harp

I am a beginner at amp mods. I have a JCM 900 that I'm trying to re-tube with EL34's. I changed the screen resistors to 1K and changed the bias resistor. I'm able to bias the thing just fine but the amp is getting hot. The plates don't seem to be glowing red, but the tubes are getting hot enough to make the face plate hot to touch. Does anyone know what might be going on?

Jesstaa

I've seen a guide somewhere that said you had to do quite a bit more than that (A few more resistors, probably to lower the heater voltage by a tad).
I'll see if I can find it, unless someone else around here has it bookmarked or something.

__________________

harp

I read in a forum the only thing I needed to change was the screen resistor and the bias resistor to pull the bias in. I was wondering if maybe I need larger screen resistors, if that would make a difference.

Jesstaa

Have you tied pins 1 and 8, I hear that has to be done, didn't remember until just then.

__________________

harp

They were already tied.

Jesstaa

They were tied in my amp too, guess it must be stocked like that, dunno why people were saying it had to be done.

The only other thing I can add is I tried to put a set of KT77's in my amp (Eurotubes told me they should almost definatly work) and they were really hot, although my Bias kept shifting, if I set it to 35, it'd drift to 40 within a few minutes.

__________________

harp

Thanks for the help man. I'll keep digging and see if I find something else that might affect things.

Jesstaa

No probs man, and once you find out how to do it 100%, tell me, I'd love to get EL34s into my amp, and I got an Electrical engineer/Marshall enthusiast down the road that I'm sure would be happy to do it for me if I told him how.

__________________

harp

I found this one amp guy that says to do the following:

From Liam on Marshall Forum:
http://www.blamepro.com/CGI-BIN/cute...=31&thread=163

In order to run EL-34's in a JCM900 equipped with 5881/6L6's, you'll
need to change a few things.
****If you don't know what you're doing, DON'T DO IT!!!!****

Discharge the caps before starting.

Change R31, R32, R33, and R34 to 1kohm 5-watt resistor

Change R30 to 22kohm 1-watt metal oxide resistor

Change R29 to 15kohm 1/4 watt resistor

Change R28 to 56kohm 1/4 watt resistor

Change R23 to 91kohm 1/4 watt resistor

Change R17 to 100kohm 1/4 watt resistor

Change R18 to 10kohm 1/4 watt resistor

Change R19 to 470ohm 1/4 watt resistor

Change R20 to 100kohm 1/4 watt resistor

Change R26, R27 to 220kohm 1/4 watt resistor

Remove C14 100pf capacitor

Change C9, C10 to 100n 250v capacitor

Fit link across large HT capacitor with red and brown wires on it,
thus increasing it from 50 to 100uf

Remove the 56k resistor in parallel with a 10nf capacitor which is
connected between the 4 ohm tap of the output transformer and the
violet wire, then connect the violet wire directly back to the 4 ohm
tap of output transformer.

Remove the 4k7ohm resistor paralleled with a 22nf capacitor (if
fitted) that sits across R6.

Fit you EL-34's, turn it on and bias the amp for around 35ma per tube.
(No signal condition)

The above mods are for the dual reverb series amps (100w and 50w)
The mods for the 2100 (100w) and 2500 (50w) SL-X amps are almost
exactly the same, except:

Change R28 to a 68K 1/4w resistor (instead of the 56k)
Change R29 to a 27k 1/4w resistor (instead of the 15k)
Leave out the change to R17

stargazer76

Very interesting thread here! I also have seen Liam's post outlining all that must be done to convert a 900 from 5881/6L6's to EL-34,s. Most say that both tubes are interchangable, as long as you can bias them properly. This may require the replacement of one or three resistors. I had to change R28 to 33k to achieve this.
Now, Bob at EuroTubes told me that all that extra stuff is not necessary to conversion. Also, where is the schematic evidence for replacing all of these components? R26-R27 from 150k to 220k? Perhaps. R28? Probably to a lower value. You want the bias dc voltage at pin 5 of your output tubes to be about -30vdc to -45vdc, tubes NOT installed. My 900-4100 is set at -34.8vdc, and the bias current is 36.6mA with a plate voltage (EL-34,s) of 488vdc. I only replaced the three resistors.
But, is there something to Liam's call for the replacement of these many components? Something we may be missing?

