Metropoulos Forum

Greetings all, so here is my dilemma. Some time ago (about 6 months) my guitar amp (originally a JMP Super Lead with with 4th preamp tube mod), developed a tremendous drop in volume of the actual signal. I opened up the amp to investigate and very quickly discovered that I was in over my head. I am currently a Junior in Computer Engineering at CSU Long Beach in CA and have since then have made this amp my pet project. Ultimately due to frustration at never being able to discern the issue I opted to do a complete rebuild into a 2203 with an extra gain stage, replacing all the components except for the OT, the PT and the choke from the original amp.
So the amp is now rebuilt as a turret board style, got it all wired in and am now having trouble with fuses blowing and the power tubes seeming to get really hot. I have checked the rectifier against the theoretical values and everything seems to be right where it should be. The thing that struck me was when I got to the OT I found a data sheet that said the impedance across the primaries should be in the 1750 ohm range. And when I measure across my primaries I get about 30 ohms. So I ordered a replacement transformer and in my excitement to get the amp cranked on again after so long I think I forgot to check the resistance across the new one. (same thing though, its datasheet said 1750 ohms).
So, got it all wired in, cranked it on, same problem. THEN i decided to check the resistance of this new OT primary line and am getting ... you guessed it, 30 ohms. So at this point I am at a loss. I checked out Jack Darr's section on OT's and it said the primary should match the load impedance of the tubes, I am using JJ EL34's and their data sheets claim that they have an Ri of 15k, now I am not totally sure what Ri is supposed to represent in this context, but it is the only resistance on the sheet.
I also was reading this http://www.geofex.com/ampdbug/outtrans.htm where it says the resistance across the primaries should be kinda low (10's to 100's) so thirty ...though really only representing 15 from B+ to each end... is low but possible I would assume.
I am kinda at a loss here and I am doing the best I can. I have put TONS of time and money into this project and have learned A LOT while doing it, but I really just miss cranking up and strumming power chords and would love to get a little help. Do I go by the data sheet??? or is 30 ohms across the primaries fine?

ps ... data sheet for the OT transformer [url]http://www.classictone.net/40-18026.pdf

30 ohms across the primaries is fine, because you're measuring the DC resistance, the pure copper resistance.

You didn't measure the AC resistance, named impedance !!!

You can see clearer now?

If not, read my signature and don't hesitate, to ask once again.

Larry


Btw. Welcome on this board

Well I suppose that makes sense as I ordered a brand new transformer and right out of the box it measured at 30 ohms again. I am supremely at a loss at this point. I cannot figure a reason at all why this thing would keep blowing fuses. I am wondering now if its maybe the types of tube I am using or something. My Modded JMP Super lead (what the amp used to be) used EL34's , as I have now rebuilt this thing I never even thought to check and see if a different tube might be required. When I turned on the amp last I got a steady hum from the speaker that was not affected by any change to the pots or the guitar jack which persisted until the fuse blew.

Nope, just read online and it seems that a 2203 would use EL34's as well. I am going to go through and do a node by node analysis and hopefully figure out what I am doing wrong.

Well your not exactly understanding what Larry is saying, it's a bit more complex and your being a little mislead
By what the datasheet is trying to convey as the term impedance is a bit convoluted. The 30 ohms you are measuring is the
Dcr across each half of the tx primary, it's simply the dc winding resistance. The 1750 ohm impedance spec'd on the datasheets is the impedance reflected to the primary from the secondary due to the square law of the turns ratio into what ever given load. The OT doesn't really have an "impedance" it just has a ratio. To measure it you would have to inject an ac signal into the primary or secondary and measure it on the other end and apply square law to compare it to its published spec. As an aside keep in mind this is only somewhat of a linear figure (that's what the datasheet specs). Since a speaker load is highly reactive it wont be that to all frequencies.

Regarding the tube data sheet regarding the 25k plate resistance, again, what your looking at is the tubes internal dc resistance, not what an appropriate anode to anode load is for what ever class of operation under given circuit parameters.