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Author Topic: New Single End Amp  (Read 3963 times)

Littlewyan

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New Single End Amp
« on: April 22, 2014, 03:47:46 PM »
Hello Everyone

Today I started looking at building myself a very small Valve Amp that would give me good tone and distortion, but obviously at a low volume (well lowish :D). Anyway I had a look at AX84 and looked at their P1 Extreme. I knew I wanted to use a 6V6 in the Output Stage and a 12AX7 in the Pre Amp so this was a good start.

So I played around with their design and come up with what I've attached. Now pretty much the only things I've changed are moving the Tone Stack to after the first triode, changing the bias of the 2nd triode and lowering the B+ to 250V (should give me 4.5W according to the 6V6 Datasheet).

So what do you guys think? I'm hoping that this will give me nice power amp distortion, however the original design looks like its based on a mix of Pre Amp and Power Amp distortion so not sure if I'll get much distortion until I really turn it up. Also I've never touched a single end amp in my life so I don't know how hard to drive it or anything!

Roly

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Re: New Single End Amp
« Reply #1 on: April 23, 2014, 11:34:39 AM »
Aww yeah, looks pretty reasonable.  Where you've got the tonestack is very traditional.   :dbtu:

I'd suggest a 0.001uF/400V, or better a well chosen VDR, across the OPT primary.

S.E. OPT's normally have a gapped core to prevent saturation due to the unbalanced DC, but a few S.E. builders here have tried the $A20 ungapped M1120 20W 100V Line PA tranny and have good reports, lotsa steel for the power; that or similar could save you some bucks.
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

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Re: New Single End Amp
« Reply #2 on: April 24, 2014, 04:58:05 PM »
What is an VDR and what would it do?

Already got an OT in mind, I'm cheating a bit in a way as ampmaker.com sells a Chassis, OT and PT which is perfect for this amp. So a lot of work is done for me already :D.

Now I've been experimenting on LTSpice with the circuit and as much as I want to keep the Tone Stack inbetween the gain stages, it does have slightly more loss than having it after. Also I've found that the output waveform appears differently for each, I've attached some screenshots of each circuit and their waveforms. Probably wouldn't be much work to swap them around if I changed my mind anyway.

Littlewyan

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Re: New Single End Amp
« Reply #3 on: April 24, 2014, 05:11:06 PM »
Or there is the original circuit from AX84.com which going by some youtube clips does sound quite JTM45ish which I do like. Only difference being I wouldn't both with a master volume, I prefer just the plain simple Pre Amp Volume.

Roly

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Re: New Single End Amp
« Reply #4 on: April 25, 2014, 02:17:21 AM »
If you look at s1 and s3 you will notice that the waveform is still settling to its final steady-state shape, so you really need to run the sim a bit longer so you can see both the initial and ultimate waveforms.

"Distortion" adds frequencies that were not present in the input (which is why guitarists love it and Hyper-Fi-ists hate it).

Symmetrical clipping adds mainly odd order harmonics.

Asymmetric clipping adds both odd and even harmonics (hence why people play around with using differing diodes in signal clippers).

A tonestack placed ahead of a clipping stage changes which frequencies clip first/hardest.

A tonestack placed after a clipping stage changes which newly generated harmonics are enhanced or reduced.

When you have two gain stages following each other it is easier to push the second stage into clipping than when you have a lossy tonestack in between, in other words it is a matter of how the gains and losses are distributed along your signal chain, e.g.;

x30 x0.1 x30
or
x30 x30 x0.1

Assuming 1V in, the first stage will produce a swing of 30V, and in neither case will the first stage clip.  In the first case this will be cut back to 3V by the tonestack (set flat) before it is applied to the second stage, and x30 will give 90V output (perhaps not itself clipping but certainly driving the output stage into clipping); in the second case the second stage will see an input of 30 volts and try to multiply it by another x30 to 900 volts of swing which it obviously can't do because of the HT limitation, so you will get a clipped wave into the tonestack.

I would certainly try it both ways, all gain before, or tonestack in the middle, because you may find one more to your taste depending on your material and playing style.

In my view you would ideally have a tonestack both ahead and after a clipping stage to give you the fullest control (but because tonestacks are lossy, about -20dB on average, this is a more complicated way to go, would require an extra gain make-up stage).

I'm sure that phatt will have something to say about this given his deep dig into the topic over the years.

I also suspect that your 680 ohm value for R12 may be a bit on the low side; you need to check the DC conditions of both triode stages give you a bit above half the supply.

I'd also add a cap following the tonestack to keep the grid current out of the tonestack pots or you may get some scratchy noises when they are moved, not dirt but DC - it's good practice to DC isolate all pots for this reason.
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

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Re: New Single End Amp
« Reply #5 on: April 25, 2014, 12:59:37 PM »
So basically a Tonestack placed ahead of a clipping stage would give you more Output Stage Distortion than Pre Amp Distortion, which is probably what I want really. I find you get better clarity with Output Stage Distortion than Pre Amp. Altho I do want to have a Marshally sound so may just keep the tone stack after the gain stages. Pros and Cons.

I did work out the bias of the Pre Amp Valve using a load line and I got 300ohms as the Cathode Resistor! Will probably use split Cathode with 820ohm Resistors on each. If you bias a Pre Amp too cold it can start to sound buzzy I think.

Roly

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Re: New Single End Amp
« Reply #6 on: April 25, 2014, 02:43:17 PM »
Quote from: Littlewyan
So basically a Tonestack placed ahead of a clipping stage would give you more Output Stage Distortion than Pre Amp Distortion, which is probably what I want really.

