Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jaymz77 on May 09, 2019, 02:14:38 am
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Hi all, long time no post...
I have a JCM800 2203 that im looking at for a friend. When it came to me 2 of the output tubes were red plating badly. Turns out it was a bad pair of tubes. Anyway, now i have a set of new Svetlanas in it and i can't seem to get the bias in the right window. I have changed the value of the resistor in series with the bias pot and i can only seem to raise the bias current, not lower it. I have been soldering various resistors in parallel with the 47k to see what sort of value it requires, i have tried values between 23k up to about 200k and nothing seems to lower the bias current below about 120ma.
Any suggestions on what it could be? Clearly whatever this issue is is the cause of the red plating tubes that were in it.
Cheers
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Reducing that resistor value is reducing the bias voltage, whereas you need to increase it.
Rather R29 or R30 value need to be increased.
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Reducing that resistor value is reducing the bias voltage, whereas you need to increase it.
Rather R29 or R30 value need to be increased.
R29 is not in the bias circuit. R30 and/or R27 values need to be DECREASED to increase the negative voltage which will decrease the bias current (make tubes run cooler)
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Thanks for the replies guys.
Do either of you have links to a more modern schematic? I only have the old ones. In fact the one i have doesn't even have an R30!
Cheers
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https://el34world.com/charts/Schematics/files/Marshall/Marshall_jcm800_lead_mstvol_100w_2203.pdf
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Doh, many apologies for my typo, of course sluckey is spot on.
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Thanks gents, ill have a look at those 2.
Why would i need to tho? If they are the standard values why would they need to be changed to give the correct voltage/ current?
Cheers
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Because the resistors in the amp now don't give you the bias range you desire.
Or maybe the bias caps are dragging the bias voltage down. Not a bad idea to replace them while you're dancing around them.
Or maybe the PT is not putting out enough voltage for the bias circuit.
Or maybe the new tubes are more finicky than old production tubes.
A resistor change will get you where you want to be. But consider changing the bias caps too. There's a lot of expensive stuff depending on that bias supply to be right.
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Thanks Sluckey,
it looks as though the bias caps have already been changed to Sprauge Atoms. They should be ok.
I did notice however that the bias supply seems to be coming from a dedicated transformer tap. Should i try taking a new supply from one side of the PT secondary?
Cheers
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As Sluckey said, replace those bias caps. Don't let the new look sway you! I'd do that first before you change anything else.
Jim
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I have a JCM800 2203 that im looking at for a friend. When it came to me 2 of the output tubes were red plating badly. Turns out it was a bad pair of tubes. Anyway, now i have a set of new Svetlanas in it and i can't seem to get the bias in the right window. I have changed the value of the resistor in series with the bias pot and i can only seem to raise the bias current, not lower it. I have been soldering various resistors in parallel with the 47k to see what sort of value it requires, i have tried values between 23k up to about 200k and nothing seems to lower the bias current below about 120ma.
Any suggestions on what it could be? Clearly whatever this issue is is the cause of the red plating tubes that were in it.
Is this a real 2203 or a reissue 2203?
What kind of tubes do you have in it?
What is "bias current" and how did you measure 120mA?
What is the negative voltage going to the grid leaks?
A bias issue at the supply would usually red-plate all four tubes, not just two.
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2Deaf,
Yes its an original.
What do you mean "what is bias current?" The current the tubes are pulling on the bias circuit. I measured with a bias probe as well as across the 1ohm resistors that i don't believe are original.
I haven't measure the screens yet.
Yes that's the strange part, only 2 were red plating.
Cheers
Ritchie200
Ill replace them if nothing else fixes the issue. I would have to order some.
Cheers
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I measured with a bias probe as well as across the 1ohm resistors that i don't believe are original.
So there are 120mA going through each cathode of the still unknown power tubes? If that's the case, I would take those tubes out until the bias issue is resolved.
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2Deaf
I've only let that happen for a short period of time while i check the changes I've made to the resistor values.
