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Old 15th Jun 2020, 9:45 pm   #1
qualityten
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Default Mullard ECL82 stereo - where has the voltage gone?

I've just finished rebuilding my Mullard 7W stereo amplifier, following the Mullard circuit and layout closely. There are two deviations arising from the fact that the transformers are from a Dynatron stereo radiogram which was working before I removed them. The deviations are: (1) the output transformers do not have screen grid taps, so I have connected those points on the tagboard to the HT, via separate 1k5 resistors. (2) The power transformer only has one 6.3V winding, without a centre tap, so I have created an artificial tap where the 6.3V supply connects with the temporary indicator lamp. Apart from the variations mentioned above, I have followed the Mullard circuit and layout faithfully and have checked and re-checked all the wiring. The circuit is available here: https://www.bonavolta.ch/hobby/en/audio/ecl82_2.htm

The amp plays well and sounds nice in both channels, even though it is clearly not working correctly. There is a loud 50Hz hum and two other error conditions (at least!). These are:

1. The HT coming from the rectifier is 90VDC, though there is 475VAC going to the EZ81 anodes. This is reflected in low voltages in the rest of the circuit.

2. There is 22VAC on the ground circuit (which is not tied to the chassis ground connection).

I would be grateful for pointers to where I am losing the HT, as it was there in when I assembled this previously on an unsuitable chassis. I'd also value advice on whether there should be 22VAC on the ground circuit.

I attach a couple of pics. I am aware that primary leads to the OPTs and some of the other HT connections are too long at present.

EDIT: I have just grounded the circuit ground to the chassis ground and both the voltage errors are corrected! No AC on the ground circuit and the HT is more or less correct. BUT the loud hum still remains Though this a less interesting problem, I'd be grateful for advice on how to correct this, as it may be related to my use of different transformers.

David
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Last edited by qualityten; 15th Jun 2020 at 10:12 pm. Reason: Voltage problem corrected after posting
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Old 15th Jun 2020, 10:34 pm   #2
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

Do both channels behave the same way?
What do you mean by chassis ground and circuit ground and why did connecting them together fix your HT fault? Is it simply that you were measuring from chassis and that was not connected to 0V?
The transformers would only cause hum problems if they are being saturated and if they were designed for ECL82's they should be OK. Not having the UL connections will increase the open loop gain and that might cause instability.
Do your DC voltage measurements match those in the Mullard document? Most important is the ECL82 pentode cathode voltages.
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Old 15th Jun 2020, 10:42 pm   #3
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

Lookin' good David.

I like your chassis idea - only 2 bends to do!

I would doubt the hum is anything to do with the transformers, unless they're magnetically coupling to the mains TXF, but then it shouldn't be a 'LOUD' hum. I haven't got the circuit to hand but it looks as if your main smoothing can is in contact with the chassis? I seem to recall it should be isolated and tied to the main grounding point. Where are the ECL82 g2's fed from as there may be ripple on the supply? If you AC ground the input valves anode is the hum still present? Same with the its g1?
If it's the same on both channels then grounding or PSU seem likely. 50 or 100Hz hum?

Andy

PS Just seen that it's 50 Hz !
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Old 15th Jun 2020, 11:10 pm   #4
qualityten
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

Thanks for these pointers. I'll respond to them after work tomorrow.
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Old 16th Jun 2020, 10:05 pm   #5
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

Thanks for the diagnostic questions and observations given yesterday.

PJL, I can confirm that both channels do behave the same way.
By chassis ground I mean the ground connection between the IEC socket and the chassis, by circuit ground I mean the ground/earth connections on the circuit diagram which are not connected to the chassis. (I'd be glad to learn the correct terminology.)
I was measuring the latter when it was not connected to 0V!
My DC voltage measurements broadly match those in the Mullard document.
  • For the ECL82 pentode, the anode voltages are 12V lower, G2 is 20V lower and the cathode is 2V lower.
  • The triode voltages of the ECL82 are more or less as per the Mullard document.
  • For the ECC83, the anode voltages are about 20V higher and the cathode voltage somewhat lower, 0.9V instead of 1.5V. I'm surprised at that, as the HT supply is a bit lower than specified (262V from the Cathode of the EZ81.

Yestertech, my smoothing can is in contact with the chassis, but it is not in contact with the three capacitor grounds. I restuffed the can with three new 47uF capacitors. I've done this before with other projects, but I may have mixed up the grounds for C15 and C12 and C112, i.e. joined the two latter ones. Grounding G1 of the input valve has no effect. I don't know how to AC ground the input valves anodes.

