This amp is a 1974 silverface model, with the master volume/pull boost control which was perhaps an attempt by Fender (by then well in to its maligned ownership by CBS period) to corner more market at the Marshall/Boogie end of the spectrum. Amp makers have spent forty years attempting to combine clean and dirty designs in to one chassis and there are some notably successful amps in this respect; this is not one of them. However, for powerful fat cleans and one of the best onboard reverbs out there these are hard to beat.
Although it worked, and still went pretty loud, this one had clearly never been serviced (all original caps, quite possibly valves too), had certainly had a very hard life (from the various numbers carved in to the chassis maybe it had been a rental amp), and sounded ‘less’ than how it should, with loose bottom end, noisy tremolo and effectively unusable reverb. Cosmetically it was pretty battered and filthy (“full of dusty dust” as was described to me), with a cab that looked from the valve location chart to be rather older than the amp itself. The only cosmetics I attended to was a thorough clean, and gluing down of some flapping bits of tolex, and the replacement of its diy screwed on castors with the proper inserting locking type. Yep, and I did polish those clever chromed Fender lean back legs.
Electronically, basically I did a full on service/renovation: new rectifier diodes, power resistors and filter caps for the power supply; complete new adjustable bias supply to replace the relatively pointless balance adjustment type in these amps; new cathode bypass caps; serviced the trem and reverb circuits, and all pots and switches; re-issue alnico Jensen P12N to replace one of the original Utah ceramic speakers which had severe cone rub; full set of NOS good quality 12AX7 preamp valves; matched full set of used but still good Sylvania-made STR 415 6L6GC Mesa-branded power valves (some of the last ‘old production’ valves to be made in the USA); installed new screen grid resistors, and 1 ohm bias check resistors on the power valve cathode pins, and set up the bias. As it was missing its trem/reverb footswitch I sourced a replacement. The upper back panel was also missing, but I didn’t bother replacing it; those four 6L6GCs together can dissipate over 120W as heat and it needs all the cooling airflow it can get.
I was prepared to ‘blackface’ it if the sound wasn’t good, but actually by this time Fender had desisted from much of the offending circuitry that was originally introduced with the silverface amps. The master volume/boost circuit pull switch I left as there are some usable sounds with it, and removing it would be quite extensive rewiring of an amp now considered vintage. It goes very loud now, but doesn’t seem ‘icepick’ as many complain of with these (maybe it got so well-used because it is one of the ‘good’ ones). It does pay to experiment with the interactive tone controls for different types of guitars. Lots of carbon comp resistors, but not much hiss considering that and the volume it is capable of. Mostly, just that characteristic and unmistakable valve thickness to the sound, and the one remaining original speaker paired with the alnico Jensen complement each other well, despite that the Jensen is much more efficient. I believe there is some ‘hair’ to it from the power valves when really pushed without the boost engaged (depends on the guitar used obviously), but to be honest it’s hard to tell as the volume level is so hard on the ears by then. Overall it does best what Twin Reverbs were originally designed to do: provide lots of toneful headroom at some serious sound pressure levels.
Of course, put pedals of choice in front, consider it like a mobile (-ish, it’s heavy) PA, and there’s pretty much no limit to what it can do – all the tone is still in there to build on. Eg Marshall Guv’nor Mk1 and it’s pretty much a tweaked JCM800; Flynn Amps’ Rory Gallagher Hawk and there’s all the boosted AC30 sound you could want; even modulation effects at the front are handled with aplomb.
Like they say about amp tone, it can be hard to add back in what’s already been designed (deliberately or otherwise) out…