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Unread 02-19-2010, 07:15 PM   #1 (permalink)
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tube swap...it worked,what would it hurt?

I swapped out the two 6550's for 6L6's in the Artist 3203 for just a few minutes to see if they would warm this thing up some, (which it did) No strange hums or noises, it worked fine and sounded better. What would it hurt if I left em in there? Could I have it rebiased for 6L6's?
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Unread 02-20-2010, 07:00 AM   #2 (permalink)
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Re: tube swap...it worked,what would it hurt?

Hi.

If the heater windings on the PT are sturdy enough for 3.2A instead of 1.8A current draw, no problems.

Depending of the circuit, You may or may not have to calculate and perhaps adjust the primary load resistance for the optimal performance.

Obviously any tubes have to be biased correctly in any circuit, that goes without saying.

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Unread 02-20-2010, 07:49 AM   #3 (permalink)
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Re: tube swap...it worked,what would it hurt?

So the 6L6's draw more current than the 6550'S? I'm desperate enough just to leave 'em in there. lol . Made the amp sound alot better. THANKS T!
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Unread 02-20-2010, 10:11 AM   #4 (permalink)
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Re: tube swap...it worked,what would it hurt?

Hi.

Nope, the 6550 draw more current, I misread Your question, sorry. Call it a language barrier if You like .

If biased correctly and the load resistance falls within acceptable parameters, there shouldn't be any problems.

I haven't personally done such a swap, so I don't know for sure.

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Unread 02-20-2010, 04:01 PM   #5 (permalink)
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Re: tube swap...it worked,what would it hurt?

Quote:
Originally Posted by dugger View Post
I swapped out the two 6550's for 6L6's in the Artist 3203 for just a few minutes to see if they would warm this thing up some, (which it did) No strange hums or noises, it worked fine and sounded better. What would it hurt if I left em in there? Could I have it rebiased for 6L6's?
I started to comment on this yesterday, but erased my post. The reason I did this was I pointed out that you are asking for advice "after the fact." Believe it or not, I have been following you and your 3203 ever since you got it. Along the way, you have started threads and I have replied to some of them.

However, this time you did something first and then you wanted to know if you could do it. Sort of like sticking a coat hanger into a wall outlet and then asking if it will shock you.

I'm just being this way, because I don't like seeing people fry their amps. You would have been screwed if you had put an EL34 in there. Just because a tube will fit in a socket does not mean it will work, or that it won't cause damage if fired up. In your case, you lucked out plain and simple. A 6550 is just a big 6L6 and the pin out is the same. The only problem is you have a hybrid amp that nobody knows anything about. I have looked at the owners manual and it says that this is a split channel combo. Is that correct? It also says that it is a 30W amp. One 6550 can easily put out over 30 watts, so I wonder if each channel has its own tube and this is cathode biased? Again, IDK. If it is made this way, you may still need to tweak some components to achieve maximum valve life. The 6L6's could be running to hot. Again, I did a search for a schematic on this amp and I could not find one.

It would be hard for Jon Wilder to help you unless he has a schematic in his data base. Hopefully he will pipe in. I would tell you not to run the 6L6's before this gets remedied, but you probably will go ahead and do it.
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Unread 02-20-2010, 04:51 PM   #6 (permalink)
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Re: tube swap...it worked,what would it hurt?

6550s have the capability to put out lots more power. However, in order for it to do so it has to be installed in a higher powered environment (i.e. power transformer has to be able to supply the current/have the voltage and the OT would have to be able to handle the current as well as provide the proper plate load). These are requirements that must be satisfied before the 6550 can run at its full capability. But in Marshalls, they're far from satisfying those requirements.

