Fender Twin Reverb 100 MV

1976 Silver Face with Master Volume and pull-boost switch

1) General.

So, what's the difference between the 100watt Twin and the earlier 85watt? Answer - none. It's exactly the same amp with some marketing fairy dust and power output measured at a much higher, unpublished distortion figures.

To be fair, everyone was at it in the Seventies. A 1000milliwatt transistor amp sounded much more impressive than 1watt and, even today, programme makers will try to make things more exciting by using 10,000Kg instead of 10Tonnes or 100Km/hr instead of 62mph. You can't accuse CBS of telling fibs though, because this amp does produce 100watts rms, but at a huge 10% distortion. It's still 85watts at the old 5% spec.

If anyone's trying to test one with a continous sine wave, you'll get about 70watts rms at clipping into 4ohms.

2) Schematic

'As found' on this chassis. Left click to enlarge.

3) Component and Wiring  Layout

'As found' on this chassis. Left click to enlarge.

4) Why have I just burnt my hand on the mains transformer??

Because it's not big enough. A very common question about Twin Reverbs is 'Should the mains transformer be this hot?' They're clearly on the edge of what you can get away with, but still seem to survive.

Below is what happens on a fairly cold day in Europe. Imagine what they do in Texas.

LEFT: Standby mode

RIGHT: At idle with no signal

Worth bearing this in mind if you change output valve types or try to bias them very hot. This is one of the amps that can benefit from having a fan blowing into the back of the cabinet at very hot venues.

5) What could possibly go wrong after ten years in a shed?

Well, this for a start. A penicilin farm all over the tag boards. Very handy for a touring musician in the Seventies, but a bugger if you have to clean it.

Bleach and washing up liquid in warm water will get rid of it after a lot of work (the insulators underneath the boards need to come out completely to do the job).

 

Dry everything off with a hair dryer or hot air gun and wait a couple of days before putting it back together.

6) Master Volume

A Master Volume control can be very useful, but this one - with a tapped, j taper track and a pull-boost switch attached to it - is a real problem. The assembly isn't available any more and if the pot fails you're stuffed. There's a glimmer of hope though, because a good number of people hated the sound and took them out. Watch out for take-offs on ebay or forums.

If the track isn't completely worn out, it's not too difficult to take the whole thing apart, re-tension the wipers and re-lubricate it. Watch out for replacements with only single pole switching, they won't work.

The tap on the track and the 120pF cap form the treble bleed circuit that allows more high frequencies through at low volume settings. If the pot has to be replaced with one without a tap, the capacitor can be connected straight to the wiper.

Just out of interest, this is the difference (measured on this amp) between Fender's 'J' taper and a CTS log 'audio' pot. Notice that the J is going to sound louder at lower dial numbers even though the maximum level is the same.

You can also see that the CTS pot does virtually nothing until you get to number two.

7) The reverb circuit

Have the burns healed up from the mains transformer yet? Good. Now try not to do it again on the 12AT7 reverb driver. It's operating near to it's power limits and could benefit from a fairly common mod.

Changing the cathode resistor brings the operating temperature down and, hopefully, extends the working life. Adding the bypass cap will adjust the gain, but the Reverb Level pot may end up at a different setting. A 12AX7 (ECC83) will also work with this mod.

It's worth checking the footswitch connectors for damage. They can stick out proud of the cabinet and get squashed if the amp is transported on it's back.

8) The dreaded vibrato tick

Most, if not all, Twin Reverbs tick in time with the vibrato oscillator. It's one of those things that drive you mad once you know it's there, but isn't really a problem in real life. One thing that does help slightly is fitting the capacitor mentioned in Fender's Service Bulletin No.9. Sometimes moving the wiring loom around can improve things but, despite what Fender say, it's generally not going to go away. If the noise doesn't interfere with the music (and making very loud music is what a Twin Reverb is for), it's not worth wasting hours trying to get rid of it.

Fender Service Bulletin number 9

 

 The ticking caused by the Vibrato is caused by improper lead dress. It can almost be cured by connecting a .01 mfd 600 volt mylar capacitor on the 10 meg ohm resistor in the vibrato circuit. (That is, run the capacitor from the junction of the 10 meg resistor and the  optoisolator to ground.)

This resistor is located  on the parts panel.

Remove capacitor across the 10 meg ohm resistor (old  modification) if in place.

If this does not produce the desired results, then the leads should be  dressed as follows and excessive lengths shortened.

 

1. Dress the leads to the vibrato speed and intensity controls away from  the tone controls and filter leads.

 

2. Bunch the leads to the components on the parts panel which connect to  the tube socket of the 7025 (12AX7) vibrato tube.

9) Biasing

There isn't a 'set bias' control on a Twin, the pot (VR14) just balances the currents through each half of the output transformer primary. To increase or decrease the actual current, you need to change the value of the 15k resistor (R53) that goes from the balance pot wiper to ground. Before doing that, just take into account the effect of additional current on the mains transformer at idle.

This amp had a newish set of TAD valves and ran quite cold at about 30mA (14watts) per valve with the factory setup. That's approx 50% of maximum power for modern tubes but probably more for designs of the period. The output waveform on a scope showed very little sign of distortion and the sound was very clean. Audio buffs will tell you that there is one optimum setting for bias current and they're right if it's for a hi-fi set up. Guitar amplifiers are a different thing and adding distortion by moving the operating point around is an accepted way to tailor the sound.

10) Speakers

Unlike the next generation, ultralinear output transformers, this one only has a single, 4ohm secondary winding. That means that, with the built in speakers plugged into the SPEAKER output, you can't (or shouldn't) connect anything else. The EXT SPEAKER output doesn't work on it's own and is in parallel with the SPEAKER output. The two cabinet setup ,then, would have to be 8ohms into each socket.

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