PCB based Mullard 3-3

I have decided to build a Mullard 3-3 amplifier in stereo, VVT are supplying the transformers, Gerry at Crowthorne Tubes has supplied the valves (always good to visit Gerry and his cats) and work supplied the PCB CAD time to design the board. (don't tell them).

Having asked a couple of questions in this thread https://www.vintage-radio.net/forum/...ad.php?t=78393 about valve PCBs and received useful answers it was about time to make the PCB, see atached photos. The LT will be a twisted pair and the grounds star wired to the input sockets. I have yet to cut out the transformer holes, I will wait for them to get proper dimensions. They should be here early next week. The idea is to use the PCB as the top chassis plate and trim with some hardwood surround. I will give it a good clean and a laquer finish on the top, you never know it may turn out looking good.

Because of the larger HT current used in a stereo job I am using an EZ81 for the rectifier and two 50uF smoothing caps as opposed to one in the original design http://r-type.org/static/3-3.htm also to reduce hum they are split with two 150 ohm resistors.

Watch this space...

MM

You never remember all of it...

The components will be mounted 'surface mount style' on the bottom of the PCB, saves all that drilling and leaves the top clean.

[percival007]

Excellent idea.

This looks like it will be an interesting Amp, please do keep us posted with updates.

Best wishes,

James.

[Valvepower]

Fantastic project

I like the idea of ‘SM’ mounted parts and the ground plane on the top.

Ground planes offer a great ground and I've used them to solve many an EMC problem.

Terry.

[G8HQP Dave]

Ground planes can cause problems for audio, as you lose control of where the currents go. Star or bus ground can be better. Treat the ground plane as part of the chassis, and keep signal currents well away from it.

Treat the ground plane as part of the chassis, and keep signal currents well away from it.

Oh I am, there is some rather nice PTFE insulated wire at work for the star earth etc. I must ask the boss if I can nick some.

Wire nicked, top laquered, holes made for transformers, components mounted, LT leads fitted. I must say the PTFE wire is a joy to use, silver plated and all (they use silver plate because tin would melt at the extrusion temperature of PTFE).

P.S. The two nylon screws are there to stop me mucking up the finish on the top.

Had loads of fun with wood and my milling machine, oops the first go didn't take into account the twist in the wood, second go, much better.

Here is the result...

[ronbryan]

I'm just a bit worried that what looks like 1/16" fibreglass pcb won't support the weight of the VVT transformers, especially if the amp gets any accidental rough handling. Are you planning any reinforcement to support their weight?

Ron

Ron,

Luckily the left right transformer mounting holes holes line up, if it does bend (perhaps helped by the heat) I will pop in a strut across the middle. A simple 'stick' would do, I think I have some 1/4 x 3/8 ali bar in stock. Even so 1/16" PCB is awfully strong, ever tried snapping a bit off a large sheet?

MM

[percival007]

A very nice job so far.

From the Valve count it looks as though you are using a Valve Rectifier? Is there then really a need for a separate H.T. Switch? The H.T. will rise gently as the Rectifier's Heaters start to do their job. At least, that's what I thought anyway. I assumed that H.T. Switches (usually found on Guitar Amps) were needed if a Semiconductor Rectifier is used. I am no expert in these matters so am happy to be educated otherwise.

Keep the picture coming!

James.

Ah the old HT switch question, it would look a bit bald with only one switch and neon (sheer vanity also I can watch the HT neon come up to full brightness). And if I wanted to mute it for a little while I could save aout a half of the power consumption. Mind you I thought I was a bit extravegant usiing valve radios until I looked at the power needed to run (even one of) the LCD TV(s)!

[Diabolical Artificer]

Youve done a good job there-very neat and compact,nice one. What preamp/controller /speakers will you use with it? Looking forward to seeing it finished,Andy.

I have yet to build the controller, not much need of pre amplification these days with a 100mV sensitivity amplifier. I think it will be remote control with an Alps motorised volume pot, manual balance and a motorised rotary source select switch, I will have to make that. I dont use the tone controls on my current amplifier so they won't feature.

The speakers are Tannoy DC4's, very nice indeed, brilliant for the spoken word.

[Diabolical Artificer]

Lovely speakers. Are you only going to use this 3-3 for Radio 4 etc? Your control unit sounds intrigueing. Will it be used for PC,CD etc,Andy.

I use the current system for TV, CD/DVD, Radio (via a STB) and Radio (Perseus SDR) via PC. Also the output goes to my MW modulator to run other radios.

The transformers arrived today, so on to wiring (see picture), after that marathon session it was time for a beer. I have the rest of the week off, powering up tommorow.

[vinrads]

Hi is the glass half full or half empty, an excellent choice by the way,I hope all goes well. Mick.

If it's not empty it's good! As to choice there is a shop just down the road that does a selection of 3 for £4.50 needless to say I have three, two now. No 'preamp' at the present so I will plug my radio STB direct and use its volume control for testing.