Now one thing to keep in mind...the transformers in a 6550 Marshall are no different from the ones used in an EL34 Marshall. As such, when biasing 6550s you MUST use the figures you would use for an EL34, meaning that if you biased EL34s to 35-40mA you would do the same with 6550s installed in an EL34 environment. Same goes for running 6L6s in an EL34 environment. The range resistor in the bias supply would have to be increased in value in order to give you -10 more volts on the control grid for 6550s and 6L6s in order for them to bias up properly. All of this has to do with the reflected impedance of the output transformer to the valve plates, load lines, 6550s/6L6s exhibiting a higher transconductance, etc etc. Stuff too complicated to explain to the average joe who has no working knowledge of electronics and amplifier theory.

Also...most 6L6s require a 470 ohm 2 watt screen resistor whereas in Marshalls you'll find a 1K 5 watt screen resistor on the valve sockets.

This is definitely one of those areas where people who aren't in the know (i.e. most tone monkeys that I know of) shouldn't be playing around with shit like this. All the guys who preach that if it will plug into an octal socket then it will work are full of shit. There are a multitude of things to know when biasing, swapping valve types, etc etc that most tone monkeys do not know and can be dangerous to you or your amp if attempted without knowing exactly how to do it, which requires a working knowledge of electronics and amplifiers. It is for this exact reason why I oppose the DIY bias kits and build it yourself amp kits. Some things just aren't as simple as 1 2 3.
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Unread 02-20-2010, 05:08 PM   #7 (permalink)
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Re: tube swap...it worked,what would it hurt?

I still don't get this MUST bias like EL34 thing...

Here's my disclaimer...
I am no tech but based on personal experience:
I have had KT88's in a 2203 and biased using 35w as a plate dissipation, not 25w. It's been over a year and the tubes still sound great. According to your theory my tubes are biased ABOVE 100% do not exceed idle current and I have had zero problems. Icing on the cake, these are the JJ's everyone says break all the time.
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Unread 02-20-2010, 05:47 PM   #8 (permalink)
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Re: tube swap...it worked,what would it hurt?

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Originally Posted by American Viking View Post
I still don't get this MUST bias like EL34 thing...

Here's my disclaimer...
I am no tech but based on personal experience:
I have had KT88's in a 2203 and biased using 35w as a plate dissipation, not 25w. It's been over a year and the tubes still sound great. According to your theory my tubes are biased ABOVE 100% do not exceed idle current and I have had zero problems. Icing on the cake, these are the JJ's everyone says break all the time.
But the issue is this. You do not have a 150 watt power tranny. On the data sheet for a KT88, they are running over 500VDC on the plates to bring the tube up to it's designated operating range for max output.

When Marshall used 6550's, it was to band aid a logistical nightmare. The bottom line, the EL34 power supply was used (and the EL34 OPT). The 6550's were ran as a 25 watt dissipation tube, not 35W. You can't create power without having power.

I've said this before, 6550's or KT88's in a 100 watt Marshall amp are not going to make 140-150 watts. They are going to make 100 watts. Also and this is a key point. A 6550 or KT88 that is running at 25W dissipation is not going to sound the same as a 6550 or KT88 that is running with the proper power supply at 35W dissipation.

The main thing is you like the way your tubes sound. Now I don't know the exact biasing method you used, but apparently it put you where you needed to be; no red plating and you are happy with the results.

All I'm describing here is the physics of this question. I have people PMing me all of the time wanting to use 6550's and they think they are going to have a 150 watt amp. It's basic Ohm's Law.
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Unread 02-20-2010, 05:55 PM   #9 (permalink)
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Re: tube swap...it worked,what would it hurt?

Quote:
Originally Posted by MartyStrat54 View Post
But the issue is this. You do not have a 150 watt power tranny. On the data sheet for a KT88, they are running over 500VDC on the plates to bring the tube up to it's designated operating range for max output.

When Marshall used 6550's, it was to band aid a logistical nightmare. The bottom line, the EL34 power supply was used (and the EL34 OPT). The 6550's were ran as a 25 watt dissipation tube, not 35W. You can't create power without having power.

