Getting JMP 2204 bias in range

[nakleh]

Hey friends!

So I'm biasing my 1978 JMP 2204 for the first time myself tonight since purchasing it. Right now there are 2 NOS GE 6550 tubes in there, and 3 new JJ ecc83 preamp tubes inside. I'm using the Weber Bias Rite.

I measured the plate voltage and got 402. the 6550s are rated at 35 watts each. so..

35 / 402 = 0.087 [at 100%]

so 0.087 x .7 = .0609, 60 milliamps.

So I then tried to adjust the trip pot inside the amp, and can only get it to turn far enough to give me a reading of 49.2. I can't get it any higher, and as a result, the amp is running cold, and it's a little on the bright side.

Can anyone offer me some insight as to why this may be happening, and how to correct this?

If I use EL34 tubes dissipation as a guide [25 watts] I get this:

25 / 402 = 0.062 [at 100%]

.062 x .7 = .043 [within range]

I'm not sure if putting El34s inside will change the plate voltage- if not, then I wonder if I can drop El34s in, and bias accordingly? How else can I get the amp within range?

Any help would be appreciated, thanks!

 

[Purgasound]

i was always under the impression that some capacitors or resistors needed to be changed out when making the jump between 6550/EL34 because of the difference in plate dissipation.
i'm not exactly sure what needs to be upgraded but if i find something solid i'll let you know, but i think this is what is causing your problem.




i think the one's that stay together are
EL34, E34L, 6L6, KT77
and if you go to these or vice versa you need to change some stuff
6550, KT88

 

[nakleh]

You're right, normally you can't just up and swap EL34s and 6550s, but I wonder, since I cannot get it in range, if the EL34s would work in place of the 6550s.

The original owner told me it has always had 6550s in it, and from what I believe these amps came factory stocked with 6550s. I emailed Bob at Eurotubes, hopefully he can offer some insight.

 

[nakleh]

I did email Bob at Eurotubes, and this is what he said ..

"You either need a hotter grade of tubes or you need to change the final bias resistor that is in series with the trim pot to one with a lower value which will move your window of bias adjustment up.

We see this with a lot of old Marshall’s. My guess is that a hot pair of KT88’s would allow you to get the bias where it needs to be. Using EL34’s, E34L’s or KT77’s would make matters worse! These draw about one third the natural plate current that 6550’s do so the bias would be dirt cold.

R26 in a stock 2204 is the final bias resistor and is 56K. Dropping this down to about a 40K will do it."

I actually ordered new tubes from them today, and he's going to send me a "hot" pair. Hopefully this will help.

 

[Purgasound]

awesome. yeah that dude really knows his stuff. i'm still trying to wrap my head around what exactly he means by "hot pair" but i'm sure it's going to sound great.
and yeah and marshalls from the mid seventies to early nineties sent to the usa came stock with 6550's. the el34's kept breaking wearing down during shipping giving marshall co. a hefty warranty bill back in the day. people were always modding the amps back to el34's though cause that's what everyone was playing through on the early rock records.
kinda cool though, cause you know that little change frustrated rockers for a few years but gave the metal guys some new inspiration.

glad you're getting your stuff worked out proper. that thing will sound sweet again in no time.

 

[nakleh]

So a quick update.. yesterday my new set of KT88's came from Eurotubes. Bob is a GREAT guy and answered all of my questions. He sent me a "hot" pair [rated 50] and told me this should get my bias within range..

Well long story short, I put the tubes in, warmed them up, and took a reading.. with the new tubes, I was *juuuuuuust* able to get my bias to about 60, which is 70% [plate voltage of 402.. 35/402 = .087 x .7 = 60mA ] Before I could not get the bias above 49! So I actually backed off of 70% a little bit, down to 67%.. hotter than before, but not too hot to blow the tubes prematurely.

I didn't actually get to play the amp after putting the tubes in [midnight in a Brooklyn apartment, enough said] but this weekend I'm going to put her through it's paces and see how much better I like these tubes.

Hopefully this will solve some of my brightness issue and add a little bit of gain on top as well. Will report back.

