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Old 21st Mar 2020, 2:16 pm   #1
MAXPOWER
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Default Mullard 520 OP Transformer

Hello
I am hoping someone can offer some advice with an issue I have with a Mullard 520 amplifier. Sorry in advance if this post this a bit long winded.

Preamble
I have recently constructed a pair of Mullard 520 mono blocks following as far as possible the standard Mullard design. The only difference between the pair is the Transformer manufacturers. The first of the pair has a Majestic Power Transformer and a Hammond Choke and Output Transformer. The second has all Transformers supplied by Majestic. The first amplifier has been in service without any issues but the second has been problematic. After each amp was completed the DC conditions were measured and a max power check undertaken, all results were as expected. When used in service the amps sound great to me.

Issue
After very little use Amp No2 suddenly stopped working, on inspection I found that the 2A mains fuse had blown and after testing I found that the Power Transformer secondary No1 410/0/410 winding resistances has changed from 50 Ohms per leg to 45 Ohms and 19Ohms. The PT was sent back to Majestic for inspection and repair and I put the whole issue down to “one of those things”. I installed the repaired transformer and added two 500mA fuses to Secondary No1 legs, thinking (mistakenly) they would offer satisfactory protection, no other testing was undertaken apart from using the amp – it sounded ok to me. Unfortunately within very little time the whole episode repeated, exact same symptoms again. The PT has now again been repaired and is back in the amplifier, this time with 200mA Time Delay fuses on each leg of Secondary No1. As part of the testing I have measured the current drawn from both 410V legs and this is where my problems started…

At idle the PT Secondary No1 currents were 150mA and 147mA per leg, this seems high to me. A max power check into an 8 Ohm dummy load gives only 6Vrms before clipping, when in this condition Secondary No1 currents rise to 180mA per leg. I have removed the OP transformer and installed the Hammond OP transformer, the idle current drops to 142mA and at max power the current is 159mA per leg, the output across the 8 Ohm dummy load reaches 16Vrms before clipping. So the diagnosis seems to point to the OP transformer being the culprit and hence overloading the PT.

The Majestic OP transformer secondary has 2 separate windings as opposed to a single winding with numerous Taps (See attachment).
I have carried out the following tests on the Majestic OP transformer:-
1. Winding ratio test.
2. Primary DC Resistance.
3. Secondary DC Resistance.
4. 1000V Insulation test Primary to Frame.
5. 1000V Insulation test Secondary to Frame.

Questions
From the bench test results I cannot find any issues with the suspect OP transformer. Could someone please advise what I am missing? I would like to understand the root cause of my amp failing, swapping components and hoping for the best is never a good solution. What tests should I undertake to find the exact issue with the OP Transformer?
Thanks in advance.
Attached Files
File Type: pdf OP Tx.pdf (110.8 KB, 104 views)
File Type: pdf Test Results.pdf (408.6 KB, 88 views)
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Old 21st Mar 2020, 6:38 pm   #2
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Default Re: Mullard 520 OP Transformer

The output transformer currents seem high, I am not sure where you are measuring the currents but the mullard books quotes the output valve cathode voltages at 32V. The cathode resistors are 470 ohm which equates to 68mA per leg for the Anode plus Screen currents. In the original the mains transformer is only rated at 180mA and 40mA of that is to power the pre-amp!
I suggest you check the static conditions of the working amp especially the cathode voltages and compare with the faulty one.

I also note that the Magestic transformer has 43% taps for the screens whereas the Mullard design suggests 20% is optimal. This would not affect the static conditions however.

Peter
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Old 21st Mar 2020, 6:55 pm   #3
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Default Re: Mullard 520 OP Transformer

Beware of measurements of currents in a rectifier. The waveforms in the anodes of the rectifier will be zero current over half of a cycle and will only carry current when the anode is more positive than the reservoir capacitor in an HT filter with capacitive input. With choke input the conduction cycle will be extended. If over critical inductance the totality of diode on time becomes a full cycle

Ordinary multimeters can be scaled to read average current or true RMS. Rectified sine will give wrong-looking readings in most meters.

David
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Old 21st Mar 2020, 7:19 pm   #4
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Default Re: Mullard 520 OP Transformer

Hi All

Thanks for the replies. The current measurements I have listed are the AC current in the 410V Legs of the Power Transformer, I have 2 Fluke DVMs one in each leg. As I have burned out the secondary on two occasions I wanted to take a measurement of the current. I will check out the cathode voltages and update. I should have realised that the secondary should have been fused.

I have this afternoon, again swapped the Output TX for a new Majestic which was destined for Amp No1 (to replace the Hammond), I am now able to achieve (over) the full 20W output wattage, but now the output sinewave is ringing (Plot attached). This plot is with 1Khz Sinewave into an 8 Ohm Dummy load. I obviously have other underlaying issues. Sorry if I am now moving the goalposts. My aim now is to keep this Majestic OP Tx installed and sort out the ringing.

