Enlighten me - what on earth is Q1 and R17 doing in series with LED4?   

...or, since you intend to build a valve preamp, go the whole hog, sidestep the problem entirely, and build a valve power amp (which is where most of the desired "valve tone" comes from anyway)?  e.g...

http://www.ozvalveamps.org/ava100/ava101lamington.htm

Well, at the risk of taking this seriously, I have to agree with R.G., I just can't see the point, and I certainly don't like going off the datasheet - the manufacturers really do know best.

One question must be, what are you trying to do, more headroom for clean, or more crunch for grunge?  These are contradictory.

If you look at circuits for s.s. amps you will notice that they typically have clipper stages that depend on diodes and don't allow the surrounding op-amps to clip, and there is a very good reason for that - they tend to sound nasty.

Mods have their place but they need to be thought through, and I'm much more inclined to first decide what I want and build from scratch.

I'm inclined to agree with Phil; the output impedance is only part of the story.  In short; a s.s. amp has a Zout of a fraction of an ohm while a valve amp might be around 5 ohm, implying the s.s. amp has an iron grip on the speaker cone and thus high damping factor, and the valve amp doesn't (i.e. more colouration by speaker and cab).  So it's more a matter of magnitude than precise value.

The B-H curve of the iron in any OPT is also a serious contributor  to "valve sound".

These topics can be discussed endlessly, but the only test that counts is what it sounds like (to your ear/taste), and the only way to know that is try it.   

If it oscillates it's more than x29, if not, it isn't.     {at 180deg phase shift the loss across the PSN is ~x29}

In the example above the gain is nominally set by 1+rf/rb (nominally because a single transistor stage won't have a lot of gain in reserve, perhaps x100 open-loop, while any op-amp may have an open-loop gain of x100,000 in reserve and so will produce a much more accurate result).  A 12AX7 has just enough gain, a 12AU7 not nearly enough.


As far as the values go, the original range suggested, up to 0.47uF (polyester/greencap) should more than do it, no need to go higher, or the complication of using NP (non-polarised electrolytics).

What Enzo said.  This is actually a 25W solid-state amp and would have been used as a PA in a hotel, factory, or etc.

Yes it should work as a guitar amp 'tho you might want to make some physical changes so you can plug in your guitar (and speaker) easily.

The oscillator is the phase shift type and if you google image search "phase shift oscillator" you will see dozens of circuits using different gain devices, valve in a Fender, transistor in most guitar amps, perhaps an op-amp, but all sharing the ladder of three CR's between input and output that is the heart of the PSO.  As long as the forward gain element can provide a voltage gain of about x30 it will oscillate.  Paste it in your hat because this is what almost all guitar amps use in some form.



galaxiex is right, doubling the cap values should half the speed, but it will do it to the whole range, so some compromise between slowest and fastest may be needed.  And don't bother trying to increase the range - you can't.  You can only shift it up and down.

Changing the resistor values to ground will also change the speed but it can get fraught because that can change the gain as well so most straightforward to simply pad up those three existing caps.

As an old retired tech I can vouch that both JMF and Enzo are professional techs giving good advice.   

You gotta have a good workspace (pref ground level unless you are pone to flooding) and a big bench with good lighting, I'll suggest LED these days over the traditional fluro.  Once you get going, start small, distribute your card to local music shops.

And have a go - collect some junker dead amps, from wherever (thrift shops etc) and try to repair.  Build your Class-A amp.  The math is valuable but it's a support to understanding - a circuit is what it is.

Buy tools and instruments as needed.  They make it easier, but in many cases a lot of diagnostics and even repair can be done by careful observation.  The vast majority of problems are physical rather than electronic, dirty pots, dirty sockets or wrenched solder joints on them, &c.

Guitar amps tend to be simpler than most electronics (unlike, say, a marine radar).  The signal goes in here, through there and there, and comes out over there; and if it doesn't it's a bit like plumbing, where's the blockage?

