100R/MRA5-L6/5U4G questions

[e093116]

Hi,

Now that the mech and other details are sorted on my 100R project, the focus is shifting to figure out why the amp doesn't make any sound.

I recapped it, and think I got everything correct, it hasn't blown any fuses or burned anything up (so far).  However, I've never never heard a pop, hum, anything...it's dead.  It's been powered up for about a total of 5 minutes.  I've done the basic things the manual suggests (pull out mute plug, etc) with no changes.

I can see "most" of the tubes glow, including the 6L6s and 5879 tubes, but the 5U4G tube does not glow or get warm.  Unfortunately, I don't have access to a tube tester.

I checked the T1 transformer, it reads 100.4 ohms across the primary, 132.6 across the secondary, and 69.7 and 63.3 across the center tap to each side.  I noticed that pins 2 and 8 on the 5U4G tube socket are a dead short (0.2 ohms on my Fluke meter), is this a reasonable measurement?

Short of having the tube tested, are there any voltages I can measure at the tube socket to determine if the tube is bad or not?  Are there any measurements I can make on the tube itself?  This is my first tube amp...

Thanks for any suggestions and happy holidays!

Bill

[robnyc]

Bill this is often due to poor contact at the Jones plug that mates the two units (amp & sel. receiver). I lightly file the male side prongs and spread them slightly so as to make better contact.

Check the 5U4 socket for reasonable tightness.

Pins 2 & 8 are the heater connections. I don't recall the impedance, but any low reading should be OK.

RobNYC

[Ron Rich]

Rob, Bill,
The plug joining the two units IMHO, should NEVER be filed. It was silver plated, and "filing" removes silver. It also is a double sided contact on each "leg", so any "bending" or "spreading" just moves the contact surface to one side, and prevents the other from making good contact,
Cleaning it with a safety solvent, then, using Deoxit 5, works well here for me, when the can directions are followed. The 5U4 is NOT powered via this plug 5.8 vac connection. It is powered independently by the "main transformer" in the amp, which does get 117 VAC thru that plug, only when the motor is powered. The 5U4's filament voltage is also fused by the 2 amp SB fuse--I don't recall the fil voltage pin numbers, but they are easy to spot on the socket--the twisted pair--- Ron Rich

[robnyc]

Some minor differences of opinion here. Lightly filed -means just that. In this case it removes the discoloration towards the ends of the prongs. In nearly all the units I have most silver is gone. The slight bend is just a flaring outward of the prongs. If done correctly it isn't enough to lessen contact face but just adds pressure once the units are mated. i've not found simple cleaning to be adequate here.

"The 5U4 is NOT powered via this plug 5.8 vac connection." If one measures only 5.8 for the amp (not rectifier) tube heaters it is too low. The amp will work, but as tubes age performance deteriorates more if the cathodes are under-heated. Even a 10% reduction will cause significant reduction.

FWIW: the filament wires on the 5U4 are not twisted.

Rob

[Ron Rich]

Rob,
In "worst case", I have found, if not previously "filed", using the safety solvent first, then an ink erasure ( mounted to a pen/pencil), will remove 99.9 % of the "tarnish". Then spraying Deoxit on the male prongs and inserting them and removing them several times, wiping them off, allowing them to air dry, makes the male pins (at least), look new. I have never had a known failure doing this ! However, IF, filed, previously, ain't nothing else I know to do but re-file, bend, and hope (or replace, if possible)! Ron Rich

[e093116]

Hi guys,

Thanks for the feedback.  I cleaned the Jones plug using a combination of the methods suggested...I measured good voltage across both set of contacts on the back side of the plug in the amp (I think I got ~119 and 6.2 VAC).  The tube socket is tight.

I still don't get any voltage across the 5U4G pins 2 and 8.  I studied the schematic and didn't find anything stupid that I messed up when I recapped the amp.

The fuse is good and I'm checking with the motor running/in record playing position.

On the schematic, I see the is a "410" associated with one of the pins...is this supposed to be 410 VAC across pins 2 & 8, or is it from the pin to ground?  (I left the books out in the garage).

Is it a worthwhile exercise to desolder the transformer leads from the 5U4G socket and check the resistance/voltage?  I have a Fluke 87V and an el cheapo analog meter.

Thanks again and happy holidays.