Reading the section on diagnosing and eliminating hum in Morgan Jones' Building Valve Amplifiers, I think the AC hum is coming from near the reservoir capacitor. Unfortunately, I don't have high voltage oscilloscope probe to verify this. Any advice on where to obtain a suitable one for looking at >200VDC?

Thanks again for help with this. Apart from the hum, it sounds great! Am I right in thinking that this is likely to be a PSU issue?

p.s. The ECL82 G2s are being fed by the HT after the 100ohm resistor connected to the EZ81 cathode.

Last edited by qualityten; 16th Jun 2020 at 10:35 pm.
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Old 16th Jun 2020, 11:13 pm   #6
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

Is it 50Hz hum or 100Hz?
when you switch off does it decay or just stop abruptly?
What happens if you short the input?
What happens if you run it without the ECC83?
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Old 16th Jun 2020, 11:39 pm   #7
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

I am not sure what you have done reg the heater supply , try a 100 wire wound pot across
the 6'3 v and the centre to earth , adjust for min hum. if you don't have a pot wire two 50 ohm resistors across the heaters, earth the junction this should tell you if the problem is the heater supply , also try decoupling the g2's with a 22mf Mick.
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Old 17th Jun 2020, 1:18 am   #8
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

I think with your hum problem I would focus on your earthing arrangements. These Mullard circuits do work pretty unfailingly if you follow all the instructions exactly, and they do give details of how the earthing arrangements should be physically wired. Worth rechecking the exact connection points and routing of all the earth connections - including where they are connected to chassis.

Mike
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Old 17th Jun 2020, 9:03 am   #9
qualityten
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

A quick reply:

It's a 50Hz hum which stops abruptly on switch off. It remains even when I short the inputs and when I remove the ECC83.

I have 220ohm resistors, with the junction to earth across the heater supply. Is that value too high?
I have not tried decoupling the G2 with caps. Mick, do you mean 22uF or 0.22uF?

And thanks for the advice to recheck earthing arrangements. I have built 5-10s and 5-20s successfully without hum, but those have been mono.

Thanks for all the interest and advice.
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Old 17th Jun 2020, 2:09 pm   #10
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

A failure of one or the rectifier diodes in the EZ81 would create a 50Hz hum.

So would a bad connector in the valve holder.

A DC HT with a large amount of ripple could confuse some multimeters. I would not trust that 90V DC HT reading.

Ideally the reservoir capacitor C15 should not share a negative/signal ground connection with the other smoothing capacitors.

The negative of C15 should go to the 0V HT winding connection by the shortest and most direct route to avoid current pulses being superimposed on the signal ground.
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Old 17th Jun 2020, 4:04 pm   #11
qualityten
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

Silicon, thank you for these further areas to check.

I did test the EZ81 with another. But the valve socket I used for it is an NOS one that has been in storage for decades, while the others are new, so I need to check connections there.

Will check all my DC voltage readings, taken with my DMM, with those on my AVO 8. I intend using a dual can capacitor for C12 and C112, and using a separate cap for C15 with a separate ground connection.

Thank you for this help, and valuable bite-sized tuition.
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Old 17th Jun 2020, 10:38 pm   #12
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

Hi!

Have you correctly identified all the h.t. and l.t. secondary connections of the mains transformer correctly?

Switch off your amplifier, wait for the valves to cool and the h.t. smoothing capacitors to discharge, allow about 5 minutes minimum for this, then using a reliable ohmmeter, measure between pins 1 and 7 of the EZ80 and between pins 1 and 7 of the EZ80 and chassis.

You should get about 200–300 ohm between pins 1 and 7 of the EZ80 and about 100–150 ohms between pin 1 and pin 7 of the EZ80 and chassis.

If your reading is much higher or infinite, you have not correctly identified the centre–tap of the h.t. secondary of the mains transformer and/or returned it to chassis earth correctly.

Once you have identified and corrected, if necessary, the h.t. supply wiring, meter between pin 2 of each ECL82 and you should get a reading of about 1350 ohms, and about half of that between pin 2 of each ECL82 and earth.

These two checks verify the correct connection of each ECL82 tetrode section to chassis to give it it's correct grid bias.

Next, use your ohmmeter between pin 3 of the EZ80 and each ECL82 pin 6 to verify the output transformer primary is correctly wired to the h.t. supply.

Repeat the test between pin 7 of each ECL82 and pin 3 of the EZ80 to verify the screen–grid feeds to each ECL82 tetrode section.

Please confirm all the above is all correct so we know there's nothing radically amiss with your h.t. supply wiring!

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Old 17th Jun 2020, 11:22 pm   #13
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

Try running the heaters off 6V dc supply if you suspect this as source of hum.

