Mullard 5-20 high DC Voltages

[Techman]

Chris - the small valves are in the 'high impedance' and sensitive to these things, part of the circuit and would have full metal valve shields fitted as standard. The output valves are in a low impedance part of the circuit, so are not affected. The high quality transformers, although unlikely to be affected, will all have shrouded windings, so the only thing to ruin this brilliantly designed amplifier is to build it on an un-shielded chassis. Built on a proper chassis as designed originally, with good quality shielded connecting cable feeding the input/s, there would be no problems with this amplifier picking up anything it shouldn't.

The good thing about the OP using a wooden box is that it's sparked off a very interesting debate and I'm sure the OP has learned a lot in the process, so it's not all bad.

[Herald1360]

I don't disagree with building on a metal chassis as original per se and its benefits for hum levels and stability, just with the idea that it would be stray RF proof. Making the amplifier literally RF immune would be a whole different game.

Looking at the original 5-20, there's a skirted holder on the EF86 but no can and not even a skirt on the ECC83.

http://www.r-type.org/articles/art-003d.htm

[PJL]

Whilst I agree that layout and chassis material is important, many commercial amplifiers are not fully enclosed for example the Dynaco ST70 even exposes the PCB and all the amplifier components on the top of the chassis, whilst home builders often construct their amps on wooden chassis.

In this case, the poster has already been persuaded to rebuild his prototype to improve the layout and wiring following suggestions here although it has not changed changed the fault symptoms. He also (unusually for a builder) has suitable test equipment and the skills to use them and has provided extensive scope traces and measurements. All the collected evidence so far points to this amp having a solid electrical fault and not one from layout or chassis material.

[Techman]

I agree that the OP has good skills and has made a very good job of the wiring and the laying out of the components. It's also true that it was only the EF86 in the first stage that was fitted with a 'hat', as this is the section where all the real problems would be caused.

All I can say to all those audio types that build these very 'pretty' amplifiers with everything exposed is that they didn't live 40 years ago at the height of the CB radio craze. Well I've got bad news for them - it's making a comeback! The thing that's surprised me is the sudden use of the now fully legal AM band. It's not just CB, which is pretty much unregulated with output power to match, but Ham radio is also seeing a big increase in interest, with legal power output up to 400 watts! These un-screened amplifiers are going to be absolutely slaughtered if someone with that sort of power strikes up anywhere near them. The only reason that folk get away with building these 'decorative' amplifiers is that they don't have, or even remember what it was like to have a 'radio operator' near by. I think a very hard lesson is going to be learned, and remember, the radio transmitter folk have every right to enjoy their hobby just as much as the audio fraternity enjoy theirs.

[Kei-1986]

The last thing I looked at yesterday was running the same test but with the EL34's in place but the control grids disconnected from the phase inverter. This way, the voltages remain as they should rather than skyrocketing. I've bought 4 belton octal sockets just in case as these vintage finder sockets are technically for 250V relays. It may or may not help deal with some of the issues.

10KHz Square - EF86 out Vs 12AX7 out - Open Loop (at pin 6)
[img]https://***********************/65535/49866381591_17f04199d6_k.jpg[/img]

One thing I noticed is that if I check the voltage at the cable that would go pin 6 on the valve socket whilst the scope is connected to it, the waveform ends up looking very similar to when it's hooked up to the EL34's. I'm guessing this is because of the meter lead capacitance.

[img]https://***********************/65535/49866695507_895ceebfa9_k.jpg[/img]


In regard to building a metal chassis. I'm looking at buying the necessary tools for drilling/punching out the necessary holes as I don't have appropriate drills/hole saws for metal in the sizes I'd need. The two panels that I have are 19"x10.5"x0.125" aluminium. (actually 6U blanking panels for server racks)

[Herald1360]

1/8" ally sheet is hard work for chassis bashing! Back in the day 16SWG or ~1.6mm was common for home built heavy stuff. Commercial stuff generally used plated steel (cheaper) but again hard work for DiY. Even lighter gauge ally was fine for stuff without heavy ironmongery to support.