Bill

[Ron Rich]

Hi Bill,
If the transformer is getting good 119 vac, THRU the connector ( check it with a 100 watt lamp, as a "Fluke", or ANY digital meter, should not be trusted, because they don't "consider" current flow, on AC voltage---El Cheapo's Analogs--who knows ?) If you can't read the filament voltage on the wires coming out of the trannie, and you know the power IS getting into the primary, then you have an open winding on the transformer.   Ron Rich

[e093116]

Hi Ron,

Minor update, I disconnected the wires from the tube socket pins 2 & 8, measured 0.3 ohms, so at least the transformer isn't open.

I then found the 2A slow blow fuse in the amp was blown. 

I replaced it with a 1 amp slow blow, and with all of the tubes removed, I measured a voltage of ~5.8 VAC across pins 2&8.

I put the tube in, it blew the fuse immediately.  I replaced the fuse with a 5 amp fast blow...it blew after a couple of seconds, the tube started to glow.

I have two 5U4G tubes, both of them measure 0.5 ohms across their pins 2 & 8.

Is it possible a shorted L2 could be part of the problem here?  Is there a typical resistance I should see for L2?

Thanks again for the continued guidance...

Bill

[jukeboxmarty]

Sounds like a short (or backwards capacitor). I would re-check your work.

[robnyc]

Hopefully, Ron's suggestions bear fruit. There is a significant possibility of a ground short in the output tran.

The small tone choke in the bass ckt can not cause this problem.

Some tests:

Remove speaker plug. This removes the center tap to ground on the power tran secondary.

All tubes out.

Measure to ground from either pin 2 or 8 -whichever is connected to the first filter. There should only be a high resistance reading to ground due to the bleeder. 

If still low, disconnect the choke.

If there is still a low resistance reading, disconnect the center tap going to the output tran primary and measure.

Again, check your work and look for anything that might have gotten squished together in handling (happens often).

RobNYC

[Ron Rich]

Bill,
Check that the 2,"guide", or "alignment" pins, from one unit, to the next, are NOT "touching something" ! I chased that around once, for hours !!
Ron Rich

[e093116]

Hi guys,

Thanks again for the feedback.

With the amp removed from the phono and all tubes out, I get ~22kohm to ground when measuring from either pin 2 or pin 8 to ground.  On the meter the resistance drops to about 10k, then climbs to 22k, I assume this is because the battery in the meter is charging the cap.

I don't think I have anything pinched or shorted accidentally...this should show on the test Rob mentioned I believe...I checked the "line up" pins as Ron suggested as well, nothing making contact there, although I can see how the bottom one could cause trouble...

When I replaced the C1-C3 caps, I covered all of the positive leads  and solder connections with heat shrink tubing, the positive cap leads are well protected from shorting.  I left the 3 "can" capacitors in place to maintain the original look.  I hooked the negative leads (striped lead of replacement cap) of the new caps to the unused "ground tabs" on the old capacitor cans, I though this was convenient and the most straightforward packaging approach.  There is nothing connected to any of the "center" terminals on the old caps, and these leads are not touching anything. 

If it's of any help, the old capacitor (in the center of the three) was marked "bad" by the previous owner...I can see this cap (from the bottom) is bulged.  Could this have damaged the transformer?

Bill

[Ron Rich]

Bill,
At this point, you are "pushing a rope". You NEED a known, GOOD, 5U4 tube. You need to replace the 2 amp SB fuse with a 1.6 to 2 A SB--NOTHING "HIGHER " !! Check your speaker plug, voice and field coils, first. I doubt the bulged cap did any damage--but "overfusing", if done, may have ??
Ron Rich

[e093116]

Hi guys,

I got some 2 amp slo blow fuses and all is well.  I'm not sure why the original fuse blew in the first place...pretty sure I checked it and found it was good.

Anyway, played a few records and I'm happy with what I've heard so far.  Both of the 8" and one of the 12" speakers were shot, rather than reconing, I replaced with the Jensen Alnicos, saw them recommended by Rob in a different thread.  

Definitely has a "50s" sound compared to my other phonos.

Thanks again.

Bill 

[Ron Rich]

Bill,
Seen that fuse "blow" when the 5U4 "flashes" ( shorts out, for a brief period of time). I would, if you have not done so, purchase a new one, and toss the tube out ! Ron Rich