mal young mod to reduce the screen voltage

[Roe]

Discussing the high b+ voltage, ****** writes:


AC/DC ran into this same problem....Especially with Malcolm's amps...
They used a resisitor in place of the inductor...This way the plate voltage remained the same, which is never the problem and this resistor lowered the voltage feeding the screens and rest of amplifier... With the screen voltage much lower the tubes won't blow up... Also not having the inductor will make the amp smoother since you avoid any possible ringing associated with the LC filter.... When you slam the strings you make a SQUARE wave in the circuit which lowers the impedance looking into the screen terminal of the tube....After the string slam the impedance goes back up higher looking into the screen terminal and this is what causes the ringing in some amps...
....
Chris

[shakti]

I believe this is standard procedure in most Hiwatt amps - a 470 ohm 10W resistor in place of a choke in the power supply. The Jimmy Page custom Hiwatt had a choke, which was very unusual for a Hiwatt. Funny thing is, one guy told me the choke was technically an improvement over the resistor, so I'm not sure what to believe...

[Flames1950]

Would be helpful to know the value of resistor for sure.

The 20H RS choke that some 45/100 and JTM100 amps used had 690 ohms DC resistance already, so I'd guess the 1.5K - 2K range?

[Roe]

I guess you might need more than 2k if you want to drop the screens voltage from, say 620ish volts (in the case of mal's amp) to 550 or even 500 volts.
I haven't tried to do the math but a 1k 25w resistor in my express drops the screen voltage almost 20volts it seems.

[shakti]

Would be helpful to know the value of resistor for sure.

The 20H RS choke that some 45/100 and JTM100 amps used had 690 ohms DC resistance already, so I'd guess the 1.5K - 2K range?

Does anyone know the DC resistance of the 352-114 Drake-style choke George sells with his chassis? I don't have my amp on hand to measure, and I'm curious of changing the choke to a 20H RS style will reduce the screens voltage? I want to preserve my GECs as best as possible...

[Flames1950]

It doesn't reduce it much. Measurements off my Super PA last week with the 20H choke were around 532VDC on the plates, about 528-529 on the screens -- and that's at pin 4, after the 1K screen grid resistors. You'll need more resistance to get the job done.

[Brian Wallace]

Would be helpful to know the value of resistor for sure.

The 20H RS choke that some 45/100 and JTM100 amps used had 690 ohms DC resistance already, so I'd guess the 1.5K - 2K range?

Does anyone know the DC resistance of the 352-114 Drake-style choke George sells with his chassis? I don't have my amp on hand to measure, and I'm curious of changing the choke to a 20H RS style will reduce the screens voltage? I want to preserve my GECs as best as possible...

That choke should be around 100 ohms.

[shakti]

Thanks, Brian!

I think I'll try your 20H RS choke eventually.

I'm curious as to why the voltage doesn't drop any more even with 690 ohms (+1k individual screen resistors) though. One of the things that sets the Jimmy Page custom Hiwatt amps apart from a regular Hiwatt is the presence of a 5H choke in place of the standard screen voltage drop resistor. But the standard value screen voltage drop resistor in a Hiwatt is only 470 ohms@10W. Still, reports say that using a resistor either alone or in series with the choke results in a lot less stress on the screens.

[Roe]

the 20h choke may help with ghosting also. it is only rated at 70ma but other chokes are not much more - they are 100ma in 50w, 100w and 200w. ****** says marshall didn't have a clue what they were doing. by comparison, a komet - a 50-60w amp - has a 200ma 10H choke