Ive never seen a 2204/ 2204 style of amp with its own bias tap on the PT before, have you?
Cheers
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All of the 2203's, even the reissue, have a separate bias winding because they use a bridge rectifier that doesn't lend itself to taking the bias supply from one side of the HT winding.
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Ok.
To be fair I'm far more experienced with 50watters, than 100watters.
I have just ordered some bias caps. If they dont fix the issue ill be at a loss as to why the current/ voltage is out of spec when all the component values are as standard.
Cheers
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Yes, take all the power tubes out and check their terminal 5 control grid Vdc.
If the red plating tubes both on the same push-pull 'side' then the coupling cap to them may be leaking dc, or their grid leak resistor dodsgy somehow.
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PDF64,
Yes they were both on the same side. Ill try the bias caps, check the grid leak resistors (they are the 1k ones on the back of the tube socket, right?) and if it isn't any of those, ill try the coupling cap.
Cheers
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The 1k 5W R35-38 are screen grid current limiters.
The grid leaks are R24,25 220k.
The coupling caps in question are C14,15 22nF.
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There is an orange wire and a green wire coming out from underneath the board. Each wire connects to two 5.6K resistors. Each of these four resistors connects to a pin 5 on the power tubes. With the power on and the amp in standby mode, what is the DC voltage on those 5.6K resistors? Speaking of the power tubes, what kind are they?
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2deaf,
Sorry for the late reply. Ive had some family dramas going on.
The new tubes are Svetlanas.
Voltage on all 4 5.6k resistors is -32.5VDC
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This is very strange indeed.
My research says that voltage on pin 5 is about right.
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maybe a touch low
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Ok,
i tried paralleling another resistor over the 27k in the bias circuit to lower its value and it did bring the bias current down closer to where it should be.
Then i thought to measure the voltage on the bias tap from the PT. It only measured 67vAC or so. this seems quite low to me.
In order to get the right bias current range I'm going to need to swap the 27k with a 10k, or lower.
Is the PT on its way out? or at least that tap?
Cheers
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67vac on the bias winding is fine. Have the bias caps been changed? How much negative voltage do you measure on the bias cap that connects to the diode?
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My research says that voltage on pin 5 is about right.
Ah. Then you must have EL34's.
Then i thought to measure the voltage on the bias tap from the PT. It only measured 67vAC or so. this seems quite low to me.
In order to get the right bias current range I'm going to need to swap the 27k with a 10k, or lower.
-32.5V with the bias pot maxed is consistent with a 67Vrms winding. Replacing the 27K resistor with a 20K resistor is likely to yield a bias range of -31V to -38V. Paralleling a 77K resistor with the 27K resistor will make a 20K resistor. Don't have a 77K resistor? Neither do I. Try 68K or 82K if you have those. Replacing the 27K resistor with a 10K resistor will likely give you a range of -40V to -48V which is kind of cold for EL34's.
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Besides getting your bias voltage sorted, its best not to replace 2 tubes at a time in these amps. better to replace all 4. The chances are that you'll end up with a pair of tubes that run hotter than the other pair, and that will smoke the PT when the amp is cranked with a square wave signal
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last night i tried putting in a 10k and while it does help the bias voltage it also drives up the plate voltage, so its not a real solution.
Why is this happening? If the bias tap is fine and all the components are stock values, where is the problem?
i have replaced the caps. its not them.
i have replaced the 220k's on the PI output. its not them.
I have fiddled with the 47k and 27k resistors. the 47k doesn't help and the 27k drives up the plate voltage.
I just want this thing fixed!!
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it also drives up the plate voltage
yup, OHM says so, IF you make bias cooler - more NEG, then you have less current, hence more B+
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I realise that it is a natural reaction but it’s not how the amp should be operating. Plate voltage should be around 450vdc with the correct bias voltage. Why can’t I get that?
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Quit questioning everyone and just do what they suggest. THEY ARE RIGHT!
