Do more preamp tubes = more gain?

[ElUnoAstuto]

I've seen a lot of amp descriptions(5150+ comes to mind) where they'll say, "it has an added preamp tube for the facemelting distortion you crave!", or something similar. Does the number of preamp tubes in someway corrolate to the amount of gain an amp is capable of? And if that's the case, would, say, a Peavey Classic 30 with 4 12ax7's have more gain on tap than a Peavey Valveking, with just 3?

One side question though: Are Peavey Classic 30's MIA? That's the only thing I don't like about my Valveking, those three dirty words: Made In China.:yell:

 

[B Bent]

Re: Do more preamp tubes = more gain?

Not always the case. Take a look at a Fender Twin and all it's preamp tubes. I guess it all revolves around the rest of the design.

 

[marvar]

Re: Do more preamp tubes = more gain?

more tubes doesn't necessarily equal more gain- often it does, but it depends on the how the amp/circuits are designed. I mean look at a Fender twin-has a bunch of tubes, but not a massive amount of gain. Some amps use separate tubes for channel 1 and 2. Some amps use tubes to drive tremolo and reverb circuits, some amps use tubes for rectification, etc. It mostly depends on WHERE the tubes are located in the circuits. If more tubes are added in the front end, or preamp sections usually it does equal more gain.

 

[kmcguitars]

Re: Do more preamp tubes = more gain?

Much like marver said each preamp (Style tube) have their own job Depending on the circuit design. It could be a extra gain stage? Think of it this way... A Marshall 2203/2204 circuit has only 3 pre amp tubes V1 & V2 are pre amp stages while V3 is for the power section Phase Inversion. Maybe not a high gain amp by todays standards? But has much more gain that a 2 channel BF/SF Fender which uses 6 preamp style tubes.

 

[SFW]

Re: Do more preamp tubes = more gain?

The more pre amp tubes you have, the more potential gain stages you have. As stated above, it really depends on the design of the circuit.

 

[frankfalbo]

Re: Do more preamp tubes = more gain?

If you think of each preamp tube as a "booster" that feeds the next tube, then if they're cascading from one tube to the next, you can more saturation and gain. But you can really achieve all the gain you need from one tube if you cascade it through itself. The Hughes & Kettner Tube Factor pedals and others like them attest to that. So in the case of the marketing language used on the 5150 (extra tube adds gain, etc.) in their circuit it is probably the case. That "extra" preamp tube is being hit for another level of saturation.

 

[joelap]

Re: Do more preamp tubes = more gain?

No, not at all. With the stock Marshall Plexi 3 preamp tubes, it is possible to hit SLO-type gains with something magical called Power Scaling.

Paging Shredaholic... I'm sure he can probably smell this thread since I mentioned the PS words.

 

[aleclee]

Re: Do more preamp tubes = more gain?

No, not at all. With the stock Marshall Plexi 3 preamp tubes, it is possible to hit SLO-type gains with something magical called Power Scaling.

Power scaling doesn't give more gain. It effectively restricts the amp's output power so the power section distorts at lower volume.

 

[ErikH]

Re: Do more preamp tubes = more gain?

One preamp tube is split in to two gain stages. Look at amp schematics like old Marshalls and Fenders and you'll see it right away. One half will drive the other half like frankfalbo said. There's different types of gain too. There's "clean" gain and there's "dirty" gain. Without any gain at all, there would be no sound.

Clean gain is where the tubes are driven before any breakup at all, see Fender Twins, etc. for that type of gain. Once the breakup occurs, "dirty" gain, the headroom of the tube has been reached. Any further breakup is just pushing it more beyond the breakup point. Marshalls are designed in such a way that the breakup point is much lower than in a Fender, though the earliest Marshalls *are* a Fender, the Bassman, just with some changes.

Depending on the design, yes more preamp tubes can mean more "dirty" gain, but not always.

Also, remember that one of those tubes is always the phase inverter which is AFTER the Master Volume (for MV amps). It's not in the preamp at all but in the power amp section. So, even though old Marshall 1959's, and even 2203/2204's, have 3 preamp tubes, the preamp actually has only 2 tubes in the circuit.

Take those 2 preamp tubes in that same amp. The first 3 stages (tube halves) are the gain stages. The last half is set up as a buffer to drive the tone stack, then on to the phase inverter and so on.

