~ Selmer TruVoice TV8/T ~ Selmer Mercury 5 ~

It's March 2020 and due to the corona virus lockdown in the UK I am once again self un-employed but the good weather is letting me get parts painted for future valve amp projects and I get to start some long overdue jobs like a Selmer guitar amp I promised to repair for a friend who works in Korea and was 'in no hurry for it'

So finally after more than a year I get to look at his Selmer amplifier starting with finding a schematic for it ~ Searching the internet and asking a few friends revealed that there is a schematic out there for the Selmer Mercury 5 which was originally the TV8 but with a magic eye tremolo indicator added and is not the clearest of schematics

Another task I set myself now that I had the time was to move my schematics and PCB layouts drawn with MicroSim 8 to a new package and as OrCad took over MicroSim and had translators for such that would be easy ~ As usual not the case and I spent a lot of time trying to recreate the style of my many symbols made over a 20+ year period

OrCad was not as advanced as I expected and little had improved in 20 years ~ one thing that was worse than MicroSim was the ability to create good looking pictographic circuit symbols because OrCad insists on using fixed grids and line styles ~ Although initially frustrating to use for drawing I eventually created some custom symbols

One symbol library that OrCad completely screwed up was my valve amp and vintage transistor symbol set ~ I would now find it difficult to quickly draw some of the clear schematics that I had made in the past and it would be extremely difficult trying to draw something like the QUAD MPX Schematic again

However I accepted the challenge of making OrCad look as clear as my old MicroSim did at the turn of the century and one of the first new schematics I drew was for the Selmer TV8 Mercury 5 ~ The reason I spend time recreating schematics is they can then be quickly edited to show modifications and they look good in web pages and documents

This is not a valve amp modification article it is simply a step by step record of how the Selemer T.V.8/T was repaired with a few changes to the original layout and some modern components where they were needed and where they would not change the characteristic of the amplifier

As handed to me the Selmer TV8 did not work at all ~ Many of the valves were low gain but the main problem was the paper sleeve waxed paper capacitors and electrolytics were all leaky

With the paper coupling capacitors removed and some new valves fitted the d.c. voltages around the valves were better but not correct according a schematic I found online and calculations I made

Almost all the carbon composition resistors were out of tolerance so were also removed as seen above ~ I had started replacing some components as they were found faulty but now with so many faulty parts removed I decided to clean and check the ground connections and valve bases and fit some new solder tags rather than just rely on the old rivets for grounding
Twistlock and multisection capacitors are still made in small quantities with high prices but it is often easier to fit a capacitor clip over the hole and use a good quality modern part which will improve performance and lifetime without affecting the sound ~ Note the new solder lugs for the smoothing capacitor ground and on the transformer for a safe mains earth connection which is more than can be said for the mains voltage selector
The output transformer T35013 mounting bracket was cleaned of paint where it contacts the chassis and new bolts with locknuts were fitted and made tight ~ The bracket is used for a ground connection of a screened wire from the EL84 output pentode anode to the output transformer (OPT) as seen on left side of picture above

I guess a problem was found in production where the EL84 anode connection fed back signal to the adjacent ECC83 and caused oscillation ~ Even with the OPT bracket firmly connected to chassis the rivet the screen connects to is not a good ground so I provided a formal connection from the wire screen and speaker output to chassis ~ Swapping the OPT primary connections would probably be as good and not require a screened wire

Rather than fit a new multisection capacitor for the HT smoothing I fitted a good quality 100µF 450V for section C15 ~ The original 3 section capacitor had C1=32µF C12=32µF and C15=16µF ~ The maximum specified capacitor load for an EZ80 is 50µF but here there is a 250Ω resistor R18 between the EZ80 cathode and C15

The other 2 sections C1 and C12 were replaced with separate 33µF 450V capacitors mounted on the main tag strip as shown above with their ground connections wired to a new chassis ground tag rather than relying on the rivets ~ With most components removed it is also possible to solder the original ground tags to the chassis if you have a powerful enough soldering iron and use a liquid flux to clean under rivets

The original mounting of R18 was across the EZ80 base from cathode pin 3 to a spare pin 8 ~ This mounting method is not good especially for an amplifier that will be carried around and thrown in the back of a van ~ The picture above shows R18 mounted away from the EZ80 base using an insulated standoff so it does not flap around so much

And finally the mains lead and 500kΩ anti-log tremolo speed control pot with copper plated bottom get fitted ~ The mains earth wire is now fitted to the new solder lug on the mains transformer mounting bolt and the control pots have their cases connected and formally wired to chassis
On my redrawn schematic I comment about filtered sound below the high pass filter section consisting C9 C10 and C11

This filter has a steep response of 18dB per octave at about 120Hz so very little Bass gets passed to the small output stage transformer

Capacitor C13 across the OPT primary acts as a low pass filter and with the high pass gives the Selmer TV8-T a peaked response around 3–4kHz

You can make capacitors C1 and C12 any value 22µF or greater and C15 can be 47µF to say 220µF ~ Note I left the carbon resistors on the input sockets and used philips mustard capacitors as a nod to classic guitar amp construction but in practice the cheap output transformer and filter and coupling values have most effect on sound

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"Well I don't really care ~ If it's wrong or if it's right"