4 Tube Amp From Scratch

[TubeNoob]

I think we need a 'food fight' forum!! ;-)

Ok guys, thanks for the input.

Two questions if i may...

1) Would having a pot there do the trick since there would be no switching? That's how i have it on my 5f2a and it works fine.

2) Since this is for a 'presence' control, what tonal differences do you think there would be between doing it on the first stage vs. the 2nd?

[R.G.]

No, I'm not... You call this "a streak"? It's just nothing more than a casual observation..

I can think of several "better arrangements", but I'm not doing that anymore.. You want better arrangements, then cut the sarcasm first, and then ; maybe ; I'll think about....

Good. You can understand sarcasm when you see it.

Remember that phrase "legend in his own mind"?

You're not the only one who can think of better arrangements. And if you're not going to offer them, there is no difference to anyone else between you having clever ideas and not having a clue, is there? Except of course having to read about how very clever you think you are. If your solutions are clever or innovative, people will tell you - unless you have hacked them all off by telling them how smart you are first.

I'm back at what I said: in *spite* of what you actually put into the forum, I think you *might* have some decent ideas. It is *only* a suspicion, because you never put them down.

This thread makes a good example. You noticed one problem in the proffered schematic. Instead of calmly and with good grace saying what it was, you highlighted it with

ah, guys !!! {tink, tink, tink}....... no one sees something with S1 ?

and offered no help. You could have said something like

S1 is set up so it will give a huge pop when it's switched, aside from the effects you may be trying to get. You may not be planning to switch it while the amp is on, but if you do, it could be a problem. As shown, it either switches in the cap or shorts out the cathode resistor to ground, which causes a massive change in the bias on the tube.

Instead of how you have it switched, you might want to put a large resistor - 100K or more - in series with the cap, and use S1 to short across the resistor. The large resistor lets the cap charge to the DC voltage on the cathode, and so the cap does not have to be charged up when the switch connects it in, and you don't get a sudden change in DC conditions.

which would have been useful (I think).
Only after some prodding could you bring yourself to say what it was you saw.

It's OK not to tell *me* your better arrangements. It's rude to tell people with a real question that you know, but won't tell. If you do that, the best people will think of you is that you're conceited. Some of them will suspect that you're only saying that you know, but really don't know. It does not have the effect of making them think how smart you are - it only hurts your credibility on everything you say in the future.

I know you get hacked at me about this, but try to understand I'm trying to help you out. If you feel like people are picking on you as you have alluded to in several posts, you might want to spend some time thinking about whether it's more likely that everyone else is in fact picking on you, or whether you contribute to that situation in some way.

[mooreamps]

I'm sorry.. I you didn't phase your response in the form of a question....... and no you are not helping.... "not one bit"..

-g

[R.G.]

1) Would having a pot there do the trick since there would be no switching? That's how i have it on my 5f2a and it works fine.

If you mean a pot where the switch is, set up as a variable resistance, so it only varies the resistance between the cathode and the cap?

Sure. Works fine. Whatever changes the effect of the cap to ground without changing the DC conditions will work OK.

2) Since this is for a 'presence' control, what tonal differences do you think there would be between doing it on the first stage vs. the 2nd?

The term "presence" is slippery. What do you mean by "presence"? Presence has usually been taken to mean how much frequency content there is around 2-3kHz; it's a tone term, not a gain term. What C1 does (if S1 doesn't ground it) is mostly to boost the gain of the stage where it bypasses the cathode resistor to ground.

That boost happens at a frequency of F = 1/ (2*pi*R*C) where R is the cathode resistor to ground and C is the capacitance. As shown, the frequency is F = 1/(2*3.14*1500*25E-6) = 4.25Hz. That is, it boosts gain for the whole audio spectrum, no differential frequency boost happens. If you made it, say 0.047uF, it will boost everything over 2.2kHz, and will increase everything from there on up. Amp tinkerers like to futz with the bypassing on the cathodes of preamp tubes endlessly.

As it sits, it is more a gain changer than a frequency boost/cut changer.

[R.G.]

I'm sorry.. I you didn't phase your response in the form of a question....... and no you are not helping.... "not one bit"..

I'm sorry that I me didn't help. So you want me not to say anything if it's not helpful?

Think about that for a while, Gary.

[Steve Conner]

I think we need a 'food fight' forum!! ;-)

Please stop fighting, mooreamps and RG!

So the switch makes a big pop and grounds the cathode. Well, some really old amps had a first stage with a grounded cathode, it was called "grid leak bias". Probably needs an input coupling capacitor to work, though.

When I tried putting pots in cathode circuits, I always had trouble with them going scratchy. I was pretty sure I blocked the DC, too.

This is an ancient circuit of mine that shows a gain control in the cathode. As the gain is increased, the bass turnover frequency goes up, helping it not to go mushy. It also contained a tone control, but only the first tube stage is relevant to this discussion.

The plate resistor is lowish as part of the overall gain structure.

[mooreamps]

Please stop fighting, mooreamps and RG!