 

[Sir Punk]

I am bringing this up. I have a 79 2204. my questions are:

1. is it always the case that 6550 draw 35w? I have old RCA 6550.
2. is it bad, or not ideal, and why does it happen that the two tubes are off by 4-5 milliAmp? one runs hotter. what is an acceptable difference.
3. my plate voltage is 410~, I need to get up to 59, but the most I get is 52 from the hottest of the two, but the hum gets really loud, I couldn't imagine going up to 59, I don't wanna burn anything.

 

[TiPiMods]

I am bringing this up. I have a 79 2204. my questions are:

1. is it always the case that 6550 draw 35w? I have old RCA 6550.
2. is it bad, or not ideal, and why does it happen that the two tubes are off by 4-5 milliAmp? one runs hotter. what is an acceptable difference.
3. my plate voltage is 410~, I need to get up to 59, but the most I get is 52 from the hottest of the two, but the hum gets really loud, I couldn't imagine going up to 59, I don't wanna burn anything.

35 w is the maximum the 6550 can dissipate. If you bias your Amp that high your tubes will wear out quite fast and the sound does not get better. you should bias between 50-70% of the maximum dissipation.
4-5mA difference is acceptable, some people even say a little mismatch sounds better, because some harmonics come out, which otherways would be canceled.
If the hum gets really loud, the plate is red glowing, which destroys the tube after a few minutes. so be careful and read some infos about biasing before you do biasing. Also you can destroy your main trannie, it's not made for delivering that much current for a long time.
If you measure the cathode current you should be aware that you measure plate+screen grid current which is a little more than plate current alone. But with 60-70% of max. dissipation everything should be ok.
Also be aware: every time you change the bias the plate voltage changes, so you always have to measure again.
good luck and be careful...

some interesting links:
Weber Bias Calculator
Bias FAQ
Tales From The Tone Lounge; Marshall Myths
http://marshall.redpt.com/clay/marshall.html
viva Analog Marshall Amplifiers Page

 

[TiPiMods]

Also IMHO: if you want to make your Marshall sound "british" eg. like a Marshall should, change to EL 34.
It's a huge difference: they breakup earlier and have a lot more midrange and overall distortion. KT 88 are very strong, loud and clean. The 6550 is a cheaper KT 88 which does not take that much voltage and current, but is also tight and clean till your ears bleed.
If you like it a little cleaner, try KT 66. They where Ken Bran and Jim Marshalls choice for the very first Marshall Amps. They sound "glassier" or more "fender" than the EL 34, but also very nice at breakup.

 

[Sir Punk]

Follow, thanks for the info. There's a bit of contradictory infos around. The 6550s have has max plate dissipation 35w (although I found in some spec sheets 40w, maybe there are different variations)

Lord Valve suggests to use 27w as max dissipation, without much explanation why.

I also noticed that with the socket probes not in the amp, the plate voltage is about 10v lower. it doesn't make a big difference in the dissipation value, but still I can't explain why.

I also don't think that there is a huge sonic difference between cold and hot. I was biasing just by ear, and yes as I would make the tubes run hotter I was getting more creamy sound, fuller, but also the loudness was going up even though I didn't not change the master volume. So I tried to compare the same bias setting at the same volume, and there was NOT a huge difference. It's really subtle and takes a lot of listening. right now I am biased around 60% or so. my tubes are also off by 7 microA.

the guy at eurotubes tells to use 35w for the 6550 and to bias at 70%. so that's more clashing infos from these "gurus". I will call lord valve though.

 

[Sir Punk]

Also IMHO: if you want to make your Marshall sound "british" eg. like a Marshall should, change to EL 34.
It's a huge difference: they breakup earlier and have a lot more midrange and overall distortion. KT 88 are very strong, loud and clean. The 6550 is a cheaper KT 88 which does not take that much voltage and current, but is also tight and clean till your ears bleed.
If you like it a little cleaner, try KT 66. They where Ken Bran and Jim Marshalls choice for the very first Marshall Amps. They sound "glassier" or more "fender" than the EL 34, but also very nice at breakup.

I definitely wanna try el34 at some point.Although I don't understand when people say that 6550 are cleaner. the only way to get some clean tones is to use the low input channel, or on the high input ch turn the volume to 5-6 and roll off the guitar.

i know 6550 = 80s american rock and el34 british 70's rock, can anybody mention some bands?

