Grampian type 531 Amplifier

[phut bang]

Been on my bench now for eighteen months or so, hoping to look at it again when weather gets warmer.

[crackle]

Its looking good.

[phut bang]

Some more pics and circuit.

I will have to put the value of components in, the circuit is from memory.

[Diabolical Artificer]

Where's the OPT, is it in good nick? Looks like it will make a half decent guitar amp if the OPT is up to snuff. Not sure I like the colour of the caps though, gets me thinking of cats and owls in boats : )

Andy.

[phut bang]

The OP transformer I am using is a Partridge 9428. It says that on the transformer. It's rated at 75W and is physically too big, so when I finally sort the amp out I will look for another transformer. I originally replaced the two 5U4G rectifiers with 127's many years ago to mount the transformer onto chassis, but have decided now to put things as were.

[phut bang]

I was thinking it would make a nice foot warmer if I do venture into shed in the winter and the caps are somewhere near the original colours, don't suppose anyone will see them though the front of the amp. It is in wrinkle paint used for car engine rocker covers, so I put a little rock and roll into it and it's good for 250 degrees C.

[rontech]

I acquired a Grampian 552B some time ago and dismantled it for spares. Everything was built on a very sustantial basis. Heavy mains transformer and opt.

Considering a rebuild sometime. With new chassis and valves.

[AC/HL]

Grampian circuit diagram B version.

[phut bang]

well i have done some silly things in my younger years scrapping this one was one of them and now trying to find bits to replace certain items like the input transformer 4 inputs 4 outputs will never be found so have to do different circuit i just drawn the circuits so everyone can see i will add another eventually with component values in another amp i scrapped was a Williamson derived one could have been a gec circuit still have mains transformer choke and op transformer might get round to making it up one day

thanks for circuit incidentally i the valve line up is 4 kt66, 2 5u4g, s130 regulator, and 2 h63 as you can see from circuit diagram the output valves are cathode biased

[Herald1360]

The 531B is fixed biassed. Four KT 66s like that ought to be good for an easy 60W, 100W if you go for it. Cathode biassed I guess nearer 60W but not being thrashed at that. It should give a good account of itself with a half decent output TX.

[phut bang]

when it was original with poor op valves it used to give a measured 60w i would be happy with 60watts plus i can remember trying a mains transformer for output a very loud 1.5 watts once i have the amp working with the present transformer i will have to find a different one as this op transformer is pysically to big for chassis

[Herald1360]

I cobbled up a few LOUD amps in my yoof using large old HT transformers as p-p output ones. Anodes to HT sec outers, HT to centre tap and series up all the heater windings to get somewhere in the ballpark for turns ratio. My noisiest one ran a pair of EL37s. You could get a large 400-0-400 250mA or so surplus mains tranny for pocket money, a proper output one was about ten times as much!

[phut bang]

i will have to alter the cathode resistor values as the amp is already drawing 250ma i think the ht is 480v otherwise the bulb on the front will pop the valves dont seem to glow red internally though

[barrymagrec]

Is the bias supply correct?

[Herald1360]

GEC quoted idle current for 4 KT66s in cathode bias is around 220mA rising to around 280mA at full chat. HT is a bit lower than 480V though that figure is still within spec limits. If the amp is UL, currents go a bit higher.

[phut bang]

i will have to to sort out everything the bias for valves etc i think i have a few impedance matching problems for the inputs as ther is a lack of signal bt the time you scope the inputs and go through the audio input transformers and then the tone control network ,if you start with 1v p-p it comes out at the same before going into the first valve and not enough overall volume ,still, enough entertainment for me to work out in the spring when its warmer