Fender Brownface repair – Super Amp 6G4A

This Fender Brownface repair was something of a labour of love here at Keld Ampworks. It’s a fascinating amp – having started life presumably in America, it’s got a 110V transformer. At some point it made its way to Belgium, where it was ‘converted’ to EU voltages, using a rather scary transformer bolted to the inside of the woodwork. It was later bought by the current owner and brought to the UK.

This was my first Fender Brownface repair. I’ve done blackface Fenders, Tweeds and Silverface fenders but never before a Brownface. Nice to have something new.

My first task was to make the amp safe. The Fender ‘death cap’ is well documented elsewhere so I won’t dwell on it. Suffice to say that it was removed, and a 3 core earthed mains lead with US plug fitted. The fuse and mains power switch were moved to the ‘live’ line. They don’t make ’em like this any more! The scary open frame in-cabinet transformer was also removed from circuit and replaced with a removable US-UK transformer. This makes the amp more ‘original’ and also safer. Double win!

Most of the preamp tubes were still good. The power valves were replaced with a new set of Sovtek 5881s and one preamp was replaced. The valve sockets were all tensioned and cleaned.

Checking the filter caps inside I saw that 2 out of 7 had already been replaced – but with underrated parts (350V instead of 500V!). Of the remaining five, three were leaking electrolyte and so after consulting the customer I replaced all 7. I was able to preserve the original filter cap covers and use them to conceal modern Rubycon parts at 700V.

At this point the amp was much more stable but had a few intermittent crackles and bangs. Many of these were sorted by replacing some coupling capacitors.

The last issues were with the ‘Vibrato’ channel. The vibrato modulation was bleeding through horribly onto the normal audio signal. This turned out to be further cathode bias and coupling issues.

If you need a fender Brownface repair, please give me a call.

Vintage Fender Tweed Repair – 1953 Tweed Deluxe

Vintage 1953 Fender Tweed Repair

This Vintage Fender tweed repair is the oldest amp I’ve worked on to date. I’m informed it was made in 1953, its a 5C3 design.

The 5C3 deluxe tube complement is 2x 6SC7 for preamp and phase inverter and 2x 6V6 running a push pull output stage to about 12W. The amp uses a valve rectifier – the stock is 5Y3. The customer was running the whole amp run from a nice meaty 240V to 110V transformer from maplins to provide the US mains voltage.

The customer brought the amp in as not working and requested an HT capacitor refit.

I noticed that the HT voltage gets to 500V at inrush before dropping to below 450V (the rating of the existing caps). Since the amp doesn’t have a standby switch this means that the caps are subject to significant stress at turn on. I experimented with a 5V4 rectifier valve to reduce the inrush current, however the 5V4 results in a higher HT with not much headroom before the 450V rating. I recommended 600V caps for the repair to allow for a good safety margin.

600V 15u/16u caps aren’t that common in these days of low voltage electronics – unfortunate for those attempting a vintage fender tweed repair! So we used 350V electro caps in pairs. This didn’t look as pretty, but functionally provides the same performance. I never recommend NOS electrolytic capacitors as electrolytic capacitors degrade even when out of circuit.

I removed the low value bleed capacitor on the primary side of the mains transformer as this is considered unsafe by today’s standards. If the cap becomes faulty and passes DC then the amplifier chassis can become live. Unlikely, but not nice! I always insist upon following mains safety procedures – even on vintage amplifiers.

The fault with the amplifier was actually a loose ground connection underneath the eyelet board – simply fixed.

One last problem – these 6SC7 valves in this particular period of vintage fender amps have a horrible tendency to be microphonic. The speaker vibrates the cabinet, the cabinet shakes the rather old valve base, which shakes the valve and the whole thing takes off in LF feedback. There’s a probably a reason Fender dropped them after this model! I first assumed that the fault was with a worn out valve but on ordering a replacement I now believe that this is likely to be a feature of all 6SC7s. The solution I found was to replace the valve base with a more mechanically rigid Belton one, to stop the valve moving in its base. Interested to hear from anyone else who’s experienced this!