Looking for help please with 1964 Trio W-41 stereo valve amp
 

Hello everyone. Thank you for taking time out of your day to look at my thread.

I'm new here and first of all should outline my skill level:
I can solder, (cables and connectors mainly) - I have built speakers and followed diagrams to put together kits of crossover boards... I can use a multimeter, & I have just bought a capacitor meter. My understanding of electronics is growing daily as I've been on a very steep learning curve for the past week with this amp. Uncle Doug on youtube has educated me a little in valve technology, and I've got hold of a schematic for the amp which at first made no sense at all, but now I've learnt a lot more, it does (to a degree). I am having great difficulty in relating the schematic to the physical wiring though as I have no layout/topology for it. I am a broadcast sound recordist of 20 years and a keen vintage hi-fi listener but this is the first piece of audio equipment I've tried to fix myself! Sadly my tiny brain really struggles with the physics and maths part of electronics. calculations of ohms, farads, amps leave me wishing I'd learnt it at school when I had a better working brain. Volts I'm better with, especially now that Uncle Doug has explained that both DC and AC can co-exist in the same wire!

I've been getting a little guidance from a retired electronics engineer via facebook and tested out voltages at certain points but we've come to a point of head scratching... here's the crux of the problem...

The phono preamp is not working... it comprises of 2 12AX7s v1 and v2 on the schematic which are heated from a filament tap from the output valves, as are the other 3 12AX7s that take care of tone controls (I think) - neither v1 or v2 are heating up however. they are wired in series and I've measured 20v going into v1 filament 1 and 10v coming out of filament 2 which feeds filament 1 of v2 (theres 10v on that terminal at v2) filament 2 of v2 however shows no voltage and the pin is soldered to the centre tap which I'm assuming goes to ground somewhere. I've noticed that the 3 other 12AX7 that are heating up, are wired for heating differently to v1 & v2 - My understanding of why is lacking here.

I'm hoping in an ideal world that someone on this forum has had a Trio w-41 on their bench, maybe with a similar problem, and can point out something blooming obvious that I can cope with using my fairly basic skills with soldering and wiring. I have no oscilloscope or signal checking equipment.
The Amp is working on the 3 inputs that bypass the preamp and sounds very nice indeed!.. I have gone to the trouble of furnishing it with a full set of new matched Sovtek valves (9 in total) so would dearly love to be able to get the phono stage working and enjoy some records with the valve tonality that I've come to learn is pretty fine indeed.
Thank you in advance for any help you can give me. Images are attached that will hopefully help...
Tim

Re: looking for help please with 1964 Trio W-41 stereo valve amp
 

As I read the diagram, there should be no wiring to the centre tap (pin 9) of V1 or V2. Current flows:-

V1 pin 5- heater- V1 pin 4-V2 pin5-heater- V2 pin 4- ground.

Check the wiring and measure the voltages on pins 4 and 5 of both valves.

EDIT. Your voltage readings look OK to me, but I'm confused by the reference to a centre tap. Centre tap of what?

Check the continuity of the valve heaters (valves removed from amp) with your meter on ohms range. It's a nice easy and safe thing to do.

Re: looking for help please with 1964 Trio W-41 stereo valve amp
 

I have not seen that before, always something to learn.

V1 and V2 have a DC heater supply and are part of the cathode bias components for the 4 x EL84’s.

Yes do has Graham suggests, I am just commenting that I had not seen this before, must have led a sheltered life.

Re: looking for help please with 1964 Trio W-41 stereo valve amp
 

Thank you both.

So I've learnt something already! The big central contact post in the centre of the valve socket is not as I mistakenly called it the 'centre tap' ....:-[
I can confirm Pin 9 the real tap is free from connection to anything on v1 and v2.

the valves are new, hand picked and matched and these 2 measure 32 ohms across pins 4&5. does that seem ok?

Can you give me any ideas please why would I not see a voltage at pin 4 of v2 if theres 10v going in on pin 5?

many thanks

Re: looking for help please with 1964 Trio W-41 stereo valve amp
 

The reason for the separate heating arrangements for V1 and V2 is to supply DC rather than AC to the valves in the most noise-sensitive positions. AC heating induces hum into the valve, albeit at low level, and the absence of this is advantageous when you need every last dB of S/N ratio, as is the case in the PU stage.

Re: looking for help please with 1964 Trio W-41 stereo valve amp
 

Hi Ted,
No problem with DC heaters for hum reduction, just not seen it from the cathodes of the output valves.

