Can I turn my broken valve jr into a cab for my other valve jr?

Strangefan

Hi all so the output transformers gone on my vj, I managed to procure another fully modded for....... 30 quid  

So can I use my broken on as an extension cab, with two 4 ohm speakers will that be OK... As I dont know anything about ohmage, 

Thanks 

usedtobe

Never mind about that.. £30??? 

Rocker

How or Why did the OP traffo fail in the first place?

Strangefan

@usedtobe people are silly and do collection only, I will happily collect!
@Rocker I would like to know myself!!!

ICBM

Yes, you can use the second one as an extension cab. It's not ideal since it will cause a low impedance mismatch on the amp, but it's harder on the output valve than the transformer.

Why did the transformer blow? Because there is a reason these amps retail for less than the combined cost of the parts!

DJH83004

Harsh but true IC !! the Chinese are able to replicate most things in this world, but they can't seem to build decent transformers 

Strangefan

ICBM said:

Yes, you can use the second one as an extension cab. It's not ideal since it will cause a low impedance mismatch on the amp, but it's harder on the output valve than the transformer.

Why did the transformer blow? Because there is a reason these amps retail for less than the combined cost of the parts!


Thanks
, it' still works just after 5 mins it goes to half volume, if I could find a spare tranny I would repair it but they're literally impossible to find in the UK  

ICBM

DJH83004 said:

Harsh but true IC !! the Chinese are able to replicate most things in this world, but they can seem to build decent transformers 

I wouldn't criticise them too much when we live in the country that made the transformers Marshall used in the DSL/TSL series .

Strangefan said:

it' still works just after 5 mins it goes to half volume, if I could find a spare tranny I would repair it but they're literally impossible to find in the UK  

Keep a look out on Ebay - a lot of people upgrade the OT and some of them sell the original. Or get one of the upgrade ones...

There are quite a lot of other amps that use the same basic spec too, it shouldn't be hard to find something that works.

ecc83

"the Chinese are able to replicate most things in this world, but they can seem to build decent transformers"

Hmm? A fair proportion of my time at B's was spent fitting transformers, OPT and PT, from various mnfctrs* and then driving the bollocks off them. Mains at +10% (253 V for UK) and  pink noise at 3dB under rated power 'forever' . Eventually a fuse blew or a valve (they are shit) failed,THEN a fuse blew!

Twas only in the last month or so of my time there that I had an OPT  fail (in an HT-60 IIRC) . All the traffs met ratio specc very well and passed heat rise tests. I even rigged a gadget to A/B them, live, with the originals and even seasoned, 'golden eared' players could not tell them apart.

No idea of the factory's 10-20s but all the boxes had 'Oriental' script on them.

*Because in business you do not want to put all your supply eggs in one basket!

Dave.

DJH83004

Fair point Dave & IC, I can only go from personal experience, of changing so many failed ones, particularly O/P txs. That said I have also changed more than my fair share of modern Dagnalls   

Strangefan

@ICBM Thats the problem bud, nowhere seems to be selling even upgraded ots to the spec of the VJ, I'll ha e to keep an eye out on the fleabay:) 

crunchman

I think I might still have a Valve JR output transformer lying around somewhere but it's one of the very early ones that had the correct impedance for a 6V6 not an EL84.

I replaced it with a Hammond (125DSE I think) and put it in a drawer somewhere.  If you want it let me know.

Strangefan

@crunchman  I'll happily pay for that thank you, would it  just need a change in valve to work then? 

crunchman

Strangefan said:

@crunchman  I'll happily pay for that thank you, would it  just need a change in valve to work then? 

It works.  It's just a little less than optimal.  If I remember correctly (and I sold the amp years ago) when they first introduced the valve junior they just copied most of it from 50's Champ circuit - including the spec of the output transformer. The problem was that they were using an EL84 power valve rather than a 6V6 so the impedance that it was seeing wasn't correct.

There were a few other issues with the early ones as well, and a whole cottage industry grew up modding them.  A year or two later they produced a revised version that incorporated the important mods that everyone was doing, and had an output transformer with the correct impedance for an EL84.  I had an early one, and being reasonably competent with a soldering iron, did the mods to it myself.

It would be a lot easier to get a replacement Hammond transformer than mod it for a 6V6 power valve.  From memory, you need to drill a couple of holes for screws in the chassis but apart from that it's a straight swap..  The valve change would have all kinds of complications.  The 6V6 doesn't use the same socket as an EL84, so you would have to replace the valve socket with an octal one.  Given that the valve socket is PCB mount, it would be a nightmare to do.  The octal socket is bigger and enlarging the hole in the PCB would cause all kinds of problems and probably cut traces on the PCB.  You would probably have to find a place to chassis mount an octal valve socket, and then runs wires to it from the PCB.

The transformer that I've got would give you a functioning amp, but with the caveat that it's probably not ideal.

Strangefan

@crunchman Ooooh that's some great info, I'm really struggling to find a suitable ot, but mainly because I don't have a clue what to look for, I make my own pickups, rewire guitars, etc but amps, never really had to open one up,, let me k ow how much you want for the ot  

ICBM

I'd be surprised if the difference in primary impedance really matters much in a low-powered single-ended guitar amp. Maximum power transfer, maximum bandwidth and minimum distortion aren't exactly critical factors .

Even a 2:1 mismatch is usually acceptable, and the difference between a 6V6 and an EL84 isn't as much as that.

ecc83

http://uk.rs-online.com/web/p/audio-transformers/1237242/

Going to be the thick end of 40quid with VAT but I never had an RS supplied traff fail in 30 years of fitting them in radios and tellies. Made by OEP and there are other outlets for their products which might be cheaper than RS.

Take no notice of the 3 watt rating, that is probably for 1percent THD and that is 'hi fi' for guitar amps!

Note! You cannot use any kind of push-pull OPT designed for two valves.

oops! Just noticed. 'on back order' But, as I said, lots of peeps do OEP traffs, in fact they are nice people and if you are really struggling to get one I bet they would help (I got two 10k line bridging traffs from them as samples some years ago) .

Dave.

Strangefan

@ecc83 thankyou for putting the effort in to finding that in have book marked the page  

4114Effects

I can get you a suitable OT for £10 plus postage. Supplier lists them as line matching with power handling of 10w, but I've used them in a few low wattage amps as a single-ended OT with great results. Multi-ratio windings so you can easily match them to any output valve. 

Let me know if you want one as I'll be grabbing a couple for myself soon. 

Strangefan

4114Effects said:

I can get you a suitable OT for £10 plus postage. Supplier lists them as line matching with power handling of 10w, but I've used them in a few low wattage amps as a single-ended OT with great results. Multi-ratio windings so you can easily match them to any output valve. 

Let me know if you want one as I'll be grabbing a couple for myself soon. 

That's great thanks, the brill @crunchman is going to give me one, but I'd anything happens to that one I'll be in touch 

BigMonka

4114Effects said:

I can get you a suitable OT for £10 plus postage. Supplier lists them as line matching with power handling of 10w, but I've used them in a few low wattage amps as a single-ended OT with great results. Multi-ratio windings so you can easily match them to any output valve. 

Let me know if you want one as I'll be grabbing a couple for myself soon. 

I've got a version1 VJ with the wrong impedance transformer that @crunchman described, @4114Effects ;do you think that the OT you can get would improve the amp? I'm thinking more to do with the sound rather than reliability as it won't get any critical use.