Fender Twin Reverb Volume/Attenuation

[siderealxxx]

The Fender Twin Reverb is a thing of beauty for its heavenly cleans (not so much for its weight!). But of course it's also incredibly loud. I can just about practice with it on 2 (1 = 0 in Fender numbering).

But the real issue I'm having is recording with it (its primary role in my studio). The reason being, the background noise (ground hum & hiss) is fairly consistent no matter where the volume is set.

What that means is, I can still only record with the vol at 2-3 (too loud beyond) but my signal to noise ratio is poor as a result. If I could record at 5+ I'd have a much cleaner signal (less noise).

What are the potential workarounds in this scenario? Would an attenuator work?

Thanks for any thoughts!

[ef37a]

The Fender Twin Reverb is a thing of beauty for its heavenly cleans (not so much for its weight!). But of course it's also incredibly loud. I can just about practice with it on 2 (1 = 0 in Fender numbering).

But the real issue I'm having is recording with it (its primary role in my studio). The reason being, the background noise (ground hum & hiss) is fairly consistent no matter where the volume is set.

What that means is, I can still only record with the vol at 2-3 (too loud beyond) but my signal to noise ratio is poor as a result. If I could record at 5+ I'd have a much cleaner signal (less noise).

What are the potential workarounds in this scenario? Would an attenuator work?

Thanks for any thoughts!

If your amp is the same schematic I have found, the AA270 Reverb Twin then the reason it is noisy is pretty obvious? There is no master volume control.

Almost all 'modern' valve guitar amps put a master VC at the front of the Phase Inverter valve stage but the Twin follows the VC with two more Triode gain stages. It would be fairly simple for a tech to insert such a pot' but then there is the problem of where to put it and whether the amp has any significant collector value?

You could try pulling the post VC valve, "V4" shown as a 7025*? If that makes the amp much quieter (but not working of course) then a master VC mod is indicated. If the amp does NOT get significantly quieter then you need IMHO to take it to a workshop.

I have not worked on a twin reverb so my comments are simply deduced from the schematic. If you are contemplating a speaker attenuator you will need one rated at at least 150W.

I am sure Wonks and others will pitch in come the morrow!

*The 7025 is actually a rather good, lower noise version of the 12AX7/ECC83.

I have just thought! Do not be tempted to change V4 for a lower gain double Triode. I seem to recall that Fender did not fit generously rated anode load resistors? A lower 'mu' valve such as the 12AT7 or worse, 12AU7 could burn those loads out as well as almost certainly making the amp sound **** to boot!

Dave.

[Wonks]

A constant level of buzz/hum regardless of the volume knob position would seem to me to indicate something going on in the power amp section. That just runs and amplifies any signal going in to it, so a varying input signal level should increase the level of noise as the overall volume increases.

So you may just need some new 6L6s and the bias checked/reset if necessary.

It would help to know just which model/version of the Twin Reverb you have, and whether it’s an older hand-wired version or a later PCB one. Fender do some silly things with cable routings at times, though that is generally pre-amp stuff (e.g. ribbon cable loops connecting boards being pushed towards valve heater cables rather than pulled away from them).

If you look on the back of the amp, there may be a hum balance pot. You could try adjusting that to see if you can reduce the hum level. The pairs of output valves should act like a differential input on a balanced circuit and any hum picked up in the wiring between the phase inverter and the power amp wiring before the valves should cancel out. But that relies on the +ve and -ve valves producing equal amplitude signals.

This is one reason why you buy matched sets of valves, but the resistors controlling the bias to the valves aren’t typically very tight tolerance ones, so you still get some mismatch in output levels. The bigger the mismatch, the more hum you get. A hum balance pot allows the bias to pairs of valves to be balanced (within limits) so you can adjust the valves to provide the same amplitude output signals so all that hum cancels out.

But it can’t cancel hum that’s picked up or produced in the preamp before the PI as that’s mixed in with in the guitar signal.

But it could be that you’ve got some failing smoothing caps. The cap brand that Fender have used for at least the past 30 years continue to provide amp repairers with steady work as they degrade quickly and often fail from a few years onwards.

You can pull preamp valves to see if pulling one kills the hum/noise. The reverb circuit is often a cause of hum. You could try unplugging the leads to and from the tank at the amp end to see if that improves things. The tank itself can be rotated 180° from its design position which puts its transducers nearer the transformers and so picks up hum.

So there’s a lot it could be. You could spend a lot of money replacing valves that are perfectly good and still not improve things, so I’d strongly suggest getting it to an amp repairer who will have good test valves they can try out, know if it is faulty caps, and ultimately get the amp as quiet as it can be.

Now whether a Twin Reverb is an ideal studio amp if you can’t crank it is another matter entirely. I’d be looking at a Princeton or similar which still gets pretty loud if necessary, but not deafening. I’ve almost bought one several times, despite it being similar to other amps I’ve had as they do sound so good.