 

[TiPiMods]

Follow, thanks for the info. There's a bit of contradictory infos around. The 6550s have has max plate dissipation 35w (although I found in some spec sheets 40w, maybe there are different variations)

Lord Valve suggests to use 27w as max dissipation, without much explanation why.

I also noticed that with the socket probes not in the amp, the plate voltage is about 10v lower. it doesn't make a big difference in the dissipation value, but still I can't explain why.

I also don't think that there is a huge sonic difference between cold and hot. I was biasing just by ear, and yes as I would make the tubes run hotter I was getting more creamy sound, fuller, but also the loudness was going up even though I didn't not change the master volume. So I tried to compare the same bias setting at the same volume, and there was NOT a huge difference. It's really subtle and takes a lot of listening. right now I am biased around 60% or so. my tubes are also off by 7 microA.

the guy at eurotubes tells to use 35w for the 6550 and to bias at 70%. so that's more clashing infos from these "gurus". I will call lord valve though.

Hi Punk
as you noticed: when you run the tubes to cold, you loose power. If you really want to go for optimum power you need an oscilloscope and a frequency generator and so on...

the socket probes often have a 1 Ohm resistance between ground and cathode because its easier to mesasure voltage instead of current ( the voltage drop across a 1 Ohm resistance is equal the current flowing through it) That's why the plate voltage is a little higher with the probe.

I would not go over 80% because some amps dont like the higher power consumption and you dont want to deal with your powertrannie. My experience is, the sound gets better to a certain point as you bias hotter, but after that point I dont hear much difference. And if an amp runs too hot, the tubes wear out a lot faster, so you will be happy only for a short time. If it sounds good at 60% it's great, your tubes will last longer.

 

[TiPiMods]

I definitely wanna try el34 at some point.Although I don't understand when people say that 6550 are cleaner. the only way to get some clean tones is to use the low input channel, or on the high input ch turn the volume to 5-6 and roll off the guitar.

i know 6550 = 80s american rock and el34 british 70's rock, can anybody mention some bands?

what you hear is preamp distortion, concerning power amp distortion, the EL 34 breaks up much earlier. But it's very loud anyway

 

[dread1]

For the 6550's I used 25w MPD. If you use 35 the formula comes out to like 70 Ma which is impossible to bias at.

 

[TiPiMods]

For the 6550's I used 25w MPD. If you use 35 the formula comes out to like 70 Ma which is impossible to bias at.

35W is MAXIMUM! You should never bias at maximum power dissipation, exept you want to extra sponsor the russian or chinese tube industrie and you have some extra money for a new amp or a new power tranny! 70% of the maximum, which is 24,5 W should be the highest!

 

[mlannoo]

Hey friends!

So I'm biasing my 1978 JMP 2204 for the first time myself tonight since purchasing it. Right now there are 2 NOS GE 6550 tubes in there, and 3 new JJ ecc83 preamp tubes inside. I'm using the Weber Bias Rite.

I measured the plate voltage and got 402. the 6550s are rated at 35 watts each. so..

35 / 402 = 0.087 [at 100%]
so 0.087 x .7 = .0609, 60 milliamps.

So I then tried to adjust the trip pot inside the amp, and can only get it to turn far enough to give me a reading of 49.2. I can't get it any higher, and as a result, the amp is running cold, and it's a little on the bright side.

Can anyone offer me some insight as to why this may be happening, and how to correct this? ...........

Any help would be appreciated, thanks!

Hello! I stumbled across this thread last night and rather than start a new thread on the topic I decided to just get this one going again ...

I have the same problem - bought a used 1980 JMP 50 2204 with 6550 power tubes. Picked up a fresh set of Mullard preamp tubes and a pair of matched Genelex Gold Lion KT88's. Having trouble understanding how to bias with these new tubes.

