circuit you are working with has a relatively high impedance, so the loading caused by your multimeter may be sufficient to create the difference you're measuring.
I've just tested the Lar-Mar master volume in one of my amplifiers, and that is exactly what happened. When the meter was used to measure bias voltage on one side of the push-pull pair, current through the matching 1Ω sense resistor rose slightly. ( I did use two multimeters for simultaneous measurement... )
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Well bias is something I seem to get hung up on. I would like to get to a place where it's easy as falling off a log. Seems the more I learn, the more I need to learn. On one hand it seems simple and the other, a deep dark hole. I know I don't like cold or cool biased sound. So the trick it get up to the edge of the abiss with your slightly out of balance tubes without sliding off the cliff. Platefire
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Well the customer has got it and reports it sounds great. Maybe I should send him a note to keep an eye on the plates. :icon_biggrin: