EH Scott Radio Enthusiasts

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I am in the process of restoring a Phantom Deluxe remote control chassis. It uses the later, long amp. For testing, I pulled the 15 inch speaker out of my other Phantom Deluxe in the Stratford, and the set plays great. Power supply voltages, set B+ and bias are all where they should be.

Now: I have two other speakers with the 7 pin plug here, a 12 and a 15. They could be from a Philly or a Phantom. I check all the DC resistances for the field connections - they are identical to the one I tested with. The DC resistance of the output transformer is a bit higher (total DC resistance of the test speaker was about 165 ohms across the outside. These two speakers read about 220 ohms. When I use either of these speakers, the neg bias voltage goes much higher than the 27v - with the Variac at 90VAC, I'm already seeing -35v. And the B+ is low, doesn't come up like it does on the test speaker.

The only thing I can think is that the output transformers are for P-P parallel 6L6s as in the Philly, creating a different load/bias than the single P-P 6L6 Phantom. As I mentioned, the field windings read exactly the same. Both the 12 and 15 in speakers behave the same. And yes, the numbers on the output transformers are different...

Thoughts and suggestions are most welcome!

Kent

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The DC resistance across the output transformer primary should not affect bias.  Shorts or leakage to ground in the field coil or output transformer may but it is unlikely that two speakers would have the same problem.  This leads me to suspect a bad contact at the speaker plug/socket.

Norman

Thanks Norman - something you said got me thinking, I wondered if the primary side had any resistance to ground. Lo and behold - the 15 in speaker read just a couple hundred ohms from primary to ground and the 12 in speaker was about 4K to ground. So BOTH speakers have bad output transformers. I have one spare, so I'll be looking for another output transformer compatible with the P-P 6L6 Phantom. I guess I'd call it bad luck.

Kent 

Kent, just for ideas, I used this for an output in a 15" pedestal with 38 ohm  VC. 

https://www.antekinc.com/as-0512-50va-12v-transformer/

It fit right in the pedestal ok. I know nothing about the Phantom specific speakers, what their VC impedance was, etc. 

Some may question how well it would work, but it sounded excellent.  It was recommended by Tom Bavis and if you know him, he knows his sh*t.   Here is what he said about it: 

The Antek has dual 120V primaries - connect them in series for a 230V, center-tapped winding. The rated 4 mA exciting current at 120V works out to about 80 Henries, 4X that or 320 Henries for the entire primary. So low frequency response should be excellent up to the saturation point, which would be somewhere over 50 watts at 50 Hz (12 watts at 25, but nothing goes that low). Current needs to be balanced in both halves of primary for best low-frequency performance.

Toroidal windings have very good coupling (or very low magnetic leakage - same thing), so high frequency response should be excellent too.

Now I have a conventional transformer with about the same windings, 2 x 12V @ 56 VA - full primary measures about 2 Henries, (awfully low, though it goes up with AC applied) and coupling is awful since primary and secondary are on separate sections on the bobbin - I estimate it would reach 4-5 KHz at - 3dB. Would work, not well.
 

Thanks Scott! I have ordered one, I'll give it a try in this application and let you know how it works out. These are also 38 ohm VC, so it should be almost identical.

Kent

Scott Seickel said:

Kent, just for ideas, I used this for an output in a 15" pedestal with 38 ohm  VC. 

https://www.antekinc.com/as-0512-50va-12v-transformer/

It fit right in the pedestal ok. I know nothing about the Phantom specific speakers, what their VC impedance was, etc. 

Some may question how well it would work, but it sounded excellent.  It was recommended by Tom Bavis and if you know him, he knows his sh*t.   Here is what he said about it: 

The Antek has dual 120V primaries - connect them in series for a 230V, center-tapped winding. The rated 4 mA exciting current at 120V works out to about 80 Henries, 4X that or 320 Henries for the entire primary. So low frequency response should be excellent up to the saturation point, which would be somewhere over 50 watts at 50 Hz (12 watts at 25, but nothing goes that low). Current needs to be balanced in both halves of primary for best low-frequency performance.

Toroidal windings have very good coupling (or very low magnetic leakage - same thing), so high frequency response should be excellent too.