[Boulevardier]

Originally Posted by Sideband

I've been dipping in and out of this thread and come to the same conclusion. The 5-20 is a tried and trusted design from a company that knew what they were doing......it was after all designed at the Mullard Research Labs to promote their 'new' valves available at the time.

I never built the 5-20 but I did build the 5-10 (which is pretty much the same design, just lower power)....in fact two of them as monoblocks and later the stereo pre-amp. I stuck rigidly to the design layout on all occasions....fortunately I did have someone who could bend the chassis for me (my dad). The two 5-10's were built as 'mirror image' using Parmeko transformers and ultra-linear. No problems at all from the start and I had the whole lot for many years from the late 60's until the mid 70's when I gave them to a friend who had them for a further 10 years or so. I don't recall any breakdowns or problems...they just worked!

My experience was much the same as Sideband’s. As he says, the 5-10 is very similiar to the 5-20. I built my first 5-10 as a schoolboy, and, being severely cash strapped, built it almost entirely with parts I already had lying around. These included a used Radiospares standard stock mains transformer with the right voltages, and a scrapped Radiospares “generic” 30W ultra-linear output transformer. Other components had mostly been salvaged from scrap radios, TVs, etc (life was like that as a teenager back in the 1970s!). I did, however, scrape together enough pocket-money to buy a decent chassis to Mullard specs, and laid it out as per those specs. Despite this “cobbling together” of components and my inexperience at the time, it worked perfectly from the first switch-on. True, I didn’t then have any equipment to do more sophisticated performance checks, but if the ears were to be believed it had no problems.

Mike

[PJL]

Originally Posted by Kei-1986

One thing I noticed is that if I check the voltage at the cable that would go pin 6 on the valve socket whilst the scope is connected to it, the waveform ends up looking very similar to when it's hooked up to the EL34's. I'm guessing this is because of the meter lead capacitance.

Yes, that will be the meter capacitance, and this is why we are hoping that the fault will clear when you replace those dodgy valve sockets.

[Kei-1986]

New sockets arrived this morning so I'm working on swapping them over. There is a difference in the cut out size. The tightness of the connections on the new belton ones are very high. The body on the finder sockets appears to be something akin to a phenolic plastic that is mildly brittle. Not a lot else that seems different about them.

[img]https://***********************/65535/49873123753_da78a55dd4_k.jpg[/img]
[img]https://***********************/65535/49873656716_999b5abdff_k.jpg[/img]

[Kei-1986]

Shock of the day, finder relay octal sockets do not make good valve sockets. There is a vast improvement in HF performance out of the phase splitter now that I've replaced the sockets.

Before
[img]https://***********************/65535/49858992581_5a314e4a35_k.jpg[/img]

After
[img]https://***********************/65535/49873981188_cf5a41f4b4_k.jpg[/img]

One thing I've finally nailed down finding with the scope is the low frequency "rumble" that causes the waveforms to move around. Not sure on the cause as it's present on the input. I guess filtering sub/infra sonic out at the input may cure it.
[img]https://***********************/65535/49873981723_69bf809417_k.jpg[/img]

I'm now using the elektor documentation to help make sure the DC conditions are right as they have listed the voltages on the claus byrith modified version which is very handy.
https://drive.google.com/file/d/1Xt1...ew?usp=sharing
https://drive.google.com/file/d/1Wav...ew?usp=sharing

I've now changed all of the resistors to match the CB design so against the original mullard component numbers, R9 is now 100K instead of 270K and R17 is now 10K instead of 15K. These are the voltages I measured during today's test with all of these values as per the elektor/CB design. (all bar the manual balance/bias)

V1
A - 98.4V
K - 0.98

V2
A1 - 287V
A2 - 294V
G1 - 89
G2 - 98.4
K - 100

V3
A - 433
SG - 431
K -31.2

V4
A - 431.5
SG - 429.6
K - 31 (moving around between 30-32V)

C15 - 454V
C12 -440V
C5 - 406V
C4 -206.5V

[PJL]

I would go back to the original 5-20 design as that has stood the test of time rather than mess around with the CB version. It's highly likely the output transformers were designed against the 5-20 circuit.

