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Old beast after tidy up (1969 ORST)

Posted: Thu Sep 05, 2019 8:28 pm
by LD50
Further to the post by Silverfox I have an update.


The amp is now up and running. Just need to soak it fully before putting two of my last NOS Mullard xf2s. It is a bit louder and cleaner than my clone as it runs a plate voltage of 525v on the outputs (vs 490 on my clone).

Re: Old beast after tidy up (1969 ORST)

Posted: Thu Sep 05, 2019 11:43 pm
by bclaire
Wow... very cool. Shoot a video once it's up and running!

Any idea what it says on the back....?

Re: Old beast after tidy up (1969 ORST)

Posted: Fri Sep 06, 2019 7:22 pm
by LD50
So far I am having oscillation issues when volume is above 2nd notch, up to that point it is, as is usual for these pretty loud and has nice touch dynamics, it is quite a brighter than usual and very loud (with the 1M chassis pot turned up to max - the sound is fatter and more full that way). It is a bit spiky with a strat but plenty of beef with my LPR0.Problems come with volume over 2.
I have cleaned and D100 treated the switching jacks and rechecked the bias (shunt voltage on the primaries are not particularly stable yet the grids are solid at -50v).
I have measured the cathode bypass caps in the preamp and first two are 200-300uF but PI one is 0uF, albeit in circuit and they should be 47uF. I have some more of the correct value on the way and will probably replace the bias caps before I risk my NOS Mullards.

Other point I note is the dropping resistors after the screens are 200R x2 in series (not stock). I replaced one with a 10k - what I have normally seen here but left the other at 200R (I've seen a couple that just have a link where the second resistor is.

Anyone else have an early one of these? I've worked on 3 now - all slightly different!

Re: Old beast after tidy up (1969 ORST)

Posted: Sun Sep 08, 2019 8:40 pm
by LD50
The issue was the ground wire from the drive pot was not making good contact with the ground lug on V2.
With that sorted everything came good.
It is, of course I am biased, fantastic. Very loud, very dynamic, at moderate volumes with boost down a warm clean Orange tone and with the volume and boost cranked very dirty too.
I am grateful to Silverfox for his assistance in selling it to me.
It needed a good clean and a scrub.
I superglued the floating paint down to the chassis
Replaced both octal sockets
Replaced the bias caps
Remade the rectifier (I used UF504, 3A ultrafast recovery diodes
Replacement front panel
New pilot lamp
NOS RS 100uF 450v filter caps (with added 220k bleeders).
I'll make some quick clips soon.