Burman 501 combo reverb problem

[Willholmania]

Hello

I've owned this amp for five years and love it to bits. So much so that I decided to treat it to a new set of valves and filter caps. I took it out of the cabinet to check which bits I needed to order then put it back together so I could continue playing through it. All was fine at first then the reverb suddenly dropped out. I imagined that one of the plugs into the reverb tank was loose but they're not. And I've checked the wires running to it for continuity and all is well.

I've got a circuit diagram for the amp but it doesn't include the reverb section. Does anyone have any fault finding tips?

Thanks a merry Xmas to all.

Will.

[pdf64]

Have you got continuity at the tank in and out connectors (they are coils)?
Any noise hum / hiss/ buzz when you the reverb up?
Any clang through amp if you tap the tank?
Can you measure any signal on the tank 'in' connector - needs input to amp!
Here are a couple of Burman experts who might be able to help
http://vintageamps.com/plexiboard/vi...p?f=10&t=86566

[Willholmania]

Hi. Yes. I've checked the tank and it seems fine - about 750 ohms on one coil and 2 or 3 on the other - which I'm told is about right. I get no clanking when I bang the cab and no strange noises at all when I turn up the reverb.

[pdf64]

Sounds like it might be something in the recovery circuit, as there's no clang or hiss. Can you identify which tube it is (ie connects to the feed from the tank output connector - 750ohm end)?

[Willholmania]

I think so yes. I think the ECC81 drives the tank and one of the 83s is the recovery. But I've also fitted a full set of new tubes, which had no effect on the reverb problem.

[Wakculloch]

If it's similar to the pro 50 then the recovery valve is V1, and the driver V2.
If you feed a signal into the RCA plug from the springline you should hear something.
If not, look at the circuitry around the recovery valve, especially the footswitch that operates by shorting the reverb volume control to ground.

[overtone]

Burman Reverb

Hi Will,
I have the same problem on one of my Burmans too & really need to get around to fixing it. Here is a link to the schematic that I use:
http://www.schematicheaven.com/barga...an_Pro-502.pdf
You need to scroll down to the last page, no. 6, to see the section for the Reverb, and there you have V7 as the driver and V6B for the recovery. This should help you check the circuit through.
I have that Burman on the bench now and will get in touch if I get a breakthrough.
All the best for the New Year!

[Willholmania]

Hi. Thanks. Let me know what find.
Cheers and happy new year to you too.
Will

[overtone]

Will,
did you check the black RS connectors for the reverb cables to the chassis while moving them around?
The connectors are the weak link on my Burmans causing the reverbs to drop out, but it is intermittent so it did not show up on the continuity test until I got to pushing things around.
I have checked the circuits and the reverbs, all are fine, but the RS connectors wobble all over the place. If I move one, while playing, not only will the reverb drop out but the volume of the whole amp will jump from normal to zero or even to five times louder. IMHO these connectors are really not up to the very high standard of the rest of the amp and I am going to change them all.
It looks like a typical problem because on the Pro 2000 combo someone had drilled the connector pins out and "hard wired" new speaker cables straight through! So check and possibly change the speaker connector too.

[Willholmania]

Hi. Do you mean those black plastic connectors that stick through the chassis? I'm away from home at the moment but from memory there are two and the cables to the reverb tank go through one of them. Is the other for the speaker then?

[overtone]

Hi. Do you mean those black plastic connectors that stick through the chassis?

Those sure are the ones that I mean you should check over. After 30 years the plastic is very brittle.
I am away from base too but have red lined an image which happens to show the reverb socket in the chassis next to the output transformer.
Yes, the speaker socket is in the same area but at the back edge.
(The image is of a Pro 2000 and the 501 layout will be similar.)
Good luck and congratulations on playing through one of these amps.

[Willholmania]

Hi Overtone

Well, seems I've been a bit of a pillock. When the reverb stopped working I took the tank out to check the coils. When I put it back I reversed the connections, despite sketching the layout (incorrectly). With it connected the right way it now works fine. It probably packed in because of corrosion on the phono plugs.

I've bought four new filter caps for the amp and plan to fit them soon - I wondered whether you've replaced any parts in yours? I was considering replacing all the caps. What do you think?

