After a pre-arrangement on the phone of a day and time, a young man (Adrian) and his father paid me a visit from Nuneaton, Leicestershire. Adrian had a Fenton-Weill Porta-Bass 15 head with a sad story behind it. This amplifier had apparently stood in someone’s shed for about 10 years or more. Now..... not only is a shed a very damp place and not the type of place one should store delicate electronic equipment but, in this case, water had actually been running into the top of the amplifier too!
Adrian continued his story explaining that when he acquired the unit, complete with its matching speaker
cabinet,
it was all covered in mildew He had cleaned-up both units thoroughly and fully dried them out before proceeding to try them out. The amplifier lit-up
OK, but after about 20 minutes there was a loud bang from the underside of the amp.
Rushing to the power socket, he disconnected it rather promptly and then took the
old amp to two or three "modern-day" technicians, all of whom basically refused to work on it!
He then proceeded to search the net for someone with the "know-how" on this type of
equipment. Upon finding this website, he then made the said phone call.
On first examination of the unit it was blatantly obvious that one of the main smoothing capacitors had blown itself apart! This actually is quite a normal and common occurrence on high-voltage tube (valve) amplifiers after they have stood for a very long time. The capacitors dry out and then, when they are hit with the voltage again, they get hot and eventually blow out the electrolyte. This can be very dangerous if someone is standing in the sight-line of such
capacitors; they usually shoot one of the ends off with some tremendous force and speed!
I blobbed a couple of new ones on whilst Adrian and his father were in the workshop; just to see if in actual fact it would power up and at least do something? This it did with about 3 watts showing on the test gear, and a most dreadful picture on the scope
too! ......Not surprising at all.
After bidding Adrian and his father farewell, I left the unit on for a couple of hours just to see if the very rusty power transformer had sustained any permanent damage through all the water ingress? It turned out to be fine,
which was very lucky. Although I had a couple of other repairs in the wings (but not
urgent), as this amp was now on the bench I just decided to continue with it.
After the initial examination, the following faults were apparent:
- Extremely bad condition due to damp damage, (I have done three or four of this model over the past couple of years but this has to be the worst one)
- Many tired/worn out components, plus 4 tubes (valves).
On closer scrutinizing the following major problems were found too:
- This amplifier has had a good few modifications in the past; the original output tubes (6BW6’s) have been replaced with EL84’s and a suitable output transformer too (different plate load). This is not a bad idea as the EL84 is still currant whereas the 6BW6 isn’t. However some of the mod was ok but the drive and bias circuits had not been changed accordingly, plus the output transformer was wired wrong!
- The inputs and feedback circuits had also suffered some strange modifications!? I have re-wired and corrected these issues.
- Furthermore a lot of damage to the printed circuit has occurred through the damp. This has impregnated the compressed board so much that it was causing major problems with a 50cls hum via the 6.3 volt heater chain, and this was “tracking across” to signal paths. I have had to remove the copper track from the circuit board completely and “hand wire” a new heater chain to remedy the problem (see photos). I have also soldered nearly every dry joint on the amplifier and re-located some of the components underneath due to the bad print on the circuit board. The print was in such bad condition that I had to chase and repair a vast amount of it.
Basic restoration tasks additional to the above included:
- Spraying & cleaning all potentiometers and tube bases, plus changed one suspect tube base.
- Checking and changing the fuse for a 2amp (it had a 3amp in here; wrong).
- Cleaning the case and front panel, together with removing all the electrolyte (blown capacitor) from the under-chassis.
- Replacing all screws, nuts, bolts, washers & hardware as need be. Also I found a set of 4 knobs more in keeping with the amplifiers age and looks.
- Replacing 4 tubes. Two ECC83’s were well tired plus the two EL84’s too. These were actually an odd pair anyway. One of the ECC83’s had been changed at some point as had the EZ81 rectifier. However, these were both fine.
- Tightening all potentiometers, jack sockets & switches.
- Virtually full re-wiring of approx one third of the amplifier, which included power socket, switch, fuse, neon and output transformer. (I have now soldered all the wires on the power socket, which is a better idea as the Bulgin socket is renowned for its wire securing screws and pins coming loose!) Also replacing the missing 6BA nut & bolt on it too.
- Though about half a dozen resistors had been changed for modern ones, I had to change all the rest of the old 20% resistors which were tired & well up in value, plus virtually all of the capacitors. (I have glue-gunned all the larger capacitors to the board now too. This takes the strain off the legs and, in the case of this amplifier, also takes the strain off the weak circuit board track. This is the modern way of doing things, anyway.)
- Removal of the non original side mounted “feedback control” (modification) and fitted a second speaker jack here to “fill in the hole”.
- Replacement of the missing “red glow chassis lamp”.
About a full weekends work……….Phew!
This one was quite a challenge but I don’t let things beat me. All good fun
though!
She’s now all back to life, working well, and at 15.98 watts too. That's
actually better than it would have been with the original 6BW6’s; this model usually makes about 12 watts RMS before clipping.
By the way, this Porta-Bass has the date of 7/8/64 written inside the chassis. The amp, or maybe just the PCB, was presumably assembled by Joan, who has written her name on the circuit board. See the two photos below:
Left, the date 7/8/64 written on the chassis and Right, Joan's "mark".
Click HERE
to see another Porta-Bass 15 that I serviced some time ago; still original and
un-touched. That one wasn’t quite so sad!
Cheers for looking, John.
And the final comment from Adrian, the amp's owner:
