Using P series valves for Projects

[bikerhifinut]

(As to old telly (P series) valves - I've made lots of experiments on NOS (allegedly) and pulls and most are serviceable in an amplifier though if AC is used on the heater expect more hum and the need for a heater lift to improve it. In most of my HiFi amps I've gone for DC feeds to the heaters but I've not bothered with the hassle of regulators a couple of diodes and a big capacitor are enough and in those cases P series valves are nicer in that the currents are lower.)
Quote from Dom.

I never thought about that aspect, thanks its given me an idea for the heaters on the PCL86 amp. 4 x 300mA = 1.2Amps, well within even an LM317's comfort zone, but I feel I'd go for an LT1085 for a bit of extra current capacity. So theoretically I could use a an 18V transformer rated at 2 amps (see where I am coming from, i can use a 9-0-9 V job from Maplins) and with a full wave rectifier and a voltage reg bingo!
I've gone off topic so if a mod wants to pull the comment ............or start a new thread with it than maybe that's better.

Andy

Split fron this thread: https://www.vintage-radio.net/forum/...285#post717285

[dominicbeesley]

I wouldn't bother with regulators personally for those instead I'd be tempted to find a 300mA bulb and make a series chain

D

[bikerhifinut]

Point taken.
However I have a couple of adjustable voltage regulator boards that only need the AC feed lying about and they would make life easy for the PCL86 heaters and I can feed them with the correct voltage. Plus I have everything isolated from the mains, makes me much happier.
I have to use these PCL86's now as i bought 10 for £16. unmarked unboxed but new and supposedly tested ok. I figure I should get 4 good 'uns at least.

Quite like the idea of using plentiful stuff and giving it a new lease of life and leaving the rare stuff for those enthusiasts with Vintage gear as replacements.
Especially valves like ECL86, there must still be a lot of rogers cadet amps out there needing these rare and expensive valves.

Andy.

It is easy, if not a bit time consuming, to put an overwinding on a toroidal transformer for the heaters.

[PE9ZZ_JO22KI]

There are kits around to fix ECL86 amps with a NOS PCL86 and a heater voltage doubler circuit that hits 13 volts nicely. Two 1A diodes (1N4001) and two 1000 uF electrolytics and a resistor for current limiting. I would have used smaller caps (100/220 uF) and no resistor. My own PCL86 Hacker RV16/19 PP kludge uses 26 volts for two tubes from a DC supply.

Tjerk, 9ZZ

[bikerhifinut]

Hi Chaps,
I'm about to start on the PCL86 project, Project "Sprog" as its based loosely on the Rogers Cadet circuit.
I'm about to make the chassis/case tomorrow. Basically it will be a rectangular "frame" made from 3"x0.5" planed timber with top and bottom plates. its a technique which worked for my last project.
Looking at the job I am in 2 minds as to whether I should go for the DC heater option or just use the unused 2 x 12V transformer @ 1A per secondary that I have in the toybox. I think i should get 13V out of it as its rated nominally 230V on the primary and a quick mental sum suggests that should give me around 13V out using the 240V mains. That gets the Valve heaters somewhere close to their nominal 14V rating.
Or I could buy a 18V transformer rated at 2 A which would regulate to the 14V with ease using an LM317 reg.
Would I be ok using just 1 regulator for all 4 Valves as its running at 1.2 amps, close to the regs 1.5A rating, or should I go for 2 regs and share the load and run 'em at 600 mA? As I have oodles of regs (thanks Keith) My instinct is to be cautious.
Secondly, The rogers circuit uses a common cathode resistor on each push-pull pair of 130 ohms. I feel that was just done to save a couple of bob on an extra resistor and capacitor, so I have plumped for separate cathode resistors of 270 ohms as the nearest preferred value to the 260 that would be necessary. (I eventually intend to fit Constant Current sinks on each output valve using an adjustable voltage reg, I feel this will help equalise the current in each section of the output transformer primary.)
Otherwise I'll keep values much the same.
Comments and advice will be most welcome. It's just the power amp section I am building.
Andy.

[kalee20]

I'd use separate cathode resistors, decoupled of course. It will be more self-balancing for the currents than a common resistor, if the valves don't match.

As to heaters, if you already have a transformer, then try it!

[Ed_Dinning]

Hi Andy, I reg will do it if well heatsinked, but if you have 2, why not.

You could of course use the separate resistors and add a 50 ohm pot with its wiper earthed and the cathode resistors connected to each end. Then balance as required!

Ed

[bikerhifinut]

Might be slightly off topic here.
As I was mulling over the circuit and a quick check of the Output transformer spec sheet, I remembered something Ed said to me and an alarm bell is ringing.
I was going to use a pair of NOS RS "Hygrade" output transformers for this push-pull PCL86 design, using the Centre tap as the impedances are just about right for PCL/ECL86.
However I think they are designed with a "gapped core" for SE use.
Will that cause problems or damage to them if I use them as Push pull transformers?
If so I can use them for a SE amp project with EL84 etc.
I can buy a pair of brand new ECL86 p-p transformers online for not an awful lot if these Hygrades are unsuitable.

Andy.

Thought... AC is OK for heaters but if the triode section is nearer ground you may get less hum. An offset 'humbucker' pot might improve things better than the original.

Having a quick look at the data sheet the heaters are in series, pin 4 is the triode end.

[Herald1360]

Originally Posted by bikerhifinut

I was going to use a pair of NOS RS "Hygrade" output transformers for this push-pull PCL86 design, using the Centre tap as the impedances are just about right for PCL/ECL86.
However I think they are designed with a "gapped core" for SE use.
Will that cause problems or damage to them if I use them as Push pull transformers?

A gapped core for P-P use is no problem though an ungapped core for single ended is.

The transformer will be bigger than strictly necessary for push pull and its performance likely less optimal.

[bikerhifinut]

Thats a relief.
By less than optimal, does that mean a frequency response issue or power efficiency?
If they would be better used in a SE project then I don't mind hanging on to them and looking around for a suitable pair of p-p transformers.

Thanks

Andy.

[G8HQP Dave]

Efficiency may be lower. This is because an SE OPT needs more turns (therefore thinner wire) to get the same inductance as a similar PP OPT, so it has greater resistance. Alternatively, it will just have less LF due to smaller inductance. An SE OPT has to make more compromises than a PP OPT.

[Tim]

I have done a couple of things over the years, a little audio amp with a PCL82 and an HT PSU with a series pass PL82. Perfectly OK, and fun to do. ISTR I used a transformer for the heater in the amp though, so H/C leakage wasn't really an issue.I happened to have a 85-0-85/15-0-15 transformer knocking round......

[G6Tanuki]

I have in the past used a pair of PL81s strapped in parallel, as the PA stage of a homebrew 3.5MHz CW transmitter. A PCL82 (or was it a PCL83?) served as the xtal oscillator and driver.

From memory I got about 20Watts RF out but wouldn't want to keep the key pressed for too long.

[G4XWDJim]

The KW 160 top band transmitter uses a PL 81 as its PA valve with a 21volt winding for its heater on the mains transformer.

Works very well too.

The 30P12 looks to be a useful valve. Anybody ever used one for a project. I think it would make a good QRP PA valve.

Jim

[G6Tanuki]

Originally Posted by G4XWDJim

The KW 160 top band transmitter uses a PL 81 as its PA valve with a 21volt winding for its heater on the mains transformer.

I guess the top-cap anode was the main reason for this choice !

(same goes for using PL500/505/509/519 series in higher-power RF stages)