Fender 6G6-B Bassman amplifier

[Valvepower]

Hello,

Re Fender 6G6-B Bassman amplifier.

For a nearly 58 year old amplifier it’s in good shape with only a few signs of previous repairs, however there is one aspect of a previous repair(?) that intrigues me… In the past there has been the removal of some the components from the tags on the original panel either by floating them or using a tag strip to isolate and secure the connections and circuitry from the existing tags. Some insulated wires, which run through the tag panel, have also been cut and new cables have been run over the tag panel.

Attached pictures.

In the past I’ve heard about tag panels in Fender amplifiers becoming conductive causing noise and suchlike, however I’ve not personally experienced this. There are some threads on a couple of forums about Fender amplifier circuit boards becoming conductive, so I figure it’s been done because of this.

I did quickly try Meggering the original unused tags at 1KV to the adjacent used tags and chassis/ground and these read infinite >999M so outwardly they test OK, however maybe in the past the amplifier was stored in a van or another damp environment, and this may have caused the problem (although I’ve not seen any signs of damp), however this amplifier has been safely dry stored in a centrally heated spare room for the last 32 years it may have dried out over this period

From experience is takes only the minutest amount of leakage to cause noise in high sensitivity high impedance valve audio circuits, so even with the Megger showing nowt there may be something present that could cause problems.

I know I’ve probably answered my own question however, the reason for the post has anyone else experienced this on a Fender or other make of amplifier.

Surprisingly, the moulded blue film capacitors appear OK, but I figure it’s prudent to change them… however, are these the American equivalent of the ubiquitous C296. Like may 'vintage' capacitors found in amplifiers of this ilk this there is a lot of folklore surrounding Fender moulded blue film capacitors.

Terry

[GrimJosef]

I once had a Quad II in for service and I found it was developing odd behaviour after maybe 20 mins warm-up. I can't remember the exact details but I recall it involved changes in the DC voltage on R7, one of the 680k grid-leak resistors for the KT66s. To start with I assumed it was gas in the valve, but no amount of valve-swapping helped. Then I started looking for contamination on the valve socket. But nothing seemed wrong there either. Eventually, after a good deal of barking up wrong trees, it turned out that an area on the paxolin tagboard which carries most of the resistors had become electrically leaky when warm. I cleaned both sides of it as best I could but that made no difference. The leakage was causing the high voltage on R6, one of the EF86 anode loads, to drive current through R7. It's still one of the strangest faults I've come across.

I did consider trying to slot the plastic to see if I could stop the current flow. But in the end I replaced the whole tagboard with one I'd bought online.

I also once had a pcb-built valve pre-amp in which used a large TO-220 FET as the upper half of a cascode. This exhibited transient leakage between the gate and drain which lasted for perhaps a minute after switch-on from cold. In that case the leakage was taking place through a surface deposit on the narrow gap between the pcb tracks. The deposit, which was a consequence of poor washing to remove solder flux residue, was actually an insulator. But it was prone to adsorbing moisture from the atmosphere when cold, and the moisture made it conductive. The FET ran quite warm and it was this which dried the moisture out after a minute or so. It then took 12-24 hours of the unit being switched off before the fault would re-appear.

Cheers,

GJ

[McMurdo]

it's always possible that someone was trying to track down a noise problem at some time in its history and did the mods first, before finding the actual cause. A bit like lifting the wires from the spare pin of a DAC90A output valve...it may or may not cure a problem but will certainly stop it from happening in the future!!

Although I'm sure its not the case with your amplifier, the modern day musical equipment forums are alive with dubious modifications and improvements that are absolutely guaranteed to make you sound like the next Eric Clapton or Rick Wakeman and prevent any possible future faults from ever occurring, ever.

I once had to fix a guitar 'head' amp (ie the amp box that sits on top of a speaker rather than built into it) which was making strange noises. Someone had fitted a set of chinese blue LED chasing rope lights inside so you could see the pretty glow from behind the front grille. Unfortunately they also wrapped around the pre-amp valves and were inducing allsorts of unwanted accompaniment to the performance. They were removed on health and safety grounds!

[Omegaman]

It's quite common to see old Fenders with the components up in the air. I'll admit I've even done it myself.
The fibre boards quite often suffer by soaking up damp and becoming conductive. The usual symptoms are random 'Frying egg' crackling or DC on the tone pots which causes them to crackle when turned.
Fender even tried to address the problem by coating the boards with wax from new. It didn't work....

[Valvepower]

Hi,

Thanks for the replies and it all makes sense.

As I said I’ve never seen any ‘stood-up’ components, however I’d heard about it. Mind you It’s got to be 40 years since I repaired Fender amplifiers in anger, and I could well have forgotten seeing this

I’ll tidy up the repair and fit a small piece of tag strip to anchor the floating connections, but still insulating it from the eyelet.

The electrolytic capacitors will also need replacing, although they are working I feel it’s prudent.

For an amplifier made in 1964 it’s in a surprisingly original condition with original transformers etc.

Regards
Terry