JCM800 2204 KT88 question

[NikkiNova]

Hi all, first post. Nice to be here.

Yet another question on power tubes in a U.S. 2204. I have a 1981 2204, it's in great shape. I had a well-respected amp builder change it out from 6550's to EL-34s a few years back. I know he did the piggy-back resistor thing, but I'm not sure if he bothered doing the entire schematic change, involving other components on the board. I can bias my amps, and have a Mullard CV4004 I throw in the V1.

But...I don't like the sound. There's a major loss of volume, and it just sounds muddy. I remember the tech was hurting from back problems when I picked up my amp, and I was too dense to ask for the GE 6550's back. (many regrets there). Maybe he did a less than stellar job due to his injury.

While I started this thread thinking that maybe I should try KT-88's, I remembered that I went to look at another 2204 that had EL-34's, and it sounded like exactly what I wanted in mine! Even though that one was a beater, it sounded better than mine with EL-34's. So, let's forget about going with KT-88's. Is there something that I need to do to mine to get it to sound right with EL-34's? I did the typical Marshall bias with my Allesandro bias meter and my Rat Shack digital multimeter.

Thanks!

 

[MajorNut1967]

Hi all, first post. Nice to be here.

Yet another question on power tubes in a U.S. 2204. I have a 1981 2204, it's in great shape. I had a well-respected amp builder change it out from 6550's to EL-34s a few years back. I know he did the piggy-back resistor thing, but I'm not sure if he bothered doing the entire schematic change, involving other components on the board. I can bias my amps, and have a Mullard CV4004 I throw in the V1.

But...I don't like the sound. There's a major loss of volume, and it just sounds muddy. I remember the tech was hurting from back problems when I picked up my amp, and I was too dense to ask for the GE 6550's back. (many regrets there). Maybe he did a less than stellar job due to his injury.

Now I'm wondering if I should try KT88's, and if any more mods are needed to do that? Or, should I keep trying to make the EL-34's sound better?

Thanks!

Well there really no need to change any components on the board just to reset the bias range for different tubes, unless you are going to permanently use one type of tube. All the bias circuit does is provide a negative voltage to the Grid. While it is an important part of the amp, it’s not a complicated circuit.

You might want to get rid of that CV4004 they are not conducive to good guitar tone. They are great for phonograph preamp. I’m going to tell you like all the other guys get told “Changing the power tubes in your type of amp is not going to bring you the Holy Grail sound. First off the power transformer doesn’t have enough output to support the KT88 properly and the output transformer has the wrong impedance for the KT88.

Why don’t you just have the amp checked out by a reputable amp tech and if its ok, just do a re-tube and a bias and see how it sounds.

 

[MartyStrat54]

Well there really no need to change any components on the board just to reset the bias range for different tubes, unless you are going to permanently use one type of tube. All the bias circuit does is provide a negative voltage to the Grid. While it is an important part of the amp, it’s not a complicated circuit.

You might want to get rid of that CV4004 they are not conducive to good guitar tone. They are great for phonograph preamp. I’m going to tell you like all the other guys get told “Changing the power tubes in your type of amp is not going to bring you the Holy Grail sound. First off the power transformer doesn’t have enough output to support the KT88 properly and the output transformer has the wrong impedance for the KT88.

Why don’t you just have the amp checked out by a reputable amp tech and if its ok, just do a re-tube and a bias and see how it sounds.

This is correct. A KT88 has been used in EL34 amps, but the KT88 is not working like it should. The EL34 is a 25 watt dissipation tube and the KT88 is a 35 watt dissipation tube. An amp designed for four KT88's is going to have a much stouter power transformer and the proper output transformer. The wiring to the tubes is usually heavier to accommodate the KT88's current draw.

How long ago did this "mod" take place? I ask, because GE quit making 6550's a long time ago.

What sort of EL34's was the beater amp running? Find out and then find out from the owner who his tech is. It sounds like that is the guy you want to go and see.

