Mullard 5-20 high DC Voltages

[turretslug]

Hanging large power resistors between tags of even an octal valve socket is a bit fraught though, especially when there is a definite need for them to be well separated because of their high potential- and they need to stay that way. An even bigger single centre-tap-to-chassis resistor can at least be strung between two likely firm anchorages i.e. transformer centre tap and chassis metal and, if the chassis lead is arranged to be short and via a sturdy 2BA etc. tag, it may even benefit from heatsinking- not the case with a valve socket already contending with the proximate effects of 10-15W of heater power plus HT loss! It doesn't have to be kept away from its mate on the other side of 410-0-410VAC, i.e. some 1200V peak either.

There are a few arguments for-and-against.

[G6Tanuki]

I'm always thinking of ways to minimise component-count, on the basis that "parts that are not there cannot become a cause of failure" and "fewer soldered joints involved in assembly mean greater profit for me".

To some extent us 21st-century types have things easy - we can get nice metalclad power-resistors with good high-voltage insulation that can be clamped to a heatsink and keep things cool.

The 1950s/60s designers of amplifiers didn't have such conveniences - but surely now that we do have access to these things it makes sense to use them?

[Techman]

The bottom line of this is that the output valves should not be red plating whatsoever as they both clearly are in the third picture in the first post on this thread.

[PJL]

In guitar amps they will run EL34B's at crazy HT voltages and dissipation - they don't last long though!

From what I can see, the AC HT voltage is lower than the figure in the 5-20 datasheet but the resultant DC voltage is too high. There are several possible causes and it might be a combination:
1. The choke is 117 ohms but Mullard specified 200 ohms (this accounts for around 12.5V)
2. The increased smoothing capacitor value is raising the HT (C15=47u, C12=10u)
3. The GZ34 is more efficient than a vintage Mullard
4. The 5-20 datasheet measurements were made with a pre-amp attached
5. The mains transformer has better regulation than the one used by Mullard

Try swapping C15 and C12 over as the 47uF as the reservoir is working the GZ34 very hard.
If HT is still too high then increase R26/R27.

Increasing the cathode bias resistors to decrease the current will further increase the HT volts.

[Diabolical Artificer]

Thanks Alan, missed that. Nice layout, you've done this before and have a neatness bug : ) Only observations are don't like spade terminals, they come loose. Wish I had a fiver for every vintage amp where they were flopping about, HH loved the silly things, that goes for motor bikes and cars too, though not a fair comparison. Not keen on those small (Vishay ?) resistors after having several go high and shunt EL34 cathodes into out of space.

Those figures are high. in my EL34 amp the EL34's are sitting at 30mA, half yours, no X over distortion. The grid lines go down to something like - 60v at 450v UL, your no where near that.
HT is high too. From mucking about with amps 420v should be able to deliver 20 watts easily.

So as PJL says above, reduce the HT, if EL34 dissipation hasn't come down much, pop larger Rk's on. Any red plating or valves sitting at high quiescent current is just shortening valve life, better to use a resistor to lose excess power, cheaper than valves.

Andy

My previous post was trying to suss if you have any oscillation or high ripple on the HT

[Kei-1986]

The majority of the resistors used in this are Dale. (which are part of vishay group) This is the BOM I put together. (eye watering really mostly because of the Sowter outputs)
https://docs.google.com/spreadsheets...it?usp=sharing

I spent some time going over the datasheet for GZ34/5AR4 and realised that there is a minimum resistance per anode that I'd not realised would need to be checked.

https://***********************/65535/...e9d37933_h.jpg

The valve wizard site also happened to have a handy formula for calculating it. (Rlim = Rsec + Rpri × (Vsec/Vpri)^2 + any extra resistance)
V Pri = 250
V Sec = 398
R Pri = 5.7
R Sec = 88.5
Result = 102.9464448

This put me a tad short of the 125 ohm minimum at 2x400V. (although they only list it for 60uF filter) I spent this morning having a good dig through my component stash and found a pair of 5W 60R vintage RS branded resistors which I've fitted in the 410-0-410 feeds from the HT fuses to the rectifier socket.

https://***********************/65535/...03ce6d1e_h.jpg

These are the measurements I got with these 60R limiting resistors in place. The numbers look significantly closer to the book figures now. I'm doing some testing this evening and will check with the camera to see if there is any sign of glow in the anodes.

