Aus.'70's 180w tube amp-what tubes won't blow?

[rodl2005]

Hi I'm a new member here, so HELLO to all! As said in the title I've an Aus made '70's tube P.A. amp I've been using as a bass amp for 15 yrs- beautifully too BTW- recently I bought a set of EH EL34's(amp takes 6 power tubes) so I thought after sending to my tech to bias & check over , all'd be sweet, but 1 & 1/2 gigs later the thing blows a fuse- the tubes can't take the nearly 700V to heat the plates(I'm NO tech BTW). Anyone have any reccomendations on what I should do?
one- reccomended putting in 6550's-four of 'em I s'pose, would nearly match the output of the 6 EL34's?
One-reccomended - new tube- KT77 work as a direct drop in for EL34's & handle 800V- but I thought EL34's were s'posed to handle 800V??
WHAT TO DO??? what to do?? PLEASE HELP!!!!

[UnklMickey]

hi rodl2005,

welcome to GuitarNuts2


this sounds a bit scary to me.

tube ratings are a bit etherial, but i believe EL34s are rated for 700v.

you will be hard pressed to find a tube that has a higher rating.

if you do, and the same outfit also has EL34s, they will be rated for more than 700v.

if you are blowing fuses now, it may already be too late.

perhaps one of those tubes was poorly made, and shorted internally after a short time of use.

replacing one defective tube might be all that is required.

or perhaps when it shorted, it also damaged your output transformer.

you might have shorted the windings in either your power transformer, or your output transformer.

hopefully there may be a less fatal cause.

many amps have solid-state diodes, used to clamp the anodes of the tubes. either preventing them from going below ground or above the P.S. voltage.

if one of these fails, it will cause your mains fuse to blow.

a couple of bucks for new diodes is all it takes to fix that problem.

also, i believe the EL34 requires 1.5 A of current for the heater.

many octal based pentodes and beam-power tubes only require 0.9 A.

if the circuit was designed for one of them, your power transformer will be working overtime, just running the heaters.

you would be using about 57 watts for the heaters, instead of 34 watts.

this may have caused a failure in your power transformer, or, if you are lucky, the increased load on the primary, combined with the normal load required at high volume output, added up to more than the fuse could handle.

if that is the case you got a second chance to get it right before something gets toasted.

180w out of 6 EL34s ?

sound pretty aggressive to me.

120w seems about right, but sometimes manufacturers push things past reasonable.


(what you didn't mention:)

what kind of tubes was the amp originally designed for?


unk

[rodl2005]

The amp was originally made for EL34's as they are/were an Aussie copy of Marshals! It's prob. more 150 w than 180w sorry Torres eng. (US based tube retailer) reccomended the KT77's ?? Any thoughts.
Thanx for your reply also.
cheers, Rod.

[rodl2005]

Whoops 4got 2 say. A diode blew after a faulty valve/or weak valve/tube. is what hapened. I just want to know what tubes(read modern day ones) will handle the amp so I can use her! Tis a lovely ol' thing. Don't mind putting KT77's in her or as one suggested-trying four KT88's(6550's)?? thoughts?
Rod.

[UnklMickey]

hi Rod,

so you've been using this amp successfully for 15 years.

then you change to EH brand tubes and have your tech re-bias.

and it fails on the 2nd gig.


now you want to change tube types.


maybe.


but i'm inclined to think the failure was cause by:

A -- a poorly made tube. (possibly all of them, because of the manufacturing techniques. possibly just one, because it got past quality control.)

OR

B -- your tech biased the amp too hot.



i think i'd have chosen JJs over EHs, but that's just my preference.


i don't have any experience with the KT77s.

but if the cause of your problem was that the amp was biased too hot.

and your tech biases it too hot with the KT77s?............................

[rodl2005]

No It was a BAD tube!!!! which in turn caused a diode to go?( as I said I'm no tech- & I may have gotten this muddled up- but IT WAS A BAD TUBE!!!this much I know) & I don't neccesarily WANT to change tubes/types...... I just want something that'll work in the amp! As I said- it ran fine for 15 yrs(5 then I got it serviced & new tubes, then 10 yrs-not as much use -& brings us to a few months ago) It wasn't sounding as I remembered so I checked the tubes for microphonics etc-it'd been 10 yrs, so I thoght I'd get a set of new tubes for her. Couldn't find any Philips tubes(which were in it B4) so I got what I thought were OK for the amp- a matched set of 6 X EH EL34's. Then after a short time it blew a fuse, & another --- so Back to the tech.
I doubt he would've biased it too hot- he's been working on these sorta things since approx 1970. & has always been OK B4.
thanx for your reply tho! All info I get is greatly appreciated & helps me make choices down the line, so again.....thanx.
Rod.