Also, in an old 800, V1 is your most important tube. Just as American tube manufacturers made "military grade" tubes, so did Mullard. I have found that the stiffer construction of military grade tubes translates into less tonal quality. I would recommend a solid, regular Mullard, Amperex or a Telefunken in your V1. Most 800 owners usually go with a slightly hotter gain tube in V1 and this is what I recommend.

 

[Purgasound]

He's talking about an '81 2204, all those in the U.S. came stock with 6550's which is the same plate dissipation as a KT88, why would the OT need to be different if that's how it came stock?

I run 88's in my 2203 and I love 'em.

I think some new preamp tubes would do you right.

 

[NikkiNova]

Thanks, Im going to do that and see what happens. I have a few vintage 12AX7's to try in there.

I'd like to stick with the EL-34's too. That beater was a long time ago, so I dont' know what it had, the guy was not that much of a gear head, so I dont' think they were anything exceptional.

My 2204 is a 1981, the first year of the JCM800 version. Those GE 6550s were likely the ones that it came with. Damn, I wish I had them.

 

[Purgasound]

It would be sweet to have some GE tubes, but they really wouldn't be worth anything if they were worn out and played to death. Power tube changes can really affect tone but I believe the majority of what you hear is coming through the preamp section and that has the most control over overall tone. Are those vintage 12AX7's worn out or have they never been used?

 

[MajorNut1967]

He's talking about an '81 2204, all those in the U.S. came stock with 6550's which is the same plate dissipation as a KT88, why would the OT need to be different if that's how it came stock?

I run 88's in my 2203 and I love 'em.

I think some new preamp tubes would do you right.

It is fine if you want to do it Viking, its just that the KT88 efficiency is compromised.

The marshalls during this time period were shipped without power tubes to the US and the 6550's were installed in New York, supposedly because of the shortage of EL34's in the US. But the rest of the world was getting amps shipped with EL34's, now do you think Jim Marshall altered the JCM 800 line just for the US? Any way so the JCM 800 was never designed around the 6550 they just willy nilly threw in the 6550 and said here to the US public. Marty can vouch for this, but we would get a lot of complaints about "these sound like shit, whats wrong with my amp? It don't sound like a real Marshall" Right out of the BOX! OT mis-match and not re-biased correctly.

So while the 6550 & the KT88 are similar they are not the same monkey! And that also the era of the late 70's to the early 80's was the time of the power tube switch but that EL34 belonged in these amps originally was just a bad thought.

 

[Jonathan Wilder]

He's talking about an '81 2204, all those in the U.S. came stock with 6550's which is the same plate dissipation as a KT88, why would the OT need to be different if that's how it came stock?

AV, as Major (pain in the ass... ) mentioned, every component was the exact same for every amp. They all came with OTs with a primary impedance suitable for EL34 valves. The US was either having a problem with EL34s coming in broken (this was the story I heard) OR like the Major stated there very well could've been a shortage of EL34s in the US. So the US distributor started just throwing in 6550s as a result.

However, an EL34 OT doesn't provide the proper plate load impedance for the 6550 to run it at its optimal operating characteristics. What he means is that without the PROPER PT and OT for 6550s, they aren't performing where they COULD be electrically and are not being used to their fullest potential, kinda like running DDR266 memory on a CPU with a 400MHz FSB.

Now, tonally, does that matter? Who knows? You'd have to properly design the power supply and output section for 6550s to find out. Tonally, the way they run in a stock Marshall may be the holy grail to your ears, whereas someone else might think that the "optimized for 6550s" version would be the holy grail.

 

[MartyStrat54]

He's talking about an '81 2204, all those in the U.S. came stock with 6550's which is the same plate dissipation as a KT88, why would the OT need to be different if that's how it came stock?

I run 88's in my 2203 and I love 'em.

I think some new preamp tubes would do you right.

