The Mercury - Page 3 - Videokarma.org TV - Video

Adam · #31 09-30-2007, 06:22 PM

Originally, the 6X5 does go to the same 6.3v winding and to ground as all the other tubes. But this new transformer has two 5v and three 6.3v filament windings, so I could change that. Two are 5v 3A, two are 6.3v 7A, one 6.3v 2.7A with an extra tap in the middle for 5v. Originally the sams says it requires 9.1A on the 6.3v filament winding. I suppose I could take the 6X5 and hook it to the 2.7A winding, and keep all the others on a 7A one, but then the 6X5 might be getting too much power, and the other tubes not enough?

blue_lateral · #32 09-30-2007, 07:17 PM

Did you say there is a third 6.3v winding? How many amps is that? If theres only two, it would seem they intended you to hook the two windings in parallel in phase to get the extra current capacity.

It is WAY better to have a separate filament winding floating for the rectifier. If you've got an extra 5 volt winding, you could use a 5 volt rectifier tube like a 5z4. This is a cathoded tube like the 6x5, but the cathode is connected to the filament, so it requires a separate filament winding. Another good solution might be a 5y3. These have a bunch of internal voltage drop, and your dropping resistor(s) would not have to dissipate as much heat.

I like to keep things original though, and so would probably keep the silly 6x5, even on a grounded line if it was the only way.

It seems you have a bunch of windings. What is the voltage of your primary winding? 117? Do you have any primary taps? Whats your line voltage? Mine is 121V.

Heres some food for thought, based on whats currently on my bench. You kinda hit the nail on the head with your comment about the 6x5 possibly having too much filament voltage.

On the old comms receiver I'm currently working on the power transformer had been replaced, and everything was running too high. I had an extra 6.3v winding, probably intended for a damper in an RCA TV chassis. I connected it in series with the primary, phased in such a way to reduce all the voltages. This brought the 5.0v and 6.3v lines back down to normal. The b+ is still too high, but only by about 25 volts. I'm gonna drop that with a resistor.

I would get the filament voltages right, and then do whats necessary to correct the b+. Maybe a winding of some kind could be used in series with the primary like I did, if everything is too high. If a fairly big 5.0v winding was unused, it might run high enough for the 6x5. You can also usually get away with overloading one secondary winding a little if you are not using all the others. If you overload it the voltage will probably come down a little.

blue_lateral · #33 09-30-2007, 07:29 PM

Is the replacement transformer tapped like the old one, or is the 6x5 b+ winding separate?

Adam · #34 10-01-2007, 06:46 AM

The old transformer had 3 secondaries: 360-175-0-175-360v 195ma/40ma, 5v 3a, 6v 9.1a.
The new one has 6: 375-325-0-325-375v 170ma/130ma, 5v 3a, 5v 3a, 6.3v 7a, 6.3v 7a, 6.3-5v 2.7a.

One of my concerns is with the 6.3v filament winding, it seems if I just use one 6.3v 7a winding the current is too low, if I parallel them it is too high. Now as for the 6X5, if these rectifier tubes using a cathode are such a problem, I could take the 2nd 5v 3a winding and put in another 5U4 instead, I would think a 5U4 would work better than a 5Y3 because isn't a 5Y3 only a half-wave rectifier.

blue_lateral · #35 10-01-2007, 07:44 PM

Actually no, a 5y3 is an 80 with an octal base. It's a full wave tube. It's also extremely common and cheap.

There isnt a problem with cathodes specifically, but most rectifiers that have them have the cathode hooked to the filament (5z4, 5v4,5ar4,etc), so a sepatate filament winding is needed just as though it didnt have a cathode.

The real reason for using a rectifier with a cathode is voltage drop. All else being equal, there will be less voltage drop across a cathoded rectifier. This leads to less 'sag' under load. This is generally considered a good thing, but in your case you need to lose all the voltage you can.

Either a 5y3 or a 5u4 should work fine, but if I was going in that direction I would use the lossiest tube I could find. 325 volts is WAY too high. You'll have to look in a tube manual, but I'll bet the 5y3 is gonna be the lossiest one. Any extra voltage you dont lose in the tube, you will have to dissipate with a resistor.... so you might as well lose all you can in the tube.

Heres what I think I would do. Leave the 6x5 for the moment. I would hook one of the 6.3/7a windings in parallel with the 6.3v/2.7a in phase. This is 9.7 amps for the 9.1 amp winding. not too bad. Use one of the 5v/3a windings for the 5u4. This leaves you a 6.3/7a and a 5v/3a to play with.

I would then temporarily scab some resistors in the plate lines of the 6x5. 4.7k at about 5 watts oughta do it. Run it up on the variac, and see how far off everything is. If everything is too high, (and It probably is) you can use one or both of the extra windings to "extend" the primary.

If you have any windings left now, you could use one for the 6x5 (or 5u4 or 5y3).

John

Adam · #36 10-08-2007, 01:50 AM

Now I've tried numerous combinations, the 360v volt source hooked to the 375v secondary of the transformer going to the 5U4 is right on at 360v. The 6v filaments are hooked up to the 2.7a and one 7a winding in parallel and that seems to work, it actually runs some tubes a little high at 6.5v, but it seems ok. The second rectifier is currently wired for a 5Y3/5U4 (they have the same pinout), using the 2nd 5v 3a winding. I figured why deal with the 6X5 if they have a history of shorting out, even if I could wire up a seperate power source to the filament so it wouldn't take my new transformer with it.
Using 2 4.7k 5w resistors to the plates and a 1k 5w resistor before the filter capacitors, the voltage was too low at around 100v, with no resistor before the filters and 4.7k before the plates that went up to around 110v, with no resistors before the plates and 1k before the filters around 190v, with no resistors before the plates and 4.7k before the filters 120v. Switching between the 5U4 and the 5Y3 only made around 5-10v difference. I think next I'm going to try around 3k before the filters, no resistors before the plates with the 5U4 (I like the way those 2 big 5U4s look in there), and also go up to 10w, the 5w resistors were getting on the hot side, not the ones before the plates, but the ones before the filters.

blue_lateral · #37 10-08-2007, 03:02 AM

Sounds like a good idea. Since you're using a 5u4, theres really no good reason to have resistance ahead of the plates. The whole Idea there was to keep the heater to cathode voltage on the 6x5 from being any higher than it was originally.

Keep us posted

John

Adam · #38 10-09-2007, 01:02 AM

I ended up using a 2.2k 10w resistor before the filter caps with the 5U4, with that it produced around 155v after the set was warmed up. Here's some pics...

blue_lateral · #39 10-09-2007, 01:29 AM

Looks great. Nice picture!

John

Adam · #40 10-09-2007, 02:36 AM

All back together, thanks again for everyone's advice and help on this set, this was a difficult one. Here's some pics of it back together. I took a pic with the back on because this is the oldest set I have that still has (at least most of) the back...

John Folsom · #41 10-09-2007, 08:08 AM

That set really looks great! Good job. This should inspire me to reatore my Mercury on of these days!