Selmer Bassmaster 50 - mains transformer

[stevehertz]

On the link below at the bottom of the page is one of my Croc Skin 50 heads: http://www.vintagehofner.co.uk/galle...ery3/sel3.html

[Stretcher]

Thanks for these links – it’s really useful to see what other people have done to restore these amps and how good they look when finished. Unfortunately mine has lost its iconic croc skin along the way, and is covered all over in fairly featureless black vinyl, but the front panel and knobs are all original.

It’s also useful to see an original type mains transformer, which looks a bit more chunky than the one in my amp. Do any of you know who might be able to supply such an item? I might make enquiries. Also good to see how people have managed wiring in the new smoothing capacitors – I have some on order.

I had a thought about the extra wire from the transformer to pin 3 of the rectifier – I wondered if this is an extra tap to give 6.3v in case the rectifier needs it, but just attached to pin 3 in this amp to keep it out of the way?

[Diabolical Artificer]

This is a fixed bias amp so the cathodes are at ground 0v

Thanks,in which case the you might want to pop a couple of 10r resistors from ground to cathode so you can check current. If you do that try and use 5w wire wounds.

300v AC should give you 400v ish HT, a drop of 55v from the original,unless you intend gigging with the amp it'll probably be ok.If that's no good Hammond do cheapish tfmrs, the nearest they do though is the 290D2EX 710v CT, you need 740v CT, see - https://www.hammfg.com/electronics/t...rs/classic/290 There's Edcor, several likely contenders - https://www.edcorusa.com/xpwr002 HT a tad under @ 720v ,see here - https://www.edcorusa.com/xpwrseries-1#/noFilterApplied for full list.

You could contact Ed Dinning or Mike Barker who both do rewinds to see if they could rewind your tfmr. It could be your tfmr is just wrong, too low voltage and too low current capability for the amp. Tfmr's with shorted turns usually tell you by humming and getting hot.

Andy.

[Stretcher]

Thanks very much for these links and the information, the Edcor transformer looks reasonably affordable for me. As you say though, it would good to see what the best is that I can get from my present transformer.

Re the current through the cathodes, what range am I looking for? The circuit diagram says the voltage at the grids should be minus 34v, at present around minus 25v. I think I have gathered that the cathode currents may be too high because this voltage is low. I can see that I might be able to increase this voltage by reducing the value of the resistor between the diode and the HT, which is 180K at present – would this be a sensible way to go?

[snowman_al]

This is what I would do, others will do it differently I'm sure.
Replace the electros on the HT line. C1, C18, C21, C22 and C27 from the schematic.
Replace the electro C25 which smooths the negative bias supply. Note this cap has positive to ground / 0 volts and use a 47uF of at least 150 volt working.
Now check the values of R43 (180K) and R44 (39k), they form a sort of potential divider across the transformer winding to feed the bias.
Now check your voltages again. The HT will likely be higher and could be the bias is higher too. You can then adjust R43 / R44 to get you around 34 volts if needed.

As built there is no adjustment for the current through the EL34s its set by the -34 volts bias. Easiest way to measure it is as Andy (DA) suggests. Add a 10ohm resistor to the cathode of each EL34 in place of the wire link to ground. You can then measure the voltage across it and by Ohms law calculate the currents. You can link them out after, but you can leave them in circuit as they will make very little impact on output power.

Here's a link to the diagram again to save swapping back and forth. http://www.vintagehofner.co.uk/selme...assmast50.html

Alan

[stevehertz]

I have a few mods that will enhance the amp once you have got it all working. As I said in a previous post, the preamp part of the amp is a 99.9% copy of the two channel Fender Bassman model 5B6 amp from the early 50s. Back in the day, guitar players soon found out that the Bassman was also an excellent guitar amp, and eventually Fender starting marketing it for use with lead/rhythm as well as bass guitars. When Selmer decided to copy the circuitry for use in their Bassmaster and Treble n Bass amps (which are exactly the same amps apart from their names!), they made just a couple of changes to the preamp. I guess in some ways this meant that it was no longer a copy!

