Rectifiers and Effects

admin · Post by **admin** » Wed Nov 30, 2005 5:04 am

I have always noticed that my VOX AC30 has a sustain and a compression effect that is quite noticeable compared to Fender amplifiers.

There are differences between VOX and Fender amplifiers of course and different speakers are also a confounding factor in my case.

A fundamental difference between my VOX AC30 and Fender Twin is the tube and solid state rectifiers, respectively.

Given that there is sag with tube rectifiers relative to the solid state rectifiers, does it follow that the sustain or "compression" effect that I may be hearing with the VOX stems largely from the tube rectifier?

soundmasterg · Post by **soundmasterg** » Wed Nov 30, 2005 2:25 pm

With the AC30, a lot of that compression effect is due to the lack of a negative feedback loop inside the amp. You can make a Fender do it to a certain extent also if you disconnect the negative feedback loop in it. The GZ34 rectifier tube in the AC30 will have the least sag of any rectifier tube, so its not really at that point where the sustain/compression effect is happening. The design of the whole power supply, including filter cap sizes, voltages, etc, all play into it. Also, the output tube types, (EL84 vs 6L6GC) the speaker choices (Celestion Blues vs JBL's I'm assuming), and the way the gain is structured in each amp all play a part. But the largest contributing factor in my opinion is the lack of a negative feedback loop on the Vox.

admin · Post by **admin** » Wed Nov 30, 2005 2:32 pm

Thanks Greg. Can you describe the purpose of the negative feedback loop in the Fender?

soundmasterg · Post by **soundmasterg** » Wed Nov 30, 2005 2:43 pm

The negative feedback loop samples the sound at the output, and injects it back into the amp in a certain place to widen the frequency response. It also reduces the gain a little, gives more headroom, and makes the amp feel more stiff and unresponsive. One of the reasons transistor amps always feel so stiff is because they often use tons of negative feedback to make the amp stable. Most tube designers try to use as little feedback as possible when it comes to guitar amps, and its a good thing for them that tubes are often stable without feedback. Most guitar amps have a negative feedback loop. Some older tweed Fenders, some Gibsons, some older Ampegs, Silvertones, Voxes, etc don't have negative feedback loops. Almost all Marshalls have a negative feedback loop, with the exception of their fabulous 18W, which has many of the same sustain/compression effects as an AC30.

If you disconnect the loop on the Fenders it can work better for blues. I regularly play a couple deluxe reverbs at a local blues jam with the negative feedback loop disconnected. Without doing that, they sound too bright and don't break up enough for a good blues tone unless they're dimed.

winston · Post by **winston** » Wed Nov 30, 2005 3:27 pm

Greg,

I have a 1970 Fender Twin Reverb. Can you tell me how to disconnect the negative feedback loop on my amp.

Thanks.

jps · Post by **jps** » Wed Nov 30, 2005 5:19 pm

Another contributing factor would be cathode bias vs. fixed bias on the output tubes.

soundmasterg · Post by **soundmasterg** » Wed Nov 30, 2005 6:16 pm

Yes, I forgot to mention the cathode bias/fixed bias difference. It has a large effect too. To make it more like the twin reverb but still leave it cathode bias, you could change to each pair having their own cathode resistor and cap instead of all 4 sharing one cathode resistor and cap. If you do this that resistor and cap values change though.

To disconnect the negative feedback loop on your Twin Reverb, you disconnect the 100 ohm resistor connection to ground that is hooked to the 820 ohm resistor near the phase inverter. The resistor is sideways on the board and should connect to the 820 ohm resistor on one end and ground on the other. Look at the schematic and layout for the Fender and you'll see it. The phase inverter is the 12AT7. Disconnect the ground end of the resistor and stick it in the air so it won't touch anything and see if you like the sound. If so, then put some shrink wrap tubing on the end of the 100 ohm resistor you have in the air and thats it. You could also hook it up to a switch to turn the effect on and off.

winston · Post by **winston** » Wed Nov 30, 2005 6:30 pm

Thanks Greg,

I'll do that after this weekend is over. I am sure that I'll like it a lot better. My Twin Reverb is far too clean for my taste right now. It does sound good with my Ric 12 but I want the amp to work well with my other guitars as well.

