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Old 8th Sep 2019, 9:03 pm   #1
Takapuna
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Default Ekco U354 conversion to P-Series valves

Hello everyone.
Whilst I’ve been a keen follower of the forum and have occasionally commented on posts this is my first original effort. I intended it to be a concise account of another Ekco U354 repair but got a little carried away! It is really a “success story” but tells of a valve substitution too. Mods please edit/re-post accordingly…

A number of favourable comments on this forum led me to look for an Ekco U354. One on eBay that took my fancy was missing one knob but otherwise OK. It had been re-listed a couple of times with an ever-reducing starting price so I jumped in. It was pictured with all valves lit up, which is both a blessing and a curse as we know and was also listed as “not working” but had a dial glass with all print in-tact.

On receipt it was obvious from its white top that the UL84 was dead. Also, the UY85 was only there to complete the heater chain with rectifying duties being carried out by a below-chassis BY127. The output stage had been reconfigured, removing the 47k pull-up resistor at the UL84 cathode and fitting a straight-forward 180R decoupled by 50uF. A search in the loft failed to locate a UL84…

Around the same time another U354 was listed that had been broken for spares. In separate lots were to be found the case, the individual valves, the speaker and the bare chassis. The chassis came with knobs and was listed with a low starting price for which I was the only bidder. On arrival the chassis was virtually un-touched other than a replacement on-off switch/volume control assembly.

So now I had two chassis. The first chassis was the later version with an in-built aerial and rearranged front end and the second one was to the original schedule.

An interesting point regarding the original chassis was how tight the valves were in their sockets. McMurdo sockets with their tuning fork contacts usually relax over time and need to be re-tensioned or replaced to get reliable connections. This was not the case here and a couple of the valves had pins that were somewhat buckled (as opposed to being splayed or bent) as if they’d suffered due to the compressive force required to insert them. Unusual I thought.

I decided to renovate the second chassis first, if you see what I mean. I had a new UY85 in my collection but no UL84. The price of a NOS UL84 is quite high and I was unwilling to make an offer for the one that had been stripped from the chassis I’d just bought! However, in the window of our local electronics store I could see new 0-9V 0-9V @30VA toroidal transformers selling for £5.00. They were nice and compact, which set me thinking…

The Ekco U354 is slightly unusual in that its valve compliment has almost direct equivalents in the 300mA “P” series: UCC85=PCC85, UF80=EF80 ( read “PF80” for the purposes of this discussion!), UABC80=PABC80, UY85=PY82 and UL84=PL84. I was the proud owner of several new examples of each of these. The U354 shares its physical chassis with the U353, the U354 having space on top of the chassis where the AM tuning gang and ferrite rod aerial are mounted in the U353. All I had to do was confirm that the aforementioned transformer would fit the space and that P-series heaters could be powered as a number of parallel/series strings connected to the secondary, wired for 18V.

A transformer was purchased and the secondaries connected in series were loaded with my trusty rheostat until a current of 1.2A, representing four sets of 300mA heaters, was indicated. At this load of just under 23VA the secondary voltage was a little over 19V, the “bonus” volt being partly due to drawing only 76% of the transformer VA rating.

So, if I fit the transformer and re-wire the valve haters using appropriate droppers all should be well. I chose the following combinations: 15R dropper+EF80+PCC85, 15R dropper+EF80+PABC80, 10R dropper+PL84 with PY82 directly across the secondary.

This was the plan, but I had to service the chassis first. The bulging smoothing capacitor did not reform and was replaced by three discrete 47uF 450V capacitors which I had to hand, along with new 820R wire-wound smoothing resistors as the originals were both approaching 1k. The DM70 magic eye had an open-circuit filament and was replaced, as was its filament supply resistor chain. I was surprised how “short” the DM70 output leads seemed to be for this application. In spite of reusing the sleeves and doing my best to copy the lead shapes I found myself pushing the glass envelope downwards in its clip, pulling on the wires with somewhat more force than was ideal to get it to the correct position… A couple of series-connected 1N4007 diodes were wired across the filament for protection just-in-case. The usual moulded capacitors were replaced as well as any resistors that were 20% or more out of tolerance. There were many of these, one of which was R10 (manufacturer’s sheet) which should be 62k Ohms yet measured 92k. I replaced it with 56k+4k7 in series, but remain curious as to what made EKCO decide that a value from the E24 series was necessary in this position as opposed to either 56k or 68k from the E12 series.

