Selmer Vanguard

[vampyretim]

Hi all,

I'm in the process of recapping my Selmer Vanguard the schematic for which can be found here.

http://www.vintagehofner.co.uk/brita...atic/vang.html

I took the fuse out of its holder and it seemed to be wrapped in a very fine wire which fell on the floor and I couldn't find it. The 2.5A fuse obviously doesn't work now, I have been to R.S to try and find a replacement to no avail, can anybody here point me in the right direction?

I have a picture of it next to a penny for scale.

The amp is also missing its back panel is there anywhere to get a replacement?

Thanks in advance,
Tim.

[ronbryan]

Tim

The fuse is a 5/8" Belling-Lee type. I assume that it is a quick blow fuse rather than a delay type as my late sixties Selmer Twin Bass 30 survives with 2.5A quick blow installed.

I can send you a couple of 2.5A quick blow 5/8" fuses for the price of the postage. The Belling-Lee fuseholder is not exactly touch proof (which is presumably why they are no longer used) and so some people replace them with 20mm types if they have space.

Is the screen-grid resistor for the 6BW6s really 47k ohms as shown on the circuit diagram? Seems at least 10 times too big.

Ron

[Herald1360]

It does look wrong. A similarly rated amp http://www.vintagehofner.co.uk/selme...ic/futcar.html using 6BW6 finals shows the 47K as decoupling for the early stages only with the o/p screens fed effectively from the same voltage as the anodes.

[vampyretim]

Hi guys,

I haven't had a chance to take the amp out and look underneath the pcb to check screen grid resistors yet. I can see that there are 470k resistors and am assuming that they go the screen grids.

I have uploaded a few more pics the input to the spring unit has become unsoldered due to the elasticated holding becoming perished, (I guess Selmer didn't think the amp would still be used this far into the future) it isn't that obvious to me where the signal and shielding portions of the cable go to.

I'm going to find some elasticated material to hold the spring unit in place am looking around the house for something suitable.

Thanks for all your help.

[ronbryan]

Tim

It is likely that the centre tap of the output transformer primary is connected to the junction of the 1.5k and 100 ohm resistors and the 6BW6 screen grids are both connected to the junction of the 1.5k, 3.9k and 47k resistors as Chris has hinted. We were merely highlighting what looks like an error in the drafting of the circuit diagram.

Ron

[ColinTheAmpMan1]

Hi Tim,

This is an interesting amplifier and circuit. I haven't seen one of these, though I have schematics for Zodiac "Twin 30" and "Twin 50" and brochures which show the fronts of these amps as well as a few other Selmer/Hofner offerings. The "Twin 30" predictably (as a British 30 watt guitar amp) uses two EL34s but my schematic of the "Twin 50" doesn't show what output valves it uses. I would guess EL34s, again (they were good for 45-50 watts in early Marshalls).

The suggested mains fuse value at 2.5 amps is probably a bit high. A Fender Deluxe Reverb with two 6V6s working on US power (120V) uses a 1 amp slow-blow mains fuse and the Zodiac "Twin 30" with two EL34s has a 1.5 amp mains fuse for British 240 volt power.

I, too, think there is an error on the schematic you have. The centre-tap of the output tranny seems to go to the right place, but I would have thought that the screen-grids would have gone to pretty much the same place in the circuit, perhaps with something like a 1k or 2k resistor in series. The voltage on the screen-grids should be something like 5 volts less than on the anodes; certainly not more than the anodes, otherwise the screen current will be much too high. You need to "reverse-engineer" the amp that you have to be certain - remember that the resistors may have drifted from what they are supposed to be, so don't just rely on the marker-bands.

I am not familiar with 6BW6 output valves, so I may not know what I'm talking about, but those 68R anode resistors look odd. What could they be for? The anodes of a push-pull output pair would normally go directly to the output transformer primary. I might expect resistors of this value as grid-stoppers to ensure stability, but "anode-stoppers"? I don't think so; perhaps the output transformer wasn't quite matched to the 6BW6 valves (as variants on the 6V6 they would have been American and maybe no British transformer company had a good match - pure guesswork on my part). It would be interesting to know what the DC resistance of the output transformer is - it shouldn't be too difficut to measure this on an unpowered amp with the capacitors discharged. If the output valves are removed (carefully - they ain't cheap!), there are no other components between the 68R resistors and the smoothing capacitors. The 6BW6 is effectively a 6V6 on a nine-pin Noval base. As I have said, they aren't particularly cheap (Watford quote a price of £14 for one CV4043, which is a special quality version).

