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OPT Selection Question

mackinthebox

Member
2010-10-19 11:03 am
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?
 
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.
 
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.
 

mackinthebox

Member
2010-10-19 11:03 am
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...
 
@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
 

mackinthebox

Member
2010-10-19 11:03 am
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 :)
 

mackinthebox

Member
2010-10-19 11:03 am
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 ;)
 
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:

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I replaced the OPTs in a Webcor 1068 amp, But only because one was bad.

The OPTs did make a noticable improvement in frequency response as the original OPTs were wound Primary/Primary/Secondary with no interleave. I used Edcor XPP10-8-8K opts as cheap replacements.

I did gain slightly on the output, but that may have just been that the resistive losses in the new transformers were lower than the original. The replacement stacks were about 30% thicker than the original ones.

That said, Webcor was not as high in quality as Magnavox. You may find the Magnavox OPTs are very good and not worth replacing unless you are trying to wring the most out of the amp, and then you will probably need something more in the line of $60/per for tranformers.

I've got a Magnavox 86-10 ($20 at Salvation Army) that I have no intention of replacing the transformeres in because they sound so good. They look like they might be Primary/secondary/primary interleave based on where the wires exit the bobin.
 
I am not looking to replace the transformers I have not but I am about to have enough parts for 2 of these amps
Im going to be missing output transformers for the second amp so im just trying to find some adequate replacements for it so I can either sell it or put it in the bedroom

for now ive ordered a new chassis, new ceramic and gold tube sockets, and I will be installing all new parts in the new chassis using the original tubes and power transformer as well as the original choke and output transformers
I figured I would leave most of the original stock components in the old chassis except for the transformers
that being the case im also going to buy a few sets of tubes over the next months to play with and will eventually rebuild the old maggie amp using replacement parts for it