Now I was at the point where I could either test the chassis by refitting the original valves from the first set, along with a new UY85 and borrowing a UL84 from another set, OR carry on and fit the transformer and complete the valve transplant before giving it a try. You can probably guess that I did the former as it made sense to confirm that I had a working chassis before proceeding. So, after a few meter checks an original set of valves was fitted and the radio powered ON through an isolating transformer. It worked well. Pleasingly so.

In spite of this, I completed my quest by removing the U-series valves and fitting the transformer top-side with a tag board terminating the connections and carrying the dropper resistors underneath. Any false ideas of extra safety from the new transformer were put to rest as I found myself making a common connection between one end of the transformer primary, one end of one of the secondaries and the chassis >>> No extra isolation there then! Chassis remains live…

Somehow the stated valve heater current vs voltage relationships did not always seem to correspond accurately to the data sheet values. PCC85 and PABC80 both have “9.5V” heaters and EF80 6.3V, so a series pair should drop 15.8V when drawing 300mA. However, rolling through my valve stock led to different results when the voltage split was measured. There are several threads on here that discuss getting heater power correct and my conclusion is that we shouldn’t look too closely! The “tight” tolerances in the Mullard technical manual are there to provide a solution that will be OK with 10% mains changes and relatively large voltage selector steps. I found the stated figures difficult to achieve whilst adjusting the droppers. I’d carefully measured the values of the dropper resistors using a TF2700 bridge (which I trust even though it has uncalibrated status) and this allowed me to use them as shunts to measure the current. The use of different DMMs on their AC voltage settings, even with an AC millivolt meter thrown in for good measure (sorry; no pun intended!) led to small reading differences when measuring the same voltage! Trying a number of new PCC85s whilst not changing anything else the PCC85 heater voltages ranged from a high of 9.47V to a low of 8.7V. The corresponding voltages across the (unchanged) series connected EF80 heater were 6.24 and 6.96 respectively, hence the sum of the voltages remained essentially constant. This variation I put down mainly to valve heater tolerances.
What I had not spotted previously was significant local mains voltage changes during the day. A range of 230V – 248V or thereabouts was seen over a few hours and added to the confusion until I became aware of it when trying to confirm the chosen values for dropper resistors. Also, the use of a 250VA mains isolating transformer gave a few extra volts on its secondary as the transformer was not fully loaded. I had to disconnect the isolating transformer at some points and measure cautiously/carefully to get a real result. Eventually I reached a compromise that seemed less than perfect but OK. Interestingly and somewhat annoyingly the heater dropper resistor values I ended-up with were those I’d selected in the first place!
Perhaps the usual series chain arrangement comprising a greater number of hot heaters leads to a more accurate nominal current… and I’ve lost some of this benefit by having only one or at most two valves connected to a 19V supply. The 19V supply has a reasonably low source impedance and acts to force its terminal voltage across the load(s) regardless. In the end I decided not to worry even though the two sets of series-connected valves PCC85+EF80 and PABC80+EF80, which theoretically should generate the same voltage split at their centre points do not.

So now I have a U354 fitted with P-series valves which, unsurprisingly, works every bit as well as it did with U-Series valves. It is very sensitive and remains on-station from switch-on. I suggest it is equally as sensitive as the Bush VHF90A which has one more IF stage. It is absolutely quiet with no hum, although I appreciate the reservoir/smoothing capacitors have been increased slightly from standard values. It has the benefit of all new valves of course but I remain impressed.

Here are a couple of pictures although there is very little to see. The added transformer and the fact that the rectifier and output valves are the same height are the only clues when viewed from above the chassis.

https://www.vintage-radio.net/forum/...1&d=1567971669
https://www.vintage-radio.net/forum/...1&d=1567971669
https://www.vintage-radio.net/forum/...1&d=1567971669

Lastly, here are a few observations and questions for comment from the knowledgeable and kindly people that I know contribute regularly:
1. Along with PY82s, I have PY80s which I could have used as rectifier. Physically, they look identical. Whilst the PY82 is described as a mains rectifier primarily with little reference to it being a boost diode, the PY80 is described especially as a boost diode even though there are rectifier curves available for it. The “valves and their habits” article that is linked to many of the Valve Museum pages states that “in a push either of these can be used as a substitute for the other”, hence I think PY80 would be fine as a mains rectifier. The low max stated (reservoir) capacitor value of 4uF for the PY80 is a little confusing though. I think this may be related to boost duties where the frequencies are much higher? Using the PSU modeller from the Duncan’s Amps page suggests that PY80 (EY80 actually) will exceed its max forward current figure with a 47uF smoother and series resistance representative of the U354, whereas the PY82 (EY82) does not. When fitted for a sort period a PY80 worked OK in the rectifier position and gave almost identical dc rail voltages. Any thoughts please?

