PP output stages - different OPT primary Z - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 18th June 2003, 01:29 AM   #1
diyAudio Member
 
Join Date: Oct 2002
Location: Israel
Default PP output stages - different OPT primary Z

Hi all,

I just want to know if i got something right:

Since the OPT effectively acts as the plate resistor in PP output stages it means that i can chose any input Z for the transformer as long as the PS can supply the needed voltage and current. For example, my amp has 10K on the OPT primaries and 4ohm secondaries, while the amp is spec'd at 36WPC - that means that the output stage draws I=(36/4)^0.5 = 3A then the output stage consumes 3/25=120mA with the 25 being the turn ratio. So if i want to use an OPT with a 5K primary i can as long as the PS can supply the needed 170mA? (same goes for voltage of course).

Cheers,
Guy.
  Reply With Quote
Old 18th June 2003, 07:27 AM   #2
diyAudio Member
 
analog_sa's Avatar
 
Join Date: Aug 2002
Location: Sofia
All these calcs are pretty meaningless without taking the o/p tubes into account - they determine the power/current through the o/p transformer. You should start everything by looking at the output curves of the tubes and determining the power for your particular output transformer/load


cheers
peter
  Reply With Quote
Old 18th June 2003, 12:45 PM   #3
Joel is offline Joel  United States
diyAudio Member
 
Joel's Avatar
 
Join Date: Oct 2002
Location: Brooklyn, NY
No tube output stage I know draws 3 amps!!!

Your formula is incorrect. What tubes are you running?
  Reply With Quote
Old 18th June 2003, 12:49 PM   #4
diyAudio Member
 
Join Date: Nov 2002
Location: North Herts, UK
Hi Joel,

I think he meant 3 amps in the secondary...

or about 125mA in the primary...

I asume both are RMS???

But as Peter says, Guy needs to work from his output tubes and topology first.

I recommend some reading on Steve Bench's site about Load Lines as a good place to start.

ciao

James
  Reply With Quote
Old 18th June 2003, 02:39 PM   #5
diyAudio Member
 
Join Date: Oct 2002
Location: Israel
Hi all,

Sorry about the 3A... the output stage draws about 120mA as James said

I am attaching the output stage of my current amp (WAD KAT88), i'm trying to understand what would happen if i use a 5K primary traffo instead of the 10K one supplied with it originally. I am trying to learn how to conclude correct things from loadlines, but it is a bit confusing.

For example with this output stage - i am looking at the pentode charactaristics of the KT88 and trying to understand - WAD used
-10V of bias (point E), 255V (point D) fed to the grid and the CT has 510V (point A) - how do i know what's the loadline? if i understood correctly (which is not so certain ) i calculate the loadline in the following way:
510/10K=51mA and then draw a straight line from the 51mA to 510V? If so where is the operating point with -10V bias? I'm totally confused, totally.

Can someone try and explain these graphs and loadlines or refer me to a source which does (i'll check out Steve's pages later today).

I want to note that gaining the knowledge is what i'm after, not the specific mod's i need to perform in order to change the OPT from 10K to 5K although it can function as an interesting example.... i think....

Cheers,
Guy.
Attached Images
File Type: jpg outputstage.jpg (25.5 KB, 185 views)
  Reply With Quote
Old 18th June 2003, 03:11 PM   #6
Joel is offline Joel  United States
diyAudio Member
 
Joel's Avatar
 
Join Date: Oct 2002
Location: Brooklyn, NY
Well, graphically, all loads have a corresponding angle on the plate curve charts. For example, drawing a line with a 22 degree slope on the 6080 charts equals a load of 2500 ohms. Anywhere on the chart that you draw a line with that angle will be showing the behavior of a 2500 ohm load. And then you can see exactly what the tube's plate current and voltage will be as it moves across that line as the signal input gets higher or lower.

I don't know the angles for a KT88, but you can figure them out by plotting some load lines. Any RCA tube manual has a tutorial on plotting them.

Joel
  Reply With Quote
Old 18th June 2003, 03:36 PM   #7
diyAudio Member
 
Join Date: Nov 2002
Location: North Herts, UK
Default ...That's a load off my mind...

Guy,

Steve Bench's pages are best.

His site is Steve Bench Site

and the load line tutorial starts at Loadlines Page 1

There are five pages they start with triodes and work through all the uses and theory. Then Steve does push pull load lines and then Pentodes.

They are the clearest explaination I have found and easy to work through.

Good luck!

ciao

James
  Reply With Quote
Old 18th June 2003, 03:45 PM   #8
diyAudio Member
 
Join Date: Oct 2002
Location: Israel
Thanks people - i'm printing the article parts as i write this

Cheers,
Guy.
  Reply With Quote
Old 18th June 2003, 08:36 PM   #9
diyAudio Member
 
Join Date: May 2003
Location: Morton, Illinois
Default Impedance...

Hi Guydot.
I hope I am not butting in.
There are some things to consider in PP operation.

1) If the output tubes are run strickly class A, triode, basically no worries. You can run almost any impedance tranny. (I cover myself here). Tetrode, pentode operation may be affected more as the screen may be subject to increased dissapation when clipping. I haven't dealt with tetrode operation in a long time.

2) If you are running the output tubes AB1, or AB2, you have to worry about the area under the curve as far as plate dissapation, and screen dissapation. Clipping is another consideration as this increases the heat dissapation on the plate and screen.
Distortion also changes with load. I don't have the figures etc at hand on this.

Hope this helps.
  Reply With Quote
Old 18th June 2003, 11:20 PM   #10
PRR is offline PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: USA
> if i want to use an OPT with a 5K primary i can as long as the PS can supply the needed 170mA?

And if the tubes will pass twice the current!!!!

> to conclude correct things from loadlines

Radiotron Designers Manual has THE definitive explanation, but there must be good pages on the Web.

> 510/10K=51mA and then draw a straight line from the 51mA to 510V?

No. The plate load on one valve, in most cases, is /4 of the plate to plate load. And the no-signal load current is always ZERO.

So the dynamic load line passes through Zero Current at B+, and B+/(Rl/4)

The actual current for each valve follows that AC loadline at full signal, bending off to intersect the bias point.

Click the image to open in full size.

Straight lines are the push-pull AC loadlines for 5K and 10K. The bent line is a possible one-valve loadline.

Assuming no grid current:

Pentodes are current-limited. We set the screen voltage to pass the maximum current needed to cover the proposed load line. If you switch to another load-line, you need to change the screen voltage or the tube will run out of current. See the 5K line above: it gives a little more current swing and a lot less voltage swing. 5K makes less power than 10K, for this tube, for these voltages. If you do a full analysis you will also find a lot of heat in the plate.

Taking the screen voltage to 300 or 400 volts might cover the 5K loadline. But I bet you run way over the plate dissipation rating. You will also need 1.5X to 2X the grid drive.

> -10V of bias

Sounds AWFUL low. I don't know any 35-watt tube that will run with 10V grid bias. It looks like 72 Watts idle dissipation in what is probably a 35W or 50W plate. Aint gonna live long.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Primary impedance output transformer Rob11966 Tubes / Valves 5 2nd June 2008 03:14 PM
output transformer primary imp. calculation frank2395 Tubes / Valves 4 22nd April 2005 07:24 PM
SE Output Primary Impedence Mike L. Tubes / Valves 6 20th April 2005 11:54 AM


New To Site? Need Help?

All times are GMT. The time now is 05:37 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2