Bronco amp - first glimpse

[Bushbaby]

Hi All,
This is the first time I've had an opportunity to supply information with regards to my new 'baby' amp imported from the US. It appears to have a arrived safely from its long journey from Houston, Texas. Its missing its 6v6gta (I have a few gt's which I can use as a replacement). At some time the 5y3gt has been swapped for a solid state rectifier - this will be returned to its original valve configuration during the refurbishment. The smoothing cap appears to be 20/20/20/20uf.

I've posted some pics to give an idea of what it looks like and I do think that it is a 68 or 69 model.

The 12ax7 are the original German Telefunkens - assuming that they work (not tested just yet).

I can certainly see a very burnt resistor (1k 1 watt) which feeds the smoothing cap.

Any thoughts?

Thanks and Regards

Rob
                           

[Yorkie]

The resistor doesn't 'feed' the smoothing cap - it's between the smoothing cap and reservoir cap and on the circuit is shown as 1 Watt 1K. It's obviously struggled to cope, and I'd be inclined to replace it with a modern wirewound resistor. A 2 or 3 Watt one would do, but personally, I'd be inclined to use a 7 Watt one which would run much cooler and is small enough physically, at just 35mm (1.5") long and 10mm square. Alternatively, if you preferred to use a tubular one closer to the appearance of the existing carbon composition one you could use a metal film 2 Watt one (eg, Maplin Code 'D1K'). The 7 Watt wire-wound one is code L1K:

http://www.maplin.co.uk/7-watt-wirewound-resistor-2179

As I said earlier, given the age of the amp, personally - though they look physically OK - I'd be inclined to change the tubular caps - especially the one between the Anode of the 1/2 12AX7 and the Grid of the 6V6GT, but if you prefer to leave them in place, I'd at least check their leakage - I think you have at least one cap tester that can test them at their rated voltage. I'd also replace the 25uF 25V electrolytic cathode bypass caps in the 6V6,and the 12AX7s, (four in total I think), but the disc ceramic caps should be fine.

That would be my thoughts.

Overall it looks in nice condition, well constructed and easy to work on.

Hope that helps Rob.

[Bushbaby]

Hi David,
I take on board what you are saying and agree with you.
I'm also a bit concerned that this amp has no earth connection too (as I am aware that many early amps didn't).

Thanks

Rob

[Refugee]

I would be very tempted to pull the remaining valves and wire a 1N4007 and a 100K 2W resistor from the HT transformer to the rectifier cathode and measure all the grids as the main smoothing capacitor reforms.
The fault currant will need to go before it sees full power for sure.

[pwdrive]

Congrats Rob, it looks like it got over the pond in reasonable shape, nice and clean too.

Lawrence.

[Yorkie]

Hi David,
I take on board what you are saying and agree with you.
I'm also a bit concerned that this amp has no earth connection too (as I am aware that many early amps didn't).

Thanks

Rob

As far as the amp itself is concerned Rob, the only mains connections are to the primary of the mains transformer via the on/off switch, and the fuse-holder. Everything else is powered from the mains transformer secondary, and thus, is isolated from the mains. I do though notice that there's a .047 cap from the mains input to chassis, so that certainly needs to be a class X or to be snipped out as it's not strictly necessary. (If the existing cap was to go short circuit (as one has just done on me in a sewing machine motor speed control!) it would put mains onto the chassis, but then the 1A fuse would instantly blow).

I assume that your main concern as to earthing will be the safety of any equipment that's attached to the input such as a guitar, and whether or not the metalwork on the guitar could become live. If you wish to (given that it has a mains transformer - not a live chassis), there's no reason why you can't fit a three core flex with the earth lead to the chassis for your reassurance.

I'm not sure what type of mains switch the amp has, but some of those featured on the website have flimsy single pole slide switches. You could fit a more substantial double pole toggle switch such as this one:

http://www.maplin.co.uk/dpdt-toggle-switch-2355

Of course if that would alter the front panel layout, you'll be very reluctant to do that.

Apart from the fact that this equipment dates back to a less safety conscious era, as with Heathkit it was initially designed for the American market on 110V mains, so they took a more casual approach to not bothering to earth equipment, even if housed it metal cabinets (which your amp isn't of course). I've got a Heathkit RF1U signal generator (metal cased) which originally wasn't earthed but I've fitted a three core flex to it. (the 'U' suffix was applied to Heathkit equipment marketed in the UK).

[Refugee]

I spotter that capacitor bu the switch.
It looks like a bomb.
Earthing the chassis will earth the transformer core and get rid of the noise so it will be of little use having a capacitor in there.

[pwdrive]

Under normal conditions that 1k shouldn't be passing much current.

Lawrence.

[pwdrive]

Something else that you might have to consider is the existing mains transformer, it's probably been designed to run at 60Hz as opposed to 50Hz, that's a 20% difference.
It depends how the transformer is designed, some 60Hz transformers will run at 50Hz without getting excessively hot, but some won't and they will get a lot hotter, maybe too hot and might result in the transformers failure.
There's no easy way round it if the transformer runs hotter than normal under normal current draw conditions due to it being run at 50Hz.
The options are to run it via a frequency converter (expensive?)
Reduce the mains input voltage accordingly, obviously the secondary voltages will be reduced, you can compensate to a certain degree by retaining the solid state HT rectifier that's already installed due to it's lower internal resistance but the valve heaters might be under run.
The other option is to replace the mains transformer with a suitable 240v 50Hz job.

I have a few receivers that were designed for 115v 60Hz, some run not too bad temperature wise but a couple run hot (hotter than one would expect) also have had two that went to meltdown when running them at 50Hz.

There is an explanation in the link below, scroll down the page when it's opened and the info is there.

http://www.allaboutcircuits.com/vol_2/chpt_9/8.html

Lawrence.

[Bushbaby]

Lawrence,
Thanks for the info. It was always my intention to run this with a 5Y3 fitted as per the original design.
I was aware of the difference with respect to the transformer at 60Hz for US voltage compared to the UK 50Hz but i did not consider heat problems - Poo!

Rob