UK Fender Service centre and general amp servicing questions.

vertigo88

So, I've got a Fender Supersonic 60 that has blown a fuse, I think because one of the power valves failed. The amp is from 2009 and still had all the original tubes until I replaced some pre-amp tubes and biased a bit hotter.

Shortly after one of the power tubes started making a bad noise so I replaced it with a pretty old JJ that I had spare. The amp was working all right for a bit until I had it pretty cranked and started making noise, that from research, I think was one or the other power tubes failing...

As this is the amps first service I was looking for advice as to if it is worth taking it to a Fender service centre? Or getting an amp tech to check that nothing has gone a miss inside? A full re-tube, bias and fuse replacement? Or just replacing the power tubes, fuse and re-bias?

Cheers

StormshadowGuitars

Forget Fender, Take it to @martinw at MJW Amps

vertigo88

Forgot to add, i'm in the Highlands of Scotland! 

Babones

Whenever you replace power valves, you should replace the full set. ie as a pair, a quad etc. I don't think your amp would have appreciated the different valves.

martinw

StormshadowGuitars said:

Forget Fender, Take it to @martinw at MJW Amps

vertigo88 said:

Forgot to add, i'm in the Highlands of Scotland! 

I'm planning to retire there.....how long can you wait?

vertigo88

Well, i've dusted off the Marshall MG15 and am actually quite enjoying playing it but can't wait to be really loud again!

DJH83004

I think it is pretty safe to say that if the HT fuse that has blown, one of the output valves will have flashed over internally, so a matched pair and a re-bias would be a good starting point. From memory the Supersonic 60 has 1 ohm cathode resistors and test points, so a direct reading in mV will give you the cathode current (but don't forget that is the sum of the anode and screen current when calculating % dissipation). Also whilst the chassis is out give the controls and jack sockets a spray and she should be good to go.

jpfamps

If you take the amp to a tech make sure they rewire it to UK 240 mains as these amps are usually wired to 230 V mains.

The Supersonic, like other Fenders, regulates the +/- rails for the op-amps in the unit using a power resistor and zener diode.

These resistors run hotter than they need to if the amp is wired to the wrong voltage.

Worse the high voltage screen supply runs very close to one of the zeners and can (and does) short out due to the insulation being compromised by the heat generated in the zener.

There is a way of rewiring the screen supply directly to the valve sockets to prevent this happening in the future.

DJH83004

All good advice from @jpfamps, interestingly the manufacturers have a dilemma in the UK as the harmonised UK mains regulation limits are 230v +10% / -6%, but if a 240v tap is available, it makes sense to utilise it. I have only seen a couple of SS 60s and although the 5W dropper resistors did run hot they they weren't causing problems unlike the HRDs which can cause all sorts of grief to the PCB and other areas. In their wisdom Fender fitted voltage regulators to the SS22

jpfamps

DJH83004 said:

All good advice from @jpfamps, interestingly the manufacturers have a dilemma in the UK as the harmonised UK mains regulation limits are 230v +10% / -6%, but if a 240v tap is available, it makes sense to utilise it. I have only seen a couple of SS 60s and although the 5W dropper resistors did run hot they they weren't causing problems unlike the HRDs which can cause all sorts of grief to the PCB and other areas. In their wisdom Fender fitted voltage regulators to the SS22

I've seen 3 or 4 SS 60s where the screen supply has shorted to the zener dropper.

A better way would be to use 3 terminal regulators.....

vertigo88

jpfamps, i'm a bit lost to actually knowing what zener droppers and other electrical components are but was wondering if you could explain the advantages of re-wiring to 240v? Is it for reliability? cheers for all the info.

jpfamps

vertigo88 said:

jpfamps, i'm a bit lost to actually knowing what zener droppers and other electrical components are but was wondering if you could explain the advantages of re-wiring to 240v? Is it for reliability? cheers for all the info.

Rewiring the amp to 240 VAC will reduce the internal voltages a bit, and so the amp will be running cooler, in fact to the design spec. 

As you can imagine this is beneficial to the long term reliability.

re the Zeners, many Fender valve amps also have stages that employ solid state op-amps, eg for the pre-amp send and switching circuits in the Super Sonic.

The op-amps run of a dual rail supply of +16V / -16V. 

To generate this the amp has a dedicated transformer secondary winding that when rectified and smoothed according to the schematic should be about +/- 28V (although will be higher running the amp wired to 230 VAC in the UK), which is way too high for the op-amps.

