JCM 800 2203 dead...

[Bruce / Mission Amps]

Many good and inexpensive transformers which have multi-tapped secondaries, use thinner and thinner wire on the higher zed secondary outputs. (More wire windings needs more space = thinner wire with more windings in the same space).
Mismatching to change the primary load back to around 3K2-4K should be OK but not a perfect match for two KT88s at 450vdc, if the OT is actually 1700 ohms.
Putting a higher load and the lower tap is the correct thing to do when going from 4 tubes to 2 tubes but you have to be carefull when running that higher load to get more power on the the thinner wires of the higher zed secondary.
A cheap tranny can just get damaged.
Also, I have found that the cathode follower triode in some of these have blown up due to the cathode being at inexcess of 230vdc and a cheap 12AX7 tube like say, a SOVTEK 12AXWA, is a turd IMHO. It can't take that much cathode voltage for too long before going open.
One more thing, and just my opinion with out trying it...
even though the power supply can push\pull the current through four EL34 power tubes and OT at 1700-2000 ohms, making +100 watts, a pair of KT88s trying to make close to 100 watts output is going to be looking for way more plate voltage across the OT to make all that power so I'm betting the amp with two KT88s won't make nearly as much power as when the four EL34s are installed.
My guess is around 65-70 watts max with a pair of KT88s.


Bruce

[stokes]

Bruce,I have run a similar test with a SF Twin but with NOS Tung-sol 6550's vs RCA 6L6GC's.With 4x6550 I got 97 watts.6L6's only got 94-2x6550 I got 48 watts.The point I was trying to make earlier was that someone posted that the 2xKT88 were going to give the same 100 watts as the 4xEL34.Althought a pair of KT88's could put out close to the 100watts it wont happen in that circuit.

[Amp Kat]

Although you can turn an amp on at zero ohms and let it idle,if you put a signal thru it,the secondary which is running at a dead short (zero ohms) will eventually fry.I gotta disagree with your that running a 2 ohm load on the OT set for 16ohms.That tranny needs to see 16ohms to run efficiently,2ohms is very close to a dead short.It will draw more current and get hotter.Gennerally an OT can stand a 100% mismatch i.e. 8ohm load into a 4ohm tap,problems arise when you go the other way.In my experience Marshalls dont hold up as well as say!

Ot's like shorts,it's opens they don't like. Power in = power out and there isn't enough difference mathematically between 4,8,16 taps to determine a sufficient power difference. Now a cheap made tranny is going to eventually fry no matter what and I certainly agree with you on the Fender/Marshall OT differences. Have you ever measured the difference in ohms on a 4,8,16 ohm tranny from common. Not very much difference at all.

[MitchK]

Most Marshalls will have two fuses. The Mains fuse and the HT fuse. I think you just missed one of them. Looks like you have an amp fitted with the fuse board as used for the Canadian market where the code requires the extra fuses. In you example “F5” is the part designation meaning “Fuse Number 5” and 500ma is the value that should be installed in that position. All the other fuses should be similarly labeled.

HTH,
Tom

Eek I hope we're talking about two different things or the person who put the fuses in this thing had to be borerline retarded. I'll take a picture when I get home from work and show the fuse board but there is says 500mA there were definitley some 5A fuses...

I am in Canada so the fuse board makes sense...

[Tom Phillips]

I think all is OK. I would expect to find both 5A and 500ma fuses on the board. The 5A fuses are in the heater lines and the 500ma will be the center tap / HT. Attached is a photo from a 2204. I frequently find incorrect value fuses installed. When you get back to your amp you will probably easily figure out which size fuse goes where. You should be able to go by the board markings. You can then correct any past mistakes.
Tom

[MitchK]

Beautiful tom thats exactly what I was looking for and yeah the values of the fuses in this amp are just not even reasonable "f2 3A" was a 1/10A 125v fuses according to etchings on the end of the fuse. needless to say its dead, but I don't know how it survived up until this point.

I don't know about those types of fuses there, the ones with the spring inside of them, the 1/10A 125V fuse in the amp as that kind but I've never seen them before, what are they called just so I know for future, and do I have to use those kind or can I just use a regular old 3A fuse just as long as its not a slo-blo?

[Tom Phillips]

I don't know about those types of fuses there, the ones with the spring inside of them, the 1/10A 125V fuse in the amp as that kind but I've never seen them before, what are they called just so I know for future, and do I have to use those kind or can I just use a regular old 3A fuse just as long as its not a slo-blo?

