Has anyone any experience with Burman amps?

[CraigGa]

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

[CraigGa]

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 low notes when gain 2 is wound up.
I don't remember it sounding like that before.

I've attached that part of the diagram if anyone has any suggestions.

Thanks
Craig

Burman Ch2.jpg

[pdf64]

Does the Vdc at V2 pin8 cathode increase much as signal level increases from 0 to heavily overdriven? At 100uF that cap seems rather a high value, leading to longer time constants.
Also the ESR of the new bypass cap may be very low, perhaps adding a little series resistance (eg 220 ohms) would smooth the overdriven tone out a little?
Lifting the bypass completely would provide an extreme example of that but it would also mess with the tone stack (as the stage’s output impedance would increase 2 or 3 times.

[CraigGa]

Thanks pdf,

The voltage is a steadyish 1.45V with low settings, but with more gain it starts to swing between 1.2 and 1.9, is that significant?

I don't have an ESR meter to check the new caps but all the old ones that I replaced all measured around 150uF.

Craig

[chief mushroom cloud]

Craig
Yea, you can do all these things everyone is saying....but dead stock...these amps are stellar sounding and don't have LF 'farting' issues. They can be made to sound pretty wooly by turning up BASS control....but everyone I know that used one had the bass turned fully off. But they make kick ass bass amps by just using LF bass cabs or spkrs.
For starters, V4 is ECC83. That schematic is a first draft and has since been updated. Tony Hunt and I went thru these ages ago, and documented not only 501s, 502s, and 2000s (plus a couple variants)...but additionally Pro4000 s, as Tony has a couple. Attached is updated schematic.
Craig, I would like to help....but I can't without the amp in front of me and hooked up to a scope. There is no way I can second guess what's happening because musicians(sic) and amp guys tend to use words that make no sense to me.
First thing to do, is to turn Gain2 full CCW or off, and put all EQ at 'noon', Presence full CCW, and everything else full up. Play a few chords. It should very much like a silverface UL twin at full stop.
If there are anomalies anywhere else in the circuits.....this is where you'll hear them. Your ears might start compressing at some point, but you'll hear problems (if there are any) before that happens. Your ears might ring a bit, but you're a musician, right? Take it like a man.
But seriously, you might try the 'chopstick' method and make sure it's not electro-mechanical or dodgy component. Make sure it hasn't been modified. Check for proper component values. Try a different valve on V2...preferrable some old or NOS...even a shitty old one is better than new mfr. unless it's microphonic. Trace the circuit with a multimeter (while amp is off, of course) and make sure it hasn't been rewired. Be aware the Gain2 is the FIRST gain level control BEFORE Gain1, in a circuit sense. Gain3 is nothing but a master vol for both channels.
Another thing to think about, is the downstream circuitry in that channel.

I used to have a 502 and a 1000 (100watter). I sold both and gave away another 502....they are just too loud....and heavy for me. I'm an old hide. And I don't like high gain at all. Just my preference. I only bought them because I was sure that Geordie Walker was the true living god of guitar and all the hell and humanity on earth would be exterminated by his scorched earth tone. But, alas, it did not come to pass. I met him and chatted....he's just a regular guy who likes playing his guitar and fucking w/ people's heads doing it. Just like you or me. What a letdown that moment was.

Burman_Pro-502_reverb.sch.pdf

[chief mushroom cloud]

btw, my email is tracynorton257@gmail.com
if you need to contact me

[CraigGa]

Thanks for your help and the great schematics, I couldn't have got this far without them.

I've not used the amp for quite a while before all this work because there was too much 100hZ hum but I'm pretty sure the distortion on G2 didn't sound like this.
I've not changed any component values and the only mods are a channel switch and feeding ch1 to the reverb but I must have done those 20 years ago.

The most obvious difference between your schematics and my amp is the 230v line, I'm seeing that around 195.
If my ohms law is correct you'd only need 0.45mA to drop 10V across a 22k resistor.