2. A couple of NOS PCC85s stopped oscillating before the high frequency end of the tuning scale was reached. They started again as the tuned frequency was reduced and the effect was repeatable. This is somewhat curious as they were “famous” Mullard or Mazda brands. I did not investigate further as I was in the fortunate position of having several valves to choose from. Perhaps this was a bit of a cop-out! I didn’t open the FM tuner box as I did not think there are any components in there that couldn’t be diagnosed by measurements from outside. Maybe C3 or C5 (manufacturers sheet) are slightly leaky?

3. The PL84 cathode current was a little high (new valve?) so I’ve increased the cathode resistor to 180R.

4. Touching the tip of the soldering iron against the dial cord has somewhat undermined it! It hasn’t melted all the way through and the damage is not in a position that has to wrap around a pulley. I’m keeping my fingers crossed that it survives. Not my finest hour!

I hope everyone is OK with this re-purposing of “TV” valves. It has a feeling of great satisfaction about it from my viewpoint and is easily reversible should I wish to do so, although I don’t see why I would. Many other sets prohibit a similar exercise as there is no such thing as a PF89, although I note that ECH81 is a 300mA device and hence may be considered to be a “PCH81”. Can anyone think of any other models that might lend themselves to similar conversion?


So that’s all. If you’re still reading this I apologise for the lengthy ramble and congratulate you for making it to the end. Thank You.

Regards,
Phil.
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Old 8th Sep 2019, 11:00 pm   #2
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Default Re: Ekco U354 conversion to P-Series valves

Nothing wrong with your 're-purposing' at all. It's your radio to do with as you wish. It's not as though it's a rare set or of any particular value. Quite a novel idea actually and well done getting it all working. It's worth noting that 'P' series, and 'U' series valves are not necessarily the same other than for heater current. If you look up an EL84 and a PL (or UL) 84 you will see that maximum anode voltages will be different. The EL84 is designed for AC only transformer fed radio's etc where final HT voltages will be higher. The PL84 was designed for TV use and series heater where there is no mains transformer and maximum HT is governed (largely) on maximum mains voltage so you will find that the maximum HT voltage for the PL84 is lower than for the EL counterpart. The same applies to the UL84. All of this may account for why your PCC85 doesn't like oscillating above 100Mhz. Check out the differences between the PCC85 and UCC85 and you may find the maximum anode volts for the UCC version is lower than the PCC version. If there is a series feed resistor to the VHF tuner you may find that reducing the value slightly to increase the overall supply, may result in your PCC85's working satisfactorily up to 100Mhz.
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Old 9th Sep 2019, 7:59 am   #3
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Default Re: Ekco U354 conversion to P-Series valves

There usually is a decoupling capacitor in the tuner unit, some sets used a ceramic but others a Hunts Mouldseal, the paper capacitor can give unusual symptoms.

The basic design was used in a MW/LW/VHF set as well as this version of VHF only, they do work extremely well.

The circuit design may be getting near its design limits at 100Mhz, the scale only goes to 100Mhz and when the set was made there were no stations above about 95MHz.
After saying that my Ferranti U1032, virtually the same set but with AM receives Classic FM on about 101Mhz.

Enjoy using it, an unusual modification but that doesn’t matter, you probably had a great deal of pleasure doing the conversion and it being a success.

Looking at the PCC85 and UCC85 specifications they look very similar but Sidebands suggestion of raising the HT a little is a worthwhile experiment.

We were Ekco/Ferranti dealers so I always like to see these sets working again.
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Old 9th Sep 2019, 9:35 am   #4
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Default Re: Ekco U354 conversion to P-Series valves

Thanks Sideband and Nuvistor for your positive replies and suggestions.
I'll keep the idea of raising the supply to the front end in mind. It didn't occur to me that PCC85 typically sees only TV IF frequencies. For the moment the resident PCC85 makes it all the way to the top end of the scale where I can tune Smooth Radio at 100.6MHz just before the end stop is reached.

The wife has allowed me to keep the radio on the end of the kitchen eating surface (the thing that used to be a table) where it sees daily service. To date, it's done many hours on Radio 4 on weekday mornings and Radio 2 at the weekend.

Phil
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Old 9th Sep 2019, 3:46 pm   #5
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Default Re: Ekco U354 conversion to P-Series valves

PL84 and UL84 are versions of EL86, not EL84.
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Old 9th Sep 2019, 11:33 pm   #6
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Default Re: Ekco U354 conversion to P-Series valves

The UCC85 and PCC85 do appear to have been identical apart from their heaters. The curves included at the end of the attached datasheet are annotated as applying to both the UCC85 and PCC85.

