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Old 2nd Jan 2021, 8:05 pm   #1
Stretcher
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Default Selmer Bassmaster 50 - mains transformer

I have had this guitar amp in my possession since around 1980, when I was a (not very good) bassist in a band. It was probably manufactured in the early 60's, and is quite a historic amp, judging by what is online about them. For the last 20 years or so, it has lived in my shed, but amazingly, it still works! Since I have retired I have finally got it out to do some restoration work on it. I have a good and accurate circuit diagram, and have been starting off with checking the voltages around the circuit. This has immediately shown up a problem - the voltage at the rectifier was 313 volts, whereas the diagram says it should be 470v. The heater voltage was also down, at 5.2v, and the rectifier heater is showing 4.2v - should be 5v. The mains voltage selector is on 245v, the other options being 225 and 205v. I have checked the primary winding with a resistance meter, and the resistances correspond with the voltages for each tap. The HT voltage seems to be steady while the amp is working. I am puzzled that the HT voltage is so much lower than specified, and also it is low by a bigger ratio than the two heater voltages. The amp seems to be all original condition, and the mains transformer is the same size, and looks nearly identical to the output transformer.

As part of my investigations, I have tried moving the mains supply onto the 205v setting. This has moved the two heater voltages up to where they should be, but the HT is still only showing 366v at the rectifier. I am really puzzled. I wondered if the transformer has been rebuilt in the past and all mismatched windings put in? I don't know much about transformers or historical repair practices, so any advice or suggestions much appreciated!
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Old 2nd Jan 2021, 8:40 pm   #2
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Default Re: Selmer Bassmaster 50 - mains transformer

If you take the valve rectifier out it would be interesting to know what the unloaded AC voltages on the two halves of the transformer's HT winding are. If you take all the valves out it would also be interesting to know if, after an hour or so's being plugged in, the transformer was warm. A significant temperature rise might indicate a shorted turn, which would be consistent with low voltages on the secondaries.

EDIT: I suppose both halves of the rectifier are working are they ? A not uncommon failure mode is for one half of the heater to go open circuit, turning the rectifier from a full-wave to a half-wave one. It's worth making sure you can see two orange spots from the heaters when you look down on the rectifier from above. Half-wave running drops the HT voltage and, because it loads the mains transformer less evenly, might even drop the heater voltages a bit I suppose.

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Old 4th Jan 2021, 7:25 am   #3
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Default Re: Selmer Bassmaster 50 - mains transformer

Lots of things can pull the HT down, leaky smoothing/reservoir caps, is there any hum? Check the HT with your meter set to AC volts. Some other short can pull HT down too, like the OP stage running too hot,check each OP valves grid and cathode readings, but a disparity of 150v ish is a hell of a lot. It could of coarse be a combination of several things. Have you also checked your mains voltage going into the amp? Could be a faulty mains voltage selector.

First thing pull all the valves and check tfmr primary AC, then tfmr secondary AC.

Andy.
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Old 4th Jan 2021, 9:23 am   #4
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Default Re: Selmer Bassmaster 50 - mains transformer

It sounds odd for a guitar amp to have an output transformer the same size as the Mains transformer, but maybe for a bass amp this was deliberate.
Maybe the mains transformer is underrated.
Try the checks which GJ mentioned above.

Mike
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Old 4th Jan 2021, 12:43 pm   #5
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Default Re: Selmer Bassmaster 50 - mains transformer

Hi All and thanks for all your suggestions.

I have done some further investigating : The mains supply voltage is 231v, which I now have going into the 200v tapping in the mains transformer.
I have put a fuse (which is actually a 12v 3w car bulb) between the rectifier and the rest of the HT circuit. Rather than pull out all the valves, I found it easier to break the HT circuit at this point.

The AC voltages at the secondary are now 311v and 312v with the HT circuit connected, and 336v and 337v with the “fuse” removed, i.e. just the heater circuits connected. This is still quite a way short of what the circuit diagram says the voltage should be, which is 470v each side.

I had a look at the rectifier, which is glowing nicely on both sides. I checked the grid and cathode voltages for the output valves – the grids (tied together) are 350v (supply is 369v) and the cathodes are connected to earth.

I haven’t pulled the valves out and checked whether the transformer warms up yet, but will get onto this soon. The “fuse” doesn’t glow atall when the amp is switched on from cold, but does light up briefly if it is switched off and on again.