[Wonks]

* There’s normally a valve chart on the inside of the speaker section, and this should have the number of the circuit used in the amp on it.

[siderealxxx]

Thanks for the info chaps! To clarify it's a 65 Twin Reverb Reissue with a PCB (2009).

The noise isn't what I'd describe as abnormal, just guitar amp background white noise and subtle ground hum (single coil pup interference aside). It's just that the noise doesn't really get louder with volume, so the SNR is a real factor for recording with this amp.

It does have faults at the moment (unrelated to the background noise) and I have a good tech for this and will sound all this out, but I'm trying to get an understanding of the situation as I know nothing about electronics!

There is a hum balance pot which needs calibrating. Also yes, the reverb is noisy too (and I use it!). Will check the orientation of the tank.

I agree it's too loud for a studio/practice amp but it sounds SO GOOD! The Princeton doesn't have the same bass response (10" speaker). Maybe the Deluxe Reverb is better, though not as clean :/

The ideal aim would be twofold... improve the SNR for recording at low levels and/or drive the valves a little harder (tone sweetens on Twins from vol 4-5 onwards) whilst attenuating volume output to preserve my windows/eardrums.

Thanks!

[ef37a]

Ah now! Re "bass response"? You could get your tech to make you up a cable so that you run the Princeton through the Twin's speakers*...don't DIY this (with your electronics numptiness!) Such a cable needs locking jacks because it must never open circuit the amplifier.

Yes indeed get the amp serviced and speak to your tech about the noise but I fear he will not be able to improve it much without radical surgery and cost. Then again, could just be something silly and cheap!

But you are right in that it is not a good recording amp, apart from the glorious 'tone'! The thing is, waaay back in the day, players were impressed by an amp that was a bit " buzzy and lively" and for sure, on a stage with a drummer&Co the noise was way below the level even a 30 watt valve amp could put out. But we are now coddled by -100dBFS and better noise floors and that ancient 'locomotive' hissing and buzzing in the corner is rather a pest!

*One of the unwanted side effects I am often told of speaker attenuators is that they mess up the "amp/speaker" interaction and this tends to be most audible in the bass region, so if you do go for a Power Soak as I prefer to call them, make sure you can return it.

Dave.

[Wonks]

A good amp tech should be able to get it to be near-silent, certainly quiet enough to record.

As to level, an attenuator will certainly do that, but you’ll need a reactive one. I have a cheap basic resistive one and whilst it certainly cuts the level as expected, it does alter the sound and makes it less responsive. But I’ve never got round to trying a reactive one.

I have a 68 Custom Reverb Deluxe and that has a good bass response and a good clean sound with just a hint of edge at a reasonable playing level. I bought it used with the original speaker replaced by a Celestion G12 V-type. I haven’t felt the need to swap it to something else, even though I generally fit Neo Creambacks as my preferred replacement speaker.

It’s a slightly different sound to the 65 Deluxe Reverb reissue, but having the ability to have reverb (and trem) on both channels is nice. The ‘custom’ channel is supposedly a tweed bassman style, so not as scooped as the second channel. Both have their place.

It was an RoHS version with a very clicky electronic trem circuit, but I swapped that out for the original GaS opto bug device, so now that is silent.

The way Fender combined the two channels to share reverb and trem is poor and relatively noisy, but there is a fairly simple mod to do it in a far better way (yet to be done).

Obviously a slight difference in general sound with 6V6s as opposed to the 6L6s of the Twin, but it’s a sound I like.

So certainly worth trying a 65 Deluxe Reverb or the 68 Custom Deluxe out.

And there’s always the Tone Master versions, which are generally considered to be very close indeed, but can be turned down to whatever level you like. Something else to try out!

[siderealxxx]

Thanks again guys, interesting stuff. Lots of options to chat through with the tech.

I'm aware there's also a half power mod for the Twin, not sure if this is preferable or as effective as an attenuator.

All options are on the table at the moment as it needs quite some work anyway.

A Princeton with a 12" speaker would be interesting (could use the Twin but I lack space for both). I'm not sure I could accept a ToneMaster though... too much of a purist and the reverb is digital :/

[Wonks]

Half-power might sound a lot, but it's only going to give a 6dB reduction in output. And it's not simply possible to just pull two valves out. AFAIK there is no impedance switch on that amp and for running on two valves need to double the impedance setting, as well as probably adjust some resistor values to keep the B+ voltage the same now there's less current draw with only two output valves.

Other options are fitting different speakers with a lower sensitivity. The standard fitment Jensen CK12s are 100dB SPL @ 1W @1m, so ones with a 97dB SPL @ 1W @1m sensitivity will improve things slightly.

