Mullard ECL82 stereo - where has the voltage gone?

[PJL]

It's an anode follower phase splitter. V3a maintains its own anode as an inverse of V2a through feedback from the potential divider between the two anodes. Decoupling R12 will completely suppress the V3a output.

[qualityten]

Thanks to those who've had the patience to stay with this project.

To recap: I'm using transformers taken from a 1964 Dynatron radiogram with an ECL82 push pull amplifier. In the absence of a circuit diagram, and having had good results building 5-10 and 5-20 amplifiers following the Mullard designs, I decided to build the 7W following the layouts in the same book.

My first build using the chassis of a HD recorder worked but hummed. I thought that my layout was defective, e.g. Heater wiring not located on the outside of the chassis and long leads to the output transformers, and so I transferred this to a chassis of the same size and layout as Mullard specifies. After debugging, it works well and sounds lovely, but still hums. The hum is definitely 50Hz.

The only departures from the Mullard book are the different transformers and the absence of the tone and balance controls.

Thanks to the discussion here, I have identified the best way of wiring the heaters and added a humdinger, with the wiper to the common ground. Turning this does not affect the hum.

The direct connection to the chassis is now close to the input sockets. The other ground connections all go to C15 neg. I still do know where best to connect the line of earth connections on the tag board to this C15 neg. Advice on this would be welcome.

[peter_sol]

I have suggested the centre of the chassis for both connections not near the inputs.
Also are you sure your HT smoothing caps (the large can) are OK and not contributing to the hum?

[qualityten]

Hi Peter, the caps measure good. I have replaced the restuffed multisection can with a known good dual section one for C12 and C112 and a single capacitor for C15, mounted alongside. That also helps me be clear about which ground C15 is, and this is the centre ground I've used.

Apart from the safety ground, I have located the circuit ground close to the inputs to try to follow the Mullard layout and book guidelines as closely as possible.

To try and make my present ground connections and necessary variations from the Mullard circuit as clear as possible I've attached a diagram.

The ground connections are marked in orange.
The 'ground bus' on the tagboards is marked in green.
The ECL82 screen grid voltage supply arrangement (in the absence of a 20% tap) is marked in pink.

Sorry it's a bit of a scrawl, but I hope the coloured diagram still helps explain the situation.

[Herald1360]

This:

http://primarywindings.com/wp-conten...Amplifiers.pdf

Link to the original Mullard book with the 7W ECL82 stereo amplifier build info may help?

I must admit I can't make complete sense of the grounding and chassis connections from the information given. There doesn't seem to be any explicit connection shown between the 0V bus link along the tag strips and 0V. It does seem to ground the can of the C15 (reservoir) / C12 (LH smoothing) / C112 (RH smoothing) multicapacitor direct to chassis and use the chassis as the common 0V for the transformer HT and LT centre taps.

Blearghh!

[peter_sol]

OK I think I have been able to follow your diagram.

Try this.

Disconnect the input socket ground from the chassis.
ground the buss bar to chassis near the smoothing can instead.
Make sure there are no other points grounded to chassis except the transformer screen and the mains inlet earth.

[qualityten]

Chris, thanks for the link. I have a well thumbed copy of the book, used to build 5-10 and 5-20 designs successfully. I just can't 'get' the grounding scheme for this one.

At present, the green marked bus is connected from 13th tag from the right to the C15, C12 and C112 ground, and then to the chassis, near the inputs. See Mullard Circuits, Ch 4 General Notes on Construction, Earth Connections, p 27. Is this wrong? If so, where else should it be?

Sorry to be the cause of mental torture. I have a busy working week, so will go quiet till the weekend!

P.s. Post composed before seeing Peter's suggestion.

[PJL]

We could really do with the scope images of the HT rails and output. This is so we can see if it is a clean 50Hz or derived from 100Hz.
Have you tried running it without the ECC83?

[qualityten]

Request noted PJL. I'll do so as soon as I can.

Yes, I did run it without the ECC83 before with no change. Doing the check again quickly again, with other corrections, there is a louder higher pitched hum (which may be 100Hz) with the ECC83 removed. With the ECC83 back in the hum is softer and sounds 50Hz again.

[PJL]

One of the issues with working on an amp with -ve feedback is it will try to suppress any hum on the output introduced by the amp. Removing the ECC83 takes out the feedback but does leave the input to V2a high impedance.

[qualityten]

Would it be worth disconnecting the neg feedback for the purposes of hum location?

[PJL]

You could try disconnecting C3 from the ECC83 anode and connecting it to ground instead. This would eliminate pickup.

Have you connected the chassis to mains earth? Try it with the earth disconnected but don't touch the chassis or anything else for that matter whilst doing this test.

It's possible the open loop gain is falling off at 50Hz so the feedback is unable to suppress it as well as it can higher harmonics.

[qualityten]

Update: after a three week delay, I have resolved most of the hum problem by moving closer to star grounding.

Over the weekend I had some time to try a few things, like removing the on/off switch wiring, supplying all the valves except the EZ81 with a separate, centre tapped, 6.3V supply and changing the route of the HT supply to the fuse and 100 ohm resistor. None of these reduced the hum.

I wondered if there should be additional smoothing caps in the screen grid supply, so I added 68uF to each side, but this only slightly reduced the hum.

But I did notice that the hum was softer on whichever side I grounded the 'virtual bus', so I followed Rob Robinette's advice for finding missing ground connections on guitar amplifiers: "I like to use an alligator clip wire and connect one end to the chassis and carefully probe all the amp's ground connections to see if I hear an improvement. Touch the ground probe to all volume pots' ground terminal, the ground side of preamp cathode resistors, etc."

This revealed that each side of the amplifier needed to be grounded to the main C15 ground. When I grounded the cathodes of of the tetrodes for each pair of ECL82s (at the ground side of R17/R18 and C13/C14) most of the hum went away. Whew!

I now need to tidy up, by moving the humbucker nearer to the power transformer and securing the extra capacitors for the screen grid supply safely, but it's now a usable amplifier, which sounds very good.

Thanks to all who offered advice and support with this puzzle.

[peter_sol]

Glad you sorted it out.The solution was more or less as I had suspected.
When I sorted my hum problem I connected the bus bar and all the other chassis connections to a screwdriver and ran it across the chassis until it was at a minimum. yes I got some sparks but ended up with a completely hum free Amp.
I wonder how the manufacturers solve these things when convention fails?

[qualityten]

Thanks Peter. I’m gradually learning what I first read here years ago. A member, who is sadly no longer with us, wrote about his Mullard stereo 5-20 build that amplifier construction is ‘almost entirely about grounding’ or words to that effect. It puzzled me at the time, but I’ve never forgotten it.

[Goodizzy]

I have followed this thread with interest, as I am considering a build. I'm curious as to how you would say the amp sounds now after a few days of listening? What are you driving with the amp?

[qualityten]

I'm glad it's been interesting Goodizzy. I still have to tidy it up and I hope that this improves the residual hum.

It sounds very good; these late 1950s P-P transformers and the ECL82s are a very musical combination. I imagine that the transformers specified by Mullard would sound even better.

As this thread shows, I've had problems with the grounding scheme in the Mullard book. Although the layout produces a nice compact amplifier, I would recommend a different layout with a conventional ground bus arrangement, especially if, like me, you do not use exactly the same transformers that Mullard recommends.

In the midst of struggling with this build, I took a break to revisit a point to point build using ECF82 drivers and phase shifters and EL84 power tubes. That one has a ground bus and has almost no audible hum, unlike this one.