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correct bias voltage
I'm a self biased guys, so...
the voltage value, is a "ballpark thing" that effectss current, a real thing is tube current, Ip
hopefully each tube as it's own 1ohm R
set the bias to MAX NEG, measure the 1ohms(value = current), and plate volts
calculate plate dissipation and compare to tube datasheet for "typical" fixed bias Ip
low, bump bias volts a smig, recheck. where the amp lands is it's "normal" operating point.
I typically set customers amps between 60 -70% of max plate dissipation idling, NO signal.
new tubes will change daily as they find their way in the world
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last night i tried putting in a 10k and while it does help the bias voltage it also drives up the plate voltage, so its not a real solution.
Why is this happening? If the bias tap is fine and all the components are stock values, where is the problem?
There is a wide range for the characteristics of those tubes. The same bias voltage, screen voltage and plate voltage will cause different EL34's to idle at different currents. There are sets of EL34's that will idle fine with your stock setup, you just don't happen to have one of those sets.
More current draw from your power transformer means more voltage sag and less current draw means less sag. When you adjust the bias for less tube current, there is less sag and the plate voltage rises. Then you have to recalculate and adjust some more and then again and then again until you are willing to call it close enough.
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Sluckey, what is your problem? Who is questioning anyone? I have done what was said and the problem still exists.
100+ mA bias current is not normal. 511vdc plate voltage is not normal.
With a normal plate voltage of 450vdc the bias circuit is pulling 100+ mA. When I lower the bias current by raising the bias voltage the plate voltage goes up to 511vdc. Neither of these situations is normal. I don’t understand what problem is causing me to not be able to get a normal plate voltage of 450vdc at the same time as a normal bias current of about 38mA. There is clearly some other condition that is causing this. So instead of being narky offer some suggestions. Geez.
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A plate voltage of 511vdc requires a bias current of about 34mA. I can’t get any lower than 38mA. This is with the 10k resistor in place.
With the 27k resistor in place I have a plate voltage of about 450vdc but I can’t get the bias current any lower than about 100mA.
Neither of these situations allows for correct biasing.
So my choice is either a red hot biased tube or a tube with plate voltage that is way too high and will eat tubes constantly. Which of these is the lesser of 2 evils?
Unless there is something else causing this which can be fixed.
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I have a plate voltage of about 450vdc but I can’t get the bias current any lower than about 100mA.
It would help me if you list each tubes current, I'm assuming 100/4 = 25mA per tube * 450vdc = plate dissipation of 11.25W/tube plenty good for a start.
Now if EACH tube is at 100mA and you have new tubes in, I'd say you have a partial short in your OT
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Shooter, thanks for that.
I should have been more clear. I’m measuring 100+mA across each 1ohm resistor that has been installed by someone else.
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So you reckon a possible OT problem?
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Now if EACH tube is at 100mA and you have new tubes in, I'd say you have a partial short in your OT
Ha! I get it. You're making fun of me because of my critique of that D-Lab guy.
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there's a lot possible, If it came to me I'd make sure with no tubes my bias can adjust between -30 to - 60
I'd make sure my PS IS rock solid
I'd surf till I found dcr values for my OT
i'd make sure I had a known good quad of tubes
i'd make sure I had a known good speaker at the proper load for the amp
i'd spend lots of time ohming everything in the PI and PA, re-flowing solder in the same area
i'd verify there is no DC creep coming in from the PI
i'd verify the tube sockets are solid
IF ALL that checks out
tube it check it call customer with my suspicions and concerns, then spent their money :icon_biggrin:
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Ha! I get it. You're making fun of me because of my critique of that D-Lab guy.
:l2:
I'm not that witty, and I smoked too much to be funny :icon_biggrin:
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2deaf,
I can tell you that before I played with the bias series resistor I could get a maximum of -34vdc. Nowhere near -60vdc. That’s why I thought the bias tap might be faulty somehow.
The only bits you mentioned that I haven’t tried is the pi coupling caps, retensioning the sockets and reflowing all solder joints.
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Clearly you need a new OT. And a new PT. And a thicker skin! :l2:
You should be able to find those for under $500. When you get it installed come back and re-read this thread. Maybe listen to some of the stuff you've been told!
