PP18_6V6 Build Summary

I reversed the image in MSPaint so I can build and test mine left to right from Mains PT on, in logical stages.
This is less tedious though takes much longer, but you learn and think more about how each stage works. Also, if there is a fault, you need only look as far back as what you did most recently. Tinning all the wires for a whole build in one go is REALLY tedious!

Mains PT
240V winding = 28.7 ohms
275V sec = 54.7 ohms (285Vac unloaded)
190V sec =35.9 ohms (204Vac unloaded)
6.3V = 0.4 ohms (6.7V unloaded)
Rectifier Diodes DC (Note same diode kink as previous build?)

0-280Vpp Unregulated DC below (10 X probe and 5V/div) on 190Vsec winding

Reservoir Cap 47uFarad

A+ ½ volt and 1V ripple for 190V and 275V windings. 272VDC and 393VDC resp.

B+ and C+ sections 190V and 275V

A+ 273V, 390V, 15V ripple

B+ 268V, 387V, 0.1V ripple
C+ 240V, 348V, 0V ripple

Pre Amp Gain = 32 and 36

I used only 1 input here (J2), and just grounded pin 7 via R1 1M resistor. This pre amp and all other stages are inverters so that the output transformer secondary is back in phase with the input.

AC Coupled Long Tail Pair Gain = 38

This amp configuration works by amplifying the difference between the two control grid inputs so as one anode swings positive, the other swings equally negative. This is achieved by fixing each grid (pins 2 and 7) at the same DC voltage via R6, R7 and R8, with the signal input at pin2. Any AC that appears at pin 7 can pass to ground via C6.

LTP Grid input 1V, 320V C+:

190V and 275V secondary gains about 34 and 38 (1V grid, 10 X probe anode 1)

Start of anode distortion about 180V and full dist 220V on 275V sec.


“The AC-coupled long tailed pair has the same function as the DC coupled version, except that it is cathode biased. It is the most common phase inverter found in push-pull guitar amps and you will usually find exactly the same design in dozens of amps by Fender, Marshall, Peavey, Mesa etc. using an ECC83. It is also called a Differential Amplifier, or even Schmitt Inverter. However, Schmitt had very little to do with the development of this circuit, so this name should not be used.

When designing, the same rules apply for the AC coupled version as for the DC coupled version. High mu, high current valves such as the ECC81 (12AT7) and 12AY7 are well suited as they operate well even at low anode voltages, providing lots of output swing for overdriving the power stage, while the ECC83 (12AX7) performs well when less clean headroom and a touch more preamp distortion is desirable. A larger tail resistor improves balance, although too large and it will limit maximum output signal swing and even lead to frequency doubling (‘swirl’). Most guitar amps use less than 50k for the tail. To cathode bias, the tail resistor is split and the voltage at that point is tapped off and applied to the grids via grid-leak resistors, in the same way as for the AC coupled cathode follower.”

Power Stage Gain = 1.2
(Note that this is meaningless in terms of voltage gain, as the power section serves only to transfer power to the speaker, by pulling more current from the DC rail through the OT primary, which gets stepped up at the OT secondary while voltage gets stepped down proportionately there, not “amplify” the signal from the previous LTP stage. The Power in the OT primary and secondary are therefore equal in a theoretical 100% efficient transformer.)
“The word “power amplifier” is a misnomer. Voltage and current can be amplified. The term “power amplifier” although technically incorrect has become understood to mean an amplifier that is intended to drive a load such as a loudspeaker.
We call the product of current and voltage gain “power amplification”.”

OT primary = 180 ohms (100 + 80)

OT sec = 0.8 ohms

A+ 258V, 353V

As I am using 6V6s in place of the EL84s in this amp (similar specs), I have to translate the pins. To use an EL34 in place of a 6V6 (check power and current ratings of your PT first!), pins 1(suppressor) and 8 (cathode) must be connected together.

The reason for this is that the suppressor grid in a valve functions by having a negative voltage compared to the anode to deflect electrons back to the anode that have been ejected from the anode due to high velocity collisions from the electrons travelling at 2000+km per second from the cathode, depending on anode voltage (due to the VERY small mass of an electron). In the 6V6 and the EL84, the suppressor grid is already internally connected to the cathode. The suppressor grid was developed to suppress oscillation at audio frequencies.

EL84 pins

2 is control grid (6V6 pin 5)
3 is internally jumped to both cathode & suppressor (6V6 pin 8; EL34 pin 8 + 1)
4 & 5 are (6.3Volts) heaters (6V6 pins 2 + 7)
7 is anode plate (6V6 pin 3)
9 is screen (6V6 pin 4)

The screen grid is at a slightly lower voltage than the anode (1k, 3W resistor, R17) and was originally developed in pentodes for high frequency radio operation, by reducing the Miller effect capacitance between the control grid and the anode, and screen the control grid and cathode from anode voltage fluctuations by having a constant voltage at the screen provided by the screen resistor R17. It also allows lower anode voltage swing than a triode so is more efficient for power amplification.(Blencowe, page 43).