[PJL]

The CB design starts with the premise of 'improving' on the 5-20 by re-wiring the EF86 pentode as a triode and putting in fixed bias eliminating the benefits of self correcting auto-bias and reduction in dissipation when driven into class B. Elektor was an electronic hobbyist publication whilst the 5-20 was designed by Mullard and has formed the basis of many professional valve amplifiers.

This may also leave you a problem with advice from the forum members as many will have experience with the 5-20 design but not the CB design.

[Diabolical Artificer]

Those waveforms still look less than optimum, that said have you tried playing music through it yet?

Andy.

[Kei-1986]

Originally Posted by PJL

The CB design starts with the premise of 'improving' on the 5-20 by re-wiring the EF86 pentode as a triode and putting in fixed bias eliminating the benefits of self correcting auto-bias and reduction in dissipation when driven into class B. Elektor was an electronic hobbyist publication whilst the 5-20 was designed by Mullard and has formed the basis of many professional valve amplifiers.

This may also leave you a problem with advice from the forum members as many will have experience with the 5-20 design but not the CB design.

It’s a bit of a pain as I had always planned on performing the CB modifications once these were complete. The amount of gain they have is way beyond what is necessary these days and the CB mods solve that. I have no intention of going over to manual bias at the moment though. The CB pcbs could be used to build the original Mullard 5-20 so I don’t see there being an issue with parts compatibility. Clearly the lundahl transformers were designed for the Mullard or something similar enough.

Originally Posted by Diabolical Artificer

Those waveforms still look less than optimum, that said have you tried playing music through it yet?

Andy.

Don’t forget this is still running open loop with no feedback. The noise on the waveform is probably worse as I’ve been forced off the table and my second test bed is right next to all the networking gear including the WiFi. I’ve not tried it with music since changing out the sockets. To be fair, ignoring the mains hum, it sounded remarkably good before but that was at probably at output levels where everything behaved itself. The mk1 sounded remarkably good and that had serious trouble with ringing and oscillation.

[PJL]

Originally Posted by Kei-1986

It’s a bit of a pain as I had always planned on performing the CB modifications once these were complete. The amount of gain they have is way beyond what is necessary these days and the CB mods solve that. I have no intention of going over to manual bias at the moment though. The CB pcbs could be used to build the original Mullard 5-20 so I don’t see there being an issue with parts compatibility. Clearly the lundahl transformers were designed for the Mullard or something similar enough.

I still think it would be best to test as a 5-20 then you can do what you want with 'modding' once you have a working setup. I am not the only one recommending you stick to the Mullard design for now.

[PJL]

What has come out from this is that the UL design does require a low capacitance valve base due to the Miller effect. Pin 4 (g2) and pin 5 (g1) sit next to each other and due to the UL design, g2 is at least -10 times the AC voltage applied to g1. The Miller effect says the apparent input capacitance is Cactual * (1 + Vgain) so a 30pF inter pin capacitance will appear to load the driver as if it were 330pF. This was evident in post 88 when the HF response was better without the valve although the inter-pin capacitance would still be in effect just g2 would have no signal.

[Sideband]

Originally Posted by PJL

I still think it would be best to test as a 5-20 then you can do what you want with 'modding' once you have a working setup. I am not the only one recommending you stick to the Mullard design for now.