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Sluckey, were you born a douche or did you have to work at it?
What part am I not listening to? You have not offered anything of assistance and I have done everything others have said, so why are you being such a dick?
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while Jay's workin on the amp, and I AM hoping you solve it
I'd like to :hijack1:
2deaf, don't let you're head get to big for SG's helmet, that Answer sheet is laid out as well as any test I've takin, and I've takin ALOT, you and Steve could make good $$'s, your works are the standard!
back to regular programing;
get it fixed yet?
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Everything I've posted has been right on target.
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No everything you have posted is just a narky arrogant smart ass load of shit. Not one helpful suggestion has come from you.
Everything that has been suggested by others I have tried.
Maybe it’s you who should re read the thread. It wasn’t me who suggested the ot was gone.
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Jaymz,
I am the resident Dousch and it is a hat I wear proudly. (this is where SG posts another picture of Ritchie Blackmore wearing a witch hat)
PDF64 suggested the following:
"The grid leaks are R24,25 220k. The coupling caps in question are C14,15 22nF"
You mentioned that you have not replaced the coupling caps yet. Change those and the resistors (if you haven't). Definitely do the cheap stuff first, eliminate any question marks before you start looking at the PT. Shooter's SOP should be everyone's SOP.
Jim
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2deaf, don't let you're head get to big for SG's helmet, ...
Not that witty? I was lucky to catch the first one. This one just blew right over my head.
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Thank you Richie2000, that is what I call a helpful response.
Yes to correct myself, I am yet to try the coupling caps and retension the sockets. I will do that tonight when I get home from work.
I just don’t get why Sluckey had to start carrying on like a flog. There is just no need for it. He started out being helpful and then turned like a cut snake.
It was someone else who suggested the PT, I merely asked if the 67v I measured was within spec.
I have some Mallory’s at home so I will replace those crappy LEGO blocks tighten the sockets and see where I’m at then.
I think there is probably miscommunication involved here from all concerned, including me.
I do understand ohms law quite well as I am an electrician of 20 + years, so I get that lower current equals higher voltage and vise versa. What I don’t get is why I have a condition that means I can’t bias correctly. I should have about 450v on the plates with a bias current of about 38mA. Whatever the condition is is causing the bias current to be over 100mA ON EACH TUBE at 450v plate voltage. Efforts to fix the high current by lowering the bias voltage have tipped the scales in the other direction and ended up with a bias current of 38mA but with a plate voltage of 511v.
I’ll try the caps and the sockets and report back.
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You young folk always mess up the expressions. It's douchebag, not just douche. If you call me a douche, I envision an IV bag with a clear liquid in it and I am not offended. I erased the example for a douchebag.
I have experienced Marshalls with 500V's on the plates before, so 511V is not that shocking. The 67V bias winding bugged me enough that I looked up the spec's on that PT and it is 98V @ 50mA. The bias circuit draws less than 1mA. I'm cleaning up my act, so I won't say anything about a bias winding rated for 50mA.
The funny thing is; 98V with a 27K range resistor gives a range almost the same as 67V with a 10K range resistor. So why is there only 67V on his PT?
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A bit like dick and dickhead, I guess. It could be argued that it’s an abbreviation.
None the less, there are too many dicks/ dickheads/ douches/ douchbags around these days. I thought the idea of online forums was to help each other, not to be dick/ dickhead measuring contests.
Anyway, I do appreciate those who are trying to help.
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Did you read the serious portion of my post?
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Ok,
Pi caps changed, resistors changed, sockets tightened, bias circuit running all stock values with new caps... same problem.
I am getting -40v on the top of the cap after the rectifier in the bias circuit. I am getting -32.5v on pin 5 on stand by and -31.5 when running.
One pair of tubes is pulling 103mA per tube and the other pair 119mA per tube. So much for a matching quad.
There is about 458v on each of the plates.
I have not a clue...
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Yes I did read the serious potion 2deaf.