6V6/EL34 pins

Discharge Time = 350V-5V
in less than 5 sec with valves in, about 2 mins with valves out.

Bandwidth graph

Relative Input/Output Deformation at speaker

Above: a little deformation of the input sine wave compared to the OT secondary output.
LED options (get the amp working first..!)

“The schematic below shows a simple preamp (albeit with some Valve Wizard tricks incorporated) and shows some fun ways in which LEDs can be incorporated, usually with no impact on the tone of the amp. The different applications are numbered: 
1: In series with the anode. For normal values of anode resistor, this will have no effect on tone / operation. The LED will slowly come on as the valve warms up. 
2: LED bias. Obviously this has the same effect as a bias resistor with a perfect bypass capacitor, so carries tonal considerations. The LED will slowly come on as the valve warms up. 
3: In series with a cathode load resistance/tail resistance. Although this will raise the cathode voltage by a tiny amount, it is usually insignificant in terms of normal circuit operation. The LED will slowly come on as the valve warms up. 
4: In series with a smoothing capacitor. A rectifier diode is also added in parallel to allow normal operation of the cap. The LED will be off normally, but will light up as the capacitor discharges at switch off, providing a warning while the cap is still holding charge 
5: A dual-colour LED in series with a smoothing capacitor. The green LED will light for a couple of seconds at switch on as the capacitor charges, and the red LED will light at switch off. Don’t use this one on the reservoir cap though, the higher inrush current will fry the LED! 
6: Ok, this one isn’t an LED, it’s a neon lamp used as grid-to-cathode arc protection on a DC-coupled stage. The neon will light at switch-on until the valve has warmed up, when it will switch off. (An LED can’t be used here due to the high reverse-voltage during normal operation).”

AC Hum = Rectifier Hash – see videos and page 76 Blencowe.
I want to point out that the problems I have below do NOT reflect on the Ampmaker build if that is followed to the guidelines that Barry has laid out for that amp – I did my own version with transformer and wiring layout of it, and I think I am suffering from magnetic induction issues getting into the amplification chain.
However – Rectifier Hash is a documented issue in many circuits so it is as well to know of it and why it happens, and the possible fixes for it, as I try to document below.
I emailed Barry and he was very curious to know why this is happening and as helpful and courteous as ever.
Initial Faults

AC Hum = Rectifier Hash – see videos and page 76 Blencowe.


Partial Cure = ground the heater centre-tap – do not connect to power cathodes as in schematic.
You will probably need to add diode switching noise suppression also in the form of a 10R to 22R, 3W resistor after the diodes, and maybe snub caps also, as in the links below. (I do as I still have considerable hash noise at 1/2 volume and the 22R, 10W I have alone has not cured it).
The resistor removed this spike from A+ in these 2 pics but its not enough as it still exists just after the diodes:
You add 450+V “snubbing caps” across the diodes to suppress switching noise as in the Fallen Angel PSU section:
(Finally found out what these 4n7 caps in the Fallen Angel schematic are for!)
This is what I have left noise wise after the 22R, 10W resistor, another 47UF reservoir cap (which will increase inrush current so NOT a switching fix, but reduces ripple from 18V to 12V). An experimental 0.47uF, 1kV ceramic cap across the diodes to see if that filtered anything (no!), and a 3.3uF, 450V cap just for the hell of it, at A+:

I guess you would need to work out the spike frequency, and find a cap value that filters that…?
Quiescent Voltages – end of build – with above hash/ripple reduction mods
Ripple at Diodes, A+ (no hash) and 6V6 Anodes (with hash – connected directly to A+) against the deformed heater 12.6Vpp sine wave – the hash spikes correlate with the PT sec inrush current as the diodes switch on for each half cycle:
Anode Hash.jpg
Diodes 343V, 18V ripple
A+ 340V, 12V ripple
B+ 316V, 0.3V ripple
C+263V, 0V ripple
Ok, after a bit of thought, and knowing that the spikes are in the amplification chain at the control grids and anodes of all stages (power anode here):
and not at A+ with the ripple here (which connects directly to the power anodes):
the only conclusion I can imagine, is that it is being picked up via magnetic induction somehow. Either via the PT transformer secondary or heater wiring getting onto the pre-amp grid wiring maybe. Blencowe states stray capacitance – p75.
Referring to the Fallen Angel schematic, the snub cap values are 4n7, so I thought I would go with that value rather than Blencowe’s 10nF values,
as it’s a production circuit.
Maplins didn’t have 4 x 4n7, 630V caps (as usual! stock 2 of each only! Duh.) only 4 x 1nF, so I tried those across each diode, but it did not filter the spikes.
Next, I removed all the mods done so far, except the 22R resistor – back to one 47mF reservoir cap, and I replaced all the 1N4007 diodes with new ones.
As hash is present at the volume and tone controls and can be filtered there, it gave me the idea to use a 4n7 cap across the control grid (LTP pin 2) to ground of the LTPair instead, and hope this value has been worked out (using f = 1/2 x Pi x RC ?) for the duration of a diode switching spike – which is very short of course – less than 1/1000 sec. A 50Hz cycle is 20ms, so the spike duration is about 20 times shorter that the mains cycle:
This actually worked quite well, but sacrificed a little tonal quality I think, as there is the more jangly mid presence “missing” it seems, though PP amps are supposed to cancel 2nd order harmonics, which is what you get in a Single Ended design, which gives that characteristic valve tone.
I videoed it here with some music through it after the addition:

and here is the cap filtered PP18 compared tonally to the Maggie – a much better result than I expected:

As I will be using the two amps together for my setup for a while, with the pedals, so the loss of PP18 tonality won’t matter – it has better bass response to make up for it anyway – I’m not too bothered right now.
Here’s what it all sounds like with the stereo FX chain – AW3, Guv2, Echohead and Zoom1202:

Looking forward to cranking them again this Thursday, and recording it this time.
After reading the Briggs PDF on heater noise, I am more convinced that there is leakage of the heater current signal onto the cathodes of the valves, which is how it could get into the signal chain.
I tried setup c: with my 4n7 filter just by moving it from grid pin2 on the LTP to heater pin 4, removing the heater centre tap from ground, and grounding the end instead, but it just made overall hum MUCH worse instead.
I returned it back to the way it was.
I have decided I will totally re-work this amp in future, by placing the transformers pins inside the chassis as the Maggie, and orienting the PT and OT at right angles – as the Maggie is – which does not suffer the hash problem, as it could also be mutual PT/OT field inductance..? This makes for much shorter link leads also which should help.
For now though, it sounds pretty good with both amps in stereo. I have also added a Danelectro flanger to the setup, which sounds nice…cover that another time.

OK – I rewired it after moving the transformers inside the body of the chassis like the Maggie etc – a lot tidier wiring now also, but there is still some mains hum/hash with this amp. The Ripple percentage is low though – 17V of a rail voltage at A+ of about 365V is about 5% – a really good figure according to Blencowe (chap3, p59) for any valve amp.

    Quiescent Voltages – After Rebuild

A+ 344V ripple = 17V
B+ 325V ripple = 0.8V
C+ 272V ripple = 0V
Pre amp anode = 135V
Preamp gain = 6.6V/0.2V test sig = 33
Pre amp cathode = 1V
Current = 1V/820 ohm = 0.001A
LTP (input) anode (pin 1) = 216V
LTP (fixed) anode (pin 6)= 203V
LTP cathodes (pins 3 and 8) = 71.7V
Current = 71.7V/56820 ohm = 0.0012A
LTP pin 2 (vol at 0) = 52.7V so grid-cathode bias = 71.7V – 52.7V = -19V
LTP pin 7 (vol at 0) = 52.4V so grid-cathode bias = 71.7V – 52.4V = -19.3V
LTP mid point (R6, 56k) = 73V
Power tube anodes = A+
Power tube cathodes = 16V
Current = 16V/150 ohm = 0.1A
Power tube 1 grid pin 5 (sig pin, 0 vol) = 0.02V
Power tube 2 grid pin 5 (fixed pin, 0 vol) = 0.03V
There is still some hum/hash with this amp, but it is bearable, though on the scope the there is only a little trace of a spike here on the power anodes:
The ripple shape on the anodes is now a different shape also, with asymmetrical charging and discharging now…?
Still deformed heater sine also:
I’ll record this amp tomorrow – 21/11/13 hopefully – seems I’ve a dodgy jack now in the cab, so cant have stereo amps or 1×16 ohm single amp…just jinxed lately it seems…
PP18 kit on Ebay – Maggie to go back up when returned and repaired. Maggie_PP18amps.jpg  https://www.ebay.co.uk/itm/251529565678?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1558.l2649
PP18  + Maggie – SOLD
Maggie Voltages after repair: maggie_schem.jpg Back on Ebay https://www.ebay.co.uk/itm/251532739469?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1558.l2649 Maggie – SOLD ————————————————————