In my experience the fuses with the spring inside are just an older type of construction. All the ones I have seen that were made like that were slo-blo types I believe. I don’t think anyone makes them that way anymore.

I think that Marshall intended to use slo-blo fuses in the mains and heater circuits and fast blow for the HT and bias circuits. That’s what I always install. It would have been nice if they specified that on the board.

Cheers,
Tom

[MitchK]

alright so it likes I'm just about ready to go with this thing. The only thing I'd really love to get some input on is the screen resistors. should I change the values?

What I'll be doing by the looks of it is ordering a quad of KT88s and a quad of 6550s probably put the kt88s in the 5150 and the 6550s in the jcm800. do you guys find much of a difference in brands? I see a lot written on each brands sonic qualities but I don't know if its worth paying 50% more for JJs or what? I was going to get Svetlana 6550s and Shuguang for the kt88s?

[stokes]

Mitch,the screen resistor should be 1K 2-5 watt.Some amps have 470ohm screen resistors,if so they should be changed to 1K.The only experience I have with 6550's are NOS so I cant help you with the current production tubes.

[Enzo]

I have a couple different 2203 schemos, and on both the HT fuse is 1A, and slow blow. When it says "T1A" or for that matter "T500ma", the T means timed - timed fuse means slow blow

[Steve Conner]

Stokes, I swear I did it and it works I made the amp with a PT and OT from an old Sansui PA amp that ran EL34s (at 550v no less!) they were almost as big as the ones in a 100 watt Fender Twin, so I reckoned they'd survive. I clocked it at 112W into 8 ohms with a 400Hz test signal. Though in the light of what you guys said, I should go back and measure it again in case I did something stupid.

To the original poster: if the power tubes glow blue and you hear sound coming from the amp chassis, but no sound from the speaker, that says to me that the speaker got disconnected. That will destroy your tubes and maybe the OT too.

[Amp Kat]

By the same line of reasoning, even four KT88s would still only give about 100w, unless you changed the loading by connecting a 8 ohm speaker to the 16 ohm tap. Then you'd probably get almost 200w, but at risk of burning your transformers out. I've managed to increase a 50w amp to 100w by this method, but can't vouch for the long term reliability.

hmmm do you mean 105 watts instead of 200 ? That's all you get by changing taps. The voltage actually goes down and the current goes up but not by very much at all. From 4 to 8 ohms is .5 ohms and from 4 to 16 is .2 so how do you get a 50% increase in power from 1 ohm of difference between 4 to 8 ohm taps ?
One thing that Stokes said that does happen is the speaker impedance changes with frequency and does reflect back to the primary but these frequencies called high or low are in the 10k range of high and 400 as low and the frequencies we play at are lower on the low end and less than 10k on the high end. Another thing that happens is the pots are logrythemic so when it dumps a ton at that 50% range that wattage stays pretty close to constant until the end and then the power supply sags and the voltage drops further and so does the power. If your tranny blows it's not because of mismatch. Some of the Marshalls however blow under any circumstances so I would have to think that Bruce's thing on size of wire is a major cause and a major concern in those OT's.

[Ray Ivers]

Kerry,

From 4 to 8 ohms is .5 ohms and from 4 to 16 is .2 so how do you get a 50% increase in power from 1 ohm of difference between 4 to 8 ohm taps ?

Can you explain this a bit more? It seems to me like you're taking resistance readings of the OT secondary and then attaching some power-output significance to them, but surely I'm misunderstanding you.

Ray

[stokes]

Those resistance measurements mean very little.what makes the difference between the secondary taps is the turns ratio between the primary and secondary.

[Amp Kat]

I'm talking about the difference in ohms between 4,8,16 ohm taps. In the perfect world of transformers lets say you put in 1 volt to the primary and measure from secondary 4 ohms to common and you get .31 volts well on paper your supposed to get .625 for 8 and 1.25 for 16 of course reflecting the primary impedance changes as a result of each. Those numbers don't jive when doing the test and they are much less then they're supposed to be. Is it because of the wire used and the permeability of the core or it's dimentions I don't know but Fender trasformers seem to handle the low loads with no problems and many others besides Marshalls and maybe I need to do the test over with other trannys but the actual resistances aren't what they should be and are much less. Certainly there are other parameters that come into play with frequency in the complex domain,transformer losses and what the speaker actually sees as it changes from anywhere from 2 to 100 ohms during operation. What I'm saying is it's not as extreme as the numbers look but I'll do somemore test. It's a great discussion and quite complex when you factor in the phase angles.