On the subject of the back-panel, I'm afraid that all of the pictures I have are only front views. Consequently, I have no idea what shape the back-panel would be. Another difficulty, if you decided to make something yourself to substitute a back-panel, is that Selmer tended to use a faux-alligator vinyl on many of their amps. I think it may still be available from somewhere like Weber, but it could be difficult to source an exact match.

On the whole, your amp looks like it isn't spectacularly ancient. Those valve-bases seem to be mounted to something like PCBs. Am I right? In that case, it might prove to be easier to put back into good order, as modern components won't look (or maybe even be) out-of-place.

Whatever you do, have a play with the amp before you start taking it apart. Try our the controls and listen for any problems first. I can see that you've taken some pictures, but keep on doing that as you poke about inside. It is so easy to forget what went where if you leave it for too long. At my age, it is even quite easy if you leave it for a cup of tea!

Regards, Colin.

[Herald1360]

Originally Posted by ColinTheAmpMan1

I am not familiar with 6BW6 output valves, so I may not know what I'm talking about, but those 68R anode resistors look odd. What could they be for? The anodes of a push-pull output pair would normally go directly to the output transformer primary. I might expect resistors of this value as grid-stoppers to ensure stability, but "anode-stoppers"?

The 6BW6 is pretty much a 6V6 in B9A packaging.

"Anode stoppers"- yes, that's exactly what they are. They'll tame some sort of HF oscillation problem associated with stray capacitance/inductance/coupling in the anode/transformer circuit. The resistance is too low to have any significant effect on the normal operation of the stage- it's only 1 or 2% of the actual load impedance.

Whatever else, they're nothing to do with mismatched transformers- these would be either off the shelf units with the correct ratio or units built specifically for the particular application.

[ColinTheAmpMan1]

Well, blow me down. You learn something new almost every day. I was under the impression that while grid circuits are high-impedance, anode circuits are low-impedance. In my uneducated (in these matters) ignorance, I would have thought that anode circuits wouldn't be subject to effects of stray capacitance. If the anodes had gone directly to the output transformer and these 68R resistors had been connected to the screen-grids and then to HT+, I would have not batted an eyelid.

My thoughts on mismatched transformers were purely speculation and I'm glad to be informed of the correct situation. I was merely guessing that since the 6V6 is/was a US-originating valve, that when put into a B9A package (British-origin?) there might be matching issues. I recall that Selmer were a British/European company originally and would have been probably more familiar with EL84 and EL34 offerings. Selmer were also somewhat cheaper than corresponding American equipment, but maybe that's down to import-duty and the economics of the time.

Colin.

[vampyretim]

Hi guys,

I haven't got the time yet to write a full response to your posts but I have some photos of the circuit board here. It is in a real state the first photo is before my attempt at cleaning off the dust, some of the tracks have cracked and somebody in the past has had a go at repairing them.

It's looking likely that I'm going to have to pull all of the solder off and re-do it as it's rusting and dirty. Great!

I did power her up with my lamp limiter in series and there was no indication of shorts in the power supply however there's no amplification not even a hum at the speaker. I have checked that the speaker works and it does.
I tried to discharge the caps by using a crocodile lead from the anode of one of the ECC83's to chassis earth and watched to see if the HT drained from the 33UF smoothing caps, it didn't I had to discharge the caps directly to chassis before I could work on it.

I will keep you all informed of my progress.

[Herald1360]

Originally Posted by ColinTheAmpMan1

Well, blow me down. You learn something new almost every day. I was under the impression that while grid circuits are high-impedance, anode circuits are low-impedance. In my uneducated (in these matters) ignorance, I would have thought that anode circuits wouldn't be subject to effects of stray capacitance. If the anodes had gone directly to the output transformer and these 68R resistors had been connected to the screen-grids and then to HT+, I would have not batted an eyelid.

Anode circuits are low impedance relative to grid circuits, but a 6BW6 still has an anode impedance of 50-70 kohm. Plenty high enough given the voltage levels for small amounts of stray capacitance to cause problems with transformer leakage inductance etc. Adding a bit of series R helps to damp down the Q of such unwanted resonances.