It’s clear to me the bias tap is running lower voltage.
Or did I miss something?
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Don't put tubes into that almost certainly damaging situation! The best case scenario is that dozens of hours of operational life have been wiped out in moments; the worst case is that they might never run right again.
Ensure bias issues are resolved before putting the tubes back in.
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What I don’t get is why I have a condition that means I can’t bias correctly. I should have about 450v on the plates with a bias current of about 38mA. Whatever the condition is is causing the bias current to be over 100mA ON EACH TUBE at 450v plate voltage. Efforts to fix the high current by lowering the bias voltage have tipped the scales in the other direction and ended up with a bias current of 38mA but with a plate voltage of 511v.
You're missing a fundamental. You are focused on 450V and 38mA as being the only correct bias condition. That's just not the case. A Proper bias condition is obtained when you have set the current and plate voltage to a safe plate dissipation for the tube. An EL34 has a max plate dissipation of 25W.This is static/idle conditions. No signal applied . Most people try to set the bias so the tube idles at 70% of max plate dissipation. For an EL34 that is 17.5W. THAT'S THE MAGIC NUMBER. This is a safe bias condition for the tube and it also sounds pretty good for a fixed bias amp.
Your 450V and 38mA gives a plate dissipation of 17.214 watts (68.9%). This is just shy of the magic 70% point. That's great but it's not the only voltage/current that will yield 70%. 511V and 38mA gives 19.4W (78%) which is a little hotter but is still acceptable. Many people like the feel and sound of an amp running a little hot and they are happy with replacing tubes every 9 months rather than every 12 months.
If you tweak the bias pot a bit more you should be able to set the current to 34mA. .034 x 511 = 17.4W (69.5%). This is also a perfectly biased condition. I realize that plate voltage will increase a bit as you adjust the bias to lower the cathode current. It's a juggling act. Everytime you change the current you must recheck the plate voltage and do the math again. You may have to repeat this several times but you will eventually arrive at the magic 70% point.
That 450V and .38mA condition is probably obtainable with a particular set of tubes. But it ain't gonna happen with your current set of tubes. Quit focusing on that and focus on this...
Plate Voltage times cathode current times 70% = good bias point. (should be 17.5W for EL34)
The above info has totally ignored screen current. Some people say the screen current is typically 10% of the cathode current (which is what you are measuring) IOW, if cathode current measures 40 mA, 4mA goes to the screen and only 36mA goes to the plate. So, if you ignore the screens there will be an error in your plate dissipation calculation. The actual plate dissipation will always be slightly less than the calculation. The err is on the safe side and that's OK with me and a lot of others.
I hope this clears up the bias adjustment for you. But by all means, quit running the current up to 100mA/tube. It will only take a few minutes to permanently damage the tubes by running at 100mA and 450V. That's 45W! From a tube that has a max rating of 25W.
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Thank you Sluckey for a detailed, helpful response.
Those high bias current numbers that I quoted are with the bias pot wound completely down. They only go up from there. Everything is so far out of balance.
Should I perhaps take the bias supply from the HT like on a 50w?
I understand what you are saying. But won’t 511v seriously eat up the tubes? 450v is supposed to be near the maximum recommend as it is.
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Maybe the 1 ohm resistors aren’t actually 1 ohm any more?
Hence messing up your current measurements.
Consider replacing them.
What info source are you getting a 450V plate recommendation /limit from? see p4 of https://bms.isjtr.ro/sheets/129/e/EL34.pdf or http://frank.yueksel.org/sheets/163/e/EL34.pdf
Maybe thinking of the screen grids?
Whatever, it's the electrode voltages at heavy / full load that really matter; exceeding voltage limits at idle or low loads isn't an issue, providing that the idle dissipation is reasonable.
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Should I perhaps take the bias supply from the HT like on a 50w?
no
I understand what you are saying. But won’t 511v seriously eat up the tubes? 450v is supposed to be near the maximum recommend as it is.
Voltage don't eat tubes. Current eats tubes. Where did you get that "450V" number? Link please.