 

[AdmiralB]

Re: Do more preamp tubes = more gain?

There's "clean" gain and there's "dirty" gain.

Gain is gain, it is neither soiled nor spotless! The reason something like a BF Fender doesn't distort much is that the amplification provided by the first tube is pretty much lost in the tone controls. The second tube recovers that signal, and drives the power section. There is neither enough gain to significantly clip the preamp tubes, or the power tubes.

A Marshall non-master has the tone controls later in the circuit, and the signal from the first tube can (if the volume is set high enough) slam the second tube hard enough to clip it. There is also sufficient drive into the power amp, due to the buffered tone controls, to overdrive the power tubes too.

Some amps use tubes as distortion devices not unlike diode clippers. The MV Marshall amps (2203/2204) and Trainwreck Express/Komets do this - they feed signal into a tube biased very cold so that one side of the waveform causes the tube to 'shut off', clipping only one side of the wave.

IMO, losses due to tone controls aside, there's a practical limit of about three triode stages (one and a half 12AX7s, say) before you have a square wave and more gain doesn't do anything.

Also, remember that one of those tubes is always the phase inverter which is AFTER the Master Volume (for MV amps).

Not always. Lots of master designs come after the PI.

 

[ErikH]

Re: Do more preamp tubes = more gain?

Gain is gain, it is neither soiled nor spotless!

Very true.

The reason something like a BF Fender doesn't distort much is that the amplification provided by the first tube is pretty much lost in the tone controls. The second tube recovers that signal, and drives the power section. There is neither enough gain to significantly clip the preamp tubes, or the power tubes.

I forgot the tone controls in those things were right there in the middle of everything in the preamp instead at the end.

IMO, losses due to tone controls aside, there's a practical limit of about three triode stages (one and a half 12AX7s, say) before you have a square wave and more gain doesn't do anything.

I agree here completely. That's what I love about the older Marshall designs. It's just a matter of the bias and how each stage is fed that'll influence the tone after that.

Not always. Lots of master designs come after the PI.

I've heard a lot of discussion on here about PPIMV design and modding amps for them but haven't come across any that are built that way. Many schematics I've seen (across the major players in the market) have the MV pre PI. Who makes them stock like that?

 

[AdmiralB]

Re: Do more preamp tubes = more gain?

The earliest MV I've ever seen is the one used on the Orange Overdrive amps, circa 1970. They're just the Graphic amps with a dual 1M pot after the phase inverter, one section per side, isolated by blocking caps on both ends. Marshall also used this circuit on the Club and Country amps in the late '70s, and the first version of the 2205/2210 JCM 800s. I think the DSL 201 and 401 use it too.

Matchless uses a variation that puts a pot after the PI and simply shuts the two sides together - rather than grounding the signal, it it self-cancels.

The 'Trainwreck Pages' in the Weber book list several types of post-PI, but I don't remember all of them.

FWIW I prefer the tone of the pre-PI, myself.

 

[ErikH]

Re: Do more preamp tubes = more gain?

I forgot all about those old Orange amps. Now those babies had some tone. Didn't know about those early 2205/2210's. They must have been really limited because the schematics I've seen have the MV pre-PI.

It sounds like an cool idea. Never played through an amp with it that way myself but I'd like to just for the heck of it. I can see where it would be useful since the PI would now be getting overdriven as if a pre-PI MV was fully open. To me it sounds like the halfway point to an attenuator built-in to the amp.

 

[AdmiralB]

Re: Do more preamp tubes = more gain?

The schem for the 2205 is in the Pittman book, it might be online also. I think that version was made in '81 and maybe '82, definitely not later than '83.

I've never seen a 2210 made to that schem, but I assume they must exist.

 

[devastone]

Re: Do more preamp tubes = more gain?

Check here under JCM800s, there are quite a few different versions of the split channel 800s, some sound better than others from what I've heard. From looking at the schematics, it's easy to believe that some sound more "Marshall" than others.

 

[ErikH]

Re: Do more preamp tubes = more gain?

I see the PPIMV on the 2205 now. It looks like there's 2 of them, one for each power tube. Pretty cool. It doesn't look like the 2210 has it, at least the schematic I see doesn't (it's from 1988 and like said, both amps were pre-PIMV by then).