Meesin'about with heaters ...

[andrew Ivimey]

I've been messing with pre-amps - can't think why; I mean, what's the point. Nothing splendid and complicated but I thought it would be `fun'. Thought I might learn something too. I started with a 26 - may as well start with something easy and it was. It sounded great ... and then it didn't :-: and nothing I could do would repair; I have hummmm and it can't be the lead acid batteries. The valves seem to need high current delivered on a tray of finest spun platinum threads and if you have a regulated power supply that may help too - I don't know. I put it on the side and moved on.

My sperrymints with 10 / 801had the gain of a bucking bronco and were also drowned by hummmmm so I took that apart but before I changed the sockets I plugged in a 47 (I have these very useful adapters). Starved of HT it was amazing! But using lead acid batteries with 1.75amp heaters is not the way to go. (45 was dull, didn't try 46 but moist eyes have been recorded by folks who have tried 46) 30 is screamingly microphonic, 31 much better. And a 27 is coming along. I love globe 27s - they look so ancient and crusty. I will work on this one.

But, taking an old chassis from the shed I thought, `12B4a', why not? Microphonic; Poing! Ting! I will reduce the gain by removing the cathode by-pass cap and plough on. It didn't sound very good to be honest. Still and what about this, some geezer, somewhere described the relationship the cathode resistor and cap as negative feedback for the valve. Well you could have knocked me down with a BMW M5 - one man's negative feedback is another's cathode bias.

So, what do I expect from something that shouldn't really be necessary in the audio chain? Magic; and addition of fairy dust, a lowering of the audio floor (so things are more clear; like an `expander') with as few components as possible, the aesthetics of a few more valves to add to the already subdued and subtle lighting in Beford Towers... and so on.

Where will this end?

[pre65]

As a fellow preamp player (temporarily suspended) I am of the opinion that to get the best out of a 26 in a pre one needs to try filament bias with a Rod Coleman filament module. Seemingly Rod can mod his normal 26 board to suit filament bias.

So my next effort might well be 26, choke loaded, filament bias and dual regulated PSU.

If that fails I shall stop trying.

EDIT. The "now me" is looking favourably at battery grid bias for 26, not through my experiences, but through those of (unnamed) "others" .

[andrew Ivimey]

The funny old thing about Rod Coleman (power to his elbow) is that back in the 1920s, 30, 40s etc etc noone used these devices and, it seemed, they didn't need them - odd that!

[pre65]

The funny old thing about Rod Coleman (power to his elbow) is that back in the 1920s, 30, 40s etc etc noone used these devices and, it seemed, they didn't need them - odd that!

Perhaps our expectations of what we want these old valves to do has changed ?

Funny how you can't get your 26 back to how it sounded at mine. Do you think the valves have gone past retirement age ?

[andrew Ivimey]

Yep, could be knackered valves - I have ancient globes and they may be a bit too long in the tooth.

[Dave the bass]

The funny old thing about Rod Coleman (power to his elbow) is that back in the 1920s, 30, 40s etc etc noone used these devices and, it seemed, they didn't need them - odd that!

Ah yeah but no but yeah... what where these devices used in? Table top radio? The limited bandwidth these radios could reproduce 'may' not have highlighted any short comings in the filament supply, possblee. Only saying.

DTB

[andrew Ivimey]

Table tops yeah that's where the venerable and wonderous 45 came from too. Good point!

[simon]

Rod's filament modules are marvellous. Apart from a quick lash up with AC for 45 or 2A3 etc. I wouldn't waste my time thinking about anything else. Easy, cheap (as it goes) and sounds great. Rod's been first class too. Just my experience of 26-10Y-300BXLS, all with Rod's modules.

[Mike H]

Table tops yeah that's where the venerable and wonderous 45 came from too. Good point!

The AC heater hum wouldn't be so intrusive if you hadn't got big OPT's and big speakers.

E.g. the typical table-top wireless's lickle speaker and weedy titchy OPT which probably rolls off below 250 Hz. Tried making an amp with one of these once, totally useless for bass.

[andrew Ivimey]

that too is true!

[andrew Ivimey]

2 nite I thought - Life or death - either it works or it doesn't. Re:- the 12B4a pre. The dustbin operatives arrive tomorrow at 7-30am.

Under the bonnet its a mess of wires all over the place ... yuk! The mains transformer is huge (from the 99p EL34 Music Angel I acquired after much shenanigans but 99p .... it still wasn't worth it amp - the chassis is now a Darling amp and this mains transformer is cooking the 12B4a pre.) and gives 250 volts with SS diodes cap, choke, cap, choke, cap onto the anode resistor.

These are idht so who cares ... A.C. heating - hmmmmmmmmmmmmmm -> D.C. heating, surely that's enough .... yuk - nope, crap and hummmmmmmmmmmmmm.

So I raised the heaters using 22k in parallel to 150mfd with 220k in series across the HT the tap going to the neg of the heaters. Silensio !!!!! and then Alexis Korner cuts through sounding as sparkling live as he never sounded in the Flamingo Club in Wardour street when I watched, awed and furiously reading a book about smoking dope, 44 years ago as I grooved to the blues - WOW!

tomorrow is another day - Let's see eh.