Opportunity to buy an old valve guitar amp

[Bushbaby]

Hi All,
Its been I while since I have posted but I am still here.

I need some advice please.
I have seen an old 1968 Fender Bronco Amp (which is just a rebadged Vibrochamp) for sale as a 'project' from the US. Its sold as a project in so much that it makes a loud hum and doesn't work at present. Its a reasonably simple circuit with 2 12ax7's, a 6V6gt and a 5Y3 rectifier driving an 8" speaker.

My immediate thought is that the output transformer could be shot.

They are nigh on impossible to buy in the UK and price is not too bad (taking postage and import duty).

Any helpful thoughts would be most welcome.

Thanks

Rob

[AlanBeckett]

Rob,

It hardly sounds (sorry) worth the trouble. Just make one. Hum is likely to be Electrolytics. Anyway, 6V6 OPTs are readily available.

Alan

[pwdrive]

Difficult to say what the cause of the hum is until you can get it on the bench, from what you describe it looks like an single ended class A output stage, if by chance the output transformer is shot you should be able to get it rewound or find a reasonable substitute, whichever way I would say the amp is repairable.

Lawrence. [EDIT: Missed Alans post]

[tiv48]

here is the circuit
Rob T

[Bushbaby]

here is the circuit
Rob T

Thanks Rob,
I dont think there will be too much trouble to repair this.

Best Regards

Rob

[AlanBeckett]

I used to build things like that when I was 14. I wasn't impressed then.

Alan

[pwdrive]

Did you twang a guitar Alan? I made a couple for mates back in the 60's using the then new double pentode, ELL80 I think it was.

Lawrence.

[Bushbaby]

Thanks for your kind support Al.
The original champ and vibrochamp amps have been used to record numerous 'classic' recordings because they overdriven at low volumes in the studio (Layla comes to mind here). They are also a good home (bedroom) amp too and furthermore they are a bit like buying and antique as they are going steadily going up in value.

When I was first playing guitar, I turned my nose up at at being offered a 'small' fender value amp - a deluxe reverb (one of the first silverfaced ones). As a kid I thought 22 watts would be far too low an output as we all wanted 100 watt amps - it was just under £100 (new). Now they are like hens teeth so much so, the fender re-issued them for lots of dosh (about £1000+).

I suppose this is my last chance to buy something that I consider 'affordable'.

Regards

Rob

[AlanBeckett]

Did you twang a guitar Alan? I made a couple for mates back in the 60's using the then new double pentode, ELL80 I think it was.

Certainly not! I just built and mended them then did a runner.

I did try an ELL80, but never the ultimate ECLL800. One of these days .....

Alan

[Yorkie]

Nice to see you on the forum Rob!

If the loud hum is emitting from the speaker, it's unlikely the output transformer is shot. Much more likely to be the 20uF 450V smoothing/reservoir caps. Given the age of the amp (45 yrs), the cathode bypass cap (25uF/25V) will most likely be u/s too, and of course, it would be a wise precaution to replace the .02 cap feeding the 6V6 ('that cap').

Lots of domestic radios from the late 40s onwards used 6V6 output stage - sometimes, as in the Barker 88, two is push-pull, so in the unlikely event of the OPT being shot, one could be culled from a scrap radio chassis.

(I'm not sure from your post whether you've bought the amp Rob, or are considering buying it - the title of the thread suggests that you're thinking about it and wondering from the description of hum, if it's worth a punt).

This is another instance where someone has an irresistible urge to switch it on to 'see if it lights up' with the risk of seeing it light up spectacularly for a brief instant, sending it to the 'scrapyard in the sky'.

Hope that helps a bit.