Go Back   Home > Forums > >

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

Got some money, going to buy OPTs for PP GU-50
Got some money, going to buy OPTs for PP GU-50
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
Old 23rd November 2020, 06:36 PM   #31
nerdorama is offline nerdorama  United States
diyAudio Member
 
Join Date: Aug 2005
Location: Spokane
Here's a post on this from years ago by Mike at Magnequest regarding his rule of thumb for calculating an acceptable bias current for an unknown transformer.

RE: Looking for a output trannie match, Scott LK72 - mqracing - Tube DIY Asylum
  Reply With Quote
Old 23rd November 2020, 06:53 PM   #32
TG is online now TG  Ukraine
diyAudio Member
 
Join Date: May 2002
Location: Kiev, Ukraine
Correction to my previous post: 56mA is the cathode current, not the plate one (forgot about g2).
Plate current is 44mA, g2 current is 12mA, DCR is 250R (each half), so extra 3.1W of heat at 90mA and 22.5V drop. Not that great indeed.
  Reply With Quote
Old 23rd November 2020, 11:00 PM   #33
Schmitz77 is offline Schmitz77  Germany
diyAudio Member
 
Join Date: Apr 2015
There is a saying here in germoney:
whoever buys cheap buys twice
  Reply With Quote
Old 24th November 2020, 10:46 PM   #34
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Got some money, going to buy OPTs for PP GU-50
Quote:
Originally Posted by nerdorama View Post
Here's a post on this from years ago by Mike at Magnequest regarding his rule of thumb for calculating an acceptable bias current for an unknown transformer.

RE: Looking for a output trannie match, Scott LK72 - mqracing - Tube DIY Asylum
Quote:
calculate the voltage drop--- 90 ohms times 150 mils would be a voltage drop of 13.5 volts.

90 ohms times 75 mils would be a voltage drop of 6.75 volts

as a rule of thumb--- your probably not going to have a problem if you keep this voltage drop below 20 volts (in the context of an ot with tube friendly impedances and power supply voltages and etc). Remember it is a rule of thumb meaning someone, somewhere will be able to find a case where it would get you in trouble--- but they should be rare cases indeed.

generally---- my rule of thumb looks at anything less than a 20 volt drop as not being much to worry about--- again--- excluding the unusual or weird case that someone might dream up.

other rule of thumb is to calculate how many watts are dissapated across the winding----

90 ohms and 75 mils would dissapate about one half of one watt of power going up in heat. Not much at all.

formula is current squared times resistance
Hmmm...

So with Ip = 0.09A per winding, each winding (half of the primary) needs to have DCR of no more than about 220 ohms.

220R(0.09A) = 19.8V dropped across the winding

The power dissipated would be

0.09squared(220R) = 1.782 watts (hot!)

So it looks like 220 ohms per primary half would be marginal. Better to aim for 150 ohms or less.

Is this achievable? I don't know... I need to measure some OPTs.
--

A pair of 8k:VC OPTs meant for Push-Pull EL84 measure about 200R per primary winding, or almost 400R from plate to plate.
I guess those would be marginal for Ip = 90mA per tube.

--

Last edited by rongon; 24th November 2020 at 10:56 PM.
  Reply With Quote
Old 24th November 2020, 11:53 PM   #35
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Got some money, going to buy OPTs for PP GU-50
Another push-pull OPT I have, with a 5k primary rated for 30W and 75mA per tube max, measures 85R per primary winding (170R plate to plate).

Specs for the Dynaco MK-III Output Transformer A-431:

4.3K Push-Pull with 33% Screen Taps to 4, 8, & 16 ohms 60 Watts
Frequency Response: 10 Hz to 100 kHz +/- 1 dB @ 1 Watt
Primary DC Resistance: 142 ohms
Primary Inductance: 40H @ 120 Hz

Hmmm, only 142 ohms. I think that's plate-to-plate too. So it does appear that a 50 or 60 watt OPT will work best for my application. The thing is, I remember what the Dyna A431 OPTs sounded like. I tried using them for a pair of 300Bs and 2A3s. They sound kind of big and dopey compared to something like the Dynaco A470 (ST70) or the OPTs from a Fisher 800C (why did I sell those off??). My suspicion is that big honkin' OPTs like the Dyna A431 have that big transformer thickness to their sound. Unfortunately, that's what's going to be required to run GU-50s safely with 90mA quiescent Ip. Am I right, or am I on the wrong track here?

Transcendar's 30W 6.6k OPT specs are:

6.6K Push-Pull to 4, 8, &16 ohms
Frequency Response: 10Hz to 100 kHz +/- 1 dB @ 1 Watt
Primary DC Resistance: 235 ohms
Primary Inductance: 65 H @ 120 Hz
Weight: 4 lbs

So 235 ohms plate-to-plate and 4 lbs each. Those are a lot lighter in weight than Edcor's 30W OPTs (6 lbs each). I wonder what that means...

It looks like if I want some headroom for high current swings, I may need a pair of big boy OPTs. Perhaps 50W rated.

--

Last edited by rongon; Yesterday at 12:02 AM.
  Reply to this post

Reply


Got some money, going to buy OPTs for PP GU-50Hide this!Advertise here!
Thread Tools

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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
groups buy TDA1541AS2 chips, money back gurantee tube-lover Group Buys 222 24th October 2020 05:52 PM
"Solomon" Solder Station - Worth the money or stick with what I've got? Formula350 Everything Else 2 2nd February 2011 12:42 AM
I require the best portable CD player money can buy. mhouston Digital Source 7 9th February 2006 07:22 PM


New To Site? Need Help?

All times are GMT. The time now is 06:20 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Copyright ©1999-2020 diyAudio
Wiki