Probably because it was the so called ‘bass’ channel, Selmer included a modification that simply robbed it of treble by shunting it down to deck via a capacitor at the output of that preamp. It should be noted that the bass channel of the Selmer is in fact the more 'happening' channel of the two. It has an extra valve stage in there and is generally a nicer sounding preamp. So yes, the bass channel ‘tone robbing capacitor’, it’s fitted on the output of the bass channel where it meets the driver stage (0.01uF, C17). This cap was not fitted to the Fender Bassman, and here on the Selmer it just serves to totally ruin a near perfect sound by crudely shunting treble down to deck. Accordingly, for guitar or bass, this cap should simply be removed to totally change the tonal characteristics of the bass channel, opening it up to become sparkly and harmonically rich.

Also on the bass channel, I recommend a resistor value change. There is a 1M (R22) resistor fitted across the last triode preamp stage. In some versions of Fender’s Bassman, this resistor is not fitted at all. I have tried a 10M with improved results, namely a slightly more open, breathier sound quality. So change the 1M resistor to a 10M one, or just remove it.

Under-chassis screening. I have had considerable success with RF noise reduction by stapling a thin aluminium screening sheet into the cabinet floor directly below the main amp chassis. The screening sheet will need to be cut away or have holes punched in the area where it sits above the under cabinet vent, otherwise the vent is relatively pointless. Aluminium foil glued in place would have the same RF reducing effect but has a tendency to tear when the chassis is removed or replaced.

I've attached a home drawn schematic that includes real voltages that may help (it was not created by me).

[Stretcher]

Andy – point taken about sorting out the capacitors before looking at the biasing. Steve – thanks for the suggested mods – looks like they will improve the amp and very straightforward to do! I am hoping that the components will arrive in the next day or two so I can get to work.

Oliver

[Edward Huggins]

I grew up with these amps in our Band. Although cheap and cheerful, one cannot fault the overall design too much. However, that treble roll-off arrangement was crude. Anyone who knows just how a Bass Guitar should sound will know there is a lot of treble and mid-range content. Just listen to the music of Jaco Pastorius. This is what gives attack and definition to the sound, especially in crowded venues.

[ColinTheAmpMan1]

Originally Posted by Techman

Absolutely!

Another thing that I would do is to check out the transformer primary tap connections to that mains selector, as with it having been replaced by someone in the past, you don't know whether they got the wiring connections round their neck when they fitted it, so don't rely on what it says on the voltage selector until you've checked it out properly.

I would also still check the calibration of your meter against another known good one - 231 volts mains sounds a bit low for anywhere in the UK these days...and a good car battery will be more than 12 volts.

I don't know where in London you live, Stretcher, but I live in Wimbledon and I have just checked our mains voltage at just a fraction above 240 VAC with my Fluke 175 true RMS DMM and my AVO 8 Mk V.

Colin.

[Stretcher]

Interesting point you raise, Colin. My understanding is that the UK's official voltage is 230v to harmonise with the EU, but the tolerance is so wide (-6% to +10%) that existing 240v transmission equipment can easily fall within the specification. It is possible that my meter is under-reading; if we took the local voltage as being 240v, then my meter would be under reading by around 4%, or 96% accuracy. I have used the meter for a while, and I feel that is sufficiently accurate for what I am trying to do here, especially if I know that the readings are likely to be on the low side. Others may disagree!

[Techman]

I've never known anyone's mains voltage below at least 240 volts these days, so if yours is lower, then that's very unusual, but possible.

You talk about possibly spending a small fortune on a replacement transformer on the basis of an old meter that you've nothing to properly check the accuracy of against, but a brand new meter (DVM) can be bought for a fiver and will be spot on as regards accuracy, so I don't understand your logic - just get a new meter and be done with it...and then go from there.

