did I blow my PT? - help!!!

[HTH]

Ok, I've had this '76 Marshall Superbass head for around a week, played it at rehearsals for the first time tonight. just playing it at low volume ('2' on bright channel) and the mains fuse quietly blows and white smoke floods out the top of the amp vents.

Some of the caps are bulging but it's not apparent whether it's one of the caps that died or the PT.

What quick and dirty tests can I do to see if the PT is dead? (PT was still warm after a 45 minute drive in the back of my car).

... Ian

[496dart]

see my post called--What causes this ? .


replace the fuse and unhook all the leads off the secondary side of the PT. then fire it up and see if the fuse blows and/or it starts humming / smoking. Be ready to kill the power of course .

[HTH]

Unhooked all the secondaries - mains fuse blows. I'll be on the phone to Marshall in the morning (probably the afternoon since I'm on holiday, heh).

Looks like I'm down £50 on a new PT only a week after buying this amp - what rotten luck eh?

[stokes]

Plan on changing those bulging caps before you put a new PT in there,it may not be apparent to you but they are shot and could very well be the cause of your first tranny going south.What about your HT fuse,why didnt that blow?Make sure it is even in there and somebody didnt replace it with a much too high value or bypass it somehow.There is usually some other problem in the amp that causes the PT to go south,and bulging caps are "bad caps" no doubt about it and no need to test them any further,if they bulge they are done.The test 496dart told you to do tells you for sure your tranny is shorted,if you read his post through you'll see hes been there.But before you put a new tranny in there you need to find out why it died,and those caps are more than apparently bad they are an obvious problem.That is an old amp and needs to be gone over completely,the fact that the HT fuse didnt blow is very unusual,and makes me wonder if it is even there.Try to find a schematic for the amp and make sure something wasnt changed especially fuses.

[HTH]

thanks for the reply Stokes, the fuses are the correct types as marked on the amp (2A mains and 1A HT).

I'll be changing the filter caps before the new PT goes in - it's a 30 year old amp afterall.

About the HT fuse though - I'm used to a 500mA rating here. do you think 1A is too high?

[Amp Kat]

Good info from above and do yourselve a favor and don't just change the bulging caps but change all of the electrolytic caps you can find. Also check for open power resistors and verify their values are correct. Check the inside of the tube sockets and make sure there is no arching marks and or on top of the sockets. If there is replace them to as it could also take the new tranny out. It would also be a good idea to use a variac when bringing it back up and do voltage measurements to the plate supply,bias and heaters instead of the quick inrush from 120 that could fry something before you have a chance to turn it off. Usually it will blow the fuse first before doing damage if there is still a problem but not always depending on the problem.

[HTH]

hey Amp Kat,

no arcing on the output tube sockets - they seem fine, could do with retensioning a little though.

should've replaced all the filter caps from the get-go when I got this amp (doh!).

The PT was a replacement in the first place - there is a possibility it was underrated from the start.

[496dart]

What ever you do , dont buy a marshall replacement power trans from tubesandmore.com . Ive had 2 in a row that seem to be over-rated . Great website , service and all, but i think they had a bad batch or.......

[HTH]

I'm in the UK - I'll be buying directly from Dagnall (if they do that) or from Marshall (when they re-open after xmas)

[Steve Conner]

500mA seems to be the usual rating for 50 watt amps. The Superbass is 100w (IIRC?) so 1A probably isn't that excessive.

Also, those four EL34s probably draw quite a lot of heater current. I think the most likely explanation for the PT death would be that the heater winding was underrated (see 496dart's thread...)

If the power tubes were badly biased and drawing too much, you'd notice red plating and bad smells at least.

If a bad filter cap was leaking and drawing a lot of current, it would probably burst and make a huge mess, or blow the HT fuse, long before the PT was damaged.

[HTH]

Steve,

I biased the amp when I got it - had previously been running VERY HOT, some of the EL34s were drawing 80mA at idle. I'm surprised those EL34s lasted at all.

I've had an offer from a very nice chap I know on the Plexi Palace bbs to sell me an original 70s JMP-era PT for my amp. may well go for that to keep it all original (the PT that blew was a replacement anyway).

[stokes]

"If a bad filter cap was leaking and drawing a lot of current, it would probably burst and make a huge mess, or blow the HT fuse, long before the PT was damaged."

Gotta disagree with that Steve.If a cap is leaking and the HT fuse doesnt blow,depending on the quality of the PT the added stress of the extra current the cap is leaking could slowly stress the PT till it shorts.His caps are bulging,which is a sure sign they are shot and could explode anytime,but in the meantime you can be sure they are leaking pretty bad.His HT fuse didnt blow,to me that is suspect.Cant find a scem for this amp,but I have seen 500ma HT fuses in 100watt amps,so that 1amp may be too big.The 2amp mains value seems too small for a 100watt amp as well.

[HTH]

Stokes,

I must admit that I'm used to a 3A mains fuse and 500mA for the HT but this amp does have 2A for mains and 1A for HT (see picture attached) and the markings appear to be from the factory.

The PT was not a factory unit though (PT had already been replaced) so it may have been less robust than the original (?).

... Ian

[stokes]

Assuming it is set up for 220vac,the 2 amp would be correct.It is a 4amp here in the land of 120vac.Keep forgeting about the house current over there.Still cant see why the HT would be 1amp tho,but that is what it says.As for the old PT, it apparently wasnt robust enough for whatever the amp was drawing.Be sure to do a complete cap job and check the power supply resistors.Change the caps in the bias supply as well and check those resistors to be sure the bias supply is good.Its an old amp a good cleaning of the sockets and pots wont hurt either,not that dirty components are what fried your PT,but it is probably overdue for a good cleaning.Let us know how you make out with the new components.New caps are really gonna tighten that amps response a ton.Use the best quality caps you can get,I'm not sure what is available to you over there,but stay away from those cheap Asian made electrolytics,they all suck.

[HTH]

wow - new caps are gonna tighten up the amp more than it already was. this amp was already punchy as hell which is why I didn't think of a cap job straight away.

anyway, I'll be giving it the complete works when the new PT gets here.