voltage tolerance

[bnew63]

If an amp design calls for 290-0-290 how close in general do you need to be to the designed voltage?

I have a 390-0-390  PT that I want to use for this application and want to use a zener diode to drop the voltage.I did some research and there are many different thoughts on this method of voltage regulation.
A zener grounded through the HV centre tap seems to be way to do this.
What wattage of zener should I use and is using the chassis as a heat sink sufficient.


Brian

[flora]

100v is a lot to drop... I think I would buy a transformer with the voltages you need.

[bnew63]

I'm on a budget so I'm trying to use what I have.
Thats why I'm wondering if I can get away with suppling the amp with 320V instead of 290V.

Brian

[bnew63]

If I was to buy a new PT would 200ma be enough power for this design

[flora]

Around 180 - 200ma.

[jjasilli]

GZ34 has a forward conversion factor of 1.3:  1.3 X 390 VAC = 507 VDC.  5U4 @ 1.2 X 390 = 468. 

But your PT may be rated for 115 VAC primary.  Actual wall voltage of 120 to 125 VAC will push up your secondary voltage by a factor of, say, 1.087 X 468 = 508.  1.087 X 507 = 551. 

Is it possible to use 2X EL34's instead of 4X EL84's?

[tubesornothing]

Although someone has given you an answer, you can get this information - for any tubes - from the datasheet.

On the data sheet there will be a section called "Typical Operation".  Just look down the list of values for the plate current, sometimes called "Ia".  In your case, then multiply by 4 one for each tube.

[RicharD]

You're actually sluffin off > than 100V.  Let's look at using (4x) 1N4007 just for a moment.

290 * 1.4 = 406 - 2 = 404V
390 * 1.4 = 546 - 2 = 544V

140V * 180mA = 25.2W  That's some heat

Now your circuit calls for a GZ34 so
290 * 1.3 = 377 - 30 = 347V el perfecto for 6BQ5's.

So lets look at the very hungry 5R4 using your transformer.
390 * 1.1 = 429 - 65 = 364v Now we're getting close.

15V * 180mA = 2.7W  manageable.

15/.18 = 83 ohms (use a 10W device)
You can sluff off this voltage either by adding a B+ voltage node or raising the center tap offa ground.  Eiter way you'll need another electrolytic cap too.  1 cap, 1 resistor, and a different rectifier tube is bound to be cheaper than a new PT.

[bnew63]

I a newbie at this so I'm trying to stick to a established design.
That being said I'm not adverse to trying new things.


Buttery
to simplify change to a 5r4
use 83 ohm resistor 10watt
add another electrolytic cap
Could you possibly draw me a quick and dirty layout of this

Sooo many options

Thanks Brian

[tubeswell]

Back to the original question, if you want to drop ~100V using zeners, you could string a bunch of 11 x 9V 5W zeners together in series, but it will take up a bit of room mounting all those zeners. (using this method they need to be mounted on tag strips of a board clear of the chassis. Cathodes (banded ends) pointing to the ground return path).

Say your HT is delivering 200mA tops, then .2 x 99 = 19.8W. The eleven 5W together gives you 55W - plenty of margin.

[RicharD]

.

[jjasilli]

I a newbie at this so I'm trying to stick to a established design.  Then maybe get a new tranny!   

The 5R4 is a possible option, but note that it limits you to a 4MFD 1st filter cap.  (Guess you could use it as a "pre-filter stage" > dropping resistor > "1st filer stage".) 

Zeners regulate voltage.  Lots of Zeners regulate voltage a lot.  It may change the tone/performance/feel of the amp.

Mr. Natural says:  "Get the right transformer"

[RicharD]

>The 5R4 is a possible option, but note that it limits you to a 4MFD 1st filter cap.

http://www.mif.pg.gda.pl/homepages/frank/sheets/049/5/5R4GY.pdf
Whoops! JJ is absolutely correct.  I knew there was a reason I never use this tube.  Change that cap in my sketch to a 4uF and you should be OK since it is now the 1st cap.  I've never used zeners in a B+ supply.  It'd be borderline ironic to use zeners in a tube rectified supply.

[tubeswell]

Re the 5R4G, even tho' the datasheet says 4uF, remember that we are talkin' about geetar amps. I have 5R4 on 20uF and they seem to work okay, and I have heard of others doing so without issues. Of course you can put limiting resistors between the HT winding and the rectifier anodes to enhance the tube's robustness if you are still concerned about it.

[sluckey]

I've had good luck with tater masher style 5R4s in three different amps with up to 40uF input filter cap.

[jjasilli]

Change that cap in my sketch to a 4uF and you should be OK since it is now the 1st cap. Yes, but a 4Uf cap feeding the plates is a whole different sounding amp.  Hence my suggestion to use it as a "pre-filter stage".  I guess you could also get away with stressing the 5R4 with a larger cap -- if sluckey & tubeswell recommend it, that's ghood enough for me.

BUT I still feel B+ may remain too high in this particular application, especially with 120VAC wall voltage.

[FYL]

The 5R4 is a possible option, but note that it limits you to a 4MFD 1st filter cap.

You may use more capacitance provided that you don't exceed 650 mA peak plate current.

[RicharD]

>but a 4Uf cap feeding the plates

No the plates are fed from the next stage which is a 33uF.