Has anyone any experience with Burman amps?

[CraigGa]

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 on 240/230V rails but I'm only seeing 190v where there should be 230V however if I remove V4 then I get 230V
Now according to the schematic V4 should be an ECC82 but I found an ECC83 in there, with the ECC83 I'm getting around 210V on the rail.

Can anyone confirm which type should be in V4 please? and also what could be bringing the supply down? I've checked the plate resistors and they are in spec.

Thanks in advance
Craig

[xtian]

Welcome to TAG.

The voltage difference you're seeing on the preamp tubes is not significant, in itself. Is the amp misbehaving or not sounding right?

Please post the schematic.

[CraigGa]

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 too.

Craig

[xtian]

Interesting amp! The amp is vintage, right? But the schematic is very modern and very nice. Anyway, first thing I notice is that all the coupling caps and cathode bypass caps are full bandwidth, meaning they won't cut any bass frequencies. This must make this amp VERY bass heavy, and prone to farting out on low notes.

The bias circuit is suspect--notice that the adjustment pot comes first. If that pot is dirty, intermittent, or fails, power tubes will red plate and burn up.

[CraigGa]

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 reworking the bias circuit?

Craig

[CraigGa]

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

[xtian]

Yes, I would update the bias supply to be fail safe.

[CraigGa]

Yes, I would update the bias supply to be fail safe.

Could you point me towards a circuit please?

Thanks
Craig

[xtian]

Read the whole page for good theory, or scroll down to the figure labeled "Standard":

http://www.valvewizard.co.uk/bias.html

[CraigGa]

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

[pdf64]

I’m not sure but I think that forum member ‘chief mushroom cloud’ is a Burman fan and is the T.Norton that reverse engineered and drew up that schematic.
So maybe try a pm to https://ampgarage.com/forum/memberlist. ... c03f227488
If it’s built on tagboard like the ones I’ve seen, a layout would probably be really handy, as, same as old JMI Voxes, there’s underboard link wires running all over.
There’s several Burmans in my area, with those cascading gain stages they had a rep as the ‘British Boogie’ when MBs were rare as hens teeth over here. Unfortunately the lack of a channel switching system was something of an impediment to their usefulness.

[CraigGa]

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 difficult to trace but somewhere in the distant past I built channel switching into it!

Craig

[martin manning]

Here is another fail-safe bias circuit. Adjust R1 to get the range centered for your PT's AC voltage and your tubes.

[CraigGa]

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

Craig

[pompeiisneaks]

I’m not sure but I think that forum member ‘chief mushroom cloud’ is a Burman fan and is the T.Norton that reverse engineered and drew up that schematic.
So maybe try a pm to https://ampgarage.com/forum/memberlist. ... c03f227488
If it’s built on tagboard like the ones I’ve seen, a layout would probably be really handy, as, same as old JMI Voxes, there’s underboard link wires running all over.
There’s several Burmans in my area, with those cascading gain stages they had a rep as the ‘British Boogie’ when MBs were rare as hens teeth over here. Unfortunately the lack of a channel switching system was something of an impediment to their usefulness.

The only problem is that our PM system is broken, he won't get an email, and will only see the PM the next time he logs in. Sadly.

I know how to fix it, but can't as I don't control the email domain, and can't setup a real email address for the ampgarage. The hosting provider used to allow email out from the app, but blocks it w/o an authenticated user to a real mail server. I think, for some reason the account level email works, but I think that uses a different process, but PM's don't, still no clue why.

Sorry.

~Phil