PP output transformer for an SE 845
 User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read Search

 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
kward
diyAudio Member

Join Date: Sep 2013
Location: Western USA
PP output transformer for an SE 845

Hello All,

I inherited some large EI type output transformers that were used in an SE 845 amp. I've examined and measured one of the OPTs and find that it is actually a PP output transformer. Brand is Northlake Engineering Inc. Stamp marks on the side of the transformer are A139-103, 454216, 17227297.

The transformer has three leads on the primary and 4 leads on the secondary. Primary: Blue, Red, Blue/White; Secondary: Black, Yellow, Orange, Purple.

DCR on primary measures as:
Red - Blue: 68Ω DCR
Red - Blue/White: 70Ω DCR

DCR on secondary measures as:
Black - Yellow: 1Ω DCR
Black - Orange: 0.8Ω DCR
Black - Purple: 0.5Ω DCR

Impedance measures as:
54.1V AC (60 Hz) applied between Blue and Blue/White leads produces the following voltages on the secondary:
Black-Yellow: 3.34V AC
Black-Orange: 2.23V AC
Black-Purple: 1.68V AC

Also, the AC voltage measured between Red and blue primary leads (when AC input is hooked between blue and blue/white leads) is 27.05V, confirming that the red lead is tapped half way on the primary.

The secondary voltages are approximately a factor of root(2) apart, so I assume:
Yellow = 16Ω
Orange = 8Ω
Purple = 4Ω

This implies a primary impedance of approximately 4.2KΩ.

These look and act like big PP output transformers, that guess is supported by the red primary lead being mid position on the primary, and the fact that it's red in color, where the end leads are blue and blue/white.

The lamination stack is huge on these: 2" width, 4.5" height, 3.75" depth. They look like they could support 80 to 100 Watts of PP power. Size-wise they are at least as big, perhaps slightly bigger, than Dynaco Mk III (A431) output iron, if that helps for the visual on how big they are.

I don't know if this will work, but if I load the 4 ohm tap (purple) with an 8 ohm speaker, it'll reflect approximately 8.4KΩ to the primary. Load line analysis of the 845 tube with 8.4K primary says with 800V B+ I can get 25 watts output, if I drive them +40V into A2.

The real questions right now are:
1. Can I really use these PP OPTs in an SE amp? (certainly the previous owner thought so)
2. Can I load the "4 ohm" tap with an 8 ohm load? What negative effects will that have on bandwidth and power?

Picture (as best I could get) of the actual unit is attached.
Attached Images
 845 SE OPT.jpg (67.0 KB, 299 views)

Last edited by kward; 18th June 2019 at 06:23 PM.

 19th June 2019, 12:31 AM #2 smoking-amp   diyAudio Member     Join Date: Dec 2001 Location: Hickory, NC Was this 845 amplifier a completed and working SE amplifier? Is there a big inductor on the chassis connected to this OT by a large cap? (connected to one end of the primary, and B+ connected to the other end of the primary) In which case it is probably operating as a Parafeed arrangement, using possibly the entire P-P primary as the SE primary. (so no DC in the OT)
 19th June 2019, 01:46 AM #3 kward   diyAudio Member   Join Date: Sep 2013 Location: Western USA The amps haven't been powered in probably 15 years. But they are supposedly fully in tact and working, although I haven't dared power them up yet since they came tubeless and I wasn't willing just yet to shell out the bucks for the 845 tubes to try them. They are BIG BIG mono blocks (probably weigh 75 to 100 lbs each) with three channels per block (meaning three 845's per block and three output transformers per block, all fed off of one rather monster power transformer). Apparently they were used in a vertical bi-amped scenario, where two of the three channels per block are paralleled at the output transformer secondaries to power the woofers and the third channel feeds the tweeter. This is per side. The other side is identical for the other monoblock. But anyway, The power supply for the three output stages is diode bridge rectified, the pos terminal of the bridge goes into a choke for a choke input power supply, and as near as I can tell, the negative lead of the bridge goes into a series connected triplet of capacitors, lower neg lead eventually going to ground. It almost looks like some sort of doubler configuration. It is definitely using the entire PP primary as the SE primary. I can't quite tell yet if there would be Quiescent current flowing at tube idle. The fact that there is a fairly large capacitor between negative lead of the bridge and ground suggests this may be true. This part of the power supply feeds only the output stage. There is a separate diode rectified power supply feeding the frontend tubes from a different high voltage secondary winding. In any case, I'm not really excited to use the amps as is, since I don't think I have any application for these monoblocks in current form. For one thing they're just too big. I was more interested in salvaging the parts and making a 20 watt two channel 845 SE amp. That's when I ran into the OPT issue discussed in the opening post. I could probably power them up at a low mains voltage, say 25V AC or something, and install a big power resistor between plate and cathode of one of the 845 sockets and see if it draws current. I also might be able to reverse trace the power supply more carefully now that I know I might be dealing with a parafeed design. If it is a parafeed design, it makes more sense to use these PP OPTs in SE form, but still it bothers me a bit that the primary is supposedly 4.2KΩ, as that seems way low for an 845 SE amp. I was thinking 10K would be more appropriate. Any suggestions for next steps to try to decode this? Last edited by kward; 19th June 2019 at 01:57 AM.
kward
diyAudio Member

Join Date: Sep 2013
Location: Western USA
It's not a parafeed.

