Handbook Amp Project – Part 5 – Output Section – Working Amp

This section was heavy going for a LOT of reasons. I think I will have to start backwards from where I am, with a working amp, as a lot was researched and some components changed to get to this point – probably in error overall from what is actually necessary – but that’s learning.
That’s the good news – it works! I will show my final schematic when I’m happy with it, which may be a long way off yet, after a few more mods. I would not have got far in understanding this stage without background reading of Merlin Blencowe’s “Designing Tube Pre Amps…” book.
This Post is just to give an idea of what has been done towards that understanding – so much is not definitive – so take that note with you as you read – errors! It is just a convenient point to do this, as the amp works, but needs tweaking, which requires more study for voicing of the overdrive maybe, which is still good as it is, but not quite what I want. The clean tone is great – as good if not nicer than the Aston – this has more options in one amp with the bright boost, and lower volume overdrive levels (a bit too early).
In the meantime, Blencowe gives the 1st chapter (essential valve overview!) and the Grounding PDF free here, as well as tons of other info:

The main amp problem is distortion setting in too early on the volume pot – about position 1. I wanted this amp to distort early, but not this early, as it has very a nice clean sound too, I want more volume here first, and ideally the distortion to be about the same volume level as the max clean sound starting as it did in the pre amp section at about volume 4. I was thinking that full volume/distortion could be kicked in with a stomp switch or a Guthrie Govan tone guitar tone section bypass button, to fixed resistor of the max volume value (1M)? In the amp.
I have even recorded it for a track I’m writing in FLS – but the Fostex MR8 and/or mics let me down sound wise. It sounds nice out the cab but, a bit transistorised when recorded – it’s lost its dynamics.
A couple of important points I want to make in general about the 2 major errors that happened in this build, the first probably cost me a 6V6 valve and the second is a good reason to put a fuse or low value resistors in the heater circuit – RESEARCH IT – or buy Blencowe’s book, and Dont Break Safety Procedures!

Major Lesson No. 1 – Again!     “Assumption is the mother of all fuck ups.”

The lesson here when checking an amp for the first time is to be sure of ALL the connections you make have the resistance values they should and all grounds are 0.3 ohms or less (on my DDM that is). The most important connection of course, is the Mains plug Earth pin to the chassis – 0.3 ohms OR less again – check it before you plug into the wall. There is no point having low ground to chassis resistances, if the wire in the plug to Earth has come loose…
It turns out the cab jack socket is a bad connection and the jack tip was in too far and disconnected! Odd I know, but once connected I got the FLS test tone. I should have tested through the cab lead with the DDM to make sure there was a speaker on the end of it and not just assume there was!
Obviously a faulty jack socket from nearly new without much use – you can’t trust anything!
Life will always do that whenever you already have enough on your plate. This is one of the most important connections for a valve amp – they MUST have a speaker load before you turn up the volume!
As I plug into the cab quite often, and that hasn’t happened before I didn’t think to check it. I could have spent ages looking down wrong avenues for valve problems that didn’t exist, for no sound out. Instead this caused me one – the 1k tone was coming from the amp itself – not the cab!
I think I was hearing the tube self resonating at 1k because it had no load to feed?
Well, it turned out it was damaged – but not dead – beware of this weird scenario too – as it sounds OK clean, but goes a bit mad when the volume is increased, as I got weird scope responses – and a drop in gain at ½ volume, but high output at none and full volume! You may not know you have a valve problem unless you turn it up. I got this sort of HF noise with the test tone:

Major Lesson 2 – Don’t Break Your Own Safety Rules!!

I found out the 6V6 tube was duff – probably due to that connection cock up at the start – hence the weird readings– by starting an anode pin DC fire when the probe slipped – seriously!
I had a nice DC arc that could only be stopped by turning off the amp. It would have happily burned out the Anode – Heater pins I’m sure.
Make a mental note of this – 6V6 Anode Pin 3 = HT maybe 200+ Volts; Heater Pin 2 = 6.3V AC but capable of delivering 3 Amps. A lot of E.M.F between a very short air gap and enough current to burn metal!
Also – you would not normally take readings at valve pins as all links should go back to a more accessible point on the board – but I didn’t have all the turrets at the start that I now require – make sure there are more than you will need before you start (unless you are prepared for stuff like this because it’s a prototype, of sorts).
This was also a timely reminder of why you don’t want to be stuck to a DC high voltage by your skin!