Who Is He Anyway ??? and why does he continue to patronize me like a 12 yo ??????

-g

[TubeNoob]

ok, let me hijack this thread back if i can!

MooreAmps: thanks for pointing out that the switch was a prob.

The term "presence" is slippery....As shown, the frequency is F = 1/(2*3.14*1500*25E-6) = 4.25Hz. That is, it boosts gain for the whole audio spectrum, no differential frequency boost happens. If you made it, say 0.047uF, it will boost everything over 2.2kHz, and will increase everything from there on up.

R.G.: Thanks for pointing that out as well as the math for it. I'll change that to .047uF.


Steve Conner: Thanks for your post. On your schematic are you using the pot as a reohstat? I am trying to recall if on my 5f2a build my pot swept from 'cap' on one side to ground on the other, so at one side it would completely bypass the cap i think.

Since i just broke my knee on Friday and am laid up with a leg-long cast now, i can't look at it. I am taking the opportunity to study tho, and am making my way thru "Electronics Made Simple" ('65, Jacobowitz) which is helping a lot.

Wanted to check back on that other question i had - what kind of differences would you expect to hear with this 'presence' control on the 1st stage vs. the 2nd?

Thanks a lot to all of you for taking the time to help me out with this...! I've learned a lot already.

[R.G.]

Since i just broke my knee on Friday and am laid up with a leg-long cast now, i can't look at it. I am taking the opportunity to study tho, and am making my way thru "Electronics Made Simple" ('65, Jacobowitz) which is helping a lot.

If you haven't yet, see if you can get a copy of "The Art of Electronics" by Horowitz and Hill. Bummer about the knee. Give it as much healing time as you can stand. Knees have a way of reminding one about past mistakes.

Wanted to check back on that other question i had - what kind of differences would you expect to hear with this 'presence' control on the 1st stage vs. the 2nd?

That's tough. It depends a lot on where and if clipping happens.

To step back a bit, if you clip a sine wave, you get a predictable set of harmonics which depend on the specifics of how you clipped it. You get the fundamental (original ) frequency, plus 2x, 3x, 4x,... times the fundamental frequency. The size of each harmonic may be from zero to something smaller than the original. If the clipping is soft, the size of the harmonics decreases rapidly as the frequency goes up; if it's hard, the size of harmonics decreases less rapidly and you get more high frequencies in the result.

Guitar outputs are not sine waves, but the general idea applies - clipping gives you a string of higher harmonics than the original frequency. It sounds both distorted and more treble-y.

If you use a lowpass filter after a clipping stage, it decreases the size of the distortion products above the filter frequency. If you use a lowpass filter *before* a clipping stage, it decreases the size of the signal above the filter frequency and so decreases the amount of distortion the clipping stage makes for those frequencies. Both affect the perceived tone of the distortion, but in different ways.

Filtering before clipping changes which frequencies get clipped and by how much. Filtering after distortion changes which of the generated harmonics you hear. For a practical review of some of this concept, you may want to read"The Technology of the Tube Screamer" at geofex.com (my own humble web page). One of the more fun things to do on a rainy afternoon is to put a graphic EQ in front of and after a distortion pedal and play with the EQs. EQ before distortion sounds very different from after, and the combination of the two is the "secret" (if there is one) of getting unique distortion sounds.

With that as background, what you're doing with the "presence" switch is increasing gain for the tube stage above some frequency determined by the cap value and the cathode resistor. Below that frequency, you get the gain with the cathode resistor, ignoring the capacitor. Above it, the gain rises to some higher level. So you're switching in a high pass filter when you switch in the cap. This makes the sound both louder and more treble-y. If you do this in the first stage, the second stage may or may not clip. But it will for sure sound a lot more treble-y. If you do it in the second stage, you will get much the same thing, but the second stage will for sure amplify the first stage's noise. If you overdrive the first tube with a gain stage pedal, then you get into the first stage clipping but amplifying the treble more than the bass.

Actually, this is such a simple thing to set up, and the results are so highly subjective, the best thing is to try it. Use a temporary switch and capacitor set on both tubes, flip the switches and listen. It'll be very interesting, and more so when one of the tubes starts clipping. You may find you want to put a capacitance across the plate resistor of the second tube to knock off some of the treble.

[TubeNoob]

Thanks R.G., I will be taking it as easy as i can on this knee. They definitely seem to haunt you later in life when mistreated!

And I'll keep my eyes open for that book.

Funny, i was going to ask you if you were "Mr Keen" or not. I know your site very well, especially your amazing article on different types of EQ circuits. I may do a post soon on a parametric tube eq circuit i found and it'd be killer to get your input.

Well, putting some temporary switches on both stages sounds like a plan. I always prefer something 'different' to something 'standard' and maybe this will give it to me.

