Thanks for the tutorial. I once connected a guitar to a Hi-Fi amplifier and (decent) loudspeakers and was wholly underimpressed by what I heard, so I then tried it into an oscilloscope and found some distinctly boring waveforms, which seemed to justify my impression. Of course, it miight just have been a not-very-good guitar played by a poor musician... (Not me, I can't play to save my life.)
The original Mullard 20Watt amp had a loop gain of 30dB using the recommended o/p tranny of the time. This put the S/N ratio around the 90dB ballpark. I know from many who have made this amp (and it's bog' simple to build) on music it sounds dreadfully flat with flabby bass. Reducing the nfb to just over half adds more looseness to the sound. About 7 yrs ago I did a new build with the Maplin o/p tranny with same EL34 tubes but I found it needed g2-a snubbers and nfb reduced by 15dB for stability. That brought up the S/N to around 75dB. Looking at my wappa 300W tube amp the sound quality is sim but obv.without the punch. With the 20watter I didn't like the sound of ECC83's as phasesplitter...with surgury a 6SN7 was slammed in lieu with lower gain but there was one caveat left. Having realised the o/p tranny was deficient and to get comparable stability and acceptable thd at the 30Hz end with 20watts, I had to reduce the interstage coupling caps resulting in 5% thd ...too high for HiFi. My original o/p tranny in the Mullard 20W circuit easily swallows 20Hz at 20watts and roughly 30Hz at 30Watts at way less than 1/3 of the thd of the Maplin version, even without nfb !!! So the designer of the Maplin tranny seems to have forgotton where to put the iron and while leaving the copper aside.
So we have it. Pushed hard, the original o/p tranny of the 20 W Mullard amp will reach 30Watts +, the thd rising to about 0.5% at 400Hz at that level.
If you have one of these Maplin o/p trannies, treat it as 50Hz cutoff with the interstage coupling caps and it will be good enough for a quitar amp. The lower nfb adds more spice to the sound and with lower nfb you can actually hear something out of the LS when it is switched on.
More anon...
d:-richj
So we have it. Pushed hard, the original o/p tranny of the 20 W Mullard amp will reach 30Watts +, the thd rising to about 0.5% at 400Hz at that level.
If you have one of these Maplin o/p trannies, treat it as 50Hz cutoff with the interstage coupling caps and it will be good enough for a quitar amp. The lower nfb adds more spice to the sound and with lower nfb you can actually hear something out of the LS when it is switched on.
More anon...
d:-richj
OK, so the website must be out of date.>>
Sorry, I wasn't clear - the Danbury transformers should still be available off the website mentioned, but Maplins have discontinued them.
I went to the OEP site but found nothing>>
http://www.oep.co.uk/ They have a catalogue you can download - I haven't done so. I spoke to the sales director a while ago and he pretty much said "tell me what you want and we'll try and better anyone else's prices". You're right that there isn't a convenient search facility on the site, but there's a good range of products.
Sorry, I wasn't clear - the Danbury transformers should still be available off the website mentioned, but Maplins have discontinued them.
I went to the OEP site but found nothing>>
http://www.oep.co.uk/ They have a catalogue you can download - I haven't done so. I spoke to the sales director a while ago and he pretty much said "tell me what you want and we'll try and better anyone else's prices". You're right that there isn't a convenient search facility on the site, but there's a good range of products.
If you have one of these Maplin o/p trannies, treat it as 50Hz cutoff >>
I've seen measurements for the 20w Danbury OPT (6.6k) that Maplins used to sell done by Terry Bateman of Rega, and it looks good and even with no peaks. I've used it in a triode amp with no global feedback and it sounded excellent - very detailed - but with significantly less bass than the version Danbury do with a thicker stack (Maplins used to sell that one as an upgrade at one point. ) This is in agreement with the last poster above. The Danbury range should be still available from http://www.livinginthepast.demon.co.uk/xformer.html
The only Danbury item Maplins still seem to stock is the 10H, 100mA choke at £6.99, and that's a very good price - useful bit of kit. Andy
I've seen measurements for the 20w Danbury OPT (6.6k) that Maplins used to sell done by Terry Bateman of Rega, and it looks good and even with no peaks. I've used it in a triode amp with no global feedback and it sounded excellent - very detailed - but with significantly less bass than the version Danbury do with a thicker stack (Maplins used to sell that one as an upgrade at one point. ) This is in agreement with the last poster above. The Danbury range should be still available from http://www.livinginthepast.demon.co.uk/xformer.html
The only Danbury item Maplins still seem to stock is the 10H, 100mA choke at £6.99, and that's a very good price - useful bit of kit. Andy
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