As the Major stated, the British version was set up for EL34's and the power and output transformer were designed around this. Due to tube breakage during shipping and/or supply of EL34's, American versions of this Marshall amp used 6550's. I'm sure you have read on Dr. Tube where the American consumer was trying to figure out why their Marshall sounded like crap and didn't have the tone of the British counterpart. The 6550 wasn't designed for the OPT. The OPT was dialed into the values of an EL34. Also, many of the American amps were biased "cold" with the 6550's. All of this added up to a really nasty sounding amp. The good thing is that Marshall responded and cleared the matter up by getting enough EL34's for their entire production output.

If you are following Jon Wilder on the forum, he is revamping a Marshall amp and is installing an OPT specifically designed to run a pair of KT66's. When you have an amp that can run a certain amount of tubes, you are stuck with the OE OPT. It may or may not match up to certain tubes. That's why someone will say, I tried KT77's but the high end sounded like crap, or it was fizzy sounding, or it won't quite bias right, even though a KT77 is "supposed" to be a direct replacement for an EL34. You can take a lot of different tubes and make a Marshall work with them, but it is actually designed around one specific tube, usually an EL34.

 

[Jonathan Wilder]

As the Major stated, the British version was set up for EL34's and the power and output transformer were designed around this. Due to tube breakage during shipping and/or supply of EL34's, American versions of this Marshall amp used 6550's. I'm sure you have read on Dr. Tube where the American consumer was trying to figure out why their Marshall sounded like crap and didn't have the tone of the British counterpart. The 6550 wasn't designed for the OPT. The OPT was dialed into the values of an EL34. Also, many of the American amps were biased "cold" with the 6550's. All of this added up to a really nasty sounding amp. The good thing is that Marshall responded and cleared the matter up by getting enough EL34's for their entire production output.

If you are following Jon Wilder on the forum, he his revamping a Marshall amp and is installing an OPT specifically designed to run a pair of KT66's. When you have an amp that can run a certain amount of tubes, you are stuck with the OE OPT. It may or may not match up to certain tubes. That's why someone will say, I tried KT77's but the high end sounded like crap, or it was fizzy sounding, or it won't quite bias right, even though a KT77 is "supposed" to be a direct replacement for an EL34. You can take a lot of different tubes and make a Marshall work with them, but it is actually designed around one specific tube, usually an EL34.

Correct...OTs are designed around a certain valve. This all has to do with the load that the valve sees due to the reflected impedance of the speaker, which is why impedance matching is so important. If this load is too low, valves will go into overcurrent and you fry your OT. If it's too high, electrons will see the screen as the easier path and cause the screen grids to go overcurrent.

The required plate load impedance is determined by the plate voltage, the effective plate resistance of the valve itself, and the class of operation. In a valve data sheet, you will find the "Plate Load Resistance" spec (sometimes abbreviated Rl (lower case L)), and it will usually show the spec from the plate of one valve to the plate of the other (plate to plate load) rather than from plate to center tap, which would be 1/2 this value.

When they make an OT, they're spec'ed with a "turns ratio". This is the ratio of turns on the Primary as compared to the number of coil turns on the Secondary. The turns ratio is equal to the voltage ratio (voltage across the Primary as compared to voltage across the Secondary). If you have a turns ratio of 3:1 (3 turns on the Primary for every 1 turn on the Secondary), the voltage across the primary will be 3x higher than the voltage across the secondary.

The impedance ratio is equal to the turns ratio squared.

Thus, different OTs will exhibit a different turns/impedance ratio for different reflected primary impedance values to accommodate different output valves. This is what manufacturers refer to by stating that a certain OT is made for EL34s while another is made for 6L6s, etc etc.

In the case of the Mercury Magnetics RS DeLuxe clone that I'm doing on the Bluesbreaker upgrade, it has 3 different primary taps for a plate-plate load of 6.6K, 8K and 9K to accomodate different valve types. According to the MM spec, you can pretty much run every valve type used in the most common guitar amps - KT66, 6L6, 5881, EL34, KT77, EL84, 6V6, 6550, KT88 and KT90.

 

[MajorNut1967]

Man you guys we are stomping out all the Marshall myths! pretty soon there will be no mystique and they'll just have a fun amplifier!

 

[Purgasound]

AV, as Major (pain in the ass... ) mentioned, every component was the exact same for every amp.

