JCM 800 2203 dead...

[stokes]

With the 1 volt test you describe,try putting the 1v on the secondary and measuring the primary from end to end (ignore the center tap).You will get much better voltage comparisons.When measuring the difference from .31v to .625v your meters inaccuracies will make a bigger difference as opposed to measuring the increased voltages you will be measuring on the primary.I usually use a .5 volt 1K signal from my signal generator on the secondary and ,depending on the tranny and the tap will get somewhere around 18 to 30 volts on the primary,much more exact when determining the turns ratio.Your meter will read these higher volts with more accuracy than trying to read the volt fractions.If it is a case where a tranny's secondary loads the .5v too much,I get out the variac and connect to the primary and raise it till I see .5 on the secondary.Gives you much easier numbers to work with.I am sure the poor guy who started the post wishes we would get back to his problem.Sorry for getting sidetracked here,Mitch.Hope you are getting something out of all this.

[Steve Conner]

Hi all

I measured the amp again, this time using a non-inductive dummy load instead of the wirewound resistor I tried previously. The OT was designed to run two EL34s into 16 ohms giving 50W, and the load I applied was 9.4 ohms (two 4.7s in series) The B+ is 475V, this is also the screen supply through a 1K resistor.

With KT88s, I could get about 36V peak before it started to clip obviously, which is only 69w The KT88s showed a small amount of red on their plates.

I also tried a pair of Sovtek 5881WXTs as a comparison, these could only wheeze out 25V peak which works out to 33W. The plates got very red.

Finally I tried EL34s and they could get up to about 30V, which is 47W, if I used the same 475V screen voltage. (I usually lowered it to 360 for EL34s.) I could see screen wires glowing bright orange though, which can't be too good

If I use the proper 16 ohm load, all of these tubes deliver about 50w before clipping.

So I guess the bottom line is that you guys were right, the KT88s are not being used efficiently at the lower plate voltage (the manufacturer rates them at 100w with no red plating) and the power improvement isn't as good as I thought. I take back what I said about them being able to pass twice as much current, too. I don't know how I managed to measure the power wrongly the first time.

[Amp Kat]

Yeah I've done the test at higher voltages and it came out the same but try this test with actual speakers and trannys and calculate the outputs of 4,8,16 into a four ohm load then switch to 8 and 16 with different frequencies and see what you get. It changes drastically more than you'd think. I too apologize to Mitch for stealing his thread somewhat and hopefully he somewhat got something out of this but it will be back with somemore numbers soon.Thanks fellows

[Tom Phillips]

... the T means timed

Cool.
Thanks for the information Enzo. Something new learned each day. I love it!
Interesting that I have been using fast acting fuses in the HT positions and have not experienced nuisanceblowing problems.

[Ptron]

I've always heard that an open speaker load was more problematic than a short. Don't some amps use a shorting jack for the speaker? I think the problem with an open load is an inductive flyback thing where the huge voltages produced can arc the power tube sockets or puncture the OT insulation. I'm not sure why a short wouldn't be a disaster. Seems like it should be. Maybe it's that the intrinsic impedance of the OT secondary is enough, at least temporarily, to keep things from getting out of control.

[Enzo]

Or at worst it pops a fuse. Far better than detsroying things.

[stokes]

An open load is a quicker death but a short for any length of time with a signal will take its toll also,with no signal it wont do harm

[MitchK]

Alright, so its dead again.

At the show I had to play I was running the amp with only two el34s. Near the end of the second set the amp croaked. I was playing fully cranked with just the two tubes and it died. soooo I looked inside and we had some blown fuses. so just now I replaced the fuses inside (which were incorrect ones. someone had put a 3a fuse in the h.t fuses the ones that are supposed to be 500mA). flick it on and POP, bzzzzzzzz. a loud pop and then a quite hum. the fuses I had just put in had blown.

Any ideas? these are the two fuses labeled on the board as h.t. I don't even know what thats supposed to stand for but I'm afraid for the transformers.

[Enzo]

HT means "high tension" meaning high voltage. These are the fuses for your B+ supply. Most common reason for them to blow is bad power tubes. Of course any trauma on the HV rail wil blow the fuse. TRansformer short to frame being about the worst, but also shorted filter cap, arcing tube socket, etc.

[Amp Kat]

Pull the power tubes and put new fuses in. Fire it up and see if the HT blows again. If it does you have a power supply problem and the caps are most likely the cause. Check you output transformer quick check : pull the power tubes and put your meter on ohms check from the two outer sockets pin 3 black lead of one socket to pin 3 red lead of the other outer socket. If you read anything from 25 to 150 ohms it's good. If the meter blinks real fast displaying OL then it's bad.

[MitchK]

Ok I'm going to go try this right now, but I'm curious about two things:

What exactly am I checking when I do that pin to pin test? whats the idea behind it?

and this is a method to check the output transformer, but the B+ comes from the power transformer, is there any way to check if its blown? what about the caps? Even if they aren't blown it would be nice to know how to check them out.

[MitchK]

UPDATE: Just went to the jamspot for a minute, didn't have time to put in different tubes and fire it up but I measured the resistance from pin 3 to pin 3 on the outer sockets. No power running through the amp, I'm assuming it was supposed to be off since we're measuring resistance? I got a reading of 2 KILO ohms even. please don't tell me my output transformer is dead.

[Tom Phillips]

Mitch,
Sorry but 2,000 Ohms is way too high for the DC resistance of the plate winding of the PT.
Sounds like a bad OT unless there is some other weired wiring (or meter) problem.
Tom

[MitchK]

not good.

I'm confused though what am I measuring am I measuring the resistance of the B+ winding on the PT or something on the OT, which one is bad?

[Tom Phillips]

The test described by Amp Kat in his 12-01-2006, 09:11 AM message measures the DC resistance of the output transformer primary winding. It's a quick in-circuit test that measures just one parameter of the output transformer. Unfortunatly, the 2000 Ohms you measures is way too high and indicates that something is wrong with the OT or the way it is hooked up.