Nonetheless, the said datasheet described the PCC85 as being an oscillator and mixer for TV receivers up to 200 MHz (not for split carrier receivers). (That is at the top of the second page.) I think that Philips did use it this way for at least one TV receiver model. Why is unclear, given that the triode pentode was viewed as being superior with “high” IFs. Still, in the USA there was continued use of the 6J6 (and probably the 12AT7) into the triode pentode, high-IF era. The “not-for-split-carrier receivers" suggests that the PCC85 might have been somewhat microphonic with noticeably deleterious effects at Band III frequencies. (The 6J6 had the same problem, although GE later offered a reduced-microphony version.) Possibly when it released the PCC85, Philips saw it as having more TV than radio applications, and wrote the data sheet that way. I guess too that offering a “200 MHz TV valve” for FM service was an easier sell than offering a “100 MHz FM valve” for TV service.

Be that as it may, the PCC85 was used as an FM front end valve, e.g. by B&O. So I think it could be viewed as being fully suitable for the application, and not simply as an adequate stand-in. (Some of the early UK TV-FM receivers had either separate FM front ends (Bush?) or separate FM front ends and IF strips (Ferguson?) so might have used the PCC85 – I’m not sure.)

The EF89 was used in 300 mA series strings, e.g. by B&O. In that case a shunt resistor (68R in some cases) was used to bring the current up to 300 mA. That technique with less-than-300 mA valves had been used – and effectively validated - by Philips in some of its early 300 mA series-string TV circuits that used valves such as the ECH42 and EQ80.


Cheers,
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Old 10th Sep 2019, 9:12 am   #7
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Default Re: Ekco U354 conversion to P-Series valves

I have been intrigued by the idea of using TV valves in domestic radio ever since I read an article might have been in The Radiophile (My memory has always been bad, age hasn't improved it) called something like 405 Radio.
I made a couple of simple sets using the ubiquitous ECL80 (cheating a bit) EF80’s and PCF80 and PL81 and of course PY82 rectifier. The results were OK nothing better!
I think that the results and implementation Phil has achieved in restoring this set is inspirational.
It wasn’t That long ago that TV valves were still being dumped I hope that’s now ceased with more interest in TV and valves in general.
Nice Job Phil

Cheers

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Old 10th Sep 2019, 1:14 pm   #8
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Default Re: Ekco U354 conversion to P-Series valves

As said- your radio to do with as you wish, not a particularly rare or valuable set and the fact that it's been kept going with a different approach using a valve series that might otherwise be short of employment is a good thing. Does it save a few watts through eliminating any heater dropper section?- a "green" approach too if so!

I'd been toying with various schemes for making an Eddystone 670A slightly more couth from the safety aspect (a metal-cased AC/DC radio with a line-connected inner chassis suspended on Paxolin strips and washers might have got through with a nod and a wink in the '50s but I suspect not any more!) and one of those could be a mains transformer with secondaries of c.150V feeding a bridge rectifier for HT and 15V- the UAF42s have 12.6V heaters, the UCH42 14V and a mooted replacement PL84 15V- I don't think there was ever a PL41, though I'm prepared to be surprised!
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Old 10th Sep 2019, 1:32 pm   #9
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Default Re: Ekco U354 conversion to P-Series valves

There was an interesting article in TV mag many years ago by one of the well-known contributors about building a valve radio using TV valves. He really went to town and used ECL80's in push-pull for the output.

I don't think there will be a problem posting the article in the forums since it's available on the web anyway unless the mods feel otherwise.

Mods please move or delete if necessary. There is one glaring error.....the EB92 should be EB91. I also think you would be justified in using an ECC84 since it's also a TV valve rather than slightly underrunning a PCC84.

I keep meaning to give it a go...….
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Old 10th Sep 2019, 2:11 pm   #10
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Default Re: Ekco U354 conversion to P-Series valves

Quote:
Originally Posted by Cobaltblue View Post
I have been intrigued by the idea of using TV valves in domestic radio ever since I read an article might have been in The Radiophile (My memory has always been bad, age hasn't improved it) called something like 405 Radio.

Cheers

Mike T
Well after about 30 seconds research!

It's in the Radiophile when it was still called the Radiogram issue 10 June 87

The article was entitled: 405-line Radio receivers.