Unfortunately Amp No1 is stripped down as I am replacing the Hammond ironware for Majestic, so I don't have a baseline.

Jon
Attached Files
File Type: pdf Majestic No2 OPTx.pdf (113.2 KB, 124 views)
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Old 21st Mar 2020, 9:08 pm   #5
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Default Re: Mullard 520 OP Transformer

No plot seems to have attached. But ringing sounds rather odd. I'd only expect it when a square wave or pulse is used as an input signal. If ring-like bursts attach themselves to places on a sinewave, then it's a sign of instability. Quad 303s are notorious for adding bursts of 10MHz-ish if the power transistors are too fast and the 'quad triplets' go unstable at some currents.

What are you using as a load?

David
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Old 21st Mar 2020, 9:51 pm   #6
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Default Re: Mullard 520 OP Transformer

The dummy load is 2 x 4 Ohm 50 Watt wire wound resistors connected in series mounted on a suitable heatsink.

Jon
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Old 22nd Mar 2020, 8:52 am   #7
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Default Re: Mullard 520 OP Transformer

Some pics of your amps might be beneficial and help us sort the issue out. As a builder of valve amps I've had my fair share of SNAFU's, it can get discouraging but if you take your time you can get to the bottom of things and get these running well. It sounds like you had a bad OPT with insulation breakdown or a SC somewhere, that or incorrect connection of the OPT, mixing up of or mislabeled OPT primaries/secondaries.

This talk of legs is a bit confusing, to measure current I'd put my meter in place of FS1 - see schematic, that or put a 10 ohm resistor between L1 and the OPT (output transformer) center tap and measure the voltage across it, if I is normal, you should measure roughly 1.4v across it ( or a tad more accounting for the screen currents as mentioned) if anymore than this switch off and investigate. To check current for the IP valve and phase splitter check V across R17, they should only pull a few mA so a voltage drop of around 30v should be about right.

When I first run an amp though I take voltage readings for each HT, EG HT1 (in your case C15) HT2 (C12), HT3 (C5) then for every valve and write down the readings, something like this V1 a, g1, k, g2, this gives a good indication of the DC conditions of the amp and should reveal any problems. In this case V3 & 4's cathode readings need special attention.

Other things to note are buzzing, noisey & hot/warm transformers, smells which could indicate higher than normal current through resistors. Also might be worth checking you have the feedback connected properly and the amp isn't oscillating.

Andy.
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Old 24th Mar 2020, 10:42 pm   #8
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Default Re: Mullard 520 OP Transformer

This look familiar.
Are you sure that you have the anode and screen taps on the transformer going to the same valve ie anode 1,screen 1 go to valve 1 and anode 2, screen 2 go to valve 2?
Are the grid wires/capacitors near the anode/screen wires?

John
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Old 24th Mar 2020, 11:54 pm   #9
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Default Re: Mullard 520 OP Transformer

The scope photos show a burst of oscillation building up, maintaining pretty constant amplitude in the face of varying signal voltage, and then dying out gradually.

A ring would start with some abrupt transient, then exhibit an exponential decay from the start onwards.

This looks like instability and resulting oscillation. As John says, check the screen tap phasing on the valves. Attention to layout is important, also you should have non inductive resistors as those screen stoppers for the output valves.

David
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Old 26th Mar 2020, 6:16 pm   #10
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Default Re: Mullard 520 OP Transformer

Also check the negative feedback is the correct phase. If your flukes are not true RMS then the current readings will be of no meaning. As Andy says, get the DC conditions right first but beware as instability will destroy transformers.
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Old 26th Mar 2020, 7:12 pm   #11
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Default Re: Mullard 520 OP Transformer

Many thanks for all the comments and suggestions. Its much appreciated.

John, With regard to the OPT (Nameplate attached) and Valve wiring, I have the two EL34`s connected as follows:-

V3 Anode = Red (labelled Anode on OPT)
V3 Screen = Yellow (labelled 43%UL)
V4 Anode = Brown (labelled Anode)
V4 Screen = Blue (labelled 43% UL)
B+ = Black

Is it worth swapping V3 Anode and screen to V4 Anode and screen wiring? i.e. Red and Yellow to V4. For refence the oscillations get much worse at low power output and eventually stop at approx. maximum power.

David, sorry of this is a basic question but what is meant by the screen stopper resistors, are these R24 and R25? The 470 Ohm cathode resistors R20 and R21 are definitely wire wound.

I will check the DC voltages at recommended test points over the weekend and check against the Mullard published values. I`ve attached a couple of photos of the amp just after it was completed in 2018 (and was fully functional).

