Amp internals

[ecc83]

And some less so.

Some of the modded amps are truly nasty hack jobs too, including a few with famous names on them...

Another beef. I am sure some of those chassis are very pricey? Why then do the builders not buy more than one fekking wire colour!

And I like well designed PCB amps too so long as the board is of decent quality and allows component replacement without tracks peeling off. One or two chassis had a lot of wires running across and into the centre of the PCB. Makes "getting at" a nightmare.

Dave.

[jpfamps]

Only on the tremolo oscillator (the other LED is an amplitude limiter for the trem).

Cool idea.

Removes the need for a electrolytic cap as well, which must be good.

I've not experimented with LED biasing, but it's popular in hi-fi.

We've been protyping a MOSFET oscillator (based on the RG Keen circuit), with good results.

[jpfamps]

Some of the modded amps are truly nasty hack jobs too, including a few with famous names on them...

Indeed.

I'm not a huge fan of the "modding" "industry". Much of it verges on vandalism in my view, not least that ex-Hiwatt.

Also covering the mods with epoxy does not aid servicing.......

The modded chassis where the component values have been obscured is also amusing!

[ecc83]

Indeed.

I'm not a huge fan of the "modding" "industry". Much of it verges on vandalism in my view, not least that ex-Hiwatt.

Also covering the mods with epoxy does not aid servicing.......

The modded chassis where the component values have been obscured is also amusing!

I entirely agree .We have seen in a thread just a few weeks ago that even the ad hoc swopping of a pre amp valve can cause damage (yes, I know the design was at fault but it did work ok with the correct valve).

Amplifier designers almost all know what they are doing. Agreed they will "voice" it to their taste but if peeps don't like "Scumbox A" they should hunt around for a different amp!

Dave.

[TheThingThatShouldNotBe]

[QUOTE=ICBM;1355958]

Obviously the more complicated an amp the harder it is - eg trying to reverse-engineer something like a Mesa from photos and even the schematic would be quite hard, since the PCB layout is not all that obvious in many places without stripping all the components off. Traditional designs are quite easy.

QUOTE]

I would also imagine that the track design of a PCB on a high gain circuit is pretty important , using a different method like point to point would throw up a whole load of symptoms absent from the PCB design.

[jpfamps]

[QUOTE=TheThingThatShouldNotBe;1361032]

Obviously the more complicated an amp the harder it is - eg trying to reverse-engineer something like a Mesa from photos and even the schematic would be quite hard, since the PCB layout is not all that obvious in many places without stripping all the components off. Traditional designs are quite easy.

QUOTE]

I would also imagine that the track design of a PCB on a high gain circuit is pretty important , using a different method like point to point would throw up a whole load of symptoms absent from the PCB design.

Absolutely.

High gain amps can suffer from oscillation if not carefully laid out.

[ICBM]

Even 'medium gain' ones can - that's what the white rubbery glue is for in original Marshall 2203s (there's some in those photos, as well as the mess in the modded ones) - a couple of the wires need to be held in exactly the right places next to either the PCB or the chassis or the amp tends to squeal at some settings.

One of the big advantages of PCB in higher-gain amps is absolute consistency of layout, better even than the very best hand-built ones.

[misterg]

Removes the need for a electrolytic cap as well, which must be good.

That's why I did it - It keeps the gain of the oscillator more consistent, especially at very low trem speeds. I've never tried it for audio, but am aware the HiFi people use it, but then they use watch batteries and other weird stuff!

I don't think the 'amplitude limiter' does very much, but the amp works so I'm not inclined to mess about with it.

We've been protyping a MOSFET oscillator (based on the RG Keen circuit), with good results.

I tried this, too, but found it was a bit of a knife edge to keep it oscillating through a decent range of adjustment without clipping (which caused audible "thumping" in my circuit). I found it easier to get smooth tremolo using a valve for the oscillator, with a MOSFET follower to drive the feedback loop. (There's also a MOSFET follower with CCS driving the tone stack after the EF86).

It sounds really sweet, even if I do say so myself!

[ecc83]

That's why I did it - It keeps the gain of the oscillator more consistent, especially at very low trem speeds. I've never tried it for audio, but am aware the HiFi people use it, but then they use watch batteries and other weird stuff!

I don't think the 'amplitude limiter' does very much, but the amp works so I'm not inclined to mess about with it.



I tried this, too, but found it was a bit of a knife edge to keep it oscillating through a decent range of adjustment without clipping (which caused audible "thumping" in my circuit). I found it easier to get smooth tremolo using a valve for the oscillator, with a MOSFET follower to drive the feedback loop. (There's also a MOSFET follower with CCS driving the tone stack after the EF86).

It sounds really sweet, even if I do say so myself!

Heh! Some will see this inclusion of silicon into the audio path as an advance and dragging gitamp design, K&S'ing into the TWENTIETH century!

Some will say "thin end of the wedge"!

KUTGW.
Dave.

[misterg]

KUTGW.

Thanks!