__________________
JCM 900-4100
JCM 800-2204
1960A/G12T75
1962RI Stratocaster

Jesstaa

If you got the advice from Bob, it's most likely correct, Bob seems to have quite a bit of experience down, and has handled all my problems quite well.

Eurotubes = win.

__________________

American Viking

yeah, my jaw dropped at the very first post. Why did you go about changing all that stuff right off the get-go? One thing you gotta know about tubes is that if the socket fits, it's interchangeable. 6550/KT88/EL34/5881/KT66 no matter what anyone tells you they will all work in the same amp as long as they can fit side by side. All the sockets are the same. In addition to that, modifications aren't necessarily required to make that switch. It all depends on the grade of tube and the window of adjustment combined with your plate voltage to get the desired bias.

So for starters, do you know how to get the proper readings for the equation on how to get the correct bias for the amp? Plate dissipation of desired tube divided by plate voltage of amp, take that number and multiply it by .7 peak performance. If you didn't create this formula then you aren't getting the proper bias. Also the plate dissipation will change depending on any tubes and resistors changed so it should be read every time the amp is biased.
The plate dissipation for all tubes is different as well...
35% for 6550/KT88
30% for 6L6/5881 (most there are some 25% but rare)
25% for EL34/KT77/E34L/6CA7
i forget the KT66 one, either 20% or 15%

Anyway, the only reason you should have to change any resistors in the amp is if the tube you are putting in is "hotter" or "colder" than the way the amp was set up. Like I said, this mainly depends on the tube first and foremost. If one pair rated at 20 is too cold and the amp won't get the tubes hot enough, then switching them out for a pair rated 10 or under may be able to be biased in the given window of the amp and no changes will be necessary.
You should never just change anything before you know what you've got to work with first. I don't think you should have to change anything at all in that amp. At least not right from the start. If anything would need to be switched it would only be the final bias resistor and nothing else. The tubes you're switching between are not so different that they should need any special work done to the amp.

Let us know more about what's going on so we can give you a hand. Hook us up with some numbers to work with, plate voltage, type of tubes, all that junk. Good luck dude.

__________________
-I'd rather be a good liver than have one...

HAMPAMP TUBE AMP SERVICES

[QUOTE=American Viking;46127]yeah, my jaw dropped at the very first post. Why did you go about changing all that stuff right off the get-go? One thing you gotta know about tubes is that if the socket fits, it's interchangeable. 6550/KT88/EL34/5881/KT66 no matter what anyone tells you they will all work in the same amp as long as they can fit side by side. All the sockets are the same.



Just because a tube fits in the socket does not mean it is interchangeable electrically. A 6V6, 6SN7, 5U4G, 5V3A will fit into the same socket as the 6L6, EL34, 6550, KT-88, etc. But you better not power up your 900 with a set of 6V6's or 5U4G rectifier tubes in it. Any tube that shares design parameters that are close or over-rated can be made to work but the pin-out must match or some socket rewiring would have to be done. Be careful and check specs and pin-outs before converting to any tubes besides what is in there to begin with. True, 6550's, KT-88, EL34's, 5881's, KT-66's and even 6L6's are somewhat interchangeable but, and don't get me wrong, not trying to be an ass, the way you worded that might lead some to believe that "if it fits-it will work" Sorry, I don't mean to be offensive, just want to clarify.

__________________
Pie are not squared, Pie are round

Due to inflation, I can no longer afford to put in my 2 cents worth.


'81 Gibson SG jr
'97 Gibson SG Special
'82 Gibson Explorer
'63 Fender Strat
'00 Fender Strat
'76 Ibanez Doubleneck (Gibson copy)
'08 PRS Paul Allender
Kramer (parts box) V
Peavey Vandenburg (parts box)
Marshall 1959
Marshall VS100R
Marshall 1960/A x 2
Line 6 Flextone III
Peavey Triumph 60
Gibson GA-5
Danelectro Cadet Model 122

American Viking

I would hope nobody would throw rectifier tubes in the output section! Holy moly! it's all good brother

__________________
-I'd rather be a good liver than have one...

harp

Hey Guys,
Thanks for all the helpful input. I'm still trying to figure out what's going on. I'm getting 470 volts plate voltage. Bias voltage at pin 5 is 40 volts, with tubes not installed. I've set the bias current is 15 ma (31.5 mv volts across 1 ohm resistor on between ground and pin 8)

It sounds great but after 15 minutes of playing, or just sitting there turned on, the face panel gets almost too hot to touch.

