Blew up my speaker during repair test, why?

General discussion area for tube amps.

Moderators: pompeiisneaks, Colossal

User avatar
sonicmojo
Posts: 669
Joined: Thu Mar 03, 2011 3:41 am
Location: Oahu, HI

Blew up my speaker during repair test, why?

Post by sonicmojo »

Scenario is:

I just replaced the PT in a JTM45 reissue with lots of previous mods.

Powered up with light bulb limiter and speaker attached, no tubes, looks good.

Add tubes. Looking okay but light limiter starts to get brighter. I realize that the bias supply is not connected to the transformer, power tube bias on my meter are acting crazy so I fix that connection.

Try again, light bulb limiter looks good, no sound. V2 is not lighting up so I fix that (solder heater joints).

V2 lights up, no sound anywhere whatsoever.....

I've been smelling a slight burnt resistor smell so go sniffing around and realize my 10" EV Force 10 bench speaker voice coil is crispy fried. Was great for 8 years of testing......

Since, I have hooked the amp to another speaker and it works fine with sound now. Where could I have blown up my speaker? I never got any sound out of the amp before the speaker died but I was poking around a lot measuring things. Could I have had some DC get to the speaker somehow? If so, it had to be quick.
---------
Bryan
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

Re: Blew up my speaker during repair test, why?

Post by martin manning »

Ultrasonic oscillation?
User avatar
sonicmojo
Posts: 669
Joined: Thu Mar 03, 2011 3:41 am
Location: Oahu, HI

Re: Blew up my speaker during repair test, why?

Post by sonicmojo »

I think that was probably it. My bias probe was going wacky for several seconds before I shut things down at one point. This was before I fixed the bias supply. I never heard anything. I should've measured the bias voltage before hooking up tubes....
You do not have the required permissions to view the files attached to this post.
---------
Bryan
User avatar
roberto
Posts: 1841
Joined: Tue Nov 27, 2007 4:45 pm
Location: Italy

Re: Blew up my speaker during repair test, why?

Post by roberto »

martin manning wrote: Wed Sep 27, 2017 12:27 am Ultrasonic oscillation?
+1
User avatar
sonicmojo
Posts: 669
Joined: Thu Mar 03, 2011 3:41 am
Location: Oahu, HI

Re: Blew up my speaker during repair test, why?

Post by sonicmojo »

Maybe the presence circuit was part of the culprit. Somewhere in my chain of debugging, I realized that the wire to the presence pot was disconnected so there was no ground on the PI tail. Perhaps some excessive DC voltage went directly to the OT tap through the negative feedback circuit and got the speaker that way.....

I know there will be some DC voltage on the presence pot in the JTM45 circuit. What should I be seeing when it is operating correctly, about 6V? This modded amp has a 100K negative feedback resistor and an additional trim pot on the back, I'm assuming it is a mod to adjust amount of negative feedback but I've disconnected all negative feedback at the moment until I'm sure of the presence circuit and presence pot is operating correctly.

As an experiment to see the voltage when it was wrong, I disconnected the presence pot again and I see upwards of 200V being fed to the negative feedback. Unless something else is wrong too, that might be what happened in addition to my bias issue. I think it may be the combination of the two things together. Do you think it is possible (in this kind of JTM45 circuit) to blow a speaker if the presence pot somehow gets ungrounded or otherwise fails. At the same time I'm pretty sure the PI shuts down so there wouldn't be a lot of signal going through, but a lot of DC voltage has the potential to go right into the OT/speaker.
---------
Bryan
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

Re: Blew up my speaker during repair test, why?

Post by martin manning »

I don't think the scenario you describe would do it. The ground path for the PI tail would be through the 100k FB resistor, the trim pot, and then through the OT secondary (~2R) in parallel with the speaker voice coil (~7R). You will see a high voltage at the top of the presence pot, but there will be very little current flowing through the FB circuit, and most of it will flow through the OT secondary.
User avatar
Littlewyan
Posts: 1911
Joined: Thu Sep 12, 2013 6:50 pm
Location: UK

Re: Blew up my speaker during repair test, why?

Post by Littlewyan »

How could the oscillation do it though? If the speaker had the correct wattage rating then how could the amp blow it?

This reminds me of people saying you could blow the speaker by touching the end of the guitar lead, but I just can't see how if the speakers can take the wattage of the amp?
Stevem
Posts: 4551
Joined: Fri Jan 24, 2014 3:01 pm
Location: 1/3rd the way out one of the arms of the Milkyway.

Re: Blew up my speaker during repair test, why?

