Matchless lighting new build using MM P18RS

[henter2015]

Hi Folks,

Im building a lighting style amp and using a MM P18RS, I was wondering if I could use the 5.8V tap for the GZ34 filaments as in the schematic attached... Or i will be better using a SS rectifier bridge

Schematic being used: http://www.ceriatone.com/images/layo...one_220308.jpg

PT info and possible wiring:

[pdf64]

I can't make out the symbols / writing in the box that's connecting to the primary?

I think you've only got half voltage to the main heater circuit; one leg (marked 6.3) is grounded, with the heaters being fed from the other leg and centre tap.
The HT winding is the bi-phase type and given the application, not suitable for a bridge rectifier.
Silicon diodes would need to be arranged in the same way as the tube diodes, in the form of a full wave bi-phase rectifier.
See The Valve Wizard

The HT winding legs should be connected to terminals 6 and 4 of the rectifier tube, rather than 6 and 5.

I don't understand the 5.8V rectifier heater winding tap; is it a typo? As it stands, it is too high for a rectifier intended for a 5V heater.
You may need to clarify that with MM.
I suspect that the MM P18RS is intended for EZ81 or 5Y3 rectifiers; the Vdc HT may be too high for good results with a GZ34, and especially silicon, rectification, when used with cathode biased EL84.

[henter2015]

Hi Thanks for that!
The primaries are connected to the switch.
As by the 5.8V rectifier tap, is strange is marked that way but on the MM docs it states 5v and not 5.8V, I will clarify with Patrick from MM.
The original Ceriatone layout is using a 18watt PT with 290 on the HT, about 10V difference, shall I be concerned?
I think these matchless ran the el84 really hard anyway but sound amazing...
I was thinking if I cant get around to install the GZ34 I could do a diode rectifier and immediately afterwards a VVR which will allow me to knock down the voltage to anywhere I want... what B+ would be the maximum I should be looking for if using diodes?
If wiring the el84s and ecc83s from 0 and 6.3V shall I ground the CT or leave it on air?
Thanks again!

[Mick Bailey]

Duncan's PSU designer is a handy tool for calculating voltages - you can compare SS or tube rectifier and select the configuration and tube types.

Quite a few amps give EL84s a good spanking - especially Matchless. But also Traynor, Marshall, Mesa, Dr Z. and more. But don't expect a long life out of the tubes. SS is going in the wrong direction - more voltage, not less. A tube has a higher internal resistance (and therefore voltage drop under load). If you wire the amp for a GZ34 there are other tube options that can bring the B+ down if you find it too high.

If you can't use a tube rectifier, plan on around 20v or so B+ increase with SS

[pdf64]

Bear in mind that a 10Vac difference will rectify to ~14Vdc difference.
OK that's not massive, but the tubes are already being pushed extremely hard.
If you are looking to run the EL84 at high dissipation (p-p cathode bias works best when close to class A), I suggest keep the idle HT below 320Vdc.
The main 6.3Vac heater line must have a dc reference to the amplifier circuit, it must not be left floating.
A convenient way to do that and minimise radiated hum is to connect the heater winding CT to the circuit 0V.

[henter2015]

Great! will work on having the HT under control then! hopefully I can use a tube rectifier on it and drop some voltage!
there are 2 6.3V taps, one with a CT (which I guess will be using for the filaments) and the rectifier ones (5/6V) one to be left unused I guess then?
Pic attached

[henter2015]

Hi Folks,

So I got a reply from Patrick at MM,

He mentioned that the 5.8V tap is unloaded so it should be about right when hooked to the GZ34 (loaded)
He as well mentioned that the CT should be floating as a VCT is already on the schematic using 2 100R.

What I dont understand is why the 6.3V CT Tap is named as 6.3V and 0, instead of 2 - 3.15V and a 0V like in other 18watter PTs.
He mentioned that is the same because is unloaded.
could you help me guys understand this better?

[Mick Bailey]

I'm puzzled why the markings are inconsistent. A transformer should be marked with the voltages at the rated load for each winding. The marking of a centre tap varies; CT and 0v are both valid in this context.

I would hook it up to the mains and measure what you have unloaded for all of the outputs. Is your supply actually 240v? This has a big difference on B+. I find that mains voltage often runs high, and consequently secondary voltages are above what they're supposed to be even loaded.

[henter2015]

I'm puzzled why the markings are inconsistent. A transformer should be marked with the voltages at the rated load for each winding. The marking of a centre tap varies; CT and 0v are both valid in this context.

I would hook it up to the mains and measure what you have unloaded for all of the outputs. Is your supply actually 240v? This has a big difference on B+. I find that mains voltage often runs high, and consequently secondary voltages are above what they're supposed to be even loaded.

Thanks, my supply is about 240-245 here in UK, I would hook it up and measure the AC on all taps and get them on paper, shall I ground the CT while so?

[Mick Bailey]

No need to ground the CT. Just measure from the CT to either side.

I was getting 247v where I live (UK) yesterday.

[henter2015]

No need to ground the CT. Just measure from the CT to either side.

I was getting 247v where I live (UK) yesterday.

Hi!

Just got all measured today, I am getting 245V here.

the rest of the voltages are:





thanks!!

[henter2015]

bump! anyone could give me a hand please?

[J M Fahey]

Not sure what the question is; voltages look right for an unloaded transformer and very slightly high mains.

And like your old School teacher would say: you still have to improve your unreadable calligraphy

[Dave H]

bump! anyone could give me a hand please?

Could you upload a readable image to the forum? When I click on the one posted all I get is a survey or Facebook offering me £153 a day to work from home.

[henter2015]

Totally agreed with the callygraphy! looool sorry about that!

Should I be concerned and try and get the HT down?

Dave here is another link, hope this one works for you:



Thanks!