Yep, pretty much.  If you want a lot of shred then put all the gain up front, but if you want clean to more moderate distortion put the tonestack in between.  But you should experiment and let your ears be your guide - that's a major reason for building rather than buying somebody else's compromises.   ;)

Similarly you can play around with the cathode bias resistor to taste, but you should start with a value that gives you about half the HT supply voltage on the anode - this will be the cleanest with maximum headroom, then tweek to taste.
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

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Re: New Single End Amp
« Reply #7 on: April 25, 2014, 07:22:33 PM »
Hang on hang on, somebody got his maths wrong...............If you go by the datasheet it actually specifies about 2.3K for -1.5V or for -1.25V 1.5K :-[

Will probably start with 1.5K on both Triodes, put the tonestack after the gain stages and have calculated the cathode bias resistor for the 6V6 to be 270ohm which will bias it at 9.5watts or there about I believe. I think if I used 820ohm it would be a bit much but like you said, let me ears be the guide. I think these are good values to start with though.

Would I be right in saying that if I bias the Triodes so they don't clip as easily then they'd be pushing out (letting out) more of the original signal so would be driving the Output stage harder?

Roly

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Re: New Single End Amp
« Reply #8 on: April 26, 2014, 05:51:02 AM »
Somewhere around 1k2 to 1k5 is pretty typical, this gives about -1V bias at a section current of a bit under a milliamp (two sections on a single cathode resistor, as it was, should be around half).

You will find a table of typical values here;

http://www.ozvalveamps.org/rcatriodes.htm

270 ohms for the 6V6 looks about right.  This is generally a 5 watt wirewound ceramic power resistor 'coz it gets hot - so don't cuddle it up to the cathode bypass electrolytic, they don't like heat.  Measure the voltage across this resistor and apply Ohms Law to find the current, then multiply by the supply voltage to obtain the power - this should be less than the maximum anode power rating from the data sheet.  {this simple method isn't exactly accurate, but the error is small and on the safe side, it slightly over-estimates the anode power}.

Quote from: Littlewyan
Would I be right in saying that if I bias the Triodes so they don't clip as easily then they'd be pushing out (letting out) more of the original signal so would be driving the Output stage harder?

Normally we bias an active device (valve/transistor/FET) so that the output element (anode) has the largest available swing above and below its resting or quiescent point.  This is the voltage that results in the output clipping against the HT supply rail (device off) and roughly ground (device saturated on) at the same time as the input is increased.  This gives the stage maximum "headroom".  If the resting anode voltage is higher or lower it will clip, run out of available voltage swing, against either the HT or ground first, wasting available swing on the other side, and reducing headroom to stage overload.

This is for "normal" audio applications, but for guitar some clipping on one side before the other may actually sound better than the Hi-Fi optimum, however the proper mid-point is a good place to start any experiments.

Note that while the device off clip against the HT rail is pretty well defined, the on clip at device saturation is less well defined with valves since they will still have some voltage across them even when saturated on - transistors on the other hand can go from off to hard saturated on, still a little soggy but much harder.  For this reason the optimum resting anode voltage in a valve stage is generally a little bit above half the HT supply, a few volts.

{Voltage amplifiers like this don't so much "push" or "let" out their signal, so much as they vary the voltage on the anode up and down and the following stage "senses" this change (normally via a DC blocking/AC coupling capacitor).  You can think of the classic triode stage as a fixed anode resistor in series with an electronically variable pot producing a rapidly variable voltage divider across the supply, the bias only required to make an imperfect real world device work properly.}

HTH
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

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Re: New Single End Amp
« Reply #9 on: April 26, 2014, 11:36:24 AM »
Thats a good link, thanks for that Roly.

Might use a 330R Resistor as I think I've got an old 330R 5W Wire Wound in my stock, will have to check. 330R will be ok as the valve will still only be operating at about 10watts.

I'm pretty much ready for this now, only thing stopping me now is money. Trying to sell my Marshall 1987xl as I no longer require it and the money I get from that will pay for this project. O I may have forgot to mention I built myself a Trainwreck Express earlier this year!

teemuk

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Re: New Single End Amp
« Reply #10 on: April 26, 2014, 02:11:07 PM »
Quote
A tonestack placed ahead of a clipping stage changes which frequencies clip first/hardest.

A tonestack placed after a clipping stage changes which newly generated harmonics are enhanced or reduced.

Spot on!

The generic tonestack, however, often introduces nothing but "low Q" high-pass, band-pass/band-reject, -and low-pass filters at specific frequencies of interest. They may be fit for overall tone correction but overall their magnitude still tends to be rather low.

With a simple design, that merely switches between gain stage + gain stage + EQ and gain stage + EQ + gain stage -structure, you may find equalisation provided by the EQ alone insufficient enough that performance of these two inherently different "modes of operation" are somewhat compromised.

For example, without excessively applied hi-pass filtering before clipping distortion the resulting clipping may not be "tight" enough or it will sound too "fuzzy" while user's preference happens to be closer to more "modern" overdriven tones. If such hi-pass filtering correction is added then "cleaner" mode of operation becomes excessively "bright" and "thin". The natural low end is lost.

As entire amp design practically relies on pre and post distortion shaping you can imagine how much is compromised when simplicity is the goal and the task of "voicing" is practically solely left on shoulders of the EQ, which is often nothing but the simple tonestack.

Have you ever seen frequency response of a generic "cleanish" amp versus generic O/D channel of a modern amp? The effect of the tone control in the entire picture is surprisingle small in the latter.
« Last Edit: April 26, 2014, 02:14:14 PM by teemuk »

Littlewyan

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Re: New Single End Amp
« Reply #11 on: April 27, 2014, 04:59:44 AM »
I haven't actually. What two amps should I compare? A Fender and Marshall?