How does it actually work?

Hello!

I was thinking about tubes recently and it occurred to me I don't actually know how my Herbie preamp works (or any other high gain amp head) and which tubes are doing what or distorting or not!

So V1 and V3 are always used, and V2 gets added for Ch3, I know that much. But each of those are dual tetrodes, so is the first half of V1 basically always clean regardless of channel? And then does it go just through one half of V3 still clean on channel 1, or does it pass cleanly through them all? For channel 2- is the first tetrode then still clean but the second half of it gets used and pushed into distortion, then V3 then cleanly amplifies it continuing on it's journey? Does V3's second tetrode get added in for ch 2+? Is there then 2 tetrodes running distorted? Does the gain knob effect one or both of them? Do all the tetrodes in ALL the tubes get used for distortion? Why multiples tubes distorting rather than 1 being pushed harder? Or is it that the first one pushed hard as possible is limited so can't distort the next since it's out of headroom, so later tubes have to run at a lower plate voltage or is the saturated tube still capable of overdriving it in the first part of the roll off and hence gets more saturated, compressed and square-wave-like? For channel 3 are both additional tetrodes or just 1 more all slamming each other?

Anyone got a mini guide to high gain amp heads? Just curious that I know so little about something I love so much!

(Whoops, meant triode, not tetrode, that's got more grids innit!)

Depends on the design/amp.

You can have seperate triodes for each channel or share them or some of them. Some designs share V1A for all channels (like Diezel) or use seperate V1As and share (some) of the others.
Then if you mix Fender-designs (preamp-wise, no cathode follower to drive the tone stack) you mix it with the Marshall-designs (CF driven tonestack) in front of the loop. Depending on the design one can either drive all tonestacks from a single CF or have separate.

Cool, cheers for the info Olaf! I've got a slightly better idea now, but still have more questions...

So is it V1A is always like a clean input buffer regardless of channel, or does the gain of it change depending on the channel and hence is sometimes used for distortion? V3B is a CF that drives the tone stack that comes last before the loop drivers, and which tone stack depends on the channel selected (different knobs!)?

Then Ch2- uses V1B for distortion, and Ch2+ uses V1B AND V3A for distortion? And the gain knob effects the gain of both V1B and V3A, or just the gain of V1B and if that's saturating V1B the output will be at a level that naturally clips V3A too?

Then for channel 3 does it add in 1 extra triode V2A? Or 2, both V2A and V2B? If it uses all 6 triodes does this then make it 4, 5, or 6 "gain stages". I've heard the Peavey 6505 has loads of gain because it has "6 gain stages". What exactly does this mean? Is that more or the same as the Herbert? (Not that I'd ever swap my Herbert for one).

Slightly less exciting part of the preamp, but does the the loop driver then just raise or lower the internal signal level (and or give the right impedance) to drive the loops with one triode V4A? I assume it doesn't need another triode between loops if they're all the same level? I assume they're always in the circuit even if you're not using the loops?

Same for return driver V5B? Just one triode? I'm guessing this bit can't be right else you'd use V4A for loop driver and V4B for return driver and save a few quid.

What does the phase inverter do? I assume an emitter follower with one half and a cathode follower with the other since it has to be balanced?

Sorry for the million questions, but I just like knowing how cool stuff works that affects my life: anything from computers, to plants, to stars and black holes, to relativistic physics (good 'location' Olaf!), to women, to RF! I don't understand people who happily live in ignorance of how all the great things that surround them and change their lives actually work.

This thread for me:

Me":2he9niw6 said:

Sorry for the million questions, but I just like knowing how cool stuff works that affects my life: anything from computers, to plants, to stars and black holes, to relativistic physics (good 'location' Olaf!), to women, to RF! I don't understand people who happily live in ignorance of how all the great things that surround them and change their lives actually work.

Click to expand...

This. Yes! I am not sure I could have said that better myself.

Sigma.Xiria":1gjzrdx7 said:

This thread for me:

Click to expand...