Voltage ain't a magic number. Current ain't a magic number. THE PRODUCT OF PLATE VOLTAGE AND CATHODE CURRENT IS THE MAGIC NUMBER!
Look at this. Look closely at VaMax…
http://tdsl.duncanamps.com/show.php?des=EL34
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This chart comes from the JMP era I think but even so http://www.kbapps.com/audio/schematics/tubeamps/marshall/ampchart.php
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I don’t know... To be honest I dont know where I got the 450v vamax from.
I just remember reading somewhere that anything too much above that is not so good for tubes.
So in other words any balance of 70% is fine as long as plate voltage is below 800v?
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BINGO!
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Maybe the 1 ohm resistors aren’t actually 1 ohm any more?
my last thought b4 falling asleep!
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Maybe the 1 ohm resistors aren’t actually 1 ohm any more?
Why? Even with 100mA flowing through them they are only dissipating 10mW. I bet they are fine.
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only dissipating
I wasn't thinking power, was thinking Vdrop.
I didn't go back n look, but seems Jay indicated they might not be original
I only had 10ohmers the last amp I did, knew it was coming, but my heart still skipped a beat seeing 200mV til my brain did the /10 :icon_biggrin:
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Jay indicated they might not be original
That's a safe statement. The original JCM-800 2203 had NO 1Ω resistors.
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If we assume that the resistor values are accurate, the recent indicated dissipations of 10mW doesn’t inform us of their dissipation history. Maybe this amp has been running its power tubes very hot for a long time; that can promote catastrophic failures, and such events can result in a lot of cathode current and hence the potential for significant over dissipation of the cathode resistors.
Best not to assume anything is good in a wonky old amp.
As dedicated low ohm meters are really necessary to accurately measure 1 ohm resistors, it may be best just to replace them.
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> where I got the 450v vamax
The 800V number is mighty bold. However many, even most, EL34 amps run near 480V. I had an Ampeg ran them at 590V, and I have seen other amps in that zone. 511V is not a tube-eater.
EL34 at 511V plate and screen targeting 50mA needs nominal -44V G1 bias for 40mA. This *will* vary +/-20% between different tubes. So you need to be able to dial-down to -53V from the bias supply. With typical 50VAC bias winding (I forget if you measured that) you should have over 65V raw DC available for filtering, no problem hitting -53V at one end of your bias trim.
You mention that voltage at pin 5 reads less than from the bias supply. This is meter loading. It would be good to know your total grid resistance and the make/model of you meter which will lead to some meter specs. If you have 225k of grid resistor and a 1Meg meter, it will read 82% of the true value when the meter is not poking the grid. This is exactly your "-40v on the top of the cap ... -32.5v on pin 5" reading. Many tube-guys favor 10Meg meters which will show less drop (~~2% in this case).
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With typical 50VAC bias winding (I forget if you measured that) you should have over 65V raw DC available for filtering, no problem hitting -53V at one end of your bias trim.
He has 67VAC on his bias winding with a stock Marshall 27K/15K/47K/22K pot bias circuit. He gets -40V at the first capacitor and -32.5V at the second as expected.
The spec. on that transformer is 98VAC @ 50mA. The 67VAC reading is the only thing that is out of the ordinary on this amp.
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The spec. on that transformer is 98VAC @ 50mA. The 67VAC reading is the only thing that is out of the ordinary on this amp.
That could very well be due to a cheap meter.
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That could very well be due to a cheap meter.
It's very unlikely because everything else is right in there for a 67V winding.
I think what needs to be addressed is why it only has 67V. It was wound that way? It has an unseen load on it? It has partially shorted? Is this a progressive phenomenon that is going to cause more problems in the future?
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It has an unseen load on it
I got a small bet placed here
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Thank you all for the input while I was sleeping.
My meter is a fluke 117. While not a top of the range fluke, still good.
67v on the bias tap is definitely on the low side. But if 511v on the plates is no issue then I’ve clearly freaked out over nothing.
