Valve preamp.

[Diabolical Artificer]

After rootling out some bits I decided to have a go at knocking up a preamp to go with the big amps I made. They need 1.5v RMS to get the full 100w OP, my PC and other music sources do a bit less than this, so the preamp will compliment them.

At first I was thinking of designing the circuit myself, maybe using EF80's triode strapped for a line level amp, but after some searching online I decided to use Patrick Turners design's for the phone and line level amp, the fettling will take enough time as it is, so decided to use an existing design. I have a lot of respect for his work, he put in a lot of research and testing and to be honest a phono amp is a complex beasty, anything I designed wouldn't be half as good as his.

I've had this big tfmr and choke knocking about for ages, see pics. The tfmr is huge in comparison to it's power output, only 80mA, so perfect for a preamp. I've connected it with an LC filter rather than CLC, to reduce the HT, with two 820u 400v caps ripple is 20uV before regulation. I'll use Patricks PSU design as a guide and fit it what components I have.

Any ideas and constructive criticism welcome, Andy.

[Diabolical Artificer]

After some reflection and some advice over on UKVRR - see - https://www.vintage-radio.net/forum/show...p?t=157121 I've decided to test a few different phono circuits. I've built one phono circuit to test so far, IE the Valve Wizard ECC83 based jobbie, see - http://www.valvewizard.co.uk/phonopcb.html . I built this using Tektronix ceramic wafers although there is a PCB available should I decide to build it, I can get the whole circuit on two 11 "pole" wafers. It seems to work ok however I have 50hz modulating the OP. With the circuit knocked together on an old chassis 50hz was at about 60mV P-P, I reduced this to 20mV P-P by lifting the ground and reduced it a bit more by moving the circuit away from the mains tfmr/PSU section and putting it in a shielded steel box, it's still too much though.

Another issue I've hit is testing said phono stage using an inverse RIAA filter. I built this one - http://sound.whsites.net/project80.htm the signal in is really attenuated, but after some thought I suspect this to be normal. I thought at first I could IP a 2mV sine, a typical cart OP but there's nowt comes out, obviously in retrospect.

I'm a little uncertain about R3's value, there are a few inverse RIAA filters I've found, all use the same R's and C's more or less, EG 833k, 75k, 3n6 and 1n, but use different value's for R3, anywhere from 600r to 3k3. More testing of my circuit is needed but suspect R3's value needs tweaking to suit one's sig gen OP Z.

I've also finished building the wood box/chassis, a nightmare to do but doesn't look too bad.

Andy.

[Diabolical Artificer]

Made some progress on this - see pics. First I decided to split the RIAA phono stage and put it on a separate chassis with the PSU and line stage in the case as shown in tother threads. The circuit I used is a simple one but slightly odd in that it's a CCDA = constant current draw amplifier, more info here - https://www.tubecad.com/2009/03/blog0161.htm and see attcahed schematic. I made a few changes subbing 470r for the Rk's and 310r for the grid stoppers.

The two valves at the back are VR150/30's, there to clamp final HT at 309v as I killed a 820u 400v cap as HT is 450v, not ideal but I used what I had to hand.

Andy.

[Diabolical Artificer]

Finished off the wiring today and apart from a few things like finding some screws, better knobs and doing a bit of tidying and testing it's finished. Oh, have to do some labels. I tried Letraset on thin brass plate, letters good but I coated it in watered down nail varnish which left a rubbish finish. I'll try the same but with laminate. See pic's.

Andy.

[BusyBee]

It's looking great Andy! Looking at your posts and responses on the other site plus the photographs here it looks like you have been busy, to say the least. I hope it sounds half as good as it looks as it will be well worth the effort.

Thank you for that little schematic above. Looking at the Tubecad site it looks interesting as the circuit is not a million miles from similar circuits I work with.

Tracy

[Diabolical Artificer]

Thanks Tracy, yes, it took me a long time to fettle, having minimal tools prolongs fabrication of the chassis. for instance the sycamore wooden box was planed down from rough 1 1/2" boards straight off the tree, took me ages with a small hand plane especially the 45 deg mitre's. Next amp I'm going to find someone with a machine.

It doesn't soon too bad, though when i tested it the first time it blew a capacitor due to it running 75v above wkg voltage, a wiring mistake.

Yep, John Broskie has some interesting ideas, the Tubecad site is well worth a read.

Are you building any amps yourself Tracey?

Andy.

[Kalee20]

... for instance the sycamore wooden box was planed down from rough 1 1/2" boards straight off the tree, took me ages with a small hand plane especially the 45 deg mitre's.

Nice work, especially as there's a couple of knots in the wood! Adds to the final appearance, but harder work to get there.

I like marrying electronics with dead plants.

Cathode follower outputs to drive cables is a good move.

[ppppenguin]

This design is OK as it has resistors in the grid of the CF stage and between the cathode and output. But beware of CF (and emitter follower) stages driving capcitative loads. They can oscillate rather well, often at frequencies your scope can't see. Capacitance at the cathode/emitter is reflected back as a negative impedance at the grid/base. Hence the need for a grid/base stopper. A bit of resistance in the output, to keep the capacitance away from the cathode/emitter also works.

[Kalee20]

It's a point well made - a grid stopper right on the grid pin is good practice.

Drawing stray capacitances, and imagining lead inductance between the grid pin and a 'stiff' source, you can see you've created a Colpitts oscillator.

[BusyBee]

Are you building any amps yourself Tracey?

Andy.

Hi Andy, I don't do much with audio, apart from little circuits but have done a little this year.

During the summer the TV sound system failed and, because the sound was never too great plus there was no obvious fault, it was outed. I did keep the speakers though as they looked useful. I concentrated on justifying that by putting together a couple of transistor amps using some power transistors I originally bought to use as high voltage regulators (before I settled on below 100V for most of my circuits). The amps were pulled together using info from various places so are nothing out of the ordinary, just organised with components I had. I was going to post here but never got round to it (photos attached). I did need to work on preamps though but, as happens, other things came up and it never happened. Your thread is pushing me a bit though but I am not thinking in your terms though. Something fairly simple. The amps are around 10W with 15W if pushed hard (well distorted). It's really the heatsinks that limit them but I only was looking for toward 10W anyway so good for the job. With higher voltage they would need bigger heatsinks but should be well capable.

At the moment I am thinking about a valve radio with 'gram' input. I do have some valves (miniature Russian 6p30b) and around 5W out. I have an output transformer which I think was for ECL86's but need to check it's suitability for an 'ultra linear' design (it is designed for that). It's very early days as I am not sure of the design but can probably get to a test circuit easily.

Tracy