There is no time saving excuse here for forgetting either, as this circuit discharges to only 4V DC in less than 5 seconds after switch off!

I haven’t failed to switch off the Standby since this, before moving a probe. Stay SAFE.

I wish I had a video of it though! Nothing on YouTube…

Before you go any further, a basic understanding of this Single Ended cathode biased configuration is required:

New 6V6 Valve Data

Finally – sensible readings…
These readings are compared to the pre amp section before the 6V6 was added, and are the current working amp values with some different component values from the original schematic, with the 15W Danbury OT, and the 190V Mains transformer secondary used.

OT primary windings options = 3k @ 16 ohms, 2.5k @ 8 ohms? The label is vague, and so is Ampmaker.com regarding values here:
Is it 3k across the whole side or from the Centre Tap each way, or 6k across the lot?
OT secondary windings options = 0, 4, 8, 16 ohm
The DDM measures about 70 Ohms DC resistance for the primary of this OT, but I need to check this value has any meaning in terms of a load resistance to get an idea of the biasing of this OT in this circuit.
(It doesn’t much, I found out, because the anode plate resistance itself for this valve is 50k ohms, so it is barely 1/700th of A+ that will be dropped across it – a few volts may be read at best).Note also that a single 6V6 is about the same power out as an EL84, 4 – 12 Watts or so depending on use.

A+ = 263V (down 14V)
A+ mains ripple = 5Vpp (same as the Aston, 20Vpp for the Marshall)

B+ = 234V (down 30V)
B+ ripple = 0.05V (same as the Aston, 2V for the Marshall)

C+ = 157V (down 16V)
C+ ripple = 0V (same as the Aston, 0.3V for the Marshall)
Vanode = 263V
Vanode ripple = 5Vpp
Vgate = 0.01V
Vgate ripple = 5mVolts Volume min
Vgate ripple/noise = 100mVolts Volume max

Vcathode = 4.8V
And something new I need to learn about:
Vscreen grid = 181V
All this effort with different scope readings makes you think about the circuit and gives me an idea of what oscilloscope views look like with HF and other odd noise sources I got with the damaged valve, what form they can take, and how the scope sees them.
Here’s a good example of the 5V anode ripple modulating the 1k Hz test tone:

Global Negative Feedback Readings

A couple of readings I’m curious of now the 6V6 is added is how things change with NFB and NO NFB:
Triode 1 AC reference at Anode, 200mV input, 1kHz input (fixed gain)
Stage Gain NO NFB link from OT = 7V/200mV = 35
Stage Gain with NFB link = 8V/200mV = 40 (fixed gain)
Triode 1 AC reference at cathode, 200mV input, 1kHz input (fixed gain)
Stage Gain NO NFB = Vcathode/Vgate = 2mV/200mV = 0.002 (this has NFB already due to the Cut/Boost capacitors and is working well – a stable DC bias more or less.
Stage Gain with NFB = Vcathode/Vgate = 2mV/200mV = 0.002
Triode 2 AC reference at Anode, 200mV input, 1kHz input, set at start of distortion as a volume reference:
Stage Gain NO NFB before distortion = Vanode/Vgate = 12V/0.5V = 24
Stage Gain with NFB before distortion = Vanode/Vgate = 12V/2.0V = 6
Triode 2 AC reference at Cathode, 200mV input, 1kHz input, at start of distortion volume reference:
Stage Gain NO NFB = Vcathode/Vgate = 0.3V/0.5V = 0.6
Stage Gain with NFB = Vcathode/Vgate = 1.6V/2V = 0.8
OT sec at cathode 0.3V = 0.5V. Ratio before NFB that would be sent via NFB resistor is therefore 0.5/0.3 = 1.666

20 X Log 1.666 = 20 X 0.22 = 4.44dB. This is less than 12dB max that could be fed back to this cathode without oscillation. This ratio may change with more volume and different frequencies.

6V6 Stage Gain NO NFB start of distortion = Vanode/Vgate = 140V/10V = 14
6V6 Stage Gain NO NFB at full volume distortion = Vanode/Vgate = 160V/52V = 3 (max at volume 6)

6V6 Stage Gain with NFB start of distortion = Vanode/Vgate = 17V/10V = 1.7
6V6 Stage Gain with NFB at full volume distortion = Vanode/Vgate = 100V/55V = 1.8

You can see the major effect NFB has on squashing Gain overall, by up to about 14/1.7 = 8 times at 1kHz.