I've said this before, 6550's or KT88's in a 100 watt Marshall amp are not going to make 140-150 watts. They are going to make 100 watts. Also and this is a key point. A 6550 or KT88 that is running at 25W dissipation is not going to sound the same as a 6550 or KT88 that is running with the proper power supply at 35W dissipation.

The main thing is you like the way your tubes sound. Now I don't know the exact biasing method you used, but apparently it put you where you needed to be; no red plating and you are happy with the results.

All I'm describing here is the physics of this question. I have people PMing me all of the time wanting to use 6550's and they think they are going to have a 150 watt amp. It's basic Ohm's Law.
Of course they won't redplate. But your power and output transformer will be greatly stressed as you've pushed the 6550 into an operating range that the supply nor the OT was made to handle.

Biasing isn't just governed by the valve type. Other factors such as the transformers come into play here. You would have to know how to read the load line, plate current and grid characteristic charts found on the datasheets in order to see what's going on.
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Unread 02-20-2010, 06:17 PM   #10 (permalink)
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Re: tube swap...it worked,what would it hurt?

DUGGER-I got one for you. You are calling your amp a 3203. That's a head. The combo is a 4203. Now when you bought this, I recall it having 6550's in it? Anyway, I did locate a schematic and it shows that this amp came equipped with EL34's. Now on the 50 watt model, they show EL34's (UK) and 6550's (USA). What tube is supposed to be in there? Did your tech say? What screen resistors are in the amp on the power tube sockets.

I can't remember, but didn't you put EL34's in this for a while?
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Unread 02-20-2010, 08:47 PM   #11 (permalink)
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Re: tube swap...it worked,what would it hurt?

I don't know what amp the OP has but I wanted to confirm that my 4203 (combo version of the 3203) and a 3203 head I had in the '90s both came with el34s and I am in the US. Also both have/had a SS preamp.
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Unread 02-20-2010, 09:52 PM   #12 (permalink)
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Re: tube swap...it worked,what would it hurt?

Quote:
Originally Posted by ant_riv View Post
I don't know what amp the OP has but I wanted to confirm that my 4203 (combo version of the 3203) and a 3203 head I had in the '90s both came with el34s and I am in the US. Also both have/had a SS preamp.
Thanks, that helps out a lot. Unless the amp has been modded he is running the wrong tubes.
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Unread 02-22-2010, 05:59 AM   #13 (permalink)
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Re: tube swap...it worked,what would it hurt?

Quote:
Originally Posted by MartyStrat54 View Post

When Marshall used 6550's, it was to band aid a logistical nightmare. The bottom line, the EL34 power supply was used (and the EL34 OPT). The 6550's were ran as a 25 watt dissipation tube, not 35W. You can't create power without having power.

I've said this before, 6550's or KT88's in a 100 watt Marshall amp are not going to make 140-150 watts. They are going to make 100 watts. Also and this is a key point. A 6550 or KT88 that is running at 25W dissipation is not going to sound the same as a 6550 or KT88 that is running with the proper power supply at 35W dissipation.

The main thing is you like the way your tubes sound. Now I don't know the exact biasing method you used, but apparently it put you where you needed to be; no red plating and you are happy with the results.

All I'm describing here is the physics of this question. I have people PMing me all of the time wanting to use 6550's and they think they are going to have a 150 watt amp. It's basic Ohm's Law.
I totally agree with you there that just putting in tubes isn't going to turn it into a higher wattage amp. I've heard that a lot before and still don't know where people keep getting that idea from.
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Unread 02-22-2010, 06:08 AM   #14 (permalink)
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Re: tube swap...it worked,what would it hurt?

Quote:
Originally Posted by American Viking View Post
I totally agree with you there that just putting in tubes isn't going to turn it into a higher wattage amp. I've heard that a lot before and still don't know where people keep getting that idea from.
It's because when people hear that 6550s HAVE THE CAPABILITY OF putting out more power, they try to put 2 and 2 together and get 22 every time because their missing the 1/2 of the equation that states that the environment in which they're installed in has to satisfy the required requirements in order for that to happen.

