Opportunity to buy an old valve guitar amp

[Yorkie]

Given its age, before I used the amp, I’d replace the .02 cap between the anode of ½ 12AX7 (pin 6) that feeds the grid of the 6V6 output valve pin 5. (‘that cap’). That cap has 200V DC on the input – you don’t want any of that on the grid of the 6V6 – all you want is the AC signal. Then I’d replace the 25uF 25V cathode bypass on the 6V6 (from pin 8 to ground), because it will almost certainly have dried out. Modern ones (22uF 25V) are a puny size, so if you want to preserve originality of appearance, you might want to stuff the old cap, but you may actually find that a modern cap won’t look out of place as the existing one is mounted on the tag-board adjacent to the 6V6, so may in fact be quite small itself.

Personally, before I meddled with the smoothing/reservoir caps I’d reform them, check the leakage and ESR and if they’re good, I’d leave well alone for now.

When you’ve done that, plug in your guitar(s)! The most essential component of any sound system is our ears – see what it sounds like – don’t take for granted statements by self-styled aficionados, judge for yourself before tinkering with it. Statements such as ‘distorts earlier and sounds a little sweeter’ that aren’t backed up with any tests are entirely subjective should be treated that with scepticism. Has whoever made that statement explained at what frequency and amplitude does a sine wave distort on this amp when fitted with 22 uF caps, and how do the oscilloscope traces compare with that of 40uF caps? I think we know the answer to that – it’s just someone’s subjective view dressed up as ‘fact’ with no underpinning science to support it.

As to replacing the 40uF cap ‘to the rectifier’ to be pedantic, the reservoir/smoothing caps aren’t ‘to’ rectifier – they’re from it. The layout of the chassis at the link below shows a three section unit in one can at the end of the tag-board, but given the long run of this amp (19 years), maybe some had discrete caps. You might struggle to get a three section unit and the old can will be too small to stuff three individual caps inside Rob.

http://vibrochamp.org/wp-content/uploads...layout.jpg

When you say ‘the bf version operated at 50V lower at the rectifier which means the amp has slight less output’, that may well be so but I doubt it. The circuit of the AA764 using 20 uF caps shows 342V on the anode of the 6V6. 50V higher with 40uF caps? I don’t think so! (The circuit says the voltage readings should be taken with an electronic meter (ie NOT a 20,000 OPV AVO 8 or whatever, but a valve voltmeter, or more likely these days, a DVM) and are +/- 20% of stated values. Resistors are +/- 10%.

That's my ten penn'orth.

Hope it arrives by the weekend Rob!!

Have fun