Mains voltage - 237.3Vac
HT voltage - 391Vac
C15 - 458Vdc
C12 - 441Vdc
C5 - 410Vdc
C4 - 165.5Vdc

V4 Anode - 431Vdc
V4 G1 - 0.7Vdc
V4 G2 - 433Vdc
V4 G3/Cathode - 30.9Vdc
Heater - 6.0Vac

V3 Anode - 431Vdc
V3 G1 - 0.6Vdc
V3 G2 - 432.5Vdc
V3 G3/Cathode - 30.4Vdc
Heater - 6.0Vac

V2 Anode 1 - 303.6Vdc
V2 Anode 2 - 303.7Vdc
V2 G1 - 88.4Vdc
V2 G2 - 80.4Vdc
V2 Cathode 1&2 - 92.8Vdc

V1 Anode - 88.8Vdc
V1 G1 - 0 (no signal)
V1 G2 - 110Vdc
V1 G3/Cathode - 2.1Vdc
Heater - 6.2Vac

[PJL]

Did you try swapping over C12 and C15 so that the 10uF is the reservoir? This should reduce the HT volts. The Beam Echo DL7/35's 5-20 clone that I have use a 4uF reservoir (C15) and 40uF smoothing (C12) and have no noticeable hum. This has the benefit of providing more reserve power than if it were fed through the choke from C15.

[Kei-1986]

Originally Posted by PJL

Did you try swapping over C12 and C15 so that the 10uF is the reservoir? This should reduce the HT volts. The Beam Echo DL7/35's 5-20 clone that I have use a 4uF reservoir (C15) and 40uF smoothing (C12) and have no noticeable hum. This has the benefit of providing more reserve power than if it were fed through the choke from C15.

Not exactly, i simply removed the 47uF capacitor and put another 10uF capacitor in its place. There was absolutely no change in the measured voltages.

I ran the amp all evening without any issues. I cannot see even a hint of glow on the anode with my own eyes, even in pitch black conditions. The camera can still show a very slight tinge in the same place as before. I have also noticed that the amp picks up stray noise from other devices in my room but no noise from itself. I'm guessing that this is an artefact of the complete wooden case not acting as an emi shield.
https://***********************/65535/...a98bfa46_h.jpg

[Diabolical Artificer]

That looks a bit better, interesting that the EL34 screen grids are higher than the anodes though, how can that be?

Andy.

[snowman_al]

Ultra-linear, so less turns on the output transformer and less volts dropped.

[rontech]

If the valve is red plating then its maximum anode current limit is being exceeded. Some current measurements might reveal something.

Whilst doing this beware high voltages on the DMM though!

[Kei-1986]

Some of the voltages were wandering around on V3 & V4 by 2V or so when I measured them. This was with the 10uF capacitor still in the C15 position. I think the value of these resistors needs to go up a bit as it’s still nudging 26W and that’s still on the 250V primary with only 237V going into it. I only used 60R as that is all I had a pair of that was close to the 100R I wanted.

I noticed that the bias calculator site takes 5.5% off the anode current number for screen current which is where I originally got my 24.8W from for 400V anode / 30.9V cathode with 470R.

[Kei-1986]

Since I have a good idea on what needs to be done on the voltages, I spent some time at the weekend looking at the stability of the amp. It seems that it likes to oscillate at 70-80KHz which based on my reading is probably the resonance of the output transformer. In order to try and mitigate some of the ringing, I've looked at adjusting the feedback network and the step network to suit my components.

These are all the scope shots I took.

Raising the capacitance of C9 by 68pF (to 398pF) didn't seem to have as much effect as I was expecting. I also tried increasing the value of R13 to 6K8 which also didn't appear to have much effect. What did have quite a pronounced effect was completely removing the step network (C1/R3) All of the ringing on the square waves disappeared. I tried adjusting R3 and C1 to higher values but this didn't seem to have the desired effect.

