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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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8th Apr 2021, 7:30 pm | #1 |
Octode
Join Date: Feb 2009
Location: Wallasey, UK.
Posts: 1,310
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Mullard 3-3 voltages query
I'd be grateful for advice on my newly built 3-3.
I recently bought an old SE output transformer harvested from a radiogram running a single EL84. I decided to assemble a Mullard 3-3 to test it and I now have that amplifier working. Although it sounds good, there are some problem voltages. I'm measuring voltages with a Fluke 87 DMM with good batteries. Specified voltages in brackets below. I have a working AVO8 to hand. I've studied all the threads on the 3-3 here and know that correct voltages are especially important for correct operation of this circuit. I have checked that the primaries of the output transformer are correct and checked and re-checked my assembly and solder joints. My power transformer has a separate 5V winding, so I'm using a GZ30 rather than EZ80 for the rectifier. After a 150R dropper resistor R15, I get 297V (310V) at the first 50uF capacitor (C9). I guess I could use a lower value resistor to raise it a bit. However, the voltage at C6 is significantly lower than it should be at 253V (290V). I'm puzzled by one symptom: when I measure the voltage at the EL84 anode (pin 7), the sound goes off and I get 250V. But when I touch the grid of the EF86 with DMM probe, the sound returns. The other problem is higher/lower voltages on both valves: EF86 anode: 48V (20V) EF86 G2: 49V (28V) EF86 cathode: hardly any voltage and nothing specified in the circuit. EL84 G2: 259V (290V) EL84 cathode: 50V (28V) Is the disappearing sound a useful diagnostic symptom? |
8th Apr 2021, 8:15 pm | #2 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Mullard 3-3 voltages query
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8th Apr 2021, 8:34 pm | #3 | |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,567
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Re: Mullard 3-3 voltages query
Quote:
These really are excellent little amps (I've built three in the past) but you really need to be careful with layout and follow the Mullard wiring as closely as possible.
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8th Apr 2021, 8:35 pm | #4 |
Nonode
Join Date: Mar 2005
Location: Nuneaton, Warwickshire, UK.
Posts: 2,038
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Re: Mullard 3-3 voltages query
Is there a smoothing choke? If so, could it be dropping too much voltage?
Aub
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8th Apr 2021, 9:26 pm | #5 |
Octode
Join Date: Sep 2017
Location: Bristol, UK.
Posts: 1,654
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Re: Mullard 3-3 voltages query
You didn’t forget to fit the 10MΩ grid-leak resistor on the EF86, did you? I’ve done that myself with this amplifier - easy to miss because in many/most input circuits you might be expecting the grid to get a DC path to ground through the volume control. But in this circuit the v.c. is AC-coupled to the grid, so the valve really needs that 10MΩ to ground.
Mike |
8th Apr 2021, 9:42 pm | #6 |
Octode
Join Date: Feb 2009
Location: Wallasey, UK.
Posts: 1,310
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Re: Mullard 3-3 voltages query
I used a chassis I had to hand, but have followed the layout in the Sterns construction manual carefully.
The 10M grid leak resistor is there. The OPT connections are temporarily quite long. Could that be an issue? I also need to get a 560 K cathode resistor for the EL 84. Can I view the oscillation on my scope? |
8th Apr 2021, 9:44 pm | #7 |
Octode
Join Date: Feb 2009
Location: Wallasey, UK.
Posts: 1,310
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Re: Mullard 3-3 voltages query
Here is a pic of the layout.
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8th Apr 2021, 10:18 pm | #8 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
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Re: Mullard 3-3 voltages query
Long connections to the output transformer could easily be the problem. I presume you mean a 560 ohm cathode resistor? what value are you using now as 50V on the EL84 cathode equates to 90mA and will burn out the EL84...
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8th Apr 2021, 10:26 pm | #9 |
Octode
Join Date: Feb 2009
Location: Wallasey, UK.
Posts: 1,310
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Re: Mullard 3-3 voltages query
Sorry, yes 560R. I’m using 470+100 in series at present. Should I try increasing it? I see the Radio Constructor article of April 1956 uses 680R.
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9th Apr 2021, 9:05 am | #10 |
Octode
Join Date: Dec 2018
Location: Llandeilo, West Wales, UK.
Posts: 1,092
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Re: Mullard 3-3 voltages query
Sorry to go against the grain, but I do not like the 3-3 design.
For every one you have working in spec, there are 2 failures... They are so dependant, amongst several factors, on the EF86 and EL84 characteristics being the same as the originals used in the 'model' amp. First step would be try several other EL84s first, then other EF86s. You do not say what value cathode resistor you have at the moment? Also a random output transformer might need feedback changes from the Mullard values. Have you tried it with the feedback disconnected? If all else fails, do what the BBC did, and AC couple the EF86 to the EL84 as in their AM8/2A circuit... Alan
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9th Apr 2021, 2:12 pm | #11 |
Dekatron
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Location: Seaford, East Sussex, UK.