When I read that 511v on the plates I had a 10k resistor as the series bias resistor and I was reading 38mA with the bias pot at minimum. In order to get the correct 70% i am going to have to lower it even further, maybe to 5,6k.
I will change the 1 ohm resistors while I’m at it and see where my balance point lies then.
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Shooter, you think there IS or ISNT unseen load? If there is where would it come from?
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If it's me, I NEED to see the bias range from 30 to 60-ish with NO tubes
once I got that, I want a plate V*I that I'm comfortable with. IF, I can't get that, I go lookin down all the rabbit holes
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What I cant get past is that this used to play well. The only life changing issue is a couple of red plating tubes? Now all of a sudden the bias is so far off we need to drastically mod that circuit? I be thinking the same that there is a draw coming from somewhere. Or those coupling caps! :icon_biggrin:
Jim
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Well, I changed the bias series resistor to 5k6 to give me the right bias range, powered up, listened to to HT fuse pop, replaced the fuse with a 2A since I didn’t have any 1A, powered up, measured the bias tap and got basically zero volts.
Clearly the bias tap is dodgy.
Can I up the bias series resistor and take a bias supply from the HT on a bridge rectified amp like with a 50w amp? Or is there more to it?
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While you are messing with bias modifications pull the output tubes and don't put them back in until you have the proper negative voltage range.
Well, I changed the bias series resistor to 5k6 to give me the right bias range, powered up, listened to to HT fuse pop, replaced the fuse with a 2A since I didn’t have any 1A, powered up, measured the bias tap and got basically zero volts.
When you say bias tap, are you referring to the wire from the transformer? If so, remember that is AC volts. Disconnect the transformer bias wire and measure the AC voltage on the dangling end of that wire. This will tell us once and for all if that winding is dodgy. And it will provide a clue about which direction the troubleshooting will take. What AC voltage do you measure?
Can I up the bias series resistor and take a bias supply from the HT on a bridge rectified amp like with a 50w amp? Or is there more to it?
It's a little more complicated than that. It requires a Class X capacitor and a resistor. Look at the simple circuit on the bottom left of page 5 of this pdf...
http://sluckeyamps.com/misc/Amp_Scrapbook.pdf
I don't advise doing this. The bias circuit is probably the most critical circuit in that amp. A lot of expensive parts are relying on the bias supply to be very reliable. That's why the 2203 uses a special separate bias winding (not a tap) to provide a simple very reliable bias circuit.
Report the voltage measurement I asked for and we'll go from there.
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While you are messing with bias modifications pull the output tubes and don't put them back in until you have the proper negative voltage range.
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It's rather frustrating that Jaymz77 keeps getting advised about this and keeps ignoring it. May as well try to fix the amp by dumping it off a 10th floor balcony.
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It's rather frustrating that Jaymz77 keeps getting advised about this and keeps ignoring it.
What is even more frustrating is the constant onslaught of suggested causes that have already been precluded by the data given.
OP: 23K to 200K resistors in parallel with the 47K resistor results in cathode currents of 120mA or more.
#8: Bias cap's have already been changed to Sprague Atoms.
#11: Cathode current was measured with a bias probe and by using 1 Ohm cathode resistors.
#20: -32.5V on all four 5.6K resistors.
#23: Measured 67Vac on the bias winding.
#27: No change with the bias filter cap's replaced.
No change with the 220K resistors changed.
#33: Measured 511Vdc on the plates.
#34: Changed the 27K resistor to 10K and measured 38mA @ 511V.
Measured greater than 99mA @450V with the 27K resistor in place.
#41: Measured -34V maximum with the 27K resistor in place.
#49: Still hasn't replaced the coupling cap's.
#53: No change with coupling cap's changed.
No change with resistors changed.
No change with the sockets re-tensioned.
-40V on the first filter cap after the rectifier with the stock bias circuit.
-32.5V on pin 5 on standby.
-31.5V on pin 5 with the amp running.
One pair of tubes idling at 103mA @ 458V.