Edit: I just had a quick look and the cheapest I could find now was around £7, but if you look hard enough there's probably still some to be had for five.

[Stretcher]

Thanks for the advice Techman. Mine is a digital meter, which I bought for around £5, in the picture below. Glad to hear that it should be spot-on.

[Techman]

If that's the meter you've been using and you've had it for a while then it may not be spot on now, so buy another one which will be spot on being brand new (they usually are). You're probably tight fisted like me, but it's just not worth making expensive decisions on the base of a complete unknown for the sake of a fiver!

[Herald1360]

Originally Posted by Techman

Edit: I just had a quick look and the cheapest I could find now was around £7, but if you look hard enough there's probably still some to be had for five.

You can still find the classic cheapy DT830 DMM for less than £5-50, UK stock but the days of two for a fiver seem to be gone.

Looks like the OP's meter is a version of it in a holster.

[Techman]

I ought to get another cheap one as a spare - I would advise the OP that it's always useful to have more than one meter anyway. I blew up my cheapy when a speaker went o/c on an amplifier that I was measuring the HT at the output valve on. Meter on 600 volts DC and HT of only a couple of hundred volts, but high voltage spikes from dirty preamp valve base contacts being amplified causing high voltage spikes in the output stage fried the meter...output transformer that was acting as the inductor survived.

The thing with the OP getting another meter is important as getting to over thirty in-depth posts about what could be wrong when we don't really know if the low voltage readings are down to a meter that may have been overloaded at some time in the past and is now giving false readings - I've had this with digital meters out of auction house job lot boxes before now...there's a reason they're in there!

The OP has his location down as London, so I wouldn't expect low mains voltage in a big city like that...but I could be wrong and it needs ruling out before keep guessing and going any further with this amp.

[Richardgr]

Has the OP tested the transformer with no connections at all? Ideally possible by removing secondary, bias, and heater fuses,but possibly necessitating a bit of desoldering.

It has to be the first step, to eliminate a possible unexpected load.

[Techman]

Originally Posted by Stretcher

Just had the amp on for 1.5 hrs with all the valves out – there doesn’t seem to be any appreciable warming of the mains transformer, so hopefully this means that there are no shorts in the windings?

Looks like he's already done this check and proved the transformer to be fine.

There's really nothing more to be said on this until it can be confirmed that the voltage readings being taken are the actual voltages. Perhaps the mains is low and his meter is reading fine, but we need proof of this before going any further.

[Richardgr]

Even with the tubes out there is a path to earth via the bias circuitry. If there is a problem there then there could be an unexpected load.

[Valvepower]

Hi,

I recently restored one of these and I concur, and it is not the original mains transformer. I had the opposite as I had a knackered output transformer.

I have seen these fitted with a similar mains transformer as used in a Roost, Sound City or Hiwatt amplifier with a solid-state bridge rectifier, I know, as I did this on a T&B 50 whilst at Roost in the dim and distant past – for the purists I stand guilty as charged, m'lud

Off the top of my head, I replaced all the power supply reservoir, smoothing, filter capacitors and bypass electrolytic. The volume pots were also beyond cleaning. The coupling capacitors were either mustards or WIMA and I left them alone.

When I last used it I noticed some noise which I attributed to dirty valve base contacts.

Nice little amp tough.

Regards
Terry

[Stretcher]

Thanks for the advice. I have now replaced all the smoothing and decoupling capacitors – see photo below. Wonderful improvement in the hum – I had to get nearer the speaker to check that the amp was on! Also some increase in the HT voltage by about 8v. I have also been experimenting with the biasing of the output valves – I don’t have a full kit of resisters (on order) but I have got closer to minus 34volts at the grids. This has raised the HT voltage by another 20v, so we are gradually getting there. I also replaced the (bypass?) electrolytic capacitors in the cathode circuits of the preamp valves – this has really improved the bass response of the amp, so I am happy all round.