lowering the plate volts ?

[jamme61]

I have an old marshall JMP (1980) it has the different transformer which runs around 450 plate volts. I like to know if there's an easy safe way to lower the plate volts to around 390? like my older jmp (1977)sounded and felt better

[dobbhill]

I would switch the power transformer for one with a desired secondary voltage. I would think that a used one could be found to save money, but I could be wrong. There are plenty of choices/options for new replacement. Brian Wallace has comprehensive data page here:
http://www.marstran.com/Historic%20Data%20Base.htm

D

[Stevem]

A 50 watt Zener like a NTE 5281k placed in the PT center tap will drop out around 60 volts from the B+ like you are looking to do.
You will need a clear space 3/4" on the top and bottom of the chassis to drill the hole to stud mount this diode and the mounting location needs to be as far away from the first gain stage as you can make it!

[pops]

The zener works well, but i prefer a large choke before the first filter cap over the zener, but the best way was posted by dobbhill, a new power transformer.

[jamme61]

would I be able to switch the voltage (450 -390) if I used the zener? The chassis would be the heat-sink for the zener? This dosent effect the heater voltage right? thanks

[Dingleberry]

Would a variable voltage regulator (vvr) work in this kind of situations?

[matt h]

(deleted)

[hans-jörg]

Hi,
I´m in a similar situation where I have (want) to reduce from 345VDC to ~310VDC for a Liverpool 2 holer with 2x EL84 and FW rectifier. So 30-40 V. I want use the IFR820 Mosfet with the Geofex B+ reducer. But I don't have by hand 1 40V Zener, but 4 x 10V (which will pull ~7V each). Can I put them in series to reach the focused 35-40V?

I think yes, but better to ask.

Hans-Jörg

PS: Somebody from European side knows a supplier where I could get such Zener for chassis mount (NTE 5281k)? I tried in German speaking area with no success.

[rp]

It's these, the large metal case bolt on, in the pict. I've used these in the (far away) past, and were easy to get from Mouser or Newark. I think they are long out of production, not available anywhere, and little info on the web other than old NTE pdf spec sheets.

If someone knows were to source big chassis mount reverse zeners please post.

[hans-jörg]

If someone knows were to source big chassis mount reverse zeners please post.

Yea!
Please post if available in Europe (private business over the pond is not so easy as it looks

I tried the Mouser number too, but no respond

Hans-Jörg

[Masco]

I'd use Aiken's low watt zener and FET soloution here.

[jamme61]

I talked to Skip at skipz circuits and he recommended this kit
http://www.skipzcircuits.com/Accessories.html only $19 I have used Skip's stuff before great guy great stuff IMHO thanks for all the info and help.

[rp]

I talked to Skip at skipz circuits and he recommended this kit http://www.skipzcircuits.com/Accessories.html only $19 I have used Skip's stuff before great guy great stuff IMHO thanks for all the info and help.

Interesting site to know about.

as for this, regarding his and the aiken circuit, This kit provides an easier method than the traditional high wattage zener diode. What could be easier than drilling a hole and bolting one part into place? Those 50W iron-clad zeners seemed chunky and reliable, the ones I used were 20+ years ago in a few SF Fenders that ran too hot when the then ex-commie tubes were total crap, and there weren't dozens of places to get drop in replacement PTs. So not sure if they are still running ok but I'm not all sure of the mosfet which you still need to bolt with some grease, and instead of one big part you have a few to fail. For all the trouble I'd just install a VVR, you're 3/4 of the ways there anyway and for all the effort you get a supposedly great attenuator. Not that this helps if you have an amp that you must absolutely limit the voltage on.

Anyway, I'd like to know what happened to the chassis mount reverse zeners? Guess only amp geeks in the late '80s with over juiced amps bought them and that wasn't enough of a market for big Asian E-conglomerates.

[hans-jörg]

Hi,
I done this B+ reduction same way like Geofex it shows.
I had at home 1 IFR820, 1x200Ohm, 1x 2k1 (2Watt) and 5x 10V/5W Zener resistors.
Could drop 35V (7V per zener) and the Mosfet doesn´t get even hand worm.
Grate!
One question only: with a VVR it is recommended to place a rubber or silicat layer between the Mosfet and chassis. In the Geofex schem (a fixed B+ reduction, not VVR) I don't see a necessity for that. Am I right? Though I put one thin rubber layer for save and because I wanted to use the same grounding point like my first cap via a wire from pin 2 (Mosfet).

http://www.geofex.com/Article_Folders/m ... tfolly.htm

Hans-Jörg

[stuben]

Did a quick google search for nte5278AK and lo and behold the parts are readily available from several vendors. I've used them in a few amps several years ago with great results.

I tried using a switch so i could switch between the higher and lower voltages and that works fine, but be careful! It is easy to red plate your tubes when switching between the lower and higher voltages. Bias your tubes very cold before switching to the higher voltage!

Go to www.ntepartsdirect.com and enter nte5278AK in the search window. The last two numbers indicate the voltage drop the part produces. An nte5268AK will drop your B+ by 30 volts, so they are available in a wide variety.

Also be sure to mount the device as close as possible to where you PT center tap was grounded.

I built an express type amp with Fender iron as an experiment nearly 15 years ago and still have it. The transformer was 340-0-340 @200ma. The reverse polarity zener brought the B+ down from 460 to 410 which made a huge difference.

I don't see why anyone would not like this simple mod. What could be easier than drilling one hole and bolting down this small device and then soldering one wire to it? I see no downside.