(inter-)stage coupling : recommended reading?

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ChopSauce
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(inter-)stage coupling : recommended reading?

Post by ChopSauce »

Not sure the title explains it well, so:

- I am both pretty ignorant in tube matter and managing to design my own guitar amp, as per Merlin Blencowe's book about pre-amps...

... & yet the book provides many thorough details about the stages themselves, it lets me in full doubt for what deals with the coupling between stages - amongst other few relevant details.

I went though other free litterature available on the internet but could not find much more details about my concerns, beyond the many practical details involved in the translation from theory to the fabrication.

Could anyone suggest me some valuable litterature. As an example, I could read very mixed comments about the tube pre-amp cookbook.

The Morgan Jones, maybe?

That would be nice to have some comments about the ressources themselves, as listed in http://ampgarage.com/forum/viewtopic.php?f=6&t=9023 (Reading Material on Steroids!)
tubeswell
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Re: (inter-)stage coupling : recommended reading?

Post by tubeswell »

Merlin Blencowe's books are excellent on this topic (and other tube-amp topics).

http://www.valvewizard.co.uk
He who dies with the most tubes... wins
ChopSauce
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Re: (inter-)stage coupling : recommended reading?

Post by ChopSauce »

Thank you very much for your suggestion and sorry - my mistake - I didn't made it clearly enough:

- I already have Merlin's book about pre-amps, which I found uncomplete in that it doesn't allow me to comfortablly design my own (EF86 based) pre-amp from scratch, with respect to the point mentionned in the title.

I found (an older 3-rd version available for free on the internet that the Morgan Jones, which seems to be much more usefull in that matter, yet I haven't had the time to read it throroughly, so far.
tubeswell
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Re: (inter-)stage coupling : recommended reading?

Post by tubeswell »

Which aspect don't you understand?
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ChopSauce
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Re: (inter-)stage coupling : recommended reading?

Post by ChopSauce »

Thank you very much for your consideration. I was tempted to answer that it is difficult to explain what I don't understand. This is rough but denotes my level of confusion.

In an attempt to answer your question, I'd say that coupling caps is the easy thing. I can manage with voltage dividers to attack the following stage as needed but I am not sure at how this impedance stuff works. The complex formalism for oscillating circuits is too far beyond me, yet I still have the maths for it. Maybe I am just fearing to much missing any crucial point.

Anyway, there's a section about that point, in Chapter 3 of Morgan Jones' book. I'll have to read it first - and redraw my plans from the begining, as well...

... but maybe you can tell me how you do it, yourself:

- it seems that many people simulate their circuits with... LTSpice - right?

In addition or instead of doing the maths on a paper, yourself?

___

So short a life and so many things to explore :wink:
tubeswell
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Re: (inter-)stage coupling : recommended reading?

Post by tubeswell »

ChopSauce wrote: Thu Sep 14, 2017 11:46 am ... I am not sure at how this impedance stuff works.
Impedance is 'resistance to AC'. In terms of gain stages in a guitar amp, we think about this as signal impedance. The signal gets 'eaten up' by the AC load (that 'loads down' each gain stage). AC load includes the parts of the signal circuit that are AC-coupled through various caps.

The signal has a source and a load. Each as its own impedance. In an AC-coupled signal circuit, this can be imagined as a voltage divider in which the source impedance forms the 'upper leg' of the voltage divider, and the load impedance is the lower leg, with the output point being the junction of this source and load (which become the signal source/input for the next gain stage). You need to calculate impedance of both the source and the load to work out how much the signal is attenuated at the voltage divider output (compared to the input). The larger the source impedance is in comparison to the load impedance, the more the signal will be attenuated, and vice versa.

Because an AC-load includes all the parts of the signal that are coupled through coupling caps, then you have to sum up the impedance presented by the various signal pathways, which includes AC pathways in series and/or in parallel. This means that with tone stacks (and filters), the overall signal impedance picture is more complicated. The resistances in a typical TMB tone stack are in parallel with each other at various frequencies through various R/C and/or C/R filters. You need to sum the bode plots for each filter to see how overall tone stack load attenuates different bands of frequencies (see the chapter on filters in Merlin's book).

