OPT Selection Question - 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 27th October 2010, 05:07 AM   #1
diyAudio Member
 
Join Date: Oct 2010
Default OPT Selection Question

How would I determine the best OPT for a given amp?
Im still learning about tube amplifiers and im about a quarter of the way through the Army education guide on thermionics and tubes but I would like to know how to determine what OPT are compatible with a given amp design?
Specifically I have a Magnavox 185AA that I will be upgrading and modifying in the near future and I was wondering if a Hammond 1629SEA would work with the amp.
The amp has 2 sets of 2 6v6 tubes and uses 15w OPTs now
The Hammonds are 30 watts

The amp specs from the schematic state that the OPT has a 6000 ohm CT where the 1629SEA has 6500 ohm

what are the factors i need to look at to determine what would work and would be compatible?
  Reply With Quote
Old 27th October 2010, 05:14 AM   #2
diyAudio Member
 
dsavitsk's Avatar
 
Join Date: Jan 2005
Location: Hartford
What's wrong with the Magnavox transformers? Unles they are non-functional, I'd tend to doubt that Hammonds would be an improvement.
  Reply With Quote
Old 27th October 2010, 05:18 AM   #3
diyAudio Member
 
Join Date: Oct 2010
Im really just curious, I dont know if the transformers are good or not yet, Im still waiting to get the amp >.>

Im curious as to if they would work in the amp and also im learning and would like to know how to pick a OPT in future if I decide to build one from scratch
  Reply With Quote
Old 27th October 2010, 11:32 AM   #4
Ian444 is offline Ian444  Australia
diyAudio Member
 
Join Date: Nov 2008
Location: Brisbane QLD
The safe operating point of the output tubes is determined by the OPT primary impedance, the B+, the bias voltage of grid 1, and the screen grid voltage. The last three are somewhat adjustable, but not the OPT. You need to learn how to draw loadlines if you want to build your own design from scratch. The Hammonds would work fine,as in, 6K vs 6K5 is no big deal at all.
  Reply With Quote
Old 27th October 2010, 12:16 PM   #5
diyAudio Member
 
jmillerdoc's Avatar
 
Join Date: Mar 2009
Location: Lawton, OK, USA
Sounds like the amp you are planning to modify is a push pull topology, the Hammonds you are considering are single ended. You need to be looking for a push pull OPT for an upgrade. You may already realize this but I am going to say it now, by "upgrading" your OPTs from 15w to 30w ones will not increase the power output In of itself.

It is conceivable the Magnavox could be a parallel SE amp but I really doubt it. Based on this assertion, the Hammonds you are considering won't work.

If you want to upgrade the Magnavox, I would leave the OPTs alone and go for some more standard modifications like replacing the coupling caps with some descent 716p orange drops, replace all of the electrolytics with new ones, use a DVM and check all of the old CC resistors and replace those that have drifted out of spec, get rid of any ceramics in the signal path and replace with a suitable silver mica or film type, replace cathode bypass caps with some Elna Silmics or Panisonic FC caps, etc...

I think you will get more bang for the buck doing the above IMHO. Save the OPTs for last once you have finished the above, you may decide you can live with the stock units. Often times vintage iron is very very good.
  Reply With Quote
Old 28th October 2010, 01:29 AM   #6
diyAudio Member
 
Join Date: Oct 2010
excellent information
after reading this alot of what ive read in the US Navy document makes more sense than it did before, Im still working on finishing that
will probably be done with it either tonight or tomorrow

@jmillerdoc: I wasnt sure if just changing the transformer would give me more output, but a transformer capable of handling more power wouldnt be a bad idea if I plan to upgrade the amp in the future
Im still not clear on what tubes make how much power and why yet but im getting closer to understanding as I read through the online resources posted on this forum

this does clear some things up though
cant wait til I get the amp and can get hands on with it...
  Reply With Quote
Old 28th October 2010, 01:51 AM   #7
diyAudio Member
 
jmillerdoc's Avatar
 
Join Date: Mar 2009
Location: Lawton, OK, USA
Quote:
Originally Posted by mackinthebox View Post

