Another new project, valve pre-amp for my turntable

[g8ina]

It's an ECC83 double triode design, 3 bottles in total. Needs 280VAC and 6.3 AC, all diodes and smoothing is onboard, so just awaiting the valves from Russia and the mains tx from China.

All silver soldered too. Case is an old Technics stereo amp, fits just nicely. I still have to work up a new front and rear panel.

[MLyons]

Wow, very nice work. Any chance of a pic of the underside?

[g8ina]

Not now sorry, its built and screwed into the now black painted case. Taking it to a friends on Sat for testing - I dont have the mains transformer yet and he does

The PCB is available in multiple ebay stores, maybe one shows the underside. Ill see if I can find one.

[g8ina]

[g8ina]

Update - New case courtesy of Bonebreaker777 Cheers Dude, its perfect.

New mains tranny arrived from China, the same damn day as the replacement vintage one I ordered off ebay did !

Got a great deal on cryo treated 12AX7, three for 21 quid inc post.

Just bought a nibbler to make the right sized cutout on the case rear panel.

More pics soon.

[g8ina]

Oh Joy - when a tool you buy *specifically* for ONE job works perfectly

Nibbler

[g8ina]

More piccies, nearly finished, just waiting for sleeved grommets for mains cable, then final wiring.

[peterb]

Very nicely finished - the hallmark of good homebrew kit. (Had to smile, you dont see those high power wire would cermic resistors very much these days, always slightly problemtatic at high frequencies because of the parasitic inductance - although if they are where I think they are on the cct diagram, they are part of the power rail filtering so not a problem!)

[g8ina]

Yep, resistor, capacitor on two voltage drops Built in HT regulator too, wasnt expecting that, but Tim de Paravicini always does a great job on design.

[peterb]

Probably need a better looking on/off switch for the front panel!

https://uk.rs-online.com/web/p/rocker-switches/3779759/

[g8ina]

It will be fine once the locknut is on

[g8ina]

finished, a bit noisier than I expected, but got that great bottle warmth

[g8ina]

After reading a HUGE thread on these amps in LencoHeaven, I decided to split the PSU out -

Power supply removed to new separate case. New zener diodes to be fitted and adjusted to give 285VDC on main rail, my HT was a tad high, using a different combination of 75v and 68v zeners should bring it down to 286VDC. Extra capacitors to be added to heater rail, plus small 0.1uF across each heater on the valve base. The old poweer switch hole will just be blanked off, or maybe fitted with a lamp.

Case with just main PCB, extra wiring to be added for power lines, extra caps under each valve holder across heater rails.


Phase Two starts here - IMG_2745 by David W. Harris, on Flickr

Power supply now in new case, extra fat caps to be added to heater rails.


IMG_2746 by David W. Harris, on Flickr

Rear of both cases, new entry hole needed for heater supply, new labels, 300VDC out from IEC socket, will input into main case via cable thru grommet.


IMG_2748 by David W. Harris, on Flickr

A few more pics on Flickr.

[Millennium]

Ok man but which 'cryo treated' (lol?!) 12AUX did you buy? I mean, you didn't go NOS for that price I'm thinking. It makes a HUGE difference having decent valves in an amp, especially a preamp or a preamp stage. I have had a Harmon Kardon A500 (70s, USA) for many years. I don't currently use it but one of the things I'm planning to do if I ever DUST IF OFF is to change the 4 12AUX and the 1 12AU7 (currently Philips or whatever).

My thing was and is valves. I have like 5 valve amps all told, probably 2-4 working (!). They are fiddly, I'll put it that way. I have a decent UK build Class AB that's quite almost (? !) loud enough and a lovely Class A that's had the expert treatment thanks to a local HiFi shop. Both of those are working and at my place as opposed to Mum's loft !

Still, the Harmon Kardon is probably my fav. It's a literal ICON. [It predates the Commodore Amiga A500, perhaps rightly!] // The output valves on that thing (and I got 10, 4 in use at a time, about 1-2 have failed) are the rare as freakanomics Hen's Teeth '7355' which was a literal marvel and one of the best output valves that was ever produced. IMHO it really stomps the KT88 or whatever that my (still good) AB amp at home uses. I love 'em. I need MORE. But....

...that's another Story!

So... get decent valves for your amp sir. Seriously. UK is fine, USA is fine, Chinesse is ok sadly, Russian can be good or ok or maybe faulty. So... do it man. Trust me!

I have too many amps not making any noise. Darn.

edit: accccctually the 12AUX in my Harmon A500 are really pretty good, they are the Telefunken ones (the good ones it seems, there were some differing plate designs) but after many hours of use I'm totally unsure if they're too warm or too useful or whatever. One thing I know is that they would be hard to replace, even like for like... I'm not even sure if I would replace them with Mullard or such to be honest. Some dilemma. I should probably buy some suitable speakers and play with this amp sometime. Currently I'm living on the edge with all solid state. Why..... ?! Perhaps the afterglow

edit: some fixes and changes 29 05 2018 21:12pm BST

[g8ina]

1x JJ, quoted as having 30 hours and 2x Electro-Harmonix, quoted as having 50 hours. I may yet invest in a fully new set of JJ as they seem to be the favoured ones right now. NOS is always dodgy, unless they come with a test report.

Nice to see another bottle fan anyway My all time fave is the 807, but thats from my radio ham days. KT88 - yeah, I'd have em Dont know the 7355 !

I'm also lucky to have a valve genius expert just 12 miles North of me


IMG_2676 by David W. Harris, on Flickr

[Millennium]

Good for you nice choices! I think you'll be fine with the JJs or the Electro-Harmonix honestly (not sure where they make any of those these days though...).

It's well worth reading up on the 7355s. The Harmon Amp I have is capable of going really loud enough in a fun and well engineered Class AB setup. I'm not sure what other stuff used 7355s but they only had 2 American manufacturers and I think in the 14-17 years I've had the amp they've really become something boutique.

Thanks for the advice on NOS tubes - a lot of the better suppliers do send over a test report with them... You can get a bargain sometimes too. If they are never used NOS, why would you still need a test report, in case they are a bad batch or deteriorate over time?