This is the GOLDEN RULE when building anything. Start with the original design....we know that the 5-20 was designed by professional engineers who knew what they were doing.....and it's been tried and trusted and used over many years. We know it works and works very well. Once you have the basic design working properly then by all means try out any supposed 'improvements'. By doing it that way you have a benchmark (the original design) on which to assess any alterations you may try out.

[Kei-1986]

Just for completeness on this, I finally finished work on the 3rd prototype last week. I discovered a few things that wouldn't have helped during my disassembly. The 220nF capacitor I thought I'd bought was actually a 22nF capacitor and had been since day one. There were one or two dodgy solder joints and crimps, mainly on power connections. The most recent discovery is the new tung sol EF806 I had bought runs out of spec as it causes untold chaos to voltages across V1 & V2which results in a lot of audible hiss. I decided to do what I could to build the "MK3 prototype" using aluminium. I had some old 19" rack blanking panels so opted to build it on that.

What with all the prior issues I'd had, I decided to just go to town on this one and make as many of the supposed improvements that the design "needs". (according to some) This meant taking the 12AX7/ECC83 out and use a medium mu high GM valve for the phase splitter. I opted to go for 6SN7/6CG7/6FQ7 in preference to 12BH7 or 12AU7, mainly down to the simpler heater connections. The power transformer had the capacity to cover the increased heater current. I also took Andy's advice and built a CCS on a piece of veroboard. A more controversial change was the suggestion of using a "combination bias" using resistors and zeners to help drop the quiescent without sacrificing much if any power. It seems to have cured the red plating issues and I can easily get the 20W that the amp is rated for. The disadvantage is there is it's not as tolerant to valve mismatch as plain cathode bias. Hopefully there is enough give in it that the ageing of the valves doesn't prove troublesome.

This is the schematic I ended up at after a few feedback network tweaks. I think there may be room for improvement with the feedback as I've simply gone for getting it stable no matter what. I think the step network may be stronger than it needs to be. The target was 17dB of feedback and I think I got about 16.3dB with 18K.
https://***********************/65535/...1309f8f6_h.jpg

CCS built on veroboard using what I could find in my component stash.
https://***********************/65535/...fa4982a5_h.jpg

100Hz at the onset of clipping. A big win for me as there wan't a hint of ringing.
https://***********************/65535/...2ae35dda_h.jpg

20Hz at the edge, again no ringing.
https://***********************/65535/...61e37273_h.jpg

1KHz sq looks quite good.
https://***********************/65535/...e351bd69_h.jpg

10KHz is showing some HF bandwidth limitation but some of that appears to be input capacitance which I think relates to a piece of foil backed card I used behind the veroboard to act as a shield. (there is slight rounding over of the input signal after the veroboard)
https://***********************/65535/...31d5f16d_h.jpg

The input veroboard was created to help fit the infrasonic filter.
https://***********************/65535/...2a001b18_h.jpg

The complete interior. It's a bit big as it stands but it made building easier. I will be working on the final construction being a bit smaller, closer to 400x250mm.
https://***********************/65535/...a1fdc5be_h.jpg

I've been using it now for a couple of days over the last week and think it sounds great. It's not easy to differentiate between it and the solid state power amp I normally use. I think since this is no longer a proper 5-20, merely based on one, a mod could move this thread into the DIY section. It's closest relatives would be the Dynaco A-420 modified mullard design or the Eico HF-50/HF-60. I'll try to keep this updated as I build the pair in their final chassis.

[Diabolical Artificer]

Glad you got there in the end, it's been a bit of a trek, I've been following your journey over on DiyAudio. I think the OPT's you used made this a more challenging build, using a different OPT to that which Mullard used, especially if it has a few quirks, means you probably had to tweak the circuit a bit. A good beefy LTP using a CCS is capable of driving recalcitrant OP stages cleanly, at least that's what I've found.

Anyhoo, glad it all worked out in the end and your enjoying your amp, you'll probably find you might have to go back and tweak the odd issue whilst it's all bedding in, it never ends : )

Andy.