NC-183D Modification; Comments

[W3SLK]

So I have not had much success finding a power transformer for my NC-183D. Hammond doesn't list it as being made and back in the day, Peter Dahl would sell you one if you sent the old one in. The cost for it in 2004 was $165 plus shipping. I would think that to have Hammond redo it would be very costly. So what I am now considering is to use an Antek AS-2T350-200VA toroidal transformer. It will give me 700VCT @ 0.250mA and 8A of 6.3 filament current. Along with that I will solid state the rectifier whereby eliminating the need for a 5.0VAC filament. Opinions or other considerations are invited.

[w4bfs]

So I have not had much success finding a power transformer for my NC-183D. Hammond doesn't list it as being made and back in the day, Peter Dahl would sell you one if you sent the old one in. The cost for it in 2004 was $165 plus shipping. I would think that to have Hammond redo it would be very costly. So what I am now considering is to use an Antek AS-2T350-200VA toroidal transformer. It will give me 700VCT @ 0.250mA and 8A of 6.3 filament current. Along with that I will solid state the rectifier whereby eliminating the need for a 5.0VAC filament. Opinions or other considerations are invited.

the National manual calls for 6.3 Vac @ 5.25A and 260Vdc @ .14A plus more for receiver powered accessories about 70W.

the receiver internal supply is cap input full wave center tap with filter inductor.

the Antek toroid spec'd will generate B+ too high even if the input filter cap is removed (about 315 Vdc).

a better choice is the AS-2T200 .  to 'dial' this one in will require some experimenting.  I assume that you are recapping your receiver if not at least the caps off the B+.  be sure to get the ones at the transformer connections as these are likely the one(s) that killed ur stock xfmr.
since you are going solid state ps I would go to a bridge with secondary windings paralleled.
leave off the input filter cap for now.  when you power up you will get around 180V of B+
this is basically ok for now and check out the radio circuits for normal voltages.  then as the circuits check out you can vary the value for the input filter to bring the B+ to the desired 250/260V desired.  start with 2ufd or so minding polarity and be Safe

[W3SLK]

W4BFS said:

the National manual calls for 6.3 Vac @ 5.25A and 260Vdc @ .14A plus more for receiver powered accessories about 70W.

the receiver internal supply is cap input full wave center tap with filter inductor.

the Antek toroid spec'd will generate B+ too high even if the input filter cap is removed (about 315 Vdc).

a better choice is the AS-2T200 .  to 'dial' this one in will require some experimenting.  I assume that you are recapping your receiver if not at least the caps off the B+.  be sure to get the ones at the transformer connections as these are likely the one(s) that killed ur stock xfmr.
since you are going solid state ps I would go to a bridge with secondary windings paralleled.
leave off the input filter cap for now.  when you power up you will get around 180V of B+
this is basically ok for now and check out the radio circuits for normal voltages.  then as the circuits check out you can vary the value for the input filter to bring the B+ to the desired 250/260V desired.  start with 2ufd or so minding polarity and be Safe

I have a 200ohm 20W WW ceramic to drop about 50V. The secondaries didn't go bad, it was the primary! I was hoping I could find the issue near cover but it was all potted.

[W1DAN]

Hi:

I may (?-gotta look and ohm it to see if it is good) have an HRO-50 or HRO-60 power transformer. Please check to see of one of these would work for your NC-183D. If so, I can drop it off at my local UPS Store and have them pack and ship at your cost. There would be no other charge for the transformer.

Dan
W1DAN

[W3SLK]

W1DAN said:

Hi:

I may (?-gotta look and ohm it to see if it is good) have an HRO-50 or HRO-60 power transformer. Please check to see of one of these would work for your NC-183D. If so, I can drop it off at my local UPS Store and have them pack and ship at your cost. There would be no other charge for the transformer.

Dan
W1DAN

I would appreciate that Dan. I have an HRO-50 that may have met the same fate. So either way I would need one. Let me know the shipping costs and I can send a cheque to you or any other form of payment you prefer.

[Bob W8LXJ]

Mine died in early 1970s.  Found a dead tube tv set, pulled transformer, checked good, still running in my 183d today.

  Bob W8LXJ

[w4bfs]

about having design center power supply voltages - the National rx uses push/pull 6V6 tubes.

Have you priced a pair (matched) lately?

[Detroit47]

Tube matching is a scam. In my opinion.

N8QPC

[w4bfs]

Tube matching is a scam. In my opinion.

N8QPC

Yes, most are done with the tube tester 'figure of merit' numbers and are of very limited value.  better than nothing.

if the tubes are matched at operating conditions by quiescent current and then equal gain, then u have something of worth

Mike = digging that Rush lyric 

[W3SLK]

To reply to a few comments:
WA1GFZ:Hey Frank! It's been a long time since we talked! Yeah, I was recommended to use the 200VA Antek on the antiqueradio.com forum. So I'll probably go that way. In doing so I remove the heat from the rectifier which creates less stress. I've heard nothing but good things about Antek so that is why I will go that route. Since you mentioned the 'Hot Rod' Valiant, I may use your audio board in mine. I may use WA5BXO's driver circuit also. Its not as complex as yours. I gutted the speech amp because of 60Hz hum on the audio. Bob, W2ZM(SK), did some serious BE mods complete with a tube of RTV. It has the 6146Bs in for modulators and a beefier HV transformer. So half the work is done. BTW my brother-in-law who you met when we had our eyeball passed away last year. He was 67, too damn young!
Mod-U-Lator.
Frank I tried to respond directly but got the 'Message Blocked' indicator.