To reduce this down to +/- 16V Fender use a Zener diode preceded by a resistor. A Zener diode when reverse biased has a fixed voltage drop across it, eg if you use a 16V Zener diode you will get 16V (more of less) across, and in this way a + and - 16 V supplies are generated.

However this method is very wasteful of power as you are not only supplying current to the circuitry in the amp, but also shunting a load of current through the Zener diode.

Assuming no current draw by the circuitry, with a 270 ohm dropping resistor you are sourcing (28-16)/220 = 44 mA of current which will be dissipating around 0.5 W in the resistor and 0.7 W in the Zener.

The Zener dissipation will be reduced a bit by the current draw of the circuitry down to maybe 0.5W.

Not only is this dissipation quite high, it will be made worse by running the amp wired to 230 mains as the voltage drop will be greater.

Zener shunt regulation is easy to implement and generally bullet proof as long as the components chosen can withstand the power dissipation, however it is wasteful.

In contrast a cheap 3-terminal integrated voltage regulator would only be required to source the current delivered to the circuit, so if so was required to deliver 15 mA (typical for 3 op-amps) at 15V from a 30V supply then the dissipation in each regulator would be around 0.225 W, ie under a quarter the waste dissipation in the dropping resistor and Zener diode, and is easily in the capabilities of TO-220 three terminal regulator without heatsinking. .

ICBM

They should just do the job properly and have a centre-tapped winding for the low-voltage rails of the correct voltage not to need wasteful throw-away regulation, and a *separate* winding for the bias supply. It's the combination these two - the bias needing a much higher voltage - that ends up giving this crude arrangement. Given the volumes Fender buy at I doubt a slightly more complex transformer would cost them much.

It's actually more understandable that they would do it this way in a solid-state amp which has a higher-voltage split-rail main power supply anyway… but quite baffling in a valve amp really, when the only LT is the +/- 15V.

DJH83004

What is even stranger is that they fit three terminal regulators for the +/- op-amp supply rails on the Supersonic 22 !!

ICBM

DJH83004 said:

What is even stranger is that they fit three terminal regulators for the +/- op-amp supply rails on the Supersonic 22 !!

Presumably because it uses 6V6s, so the bias voltage is much lower and it doesn't need the big voltage drop like the 6L6 amps do.

DJH83004

@vertigo88, I managed to find a photo of the last SS60 I looked at, ignore the red circle, (which was the problem with this particular amp), check that the main cable loom going across the printed circuit board is well away from the two large white ceramic resistors and the black zener diodes at the side of them.

vertigo88

I ordered some replacement fuses to see if I could locate the problem, hoping it was tube related... The amp blows a fuse with no tubes in, definitely tech time, I think?! 

vertigo88

I opened up the amp for a look and found these burnt looking connections, on the end of the green wires, coming from the bigger transformer on the left hand side...

http://i1094.photobucket.com/albums/i457/vertigo888/DSC_0390_zpsxkyyk7sh.jpg
http://i1094.photobucket.com/albums/i457/vertigo888/DSC_0391_zpscssvivqt.jpg
http://i1094.photobucket.com/albums/i457/vertigo888/DSC_0392_zpsktn43qaq.jpg
http://i1094.photobucket.com/albums/i457/vertigo888/DSC_0393_zpsvo3wa8xl.jpg

I've circled the bit I thought look dodgy with red in the first two photos, can anyone tell from looking if this might be the problem? Cheers all

ICBM

If it blows the fuse with no valves in, there is one final thing to try… remove the bulb from the front panel light. Also have a good look at the holder and check that no part of it is touching the floor of the chassis.

The burnt-looking connections will be the filament supply coming from the transformer, which feed both the valves and the light. The reason they have overheated is probably just because the contact area of the push-connectors used is too small for the relatively high current carried on these, so there is enough resistance to generate significant heat - but that would be increased drastically if the bulb is shorted or if the holder is shorting against the chassis.

In fact, to check it's nothing to do with the filament loom at all, the easiest thing to do is just to temporarily pull off both those connectors. If the amp now still blows the fuse, that's not the cause and it's definitely time for a tech to have a look.

DJH83004

As ICBM says, the green wires (P13/14) are the valve heater and lamp supplies from the power TX, which have their fuse in front (10amp). Clearly the connectors have been overheating, and this does need addressing, but suspect is not the cause of your mains fuse blowing. Does the fuse blow with the valves out AND standby switch off, this is important to determine if it something in the HT circuit (rectifier diode, cap etc.) or related to the power transformer or remaining circuits. At this stage a tech would probably pull all the connectors P11 - P17 off the board from the TX to completely isolate the secondaries and try the fuse again.   

vertigo88

The fuse only blows once standby has been turned on