The filter caps in the power supply act to 'shunt' any AC (that would otherwise appear in the power supply rail) to ground. Therefore, for the purpose of calculating AC load on the signal from a gain stage, this means that the power supply rail is 'inert'. If there is no resistance between the signal source and the power supply rail, then the signal would be prevented (in the same way that grounding the signal would prevent the signal). Therefore, as far as the signal is concerned, the power supply rail and the ground return are one and the same. So in drawing a Thevenin equivalent schematic, any AC-resistance between the signal and the power supply rail, and any AC-resistance between the signal and the ground return, are in parallel.
ChopSauce wrote: Thu Sep 14, 2017 11:46 amThe complex formalism for oscillating circuits is too far beyond me, yet I still have the maths for it.
Merlin does not cover oscillators in his book. If you want to know how the C/R filters in oscillators work, you need to look elsewhere (as you surmise). Merlin covers these briefly on his website. Randall Aiken's site has a good explanation.
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ChopSauce
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Re: (inter-)stage coupling : recommended reading?

Post by ChopSauce »

(while reading :) thanks a lot!

Your explanations are really helpfull -
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roberto
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Re: (inter-)stage coupling : recommended reading?

Post by roberto »

I suggest you also Aiken as a short but exaustive introduction to the argument:
http://www.aikenamps.com/index.php/desi ... amplifiers

I also suggest you to do some trials with different values. Don't be shy with the values:
From 470p to 100n, it has been used almost every value in guitar amplification.
It is really not needed to amplify all the audible range, nor the whole guitar range.
ChopSauce
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Re: (inter-)stage coupling : recommended reading?

Post by ChopSauce »

Thanks Roberto!

Sorry - my bad: it seems that I failed to provide you with important details.

- I am bothered by frying (red-plating) my power tube, despite I biased it on the cold side.

The amp is a kind of vintage Vox AC4 revised as per Merlin's Blencowe suggestions pertaining to the EF86 settings (thereby somewhat loosing the "pushed EF86" tone, as a side effect).

Well that EL844 was probably not worthy, and the russian EL84M-alike seem to hold that well, but I suspect I have pushing the grid to over-dissipate and I am looking for what the trouble was and I still didn't find it...

... so I am like a blind man left alone in the bazaar and (I have no scope to check... :| ) any effort to help with dissipating my trouble was most welcome 8)
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Malcolm Irving
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Re: (inter-)stage coupling : recommended reading?

Post by Malcolm Irving »

A leaking coupling cap onto the grid of the EL84 can do that. It puts a positive DC on the grid which overcomes the
bias. You can check the DC voltage at the EL84 grid - it should be close to zero. Anything approaching, or above, 1 Volt
could indicate too much DC leaking through the associated coupling cap.
Last edited by Malcolm Irving on Thu Sep 21, 2017 6:06 pm, edited 1 time in total.
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roberto
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Re: (inter-)stage coupling : recommended reading?

Post by roberto »

I would also ask you to specify the mods you have done to the original circuit (better a separated thread?).
This is the original one : http://www.korguk.com/voxcircuits/circuits/ac4.jpg
Here with voltages: http://music-electronics-forum.com/atta ... 76-ac4.pdf
You can print, write down the mods and scan, then post it again, including voltage readings.
ChopSauce
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Re: (inter-)stage coupling : recommended reading?

Post by ChopSauce »

Thank you very much for the tip Malcolm!

And yes Roberto, I'll post the schematic as soon as I'll have it updated to match the "working" thing, but it is an important derivation from the original AC4 schematic.

It is is very much inspired by the "Z4" thread on the projet G5 forum : http://www.projetg5.com/phpbb3/viewtopic.php?t=5347

For the impatients - if any - a link, in french: http://www.projetg5.com/phpbb3/viewtopic.php?f=9&t=5553
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roberto
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Re: (inter-)stage coupling : recommended reading?

Post by roberto »

C'est vraiment trés jolie!
Le schéma c'est le même que tu utilise maintenant?

Nice amp! Is that schematic the actual one?
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pompeiisneaks
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Re: (inter-)stage coupling : recommended reading?

Post by pompeiisneaks »

roberto wrote: Fri Sep 22, 2017 9:04 pm C'est vraiment trés jolie!
Le schéma c'est le même que tu utilise maintenant?

Nice amp! Is that schematic the actual one?
Quoi? Vous êtes en Italie, mais vous parlez français? :D Je suis confondu! :)
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roberto
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Re: (inter-)stage coupling : recommended reading?

Post by roberto »

I have some english speaking customers, mais aussi beaucoup des clients qui parlent français, y alguno tambien español.
Quasi nessuno che parli Italiano. So I had to improve all four.
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