I've got to say that the thought of putting Gain 3 to full fills me with fear so I'll wait until the wife is very far away

Thanks
Craig

[CraigGa]

I've got to say that the thought of putting Gain 3 to full fills me with fear so I'll wait until the wife is very far away

You'd need to have played one of these beasts to know what I mean

Craig

[chief mushroom cloud]

You'd need to have played one of these beasts to know what I mean

I do.

[chief mushroom cloud]

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.

Craig, that 230v is measure with amp on and at idle (no signal). If you pull V4 and the B+ jumps back up to ~230V....there is something going on with the circuitry around V4-A. Check the cathode resistor value w/ a good multimeter. Check to make sure the 1M grid bias is connected to ground. I read that you replaced cathode caps but did you replace that particular one off pin3?
Does the 240V also drop on the other side of the 22k B+ dropping resistor? That is connected to V4-B which is summing and presence. check those cathode resistors as well...the 3.3k in particular.
Also, try more tubes to eliminate bad tube possibility.
Double check the plate resistor values w/ yer multimeter.
Also, clean the living shout out of the socket...contamination causes SARS....and we don't need any more SARS Use carburetor cleaner. It also makes a great hand sanitizer. Kills that SARS dead.
Again, without a scope on it, you're on a search and destroy mission with no idea who or where the enemy is. O-scope is night vision....haha!

[chief mushroom cloud]

have you also pulled and looked at the speakers recently? it's not related to the b+ anomaly, but a 502 I had, the Goodmans had deteriorated completely at the surrounds....it still worked...but sounded funny when I ran it flat out.......I prob made it worse

[CraigGa]

Thanks again,

I'll look at those points over the weekend but I did notice that I get back to 230v if I pull V1 instead, do you think it's possible that one of my new filter caps is faulty?

I've not checked the speakers but I have tried it into another cab and it's just the same but I will give all the sockets a good clean and switch tubes about too.

I do have a USB scope so I will have a look at what's going on in there.

Craig

[tony hunt]

Late to chime in,
Tracy' said it all and more, follow his advice.

but I'd add that I never liked Gain 2 beyond about three on the 2x KT77 amps
and never above five (noon) on the quad KT77 models.

Never had any of these fart out on low notes, but always roll bass right down with a six string.
The lows should "come out you solar plexus" as one bass player aptly put it.
The speakers need to be up to the punishment.

If you suspect a filter cap and have no mains hum and no scope you could shotgun the two later 32uF filters.
Mostly I used RIFA on the mains filters.

What a fantastic stage amp. They work really well with old Gibson P90s up to mid sixties.

[CraigGa]

Thanks Tony,
I've kind of lost the mojo with this but seeing as I've got time on my hands I should delve in again.

I'm assuming that Tracy is Chief Mushroom Cloud?
I tried his suggestion of settings and it sounded pretty dirty but my acoustic guitars were all shouting to turn it down

I hear what you say about Gain 2 but I've always run it with Gain 1 on 3, Gain 2 on 7, Gain 3 on 1 and the tone controls around 6 or 7 but now it's giving this unpleasant distortion which could be intermodulation distortion.

Someone previously said not to get too hung up on preamp supply voltages but it doesn't seem to add up.
400V - 240V across 47K = 3.4mA
but then
240V - 230V across 22K = 0.45mA
which doesn't seem much to feed 6 triodes.

I'm wishing that I'd done some measurements before I did the filter caps now.

Craig

[tony hunt]

Yes, Tracy is the Chief.

Here's a few simple suggestions that spring to mind due to how you push the first section with Gain 2:
Did you move the wire-dress on V2? It is very close to the Reverb send/return plug which could be making things funky.
Do you have the tube screen can fitted to V2?
What brand of tube are you running in there? Tracy was pointing to that too.
The ECC83 Mazdas used originally were made by RFT in East Germany. They were and still are a good choice.