[overtone]

Will, that really is very good news to have found that it is such a simple problem. Perhaps some De-Oxit on the connections will help the long-term performance too. I did not notice any corrosion on the reverb phono plugs as these are soldered onto the reverb tank on mine.

Regarding the cap-job: I think you have to decide whether you are keeping a museum piece or running a reliable piece of equipment. If you are replacing the filter caps most people will advise you to go and replace all the electrolytic caps at the same time, which I am now about to do.

The Burmans here are all still running the original electrolytic caps after 30 years: red and blue "RS" for the filter caps, blue "Phillips" for the bypass caps and an "Ace" which I forget what it is for. The 140W sounds fine, but the 70w 502 does not sound as it used to. So I have just taken delivery of F&T filter caps, but I have not found the 100uf/10v bypass caps in axial form, they all seem to be radial cans these days. So I have taken delivery of a load of low ESR Nichichons, or what ever they are called, to try out. They look horrible.

My advice is to hang onto anything original that you take out and put it safe to one side, so later it can be taken back to stock if someone wants it that way.

There is the alternative of “investing” in an ESR meter to check the caps in circuit first, but personally I would whip them out and get some decent new production caps in there so that you can get on and use the amp as much as possible. Remember to discharge the caps first.

Frankly, apart from the electrolytic caps & tubes and maybe the chassis reverb & speaker connectors, these things are going to out-live all of us by a couple of centuries.

By the way, which replacement valves did you decide to go for?

[Willholmania]

Hi again.

I've got some F&T twin 32uFs and some RS 220uF - they're not as tall as the originals but they'll fit in the original clamps. I'll dig out a part number if you're interested but you can easily find them on RS's website.

The preamp valves are JJs and I bought a matched pair of Tube Amp Doctor 6L6s - all from Watford valves. To be honest the valves don't seem to have made any difference but at least I know they're all new. I'm planning on inflicting myself of the general public again (after a 15 year break) and I want the thing to work.

I was considering a swap to EL34s as I've always used them in the past and love the sound but I wasn't sure how to rebias the amp to suit and besides, it sounds frankly fantastic as it is. Do yours still run KT88s? What difference does that make (apart from expense!)?

By the way, what I'd really like is a 502, so I could mod it with a simple channel switch circuit. I'm pretty sure that would be the last amp I ever bought.

Cheers

Will

[overtone]

Hi Will,

funny that, I am in the same position, not having played properly for well over twenty years now. I joined this great forum to learn how to service the Burmans myself, because out here in Germany I have not found a tech that I am happy with.

To get back to your points:
1. Thanks, I have two full sets of new F&T filter caps all ready to go in and next I am going to get my head around biasing these things.

2. Did you say 6L6s in a Burman? That is really something to think about, I have never heard that combination, but for sure it must sound fantastic. I discovered the Burmans after my twin reverb went down on stage and I had to borrow a strange amp with three red volume controls. I had no idea and just set those three red controls all to lucky 3. That first chord spun me right around 180° and then I spent the whole of the gig grinning at the amp with my back to the audience.

3. They were fitted with KT77s, not KT88s. The Pro 502 that I found in 1985 was running the original Golden Lion KT77s and I loved how touch sensitive they were. That set up was very, very loud but at the same time totally controllable and responsive. A German tech changed them out for Sovtek EL34s and since then that amp sounds nowhere near as good, but it could just be the biasing because the Pro 2000 with a quad of Valvo EL34s sounds absolutely phenomenal. Some say the KT77s have a tighter bottom end, but I need to get these amps set up properly and roll the KT77s & EL34s around before adding my 2 cents.

4. Concerning the cost effectiveness of KT77s, I think the priority is to be really sure that the speaker investments are right before going down that road.

5. The Pro 502 has an extra clean channel, called "Channel 1", and the main one is called "Channel 2", that's the one with the three red gain controls and much grinning. The tone stack is positioned differently in Channel 1 and for years I underestimated it. These days I find myself grinning more by just going straight from the guitar into that clean channel. Personally, I would do the switching with an A-B-Y box and keep the amp as it is with no mods.

Let me know when you find your 502, I remember an Eighties reviewer writing that these things should be on the National Health Service…

Best, etc., ‘tone