Here's the schematic of one of the woofer channels, as best I understand it at this point.
Attached Images
 Reverse Engineered Schematic DRAFT.jpg (445.0 KB, 259 views)

Last edited by kward; 19th June 2019 at 02:47 AM.

 19th June 2019, 03:21 AM #5 cerrem   diyAudio Member     Join Date: Jun 2005 Location: San Diego, CA Based on the photo..... The E-I laminations look to be a 2" stack of M6 29 gauge 150 size...with a 1:1 alternate interleaving...this would indicate that it can not tolerate any significant DC offset.... Therefore I would conclude it has a LC ...ie capacitor DC blocking.. unless I am looking at the PT in that picture ???
 19th June 2019, 03:22 AM #6 smoking-amp   diyAudio Member     Join Date: Dec 2001 Location: Hickory, NC Hmmmm, sure looks like that would need an air gap in the OT. There are special I laminations that have a center/side cutout gap that can be used to make an air gapped E-I xfmr. with close packing (no visible gap on the outside of the OT) But one would expect those to be stacked with the I laminations all on one side. Your xfmr appears to have interleaved laminations. ????? The only way to tell if there is a gap would be to measure the primary inductance at some reasonable AC excitation. Typical L meters use a very small test signal where lamination permeability would be low. I would put 24 VAC 60 Hz across the primary and measure the AC current. Can calc. the inductance from that. (be sure to use the primary for this test, AC on the speaker windings could generate some dangerous high voltages on the primary side) A non-gapped OT will show hundreds of Henries of L primary, while a gapped one will likely be in the tens of Henries. Another (distant) possibility would be the use of a DC current source across one of the secondary windings to neutralize the DC primary cu;rrent in the OT. Probably some well-heatsinked power transistor used for that and a LV DC power supply for it. Last edited by smoking-amp; 19th June 2019 at 03:35 AM.
 19th June 2019, 03:35 AM #7 kward   diyAudio Member   Join Date: Sep 2013 Location: Western USA Smoking amp, how do I do this? Can I put say a 1 ohm resistor in series with the primary to measure current? I assume I need some sort of resistor load on the secondary for current to flow on the primary? Or should the secondary be unloaded? Once I measure current what math do I use to get primary inductance?
kward
diyAudio Member

Join Date: Sep 2013
Location: Western USA
Quote:
 Originally Posted by cerrem unless I am looking at the PT in that picture ???
It's one of the output transformers.

 19th June 2019, 03:44 AM #9 smoking-amp   diyAudio Member     Join Date: Dec 2001 Location: Hickory, NC A one Ohm series resistor should work fine to measure the AC current. Leave the secondaries un-connected. XL = Vac / Iac XL = 2 pi f L so L = ( Vac / Iac ) / ( 2 pi 60 ) If the Vdrop on the 1 Ohm resistor is too small to read accurately (DVM), use a higher sense resistor. With a much higher sense resistor you could also read the Vac directly across the primary to eliminate the Vdrop factor due to the sense resistor. Last edited by smoking-amp; 19th June 2019 at 03:51 AM.
 19th June 2019, 04:09 AM #10 kward   diyAudio Member   Join Date: Sep 2013 Location: Western USA Right. Thanks. I'll try this...tomorrow. Thank you.

 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 Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Site     Site Announcements     Forum Problems Amplifiers     Solid State     Pass Labs     Tubes / Valves     Chip Amps     Class D     Power Supplies     Headphone Systems Source & Line     Analogue Source     Analog Line Level     Digital Source     Digital Line Level     PC Based Loudspeakers     Multi-Way     Full Range     Subwoofers     Planars & Exotics Live Sound     PA Systems     Instruments and Amps Design & Build     Parts     Equipment & Tools     Construction Tips     Software Tools General Interest     Room Acoustics & Mods     Music     diyAudio.com Articles     Car Audio     Everything Else Member Areas     Introductions     The Lounge     Clubs & Events     In Memoriam The Moving Image Commercial Sector     Swap Meet     Group Buys     The diyAudio Store     Vendor Forums         Vendor's Bazaar         Sonic Craft         Apex Jr         Audio Sector         Acoustic Fun         Chipamp         DIY HiFi Supply         Elekit         Elektor         Mains Cables R Us         Parts Connexion         Planet 10 hifi         Quanghao Audio Design         Siliconray Online Electronics Store         Tubelab     Manufacturers         AKSA         Audio Poutine         Musicaltech         Holton Precision Audio         CSS         exaDevices         Feastrex         GedLee         Head 'n' HiFi - Walter         Heatsink USA         miniDSP         SITO Audio         Twin Audio         Twisted Pear         Wild Burro Audio

 Similar Threads Thread Thread Starter Forum Replies Last Post Fusion916 Solid State 7 12th January 2013 11:33 AM hilbert_mostert Tubes / Valves 12 8th March 2009 05:10 PM aldovan Tubes / Valves 15 6th September 2008 10:18 AM freddymac406 Tubes / Valves 3 21st April 2007 06:15 PM kelticwizard Everything Else 11 25th March 2007 05:17 AM

 New To Site? Need Help?

All times are GMT. The time now is 09:23 PM.