jcm800 2204 50W - blowing tubes

[MikeH]

I'm working on an amp for a friend of a friend here, this is the story:

-Amp was run without a load (actually he said, he plugged the cab into the DI accidentally, never the less, no speaker plugged in)
-One power tube socket arced, both tubes burned up. HT fuse blew.
-Had his EE friend (not an amp guy) help him replace the melted socket - I checked the connections they made, all are correct
-They then fired it up again, and he said "The tubes just burned up". HT fuse blew again. He swears they had a cab plugged in that time.

Now I'm thinking the OT is toast, I measured the resistance across the taps on the OT, and I get around 40 ohms. Which is reasonable from what I read, but I don't know if it's correct for this amp. The OT is a drake. All leads have a very high resistance to ground (Hundreds of Ms). Nothing glaring at me that says the OT is dead. Any other simple checks I can do?

I took the following voltages with no tubes in, and using a halogen bulb current limiter (Voltages were very high without the limiter and no tubes, so I have been using the limiter at all times)

Plates (pin 3): 498 volts
Outer taps of OT also 498v
Center tap: 498v
B+=499v

I get 499 and 500v on pin 4 of each tube socket. Higher than the B+, but probably just due to voltage fluctuation at time of measurement.

Bias voltage pot ranges from -38 to -46, which is not to spec according to schematics, but could that be due to no tubes and using a limiter?
I checked all resistors on and around power tube sockets, as well as the bias section, and all were within tolerances- although the bias pot was only reading 18K, as opposed to 22K.

HV secondaries on the PT are at ~720v Ac
LV secondaries are at 6.9v ac (high but still reasonable, yes?)

No shorts in filter caps or rectifier diodes.

I'm a little unsure how to proceed; I have replacement sockets which I'll be putting in, but I want to make sure I don't cook any more tubes- any advice?

[MikeH]

Something occurred to me 2 seconds (literally) after posting this - I did not measure the choke - what kind of resistance should I see across the choke? Its around 100 ohms

[Prairie Dawg]

Are they a known good set of tubes or maybe related to the problem initially? When tubes brew up it is usually a bias related problem.

[big_teee]

Voltages seem pretty high.
Here is the voltage chart I made on my 2204 Clone, I made last year.
Fire it up with the Dim bulb tester.
Tube Voltage Chart-2.xls
Good Luck,
Terry

[MikeH]

Are they a known good set of tubes or maybe related to the problem initially? When tubes brew up it is usually a bias related problem.

Now that I think about it- he tried 2 sets of new tubes. After the first blow up, he tried using a new pair of tubes in it before replacing the socket. So it has cooked 2 new pairs of tubes before it got to me.

[MikeH]

Voltages seem pretty high.
Here is the voltage chart I made on my 2204 Clone, I made last year.
Fire it up with the Dim bulb tester.
[ATTACH]17535[/ATTACH]
Good Luck,
Terry

Thanks Terry; I'll have to check it out tomorrow because my computer at home is a mac and I don't have excel or anthing that will open xls files :/

Generally voltages run a lot higher without tubes in the amp, so I'd expect them to be higher in this case

[big_teee]

Thanks Terry; I'll have to check it out tomorrow because my computer at home is a mac and I don't have excel or anthing that will open xls files :/

Generally voltages run a lot higher without tubes in the amp, so I'd expect them to be higher in this case

How about Adobe?
T
Tube Voltage Chart-2.pdf

[MikeH]

Thanks for the file- so looks like my voltages are not that much higher; around 10v on average. That seems perfectly reasonable to me with no tubes pulling any current.


I'm at a loss here... any advice on what to check next?

[jrfrond]

Well, if it was on my bench, I'd clip in my test OT to confirm my thoughts, which in my head is a burned PT. I'd connect the output of the test OT directly to a load, bypassing the impedance select switch.

If there was enough transformer saturation to cause a socket meltdown, it's a good bet the OT is cooked. I'd also replace the OTHER socket as well, plus the SG resistors, grid stoppers and grid leak resistors, just to be on the safe side. Tubes are WAY too pricey to use as fuses!

[MikeH]

Well, if it was on my bench, I'd clip in my test OT to confirm my thoughts, which in my head is a burned PT. I'd connect the output of the test OT directly to a load, bypassing the impedance select switch.

If there was enough transformer saturation to cause a socket meltdown, it's a good bet the OT is cooked. I'd also replace the OTHER socket as well, plus the SG resistors, grid stoppers and grid leak resistors, just to be on the safe side. Tubes are WAY too pricey to use as fuses!

Thanks - What makes you think the PT is bad? I'm getting normal voltages. I don't have an OT to use for testing, how would using a test OT confirm a bad PT?

I too suspect a burned up OT, but usually the resistance on the windings or voltages on the taps get pretty screwy when that's the case. This one seems normal to me.

[g1]

I'm pretty sure he meant OT.

[Enzo]

Resistance is only one tool in the kit. A single shorted turn on a transformer is enough to make it not work right, but you;d never detect that with an ohm meter. Likewise, arcs between windings can easily occur, but of course by their nature they only happen at voltages your meter cannot provide, so again, an ohm meter never detects such problems.

John has the idea, clip in a test OT. All we want to know at this point is whether the OT is causing the problem or not. SO if we substitute in just about any good OT, it will tell us that. I don;t care at all about impedance mismatch, fidelity and power transfer are not the issue for this test. I have an old Fender Bassman OT sitting nearby. I use it for everything. Disconnect the old OT and clip the test one in its place. I don;t even worry about the secondary - couple clip wires directly to a bench speaker. Leave the original secondary still wired to the output jacks. And the NFB won;t be missed for this test.

[MikeH]

Resistance is only one tool in the kit. A single shorted turn on a transformer is enough to make it not work right, but you;d never detect that with an ohm meter. Likewise, arcs between windings can easily occur, but of course by their nature they only happen at voltages your meter cannot provide, so again, an ohm meter never detects such problems.

John has the idea, clip in a test OT. All we want to know at this point is whether the OT is causing the problem or not. SO if we substitute in just about any good OT, it will tell us that. I don;t care at all about impedance mismatch, fidelity and power transfer are not the issue for this test. I have an old Fender Bassman OT sitting nearby. I use it for everything. Disconnect the old OT and clip the test one in its place. I don;t even worry about the secondary - couple clip wires directly to a bench speaker. Leave the original secondary still wired to the output jacks. And the NFB won;t be missed for this test.

I see... I suppose I SHOULD have a test/emergency spare OT on hand anyway. I guess it's off to ebay to track down the cheapest 50w OT I can find...

[big_teee]

I see... I suppose I SHOULD have a test/emergency spare OT on hand anyway. I guess it's off to ebay to track down the cheapest 50w OT I can find...

I put classic Tone transformers in my 2204.
About the cheapest thing going.
Marshall Style 50W Output Transformer 4/8/16 ohm 40-18025 - Amp Parts Direct
Paper bobbins made in Chicago.
Good Luck,
T

[R.G.]

Pulse-inductance shorted turns tester is cheaper if you have to go buy a spare.