There is a clear hum from the speaker, which would be nice to sort out. Would new smoothing capacitors see to this? Any new diagnostical ideas? Thanks again

PS, here is a photo of the amp – both transformers look enormous to me, but I was brought up on transistors! The mains transformer on the left does have the shinier covers, so maybe this has been modified up at some point since the amp was built.
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Old 4th Jan 2021, 12:44 pm   #6
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Default Re: Selmer Bassmaster 50 - mains transformer

Might be worth checking your meter as well. Just to cover all bases.

David
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Old 4th Jan 2021, 2:27 pm   #7
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Default Re: Selmer Bassmaster 50 - mains transformer

That is a replacement Mains transformer. The heat shield was bolted to an 'open' frame type transformer originally. You might find four holes in one end?
It's a fixed bias amp so check the schematic for the grid 1 (pin 5) voltage on the EL34s...
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Old 4th Jan 2021, 4:03 pm   #8
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Default Re: Selmer Bassmaster 50 - mains transformer

Quote:
Originally Posted by snowman_al View Post
That is a replacement Mains transformer. The heat shield was bolted to an 'open' frame type transformer originally. You might find four holes in one end?
It's a fixed bias amp so check the schematic for the grid 1 (pin 5) voltage on the EL34s...
From the "newish" appearance, I agree with snowman_al that the mains transformer is a replacement. Was the amplifier like that when you used it? Can you recall having any repairs done on it while in your possession? If it is a replacement transformer, then it might very well be that it is the wrong specification and your measurements are correct. It seems likely to me that if the original mains transformer was replaced, then it might well be of a lesser spec.; I think that really high voltage mains transformers were probably a bit scarce at one time. I think you may be able to get a transformer that would suit it better nowadays, but at a premium price!
Colin.
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Old 4th Jan 2021, 4:29 pm   #9
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Default Re: Selmer Bassmaster 50 - mains transformer

Back in the day, Selmer was a cheap and cheerful brand, often the maker of the first amplifier a teenage guitarist saved-up for. Having a mains transformer wound to push the output valves to their voltage limits is expected so they can claim the biggest bang for the buck. But also having the transformer run beyond reasonable current limits is also expected.

So finding one whose mains transformer needed replacing is not at all a surprise.

When someone goes looking for a replacement transformer and needs one with the same lam stack in order to fit the chassis, what's available will be somewhat less adventurously rated than the original.

David
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Old 4th Jan 2021, 6:34 pm   #10
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Default Re: Selmer Bassmaster 50 - mains transformer

I have searched some of the sites that I am aware of and I found a schematic for a Selmer Bassmaster 50 at http://www.vintagehofner.co.uk/selme...assmast50.html.
It shows the AC voltages on the mains transformer as 370-0-370 and the DC voltage on the rectifier (GZ34) cathode as 470 VDC. Are you sure that you have the right schematic?

Colin.
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Old 4th Jan 2021, 6:49 pm   #11
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Default Re: Selmer Bassmaster 50 - mains transformer

Thankyou all for your speedy replies. The thought about the multimeter did occur to me, so I checked the DC out against a car battery, and the AC against the mains, and both readings were fine.

Snowman_al, there are holes in the shield – there are 4 about 8cm apart in a square at one end of the plate. I checked the pin 5 voltages, they are both minus 24v, should be around minus 29v according to the diagram.

The amp is pretty much as I bought it, from a music store in an arcade in Leeds, for £100 if I remember rightly, so replacement of the mains transformer would probably have been done by the shop. I don’t fancy paying premium prices for a new transformer, and I probably won’t sell the amp anyway, but is there a way of adding extra windings to the secondary to bring it up nearer spec? If not, I will probably leave things as they are.

I was considering buying some new smoothing capacitors, although maybe the old ones aren’t so much at risk considering the reduced voltage, but would they reduce the hum? It would be nice have the amp near silent if possible.
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Old 4th Jan 2021, 9:19 pm   #12
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Default Re: Selmer Bassmaster 50 - mains transformer

Ha, Selmer croc skin 50 amps. I bought my first in 2001 and carried on buying until I had 12 of them. Accordingly I've restored a dozen of them. The three main smoothing caps on top of the chassis will need changing as will the HT dropper/smoothing resistors between them along the HT rail. There are a number of electrolytic caps on the component board that will also be faulty. Make sure that the rectifier is a good one. Measure your mains, you may be better off using the 225v tap if it's low enough. You should find that the HT voltages will improve if you address all of these issues.

These are a truly great amp that was never at any time cheap and cheerful. However, once Vox started to give amps away to the Beatles et all, they became unfashionable. They are based (99.9% copy preamp wise) on the hallowed Fender Bassman two channel version but with EL34 output bottles. To many guitarists, this is a holy grail union of the best of both preamp and output stage tonal and distortion characteristics. When pushed hard, EL34s distort in a harmonic, sweet way with a concentration in the mids area. Conversely, although they provide bigger, better bass, 6L6s (as used in the Bassman) have a tendency to sound 'flubby' and lacking in focus when driven into distortion.