Other pair of tubes idling at 119mA @ 458V.
#73: A Fluke 117 was used for the reported measurements.
#77: 1A HT fuse blew when the 27K resistor was changed to 5K6.
Bias tap was measured at basically 0V with the 1A HT fuse replaced with a 2A fuse.
#79: It was suggested that his approach is equivalent to dumping the amp off a 10th floor balcony even though it has already been abundantly demonstrated that he does not respond well to derogatory statements borne out of exasperation.
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Oops! Sorry I missed the “PI caps changed”.
Jim
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pdf200, you are right, i do keep missing that.
I shouldn't need to explain myself but i will anyway.
My Mrs left me 2 weeks ago and i have been like a ship without a rudder ever since. I have not been in a good way. Its only the last few days where i have stopped bursting into tears at the drop of a hat.
So you are all correct, i am all over the place at the moment, but i have good reason, so i would appreciate a bit of understanding.
On top of my home issues i have committed to fix this am for a friend of mine. The amp belonged to my friends cousin who recently died, he just wants the amp to be in working condition so he can sell it.
So, unloaded bias tap voltage is 69.5vAC. Yes, i was measuring DC, rookie error, but as I've explained above, I'm not exactly thinking clearly at the moment.
Perhaps the problem is obvious to some of you, and maybe it would be to me if i was thinking clearly, but its not, and i would appreciate all the help i can get to put this issue behind me.
Thank you.
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So, unloaded bias tap voltage is 69.5vAC.
There should be 100VAC on that bias winding. The proper fix is to replace the power transformer. Then you'll be able to put the original resistors back in the bias circuit and have the proper bias adjustment range.
I'm sure the four output tubes have been damaged due to the 100mA they've had to endure, so a new set of tubes would be proper also.
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Thank you Sluckey,
It seems i may just have a PT sitting here.
About 2.5 years ago i purchased a 1976 JMP50 1987 Marshall that had its original PT replaced. Anyway long story short, the amp had an issue at one point which i can't remember and during the course of repairs i became aware that someone had used a JCM800 style 100w PT in it and converted the 50w amp to run with a bridge rectifier.
Anyway i pulled that transformer out and replaced in with a Marstran PT running the year correct voltages. The amp kills now.
Anyway i still have that PT sitting here. Ill make sure its suitable first and then maybe try installing it.
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Yikes, sorry about your situation; working on amps can be good therapy but if your mind drifts from the task in hand (as may be understandable) it would be best to take a break.
Building yourself a light bulb limitier would be a really good idea before the next power up https://robrobinette.com/5e3_Modifications.htm#Light_Bulb_Current_Limiter
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Wow, yikes is an understatement. One foot in front of the other in life and it seems, this amp as well. Sounds like you have acceptable iron for replacement. Shame to put all this work in and have it go down the road - although at this point you might be ready to help it along! Hang in there.
Jim
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Thank you all.
I can report that by sheer fluke, the PT I had sitting here was the correct one. I have installed it and the amp now works fine. Plate voltage about 447v, biased at 39mA. I’m going to hang onto it for a couple more days just to test it but it seems it’s problem solved.
Thank you all for your help, sorry for my tantrum. Now to sort my life out.
Thanks again.
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Now to sort my life out.
been there, must here probably have
slow n steady, learn n work through the 5 stages
when you get there, take up rock climbing and fast cars :laugh:
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YAY ! :icon_biggrin:
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Shooter,
I’ve always wondered what a mid life crisis would look like for me since I already have 4 guitars, 4 amps and 2 motorcycles!
Maybe I get all sensible in my old age?
Thanks your your help and advice.
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Maybe I get all sensible in my old age?
I was sensible the 1st half, didn't work out so well, but once I found Rockclimbing, Jeeps, rallycars, and down in their luck street alcoholics to help, I was in the zone :icon_biggrin:
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Maybe I get all sensible in my old age?
Nope.
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Nope.
:l2:
spoken like a true jaded 'ol man? :laugh:
Jay, need more PM me
dave
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Thank you very much