@jmillerdoc: I wasnt sure if just changing the transformer would give me more output, but a transformer capable of handling more power wouldnt be a bad idea if I plan to upgrade the amp in the future
Im still not clear on what tubes make how much power and why yet but im getting closer to understanding as I read through the online resources posted on this forum

this does clear some things up though
cant wait til I get the amp and can get hands on with it...
It would be a very difficult undertaking to upgrade the circuitry to create more power out of the existing design......read on, it will become more evident as you go. If you want more power build an amp from scratch, there are many many great designs out there. You should consider Bruce Heran's Oddwatt series that uses the KT88 tube in PP, it creates about 30w and is a very simple circuit. Check out the sister site to this forum, DIY AUDIO PROJECTS - Do-It-Yourself Hi-Fi for Audiophiles, low at the Oddblocks, they can be built for half as much as the kit sells for.

Again, it is generally a waste of time, resources, and donor amp to modify the amp to the degree you double the power. Best just start with the amp that has the power you want and upgrade the stuff I mentioned earlier.

Good luck!
Jeff
  Reply With Quote
Old 28th October 2010, 02:25 AM   #8
diyAudio Member
 
Join Date: Oct 2010
I didnt think I could double the power but im definitely going to upgrade some of the smaller components such as what was previously suggested on this post
Im probably not going to touch the OPT but wanted to know how to select one if the matter came up (ie if the ones it comes with turn out to be bad)
The better I understand the amps design and capabilities the better I will understand how to make my own

I dont think I will need more than the 15 to 20 watts I will probably get from this amp but I do plan to build my own amp at some point in the future

thanks again for all the info, very helpful!

on a side note, ever use the Acoustic Research M3 Holographic speakers? Im about to get myself a pair for dirt cheap and was wondering how well they would work with a tube amp such as the one im about to obtain
  Reply With Quote
Old 10th November 2010, 03:17 PM   #9
diyAudio Member
 
Join Date: Oct 2010
Im curious if a pair of Tomiko toroidal output transformers would work with this amp
theyre 5k primary with 4 and 8 ohm secondary
what would be the harm in using a transformer with a lower primary resistance?

the ebay auction says they were used for a 300B single ended amp originally

thanks again in advance
  Reply With Quote
Old 10th November 2010, 03:28 PM   #10
diyAudio Member
 
Ty_Bower's Avatar
 
Join Date: Mar 2007
Location: Newark, DE
Quote:
Originally Posted by mackinthebox View Post
what would be the harm in using a transformer with a lower primary resistance?
A lower primary DC resistance should result in improved efficiency. There will be less copper loss in the primary. The low resistance might be due to heavier gauge wire used in the winding, or it might be due to a shorter winding. Heavy gauge windings necessarily mean a bigger overall transformer, which carries its own implications. A shorter winding should offer lower primary inductance, which might be bad for low frequency extension.

Lowering the primary impedance has the effect of rotating the load line clockwise, making it more towards vertical. Generally speaking, a lower primary impedance will deliver more power at a higher level of distortion. A lower primary impedance should also keep you farther away from the "knee" found in pentode curves, which is a good thing for the safety of the screens. Here's a sketch, although it isn't your tube nor your topology:

Click the image to open in full size.
  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
OPT question Andrewbee Tubes / Valves 3 11th May 2009 11:35 PM
opt question skyraider Tubes / Valves 6 23rd June 2005 06:36 AM
Question on OPT MlinarS Tubes / Valves 2 26th February 2005 12:55 PM
Leak tl/25+ OPT selection avxt Tubes / Valves 12 21st September 2004 01:22 AM


New To Site? Need Help?

All times are GMT. The time now is 10:41 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