Now I have a conventional transformer with about the same windings, 2 x 12V @ 56 VA - full primary measures about 2 Henries, (awfully low, though it goes up with AC applied) and coupling is awful since primary and secondary are on separate sections on the bobbin - I estimate it would reach 4-5 KHz at - 3dB. Would work, not well.
 

Using a power transformer for tube output does work surprisingly well, there is a lot of talk about saturation and frequency response, I have even got away with using a small txt in a single ended application, (240v to9 9v), its all about primary to secondary ratios.

I have used ordinary E-I lamination transformers in the past for push pull, but not a toroid please let us know how well this works.

Scott - I got the transformer today and it also has a dual secondary - two 12v outputs. To get the 38 ohm match to the VC, did you use one of the secondary windings or both (and in parallel or series)? How did you wire it in the pedestal?

Kent

Turns ratio is equal to the transformer primary voltage divided by the secondary voltage (four combinations available).  The impedance ratio is the turns ratio squared.  The voice coil is 38-ohms.  The original Scott output transformer was on the order of 3000 to 3300 ohms for the Philharmonic (PP Parallel 6L6 tubes, fixed bias, Class AB1, 50+/- watts) and twice that for the Phantom (pp 6L6 tubes).  If you hook the plates to the 220-volt primary (pretty much have to for push pull), the impedance ratio to the 24-volt winding is approximately 3200-ohms.

Norman

Never occurred to me such a transformer could serve as an output transformer. And for a lot less money than current output transformers with sufficient wattage (Antique Electronic Supply ) if they had one for a 38 ohm VC.

What about the wattage? the toroidal xfrm handles up to 50 watts of audio?  which exceeds Scott specs for either Philly or Phantom.

Scott specs-  Philly claim greater wattage than the later 6L6 Phantom amp.

From Norman's comments, then Phantom and Philharmonic would use different output transformers on account of the two or four 6L6 set up?

Never occurred to me such a transformer could serve as an output transformer. And for a lot less money than current output transformers with sufficient wattage (Antique Electronic Supply ) if they had one for a 38 ohm VC.

What about the wattage? the toroidal xfrm handles up to 50 watts of audio?  which exceeds Scott specs for either Philly or Phantom.

Scott specs-  Philly claim greater wattage than the later 6L6 Phantom amp.

From Norman's comments, then Phantom and Philharmonic would use different output transformers on account of the two or four 6L6 set up?

The same transformer would work for the Phantom by using one of the 12-volt windings to drive the voice coil

 During WWII when parts were scarce bell transformers were used for replacement output transformers.

Norman

I thought that I would post this, it ties in with the comments Norman made earlier,

found this years ago, cant remenber where but it was pre internet days.

Decide what primary impedance you want. It might be 5200 ohms for an EL84. (6BQ5)

Divide by speaker impedance, it might be 8 ohms.
5200/8 = 650

Find the square root, because you need to find the TURNS ratio from the IMPEDANCE ratio, and one is the square of the other.

sqaure root of 650 = 25
So turns ratio of transformer is 25:1

Divide the primary VOLTAGE of the transformer by the turns ratio. for a 220V transformer:

220/25 = 8.8V
So you would need a 220 to 8.8V transformer.

You can work backwards of course too, so:
A 220V : 9V transformer would be close to 4700 ohms primary impedance.
A 220V : 6V transformer would be 10000 ohms primary impedance, also pretty good.
The 220V : 12V transformer would be 2600 ohms, too low! BUT connect to a 16 ohm speaker doubles it to 5300 ohm,

However, it will sound BAD for hifi.
Its ok for LoFi, like guitar amps.
For a single ended design, use a transformer capable of twice the wattage than you need, so for an EL84 say a 12VA transformer- nice and big.

Daspite the comments made by the author, this does work, and sounds surprisingly good, just what is needed to get out of the problem of no transformer

Mike

Kent, just what Norman said.  The impedance of the speaker is reflected to the output tubes after being multiplied by the transformers impedance ratio. 

I could not hear any difference swapping speakers on one of my Philharmonics using this AnTek transformer and going back and forth.  Then again, I was only listening to AM radio.  I am not an audiophile.   But it sounded good and I think you will be happy.  Based on what Tom said, a toroidal transformer will perform better than a conventional power transformer by a good margin. 

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