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Re: Mullard 3-3 voltages query
The EF86 anode load looks like 1.5M not the 1M in the Mullard design?
The EF86 anode voltage reading in the Mullard documentation is lowered by the 2M AVO 8 input resistance. Even the 10M of your DMM will have an impact. You need to make sure it is not oscillating as this will affect the DC measurements. If it is stable then the EF86 is probably weak. |
9th Apr 2021, 2:46 pm | #12 |
Heptode
Join Date: Jul 2009
Location: Stoke-on-Trent, Staffordshire, UK.
Posts: 719
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Re: Mullard 3-3 voltages query
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9th Apr 2021, 2:58 pm | #13 |
Dekatron
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Re: Mullard 3-3 voltages query
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9th Apr 2021, 3:58 pm | #14 |
Octode
Join Date: Dec 2018
Location: Llandeilo, West Wales, UK.
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Re: Mullard 3-3 voltages query
Well spotted!
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9th Apr 2021, 4:40 pm | #15 |
Octode
Join Date: Feb 2009
Location: Wallasey, UK.
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Re: Mullard 3-3 voltages query
Pete, I owe you a beer!
It was the one thing I didn't worry about, even though Mike (Boulevardier) drew my attention to it yesterday. Thanks to all. What a pleasure to see all the correct voltages slot into place. And it does sound good with the large OPT. I attach pic. Since last night, I had already tried: 1. raising and lowering the value of the EL84 cathode resistor; 2. swapping the OPT secondaries, following Morgan Jones' advice on oscillation; 3. shortening the leads to the OPT (necessary anyway); 4. tried a number of different EF86s and EL84s. This a bit of a milestone for me. With much help from members here, this non-engineer has now built all the Mullard amps in the book (5-20, 5-10, 7W stereo and now the 3-3). David |
9th Apr 2021, 5:04 pm | #16 |
Octode
Join Date: Sep 2017
Location: Bristol, UK.
Posts: 1,654
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Re: Mullard 3-3 voltages query
Congratulations, David! Well worth the perseverance. I think, for all their simplicity, these are jolly good little amplifiers. I've often wondered if, if you put one beside a 5-10 and equalised the gains, it would be difficult to tell them apart on an A-B blinded test. And, using the sensitive loudspeakers of its period, 3W is quite a surprising level of noise in a domestic environment.
Mike |
9th Apr 2021, 5:12 pm | #17 |
Dekatron
Join Date: Sep 2015
Location: Southwold, Suffolk, UK.
Posts: 8,327
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Re: Mullard 3-3 voltages query
Wow - that's a massive OPTX for a single-ended amp like this!
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Edward. |
9th Apr 2021, 7:23 pm | #18 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
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Re: Mullard 3-3 voltages query
Hi David, you can't stop now. There is a Philips Tech library book by Rodenhuis
"Hi Fi Amplifier circuits" with yet more of them for you to build Ed |
9th Apr 2021, 10:49 pm | #19 | |
Octode
Join Date: Feb 2009
Location: Wallasey, UK.
Posts: 1,310
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Re: Mullard 3-3 voltages query
Quote:
Ed D, I downloaded the Rodenhuis book some years ago and do consult it, but I still turn to the Mullard book first as it's a print copy. I'm contemplating a Maplin Millennium type project next. I have the OPTs and a huge Leak PT. |
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10th Apr 2021, 8:37 am | #20 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
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Re: Mullard 3-3 voltages query
I use a 10k 10 turn pot to find the right R FB value; connect between IP & OPT, set at 10k, start lowering until the amp starts to get unstable, then back off a bit. To find the C, get a capacitance decade box,IP a 1khz square wave to the amp, monitor OP on a scope, play with values till you get a nice sq wave, then do the same for 10khz. This is the simple procedure.
To sort an amp properly you need to do lots of sweeps from 20hz (lower if you can) - 100khz at various OP levels with a resistive load, then a capacitive load.This reveals it's instability points, what your looking for is where the OP peaks or where THD shoots up. When you look at your results you'll see something like "peaks at 48.3khz & at 58khz" or peaks at 10hz & 42khz. You then select your FB RC values to form a HPF at the problem frequencies using 1/(6.28 * f * r or c). It's also a good idea to pop a Zobel or Bucheret network on the OPT secondary using the same data & method. What your doing is putting RC filters in various places of the amp to stomp on any instability or silly behaviour. This is why you'll see an RC in parallel with the IP stage anode resistor or small value (picofarad) caps in parallel with other parts of the circuit in an amp,broadly speaking that is. This results in a stable amp, stable with all loads. It can be very time consuming but it's worth it. Andy.
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