Using my Weber Bias Rite I got these readings with the old Chinese 6550 tubes that came with the amp.
A - 41mA @402v
B - 27.5mA @ 389v

And with the new Genelex KT88's:
A - 29.5mA @ 400v
B - 30.6mA @ 400v

My actual question is (I guess) ... What number am I shooting for when I increase the bias trim pot? Using the Weber Bias Calculator I concluded this:
Weber Bias Calculator

Class A/B - KT88 tube - 400v plate = 61.2mA bias
(Bias Settings For Safe Plate Dissipation - Class AB 70%, Class A 90%)

I can't come near 60mA with the bias trim pot fully cranked! - So I'm a bit confused.

I've read conflicting info on the KT88 tubes. Is it 35 or 40 watt max? I would need to know this before I can generate an accurate number using the Weber Bias Calculator.

The other problem I found was that KT88's are not listed on the "Jim Jones Bias Tables". The 6550's are 35watt so I'm assuming KT88's are as well?

Appreciate any insight. Thanks.
mlannoo

 

[Marshallmadness]

I have a 4010 combo with 6550's and I am only running about 48ma. It actually sounds really, really good so I just left it. That's as high as the bias trim pot would allow but I also have slightly lower wall voltage here at the house too. Discovered that by accident working on the same amp at another location. I have a 2204 with el34's and I only run 60% on it. I might actually even go lower because it's almost too warm and thick.

 

[RussBert]

Hello! I stumbled across this thread last night and rather than start a new thread on the topic I decided to just get this one going again ...

I have the same problem - bought a used 1980 JMP 50 2204 with 6550 power tubes. Picked up a fresh set of Mullard preamp tubes and a pair of matched Genelex Gold Lion KT88's. Having trouble understanding how to bias with these new tubes.

Using my Weber Bias Rite I got these readings with the old Chinese 6550 tubes that came with the amp.
A - 41mA @402v
B - 27.5mA @ 389v

And with the new Genelex KT88's:
A - 29.5mA @ 400v
B - 30.6mA @ 400v

My actual question is (I guess) ... What number am I shooting for when I increase the bias trim pot? Using the Weber Bias Calculator I concluded this:
Weber Bias Calculator

Class A/B - KT88 tube - 400v plate = 61.2mA bias
(Bias Settings For Safe Plate Dissipation - Class AB 70%, Class A 90%)

I can't come near 60mA with the bias trim pot fully cranked! - So I'm a bit confused.

I've read conflicting info on the KT88 tubes. Is it 35 or 40 watt max? I would need to know this before I can generate an accurate number using the Weber Bias Calculator.

The other problem I found was that KT88's are not listed on the "Jim Jones Bias Tables". The 6550's are 35watt so I'm assuming KT88's are as well?

Appreciate any insight. Thanks.
mlannoo

Old thread is old!


There are many threads regarding this issue. I'll try to share my summary of them.

Your amp was designed around EL-34 tubes. Marshall's distributor was having a lot of warranty issues with the EL-34's available at that time, so It was decided by the bean counters that 6550's would give longer tube life and cut down on warranty repairs...

So, All the amps sent to the USA were equipped with 6550's. Why only the USA? I dunno

Now the transformers were still optimized for EL-34's. The 6550's were biased cold and held up just fine. Now the amp's trannies and circuit were never designed around getting the most out of 6550's and trying to do so taxes the power transformer. There's just not enough current to drive the 6550's at 70% dissipation.

So many tech it seems still treat the 6550 as if it were a 25 watt tube, and bias accordingly.

I'm sure folks with far more experience and insight will chime in here...

 

[mickeydg5]

Hi Mlannoo

Russbert is correct. The power stage in those amplifiers were designed around the EL34 tube. There were problems with transport and reliability of EL34s in the amplifiers for a certain period of time so the US requested the more robust and reliable 6550 as a substitute. So, those amplifiers are not setup to make full use of 6550/KT88 tubes and should be biased more like an EL34 of 25W rating.

 

[mlannoo]

Hi Mlannoo

Russbert is correct. The power stage in those amplifiers were designed around the EL34 tube. There were problems with transport and reliability of EL34s in the amplifiers for a certain period of time so the US requested the more robust and reliable 6550 as a substitute. So, those amplifiers are not setup to make full use of 6550/KT88 tubes and should be biased more like an EL34 of 25W rating.

Any considerations when switching to EL34's from the 6550/KT88? Can I just pop them in and bias accordingly? thanks.