Still looking for any clues, and I'm still not real sure what too look for.

American Viking

First off I strongly suggest you get a bias probe...
Second off, 30/470=0.063, 0.063 x 0.7=0.044 for 6L6
25/470=0.053, 0.053 x 0.7=0.037 for EL34
Your bias current should be 37mA
I'm not sure you are having the correct formula so here it is
A/B=C CxD=Desired Bias
A=plate dissipation of the tube (25% for EL34, 30% for 6L6, 5881)
B=plate voltage checked on pin # 3
C=Do not exceed limit of bias
D=0.7, multiplying by this gives you the normal starting point for proper bias 70%

As far as checking the bias with the tubes not installed... the reading doesn't really matter because it doesn't fit into the bias equation and on top of that that's really not safe. You can only check the bias when the amp is On, off of standby (play mode) and has to have a load (a cab) hooked up to it. When you pull tubes you are changing the impedance of the amplifier output so you're messing your transformer up by doing so.


What you really should do is return the amp to it's stock values, put back all the stuff you messed with because your amp is running a "cold" bias right now and should be getting hot but not that hot. I'm not sure what's going on but like I said before it was not necessary to change out all that stuff right off the bat. You should do some more research on this because the first place you received your information definitely seemed to lead you in the wrong direction at first.
Keep us updated on everything and we'll try to help you get this thing back to 100%.

__________________
-I'd rather be a good liver than have one...

Ken

Okay...look, for all the hassle of changing parts, just sell the amp! There's no mystery why JCM 900's are not in big demand: they aren't really that good compared to other Marshalls. This isn't just my personal experience too (I've played through a 900..not thrilled); try selling one and see how hard it is to get a decent price! JCM 800's sell easily as do JCM 2000's. This is the market talking. So do the best you can and upgrade!

Stop trying to "fix" a JCM 900. Sell it to some kid who wants a tube Marshall and buy a JCM 800, JVM, DSL/TSL, etc. and you'll save money and have a better amp.

Ken

harp

American Viking,
Thank you for the reply. Let me back up a little. I never said I did all the things Liam suggested. All I did was change the screen resistors to 1K5W and changed the bias resistor to be able to pull it in. I realize it's biased cold right now, that's why I'm so confused. Why is it getting hot? I only wanted someone to tell me if there's something important that I missed (possibly something on Liam's list.) What else makes tubes hot, if they are already biased too cold?

American Viking

It's the tubes that's making it hot, right? Have you checked to make sure nothing else is warm like the transformers or something? It's probably not it but just something simple to check.
Tube amps do get super hot when playing loud, my 800 steams when I play it on 3 or 4 for about a 30 minutes. I just don't notice it unless I have to move it right after I play. If you're just playing at home and are worried about it you can stick a fan behind it and take the back panel off. Some people actually take a hole-saw and cut a hole in the side of the amp cabinet and add a fan to the amp.

__________________
-I'd rather be a good liver than have one...

Jesstaa

Ken, if TS likes his amp, then why should he sell it? Personally I love my JCM900, one of the best Marshalls imo, it has a really nice chunky OD channel, with a nice secondary channel with either great cleans, or a vintage kinda crunch. JCM800s are good, but they lack a second channel for cleans (A second amp is good for cleans, with any Marshall, but it's nice to still have it there) and 800's need a boost for metal, JVM's are unreliable, as they're part of the newer range of Marshalls, which use low quality parts, and same for the 2000 series.

__________________

harp

I like the amp, and will actually feel guilty for selling it to someone in it's current state. It gets crazy hot. I checked the transformers and they don't seem hot at all. It's definitely the tubes. Seems to be both preamp and power tubes getting hot, but it's not red plating.

harp

I just thought of something. Could I be getting AC voltage somewhere it's not supposed to be like across the grids or screen. Would this cause excessive heating? How would I check for that?

woodddj

i agree, i love my 900 also. the 900's are very underated. it is true, the 800's sound great, but i think the 900's sound just as good, but it its own way. not everyone like the 900's or the mg's or the avt's and some might say even the 800's. but what it all boils down to is personal preference. i tryed the dsl and vintage modern and i didnt like them, i tryed the 900 and loved it, so i got it.

__________________
AVT50X and AVT20X MG15MSII

stargazer76

harp dude,
The tubes get hot normally, how do you determine they are "too" hot? Maybe your just fine. As long as your plates are not melting....