Just to mention an easier NFB option, Blencowe gives another simpler link for global NFB in a single ended amp, where you don’t have to work out resistor values, and 100% of the OT voltage is fed back, to give a closed loop gain of about 0.5 (feedback factor of 5.6dB), so the headroom is doubled. The cathode goes to ground via the OT secondary.
Read the Screen Grid section to get an idea how the resistor is calculated and why a 2W rating for it is sufficient in most cases:
I am going to have to confront Transfer Characteristic Curves to understand biasing properly, and how it affects distortion onset for both sides of the waveform (top or bottom only clipping etc.):

Ok, the basic problem with this amp is the very low volume (barely position 1) distortion of the 0.2V input signal, giving barely 15V undistorted at the 6V6 anode with no NFB and 17Vpp undistorted with NFB.
I suspect the 6V6 input impedance is too low and is dragging the Triode 2 output down? As a test of this, I doubled the 6V6 grid leak 220k resistor to 440k. No change there.
Note the gate and anode are IN Phase for the 6V6 unlike the pre amp inverters.
Global Negative Feedback Link

I traced through the whole circuit to check the phasing relative to the pre amp input, to know what phase the OT secondary is for Negative Feedback addition as the amp was originally working without it, as I had feedback problems at the start with it connected, so left it out to continue the build.

You may not need NFB initially as the amp will probably work without it, except for more noise, as stated by Lenard Audio, and you may prefer its dynamic liveliness depending on your style. The Maggie works without it with my component values. I found it too uncontrollable with pick strength dynamics, and the trebles were ear piercing. When added, It certainly did improve overall performance – lower hum noise, and to flatten bandwidth – with a reduction in Gain at some frequencies of almost 60V/20V ratio prior to NFB, at the anode, as below:
Note the logic behind the NFB. It is added at the cathode of a pre amp IN phase with what is already there, so adds to the amplitude there. This then opposes (lessens) the difference in voltage between gate and cathode, even more so than an un-bypassed NFB cathode resistor does already, as the gate input rises, reducing overall gain. If you get this the wrong way round, then you will get runaway positive feedback which causes the tubes to scream. Turn off immediately if you hear this, to prevent tube damage, and reverse the OT windings on the primary or secondary, whichever is easiest – at the OT. Double check your prior gain stage calculations (see Valve Wizard’s numerical example on his site) to ensure you have used the right value NFB resistor to give between the stated 6-12 dB range NFB only (Page 185 of Blencowe’s book).
OT Secondary and Cathode Voltage Relationship
Reading Blencowe’s book, he describes the NFB calculations, which are too complex for an example to detail here, as he states that the gains of the previous sections are required, and that a maximum of 6-12dB NFB can be applied before the amp becomes unstable again.
There is a numerical example on his website here:
I would have preferred an explanation in terms of mV proportions between the cathode and OT, as then it is easier to apply proportional resistances when building your own amp, and to avoid more complex maths. I will give an overview in terms of the figures I measured directly – as it works in this amp at present, with the original schematic 68k NFB resistor value used with the Danbury 15W OT – so you can get an idea of what happens without too much maths, which would involve from scratch – these parameters, from WikiP – the Universal Feedback Equation:

Then the gain of the amplifier with feedback, called the closed-loop gain, Afb is given by,

Without getting involved in that, as I don’t have the prior stages readings anyway, NFB can be understood in principle below using the actual working NFB voltages from this amp.
The equation relating dB to voltage is:

The NFB link is below, from the OT, via R14 – the 68k – to the top of R15 Triode 2 cathode.