Sometimes I wish valve amps costed about as much as high performance engines to fix. Maybe then some of these guys would think twice about trying these supposed tone tweaks from the self proclaimed shade tree valve amp mechanics. Or at least have the common sense to do some research as to the validity of what they read.
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Unread 02-22-2010, 11:21 PM   #15 (permalink)
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Re: tube swap...it worked,what would it hurt?

+1

The first thing that came to mind when I read that was a line from Dawn of the Dead when Peter picks up a super expensive rifle at the gun store...

-The only person who could miss with this gun is the sucker with the bread to buy it.

Sometimes I think the same thing about people that can afford a vintage amp, with rare NOS tubes, an attenuator, and then deep fry it all with a Variac...
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Unread 02-22-2010, 11:37 PM   #16 (permalink)
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Re: tube swap...it worked,what would it hurt?

What happened to Dugger?
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Unread 02-23-2010, 04:28 AM   #17 (permalink)
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Re: tube swap...it worked,what would it hurt?

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Originally Posted by MartyStrat54 View Post
What happened to Dugger?
His amp came alive and kicked him in the face for fiddling with it.
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Unread 02-23-2010, 07:22 AM   #18 (permalink)
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Re: tube swap...it worked,what would it hurt?

Hi.

Quote:
Originally Posted by MartyStrat54 View Post
What happened to Dugger?
Perhaps he got a gig?

Or a life?

Or both .

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Unread 02-23-2010, 12:06 PM   #19 (permalink)
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Re: tube swap...it worked,what would it hurt?

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It's because when people hear that 6550s HAVE THE CAPABILITY OF putting out more power, they try to put 2 and 2 together and get 22 every time because their missing the 1/2 of the equation that states that the environment in which they're installed in has to satisfy the required requirements in order for that to happen.

Sometimes I wish valve amps costed about as much as high performance engines to fix. Maybe then some of these guys would think twice about trying these supposed tone tweaks from the self proclaimed shade tree valve amp mechanics. Or at least have the common sense to do some research as to the validity of what they read.

So, is the gospel here stating that if I have a USA model 2203 Marshall that came equipped with 6550's, I still have to assume an EL34 operating max dissipation of 25 watts instead of the typical 35 watts for 6550's in order to bias the amp correctly?

Then these amps run the 6550's cold, and as a result they wouldn't sound as good? So, it makes sense then to get the most out of these amps, to bias them for EL34's?

Also, then the info on the Eurotubes site is incorrect, because they don't take into consideration the transformer types in the amps. I don't know if you're familiar with the biasing videos and the accompanying information on biasing, but basically they outline the max dissipation for a given tube divided by the plate voltage multiplied by 70% for biasing.

I don't want to stress my transformers, but i biased my 2203 with 6550's by: 35watts/plate voltage = Y.........Y multiplied by 70% gave me my biasing value. I had plenty of range on the trim pot, so I assumed I'm o.k. The tubes don't red plate and the amp sounds good, but I don't want to be silently killing the trannies.

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Unread 02-23-2010, 03:50 PM   #20 (permalink)
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Re: tube swap...it worked,what would it hurt?

Oh crap, I'm so used to having mediocre responses that I have'nt checked this in a while. Marty thanks for everything ,you've been an Ace, you too Jon and everybody. I'm forever thinking I can make a silk purse out of sow's ear! The amp came used with 6550's ,my tech put in on the scope and showed me the bias curves were perfect. He slapped in some EL34s and showed me where the bias was wrong. In a last desperate attempt to warm this thing up I tried the 6l6's.(just for a couple of minutes) This amp is shit. I would'nt care if it blew up today. It sounds like a AM radio compared to a FB Bassman. You guys are way over my head with some of this stuff anyway. I going back to concentrating on playing my guitar. They all tell me I could plug into a anything and sound good. (Marty it is the head version)
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Unread 02-23-2010, 04:02 PM   #21 (permalink)
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Re: tube swap...it worked,what would it hurt?