The marshalls during this time period were shipped without power tubes to the US and the 6550's were installed in New York, supposedly because of the shortage of EL34's in the US. But the rest of the world was getting amps shipped with EL34's, now do you think Jim Marshall altered the JCM 800 line just for the US?

Actually Marshall did make amplifiers different specifically for the U.S., just like they made different ones for Canada.

As shown here (lower left corner) in this JCM800 schematic, under the 56k final bias resistor it says 47k for U.S.A. in parentheses. This is also still true for JMP's through the 70's as well. I want to say the earliest I saw was a '74 with the 6550's but I'm not sure when they started exactly.
http://www.drtube.com/schematics/marshall/jcm800pw.gif

Just like Canada's Marshall's were packed full of internal fuses and had different taps on the OT, made specially for the Great White North.

As for the reason for the 6550's, the U.S. distributor, Unicord, was tired of having the EL34 stocked amps not make it through their warranty period due to tube problems which they associated them with shipping and getting knocked around. I'm pretty sure they shipped with the 6550's straight from the Marshall factory, but that's really inconsequential either way.


On a side note, the U.S. and the U.K. were the only places in the country to get legit Marshalls all around. In other parts of the world you could still get back stocked 2204's and 2203's with JMP faceplates and the old style headboxes in the mid to late 1980's. Marshall still had these materials left over. I know there are a bunch of those in Canada, and a gentleman from Scandinavia showed me a JMP 2204 from 1984! As we all know, once the Rose/Morris distribution ended in 1980, so did the JMP and the birth of the JCM800. It is possible, but rare to find a 1981 JMP in the U.S. and the U.K.

 

[Purgasound]

Another question about the OT's...

If they're designed around a specific valve, what does this mean for using KT88/6550 in an EL34/6CA7 amplifier? Or even 6L6 in an EL34? What does this mean for the impedance? Especially when some amplifiers come stock with the option to be EL34 or 6L6 via the little switch that changes one of the resistors in the bias circuit.

 

[MartyStrat54]

Wow, I didn't think that the EL84 was hanging in with that crowd spec wise, but I went to NJ7P and on an AB EL84 pair it's 8K.

 

[Purgasound]

Wow, I didn't think that the EL84 was hanging in with that crowd spec wise, but I went to NJ7P and on an AB EL84 pair it's 8K.

I guess that's why those little THD Yellow Jacket sockets work... Have you seen those?

 

[MartyStrat54]

Yeah, my buddy Josh used them, but he had problems with them and decided to stop using them. There was a power resistor encapsulated in the plastic base and it would get so hot it would make a large bulge on the side of the base. Apparently, they are not supposed to do this.

 

[MajorNut1967]

Actually Marshall did make amplifiers different specifically for the U.S., just like they made different ones for Canada.

As shown here (lower left corner) in this JCM800 schematic, under the 56k final bias resistor it says 47k for U.S.A. in parentheses. This is also still true for JMP's through the 70's as well. I want to say the earliest I saw was a '74 with the 6550's but I'm not sure when they started exactly.
http://www.drtube.com/schematics/marshall/jcm800pw.gif

Just like Canada's Marshall's were packed full of internal fuses and had different taps on the OT, made specially for the Great White North.

As for the reason for the 6550's, the U.S. distributor, Unicord, was tired of having the EL34 stocked amps not make it through their warranty period due to tube problems which they associated them with shipping and getting knocked around. I'm pretty sure they shipped with the 6550's straight from the Marshall factory, but that's really inconsequential either way.


On a side note, the U.S. and the U.K. were the only places in the country to get legit Marshalls all around. In other parts of the world you could still get back stocked 2204's and 2203's with JMP faceplates and the old style headboxes in the mid to late 1980's. Marshall still had these materials left over. I know there are a bunch of those in Canada, and a gentleman from Scandinavia showed me a JMP 2204 from 1984! As we all know, once the Rose/Morris distribution ended in 1980, so did the JMP and the birth of the JCM800. It is possible, but rare to find a 1981 JMP in the U.S. and the U.K.