__________________
JCM 900-4100
JCM 800-2204
1960A/G12T75
1962RI Stratocaster

harp

I wondered if I was just being paranoid, but the knobs are actually so hot that I can't leave my fingers on them very long. The face plate itself is even hotter.

HAMPAMP TUBE AMP SERVICES

It doesn't make sense that the knobs should be getting hot and the chassis wouldn't heat up from the tubes that much unless the heat was somehow getting "sinked" to the chassis through the tube retainers or some other physical means. Do as American Viking has suggested and remove the back cover and see if that helps reduce the heat. Check all externally mounted parts (transformers, chokes, filter caps, etc). If you truly think it is hotter than you think it should be then ask someone who uses tube amps or take it to a tech, just don't let paranoia cause you to end up spending a lot of money. Get advice from someone who would know. Is there any way you can check the temperature of the chassis and call the nearest Marshall service center? They should be able to tell you if it is excessive.

__________________
Pie are not squared, Pie are round

Due to inflation, I can no longer afford to put in my 2 cents worth.


'81 Gibson SG jr
'97 Gibson SG Special
'82 Gibson Explorer
'63 Fender Strat
'00 Fender Strat
'76 Ibanez Doubleneck (Gibson copy)
'08 PRS Paul Allender
Kramer (parts box) V
Peavey Vandenburg (parts box)
Marshall 1959
Marshall VS100R
Marshall 1960/A x 2
Line 6 Flextone III
Peavey Triumph 60
Gibson GA-5
Danelectro Cadet Model 122

harp

Update,
I just took this amp to a tech. He looked over the work I had done in converting to el34's (changing screen resistors and one bias resistor.) He confirmed everything looked good there. He looked over both boards with a magnifying glass and found nothing amiss. He checked the output transformer and found one side to be putting out almost no current, around 5-6 mA and, and jumping around a lot, while the other side was just fine. He said the OT was shot and this would definitely cause the tube heating problems. Does this make sense? I'm trying to understand how lack of current could cause heating.

BTW thanks for all the very helpful inputs!

dmnall

Harp,
I am not an expert and I had to do some looking into, if the OT is the culprit and is causing the whole chassis to get hot, that is not a good sign and what I have been reading 100 - 120 degrees f is usually where they will be working and anything 130+ degrees f is showing a sign it is about to die... Plus Mercury Magnetics sells replacement OT and PT for $ 250 each, so it is not too bad to replace them....

Cheers,
Charlie

__________________
Charlie

Amps
1994 Marshall JCM900 2100 SL-X 100 watt Head
Genz Benz 4x12 cab *4x 75 watt celestion speakers
Line6 Spider 3 30 watt with 1x 12 celestion and FBV Express Pedal *late night practice amp*

Guitars
1985 Fender Stratocaster '62 reissue Fiesta Red
2006 Gibson Les Paul Studio Alpine White Gold HW
2005 ESP LTD MHB400 Black *Baritone*
1992 Jackson Professional Rhoads Pro Dark M. Blue
2006 Martin D16 GT

Ken

To reiterate my thought...you'll be WAY more into this amp money wise than just buying a used DSL...and even if you fix it it's still just a JCM 900...

Ken

dmnall

Hey Ken,
I agree that replacing OT is going to be pricey because of the cost of purchasing the amp, part is not too bad but the labor is going to drive it up even more,however, even saying that it is still just a JCM 900, is something I do not understand.. I personally think the JCM 900 amps are great, sure not as popular as the 800 and not as versatile compared to the 2000 series, but the 900s are not bad and they are starting to get the recognition they finally deserve.. Go check out the jcm900 mk3 vs sl-x and take a listen to my sl-x tone..

Cheers,
Charlie

__________________
Charlie

Amps
1994 Marshall JCM900 2100 SL-X 100 watt Head
Genz Benz 4x12 cab *4x 75 watt celestion speakers
Line6 Spider 3 30 watt with 1x 12 celestion and FBV Express Pedal *late night practice amp*

Guitars
1985 Fender Stratocaster '62 reissue Fiesta Red
2006 Gibson Les Paul Studio Alpine White Gold HW
2005 ESP LTD MHB400 Black *Baritone*
1992 Jackson Professional Rhoads Pro Dark M. Blue
2006 Martin D16 GT