Quote:
Originally Posted by dugger View Post
Oh crap, I'm so used to having mediocre responses that I have'nt checked this in a while. Marty thanks for everything ,you've been an Ace, you too Jon and everybody. I'm forever thinking I can make a silk purse out of sow's ear! The amp came used with 6550's ,my tech put in on the scope and showed me the bias curves were perfect. He slapped in some EL34s and showed me where the bias was wrong. In a last desperate attempt to warm this thing up I tried the 6l6's.(just for a couple of minutes) This amp is shit. I would'nt care if it blew up today. It sounds like a AM radio compared to a FB Bassman. You guys are way over my head with some of this stuff anyway. I going back to concentrating on playing my guitar. They all tell me I could plug into a anything and sound good. (Marty it is the head version)
If you wanna "warm it up" you can get it converted to run EL34s. But NOT without the conversion.
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Unread 02-23-2010, 04:10 PM   #22 (permalink)
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Re: tube swap...it worked,what would it hurt?

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So, is the gospel here stating that if I have a USA model 2203 Marshall that came equipped with 6550's, I still have to assume an EL34 operating max dissipation of 25 watts instead of the typical 35 watts for 6550's in order to bias the amp correctly?
Yes. The plate-plate load that an EL34 output transformer provides does not satisfy the plate load requirement for 6550s to run at their optimum potential. Again not everything about a valve amp is about tone contrary to what people would like to think.

Quote:
Originally Posted by ironlung40

Then these amps run the 6550's cold, and as a result they wouldn't sound as good? So, it makes sense then to get the most out of these amps, to bias them for EL34's?
Not necessarily. While 6550s cannot run at their fullest potential electrically in an EL34 environment, the tone they produce running below their rating may be a tone that you like. And valves running below their potential is never a bad thing electrically. It just depends on what kind of tone you're after.

Quote:
Originally Posted by ironlung40
Also, then the info on the Eurotubes site is incorrect, because they don't take into consideration the transformer types in the amps. I don't know if you're familiar with the biasing videos and the accompanying information on biasing, but basically they outline the max dissipation for a given tube divided by the plate voltage multiplied by 70% for biasing.

Ironlung40
One thing about Bob @ Eurotubes that everyone fails to see is that Bob is not an amp tech by trade. He openly admits this on his site. If you doubt me, click this link and read the second sentence in the article -

euro-h

Now that being said...no he does not take the transformer into consideration and this can be a bad thing. More than likely he's never had any failures either...but there again, I left the "yet" part out. Some things may not cause problems right out the gate, but may in the long run.

The optimum bias point for ANY valve is not just governed by the 70% of max dissipation rule (for the record the 70% rule only applies to Class AB amps...Class A amps are biased at max dissipation). The output transformer plate load also comes into play in this, and this is another reason why it's very important to not mismatch your impedance (throws the plate load off and as such affects the operation point where the bias places the valve in...lots of technical data).
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Unread 02-23-2010, 04:42 PM   #23 (permalink)
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Re: tube swap...it worked,what would it hurt?

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If you wanna "warm it up" you can get it converted to run EL34s. But NOT without the conversion.
OK Jon, in the simplest terms for dummies ,what is involved in "the conversion"? My amp guy will know but I'd like to also.
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Unread 02-23-2010, 04:56 PM   #24 (permalink)
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Re: tube swap...it worked,what would it hurt?

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OK Jon, in the simplest terms for dummies ,what is involved in "the conversion"? My amp guy will know but I'd like to also.
Well assuming your amp already has 1K 5 watt screen resistors and pin 1 and pin 8 are both tied together already, the range resistor that limits the negative bias voltage adjustment range would be all that would need to be changed. This will allow the EL34s to bias up properly in that amp.

