Matchless lighting new build using MM P18RS

[Dave H]

That link works great thanks.

To reduce the HT use a lossy rectifier (EZ81 or 5Y3) and fine tune it with resistors in series with the rectifier's anodes.

EDIT: My El84 amp has B+ 336V, EL84 plate 333V, screen 318V, cathode 11V, cathode resistor 270R (per tube) and the plate current is 35mA. That's 11.27W plate dissipation.

[J M Fahey]

1) now that's readable

2) +V is already high on the original design, you have *apparently* 20VAC above that meaning ~30VDC above.

That said, I don't understand, at all, why would you use a transformer designed to feed big bottle tubes (5881/KT66/EL34) and capable of generating +450V +B for a conservaatively rated 30W amp , to feed a puny (by comparison) small bottle 15W amp needing +350V

[Zozobra]

If you're going to use SS rectification than ampmaker sell a great little PT which is suitable for your needs:
Amp Maker: Guitar amp kits and parts :: Power transformers :: 0-190-275V 20W power transformer

[Mick Bailey]

Mercury lists that transformer for a Marshall 18W and it's the RS version, so that would be a general-purpose transformer at the time, pressed into service by Marshall in the days before they specified their own transformers and just bought what they could get hold of.

If it were me, and that's what I'd got to hand, I'd build the amp, power it up and measure the voltages then fix those that were too high with a mosfet dropper or Zener, and back-to-back diodes on the heater supplies if they were way over when loaded. If the transformer is intended for, and works in an 18-watter, then it should work in the Matchless.

[henter2015]

Hi Thanks all,

I guess for £30 I would go for the ampmaker transformer, which gives me about 405V DC rectified, 275V AC (unloaded). about 355V loaded (as per the docs on their site with 2x12ax7 and 2 el84s). the lighting calls for about 340V-335V so I guess as it has an extra ecc83 it will be on the ballpark.
Any recommendations on what diodes to use on the bridge, I have 1N4007's lying around?
What tonal differences shall I expect? louder and tighter? would I miss the sag of the rectifier valve at all?

I will put the MM on sale on ebay, if anyone interested give me a shout!

[Mick Bailey]

would I miss the sag of the rectifier valve at all?

Only you can answer that. If you were using SS, 1N4007 is a good choice.

For me, I like a tube rectifier and if you're building a clone then you need to stick with that, otherwise it's something else. Like cloning a sheep and it having human lips.

Tube rectifiers also give more scope for fine-tuning the sag characteristics and B+.

Here's some useful information on voltage drops etc;

http://www.fourwater.com/files/fullrect.txt

[Dave H]

I guess for £30 I would go for the ampmaker transformer, which gives me about 405V DC rectified, 275V AC (unloaded). about 355V loaded (as per the docs on their site with 2x12ax7 and 2 el84s). the lighting calls for about 340V-335V so I guess as it has an extra ecc83 it will be on the ballpark.
Any recommendations on what diodes to use on the bridge, I have 1N4007's lying around?
What tonal differences shall I expect? louder and tighter? would I miss the sag of the rectifier valve at all?

I have one of those ampmaker transformers. With my high (nearly 250V) mains voltage the B+ was 370-375V. The EL84s were runnig far too hot so I used a 220R 10W resistor in series with the secondary to reduce the B+ to 330V. At 375V it's not really any louder, just a little brighter. I prefer the warmer sound and 'feel' that it has at 330V. The series resistor will give some 'rectifier' sag but I'd be inclined to do as Mick says and use your MM PT with a valve rectifier (and series resistors if necessary).

I fitted the ampmaker PT to have the half power option but I was getting some rectifier buzz using a 1N4007 bridge so I went back to my original PT which doesn't produce any buzz. I should have tried UF4007s but I didn't have any. This won't be problem with a valve rectifier.

[henter2015]

I guess I will give a go to the MM, measure everything loaded without valves and report back! thanks a lot really appreciate your help!

[Justin Thomas]

I built a Spitfire clone. I've got ~355V on the EL84s, which are JJs. I just don't bias them to the point of insane. No problems. And, EL84s are cheap, any way!

Justin

[Mick Bailey]

The Marshall 2020 runs 400v on the plates. Some Traynors are also in this region. Can't recall what the Dr. Z Maz 18 runs at, but it's hot and tubes last about 6 months of pro use.

[Dave H]

The Marshall 2020 runs 400v on the plates. Some Traynors are also in this region.

Why do that? Call me a conservative (with a small c) if you like but I just don't like the idea of running a 12W tube at 15W+ in a class AB amp.

[g1]

I believe the ones that run at high voltage (at least the Traynors) either lower the screens or cool the bias to keep them within "normal" dissipation limits.

[Justin Thomas]

To paraphrase an Enzoism: volts don't kill tubes. Watts do.
IOW, if your plate voltages are a bit high, fine. Just don't ask them to dissipate more than is needed to sound good.
That said, as g1 said, the screens are an issue for modern-production tubes...

Justin

[Dave H]

I was thinking of the early Marshall 20W. It has a B+ of 390V and a common 125R cathode resistor.

[ThermionicScott]

Why do that? Call me a conservative (with a small c) if you like but I just don't like the idea of running a 12W tube at 15W+ in a class AB amp.

Me too. I shake my head whenever someone takes great care to replicate the original specs on a classic amp, and then has to add Zeners and wirewounds to keep the plates from glowing. Sheesh, you could have done it right the first time and used a different PT...