The Amp Garage

So I have now replaced the bias circuit, changed output valves, changed all the filter and cathode bypass caps, replaced a couple of plate resistors, fitted grid stoppers on the inputs and touched up the solder joints on the valve bases! It's sounding pretty good apart from the buzzy distortion on l...

martin manning wrote: ↑Thu Mar 12, 2020 2:33 pm So nothing but a new fuse and pilot lamp to sort it out? That bulb had nearly 40 years in, I guess it was tired.

Yes it was a simple fix but a major panic.

Craig

So I've added the failsafe bias circuit and new KT77s. I calculated that I needed about -40V and the range came out at -30V to -50V so pretty much spot on.

See my other thread about the associated adventures though


https://ampgarage.com/forum/viewtopic.php?f=6&t=33085


Craig

I don't know what the policy on swearing here is so I moderated myself. After over 40 years working in electronics and telecoms I know a thing or two about Sods (Murphy's) law but this takes the biscuit. I'm working on my 40+ year old Burman Pro 502, someone in my other thread said that I should rew...

Thank Martin,
I'll see which design fits best on the available tag board space.

Craig

Thanks for that advice, it certainly explains why one of the filter caps had the end taped up in the chassis mounting clip.

Luckily the Nichicon replacements I used don't test the same, I've put a piece of insulating material - well a plectrum, in there for good measure anyway.

Craig

Thanks for the info pdf, Actually I've just found the owners manual in the loft and it states 6 x ECC83 and 1 x ECC81 (reverb driver) so it's possible that the one used for the schematic had an incorrect valve in it. As you said, it is built on tagboard with underside linking wires so it can be diff...

Thanks for that xtian,
I can see now that if my pot went noisy the bias would disappear and the o/p tubes would be full throttle whereas in the "Standard" circuit R3 would save the day.

That shouldn't be too difficult to implement while I wait for the new tubes.

Thanks
Craig

xtian wrote: ↑Sun Mar 08, 2020 9:20 pm Yes, I would update the bias supply to be fail safe.

Could you point me towards a circuit please?

Thanks
Craig

xtian wrote: ↑Sun Mar 08, 2020 6:28 pm Interesting amp! The amp is vintage, right? But the schematic is very modern and very nice.

Yes it looks like someone reverse engineered the schematic, that's why I was wondering whether V4 was correct or not.

Craig

Yes it's vintage alright, they were hand built in Newcastle, England in the late 1970's - early 1980's. I've had it since new probably around 1979, the filter caps I removed are dated 1978! I don't think it farted the last time I used it but I didn't have a Les Paul then! would you recommend reworki...

Thanks xtian, I've attached the schematic, yes it's sounding kind of farty on low notes when gain 2 is cranked up but I've just noticed the power tubes are way out of balance so I've ordered a new pair of those. It's about 40 years old and not been used much lately so I've changed the filter caps to...

Thanks for having me aboard guys, I've often thought about building my own amp but have yet to get around to it. I'm currently trying to get my Burman Pro 502 back in good condition and am struggling so I'd be grateful if anyone can help out. According to the schematic I have, the preamp tubes run o...