Dumble 50 watt 4 x 6V6 high plate voltage

[petermerlot]

Hi All,
I have modded my 50 watt Dumble to a HRM and then gone further with adding 2 valve sockets and running 4 x 6V6 tubes. I have it Biased low as I am getting 480 Volts across pin 3 and earth (Tube installed). Can any one out there show me by way of illustration how to lower the plate voltage. I have thought of using a 5w 50v Zener but I am not sure if 5w is high enough or just where to connect it. I was thinking maybe connect it where the wire comes of the 22uf 500V cap that goes to pin 4.
Any help is appreciated.....

[jon]

Search for Mosfet amplified Zener Diode. I think that this will be what your looking for.

Edit:

http://www.aikenamps.com/Zener2.pdf

[petermerlot]

Thanks for the link, That is a good start. A bit technical for my tiny brain but I think I can nut it out in time. So 2 different approaches to this problem.
Regards
Peter

[wjdunham]

Hi All,
I have modded my 50 watt Dumble to a HRM and then gone further with adding 2 valve sockets and running 4 x 6V6 tubes. I have it Biased low as I am getting 480 Volts across pin 3 and earth (Tube installed). Can any one out there show me by way of illustration how to lower the plate voltage. I have thought of using a 5w 50v Zener but I am not sure if 5w is high enough or just where to connect it. I was thinking maybe connect it where the wire comes of the 22uf 500V cap that goes to pin 4.
Any help is appreciated.....

You can find high-power Zener's at Mouser. I use them for exactly this purpose, they mount directly to the chassis and come with a paste for better thermal contact with the chassis. Use the 56V 50W NTE diode and you'll be fine Not cheap but very effective. Put the diode in the center tap (ground) of the transformer secondary.


Bill
www.sebagosound.com

[M Fowler]

Reverse Zener diode (AK) they work very well to drop some voltage.

[petermerlot]

Thanks Bill
So I can use the first option as per link
http://www.aikenamps.com/Zener2.pdf using mosfet

or

try the 56v 50w Zener. I have 480volts DC at pin 3 and idealy would like to drop 100v and make this switchable so would a 100v100w zener be ok?
What is the safest and best option,
Thanks in advance

Peter

[Structo]

Yes, I used a 25 watt Zener on my organ conversion amp to drop about 35 volts if I recall.

It is a stud mount diode that uses the chassis to dissipate heat.

You connect the center tap of the power transformer high voltage secondary\

to it.
I believe also you have to get one that has the cathode on the stud that mounts to the chassis.

[petermerlot]

Thanks Tom, I tried Mouser and that 56 volt and 100 volt Zeners are not available on line. I will search out some other suppliers. It seems that the high power Zener is the way to go though instead of the low watt mosfet design

[talbany]

Thanks Tom, I tried Mouser and that 56 volt and 100 volt Zeners are not available on line. I will search out some other suppliers. It seems that the high power Zener is the way to go though instead of the low watt mosfet design

https://taweber.powweb.com/store/vdump.htm

T

[Structo]

Yeah it works well.
I got mine at a local surplus place.
I think I paid around $18 for it.

The key thing is the polarity and the power handling at the volt rating you want to drop.

You need a reverse polarity (cathode to case).

What kind of 6V6GT are you running?

JJ 6V6GT's are known to take high voltages.

Leo Fender used to subject his 6V6GT amps to over 400v on several amps including the Deluxe Reverb, Harvard, Princeton Reverb, etc.

Here are some Zeners at Newark Elec.

http://goo.gl/1mPK4

[M Fowler]

Make sure you note the ordering instructions: • Reverse polarity with cathode-to-case by designation R suffix in part number, e.g. 1N3305RB, etc

[petermerlot]

Hi Tom, I am using Tungsol 6V6GT at the moment.
I founs a supplier but I have to check that it is the correct polarity.

In my case I am not wanting to lower the voltage across the pre amp tubes, only the 4 x 6v6 Tubes so I am thinking of conecting a 68v 20w Zener from the wire comming of of the 20uF 600V capacitor that goes to pin 6 and subsequently branches to all the other PIN-6 6v6 tubes. What are your thoughts on that? See attached:D

[rock_mumbles]

Will the zener in the PT center tap mess up the bias supply voltage?

[ToneMerc]

Will the zener in the PT center tap mess up the bias supply voltage?

Yes,especially if the bias is fed from the HT winding, but he would be reducing the B+ to where it should be anyway.
TM

[ToneMerc]

Hi Tom, I am using Tungsol 6V6GT at the moment.
I founs a supplier but I have to check that it is the correct polarity.

In my case I am not wanting to lower the voltage across the pre amp tubes, only the 4 x 6v6 Tubes so I am thinking of conecting a 68v 20w Zener from the wire comming of of the 20uF 600V capacitor that goes to pin 6 and subsequently branches to all the other PIN-6 6v6 tubes. What are your thoughts on that? See attached:D

You have been given the correct location where to install it, in the PT center tap. Any method at pin 6 only affects the screen voltage.In the end you might have to change resistors in the dropping string to obtain the correct voltage range anyway. Also, look at it this way, all you are doing is reducing the PT closer to what's called for anyway.

TM