Gain

[cyclone]

I think you and I are already building similar amps My current favorite design, and my personal amp, is three stages with no master volume into an LTP driving a pair of EL84's. I run the amp with the volume on eight or nine most of the time and use a "bright cap" type of circuit on my guitars volume pot so that I can get chimey cleans by turning down my guitar. As it happens, A bigger than average OT combined with a deep combo cabinet loaded with a Vintage 30 makes for a very acceptable "do all" tone that fits well into most live mixes and still has bottom and top end dynamics. Even if I had post distortion EQ (which I don't) I wouldn't really have a reason to adjust what I have now. That was an important criteria for me with this design. Sound familiar?.

Wow, thats EXACTLY what I'm doing! Same cab and speaker etc. ( using a WGS et 65 celestion knockoff)

[Chuck H]

I no longer have a schematic for my amp. I lost it to a computer virus. But I did have the layout w/component values on a backup. I'll show you mine if you show me yours. PM of course.

[cyclone]

For sure, I need to update as my schem has been "superceded" and I added bias wiggle tremolo to the design which is working out really well. I'll do that tonight in the hotel and send you something.

Dang, I just ordered a faceplate from Front Panel Express, I coulda put an hpgl graphic of camo pants and a chain wallet on there for the gain control!! LOL too funny!!

I can't make up my mind what to call the volume control but settled on Gain for some reason. It's cheaper to engrave than Volume, I'll go with that, Leo would approve

[R.G.]

What the guitar player really wants is high sensitivity on the input. Not high gain. These are frequently mixed up and confused.
What the guitar player really wants is distortion from the output tubes being over driven, not the preamp.
Amps that have preamp overdrive are just trying to imitate the REAL sound of over-driven output tubes.
Output tube overdrive is the real thing. All imitations (especially "effects" pedals) fall short of the real sound.

Is that an objective fact, supported by independently verifiable research, or should you have sprinkled in a few "IMHO" disclaimers?

It's true that overdriven output tubes have a REAL sound; so do overdriven preamp tubes. All sounds are REAL if normal-hearing people can hear them.

And that there can be no disagreement about matters of taste. There can be even less implication that one knows what someone else wants.

It's probably true that the (meaning at least one) guitar player does want high sensitivity on the input, not high gain, and that the (meaning again, at least one) guitar player does want output tube distortion. I suspect it's true that there are counter examples as well.

I'm pretty sure that "the guitar player" doesn't mean "all guitar players" or even "statistically valid most guitar players" because (1) I'm pretty sure that the opinion research has never been done, and no, I can't prove that it hasn't been done, since it is logically impossible to prove a negative; and (2) I've had a number (meaning in this case more than one) of guitar players not be able to tell the two apart on listening tests, so there are counter examples to the "all guitar players" implication.

One of the great informal lessons on getting ahead in my prior career was to learn to speak confidently and in absolutes about things one had no real knowledge of. It's how they minted vice presidents. I generally flunked that lesson, as I was nauseated at trying it for myself as a result of seeing the real life application of previous graduates of the course. But it did give me a finely tuned sensor for seeing the process happen. All these bells ring and flags wave when it happens. Weird, huh?

[Chuck H]

Well... To be fair...

What SGM said was true on two occasions in history. Once before master volume amps were popular and had the bugs worked out and then again from about 2000 to 2005 when all things boutique and vintage were considered the holy grail. So I can see where someone could form that opinion. It's just that his perception goes back to irrelevant cultural trends that now constitute a much smaller demographic.

And power tube clipping DOES sound different from preamp tube clipping. But the generic thinking of "preamp tube clipping sounds buzzy and thin" is a very dated notion. Both are used very effectively by different players every day. Each still has it's proponents. and that, at least, makes sense.

If I had to choose the type of clipping MOST guitarists prefer I would have to nod to preamp tubes. The bevy of master volume, channel switching 20W to 50W amps that sell to working musicians is enough testimony by itself. But you can add to that all the cheap, Chinese made uber gainers that the thrash monkey kids line up to buy. Few of the latter and former will ever see much power tube clipping and they comprise the majority of amp sales. Even before the monster gain amps, the 20W to 50W channel switching combo has been the popular choice for both bedroom and club for a couple of decades!

Relatively few players actually trouble with the hassle of power tube distortion simply because it's loud and most attenuators are expensive. Power scaling amps haven't had enough presence (and still don't) in the market place to change any of the above observations IMHO.

[bob p]

What SGM said was true on two occasions in history. Once before master volume amps were popular and had the bugs worked out and then again from about 2000 to 2005 when all things boutique and vintage were considered the holy grail. So I can see where someone could form that opinion. It's just that his perception goes back to irrelevant cultural trends that now constitute a much smaller demographic.

Wow, that was harsh. Accurate? Maybe, but definitely harsh.

[Chuck H]

Sorry if I offended anyone. I've been known to shoot right to the point with an utter absence of diplomacy. Most people tell me it's a bad thing. I usually go with the "don't shoot the messenger" position. I take a few shots for it anyway.

If it softens things at all, I'm a proponent of power tube clipping. I've been through many amps and more mods and circuits. "My" thing definitely includes power tube clipping. So SGM had me pegged anyway.