I measured 30mV at the cathode for a particular volume pot setting, with a corresponding 80mV at the OT secondary. Putting these in the dB equation I get:
20 X log (80/30) = 20 X log 2.666 = 20 X 0.425 = 8.5dB NFB is in operation.
This seems a perfect compromise between 6-12dB recommended. It works for this particular setup, as I have changed the 6V6 cathode resistor to 100R from 550R, which I did to improve my earlier biasing problems I thought I had – probably didn’t – learning again. I based this on the 6V6 spec. sheet, and drew lines using MS Paint, for my value of A+ (about 260V), for an average use as quoted on the sheet – about 50mA max current swing – to try to get an overall understanding of the voltage swing either side of max A+, but equal for both positive and negative sides, so the output stage amplifies cleanly, and doesn’t contribute to the distortion. This comes from Triode 2.
NFB has improved tone across the volume range also, as before NFB I had strange hi-mid “quasi-oscillatory” overtones coming through – like the valve was on the verge of vibrating or something – ghostly metallic. It has reduced the overall gain also, as I have less volume increase (none worth talking about really) once distortion has started, from about Volume 6 up to full volume – a sort of compression I guess you could say.
This NFB situation was lucky for me, as I was a bit stuck working out what value of resistor I would need for my particular circuit with the Danbury OT. I guess it can’t be that different from the Weber WSE15 in the schematic.
The 6V6 bias seemed a bit off to me before, with the default Maggie resistor values, with 14V at the 6V6 cathode when it had a 550R resistor there – but maybe not that bad now I have read the Datasheet. I replaced the 550R with a 100R anyway to reduce the cathode voltage to 4.8V.
Now I can look at Screen Voltage, and a further bias shift, to get the volume level problem fixed, then EQ capacitors to try to find a tone I like. At least in this state, even if I closed the lid now, I have a working amp with a useable distortion (and clean tone though at very low volumes).
For me to refer back to in future here is the current circuit, with my resistor/capacitor values, and measured voltages. At least you know what should “work” with the ampmaker.com Danbury Mains and 15W OT transformers if you want to experiment.

General Operation of Output Stage

There is a detailed description of each components function here:
and here:
so pointless me trying to confuse things…even if I understood them all myself at this point – which I don’t!
The important thing now the amp “works” is tweaking it to get the distortion happening at the desired volume, and the tone how I want it. These tweaks will take time and research no doubt.
Getting the bias operation point in the middle of the voltage swing is the key, so it is symmetrical and clean first then altering the bias for desired clipping will be the trick – but within SAFE parameters that don’t damage the valve – having now read Valve Wizard’s info.
I suspect that my volume level/distortion issue is a triode 2 output impedance to 6V6 input impedance mismatch at this point, with the 6V6 stage dragging the output of the triode 2 stage down to far too early but need to research this more?
Before the addition of the 6V6 stage I got distortion at Triode 2 at about volume position 4, but now I get it at 1. Tonal adjustment has many options from inter-stage and NFB cap values to Tone section overall design.
Section Summary

There are many things to mention in this stage – where to start?
It was actually the start of being a beginner in really understanding amp operation, with a lot of technical information to digest, let alone understand. It worked out quite well again for me without it being too much to overwhelm, but more than enough to swallow in stages. It takes time to get much of this stuff clear in your head, but it is rewarding and addictive. There is still a long way to go in finding that perfect overdrive tone I’m looking for, but it is a noble quest! Understanding tone adjustment may mean getting into heavy maths, and actual amplifier design as in the last section of the Aiken page:
I may start to look at design from scratch in the next build, now I have Blencowe’s book, but need to do more reading and research on the subject first.
Going back to electronics basics with load lines on Transfer Characteristic Curves (plotting the basic performance of a valve or transistor) would be a helpful but time consuming thing to refresh also, if you want a more in depth understanding of amplification.
For that reason I really advise buying a book on the subject if you want to mod your amp but understand why the mod works, and if you want to design one from scratch it is an absolute necessity.
This build was a good place to start really learning for me, as someone with a technical background, a basic qualification in electronics and one successful “join the dots” amp build already under my belt, but it was still tough going to research to find the cause of my problems for the particular components I had, that weren’t exactly as the Maggie schematic.
My experiment with the grounds returning to a lug that is used only by the components of that stage seems to work, as there is little mains hum with this amp worth noting with NFB. The high frequency hiss outweighs hum at full volume with no input signal, but guitar plugged in.
The transformer layout may have helped also, as well as having the input at opposite ends of the mains wires. This has proved better with three ground lugs with this design than the Aston (small chassis though) which had 3 grounds also, but was quite noisy until I redressed it with only one ground (not quite a bad as the Marshall – what is!? That pesky single rectifier diode..!), which is a big improvement – the Aston is now about as noisy as this Maggie. The info for that experiment came from the Grounding PDF.
There is a really detailed section on the importance of wiring layout in a build, in Blencowe’s book – you HAVE to get a good book on this subject to both understand valve amps and use the best mod tips and keep noise to a minimum.
Further Reading