I think this is the original design;

https://frank.pocnet.net/other/Mulla...rd2Stereo2.pdf
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Old 18th Jun 2020, 11:22 am   #14
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

That's not the 7WPC p-p ECL82 circuit.

https://www.radiomuseum.org/r/mullar...lifier_7w.html
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Old 18th Jun 2020, 12:06 pm   #15
qualityten
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

Thank you for the further pointers. I appreciate the time and interest it represents. Apologies for the length of what follows.

I think my problem is related to the fact that I'm following the Mullard schematic (incidentally not the SE design linked to by Restoration73, but a push pull circuit) but am not using the power and output transformers specified by Mullard. My PT only has a single 6.3v LT winding with one end connected to the centre tap of the HT supply. The output transformers do not have screen grid taps either.

I attach a picture of the power transformer. The HT and centre tap are clearly correct. I am less clear about the LT connections. At present, I'm supplying it all from the same winding, described above. I don't see how else to do it, and regret that I didn't record the original power supply arrangements very carefully.

Despite this, the Dynatron radiogram ran an amplifier and FM/AM radio section from the same transformers without hum. When I first attempted to move the amplifier to a more compact chassis over a year ago, successfully, but with some hum, I sought the circuit diagram for this circuit here and elsewhere, but without success.

Given the different transformers, the absence of detailed guidance in this chapter of the Mullard book and the fact that this particular design does not use a busbar, I am confused about how best to arrange the earth connections.

I think the principle is: One chassis earth connection close to the inputs. But, does this mean I should not use RCA sockets that are insulated from the chassis, as I have done?

Other earth connection questions, physically on the chassis:
Where is the 0v reference point?
Where should the IEC socket earth wire connect to the chassis?
Where should C15 earth connect to?
Where should C12 and C112 earths connect to? The multisection can is visible just above the white 100ohm resistor in pic 3.

And where should I connect the earths of all the other electrolytic capacitors: C8/108, C5/105, C13/113, C14/114, C7/107.

I have checked and rechecked the tag board and its connections to the valves and capacitors on the Mullard diagram (as far as these are given), but wonder whether my different power transformer requires a different layout or more decoupling somewhere?

Last night I wondered whether to add a busbar to make checking the earth connections easier, but I'll now wait for advice on the earthing arrangement. Sadly, I don't think this particular chapter of the Mullard book is available in full on line, but I can post a picture of the tag boards and layout, if this would help.

I also thought I'd wait for this before doing the checks Chris Williams mentions. Thanks for them!
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Old 18th Jun 2020, 12:21 pm   #16
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

Can you clarify something?
In post #1 you said, ''The power transformer only has one 6.3V winding, without a centre tap, so I have created an artificial tap where the 6.3V supply connects with the temporary indicator lamp.''
Yet in the last post you have, ''My PT only has a single 6.3v LT winding with one end connected to the centre tap of the HT supply.''
Which is it? You cannot have both?
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Old 18th Jun 2020, 3:06 pm   #17
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

Sorry wrong link, built that one, also this one, many years ago;
http://www.r-type.org/articles/art-162.htm
I remember one of the Dansettes had this circuit. No ultra linear taps one the basic
output transformer. In hi-fi, the 2 x ECL86 circuit seems more prevalent. I hope you
can get it working OK. The technique used in the Mullard 5-10 was to have a continuous
earth bus bar running from input to output. I confirm that worked !
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Old 18th Jun 2020, 3:33 pm   #18
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

Hi!

https://www.google.com/url?sa=t&sour...mdzLRNG9gWIVng

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Old 18th Jun 2020, 3:46 pm   #19
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

Okay, read your post # 9 now and looked at the pictures in more detail, I see a choc bloc by the mains (?) switch with 2 resistors that I assume is the faux centre tap, yes?

For that to work the 6.3 volt winding must not have either side connected to ground/ 0 volts. It has to be just the junction of the 2 x 220 ohm resistors. Out of interest, they should be close to the transformer and the heater wiring go out from there. My choice would be 47 ohm 2 watt resistors too.

Your original problem was you had missed a ground wire. Can you show where you fitted the one that fixed the 'voltage' fault? And give an idea how it connects to the other components?
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Old 18th Jun 2020, 3:48 pm   #20
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Default Re: Mullard ECL82 stereo - where has the voltage gone?

Snowman Al, this is what I mean about the 6.3v LT supply.

The 2 x 220ohm resistors were originally 2W resistors connected at the transformer, earthed to the CT of the HT terminals, but I read somewhere that this virtual tap could be connected near the indicator lamp.
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