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Saturday, 9 April 2016

Junque Box Boutique Amplifier.

After repairing Norman's Rickenbacker amplifier, he came to me and said "Andy, you can't keep repairing guitar amplifiers if you don't own a guitar"

Fair point.

So he very generously provided me with such.

Er ...

... yeah ...



So, after repairing the recent Marshall amp, I managed to wrangle the opening bar or so of "Matchstick Men" by Status Quo out of it, then Steve came along and collected the Marshall, much to the relief of the neighbours.






What's needed here is an amp to call my own. Shall I be going out and purchasing such? No. Shall I be looking round in the workshop to see what we've got kicking about that I can cobble together at minimal cost? Read on ....

Whilst I'd love to make something really big and loud, purely out of engineering interest, I'm sure the neighbours wouldn't.

So, a few bits and pieces were gathered together. I managed to score some unusual output valves for not a lot of cash from eBay seller seemoreitems. Some wire-ended N78 pentodes. I had a couple of mains transformers that I could lash up to get heater and HT from, and a rather nasty 8" 6 ohm Sansui "Woofer" from the 80's....



I was going to need to order a few things, namely a couple of chassis boxes, and an output transformer. I gave Philip at Bluebell Audio a ring, and bought a Hammond chassis and a 125D Push-pull output transformer.



So, gather the items together and see what transpires...


First up, the power supply. I orginally toyed with the idea of using two 6V transformers back-to back, but this was going to end up being huge. I found a nice 20VA 6V transformer, and a 50VA isolating transformer.. the junk box provided a bridge rectifier, and a couple of caps, all salvaged from a duff switched mode computer PSU



 Top side of the completed power supply....












And the rectifier and filter...

Note the tags on the top of those transformers have mains on one side, and our un-rectified HT on the other, we'll need to make a cover for that later...







Smoke test gives around 280V DC off load. Let's push on with the amp itself....

I'm going to start with the output stage and work my way back to the input, making it up as I go along!

First off is to look at the N78 datasheet, which can be found on the wonderful R-type.org website.

So, a pair in push-pull, class AB1, with 240V on the anodes (allowing for a little sag from our cheap PSU), should give us a nice healthy 5 watts or so, with a 9 kilohm anode to anode load. That's quite a lot for a small bottle, no wonder the N78 has a reputation for running hot.

So, how to mount our wire-ended valves?


I did contemplate mounting in a stainless steel cable gland, but it's a bit ugly, and I'm concerned the valve won't be able to radiate enough heat. I ended up drilling out some B7G bases, and feeding the wires though the holes and soldering them to the tags. Perfect. Be careful with the wires, if one breaks off, you've had it.





I mounted the output transformer, and started marking out for the valve bases.

I'll cut three for now, one each for the N78's and a B9A base for an ECC83 (or something) for the phase splitter.







Holes cut and tapped ....



Valves mounted....














Heaters wired up using twisted cable, it helps to keep the hum out. .... so it must be time for ....
The arty valve shot ...

One side of the ECC83 (12AX7) will perform phase-splitter duties, whilst the other will be our pre-amp.









After I got the phase splitter operating, I added another ECC83, this time as a cathode-follower to drive a tone-stack. I shamelessly pinched the design for the tone stack from Marshall, who stole it from Fender...

The other section is another gain stage...







I added some pots, a mains switch and the input jack... eventually it's my intention to mount the amp upside down, with the valves hanging down, so the layout seems wrong. Controls are (from left to right in this picture) "Drive", Bass , Mid , Treble, "Colour" and Gain.






Testing at this stage revealed it needed more "oomph" for that overdrive rock sound... Let's add another valve! ..  another ECC83.

Now things were sounding good... but left me with a problem, I had an unused triode section.

I thought about adding yet another gain stage, but that would just be silly, and probably noisy too... so I set about thinking about a tremolo.



I looked around the valve wizard website, which is a fantastic resource, and cobbled together a tremolo oscillator... I was hoping to couple the cathode of the oscillator to the cathode of the input gain stage to produce the effect. It didn't work...

What I needed was a method to modulate the volume... I had a cup of tea and a think. What about a small FET?



I had a look through the FET drawer, and selected a likely candidate,a BS107 in this case, but a 2N7000 would do nicely.) Connecting this temporarily to the volume pot worked well enough. I added depth and speed pots to the rear, as well as a foot switch plug, to switch the tremolo on and off.






Tidied up the wiring, and added a pink LED (because blue is sooo last year).













Now to tidy the workshop, and think about a cabinet....

Incidentally, this picture was taken with only the heater supply on, to show the valves lit up, and the LED working. Never, ever run a valve amp without a speaker or dummy load connected.






Here's the schematic...



Let's have a little walk through...

The signal enters at J1. C1 is used to block DC, as the grid of the valve is around 1.5V above ground, to set the bias of the valve. R1 is the grid leak, and also sets the input impedance. R4 is the grid stopper resistor, and, in conjunction with the miller capacitance of the valve and C4, stops the thing trying to oscillate or pick up radio. The signal is amplified by V1A, and emerges at the anode (plate if you're in the US). Anode load is provided by R8. The signal is coupled by C6 to the volume control, R10. The signal is then coupled to the drain of Q1, which is the tremolo modulator FET (more on this later..). The signal then passes via another coupling cap, C17 to another gain stage, V2A, in the same manner as the first. The output of the anode at V2A is directly coupled to the grid of the cathode follower V2A, which has no gain, but has a low output impedance to drive our tone stack. The tone stack consists of three filters, created by C11,12 & 13, R18 and the pots, which are used to control the amount of attenuation of each filter. The output of the tone stack is connected to the drive pot, coupled to V3A in a similar manner to before. The cathode of V1A has a pot connected, so we can change the bias of the valve slightly. This has the effect of altering any clipping that may be occurring, and, in a guitar amp, we want a bit of distortion! If the tube is biassed "cold" i.e. less current flowing (larger value of cathode resistor) the distortion has a "bluesy" sound to it, if the tube is biassed "hot" , it's more of a "rock" sound. The effect is subtle. The output at the anode of V3A is coupled by C17 to V3B, which forms the phase splitter or phase inverter. The grid of V3A is fixed at ~53V by the bias network, formed by R29 & R30. An inverted signal appears at the anode, and a non-inverted signal at the cathode. Note the anode and cathode resistors are the same value, ensuring balance (as long as the load is symmetrical). The inverted, and non-inverted signals are passed by their respective coupling capacitors, C18 & C19 to the grid of the N78 output valves. R33 (R34) provide grid leak, and R35 (R36) are the grid stoppers. Anode load is provided by the output transformer. R44 & R45 are feeding the screens. This resistor stops the screen grid from exceeding it's maximum dissipation during overdrive conditions. V1B forms a phase shift oscillator, and operates from about 2Hz to 10Hz, speed being controlled by R2. Each capacitor, which it's resistor, provides a phase shift of 60 degrees, and the inverted signal from the anode is inverted a further 180 degrees, forming the oscillator. The output from the anode is also coupled by a large value capacitor, C7, to R12 "depth" control which is used to feed the gate of the modulator FET, Q1. via the potential divider R13 & R14. The source of the fet is connected to ground, and the drain to our audio line, between C9 and C10. When the FET is biassed on, the audio is coupled to ground via the FET. Shorting the gate to ground switches the FET off, and switches off the tremolo effect.

How does it sound? Great... let's rock ...

.. now to learn to play the guitar !






















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