Author Topic: Magnavox 139F OPT help  (Read 507 times)

Motorola Minion

  • Hero Member
  • *****
  • Posts: 594
  • Southern Pennsylvania
    • View Profile
Magnavox 139F OPT help
« on: October 18, 2016, 01:39:00 PM »
I am working on a CR198 I got from another member to go into a Mayfair. This is a 1948 radio chassis with two 6L6 output tubes. I got a 12" PM speaker with it but no output transformer. A Hammond 1620 is over $100 and I can't put that much out to restore a lower end model.

My question: I have a Magnavox AMP142BC chassis that has what I think is a 20 watt OPT made for four paralleled push-pull 6V6 tubes. Is the impedance of this primary within range of what a pair of 6L6 tubes need?
I will use a University 6201 12" speaker with coaxial horn.

I have the same radio chassis in a Berkeley and measured the DC resistances of the opt primary and paralleled speaker field(s) to determine what I need. The entire primary measures 96 ohms! I know that is misleading and does not solely determine impedance. I do have a TV filter choke that can sub-in for the 500-ohm speaker field. :D

Reading the tube data, a 6L6 push-pull class A1 rating is based upon 5000 ohms plate to plate effective load resistance and the 6V6 is based on 10,000 ohms. If 6V6 are in parallel, resistance would be halved, correct?
Tubes - Magical - Tubes

Dave

voxACthirtee

  • Full Member
  • ***
  • Posts: 115
    • View Profile
Re: Magnavox 139F OPT help
« Reply #1 on: October 18, 2016, 02:36:49 PM »
it should work, your idea is basically correct.
As it turns out the load 4 - 6v6 see is in the range of what 2-6L6 wants to see.

Its never that simple, but it will work. If you see enough amps, you'll note that there isn't a set impedance. Different manufactures use different plate voltages and different impedance OP transformers. So as long as you are close. That being said 2-6L6GB will want to put out more than 20 watts in most cases. You should go with a 6L6B or a 5881, both lower output 6L6 types. Also, the Sams should have the exact impedance of the 142 op transformer, and the Sams from the one you are working on will have the impedance of what was originally in there. In the "parts listing" part of the Sams the transformers show what impedance they are, and what secondaries they have.

I just wrote in the post below this about OP transformers.
The only thing an ohmeter is good for on these is identifying unknown transformers primaries/secondaries/center taps. The continuity setting helps you find dead shorts as well.

Any actual ohm reading is basically irrelevant. Companies don't wind transformers the exact same way so ohm readings won't translate.
The transformer is windings designed as ratios, not resistance.
The 5k of the transformer is the impedance the tubes want to see, based on the "ratio" of windings needed to have a 4/8/16 ohm output tap.
There's a formula where you put voltage(1v for example) through the transformer and measure it on the secondary, do the math, and you know the ratio and thus the load the tube will see. You can also use this way to identify what ohms the secondaries are if they are unknown.

Motorola Minion

  • Hero Member
  • *****
  • Posts: 594
  • Southern Pennsylvania
    • View Profile
Re: Magnavox 139F OPT help
« Reply #2 on: October 21, 2016, 09:32:09 AM »
Thanks for the confirmation. I tested the winding resistance of the trans from the AMP142BC and its a near match to the one in my Berkeley ;D

I do plan to use the 6L6G rather than the GC for originality and the reason you stated.
Tubes - Magical - Tubes

Dave