Unless we're talking about an AC30, which i'd take totally 'standard'! I am so bummed i didn't get to that AC30 simulator you used to have on your site in time before the fella sold the patent! I do have a schematic, but it's missing some values. DRATS!!!! I had a guy in LA pull out an AC30, Fender Bassman, Champ and Marshall JTM-45 (all of them originals) from his priceless collection of about 50 amps, just for me to check out their sounds. That AC30 (to me) stood out leaps and bounds over the others.

Thanks for your highly informative post.

[Steve Conner]

Sad to hear about your knee!

I can second "The Art Of Electronics", it's easily the best general electronics book anywhere.

The pre/post distortion EQ thing that RG mentions is right on. Indeed in that circuit I posted, the tone stack shown is the pre-distortion EQ.
Here is the post-distortion EQ...
Dual parametric EQ

another tube parametric circuit for you to ponder

And yes the pot is used as a rheostat.

[jmaf]

+1 for The Art of Electronics

If you want specific tube guitar amps, Gerald Weber's books are as practical as they come. I bought all of them. It's like a guy narrating his repair shop experiences, tube amp A does this, if you do this it fixes that, etc. One of his books has a few pages by Ken Fischer of Trainwreck, the tricks he shared there are worth the book. He has a DVD where he's showing you around tube amp stuff, there are clips of it on youtube.

Also, Kevin O'Connor's complete book set was one of my first investments.

Last year I got run over by a car and spent 7 months in bed. Been there, know how it feels. Wish you a speedy recovery and happy holidays.

[TubeNoob]

Hey Steve - thanks. I'm taking it as an opportunity since it's allowing me to do so much reading! I managed to find that book, looks like a good intro for me. What im digging about this book i mentioned is that it goes in depth on tube circuitry. I love the section about 'Things to come' which discusses Lasers, Masers and Semi-conductors! Old skool style.

I was familiar with the Pre and Post eq'ing technique on distortion, but i have only tried it in software (i dont have enuf outboard eq's for that yet!).

Cool schematic on the tube eq - its only the 2nd one i've ever seen and i did some fairly thorough searches.
I'll do a thread on tube eq's soon. What are your thoughts on the sound and control of yours by the way?

Checked out your album "Way of the bedroom worrier". I really dug it. Especially Pictures of the Floating World. Great keyboard line in there. Some stuff reminded me of Future Sound of London, but it certainly spans much more than that. What fuzz did you use on "Clydeside" on the rhythm guitar towards the end? Very smooth - was that the pre-post eq in action?!

U still have that JX8P? I love Rolands from the 80's. And I dig softsynths but can't part with my tangible ones. My main axes are an MS-20 and a SH-101 both of which i've modded, tho i do have a bunch of polyphonic/digital ones (JD800, MS2000, JP8000, SQ-80, etc). I am taking a look at that new Arturia Lab synth and software - incredible price for all that.
_____________________________________________________

Hey Jmaf - thanks for the good wishes. Wow - 7 months, ok i won't complain about 7 days then! I have heard much about Weber's books, i will have to see about getting into some of them. I'll also check YouTube for some of those vids as a little preview. Thanks for the recommendations...!

[Steve Conner]

Hi TubeNoob

I built the amp with the tube parametric EQ 10 years ago, and I haven't taken it apart or modded it into something else. So either it sounds good, or I am lazy, or it just makes a nice ornament for the living room.
scopeblog » Ninja Töaster

The EQ took a lot of design effort, and it has always "just worked". It only has 12dB of boost or cut, so it isn't capable of really extreme EQ. But it is plenty good enough for making a scooped-out metal sound, or a Jaco style bass tone. The noise floor isn't quite up to the standard of a piece of studio gear, though it's quieter than most vintage guitar amps. I improved it a bit by changing the op-amp to a MC33079.

It was my only amp for years and most of the guitar parts on the album were done with it, except for the smooth-sounding fuzz. Ironically, IIRC, that was a solid-state distortion pedal straight into the mixer. I think it was a Marshall Shredmaster that I modded.

I got rid of nearly all of my electronic music gear, including the JX8P. All I have left is a Roland 606 drum machine, and a TB303 clone with MIDI. I am highly tempted to buy a Moog Little Phatty.

PS, my fanbase just doubled, I am delighted!

[jmaf]

Hey Jmaf - thanks for the good wishes. Wow - 7 months, ok i won't complain about 7 days then! I have heard much about Weber's books, i will have to see about getting into some of them. I'll also check YouTube for some of those vids as a little preview. Thanks for the recommendations...!

Oh man, what envy. SEVEN days? Bah! You'll be back on the workshop in no time. I watched every Nat Geo program there is...wanna know about the most poisonous jellyfish on Earth? Komodo Dragons? How houses are built on ice?

Weber's are didactic, he's not into math and instead of showing you maxwell's equations he'll say "'65 Fender Deluxe Reverb runs tubes real hot, that is because of A and B and here's how you can do C and D". He's opinionated and some folks don't like that, understandably, but if there's a place to start it's there IMO.

I also forgot Merlin's books: Merlin Blencowe's Books and Publications Spotlight

And you can get tons of info here: How to design valve guitar amplifiers