I'll let you know how I made out once it's done.

soundmasterg · Post by **soundmasterg** » Thu Dec 01, 2005 1:09 am

Your Fender probably could havfe some mods done to convert it to be more like the blackface Fenders and that would go a long way towards making it sound better too. They changed a lot of things on the late 60's early 70's twin reverbs, and they sound nothing like the earlier blackface ones. Lowering the voltages on the preamp tubes, redoing the layout to get rid of long grid wires, taking tone sucking caps out of the circuit that they added....that will help a lot. Once you've done that, disconnecting the NFB loop is icing on the cake, but you may find that if you disconnect the NFB loop now, it will sound better, but not good enough. Let us know how it sounds though.

admin · Post by **admin** » Thu Dec 08, 2005 2:48 pm

Is it possible to have a solid state and tube rectifier in the same Twin Reverb amplifier with a switch to select on or the other or would there be other complex problems with such an arrangement?

soundmasterg · Post by **soundmasterg** » Thu Dec 08, 2005 3:46 pm

Yes it is possible. Mesa Boogie did this with the Dual/Triple Rectifier series, and even patented the idea, even though people have done it before. They like to patent things that are common knowledge in the amp world apparently.

Anyway, the only thing with this arrangement is to make sure that you switch from one rectifier to another when the amp is off. If the standby is setup like a Fender, you could get away with switching when in standby, but since some amps aren't setup this way, it is safest to do when off. On your Twin Reverb, you would need two GZ34 rectifier tubes to be able to use tube rectifiers, and they would be close to the limit of their capabilities. The plate voltages on the pwoer tubes would drop by about 20v, and it would make a little less power as a result. There would also be a slow turn on time which would protect the tubes a little from cathode stripping. The main problem however on your amp is where to put the tubes, as there isn't much room.

admin · Post by **admin** » Thu Dec 08, 2005 6:09 pm

Interesting Greg. Thanks for your comments. I am wondering what the difference in sound might be between the solid state and tube rectifier circuits in the Twin Reverb?

soundmasterg · Post by **soundmasterg** » Thu Dec 08, 2005 6:44 pm

Well the GZ34 is very close to solid state, as far as tube rectifiers go, and in their performance. You could use the 5U4GB also, (you would need two of them), and that will have quite a bit more sag, and the plate voltages will be down around 50v or so from where they are now. Also, you would have even more of a problem finding room for the tube rectifiers as the 5u4GB is much bigger than the GZ34.

It would be the same as any amp that has sag and lower voltages....the sound is browned up a bit, the power output goes down, and when you hit the notes, they won't react quite as quick. Also, the bass will be a little flubbier and you'll have more compression. Good for blues...but there are better amps to try that with than a Twin Reverb.

trancedental · Post by **trancedental** » Thu Dec 08, 2005 8:03 pm

Blackface it & do the feedback mod as well it will sound warmer & break up earlier.

There's a mod about which disconnects the vibrato which adds gain as well either by unsoldering a wire or fitting a on/off pot.

Other mods about I've even heard of people putting a quad of 6V6's in! Only JJ's as they are supposed to take the high voltages, nothing complicated just a few part value changes.

soundmasterg · Post by **soundmasterg** » Thu Dec 08, 2005 9:21 pm

The only problem with using the JJ 6V6's in there as opposed to the 6L6's that it came with are that the primary impedance of the output transformer isn't correct for a quad of 6V6's. It is setup for a quad of 6L6's, which want to see half the impedance that a quad of 6V6's do. If you used a quad of 6V6's in there, you should be running the speakers off a tap that is double what the twin reverb is using now. So if it has a 4 ohm tap, use an 8 ohm instead. And if it is using a 2 ohm tap, use a 4 ohm instead. I'm not sure if the stock output transformer has those other taps or not.

The JJ 6V6 can take up to 500v according to JJ's literature, so it could handle the voltage level, as long as you can bias it somewhere similar to the 6L6's. Since the 6V6 has a 14 watt max dissipation and the 6L6GC has a 30 watt max dissipation, you won't be able to get the bias current up to where a 6L6 quad would be. This will make the plate voltage go up, which may then exceed the 500v max rating of the JJ's. But you would have to try it and see if it would work, assuming again that you have a higher tap to work with.