Leaking capacitors

Fender Hod Rod Deluxe Repair

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Because the Hot Rod Deluxe is the world’s most popular valve amp [citation needed], I get a Hot Rod Deluxe Repair arriving quite regularly.

The amp has two common faults, which are well documented:

The most common fault is the Low voltage power supply (LT supply) failure. This powers the opamp driven reverb and effects loop. Fender create their 16V low voltage power supply from a 33V and create significant heat in the dropping resistors and zener. This heat then causes the copper to delaminate from the PCB, leading to crackles and bangs and occasionally complete signal dropout. You can see the heat issues on this picture, taken with my thermal camera. The dropper resistors are getting hotter than the power valves! (Update 2020: repair kit available here): Hot Rod Deluxe Repair Thermal

On older hot rod deluxe repairs there’s some PCB retracking work to do to repair the PCB damage. On newer amps I can just take preventative action – replace the 5W dropper resistor with a 5W part raised off the PCB with ceramic spacers and replace the replace the zener also raised off the board.

The second common fault is the grey ‘IC’ (Illinois capacitor) electrolytic filter caps which are prone to failure. Fender use these presumably because they’re the cheapest 450V axial caps around. In fairness to Fender, there are probably hundreds of Hot Rod Deluxe amps using these caps that are still working, but there are also a lot that fail. I use a mix of quality F&T (German) and Nichicon (Japanese) capacitors to replace these parts and I recommend replacement on all Fenders when I’m already removing the board.Hot Rod Deluxe Repair Caps

I overrate all caps significantly The cost increase is only a few pounds but leads to better performance and improved lifespan. The most important caps are C33 My preferred configuration is:

C36 (overrated by 100V)
C35 (Overrated by 50V)
C33 (overrated by 470V using 2 series caps!)
C31 (overrated by 470V using 2 series caps!)

Update Sept 2020: At the time of this post (2018), great quality Nichicon and F&T axial caps were still available. 2 years later, Axial caps are going the way of the dodo so I’ve switched to using high reliability radial RubyCons with a special adapter board  (which I’ve made available to others as a repair kit available here).

The other thermal ‘weak point’ is the footswitch circuit, which gets the same treatment as the other hot resistors in the LT supply.

If you need a Hot Rod Deluxe Repair, please contact me.

Hot Rod Deluxe repair Dropper resistorsHot Rod Deluxe Repair ZenerHot Rod Deluxe Repair Footswitch Dropper

Fender Brownface repair – Super Amp 6G4A

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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.