Dumble 50 watt 4 x 6V6 high plate voltage

[petermerlot]

Yes of course, Pin 6 is the screen, not the Plate and that's what needs lowering. Thanks...

[petermerlot]

I only have a 68 v 20 Watt Zener and was thinking I could just lower voltage to the 4 x 6V6 Tubes

[Mark]

http://www.el34world.com/Forum/index.php?topic=10250.0

This thread gives a decent explanation of the zener circuit. If you have a separate bias winding there will be no change, if your bias comes off the secondary winding supplying the amp then there will be a change in bias voltage.

[petermerlot]

Hi Mark Thanks for the reply, infact thank "all" of you for your replies.
The power transformer has its own 60V tap so that should be unefected.
This amp started as a stock 50Watt Ceriatone OTS, then I modded it to a HRM and then further mods led to adding the 2 extra sockets and installing 4 x 6V6 Tungsol Tubes. I have it Biased at around 13.5mV at the moment and it sounds great, and when cranked the tubes are not glowing to the point of melt down but get to about 35 to 40Mv. I am concerned about the 480V plate voltage though. I would prefer to lower the plate voltage on the 4 x 6V6 tubes only though and leave the preamp tubes as they are (Hot). This amp is sounding great though

[ToneMerc]

Another method is to put a 100-200 ohm 25W metal fin power resistor in the PT center tap as well.

TM

[petermerlot]

Thanks TM,
"Any one else want to way in on this method"

[Structo]

Yes there are several ways to drop the voltage.

But, when you drop the B+, you will have to adjust your dropping string resistors lower to keep preamp and PI voltages where they need to be.

Just part of dialing an amp in.

[petermerlot]

Well I connected a 75Volt 20watt zener to the centertap on the primaries and it dropped about 30 volts for a minute or so and then failed. it now has continuous polarity through the diode. Maybe a 50watt will do the trick.

[Mark]

it now has continuous polarity through the diode

So you mean it is short circuited.

Did you have a headsink for the diode?

I'd imagine it got extremely hot before it blew?

I'd try and work out the wattage device you'll need.

P=E*I

So if you know the voltage you want to drop, it should just a matter of measuring the current at your centre tap. I'd put you your meter on AC amps, put one lead on the centre tap and the other on the earth point. It would be preferable to run a signal into it to find out your maximum current.

It should then be a matter of multiplying the voltage by the current flow.

75 volts * 0.266667mA = 20.00025 Watts

I wouldn't have thought you would have been drawing that much current. It is quite surprising.

[petermerlot]

Hi Mark, As far as a heat-sink goes I have a good "L: Shaped piece of thick aluminium and a fined type heatsink all with white paste so it should not be a heat problem I will get some alligator clips and try and measure the Current.
I will find a 50w 100v and try that though, That should be big enough and dropping 100v would be fine for this application, that would put me around 390v at pin 3

[Mark]

Post you current draw. Do you have a signal generator to feed into the amp?

A load would be great so you don't have to listen to the test signal.

http://www.valvewizard.co.uk/siggen.html

[Structo]

Peter did you use a reverse polarity Zener?

As in, cathode to case/ ground?

If your chassis is aluminum, I would just mount the diode to the chassis if there is room.
Don't put it near any filter caps, due to the heat.

Also, put a .01uF 600v cap across the anode and cathode of the diode to squelch any noise.

[M Fowler]

Ditto Tom,

I've tried to post the fact of using the reverse Zener diode (AK) but not sure my words have been heard.

Mark

[petermerlot]

Hi Mark, Your words were heard and I did ask for a reverse Zener but it was an old Philips one and I must admit that it did not have the prefix of "R" on it. I will get another and try it. I will also use your suggestion Tom. Thanks again,
Peter

[renshen1957]

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

Hi

Remember the Higher voltages are idle voltages, the 420V in a deluxe under a full load, drops to as low a B+ of 378V.

Kelley ran 4 6V6GTAs at 490V idle which would drop under full load to around 423V. His FACS was rated at 60W.

If you could find some Bendix, Tong Sol, or Centron 5992, these tubes wouldn't break a sweat (or easily go into distortion) at that voltage. Cost more than most will pay, currently $168.00 USD per pair on ebay, other tube sellers charge twice this amount.

Why not try the amp as it is with 6V6 such as JJ, or the Russian 6V6 EH-Gold Lion Reissues-Tong Sol, or TAD 6V6GT-STR, all have been reported to run on higher voltages than you mentioned.

Also JAN Philips, GE or Sylvanian 6V6GTA are available, and cheap for a NOS tube, currently about from 50-100 per pair, and cheaper than the 5992.

Best Regards.

Steve