One thing about 6550 equipped Marshalls...the transformers are the exact same ones they use in the EL34 versions. So technically 6550s are being installed in an EL34 environment, however they're increasing the negative grid voltage to allow for 6550s to bias up in an EL34 environment. So this would be all that would need to be changed.
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Unread 02-23-2010, 05:03 PM   #25 (permalink)
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Re: tube swap...it worked,what would it hurt?

Thanks Jon... probably will go for this. My biggest problem has been this guy is kinda old and doesn't like to work in his basement when it's cold! Nice guy though. No need to RE:
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Unread 02-23-2010, 06:55 PM   #26 (permalink)
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Re: tube swap...it worked,what would it hurt?

Wait a minute. Hold the presses. I thought I had just stated that the amp came with EL34's. This was verified by ant_riv in his response. Why are the 6550's even working in this amp, if it was wired for EL34's? Are you saying that someone modded the amp over to 6550's?

I would really like to know the answer to this. I guess if you had a real schematic with part numbers, you could see what the OPT P/N is. The Dr. Tube schematic shows EL34's.
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Unread 02-23-2010, 07:20 PM   #27 (permalink)
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Re: tube swap...it worked,what would it hurt?

Marty ol' buddy...I'm fried! After looking in the amp previously, amp tech said to bias this amp he would like to install a trim pot and some other things...so it has not had anything done to it from stock. He showed me on his scope where the curves met where one tube was "handing off" to the other and they were dead on with the 6550's. You stated the 50 watters(USA) had 6550's, he tested this thing and it was putting out 30 watts like it was suppossed to so it is the 30 watter. NOW my biggest question... would Marshall install GE tubes in an amp, UK or USA?? That's what this thing had in it when I got it. The 6550's really don't even sit down all the way on the tube base due to the spring holders. I think someone just popped em in there and went with it. Gonna let him "convert" it back to EL34's when it gets warm and I can afford it, if it still sounds like shart, I'm gonna pull a Pete Townsend on it one night at a gig! lol So your saying if EL34 was the original and no mod was made, the 6550's would'nt work at all or it would just sound like shit like it does now?
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Unread 02-23-2010, 07:59 PM   #28 (permalink)
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Re: tube swap...it worked,what would it hurt?

When I had a TSL60 head, I decided to bias the amp myself after checking out the Eurotubes site. It was simple on the TSL because the bias points are on the back of the amp (no taking anything apart) and you only use one bias point because they are shared. All I used was a multimeter and was able to easily read the data to get to the Marshall recommended bias settings.

What could I have harmed?

Now that I have a JVM, I probably won't bias it myself because I will have to take it apart to get to the bias switch. Maybe Marshall did this on purpose?
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Unread 02-23-2010, 11:37 PM   #29 (permalink)
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Re: tube swap...it worked,what would it hurt?

I don't know dugger. I am baffled by this. My info says that the amp should have EL34's. The tube pin out is different. The 6550 is just a big 6L6 and they have the same pin out. The EL34 is different. To me, it seems someone must have modded the amp to use 6550's, but I wouldn't know why? With some of these old amps, you never can tell what's been done to them.
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Unread 02-24-2010, 05:09 PM   #30 (permalink)
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Re: tube swap...it worked,what would it hurt?

Just to remind you ,I did also pop in some El34's one time. When you say "pin out" does that mean the wrong tube would'nt even fit in the holes? The 6l6's are missing a pin the 6550's have. Just gonna have him fix it for the EL34's someday...bias pot ect. Don't think it's ever gonna be something I want with that ss preamp. Damn thing has got plenty of power though!(and looks so cool too) Going to a big guitar show this weekend and take it with me to possibly trade. Who knows, they're might be an SLX in my future. Thanks again Marty!
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