Metropoulos Forum

Hello everyone!

I've bought a MM Radiospares P18RS power transformer for my 18watt TMB project, thinking that this would be the best choice.
When the transformer arrived, I found that the voltage at the secondary HV taps is 316VAC whilst the norm in all the other schematics 290V.

Mercury told that this is a perfect clone of an original RS PT as found in vintage Marshalls, that all the voltages was +/- 20% and that they prefer this configuration which gives a clearer and more articulate sounding amp.

316VAC should aproximately raise to 370VDC of plate current which might be considered a bit on the high side for EL84s, which are normally driven at 340...

Is there anyone out there who has faced already such an issue? Reading the TAD EL84s datasheet I've found that they have a plate voltage limit of 420VDC, but many says that you shouldn't override 340... I'm confused, I wouldn't abandon the schematic I'm following which is the most used (BYOC/Ceriatone) but I also would not build an unreliable amp.

Thanks for anyone chimes in with a comfort word!

Fabio

There is so much going on in the tube guitar amps that I don't really think that some one was really reading datasheets... VOX-es runs their el84 at 17 watts, when the data sheet says no more than 12 watts, screen voltage on kt66 in marshalls are way higher than is suppose to be, AMPEGS with 6550's push them so hard to get 300 watts out of them and people do not understand why they have to retube thier amps every year or two. 6x6550 are not so cheap.
Back to your question... old mullards could run up to 500v on the plate and 300v on the screens and no more than a 12 watts on plate dissipation. 12 watts = 500v x 24mA. If the plate current if 24mA or less - tube will work properly. More power out of el84? JJ's or Sovteks EL84M. These are more powerfull than other EL84, which means that they are not exactly a regular El84, but some thing similar, very similar

First I wouldn't use Murcry stuff ever. Not worth the price and the sales pitch. Looks like they have 2 power transformers.. one with low b+. I also don't like how they don't give specifics about their products on their site. Their output transformers don't specify the build (paper bobbin ect) . Rant over.

I like normal or lower volts on el84 and all power valves. Higher volts gives a brighter sound and crisper tone where as lower is not so brightly piercing and more "chewy" in the over drive. My 18 watt is at 340v and honestly Id prefer if the volts were lower. But have not done a comparison. The amp is very bright as is and could imagine the tone at higher volts. I don't use the tone knob as that is a treble cut and the tone becomes to dark when used.

The 18watt is the precursor to the SS 20 watts. Those I do belive had 370v.