What I did notice, not that I tested this when it was standard, is that sub sonic inputs past a certain level definitely caused the amp to oscillate quite badly. Now I understand that I could simply filter this out by fitting a ~0.01uF capacitor in series from the input to R2. I wonder if this is more of a "sweeping it under the rug" type of fix. I also wonder how much of the stability issues I am seeing are from the changes to the layout vs the potential differences in my components. R14 and R15 have a much longer path to ground than they would in the standard design as they are on the tag board. The output transformer primary wires are a bit longer too as are the wires leading from the phase splitter to the tag board.

OPT Primaries
https://********/p/2iLZogM

Phase split long wires
https://********/p/2iNVjSW

Something else that crossed my mine during all of this is that R11 & R12 in mine are to all intents and purposes identical as they are 1% parts. Mullard say R12 should be ~3% larger in value.

[bikerhifinut]

OK I am a bit late into this post, apologies but I've had some family health issues to deal with.. nuff said.

I've had a fair bit of experience of late with EL34 power amps and frankly assuming your EL34s are on spec and reasonably robust, 450volts give or take on the anodes is within spec. The leak TL25+ figures are for 452V measured with an AVO8. That'll give you roughly 35 watts at clip, I measured 37watts on my original Leak TL25 plus clones. On those amps I used the 5-20 output stage, 470R cathode resistors rather than the 440R that Leak specified, that's still inside a 10% tolerance for the resistors. I used the 680k gridleak resistors that Harold Leak did in his design rather than the 470k of the Mullard design. The Leak really squeezes the EL34 hard but during the couple of years those monoblocs were in regular use I had no unexpected failures.
The current version is a stereo job using a different transformer with silicon rectifiers for around 410V HT, this gives around 32 Watts at clip.
The GZ34 should be perfectly fine at those voltages, It's supplying around 150 to 160 mA for a typical push pull EL34 amp in cathode bias.
You should be measuring around 35V on the cathodes again give or take due to valve variations. Around 70mA per EL34 is ok.
I don't think its worth the mess around to try and achieve the "perfect" set of voltage measurements on this sort of design and as long as you are in the "ball park" with the figures I'd not worry. By the way the reason the TL25 was specced at 25W was because that was the output that the distortion was still inside Harold leaks claims, marketing isn't a new idea!
As to the redplating, I bet you are using Russian EL34 in small glass envelopes? Joe Bog put me wise to these and I agree there that they might be ok at sub 400V HT but when used at more "traditional" voltages they can struggle. I moved over to the much maligned but rather cheap if you shop around Chinese34s I suspect made by the Shuguang factory. I used a matched set of rebranded "Brimars" but the RS jobs look identical so check prices as they vary.
The Chinese valves have so far given me no problems.

I'll not delve into the 5-20 arguments, I took a conscious decision to go down the Leak adaptation of the design with its capacitor coupling and I also redesigned (if that's the right word) the input stage and global feedback to get the input sensitivity right down, in fact the final version is running at about 1volt sensitivity but it suits me fine and the hum and noise levels are now inaudible, Peter Walker knew what he was doing when he designed the Quad11 to have 1.5V for full output. The 5-20 and leak designs date from an age when sources weren't as high out put as today.

Finally (deep breath) don't worry about the anode resistors on the longtail pair phase splitter, there may be an advantage in the different values but the amp works fine as is. Leak always did use 91k and 100k on the phase splitter anodes but those would have been hand picked resistors for that purpose. By all means use 1% resistors there and get them technically perfect but it will not affect any measurements that materially affect the amp.

Best regards

Andy.

PS if you haven't got the leak circuits and relevant info drop me a PM. They make a useful cross reference.

[Diabolical Artificer]

"That looks a bit better, interesting that the EL34 screen grids are higher than the anodes though, how can that be?".....Ultra-linear, so less turns on the output transformer and less volts dropped. " Yes Al, but they were the other way round in post #1.

Re the oscillation you could reduce the HF gain by putting a 2200p and 1k 5w in series from each anode to ground, R goes to gnd, cap to anode.

Re the Vishay/Dale 2w resistors I had several 10r go as high as 1m +, others several hundred k, that's just sitting at below 1v or as high as 2/3v, so well below their power dissapation, if you start getting odd voltage readings check their resistance.