[loudthud]

On a more serious note, I've been trying to measure the gain of my latest project. It's not easy. The number came to 7900 from input to a 4 ohm load. Input was 1.9mVrms at clipping. I don't think the gain is super high, about the same or a little higher than a 5F6A. The preamp doesn't clip very hard if you turn down the Master Vol. I may have to add clipping diodes for the players that need that super saturated low volume tone.

Now I need to measure some vintage amps to get a frame of reference.

Any of you guys have the equipment to do the measurement? I think the low tech way would be to contruct a 1000:1 attenuator to use on your generator so you can have an input big enough to measure.

Edit: To be fair the gain is 78dB at about 100uV input and falls to 75dB at clipping. 75dB is a gain of about 5600. The amp has 3dB of compression.

[JoeM]

Hey!!, where did you find that?
It's my 1976 to about 1980 "bread and butter" model, sold almost 1500 of them (plus matching 2x12" cabinets) and definitely helped pay for my house.

Many of them are still found all over the place, (35 years later) and I'm probably making a "reissue" batch to test waters, since some people claim "I'd buy another if you made one now".

Ok, let's see if they let their wallet repeat what their mouth says.

Is this an SS amp, Juan?

[J M Fahey]

Is this an SS amp, Juan?

Yes.

I started making tube amps in 1969 , straight from Jack Darr's book schematics, progressing from a single 6V6 Gibson GA(5?) to Fender Bassman 50 to Bassman100 clones (*no* reverb tanks available in Argentina so no "XXXX-Reverb")) until 1972, when we had a *big* Economic Crisis, , Argentina defaulted its U$ debt ..... and ***nothing*** got in, specially tubes.

Whatever you got was bootlegged.

Now, a guy can bring inside a suitcase, say, 4000 12AX7 and make good money on them even paying a round trip Buenos Aires to NY or Miami, but only 200 or so 6L6/EL34, because "larger glass = easier to break" : the numbers do not add up.

So we had a reasonable supply of preamp tubes, but **no** power tubes, period.

So everybody and his brother switched to locally made 6DQ6 , from "Deltron", an RCA plant which also made 5U4 and a couple more BW TV tubes.

I found them *horrible* (they are designed for SMPS duty, after all) so I got into SS, which was cheap and plentiful.

Now, I could hear plainly that the sound was not the same as my tubed amps, so I started testing and experimenting non stop until I coaxed good sound out of them.

Been doing so for over 40 years now

FWIW there's musicians who *sold* their Ampeg SVT or Marshall heads or whatever to buy some of my SS stuff, so I must be doing a couple things well



EDIT:

On a more serious note, I've been trying to measure the gain of my latest project. It's not easy. The number came to 7900 from input to a 4 ohm load. Input was 1.9mVrms at clipping. I don't think the gain is super high, about the same or a little higher than a 5F6A.

1.9mV means a very sensitive amp and easy to clip.

Maybe you didn't measure it properly.

FWIW I consider single coils=50mV and HB=100mV RMS , to cover relaxed players, thin strings, etc.

Any higher sensitivity helps crunch, if needed.

A classic Marshall (Plexi, MK2, etc.) is 5 to 10 mV sensitive, that's why LP and SG were popular with them, and Strats required pedals.

The preamp doesn't clip very hard if you turn down the Master Vol. I may have to add clipping diodes for the players that need that super saturated low volume tone.

Or add 1 extra stage.
What JCM800 and Mesa Mk1 did: add an extra tube to the conventional Marshall or Fender amp most popular at the time.

Now I need to measure some vintage amps to get a frame of reference.
Any of you guys have the equipment to do the measurement? I think the low tech way would be to contruct a 1000:1 attenuator to use on your generator so you can have an input big enough to measure.

That's the high tech way

In the old days *good* Labs (not your typical TV repairman) used a dB calibrated, switchable precision attenuator.

But you can use a 100:1 (220K/2K2) to good effect; maybe a 1000:1 (220K/220r) to test very high gain "Metal" preamps.

Edit: To be fair the gain is 78dB at about 100uV input and falls to 75dB at clipping. 75dB is a gain of about 5600. The amp has 3dB of compression.

If you lift NFB and drive it hard you'll have far higher power tube compression.

Preamp tubes have to be driven very hard and compress very little, that's one of the reasons for those cascaded gain+attenuator nets one after the other.
They are ear tuned so they work at the "soft spot" and compression adds up.

[Steve Conner]

Each tube stage has a gain of about 50, except the cathode follower and the output stage which are both roughly unity gain. The 5F6A has three gain stages including the PI, so we would expect a maximum gain of about 12,500. Loudthud's figure of 7900 is in the ballpark.

50 watts into 8 ohms is about 20V RMS. If the guitar pickup has an output of 200mV, then we can see the amp really only needs a gain of about 100. The 3 gain stage circuit has about 100 times more gain than it needs, it can potentially overdrive itself by about 40dB.

Of course tone stacks and feedback loops will reduce the raw gain on tap, but those could be bypassed if an amp designer was intent on getting the maximum gain from 3 stages.