And lower gain preamp valves might get you more controllability. From the schematics for the amp, Fender appear to use 1/2W anode load resistors (most others are 1/4W as standard) on the preamp valves. I don't know if this is safe for say a 12AU7 but I believe 2W or 3W rating would be more normal.

[Moroccomoose]

Would a volume pedal in the FX loop help here? I have a Blues Deluxe Reissue and I bought a widget comprising a potentiometer and two 6.35 plugs spaced to fit directly into the send/return of the FX loop. It works quite well as a faux master volume.

Stu.

[James Perrett]

A Princeton with a 12" speaker would be interesting

There's nothing to stop you trying a 12" speaker with a Princeton by just plugging it into the Ext Speaker socket. I don't feel that mine is lacking in bass but that may be down to the Celestion speaker driver that is currently in there which was fitted after the original broke.

[ef37a]

Cracking solution Stu! IF! The amp had an FX loop...AFAICT it doesn't.

If I might lower the tone considerably for a mo'? My son has a Bandit 112 combo and he had a similar problem. Not noise but a very **** or bust gain control and he found it hard to set a socially acceptable level in his flat. Pop sent him a pot in a tin and some cables and everyone is now happy!

Dave.

[ef37a]

And lower gain preamp valves might get you more controllability. From the schematics for the amp, Fender appear to use 1/2W anode load resistors (most others are 1/4W as standard) on the preamp valves. I don't know if this is safe for say a 12AU7 but I believe 2W or 3W rating would be more normal.

The anode load of interest Wonks is R11 and is rated at 1/2W. Worse case scenario, full 333V 'X' supply on it and Rdiss is almost 2W. It is not simple to know how much current a 12AU7 would pull but I would not risk it. A 12AT7 is probably OK but then mu only goes from 100 down to 60.

But Fender do at least seem to have learned? The loads for the 12AT7 PI are 1W.

Dave.

[Wonks]

Cracking solution Stu! IF! The amp had an FX loop...AFAICT it doesn't.

Indeed. It is a reissue of a 1965 amp (but PCBs instead of hand wired boards). No FX loop in those days. The few pedals that were about then all went in front of the amp, including the various tape echo units around like the Copicat.

There is the option of using input 2 instead of input 1, for '6dB less gain' as the manual states, but this is at the expense of a much lower input impedance (136k instead of 1 Meg) and a much duller sound from guitars with passive pickups. Less of an issue if you have a buffered pedal or any operating pedal in between the guitar and the amp. But then you probably simply have to turn the volume control up in order to get the amp to the point where it's very slightly distorting to get a more interesting clean tone again.

[siderealxxx]

There is the option of using input 2 instead of input 1, for '6dB less gain' as the manual states, but this is at the expense of a much lower input impedance (136k instead of 1 Meg) and a much duller sound from guitars with passive pickups. Less of an issue if you have a buffered pedal or any operating pedal in between the guitar and the amp. But then you probably simply have to turn the volume control up in order to get the amp to the point where it's very slightly distorting to get a more interesting clean tone again.

I do this already even though I have single coils. Otherwise it's really too loud even at 2. I'd be curious to compare the tonal quality between the two inputs, though I do have a buffered input. Will test this at some point.

[siderealxxx]

Some interesting/technical observations here. I must admit the world of electronics is a complete mystery to me! Will relay these thoughts and discuss with the tech - thanks

[Wonks]

The 40W Hot Rod Deluxe and Blues Deluxes have similar ‘lots of early volume’ characteristics. 2 is too quiet and 3 is too loud. Somewhere around 2.5 is the sweet spot, but 1mm extra movement of the knob pointer tip can ruin it.

It seems to be put down to the sales/marketing types liking the fact that it made their amps seem louder than competitors “look how loud it is and that’s inly on two”, without considering what a PITA it was for the people who then had to use them in a band situation rather than a music shop. Fitting a log rather than a linear volume pot helps a bit, (as a friend had it done either by or through Bill Puplett testified) but only a by a bit. Still very sensitive.

[Wonks]

This could be a solution for home recording - a Celestion Peacekeeper speaker (new to the market).

https://youtu.be/LSAJ770B-nc?si=2ollpigx5G79_15T

It's unlikely to sound exactly like the speakers in the twin, but with a sensitivity of 86dB SPL, it's 14dB less that the 100dB SPL of the current speakers (roughly 1/4 of the volume).

You'd need a 4 ohm load, and as they only seem to be available in 8 ohms and with a definite 50W power handling limit, you'd need two of them. They seem to be about £199 each, so not cheap. Allow £200 for a suitable enclosure (I'm assuming you won't want to replace the existing speakers) and you're up at £600. Not much more for a used Princeton or a Deluxe of some sort! You could also get a very good inductive load box for that sort of money.