I have just one croc skin 50 left now, and no plans to sell it!
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Old 4th Jan 2021, 11:51 pm   #13
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Default Re: Selmer Bassmaster 50 - mains transformer

If it's been living in a shed for all those years the mains transformer is likely to be damp and that may be contributing to your problem. I would also suggest you do not run it on a lower mains tapping. As others have suggested you need to see if the transformer is getting hot without any valves in and recheck the LT voltages.
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Old 5th Jan 2021, 12:05 am   #14
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Default Re: Selmer Bassmaster 50 - mains transformer

Quote:
Originally Posted by PJL View Post
I would also suggest you do not run it on a lower mains tapping. As others have suggested you need to see if the transformer is getting hot without any valves in and recheck the LT voltages.
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.
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Old 5th Jan 2021, 1:11 am   #15
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Default Re: Selmer Bassmaster 50 - mains transformer

The amplifier valves have 6.3V heaters but the rectifier has a 5V heater.

One of the most important readings you need to check is the cathode current of the EL34s.

If the reservoir capacitor is open circuit you could get strange readings on a digital voltmeter at the rectifier cathode.

A photograph of the underside would be useful.
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Old 5th Jan 2021, 7:55 am   #16
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Default Re: Selmer Bassmaster 50 - mains transformer

335v x 1.414 = 473v, with a GZ34 Vf is 30v ish according to the datasheet, so you should have 440v not 470v. Two EL34's PP should only need 400 - 450v for 50w, depending on Vg2 that is, these are all rough ball park figures, so it sounds like something is pulling the HT down.

Try connecting your DMM set to DC mA across the fuse, red lead into mA from the rectifier, black lead to first cap,it should read about 150 - 200mA. As stated earlier check the HT with your DMM set to AC volts, it should be about 20mV tops. If you get higher readings that indicates with the hum at the speaker leaky caps or another short.

Another thing to check is the secondary resistance of the mains tfmr, the EZ34 datasheet gives 75 - 100r, cap 60u max.

Quote:
One of the most important readings you need to check is the cathode current of the EL34s.
Which is why we need to know Vk as well as Vg1. Stretcher gave us G2 in answer to my questions in post #3. We need to know the voltage at pins 5 - G1 and pin 8 - cathode. Pin 5 should read something like 0.01v ish, pin 8 should read 30 - 40v ish.

Old caps that are leaking electrically cause hum, 100hz hum, they can go bang or cause other damage, changing them would be an excellent idea.

Andy.
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Old 5th Jan 2021, 11:16 am   #17
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Default Re: Selmer Bassmaster 50 - mains transformer

Andy (DA),
This is a fixed bias amp so the cathodes are at ground 0v. There is a negative voltage at the grid 1s (pin 5). There is a schematic posted at #10 by Colin.

Strecher has -24 volts on pin 5. Which is a little low and will be drawing more current through the EL34s, and sapping the HT voltage. That with leaking smoothing caps is an endless loop...
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Old 5th Jan 2021, 12:46 pm   #18
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Default Re: Selmer Bassmaster 50 - mains transformer

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?

I returned the mains selector to the 240v position and took some measurements. The HT voltages showing at the rectifier socket were 298v and 299v. This is supposed to be 370v each according to diagram (sorry, I said 470 before), but I presume this would be with the HT circuit connected.

The heater voltage was showing 6.1v. The voltage showing between pins 8 and 2 of the rectifier socket was 4.9v. There is another wire from the transformer going to pin 3, which isn’t shown on my diagram (the same one as Colin has posted). This is also showing a voltage, 1.6v between pins 2 and 3, and 6.9v between 8 and 3. Anyone know what this is?

With all the valves back in, I wasn’t able to check the current across the fuse, because this range doesn’t seem to be working on my meter, however, the bulb doesn’t glow atall. It should be at full brightness with 250mA going through it.

I have attached a photo of the bottom of the amp, with most of the capacitors labelled.
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Old 5th Jan 2021, 1:15 pm   #19
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Default Re: Selmer Bassmaster 50 - mains transformer

The valve socket tag 3 will be an anchor tag, there's no pin 3 fitted on the GZ34 so far as I can make out.

Lawrence.
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Old 5th Jan 2021, 2:25 pm   #20
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Default Re: Selmer Bassmaster 50 - mains transformer

Just found this photo, no pin 3 connection:

http://www.chambonino.com/work/selmer/selm7.html

Lawrence.
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