General hi fi amp design – tips for Pro quality design, build and tidy wiring
I purchased this book:

Researching the Volume Problem

I could KICK myself for not studying this earlier! It may have made me look at the pre amp section in more detail before moving on, and understand why the triode 2 distorts at the volume levels its does.
Blencowe’s book made me look at Load Lines for this section finally, and that’s when I realised the volume level would improve when the valves were being driven at higher voltages – provided by changing to the 275V mains secondary, from the current 190V winding. I saw my issue after looking at the Load Lines for the 6V6:

I drew the green load line according to my A+ and a max average use from the sheet of 50mA, to get a bias point for the grid at about -5V. This is why I changed the 6V6 cathode resistor from 550R which had 14V across it originally, to 100R which now has 4.8V across it. This may have been wrong, as the usual use on the Datasheet says -12V grid bias
6V6 Independent Input:

I checked the 6V6 section with my resistor value (100R) with an independent signal at the gate, and it swings cleanly from 0V – 12V at the gate to give 0V-20V at the anode with no distortion.
V Gain = 20/12 = 1.67 – but only power out is meaningful for a power tube (according to Web forum).

This tells me there is no distortion from the 6V6 stage in any way.
Now I have to go back to the Data sheet for the 12AX7 to see what difference the C+ drop has made to their performance, since the 6V6 was added to the circuit. Now, I measure a 0.7V DC cathode bias voltage on Triode 2, with a C+ of 132V.
With 1V AC at the gate of T2, I get 20V out – Gain = 20 distorted
The cathode current is 0.7V/1500 ohms = 0.000 47mA – tiny.
Drawing a load line from this info on the 12AX7 datasheet shows the cause of the problem:

The load line for a 100k anode resistor, at 132V C+ would give about 130/100 000 A = 1.3mA, so would at least put the load line as below in blue:

This would be a little better swing before distortion if biased at -0.5V, or 60V at the anode.
I need more voltage at C+, but the option is extreme going from 190V AC to 275V Ac windings on the mains transformer….Hmm. not sure about some 250V and 25V capacitor ratings now..
This takes the calculated peak AC to 275 X 1.414 = 389V at A+
Currently it is 190 X 1.414 = 268V – 263V actual.
389/268 = 1.5 times greater.
If I assume all voltages rise at this proportion across all components on the 275V winding it will give me an idea what to expect.
Allowing a round 80V possible drop for C+ after the 275V winding is connected, this would better centre the load line with 100k anode resistor more proportionately, possibly here:

This should delay distortion at Triode 2, and improve the volume level at which distortion happens.
Let’s go and try it!
Well, it worked ok, and no capacitors blew.
It sounds great! Very loud now, and too much for the bedroom, so I will put a 190/275V switch option in so that I can have distortion at practise levels, and more clean for longer and louder on the 275V position. For now, I’m just gonna play it for a bit like this, while I have the Marshall to do also.
Here is the final circuit with the measured voltages:

Really pleased with the way this turned out, as it is different enough from the Aston, because of full distortion at 190V, more tonal options, and it’s a bit louder on 275V obviously.
This is an amp that could be gigged with no need for a separate distortion pedal – just using the guitar volume pot between clean and dirty – cool!
I will do a Post on Load Line basics soon while it is fresh, so I don’t forget too much of it.
Project Summary

What can I say? Just chuffed! I really like the sound of this amp – both the clean and dirty, with and without the tone boost – lots of options here.
It was tough in places, getting bogged down in theory and research, but I have learned much more than I thought I would – building an amp in this way – stage by stage – as I had to read a lot of Blencowe’s book to get it finished to this point.
There are a lot more mods to try if I want, or just rewire it tidily to complete it safely and well.
It has given me the confidence to jump straight to looking at the Marshall JMP 100W, and building a clone of that, whilst under repair/renovation.
On the whole, another project well worth doing if you want to understand what can be done to alter tone and distortion of your amp, and more importantly – why. The basic principles you will learn from that will stand you in good stead for understanding all other technical gear, input and output stages, impedance matching, FX pedals, and why the amazing valve is so nice for the electric guitar, or other instruments – even vocal mics.
A classic line from Blencowe goes something along the lines of:
“Don’t think of a valve as just an amplifier, but as a tone generator.”
It’s those harmonic overtones that add the sparkle to a guitar sound.
Amazing things.