My copy is likely in Poole so I cant get to it until next week

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Old 11th Sep 2019, 12:59 am   #11
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Default Re: Ekco U354 conversion to P-Series valves

Quote:
Originally Posted by Sideband View Post
There was an interesting article in TV mag many years ago by one of the well-known contributors about building a valve radio using TV valves. He really went to town and used ECL80's in push-pull for the output.
Yes, most interesting. And both the EF80 and EF91 are used in the same circuit, which relates to this current thread: https://www.vintage-radio.net/forum/...d.php?t=157399

Also noteworthy is that some of the circuit elements had commercial precedents in post-WWII radio receiver practice.

The autodyne pentode AM frequency changer was used by GE (USA) in some of its models, usually with a 6/12AU6. GE also used a pentode driven by a separate triode oscillator. RCA used the 6/19X8 triode pentode as an AM frequency changer. The 6X8 was primarily a VHF TV frequency changer. Thus one could say that using an E/PCF80/82 would be a validated alternative approach to the autodyne EF80.

Other possibilities for the frequency changer job include the EH90 and ECH84. The EH90 heptode was the European version of the 6CS6, originally developed as a gated amplifier for use inter alia as a colour TV synchronous demodulator, as was the similar 6BY6. Both were also used as noise-gated sync separators. And I think both were found in American HF receiver practice as mixers, both autodyne and separately excited. Philips adopted the 6CS6 as the EH90 for use as a quadrature locked-oscillator FM demodulator for TV sound. For the noise-gate sync separator job it developed the ECH84 as successor to the ECH83 (and before that the ECH81), both radio valves pressed into TV service. Mullard proposed the ECH84 heptode section for use as a synchronous demodulator combination for FM multiplex decoding, so it had mixer credentials.

The EF85 as an AM IF amplifier was routine (in British and European AM/FM receivers), but an EF80 (or similar) as a final IF stage in a domestic AM receiver might be hard to find. On the other hand, the EF91 was used as a final, non-agc’d IF stage in some HF communications receivers, so provides an indirect precedent. The EB91 was used as a demodulator and AGC rectifier in some AM receivers. The triode-strapped EF91 as AF amplifier was found in some Dynatron equipment, such as the T99/LF612 and T139/LF613 chassis. And Mullard published a push-pull AF amplifier circuit using an ECL80 pair.

I suspect though that it would be difficult to find a precedent for a cascode (valve) AM RF amplifier in a domestic AM or AM/FM receiver, so that choice was quite innovative. Perhaps it existed in US practice, where using the same valve as both AM and FM RF amplifier was not uncommon. The only examples I am aware of all used pentodes, but as the cascode FM RF amplifier was more common in US practice, it is at least conceivable that hiding somewhere is an AM/FM combination. The cascode (typically the ECC189) RF amplifier was found in late valved HF communications receivers, so that was an indirect precedent. Alternatively the EF85 could have been used as RF amplifier, and that did have precedent in AM domestic receivers (as well as in HF receivers). In fact, the EF80 (as well as the Mazda 6F1 and 6F13) had also been used as RF amplifiers in domestic AM practice, with the EF91 used in HF receivers. In the US, RCA used the 6CB6 as AM and FM RF amplifier. The 6CB6 was a high-slope sharp cutoff pentode developed primarily for 40 MHz TV IF service.


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Old 11th Sep 2019, 8:18 pm   #12
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Default Re: Ekco U354 conversion to P-Series valves

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(Some of the early UK TV-FM receivers had either separate FM front ends (Bush?) or separate FM front ends and IF strips (Ferguson?) so might have used the PCC85 – I’m not sure.)
Yes...Ferguson (I think the 436) used a separate FM front-end which used PCC85 RF amp, mixer osc. 2xEF80 I.F and an EB91 ratio detector.
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Old 12th Sep 2019, 8:36 pm   #13
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Default Re: Ekco U354 conversion to P-Series valves

Looking again at my stated imbalance of the two heater strings with EF80s, V1 + V2 and V3 + V4, this will be due to PCC85 having a 9.0V (as opposed to 9.5V) heater. I was working with 9.5V for both PABC80 and PCC85 hence expecting them to share volts in a similar manner. The Valve Museum summary box shows the 9.5V figure but the attached data sheet states 9.0V. Another 1.6 Ohms or thereabouts added to the dropper would line things up better but I don't feel compelled to reach for the iron after all I said about not worrying too much!

Yes, the radio is a little "greener" by virtue of the heater section of the mains dropper no longer being in use. For 240V operation this will be 100mA through 650 Ohms or 6.5W.

Thanks again for the kind words and interest. Especially Mike T at post 7. I've never been inspirational before https://www.vintage-radio.net/images/smilies/cool.gif

Phil.
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Old 12th Sep 2019, 8:37 pm   #14
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Default Re: Ekco U354 conversion to P-Series valves

Need more practice with the smilies though....
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