Selmer - Treble 'n Bass 50 Mark II  [schematic]

Tube or solid-state, this section goes to eleven!
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modman
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Post by modman »

I bought this wreck of a Selmer TnB50 for hardly any money - but may still too much. I knew it wasn't working and humming loudly, I wasn't informed of the fact that it was humming without a speaker connected. The head cabinet and OT are non-original, rectifier socket is empty and has a solid state rectifier installed.

3A and 5A fuses where you want to see .5A and 1.5A. I put in the correct values, removed the tubes and fired up the amp again, HT .5A fuse blew again.

Three prong plug cut off, two prong put on. I read this as: a tech declared this amp a loss for PT issues and cut of the plug to avoid anybody plugging it in. It then ended up in the hands of somebody who didn't know to read these signs and put on a 2 prong, leaving the ground unconnected - other side is soldered to the chassis inside the amp. I re-installed a 3 prong plug and this really did the amp some good.

Trying the amp again without new HT fuse gave me some voltage in the 350V range on the power rail, but no filament voltage on the tubes. The 5V heater winding for the rectifier seems to be ok, delivering 5.9V when nothing is connected.

The problem is in the rectification as I see things now. The amp now has a full wave bridge rectifier installed....
rectifiers.gif
rectifiers.gif (8.76 KiB) Viewed 3954 times
However, the negative DC leg of the SS rectifier is in this amp, not grounded. Should I convert to a full diode rectifier?

The power transformer has a CT that connects through the .5A HT fuse to the CT of the 6.3V winding and further to ground. Is the lacking of ground on the power/filament CTs what's causing the lack of filament voltage? I feel I should first make sure I have filment voltage before anything else...
powersection selmer.jpeg
powersection selmer.jpeg (21.57 KiB) Viewed 3954 times
There are a few different options I'm considering:
Risking another fuse at this point
Removing the SS rectification and put everything back to stock.
Selmer Treble 'n Bass Mark II schematic
Selmer Treble 'n Bass Mark II schematic
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deltafred
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Post by deltafred »

Unless you also removed the SS rectifiers then the HT will still be present so any faults on that can cause the HT fuse to blow. (Leaky/shorted smoothing cap being the prime suspect.)

If it were mine I would -

1. Use 2 SS rectifiers as in the bottom left diagram in rectifiers.gif

Because the transformer has a CT you cannot use a 4 diode full wave bridge (as in top right) unless you use half the transformer (otherwise you would get twice the voltage) and remove CT ground connection because the -ve bridge connection is grounded. This is not a particularly good idea because half of the transformer secondary winding in now supplying all the HT current rather than it being divided equally between the 2 halves.

I never did like tube rectifiers and there are far better ways of achieving the same result. If you want tube rectifier sag then you can put an appropriate value resistor in series to give you some volts drop under load.

2. Connect the HT centre tap to the ground and put the HT fuse in the HT supply between the rectifier junction and the 32u capacitor / choke junction.

Putting fuses in ground lines is rarely a good idea.

- as always IMO and YMMV.
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Post by uncleboko »

I used to have a Selmer Selectortone - the version with a fairly useless 15" speaker in mid 60s. I put it through a pair of Jensen Blues which gave an immediate boost in volume. Then a friend came up with the idea of boosting my guitar level through a mic preamp.....................................wow, serious overdrive!!

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Post by modman »

Thanks so much for the advice. The only reason for reverting back to the tube rectifier, would be to restore some originality to an already butchered up amp. I forgot the pics:
Selmer chassis 1.jpg
Selmer board.jpg
Something's happened to the amp - the board in broken in half:
Selmer board broken.jpg

I used 2 1N4007 diodes for the rectification, put HV fuse immediately after that, then on to choke and first filter capacitor. The 32uF were looking really swollen. Put the HV center tap to ground... but could not locate the filament CT. However, measuring the filament wiring I have continuity to ground...

Then there is the brown mystery wire, coming off the primary of the PT, but having no continuity with any of the other primary wires making up the voltage selector. This brown lead was soldered to the first filter cap :scratch: It is not the center tap for any of the secondaries. One of the primary leads of the PT is brown and soldered to the voltage selector - two brown wires on the same side of the PT?
Selmer power section detail.jpg
Could put in a new fuse and try it...
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Post by deltafred »

The brown mystery wire could be an electrostatic screen (usually a thin sheet of copper insulated on one side so as not to create a shorted turn) between the primary and secondary windings to prevent mains borne interference being capacitively coupled to the secondary (if I remember correctly). It is usual to connect it to ground.
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Post by modman »

I did fire it up and was happily suprised it didn't blow the fuse again. Measured about 500VDC as B+, there was filament voltage, so plugged in some tubes, and the beast was alive! It sounds big and bright, as expected. I like the bass channel for guitar, especially for single coils. Anyway, meanwhile, the bias was still at -55VDC; I replaced the electrolytic in the negative bias circuit and also updated the resistor that according the schematic was supposed to be 180k, but measured as 120k. Then the bias was -33VDC. Plate voltage in the preamp and PI: V1=180-90VDC; V2=120-30VDC; V3; 203VDC. Power tube plates: 440VDC

Still testing with the old, sacrificial tubes,

I connected the electrostatic screen mystery wire to ground, but couldn't really compare the noise level, too late for the neighbors... More measurements tomorrow.
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Post by deltafred »

Good to hear that it's going again.

The very first amp that I repaired was a Selmer PA100 in about 1967. Someone in my school band picked it up for peanuts because it was faulty, I fitted a replacement OT and we used it until we finished school and went our separate ways.
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Post by modman »

I had never heard of or encounter an electrostatic screen before, it's also not indicated on the schematic. Found this great site with more info: https://www.sciencedirect.com/topics/en ... tic-screen

I connected it to ground, but this did not improve noise or hum in my opinion. However, surfing the Selmer fakebook group, I came across a similar chassis to mine. There were a few changes within the TnB Mark II - the transformer without bell cover is used from late '66 onwards. For this layout, the filter caps are mounted next to the PT.

My unit is 44971, and the mustard caps are D6N - 4th Q of 1966

It's serial 42393, and the mustard caps are B6N - 2nd Q of 1966
Screen Shot 2021-08-02 at 14.40.20.png
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Selmer TnB serial 42393.jpeg
228252178_2049441185194744_1486950813610895244_n.jpeg

Looking at the power section, it does seem like the brown wire goes to the ground point near the filter cap:
Selmer TB 42393 power.jpeg
TB 42393 power detail.jpeg
TB 42393 power detail.jpeg (40.61 KiB) Viewed 3624 times
Went ahead and ordered a 20uf+20uF+20uF+40uF to replace the other can cap, don't understand why I didn't do that the first time. It's still humming quite a bit, trying to remember the words, it's been a long time. Don't know what you can expect from electrolytics after 56 years.

I'll share some sounds soon: it's loud, clean, chrystal clear highs, heavy round bass. And this is without giving the preamp section a good look.

Thanks so much Fred!
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Post by modman »

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Post by candletears7 »

Sounds fantastic! Well done on persevering with this great old amp.

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