N8QPC said:

Tube matching is a scam. In my opinion.

I agree 100% Bob. My friend Eric, WB4VVI(SK) taught me that a long time ago!

W4BFS said:

about having design center power supply voltages - the National rx uses push/pull 6V6 tubes.

Have you priced a pair (matched) lately
Yes, most are done with the tube tester 'figure of merit' numbers and are of very limited value.  better than nothing.

if the tubes are matched at operating conditions by quiescent current and then equal gain, then u have something of worth

Mike = digging that Rush lyric

I have a nice supply of 6V6's that I recently found in a box of tubes I had for years. So long as you don't abuse them, they will last forever.
BTW I'm a big Rush fan and thought the lyric was 'apropos'!

[W1DAN]

Mike:

The transformer I have looks to be an HRO-60 power transformer as it has the 11VAC winding for the regulated filaments. I tested the transformer and it seems to be good, no promises. Old style with no end-bell. Please be sure whether this will work in your NC183D or HRO-50 (that does not need the 11VAC winding).

I went to my local UPS Store and they quoted $42.04 to box and ship to Danville PA. Or I can bring it to NEAR-FEST.

I am a big fan of removing the 6V6s as they draw almost half the current in an HRO-60. I then use an external amp tapped from the volume or limiter potentiometer.

Dan

[W3SLK]

W1DAN said

Mike:

The transformer I have looks to be an HRO-60 power transformer as it has the 11VAC winding for the regulated filaments. I tested the transformer and it seems to be good, no promises. Old style with no end-bell. Please be sure whether this will work in your NC183D or HRO-50 (that does not need the 11VAC winding).

I went to my local UPS Store and they quoted $42.04 to box and ship to Danville PA. Or I can bring it to NEAR-FEST.

I am a big fan of removing the 6V6s as they draw almost half the current in an HRO-60. I then use an external amp tapped from the volume or limiter potentiometer.

Dan

I would say "Winner, winner, chicken dinner!", Dan. Let me see if anyone is passing nearby me on the return trip from NEAR-Fest and get it via 'Pony Express'. Hopefully that will save us both the expense of S&H. If not, then I'll send you the money to ship it. Either way it is greatly appreciated. The only amp I have is a Heathkit AA-14 with a pair of 6BQ5's in P-P. I found it and the matching speaker after an auction. It was sitting on the curb for the trash! Not exactly a McIntosh or Dynaco but it will suffice, (not that I would turn down one of those mono-block amps!).

[W1DAN]

Mike:

OK, let me know what delivery method you would like.

For one of my HRO-60's I have solid-stated the HV rectifier, removed the audio 6V6's, clip-leaded from the top of the limiter pot to a 100k resistor then a 1uf mylar capacitor. This feed the instrument (hi-z) input of a tabletop powered speaker similar to a Fostex 6301. The audio is cleaner, louder and with more headroom.

I also reduced the rear chassis fuse from 2A to 1A to help save the power transformer. On another HRO I added a 1/4a fuse from the HV secondary center tap to ground.

Dan

[W3SLK]

OK I'm resurrecting this thread rather than start a new one. I have since purchased an Antek transformer and have finally gotten around to installing it. Here are the results: The 200VA with the 2 350AC 2ndary was unavailable. I ordered the one with the 300VAC windings. I was all set to do the solid state mods when it was pointed out that it was not a good idea to introduce B+ on tubes that have not sufficiently built up their electron space charge around the cathode, (1-2 seconds). So I placed another winding around the toroid to facilitate a 5VAC filament for the 5U4 rectifier. Measuring the voltage on the bench with just the rectifier produced 275VDC prior to the connecting it to the receiver PI filter. After installing it, I blew a (2A) fuse due to the fact I shorted out both filament windings, (colors were not the same as shown on the internet vs. on the transformer itself). When I got that ironed out, I saw I was still blowing the fuse. I put my Fluke 85 in the 120VAC line to monitor the current and found that when I was at ~25VAC, (on the variac), I was measuring 2 amps! I systematically went through the 2ndary windings to see where I had the problem. I found that since I am using both the 300VAC windings, I tied 2 of the windings together to act a center-tap for the rectifier. I learned here that phase matters here. Switching the one winding I powered it up and got > 500mA when warmed up. Measuring the B+ at the output of the filter, which goes directly to the screens and the plates, (via the audio output xformer), to the 6V6s I have 248VDC. Which is slightly less than the 260VDC in the manual. When I turned the RF gain all the way up it reduced to about 245VDC. Switching the band switch, the B+ drops down to about 236VDC. I hooked up an antenna and speaker, switched the AM broadcast band and was rewarded with rich audio from a far station, (I didn't catch the stations call letters). A disclaimer here: the manual says the voltages are measured at 1000Kcyc. It is showing between 5~10% from one end to the other. I don't know if they are the originals but the bypass caps appear to be multi-colored 'black-beauties.' I may wind up replacing them and possibly get a little more voltage due to leakage.