Marshall 100watt Plate voltage

[eddiep1966]

I built 100 watt clone the voltages seem a bit high. Plate are 560 volts, its a 1959 circuit but i have never seen with this high of plate voltage. Its running el34s. A friend of mine is 65 and had played Marshall 50 watts and 100watts all his life and he said its the best Marshall 100 watt he has ever heard. It does sound amazing and the preamp mods is heavy. But just wondering if the voltages are common.

[Colossal]

Out of curiosity, what are the screen voltages in that amp (if you have measured them)? There was a hot-rodded Marshall designed by Mark Cameron some years back which was a stunning amp. I believe it ran high plate voltages (over 500V), if I recall correctly.

Also, what bias voltage and current are you running in your amp?

[eddiep1966]

Thanks for the response Yell I did measure them but forgot what it was I'm old LOL but I'm going to make a voltage chart and post it. Its bias now at 65% 29.2ma (25/556=AA AA X 0.65 = 29.2). I have all ways liked my tubes a little cold. Bais volt will have to measure. I'll try to get them voltages posted tomorrow or Sunday. Had another amp come in to be worked on.

[jbrew73]

What power transformer did you use in the amp? Some Marshall’s had really high plate voltage don’t they?

[eddiep1966]

Hammond 290hx. With a Tranny 350v I just didn't expect it to go that high. I'm not a power supply designer so their is thing i don't understand.

[Roe]

the first 1959s did have 560v, see here https://www.drtube.com/schematics/marshall/1959t-66.gif

This requires a high Z (here 4k). the problem is screen overdissipation

[jbrew73]

Hammond 290hx. With a Tranny 350v I just didn't expect it to go that high. I'm not a power supply designer so their is thing i don't understand.

Based on the transformer specs that you used I would not expect that high of a b+ either.

How are you measuring idle current? Is there any way that your current is 29.2mA per side vs per tube?

[martin manning]

Hammond 290hx. With a Tranny 350v I just didn't expect it to go that high. I'm not a power supply designer so their is thing i don't understand. :)

What is your line voltage, and which primary tap are you using? With the secondary voltage where it is now and typical 1k7 load line, I think you will be going through power tubes quickly. The PT you are using will have little sag under load, there is only 5% between the unloaded voltage and voltage at 450 mA. Beaucoup sag is what enabled the high voltage amps to get away with it.

[jabguit]

Thanks for the response Yell I did measure them but forgot what it was I'm old LOL but I'm going to make a voltage chart and post it. Its bias now at 65% 29.2ma (25/556=AA AA X 0.65 = 29.2). I have all ways liked my tubes a little cold. Bais volt will have to measure. I'll try to get them voltages posted tomorrow or Sunday. Had another amp come in to be worked on.

You're not that old if you use LOL.....

[eddiep1966]

29.2ma is per tube. Your right Roe I do need to check screen dissipation think I’m having memory loss. Line voltage is 126v most of the time. Common and 120v is what the line is connected to. This is the schematic that the base amp was built from. Power supply and power section was unchanged form the schematic. But now that I have went back and look at the schematic again wouldn’t the center tap of the 350v secondary connection be a voltage doubler?https://www.drtube.com/schematics/marshall/1959u.gif

[martin manning]

… But now that I have went back and look at the schematic again wouldn’t the center tap of the 350v secondary connection be a voltage doubler?https://www.drtube.com/schematics/marshall/1959u.gif

The schematic shows a full wave bridge rectifier. The center tap of the winding is connected to the center of the reservoir capacitor stack, which insures that the two caps each get half of the B+ voltage.

[sluckey]

...wouldn’t the center tap of the 350v secondary connection be a voltage doubler?

No. The circuit is a common full wave bridge rectifier. The center tap connection simply forces the voltage across each of the two series connected reservoir caps to be equal.

EDIT... Sorry Martin. I did not see your post until after I posted.

[martin manning]

No worries Steve.

...wouldn’t the center tap of the 350v secondary connection be a voltage doubler?

Some people call a FWB a voltage doubler, but it is two superimposed half-wave rectifiers.

[Guy77]

I have used that 290hx in a bunch of 100 watt builds and I usually get about 455v - 460v on the power tube plates once I take the amp off of standby with a 120v wall voltage.
That transformer should not be giving 560v on the power tube plates even with a 126v wall voltage.

Something seems off here.

G.

[martin manning]

Indeed, the loaded spec is 175-0-175, and so the loaded voltage should be 2x175x1.414x126/120=519.6 or in reality probably around 500V.