Burman 502 issues

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Burman 502 issues

Postby Krohurte on Sat Jul 01, 2017 11:31 am

Hey all, before I run to one of only two amp repair shops in the most expensive town in my quite expensive country, I ask here, because maybe it's easy to DIY.

So in the midst of playing, the dirty channel gave up. This amp has cascading gain stages. There's gain 1 & 2 and gain 3 taking somewhat of a master role for both channels. Now there's still a tiny weeny sound and dialing gain 2 & 3 up and down does have an impact, even with gain 1 down to zero. But turning up gain 1 doesn't do anything anymore. I checked the pre amp tubes by putting them all into another device (a mesa boogie v-twin preamp running on 2 tubes), and that one ran fine with any of those tubes. The amp's clean channel still runs fine (has it's own gain dial and uses gain 3 as a master as mentioned above), so the amp's master section and therefore the powertubes seem to be fine to my understanding.

I haven't opened the amp to check yet, but could it be just some lose wiring? Any soldering point disconnected?

And while I'm at it: I discovered the 6 pre amp tubes are all different. Anyone knows what pre amp tubes would be best for this amp?
Last edited by Krohurte on Sat Jul 29, 2017 12:45 pm, edited 1 time in total.
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Re: Burman 502, no sound anymore on dirty channel.

Postby stratele52 on Sun Jul 02, 2017 9:14 am

Schematic here;
http://www.schematicsunlimited.com/b/bu ... A5MQ%3D%3D

All preamp tubes are ECC82 and ECC83
JJ's brand are good tubes

You must check all of those tubes.
If all tubes are good,( be shure ) there is nothing more you can do by yourself.
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Re: Burman 502, no sound anymore on dirty channel.

Postby Krohurte on Mon Jul 03, 2017 11:04 am

I already have that schematic, thanks. :)
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Re: Burman 502, no sound anymore on dirty channel.

Postby Mr. Foxen on Sat Jul 08, 2017 5:26 pm

No ECC82 in my 502s. ECC81 in the lower of the three together when they are on the right, chassis valve side up. do you know both halves or each valve is used in the Mesa? Not ideal way to test.

Greg said the valves are just a component, as long as the work right, don't overthink them. Which is fine with pres, just beware JJ KT77 are not KT77, so might as well use EL34 instead of those. JJ pres have never survived long in Burmans I've dealt with, they do seem to chew through a low of pre valves, but might partly be to do with a guy loving putting more JJs in even though they don't last.
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Re: Burman 502, no sound anymore on dirty channel.

Postby Krohurte on Sun Jul 09, 2017 9:00 pm

I have Genelex GL KT77. Not sure atm about the pre valves. The amp is at a repair man's right now so I can't check. I don't know how the mesa v-twin is using the valves... I also have a one pre amp tube Orange tiny terror lying around. Could have used this one to test those tubes, I guess. But we'll see what the tech guy comes up with.
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Re: Burman 502, no sound anymore on dirty channel.

Postby Krohurte on Fri Jul 28, 2017 12:53 pm

Tech repaired amp, there was loose wiring in channel 2 Gain 1.

10 mins later everything started to crackle and distort, also on channel 1. Checked the pre valves again, still working. One powertube (genelex KT77) started to ring. Also by tapping onto the amp there's lots of crackling to be heard. When playing (bedroom volume) some notes distort, others don't, while playing scales on both channels. Probably some freqs cause more trouble than others... Also by tapping onto the dials, even with all down to zero, there's beeping and fizzling like alien radio signals from outer space. Oh well, tech took it back to make a do over for free (except if changing parts is needed). He suspected the first problem caused the follow ups eventually.

Is there anything specific to look out for in those Burmans' inner workings? I guess besides their uniqueness, they're still built like, ehm... a guitar amp?

oh and could someone enlighten me about the active mid control? So there's passive bass & treble and active mids? Also what's the default setting then, way down or noon for cut/boost?
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Re: Burman 502 issues

Postby Mr. Foxen on Tue Aug 01, 2017 4:43 pm

Sim of the mid control:
Image

Boost only. My favoured settings is pretty much everything in the middle.
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Re: Burman 502 issues

Postby Krohurte on Tue Aug 01, 2017 5:51 pm

Thanks!

I keep the bass dial at 2 tops. Mids at 3, trebles maybe 4. Maybe due to my downtuning to drop c (gauge 10-46 though) farty distortion creeps in with bass over 2. Especially on neck pickup.
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Re: Burman 502 issues

Postby Mr. Foxen on Fri Aug 04, 2017 1:07 am

They'll fart out guitar speakers real easy as no bandpassing going on like most amps, huge output transformer. I play bass into mine, so it isn't detuning in itself.
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Re: Burman 502, no sound anymore on dirty channel.

Postby Krohurte on Wed Aug 09, 2017 11:23 am

Mr. Foxen wrote:No ECC82 in my 502s. ECC81 in the lower of the three together when they are on the right, chassis valve side up.

You mean the three on the right of the aligned six? => https://ampstack.files.wordpress.com/20 ... 01-003.jpg

And all others ECC83 then? Is it a taste of sound decision or is there something technical about it? Any preferred brand? What about cheap Shuguangs? I also have some new/unused Marshalls lying around. There was quite an old in depth 502 review online recently where I think it was mentioned which ones were stock back then, I can't seem to find it anymore. Almost all ECC83 in this 501, though https://www.youtube.com/watch?v=jXvZoGaz7V0 I guess the 81 is for the reverb. So why put 3 of them in the 502?

Asking because according to this:
Small-signal (preamp) tubes (12AX7, 12AU7, 12AT7, 5879, etc.): You may hear a crackle for a short time after the amplifier is turned on. Then it quiets down. Or the amplifier may "howl" at high volumes, or buzz on certain notes all the time. These problems get worse, and eventually you will be replacing one or more tubes.
(from http://tubefreak.com/tubefaq.htm )
Besides having one power tube go down, that's pretty much what happened after I got it back from the tech. Before there was only loose wiring which he fixed. So my pre valves checking in a Mesaboogie V-twin, which only lets you check 2 at a time, may not have been very thorough. :think_1: and now I'm thinking about replacing all pres just for the sake of it.
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Re: Burman 502 issues

Postby Mr. Foxen on Tue Aug 15, 2017 12:47 am

That is a 501. 502 has a group of 3 pres together. One of them is an ECC81, but seems having looked at a bunch of mine, might vary as to which. I find the shuguangs fine, kinda bright, probably from bit of distortion, probably a good thing, easier cutting top. If you have microphonic valves, can swap them about to position where it doesn't show up.
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