For some interesting reading on 6BW6 see here:

http://frank.yueksel.org/sheets/184/6/6BW6.pdf

Amongst other snippets, in the first paragraph it states that its characteristics are the same as the 6V6GT.

That PCB looks like it could do with a good wash- if you can get just the PCB on its own with no obvious water traps like pots or relays a good scrub with hot water and a bit of detergent, followed by careful rinsing with clean water, then meths and a dry off in a low (50C or so) oven would improve its looks considerably. For some justification for this apparently draconian treatment have a look at some Tek history!

[vampyretim]

Originally Posted by Herald1360

That PCB looks like it could do with a good wash- if you can get just the PCB on its own with no obvious water traps like pots or relays a good scrub with hot water and a bit of detergent, followed by careful rinsing with clean water, then meths and a dry off in a low (50C or so) oven would improve its looks considerably. For some justification for this apparently draconian treatment have a look at some Tek history!

That sounds like a massive amount of work and probably beyond my capabilities at the moment.

Stripping every component off the board and then putting them back on again gives me a headache just thinking about it.

It probably is what it needs however it just makes me nervous thinking I'd make the problem worse.

Is there any other way of cleaning the board?

[Herald1360]

Do what you can with a vacuum cleaner and a dry paintbrush. Really gungy areas use a small toothbrush and some IPA or meths. As long as it's clean enough to see problems / tracking issues it should be fine.

[dotfret]

I was passing by and found this interesting - what happened next?

A couple of comments - the 6BW6 was a Brimar invention, and as such was usually supplied in the US relabelled as Heintz & Kaufman. Similarly, Brimar made 6BR8 - the clue is in the name, BRItish Made American Range = Brimar. The latter is actually an ECF82 with a different pin-out, so it is not a daft choice, really. The "odd" valve selection made the amp owner seek out "genuine" replacements, so it meant after-sales for the amp dealer.

It is not American influence, but all the Selmer amps reflect a significant tie to Brimar valve supply. Might date back to the fact that Brimar were first in GB with the 12AX7, Philips didn't turn out the ECC83 until a couple of years later.

And those "anode stoppers" are recommended in the data sheet quoted above to prevent spurious parasitic oscillation, so someone got the right answer.

[vampyretim]

Hello,

to be honest with you I don't like this amp at all. It's badly designed and almost impossible to take voltage readings. I wouldn't recommend this amp. It's on a shelf in my workshop and after lots of time and effort I still don't think it sounds right.

I wish I hadn't bought it.

Tim.

[trobbins]

Are all the cathode and anode voltages showing acceptable levels. Are the output stage anode currents balance ok?

V3A may need a grid stopper if input level is typically high and cause some unpleasant overdrive.

The 2.5A mains side fuse is probably to allow 200VAC operation with hot turn-on. I can only recommend that the 300-0-300V CT lead be fused, and a series 1N4007 put in series with each valve diode, to better cope with ageing parts failing.

[vampyretim]

Hi all,

I started a thread a few years ago about this amp and for some reason it's been closed.

To summarise I bought a battered, old, Selmer guitar amp that was in a terrible state.

I've had it on my shelf for a few years and have now got round to finishing it.

I recapped it a few years ago back when I would replace electrolytic caps as a matter of course.

The pcb was covered in dust and the solder was very dull. The board had also snapped in places.

Somebody had attempted to repair it (badly I must add) at one time.

I have removed all the old solder and cleaned off the board. I've replaced the valve base of the EZ81.

I have now got the amp sounding a lot better but I have distortion and I suspect the speaker. There's a small hole in the cone right on the edge but it doesn't seem big enough to be the cause. Is there anyway to test for a voice coil coming off centre?

I'm going to put another speaker on the amp tomorrow but I would like to keep the original speaker if I can. I don't usually get sentimental about original parts but in a vintage guitar amp the speaker is a huge part of the sound.

Many thanks,
Tim.

[dragonser]

Hi,
well sometimes you might be able to check for an off centre voice coil by very carefully pushing the speaker cone up and down and see if you can hear a rubbing sound. But I have sometimes found that trying another speaker is the only way to be sure...
if you have a resistive load and an oscilloscope at least you can check the waveforms on the output of the amplifier.
regards Peter B

[vampyretim]

Hi Pete,

I've already checked a waveform through the amp and all seemed fine which is why I suspect the speaker.

I guess what I really want to know is is there a way to repair a speaker that comes off centre?

And what other fault in a speaker could give me distortion?

Many thanks.