[UnklMickey]

rodl2005 said:

No It was a BAD tube!!!! which in turn caused a diode to go?....

okay Rod,

but the real question here is:

why was that tube bad?

was it just a dud that got past Quality Control?

are all EH brand EL-34s more likely to fail than the Phillips EL-34s you used successfully for so long?

would you have better luck with JJs?

do you need to change types?



gee, i guess that's actually 5 real questions here.

but if you knew the answer to the first one, it would also answer most/all of the others.



unk

[sumgai]

Ah, time to chime in here.

First, Rod..... to these here forums!

Next, unk is right (as usual), you don't need to go skipping all over the map to find tubes that will work correctly in your amp's design. Phillips was good, but they are no more, sad to say. That means that you get to play the Search game, something that nearly all guitarists do now.

Whether you take someone's advice about a particular tube type (or not), the main thing to remember is, no one tube type, be it 6L6GC, or 5881, or whatever, is built by every factory in the exact same manner. In fact, just the opposite is par for the course - you can line up a 6L6 (or a pair of them) from Sovtek, JJ, Ruby, EH (a rebranded tube), or GrooveTubes (ditto), and you'll find some striking differences.

For one, there is no hard and fast requirement to adhere to the original design specs, as published and endorsed by the big names of yesteryear. (GE, Sylvania, Mullard, Telefunken, RCA, etc.) So even if the book says the plates should be able to handle 600 volts, there's no guarantee that the tube you snatched up at GuitarCenter will indeed be able to handle 600 volts.

Which brings us to other tube types. For better and/or worse, there is a great number of "equivalent" types out there, and the designed differences are small. IMO, said differences were made strictly for marketing purposes, and did nothing to set the tube apart as being particularly noteworthy. Such is the case with KT66 and EL34 types. For all practical purposes, they are the same thing, they just came from different companies, that's all.

But what about other types? Fair question, let's take a look.....

6L6 and 5881 are essentially the same thing, the latter being a "militarized" version of the former. More rugged, or so they said.

6550 and KT77 are again about the same thing. A little more stout, they should have higher ratings across the board.

The granddaddy of them all, the KT88. No equivalent here, this one was good for 145 or 150 watts a pair, in Class AB₁. But the heater current was as great as the entire current draw for a Deluxe Reverb running full out!! The transformers were massive, to say the least. I believe that the Marshall Major used a set of these, but I can't recall now, and I don't seem to have a schematic to hand. Don't pick up one of these amps without a doctor and a chiropractor standing by!

(Just checked on the 'net, yes, the Major used a quad of 'em, rated the output at 200 watts. Mercy me, that's a lot of oomph!)


The blown diode was a protection device meant to keep your transformer from blowing (via what we call a "crowbar" circuit). Be glad it went 'poof', and not your OT. But be aware that something in the amp itself could have caused the tube to go bad, after it had been used for a short while. After all, it is 25 or more years old, so just for drill, have your tech give all the capacitors a good going over. I'd replace the power supply filter capacitors out of hand, right now. Cheap at this stage of the game, before they take out some other expensive component too.

On today's London Exchange Market, the foregoing is worth 0.0062 pounds Sterling. ;D

HTH


sumgai

[rodl2005]

Thanx SUMGAI yep thats pretty much what I had worked out re the AMP & the tubes! The old & new CL -SVT by Ampeg uses 6 x 6550's for 300w, & the new Marshal(by new I'm meaning out now -not in the '60's, 70's etc) VBA 400 uses 8 of 'em for 400w
So thanx for your info & for the welcome. No doubt U will hear from me sporadically-when I get time & or troubles & or new gear. It's great to be able to get info from regular musos etc-NOT SALESMEN!!!!! !!!!!!!!! Where I live(Tasmania- Australia) it's an Island state-so we're isolated from seeing/trying lotsa different gear, so these forums help a HEAP!!! ;D
I'll keep trying with the AMP- it's at the techs house now!! Will let U know how she proceeds as it goes.
Cheers,
ROD.

[rodl2005]

Well, it's been a while but I saved up some $$$ & sent it to Eurotubes-USA- & bought a sextet of JJ KT77's !!! They arrived here in Tasmania, Aus. in 1 week from me ordering online!!!
Faster than most things get around from capital cities in this country!!!

I had my tech - excellent tube tech too- Pete Hutchinson- Bias it t'other day & I gave her a run last night!! All worked fine & dandy- sound was good too- possibly a li'l more punchy than B4-to be expected tho!
Will write another post after a gig or two & review the JJ KT77's.

I'm just REAL glad to have my "ol' girl" up & running again!!

Cheers,
Rod.

[rodl2005]

The JJ KT77's are running reliably & sound great. Eurotubes reccomended HIGHLY!!!