Need to be schooled in a little Amp theory

[yalesmith]

Question for the folks…I had originally ask a similar question on the TW forum regarding a Rocket build, but it is essentially this. How can I reduce gain in an amp without reducing a bunch of volume? I had the same issue with a an old Meck PA amp that I worked on Reducing the resistor on the grid works but the volume gets chopped a lot. Is it as simple as reducing plate voltages on the preamp tubes? I Probably should study fender schematics, there might be some answers there. Any help would be awesome

[Milkmansound]

are you asking about headroom before distortion?

[Gaz]

Yeah, Fenders would be a good example to look at. For the most part, they simply have less preamp gain, which allows them to stay cleaner on the dial before breakup. Twins, for example, at 80 watts will 'cut your head off', and come off very loud and clean. Technically, a vintage Marshall puts out 20 watts more clean power, but doesn't sound as loud clean (in my experience, at least) because it has more preamp gain.

This seems to be a psycho-acoustic thing, that a 80 watt amp can sound louder clean than a 100 watt amp, the only difference being preamp gain. So yes, reducing preamp gain to a certain degree will give you more headroom in the preamp, but keep the power level the same. You just have to be careful, because if you reduce too much gain the PI won't be able to drive the power tubes to full power.

The best way to experiment is with a scope, a sine wave generator, and a meter so you can see when power actually drops. A dummy load is probably a good idea too so you don't have to blast a 1Khz sine wave out of your speakers.

[yalesmith]

Fender Deluxe vs. Rocket Pre-amp voltages

[Gaz]

Sorry I kinda missed the part about preamp voltages. You can get a little headroom by raising preamp voltages, but that's not the first place I'd look in a Rocket. Here's what I've personally done in a Rocket, and would do in this order:

Reduce first plate resistor to 100K

Remove 2nd stage's bypass cap (if you have one)

Add a pre-PI or bootstrapped master volume to limit signal to the PI

Add a "slave out" and run it into one of these http://www.electricamp.com/power-unit-tube-amp.html

[Phil_S]

What is it that you really want to achieve? Say it plainly. Do you want more clean headroom before it starts to overdrive? That sort of thing?

Preamp gain is governed by plate voltage and bias. Here's what the RCA manual shows. That may or may not hint at what you really want sound-wise.

[yalesmith]

Yes more clean head room before overdrive - it does not seem like you have to push a rocket much before it breaks up.

thanks

[Firestorm]

How can I reduce gain in an amp without reducing a bunch of volume? ... Is it as simple as reducing plate voltages on the preamp tubes?

Preamp supply voltages don't affect gain much at all. They do, however, affect headroom (how big a signal you can swing before the stage clips). If the cathode resistor of the stage is fully bypassed, its gain is almost exclusively controlled by the plate resistor, which in the Rocket's first stage is 220K, a value you almost never see in a Fender (except the Blonde Bassman, which is a different animal). Did you build yours with R5 bypassed? If so, try it without.

It's difficult to compare a Deluxe Reverb to a Rocket because they are very different topologies. The Fender amp that most closely matches a rocket is the 5F6A Bassman or the 5F8A Twin. In these amps, the first stage plate resistor is "only" 100K (slightly lower gain) and the first stage tube is a 12AY7 (much lower gain). You can try these, but at some point you risk reducing preamp gain too much for it to sound like a Rocket. More subtle effects can be had with the suggested split plates. Or you can try changing R17 and R18 to 150K or even 100K. Changing R8 to 220K will also reduce gain a little (edit: I should have said "increase loss," since tone stacks don't have gain, per se), but also change the way the tone controls interact.

[Gaz]

Yes more clean head room before overdrive - it does not seem like you have to push a rocket much before it breaks up.

thanks

It's true, and I think there is a misconception about the Rocket being a clean amp or something. Yes, it does do cleans, but it's a dirt amp, no doubt. At home, the Rocket does cleans loud enough to land you a divorce, but with a loud band, it's like, 'is that all you got?!'

I just remembered the Rocket doesn't have a voltage divider before the PI (where a master volume might go). I actually found the amp unusable without one. I added a 220K/220k myself, which is what the Zwreck uses. The Matchless LIghtning has a 220K/100K divider, but uses a bypass cap on the 2nd stage. I personally prefer the former with the 220K and without the bypass cap.

[yalesmith]

I agree the amp is un-usable now....the votage divider I will try but i need to do some research regarding it - not clear on what it is. I have built a liverpool, express and a 5e3 to terrific sucess. This build has been a headache.

Thanks

[Gaz]

Ok, see R10 coming off the Treble wiper? Just add another 220K resistor to ground after it, which forms a voltage divider going into the PI. This'll cut down the signal a bit. And if that's not enough try making the resistor to ground smaller like 100K (as in the Matchless Lightning). I think any smaller may lower output power, but it something you can easily judge be ear.

[yalesmith]

i read up on Voltage divider and it appears that to add a pre PI voltage divider to the rocket i would need to replace R12 with 2 220K resistors in series?

[Gaz]

I dunno if I have an old schematic or something, and I don't know what you read, but all you have to do is exactly what I said in my previous post: add a 220K resistor to ground after the 220K resistor coming of the the Treble pot's wiper.

A voltage divider is two resistors in series to ground, and the output is taken from the junction of the two resistors.

[RJ Guitars]

I love Rocket amps, the clean sparkle is probably the best of any amp I've heard. The circuit also has a lot of gain in it and I have found that they start to break up really early and move into the sweet crunch and just get sweeter and crunchier as you approach 11. I don't play in any situations any more where I need more volume or headroom, but I do agree they go over the edge very early.

Voltage dividers are a good idea and one form that has worked well in Rocket amps for me is a variable one that is known as the "PIG" control - Phase Inverter Gain. http://www.rjguitars.net/PIG_Control.jpg . By limiting how much signal you feed to the phase inverter, you can make a very clean amp out of a Rocket with this add on.... you'll probably recognize that it is a form of master volume that Ken Fischer mentions in his Trainwreck Pages, the insert he had into Gerald Weber's first book. If you search "pig and control" on this forum you'll find more info on this idea.

Also, another good read on headroom is here... smart fella's helping us understand things a little bit better here. http://www.ampgarage.com/forum/viewtopic.php?t=10489

[Jana]

Maybe try a 12AU7 in one or more of the preamp tube slots.