Hmm I did not know all of this. I was aware that Marshalls had to conform to each Countries safety & electrical standards which explains some of the extra and internal fuses, for instance the amps made for Swedish sales were required to have a filament fuse.

And so the Canadian Marshalls had different taps on their Output Transformers, for what purpose? And what was the part # for the Canadian specific Output Transformers?

I find your point here interesting and I quote, ” As for the reason for the 6550's, the U.S. distributor, Unicord, was tired of having the EL34 stocked amps not make it through their warranty period due to tube problems which they associated them with shipping and getting knocked around. I'm pretty sure they shipped with the 6550's straight from the Marshall factory, but that's really inconsequential either way.”? Are you saying that Unicord was aware that during shipping from the UK to the US that the tubes could or would get damaged or that they were getting damaged? Yet you say that Marshall probably shipped the US amps from the UK Factory with 6550’s? Well if Unicord was aware that tubes get damaged during shipping, then why would they double the chance of shipping damage by shipping the 6550’s (because as I understand it 6550 were US made?) from the US to the UK to be installed and then shipped back to the US a second time? Or are you saying that the 6550’s were just so much more durable they could take the double shipping? Or were the 6550's made in the UK?

So the US and the UK were the only two countries to get legit Marshalls. What is a legit Marshall?

 

[Purgasound]

Hmm I did not know all of this. I was aware that Marshalls had to conform to each Countries safety & electrical standards which explains some of the extra and internal fuses, for instance the amps made for Swedish sales were required to have a filament fuse. And so the Canadian Marshalls had different taps on their Output Transformers, for what purpose? And what was the part # for the Canadian specific Output Transformers?
I find your point here interesting and I quote, ” As for the reason for the 6550's, the U.S. distributor, Unicord, was tired of having the EL34 stocked amps not make it through their warranty period due to tube problems which they associated them with shipping and getting knocked around. I'm pretty sure they shipped with the 6550's straight from the Marshall factory, but that's really inconsequential either way.”? Are you saying that Unicord was aware that during shipping from the UK to the US that the tubes could or would get damaged or that they were getting damaged? Yet you say that Marshall probably shipped the US amps from the UK Factory with 6550’s? Well if Unicord was aware that tubes get damaged during shipping, then why would they double the chance of shipping damage by shipping the 6550’s (because as I understand it 6550 were US made?) from the US to the UK to be installed and then shipped back to the US a second time? Or are you saying that the 6550’s were just so much more durable they could take the double shipping?
So the US and the UK were the only two countries to get legit Marshalls. What is a legit Marshall?

lol, you ass! ahha

I don't think the Canadians had different OT's, but they didn't wire the 16ohm taps in, just the 8 and 4. I don't know why but that's just how it went.

Of course I know difference in one resistor doesn't really make it a completely different amp, I was just sayin...

What I meant by the legit thing was just that those two countries kept up with the normal trend, as in neither one of those have any of the eighties mystery JMP's and so forth that doesn't correspond with the typical year by year changes in the Marshall brand. I didn't mean to say that other Marshalls aren't legit, just that some countries got shipped leftovers while others didn't. I don't know why, but I guess in turn that makes those mystery Marshalls, rarities.

You know I love you brother!

 

[MajorNut1967]

lol, you ass! ahha

I don't think the Canadians had different OT's, but they didn't wire the 16ohm taps in, just the 8 and 4. I don't know why but that's just how it went.

Of course I know difference in one resistor doesn't really make it a completely different amp, I was just sayin...

What I meant by the legit thing was just that those two countries kept up with the normal trend, as in neither one of those have any of the eighties mystery JMP's and so forth that doesn't correspond with the typical year by year changes in the Marshall brand. I didn't mean to say that other Marshalls aren't legit, just that some countries got shipped leftovers while others didn't. I don't know why, but I guess in turn that makes those mystery Marshalls, rarities.

You know I love you brother!

I was just rattling your cage mate! Your a good bro AV.

Cheers LOL