Fender Super Twin.

[DrNomis_44]

Jealous. Have fun

I just got back home this afternoon, we had a great time and managed to record a couple of rough-jams, my mate played drums, but since his kick-drum pedal wasn't working he had to make-do without a kick-drum, the Super Twin sounded great through his Quad Box, and the footswitch functioned flawlessly, my pedal board sounded great too, I'm getting a pretty good range of sounds out of it, I might need to do some maintenance-work on my Univibe pedal since it was a bit temperamental when first powered-up, not sure what the cause might be, the dual-gang pot in the speed controller foot-pedal might need a spray with some electrical clean and lube to sort it out.

[DrNomis_44]

Just doing some minor servicing-work on my Fender Super Twin amp at the moment, one of the input sockets seems to go a bit intermittent every now and then, also one of the octal sockets for one of the six 6L6 power tubes seems to have some contacts that need re-tensioning.


So, here's what I'm going to do:


1 Replace both of the stock input sockets with a couple of new ones.

2 Replace all the Carbon Comp input resistors ( 2 X 33k and 1X 1M) with some new 1W Metal Film resistors, mostly cause I've got them, secondly, Carbon Comp resistors are notorious for generating lots of hiss-noise, secondly they go high in value over time.

3 Re-tension all the contacts in all the octal-sockets of all six 6L6 power tubes.

4 Spray all the control pots and bright switch with some Electrical Clean and Lube.


Hopefully after all that it should be all good.

Update:

After studying the schematic for my Super Twin amp, it appears that when you plug into input one, the signal goes through one 33k grid resistor, if you plug into input two the same thing happens, the signal still goes through one 33k grid resistor, what I believe should happen is this.


Plug into input 1, the signal should pass through one 33k grid resistor.

Plug into input 2, the signal should then pass through the two 33k grid resistors connected in parallel with each other.

Or am I mis-understanding it?, I don't really understand why Fender would design the input wiring so that no matter which input you plug into, the result is always the same, why have two inputs then?

My understanding is that the two inputs are supposed to give two different levels of gain, or, to attenuate the signal level coming from a pickup with a hot output, such as a humbucking pickup.

[JimC]

why have two inputs then?

Back in the day, at amateur level, more than one instrument per amp was fairly normal.

[DrNomis_44]

Back in the day, at amateur level, more than one instrument per amp was fairly normal.

Ah rightio, that makes more sense then, I'm going to mod the inputs so that you can get different gain-levels like you do with Marshall amps.

[Dacious]

It weighed 45kg as a combo. They came on casters.

If it's old check out the filter capacitors - if they're leaking goop it's imperative to change them. If one lets go they're messy and the electrolyte is sticky and corrosive. If it shorts it can take expensive bits with it. They were around from 76-82.

Decent master volume. The distortion is a love it or leave it.

This is the non-reverb version?

[DrNomis_44]

It weighed 45kg as a combo. They came on casters.

If it's old check out the filter capacitors - if they're leaking goop it's imperative to change them. If one lets go they're messy and the electrolyte is sticky and corrosive. If it shorts it can take expensive bits with it. They were around from 76-82.

Decent master volume. The distortion is a love it or leave it.

This is the non-reverb version?

Yep, it's a non-reverb Super Twin, it still has the foot switchable 5-band EQ, and built-in distortion that I don't really care for, I'm going to have a look under the doghouse cap-cover to see what condition the main supply filter-caps are in.


The two original input sockets were a bit worn-out and definitely needed replacing.


Update:


I had a look under the doghouse/main filter-cap cover, and spotted a 16uF/500V electrolytic cap that looked like it had leaked some dark looking goo, so, until I can order a complete set of new caps for the Super Twin, it's going to remain on my makeshift workbench, the input jack replacement seems to have gone well though.


If I'm going to replace that 16uF/500V electrolytic cap that leaked, I may as well cut-to-the-chase and do a complete cap-job on it.

[Simon Barden]

Indeed. The ESR of almost all of the electrolytic caps (if original) is almost certainly a lot higher than it should be and even though there might be no obvious leakage from the caps, most are probably well past it.

Most two input amps had one that was a lower sensitivity than the other. No idea why, as the lower sensitivity input generally sounded rubbish compared to the standard input. I think the input impedance was generally lower so as well as less volume you got less treble (at least with a passive guitar plugged straight in).

[DrNomis_44]

I seem to remember reading somewhere that electrolytic caps are reckoned to have a working-life of at least ten years or so before they go faulty, well at least those vintage caps anyway, I had been noticing that the amp did hum a bit, just a slight amount of background hum, so I'm guessing that the 16uF/500V cap I spotted that had some dark coloured goo leaking out of it, is probably the culprit, tube circuitry operates at high impedances, so if a supply filter cap goes high in ESR that affects it's ability to filter-out mains hum by bypassing it to ground, remembering that caps block DC and pass AC as part of their operation, when a cap goes high in ESR, it effectively looks like it has a resistor in series with it, and it's ability to pass AC is reduced.

That's not the only way a cap goes faulty, I've seen/heard at least one electrolytic cap literally explode, another way a cap goes faulty is if it develops a short-circuit, or partial short-circuit between it's plates, in a tube amp, a shorted, or partially shorted cap can damage more expensive parts, like the power and output transformers, a partially-shorted, or leaky cap can cause something called Red-Plating in power tubes, where the plates or anodes start to glow red due to too much current flow, the plates/anodes can literally melt if there is way too much current flowing in the tube.

Normally, when an electrolytic cap is brand-new, it's ESR is low, but when the cap's electrolyte dries out, the ESR goes sky-high, this is usually caused by a rupture in the rubber bung that seals the positive end of the electrolytic cap (RT style cap).


For those of you unfamiliar with the term ESR, it is short for Equivalent Series Resistance.

[McCreed]

Re: 2 inputs

I'm no authority on this, but my understanding has always been that it is a carry-over from the 50's (particularly the Fender Tweeds). The inputs were labelled "INST" and "MIC".

The "MIC" input had/has a -6db pad to allow a Hi-z microphone to be plugged in without feeding back. So the amp could be used essentially as a vocal PA and an instrument amp. Over time, manufactures just dropped the "mic" label but kept the input padded.

At least that's what I told many years ago.

[Simon Barden]

But the Inst and Mic inputs had their own gain controls, at least on amps like the tweed Deluxe, so were really two-channel amps with a common basic EQ as opposed to two inputs for one channel.

The two inputs with different sensitivities on a channel would probably have been to accommodate single coils and humbuckers/P90s from the times when the guitar sound was supposed to be as clean as possible, before overdriven sounds were embraced and desireable.