Post by Stevem »

X2 ultra osc!
Your lucky your output tubes did not melt down from the no bias condition otherwise those would be on your replacement list also!
Just out of curiosity, what took out the previous PT?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
User avatar
sonicmojo
Posts: 669
Joined: Thu Mar 03, 2011 3:41 am
Location: Oahu, HI

Re: Blew up my speaker during repair test, why?

Post by sonicmojo »

Stevem wrote: Wed Sep 27, 2017 12:57 pm X2 ultra osc!
Your lucky your output tubes did not melt down from the no bias condition otherwise those would be on your replacement list also!
Just out of curiosity, what took out the previous PT?
I think I am lucky I didn't do worse. This is a repair for a friend of a friend so I have no history. The amp had EL34s in it which probably is too much so maybe that did in the transformer. Going to put 6L6s in it instead. When the amp came to me, the bias supply and presence pot were disconnected and I should have noticed. I dropped in the exact replacement transformer being careful to get the wires right without any documentation I could find about it. This tranny has no wires except the black ones for the 5V. I had to special order it so it took over 3 months to get.

It was a Mark Cameron (name on chassis edge) modded amp at some point, lots of replacement parts, etc. Very messy inside. must have been an experimental amp with lots of changes over time. Some pots are push-pull but nothing connected to the switches anymore except residual solder. Probably sitting and been dead for years before I got it to fix. I don't like to repair modded amps but my friend wants to trade the work I do for car work from his other friend....

Just replaced the presence pot which was very, very noisy but I think I have the amp working mostly right again. Not going to try to figure out all the differences from stock....
You do not have the required permissions to view the files attached to this post.
---------
Bryan
R.G.
Posts: 1223
Joined: Tue Dec 02, 2014 9:01 pm

Re: Blew up my speaker during repair test, why?

Post by R.G. »

The obvious answer to self-oscillation is a scope, but that's a very finely honed and ornate axe when maybe a machete would do.

I've thought a couple of times about making a circuit that would detect signal above cutoff frequency, perhaps audio, perhaps 10K or so, or maybe just all signals over 60 Hz, and light a light if there was anything there. You'd clip on a resistor dummy load and this tell-tale before risking a speaker. Probably good enough for many tasks.
User avatar
sonicmojo
Posts: 669
Joined: Thu Mar 03, 2011 3:41 am
Location: Oahu, HI

Re: Blew up my speaker during repair test, why?

Post by sonicmojo »

This amp is now done and gone now. I thought I was being careful with the limiter as I would normally do especially after a transformer change, plus the fact this amp was a messy unknown quantity. Most times I just plug repair amps without any limiter into my trusty EV Force 10 - hundreds of amps over the last 4 years or so. An oscilloscope and dummy load to check for oscillations is not my first step for sure but I hear you on that.

If I had simply checked the bias voltage before plugging in the tubes, this probably would not have happened. Lesson learned. For sure this is the first amp I fired up on speaker without the bias supply connected. I've seen some bias supplies that are shorted out due to a bad cap but that probably prevents other things from occurring - such as fried speakers.

In this case also I can't help but wonder if the limiter saved the fuse and killed my speaker instead.....

Done, new speaker cone kit ordered. Let's forget this ever happened...
---------
Bryan
User avatar
TUBEDUDE
Posts: 1656
Joined: Thu Jan 04, 2007 5:23 pm
Location: Mastersville

Re: Blew up my speaker during repair test, why?

Post by TUBEDUDE »

It lived a good life. Was the spider worn out also, or was that just a byproduct of the coil removal?
Tube junkie that aspires to become a tri-state bidirectional buss driver.
User avatar
sonicmojo
Posts: 669
Joined: Thu Mar 03, 2011 3:41 am
Location: Oahu, HI

Re: Blew up my speaker during repair test, why?

Post by sonicmojo »

The spider was okay. i wasn't very neat about removal. It will live again!
---------
Bryan
Stevem
Posts: 4551
Joined: Fri Jan 24, 2014 3:01 pm
Location: 1/3rd the way out one of the arms of the Milkyway.

Re: Blew up my speaker during repair test, why?

Post by Stevem »

These amps power supply's where made for 6L6 type tubes, so if 34s where ever in there with there need for a full 2 amps more of heater current, then that's what smoked the PT!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
User avatar
67plexi
Posts: 780
Joined: Sun Apr 09, 2006 8:34 pm
Location: Juneau Alaska

Re: Blew up my speaker during repair test, why?

Post by 67plexi »

Not sure but I had a rash of fried voice coils this week Bass amps all new speakers used 4-6 times We make you great speaker for you in China.................

Thank you Ampeg. :P
Post Reply