jcm800 2204 50W - blowing tubes

[MikeH]

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

RG - thanks so much for the suggestion... er, now what the hell is that exactly?

I'll just go ahead and google it.

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

Thanks for the tip - this is actually the one I'd had in mind; I've seen them on ebay for 55 bucks.

[MikeH]

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

Is this what you mean? blueringtesterresult

[jrfrond]

I'm pretty sure he meant OT.

Oops! Yeah. The "O" and "P" are right next to each other, and I am far from a virtuoso on the QWERTY keyboard.

Anyway, substitution is a valid troubleshooting technique and the only real way you will find out if that's your issue. Just about every tube amp tech has a transformer sitting close by for subbing. Mine even has alligator clips soldered to the leads, which makes short work of clipping it in.

[g1]

Is this what you mean? blueringtesterresult

http://www.geofex.com/FX_images/xform_test.gif

[MikeH]

http://www.geofex.com/FX_images/xform_test.gif

What kind/value of neon bulb do I need for that? I thought neon bulbs needed hundreds of volts to turn on, like the ones in fender-style vibe circuits.

[Gtr_tech]

Neon ionizes at around 70v. NE-2 neon lamps are cheap and easy to find.
Neon Lamp with Resistor (Pkg of 3)-The Electronic Goldmine

[MikeH]

Neon ionizes at around 70v. NE-2 neon lamps are cheap and easy to find.
Neon Lamp with Resistor (Pkg of 3)-The Electronic Goldmine

Oh.... I.... See.... Thanks!

[Gtr_tech]

It might be easier to use a panel mount neon assembly like this:
Solico Orange "Jewel" 120VAC Indicator-The Electronic Goldmine

Thats what I did....

[MikeH]

In the interest of time (I'd like to get this together tonight), would the typical neon assembly from radioshack like this work: 120VAC Neon Lamp Assembly (2-Pack) - RadioShack.com

[52 Bill]

In the interest of time (I'd like to get this together tonight), would the typical neon assembly from radioshack like this work: 120VAC Neon Lamp Assembly (2-Pack) - RadioShack.com

Maybe. All neon lamp assemblies that are designed to work on 120 volts have a resistor in series with the neon lamp. It may be that the RS units have a 47k resistor inside there or it could be a 100k. So it might work perfectly or it might not. In any case don't add another 47k series resistor to the circuit.

[Zer09]

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?

Mike,
I'm an expert by no means but 30 years of playing have tought me a couple things....if you leave an EL34 amp unloaded while playing you will cook the screens (hopefully before the OT), and if an EL34 fails, check the screens and screen supply. It's simple and cheap to check and may save some aggravation. Learned this the hard way. I say this because I see no screen voltages in those that you gave, your other voltages are high, and it's the weak link on an EL34.

[MikeH]

That would be pin 4 correct? It's listed above- although those measurements were with no tubes installed.

[Zer09]

That would be pin 4 correct? It's listed above- although those measurements were with no tubes installed.

Saw that this morning. Your numbers seem so high to me (even unloaded) because the 800 I worked on earlier this year for my brother was an earlier one with the 345v PT secondaries and had a much lower b+. His voltages were more in line with the original schematic (365v on plates). Apparently they started using higher voltage PT's at some point.

[big_teee]

Saw that this morning. Your numbers seem so high to me (even unloaded) because the 800 I worked on earlier this year for my brother was an earlier one with the 345v PT secondaries and had a much lower b+. His voltages were more in line with the original schematic (365v on plates). Apparently they started using higher voltage PT's at some point.

If you look at my chart above, mine are as nearly as high as his with the tubes in it.
My B+ reads 510v In standby, and 492 with a load.
He should be fine.
I would be tempted to put the tubes in it and fire it up with a Variac, or Dim bulb tester, and read the voltages that way.
T

[Zer09]

If you look at my chart above, mine are as nearly as high as his with the tubes in it.
My B+ reads 510v In standby, and 492 with a load.
He should be fine.
I would be tempted to put the tubes in it and fire it up with a Variac, or Dim bulb tester, and read the voltages that way.
T

Yeah, it looks like most are that way. Only the very early ones had the lower voltage PT. I agree about the variac. It would also be nice to find out how long it took for the tubes to "burn up". Was he playing, did they bias the new tubes, etc.. If it took some time I'm wondering if something was damaged in the bias circuit and he lost bias. What kind of voltage range do you get from your bias pot? That seems like an awfully narrow range that he gets (-38 to -46).