Andy.

[PJL]

The apparent instability at low frequency does not appear to be 80KHz and may be the HT collapsing. C1/R3 are there to improve stability and I am not sure that you should be drawing conclusions based on observations made at overload when driving a purely resistive load. Removing C1/R3 might allow the feedback to suppress ringing but that is not telling you how it will behave with a capacitive load.

[Kei-1986]

Originally Posted by bikerhifinut

OK I am a bit late into this post, apologies but I've had some family health issues to deal with.. nuff said.

I've had a fair bit of experience of late with EL34 power amps and frankly assuming your EL34s are on spec and reasonably robust, 450volts give or take on the anodes is within spec. The leak TL25+ figures are for 452V measured with an AVO8. That'll give you roughly 35 watts at clip, I measured 37watts on my original Leak TL25 plus clones. On those amps I used the 5-20 output stage, 470R cathode resistors rather than the 440R that Leak specified, that's still inside a 10% tolerance for the resistors. I used the 680k gridleak resistors that Harold Leak did in his design rather than the 470k of the Mullard design. The Leak really squeezes the EL34 hard but during the couple of years those monoblocs were in regular use I had no unexpected failures.
The current version is a stereo job using a different transformer with silicon rectifiers for around 410V HT, this gives around 32 Watts at clip.
The GZ34 should be perfectly fine at those voltages, It's supplying around 150 to 160 mA for a typical push pull EL34 amp in cathode bias.
You should be measuring around 35V on the cathodes again give or take due to valve variations. Around 70mA per EL34 is ok.
I don't think its worth the mess around to try and achieve the "perfect" set of voltage measurements on this sort of design and as long as you are in the "ball park" with the figures I'd not worry. By the way the reason the TL25 was specced at 25W was because that was the output that the distortion was still inside Harold leaks claims, marketing isn't a new idea!
As to the redplating, I bet you are using Russian EL34 in small glass envelopes? Joe Bog put me wise to these and I agree there that they might be ok at sub 400V HT but when used at more "traditional" voltages they can struggle. I moved over to the much maligned but rather cheap if you shop around Chinese34s I suspect made by the Shuguang factory. I used a matched set of rebranded "Brimars" but the RS jobs look identical so check prices as they vary.
The Chinese valves have so far given me no problems.

I'll not delve into the 5-20 arguments, I took a conscious decision to go down the Leak adaptation of the design with its capacitor coupling and I also redesigned (if that's the right word) the input stage and global feedback to get the input sensitivity right down, in fact the final version is running at about 1volt sensitivity but it suits me fine and the hum and noise levels are now inaudible, Peter Walker knew what he was doing when he designed the Quad11 to have 1.5V for full output. The 5-20 and leak designs date from an age when sources weren't as high out put as today.

Finally (deep breath) don't worry about the anode resistors on the longtail pair phase splitter, there may be an advantage in the different values but the amp works fine as is. Leak always did use 91k and 100k on the phase splitter anodes but those would have been hand picked resistors for that purpose. By all means use 1% resistors there and get them technically perfect but it will not affect any measurements that materially affect the amp.

Best regards

Andy.

PS if you haven't got the leak circuits and relevant info drop me a PM. They make a useful cross reference.

I have the hifi engine copies of the Leak Circuits. They do indeed look pretty similar but definitely have some big differences I've also got a pair of JJ KT77's which I've been trying today. They seem to perform every bit as well as the Tung Sol's with the benefit that I cannot detect even a hint of glow to the anodes. (yay) Brimar were already on my list but I don't think I can necessarily get them at present what with most places being shut.

Originally Posted by Diabolical Artificer

"That looks a bit better, interesting that the EL34 screen grids are higher than the anodes though, how can that be?".....Ultra-linear, so less turns on the output transformer and less volts dropped. " Yes Al, but they were the other way round in post #1.

Re the oscillation you could reduce the HF gain by putting a 2200p and 1k 5w in series from each anode to ground, R goes to gnd, cap to anode.

Re the Vishay/Dale 2w resistors I had several 10r go as high as 1m +, others several hundred k, that's just sitting at below 1v or as high as 2/3v, so well below their power dissapation, if you start getting odd voltage readings check their resistance.