Re: looking for help please with 1964 Trio W-41 stereo valve amp
 

Look under run to me, wonder what the Ik is.

Lawrence.

Re: looking for help please with 1964 Trio W-41 stereo valve amp
 

Has Graham states those voltages look ok although the valves are under run, is it the case of the under run heaters not glowing very brightly and hard to see?

Check the voltages on the anodes of V1and V2 with a high impeadance voltage meter, if the are near correct and the HT to the pre amp is correct, circuit states HT 140v then the valves must be conducting.

Crossed post, OK Lawrence, like minds,

Re: looking for help please with 1964 Trio W-41 stereo valve amp
 

What you are seeing is perfectly correct. The valve heaters are in series, so there should be 10V across each heater. There is. Measure the voltage between pins 4 and 5 of the individual valves to prove the point. There's no voltage on V2 pin 4 with respect to chassis, because that's your reference point. At least I assume it is?

I would suggest that the valves are in fact lighting up, but you can't see it. Try looking in the dark from various angles.

Re: looking for help please with 1964 Trio W-41 stereo valve amp
 

Once you're satisfied that the valves are in fact lighting up, the next step is to measure the voltages on the other pins of V1 and V2. Post the results here and we'll analyse them for you.

Re: looking for help please with 1964 Trio W-41 stereo valve amp
 

10V seems a bit on the low side to make the heater glow nicely ?

If you see 10V at the mid point of the heater chain then one or both of the output stages must be under biased.

dc

Re: looking for help please with 1964 Trio W-41 stereo valve amp
 

I think the circuit shows 21 volts on the EL84, if that is correct, it’s a bit hard to read, then the design was to under run them.

Re: looking for help please with 1964 Trio W-41 stereo valve amp
 

It does indeed say 21V on the EL84 cathodes. I've downloaded the diagram from the original source and zoomed it.

Re: looking for help please with 1964 Trio W-41 stereo valve amp
 

Lower cathode temp=less noise? Just a nutty theory.

Lawrence.

Re: looking for help please with 1964 Trio W-41 stereo valve amp
 

The circuit says 21V on the heaters, so the output valves may be a bit light on total cathode current (-5%?). The preamp heater supply is a bit of a juggling act with some cathode current (35mA) flowing through the bias resistors for the output valves and only what's left through the preamp heater chain. Fine with all new output valves but will go "off" as the valves age.

Re: looking for help please with 1964 Trio W-41 stereo valve amp
 

More than likely Lawrence, But I think what's needed here is a bit of practice rather than theory. Looking for lighted valves and measuring HT voltages.

Theory does suggest that the valves are in fact lighting up or at least heating up. There's a volt drop across the heaters which means they're resistive and resistors work by generating heat and/or light.

Re: looking for help please with 1964 Trio W-41 stereo valve amp
 

Thanks again.

The voltages I'm measuring on the anode pins 1 and 6 of v1 is 98v pin 1 and 66v pin 6

...and on v2 95v pin 1 and 68v on pin 6

This is with no audio input signal.

I've also sat in a darkened room with it now and there is definitely no glow from all 5 ECC83s BUT I'm pretty sure they are working... all the tone controls work and if I put low/mid vol iphone headphone feed into the phono preamp sockets it will process the current. It will start to distort at v1 & v2 if I go above 6/10 volume. I did this for a very quick test to see if someone had bypassed preamp to get an extra line input in the past.

Re: Looking for help please with 1964 Trio W-41 stereo valve amp
 

Those readings are correct and indicate that the valves are working, which could only happen if their heaters were working.

I note that the pre-amp is only used for Magnetic Cartridge and Tape HD (head?) inputs.

Have you cleaned the numerous switch contacts round the input by applying Servisol?

Check the anode resistors R3, R5, R103 and R105 with a meter on ohms range.

Re: Looking for help please with 1964 Trio W-41 stereo valve amp
 

Thanks Graham,
here are the resistor readings...

I've not removed them from the circuit btw.

R3 flashed up 198k ohm as it's best result... but no steady reading
R5 reads 76k solid
R103 flashed up 136 to 145k and no steady reading
R105 reads 57k

not great ?

Re: Looking for help please with 1964 Trio W-41 stereo valve amp
 

Perhaps a silly question,, but were these readings taken with the amp powered off? They should be.

You should be able to get steady readings on R3 and R103 (270k), as these components are not shunted.

R5 and R105 are shunted by what appears to be a feedback loop, so may give incorrect readings.

Varying readings indicate that the meter is charging up capacitors in the circuit, but they should stabilise if you wait long enough.