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**stokes** · 12-10-2006, 05:05 PM

The current production Celestion vintage 30 is a 60watt speaker.A 100watt combo into a 60 watt spkr.is surely going to blow the speaker.Do it enough and the OT is likely to go next.I have found Marshall OT's to be less tolerant than others.I think the OT should be disconnected and tested thoroughly.I would also recomend not trying to force the 60watt speakers to handle the 100watts that amp is capable of putting out.

**Teleman55** · 12-10-2006, 05:18 PM

The owner told me that on the road he uses 4X12 extension cabs,the speaker that is in ther now is a Mesa Black Shadow Celestion-90wt.By the way sorry for posting here I meant to be in the repair forum.

**stokes** · 12-10-2006, 06:51 PM

Your original post only mentioned 1X12 Celestion Vintage 30.Where exactly is it blowing the speakers?If it is amtter of him using a severe impedance mismatch any damage would more likely occur in the amp,rather than blowing a speaker.You are going to have to give more specific details as to what he is using and what is blowing.

**Teleman55** · 12-11-2006, 02:52 AM

Thanks Stokes; I spoke to the owner tonight here is what I know.Has play the amp for years with the Vintage 30 that it came with.During a gig the amp died and he noticed a tube fault LED was lit and he replaced the tubes with Mesa EL34s and the dead speaker with the Mesa 90wt.(he is sponsored by them).He played the amp for a day with no problems,went to a gig and the speaker died and he brought it to me.
I did some more testing,on both my Peavey DB210 and Hot Rod Deluxe I get a resistance measurement at the speaker jack.The Marshall measures .0 ohms.Using a light bulb current limiter shows no short,first with no power
tubes and then with them installed.I checked some voltages(OT plates-490,bias_-46v) and when checking the filament supply I rested my probe on the chassis and found 231mv of AC,checked the speaker leads and found the same thing.That is as far as I have gotten today.

**Enzo** · 12-11-2006, 04:17 AM

If the OT speaker secondary winding were shorted, it wouldn't be blowing speakers, it would be putting less out to the speaker. If the OT primary shorted the high voltage to the secondary, it would blow the fuse right now. I really doubt the secondary winding is involved in any speakers blowing.

Ohm readings tell you little. Most output windings measure less than an ohm. Your PV amp is most likely a 16 ohm winding which will show more than a 4 ohm, but still not much. There are relatively few windings on that secondary and the wire is heavy. That is a formula for low resistance. If someone used more turns or lighter wire, then the reading will be higher. It is hard to damage the secondary winding.

Have you seen any of the blown speakers? Cut one open and look at the voice coil. DO all of them if they are available. If the coil is warped, darkened, burnt, then it was exposed to exces power over time. If the coil looks like new, then some impulse more likely killed it.

He may have got away with that 60w speaker and the 100w amp, but did anything change in the band or his rig? Maybe he is turning up more now?

Look for instability in the amp. If the amp goes into RF oscillation, it will be cranking 100 watts of RF out to the load as hard as it can. You wouldn't hear anything, and the speaker can't move that fast. SO all that energy just goes into heating the voice coil.

What model amp is this? Does it have the little 22pf disc cap between pins 3 and 4 of the end power tube - the one closest to the power tranny? See if that is broken, missing, damaged.

Does the guy run straight or does he use effects either in front or in the loop?

**stokes** · 12-11-2006, 03:50 PM

Have you tried the amp with a known to be good speaker?As Enzo pointed out,I would check those blown speakers,if for nothing else to be sure the speakers are actually blown,and there isnt a problem in the amp.Check the output with a moderate signal going into the amp set at a moderate level.If you see an unusually high output,this could indicate an oscillation as Enzo described.If everything in the amp is okay,I would have to say it is just a case of a 60watt or 90watt speaker not being able to handle the 100watts the amp is putting out.Using that amp with one speaker that is under rated for the output of the amp in a situation with a drummer and bass player and whatever else in the band you are certainly going to blow the speaker.It is possible that blowing the speaker a couple of times did damage the OT,but I have to agree with Enzo that a damaged OT is not going to blow a speaker.Check the amp thoroughly and when you are satisfied the amp is okay,tell him to get a 150watt speaker if he is going to use the amp as a combo with full power going into one speaker.

**Teleman55** · 12-12-2006, 03:20 AM

Thanks for the replys-Tonite I plugged in a JBL e120 and played at about 1/4 volume in both High and Low power modes with no problems for about 15-20 minutes.No problems,the amp functioned fine.I will see if he will let me do a speaker autopsy,I have the Celestion 90 in the cab.Ther is no cap on the power tubes.The amp is Model 4101-serial #Y00010-The owner says it came with a parchment attesting to its being the tenth amp manufactured in this line.
I have a Hitachi V-212 20mhz O-scope and a Heath/Schlumberger Audio generator to test with but I have limited experience with them.I could use some guidance.What about the 231mv of AC on the chassis does this indicate a leaking cap?

**stokes** · 12-12-2006, 05:09 AM

How are you reading this ac voltage on the chassis?Where are you connecting the DMM leads?Obviosly one to the chassis,but where is the other lead?I dont think you need the scope or signal generator to check anything.If the amp is okay the problem is running the amp into a speaker that is not rated at least 100watts.To put out 100watts the amp doesnt have to be maxed out.If you have a 150watt dummy load you can put a 1k signal at between .15 and .25 volts into the input and measure the RMS ac volts on the dummy load and you will see where it reaches 100watts.He may have been lucky using the speakers you mentioned but sooner or later putting 100watts into a 60 or 90watt speaker its gonna blow.In that amp I would use a 150watt speaker,at least.At full bore its going to put out more than 100watts.If you dont have a dummy load you could run the test I described with a speaker but it will be quite uncomfortable.Just read the volts across your speaker terminals.If you are using a 16ohm speaker,when you see 40ac volts you have reached 100watts.You must have a True RMS voltmeter,and I dont know if you will be able to stand a 1k signal at 100 watts.I have done it with a 30watt amp and it is brutal.

**Teleman55** · 12-13-2006, 04:34 AM

Stokes-I had my ground connected to a transformer bolt and measured the AC on the center of the chassis and on the speaker leads.I will try your recommendations tomorrow.

**stokes** · 12-13-2006, 04:49 AM

When you connect your lead to the transformer bolt and the other to the center of the chassis you are connected to the same source-both are at ground you are not getting a true reading.I suspect when you touch the speaker lead you are touching the ground lead,again both leads are at the same source.A leaky cap is not going to leak ac voltage,they pass ac and block dc.That .231vac reading you see is a ghost,forget it.

**Enzo** · 12-13-2006, 05:02 AM

Well, they are both places chassis ground, but the chassis is not a zero ohm impedance. That is why we make star ground wiring arrangements. The small resistance in the chassis is still enough to cause a voltage drop at high enough currents. It has nothing to do with grounding caps, but I can believe a quarter volt from one end to the other. After all, ground loops happen when everything is connected to the same ground.

This would be a normal condition.

**stokes** · 12-13-2006, 05:26 AM

Yeah,I see where you are coming from,Enzo,makes sense,but wouldnt that current be more likely to show as dc?But like you say it aint nothing to be concerned with.

**Teleman55** · 12-14-2006, 02:21 AM

I will post again after I get to check the speaker voice coils.I played the amp tonight after letting it warm up and then running a 1k signal into it for about 10 mins..Of course with the family home I can't really crank it up.

**Enzo** · 12-14-2006, 04:00 AM

You need a dummy load, and I don't mean the wife...

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