Hi
Yes indeed Andy has pointed out a further error with the voltage rating of the tone correction cap on the primary of the output transformer, it would likely fail when the amp is working and at high volume.
Thank you all and this (small) project has been a massive learning curve for me all round. It will now make my approach to projects to be taken one step at a time , double check what I am doing/have done and the effect its had on the circuit.
However, I no need to complete this one so I will move forward on the advice I have been gratefully given.
Best Regards
Rob
I have taken the OPT cap off the primary connectors and also disconnected the leads into the amp. The resistance readings across the primary connectors were recorded as 326R and either one to ground had an infinity reading. The secondary connectors together were recorded as 0.5R and again to ground as infinity. Does this indicate that the OPT is ok?
As a recommendation from Trevor, I am also going to upgrade the coupling cap from 1000pF to 1500pF. The original one was a red Hunts 500pF 600V.
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Hi.
If the original cap is 500pf it must be to limit the bass response. A 1000pf or 1500pf (1nf or 1.5nf) will improve the bass. If its not for a bass guitar I'd probably leave it then.
The output transformer seems about correct but of course there could be a shorted turn or turns in the primary.
I am pleased to say its working. - thanks to all. My initial thoughts are that it sound a bit like its down a tunnel, there is no real depth to the overall sound output. However, as I indicated it is indeed fully functional.
The lack of depth may be due to the low value of coupling cap to the EL84 namely the 500pf.
500pF and 680k gives a high pass filter with 3dB down at almost 3kHz. No wonder it sounds odd.
I changed it for a 1000pF ages ago as the 500pF was an old red Hunts cap but its still not great. The bass is not that deep. I think I may be expecting too much from it for what it is.
One thing I have noted is the tone control doesn't appear to do that much and the crackling from the pot itself is terrible - deoxit spray time I think.
I really wish I had the space for an isolation booth or room acoustic insulation so that I wind up the output but alas, I just don't have the space at home! I think this is always the dilemma/challenge with any valve based guitar amp (or a good hifi system too).
Best Regards
Rob
1000p will only change the 3dB point to 1.5kHz. 10n would move the 3dB point to 150Hz, 100n to 15Hz.
Anyone who checked my calculations will realise that I forgot to put in the 2*PI factor. Divide all frequencies by about 6. So about 250Hz with 1000p, 25Hz with 10n.