LOL. I get the principles of how all the tubes and circuity bits work on a component level so I'm obviously ahead of the "Guitar -> Glowing Magic -> Wonderful Sound" level, but each of those 6 preamp tubes has effectively 2 little amplifiers in (2 triodes, kinda equivalent to 2 vintage transistors in each). I have no idea what each of them are doing though at a 'system level' for each channel.

This is mostly out of curiousity, partly cuz it'd help fault-finding duff tubes (one of mine went crap recently), and I guess for the tube geeks amongst you it'd be good to know which ones are only being used slammed for distortion and which ones want the clearest cleanest jangle too. But I'm guessing by deduction half of V1 and V3 are used for clean, the other halves of each are used for distortion (so good all-rounders I guess unless you have specially selected unbalanced wonky valves!) and V2 for ch3 distortion only.

I would suggest that you look at the schematics of simple amp first. Say a 2204. It has 3 gain stages and 1 cathode follower which drives the tonestack (= 2 preamp tubes = 4 triodes).
A simple Fender Blackface f.i. just has two preamp triodes. Now how to combine a crunch and a clean amp?
One could easily just split the signal after the input jack and run both preamps parallel and just "decide" at their end which one to feed into the power amp. But to save parts/costs/room inside the amp and so on, one mixes things and uses f.i. relays which add triodes and their parts here and there . . . .

To make things a bit complicated: A CAAA OD100 has a clean channel with three stages - and a crunch channel with three triodes. So the parts (actually it is the working conditions of the tube) can be varied so much that both extremes can be achieved: clean or distorted.

A 5150 has six stages, which means that before its tonestack there are six triodes amplifying the signal. Not necessarily this has to be more gain than 3rd gear of a Herbert (which has "just" 4 stages) - it is all a question of the conditions the triodes are running. Those are affected by the parts around the tube, its plate, cathode and what is placed before (grid) and after the tube, also the voltage inside the amp has a huge effect . . .

I would also suggest that you google "Valvewizard" - a nice bacis tube amp explanation.

duesentrieb":8rjq05l5 said:

I would also suggest that you google "Valvewizard" - a nice bacis tube amp explanation.

Click to expand...

Cool. Will do.

So Herbie has 4 gain stages? Is that is 4 gain stages for all channels, all the time, + and - modes, and it uses v2 just to get 2 triodes configured within a different bit of circuitry for channel 3 instead of 2 of the others?

There are 4 preamp tubes for the preamp which includes the cathode follower (-1), which makes 7 gainstages total.
Channel 1: 2 of them
Channel 2: 3 of them
Channel 3: 4 of them
So two gain stages are shared between the channels. Which is the very first one (V1A) for all channels and one triode which is used for channel 2 and 3.

Here's one of my amps, a 50 Watter with 5 ECC83 and midi, dual master vols and a switchable loop . . .

V1A for all channels
V1B 2nd for the clean channel

V2A shared for crunch and lead (gainpots switched) = 2nd stage for both
V2B 3rd stage for lead

V3A 3rd for crunch and 4th for lead
V3B driver tonestack

in front of the loop are relays which switch the three different channels into it.

V4A loop send
V4B return driver

V5 phase inverter

Forgot to say, cheers Olaf! I had a brief look at that, but when I'm not so busy and have more thinking time I'll sit and have a good analyse. Very informative.

Is there a particular reason you add the extra lead triode between the two crunch ones, rather than on the end of them, or is that just personal choice and it's easier for the circuit layout? I assume they're all saturated so the level out the end to the next part of the circuit is the same?

The reason is that the last tube (two triodes) in the dist. chain can't be changed, so any variation must be made before the signal enters the last tube which is a gainstage AND the driver for the tonestack.

I'd love to build an amp one day for fun. Never enough time though. So in the mean time I'd just like to know how the one I have works.

I have thought about buying an amp kit or an old basic head and experimenting with some mods, but I know it'll end up collecting dust and costing me money I don't have. Wouldn't really be sure where to start "making it better" though! Maybe in about 30 years when I'm 65 and retire!

Wonderful! Keep it coming guys.