Hello everybody. OK….erm….well let me see here. I vaguely remember the name West Fillmore from somewhere back in the seventies. However this is the first time I’ve seen one in the flesh and better still…gotten the chance to work on one!
Pete the owner, along with his band tour the States from time-to-time and when they do they try to bring back some big bass amps. On their last visit to the US they picked-up an Ampeg SVT Mk1 (but with the 6550 output tubes) and this West Fillmore. I’ve got the job in bringing them both back to life so…..starting with the West Fillmore.
On first removing her from the case I had to laugh at the pretty scruffy wiring….it almost looks home-made! As is often the case with big American tube amps….the output transformer dwarfs over the power transformer (a bit like the Sunn 2000s). The idea of this is to give a better low frequency saturation point for bass instruments…this does work but personally a well worked-out transformer of a little smaller (and lighter weight) is just as good and I beg-to-differ whether the difference could actually be heard? Putting both transformers at one end of the chassis is a hideous idea, the weight at the one end is ridiculous!
Anyway folks….enjoy the article and I have made any further comments on the photos’ captions.
All finished here. Pete’s going to put a back grill on her himself.
As first received…..full of dead spiders and cob-webs……..and I hate spiders!!
The power supplies board. Four diodes for the main HT. One for the negative bias; and two more with a smoothing/filtering network to supply the pre-amp tube (ECC83) with 12.6 volts for its heater on DC.
When she first came in there were only three of the four output tubes present. These were 6L6’s, two of them were nice Sylvania and the third was an RCA metal! The metal ones are really quite bad…they can’t take voltages much above 360 plate & 270 grid 2. It’s no wonder then that all four of the 50 ohm, 5 watt grid 2 resistors were open-circuit…even though somebody has already changed one of them previously. The tubes should be KT88’s anyway! The amp is running in Ultra-Linear with around 560 volts on the plates & screen grids.
Not exactly neat wiring!
I was however very surprised that apart from the two main smoothing caps, the four screen grid resistors and the splitter anode resistors (of which I changed out of choice for the balance pot)….there was actually nothing wrong with any of the other components!
As first received. One of the main smoothing caps had snapped-off and the other had been bodge-fitted….they were both completely kaput anyway. Their value was 250uf @ 385 volts. I have replaced them with 470uf @ 450 volts and these are half the size…….thank-God for modern components. The third triple cap in the photo was fine.
Rear view as received.
Underneath finished; after all necessary work.
The drive was a little uneven so I fitted my usual mod with a 20-turn pre-set pot to the splitter anodes….always works perfect for balancing the drive spot-on!
New 47 ohm screen grid resistors fitted my own way!
This amp doesn’t have an on/off switch. It has the very common US grounding polarity switch and a 2-core cable. I removed all of this and put a nice chunky on/off switch in place of the grounding polarity switch…..along with the correct 3-core power cable and a beefy 3-pin US plug. The standby switch is illuminated from the rear which doubles as the “on” light and switch.
A couple of interesting points here. It had been brought to my attention that the impedance selector switches can be troublesome on these. On checking this; the switch mechanism is very firm and with a positive click however, the actual contacts are in-fact a wafer switch which is really bad news! The current here is a damn good few amps…far more than a wafer switch can handle! I wouldn’t even use a wafer switch for speaker switching on a 50 watt amp let alone 200 watts! This being the case; I disconnected the switch altogether and used a pair of the four output jacks for the 8 ohm output, one for 4 ohms and one for 2 ohms. Now…before any of you spot the fact that the panel states 4, 8 and 16 ohms….this brings me to my next point. When I have done all the work on an amp and are ready to try it, I always fire them up without the output tubes to make sure all voltages are present and correct…especially that there is bias on all the tube bases. Once I am happy with things I put in just a pair of used-but-good tubes to get the amp(s) running. I also put the dummy load on double impedance….as-in have the amp on 8 ohms but the load on 16 (or 4 ohms on the amp with the load set at 8)….this gives the same effect of raising the plate load by a factor of 2 and makes the load correct for just the two tubes. I powered the amp up and she all came to life however, she was only doing about 78 watts from the pair of tubes and I wasn’t happy with the looks of the waveform….it didn’t look like the tubes were pulling full current? Switching the dummy load to 8 ohms and with the amp still set at 8 ohms, brought the power right up to about 112 watts from just the pair of tubes…..perfect yes but….this shouldn’t have happened!? After now fitting four new GEKT88’s my suspicions were confirmed. With the amp and dummy load set to the correct impedance (any of them….4, 8 or 16) she was only managing about 130 watts! Switching the dummy load to half of any of the three impedance choices saw a consistency of just under the 200 expected watts! Bottom line here….this amp has 2, 4 & 8 ohm outputs….not 4, 8 & 16 as stated! Reason, well there are a couple of possibilities. The first is that someone got their maths completely wrong when designing this transformer and it happens to be correct (or pretty damn close) by halving the load or….it was deliberately designed to be used on 2, 4 & 8 ohms but the switch plate has been wrongly printed? Who knows?
All finished here along with a new set of knobs that are almost identical to the originals. All interesting stuff! Cheers, John.
From Pete - West Fillmore Bass 200 - July 2011
"Hey John, just wanted to say thank you for spending time with me on Friday and going through the ohmage maths for the West Fillmore. I am so pleased you managed to bring the West Fillmore back to life. It now produces the most incredible tones, and is true to the name given to it in the 70s, 'the earsplitter'. It doesn't get much louder than this other than maybe an Ampeg SVT, or V9.
Thank you so much again, there is a very good reason I use you John, it's simply down to one fact, other than that you are good guy, you are undoubtedly the best amp tech in the UK!