Andy.

OK, I will keep an eye on those dale resistors. It may come to limiting HF gain but I'd like to avoid making big changes to the original mullard design if I can.

Originally Posted by PJL

The apparent instability at low frequency does not appear to be 80KHz and may be the HT collapsing. C1/R3 are there to improve stability and I am not sure that you should be drawing conclusions based on observations made at overload when driving a purely resistive load. Removing C1/R3 might allow the feedback to suppress ringing but that is not telling you how it will behave with a capacitive load.

I don't think the HT is collapsing. The observations on the step filter have been made with normal signal levels and overload, both sine and square, although admittedly, only with a resistive test load.

I've tried putting a 0.01uF capacitor on the input and the results have been very good in the low end but a strange artefact (not sure if it is related) is that the EF86 output starts to distort with a 10KHz or higher sine wave input at ~180mV which is giving an output power around the 14W mark. I'm also seeing 180 degree phase inversion by 50KHz with the feedback reconnected, which is 20KHz sooner than it would when run as open loop.

I am giving serious thought to the possibility that either my layout is wreaking havoc with this thing's stability or the output transformer just isn't up to scratch for the amount of feedback employed. (which I doubt considering they are Sowter specific to this design, they are huge, they weigh a tonne and they cost a small fortune) I'm thinking I'm going to have to build the second unit on another spare bit of wood but follow the mullard layout verbatim where I can.

[Kei-1986]

I decided that I would probably end up sinking more time than necessary trying to debug the amp when the most likely cause of the problems was the layout. So, the Mk 2 Version, when all else fails, do it by the book. This is pretty much as close to the book layout as I can get. I've left the chassis about 1.5" wider as this will give me room for some wooden supports without affecting internal space. I've kept the component locations within a mm or two of their original positions and the valve sockets are orientated exactly as per the book. The capacitors may create some fun with the wiring but I don't think it'll be far off standard. The two capacitors that live between the 9 pin valves will be the hardest as it's supposed to be a multi-element electrolytic which I don't have. I chose to go with a pair of 10uF MKP caps originally so I foresee either some major fiddling or me buying a multi-element cap. I also see me having to trim the tag board as it's a bit wide and tall to fit in and permit space to run a ground bus around.

[IMG]https://***********************/65535/49813499213_7dad9b442c_h.jpg[/IMG]

The connections to the output transformer from the output valves in the standard design are significantly shorter and the B+ runs in the opposite direction too. (I have a sneaking suspicion that this may be a big contributor to the stability issues I was having in the Mk1) I'm also planning on connecting up the mains switch this time. I may even rectify the one 6.3V heater line in order to get the power LED to work too. (Though I'll probably save that for when I prove if it works stably or not)
[IMG]https://***********************/65535/49813499493_289998b5b1_h.jpg[/IMG]

[Diabolical Artificer]

Your layout looked ok on MK1, as a builder of valve amps I've run into stability problems a few times, once by moving a gnd two inch's. The HF sort I usually fix as described, RC from anode to gnd to reduce HF gain, same with V1 anode, not perfect but my grasp of nodes, poles and imaginary numbers is skant.

Your OPT won't be exactly the same as that used in the 5-20 prototype, so your probably going to have to tweak the circuit. this after keeping PSU wires etc as short as poss, component leads short etc, you know the score. High current gnd's like OPT and OP stage on gnd bus first, lower I stages last.

As regards LF instability read what it says in the Radio Designers Handbook, a good PSU helps here as David has mentioned here and in other threads.

Good luck, Andy.

[Kei-1986]

Progress on the MK2 has been pretty good so far. I've concentrated on the woodwork side of things using yet more scrap bits. The chassis is basically complete. I've decided to put significantly more tag strips in this time to help distribute the wiring about more tidily and following the mullard layout pretty much to the letter. All the large can capacitors are mounting onto the side panels with the terminals being kept as close as possible to their normal locations. I also shrunk the tag board down significantly. There is so much more space inside now, it's much easier to work on.

[img]https://***********************/65535/49823305327_e774ebaad7_h.jpg[/img]