Be orderly and methodical, know when you are out of your depth (a lot initially) and be like a few of the other guys around here and ask for help.  No shame, none of us were born knowing.

All the best in your venture, age alone is no impediment.   

ps - you'll find Jack Darr on my site here;
http://www.ozvalveamps.org/jackdarrhandbook.htm

Thank you guyz.

"Don't just stand there saying 'short circuit' - lengthen it!'"

I'm pleased to hear that I've helped people, I think of it as giving back for the many engineers and techs who have helped and enlightened me over the years, and there is so much to learn, not so much about the technology 'tho that is considerable, but about technique, tips and tricks that save time and help sort the fly *s!!t* from the pepper - and linear diagnostic thinking, finding the real root cause (particularly in DC coupled solid state output stages).

I've never lost through teaching or explaining.  I get to review and renew (and get to be expanded and corrected by JMF, Enzo, ...  )


The sitrep is that the chemo stopped working and was making me very ill, so discontinued.  Secondary peas (2x 7mm) have appeared in the front and rear of my brain (oh no, not the brain), so now it's spot radiation daily for two weeks followed by brand new experimental immuno-therapy which hopes to unmask the cancer to my immune system.  The up side should be fewer side effects, the down side is that its effectiveness ranges from 100% to 0% depending on the patient.  Fingers crossed.

Still a bit weak and wobbly but no pain and in good spirits, and very grateful for all your good wishes and kind thoughts.

68k + 10%

68k + 6.8k = 74.8k

So 76.7k may be out of spec if you are using an expensive and accurate ohmmeter, however it is unlikely that it is, and even if so that it would seriously effect the circuit - there are simply too few places where such a difference wouldn't be within the tolerance of the circuit (or they would have originally fitted a 5% or 1% component, and they don't).

Measure a known new one(s) of any values(s) and see if they all come up reading a bit high.  If so, relax.  If not we are talking a fraction of a cent - replace it.

Audio circuits just ain't that critical, +/-20% or more would still cut it fine.   

Quote from: Enzo on November 17, 2015, 12:38:39 AM
I had never seen light up shafts on single pots before.

Ditto - looks really cool.   

Quote from: Enzo on November 17, 2015, 12:36:47 AM
Why not take it to a guitar repairman for an assessment?

A couple of minutes with somebody who really knows what they are doing will tell you all you need to know.  A proper setup generally is generally cheap and can transform a poorly set up guitar.

Quote from: J M Fahey on November 09, 2015, 12:58:38 AM
Ouch !!! the second schematic is 'orrible

A work of art almost in its wonderful obscurity.   

Quote from: phatt on September 07, 2015, 07:55:57 AM
This might sound stupid,,, but Why?
Why do you need all 3 options?
Phil.

...and what happens to the cab air loading on the driven speaker in the 8 ohm position where only one speaker is driven?   

{The Last Post - there is no right place to post this, but to save cluttering up the Limiting Lamp tread...}


Since the question has now been asked...

In May I was diagnosed with terminal lung cancer.  It has taken a rapid toll on my energy level and I've had to clear the decks and savagely triage my various activities, pass on my baby OzValveAmps.org, drop out of forums, and focus on family and friends in the estimated 6 to 24 months left to me.

"Where there is life there is hope", certainly, and I'm getting excellent and intensive cutting edge care from the best oncology unit in the State, but realistically, I'm not walking away from this one, so my partner and I intend to have the best time possible in what remains.

While I'm still very weak the team have already restored some of my strength and feel that they can restore enough for Bev and I to take a last farewell road trip to visit old muso friends, to have slap up dinners, to play, sing, laugh, cry, and move on.

Rest assured that I'm not in any pain, and that I am mentally composed about the situation.

So thanks for all the interactions and problems to solve, it has been an enjoyable trip.  I'll still be dropping by to see what's happening, but I won't be posting much, if at all.

Cheers,

-Roly