Can I just ask for an opinion on the Majestic OPT secondary configuration, the Hamond OPT has a single Secondary winding with Taps at 4/8/16 whereas the Majestic has 2 separate windings which need to be connected in series/parallel combinations to achieve 4/8/16.
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Old 26th Mar 2020, 7:16 pm   #12
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Default Re: Mullard 520 OP Transformer

This is certainly not the symptoms of positive feedback.
As David and I say it is oscillation of some sort and most likely in the output stage(s).
If you require non inductive resistors for the screen grid stoppers, RS, Farnell and CPC do them.

John

Last edited by John Caswell; 26th Mar 2020 at 7:17 pm. Reason: predictive typing
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Old 26th Mar 2020, 9:20 pm   #13
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Default Re: Mullard 520 OP Transformer

Mullard gave diagrams of how to route the connections of their amplifiers, and these were the results of some careful insight and careful trials.

The photos show an amplifier that looks to be a long way from Mullard's wire locations. Just this could be enough to create the oscillations.

David
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Old 26th Mar 2020, 10:59 pm   #14
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Default Re: Mullard 520 OP Transformer

Second that. It looks very "nice" but nothing like any period valve amplifier I've ever seen! Something of a triumph of presentation over substance, as they say.....
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Old 27th Mar 2020, 12:52 am   #15
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Default Re: Mullard 520 OP Transformer

The gain of the amplifier appear to be roughly correct 200mV=16W. The oscillation is at 162kHz and that would be around where the output transformer is introducing significant phase shift. It is interesting that it is asymmetric and occurs when dV/dt is greatest.

I guess the question is, does the instability show at lower volumes?
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Old 27th Mar 2020, 11:51 am   #16
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Default Re: Mullard 520 OP Transformer

Hi,
The screen stopper resistors are low value resistors 100R to 470R connected, preferably onto the valve base between the valve and the transformer wire.
It is quite common for oscillation to occur with high gm output valves and this is a tried and trusted method to "tame" the oscillation. Start with 100R 1/2Watt types first
I must agree with David re the layout, it really needs tidying up.
The other thing that just struck me is to measure the output transformer primary windings as follows - starting with either anode winding the resistance should increase as you go along the winding eg anode1 > screen1 > Centre tap/HT > screen2 > anode2.
I have seen and had transformers where the two screen windings had been mis-labelled with dire results.
I really do not think in practice, it matters too much whether there are multiple secondaries or a single tapped secondary. There are sure to be changes in distributed capacitances but would venture to say in real life this are negligible. My own transformers are Hammonds with tapped secondaries

John

Last edited by John Caswell; 27th Mar 2020 at 11:52 am. Reason: Addnl info
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Old 27th Mar 2020, 12:56 pm   #17
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Default Re: Mullard 520 OP Transformer

It does look as though you have enough wire in that amp to make 5 more. All wires should be as short as possible and go directly to the valvebase or other point. Yours seem to go all over the place and are far too long to get a stable amplifier. Those huge terminal blocks are another potential source for instability. Mullard spent time and money getting the amplifier layout correct and to deviate will almost always result in disaster. Look closely at the original Mullard layout to see how it should have been built.
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Old 27th Mar 2020, 2:22 pm   #18
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Default Re: Mullard 520 OP Transformer

Vidjoman - totally agree. Mullard went to a lot of trouble in their labs to come up with physical/wiring layouts that would guarantee stability. If you don't have a lot of knowledge/experience and some expensive test equipment, you deviate from that physical design at your peril... Those huge connector blocks extend the wiring in long loops into places it shouldn't go. Sometimes, "neat looking" definitely isn't beautiful! Yes, yes - look at the original Mullard blueprints!

Mike
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Old 27th Mar 2020, 5:59 pm   #19
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Red face Re: Mullard 520 OP Transformer

Thanks for the all the advice, being totally honest I am feeling more than a little embarrassed. I’ve not paid attention to the Mullard layout and wiring and can only acknowledge that on reflection this is a major oversight on my part.

From a wiring perspective a complete rewire can be undertaken when I understand the implication of following Mullards wiring guidance in conjunction with my component layout. So for now it’s some belated reading, followed by a spell in the workshop.

I do appreciate the comments and help from the wealth of knowledge on this forum.

As Arnie said I`ll be back!

Jon
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Old 27th Mar 2020, 10:05 pm   #20
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Default Re: Mullard 520 OP Transformer

It's a learning exercise!

Those of us who went "Oooo-Errr, it's the layout" all learned the importance of such things by getting it wrong and getting our fingers burned.

Welcome to the club

Valve amps involve high stage gains, and rather high impedances and so layout can be touchy. transistors have their own problems.

David
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