Surplus store score! Collins 26c and 2x Gilfillan Bros/Bendix filters pics

[plumsolly]

Cool. A few more questions before I power it up. For the output, its balanced and I just attach ground to the C.T. terminal? For the input, there are only two terminals, but there is a ground connection and C.T. connection on the input strapping terminal board. From the manual: “The center tap of the input transformer is brought out to the terminal marked C.T. The terminal marked G is connected to the chassis of the amplifier.  If the input line is balanced to ground, it may be desirable to join these two terminals.” So, should I join ‘C.T.’ and ‘G’ and also connect the input ground to ‘G’?  Also, can/should I attach ac safety ground to ‘G’? As for testing for DC, I just have my meter set to read dc and check the junctions of C6/R13 and  C7/T4 against case ground? What is an unreasonable amount of DC?

Thanks a ton, I wont have been able to have done it without you. Best, Ben

 

[emrr]

Cool stock certificate, thanks!

You don't want or need to CT ground in or out.  Unless you are driving a couple of miles of line to your transmitter.  I would doubt that you have any circuits that are actually balanced to ground. 

G, B, and preamp - are all tied.  You would never run AC safety ground directly to the terminal closest to the input.  You would tie it to B. 

Any DC beyond a few mV is unreasonable.  I would disconnect those caps and put new ones in across the terminal strip.  Not doing so may be fine, but is the most expensive gamble you can make here. 

 

[plumsolly]

Ok so: Leave input CT and G unconnected. Connect input shield ground to G. Connect output shield to ? As far as the caps, I'm thinking that if I want to use the originals, I am going to have to check them in circuit sometime, and If I don't have to de/resolder those old leads another time that would be good... Do you think it would be foolish to give it try with the old ones if I monitor dc thoroughly and power it up slow on the variac?

Thanks and best, Ben

 

[emrr]

Try them, and monitor DC for at least an hour.  Even better if you have 2 meters, and can hook them both up. 

Connect output ground to closest available terminal.  Input shield is better left unconnected; let the proceeding device shield termination be the only one.  Have shielded wire connected at your connector end only, so it completes path from previous device right up to 26C input connection.  You generally only want input shield connected to device with microphones. 

 

[plumsolly]

I powered it up! But, unfortunately when I got about 30v on the variac the dc at c6/r13 started rising quite quickly - it got to about 2mv before I shut it down. So now I am on the hunt for a .25uf/600v cap on Saturday afternoon in a smallish town. Rats. Best, Ben

 

[emrr]

2 mV initially is no big deal.  You will see it as the cap adjusts.  You don't want to see much long term.  I am surprised you were seeing anything at all with variac at 30VAC; usually you don't get rectifier filaments to light up until you approach 50-60V.    You don't need a 600V cap either, right?  Look at the max voltages; under 300VDC. 

 

[plumsolly]

Ok. I turned it on again and after a few secs, it went down. Then I turned the variac up another 10v or so and it jumped to 50mv and shut it down. Should I let it run a bit and see if it decreases?
Thanks, Ben

 

[emrr]

Yes.    You may even see a few volts momentarily, but you should never see it other than momentarily.  Presence of voltage indicates some amount of current passing through transformer windings which shouldn't see any. 

 

[plumsolly]

Ok. Ive got the variac @ ~35v now and it went down quite a bit, but then hovered around 9mv for 30secs-1min. Keep going? Sorry for all the questions, I want to make sure I don't damage anything. Best, Ben

 

[emrr]

You should just replace the caps, if you are unsure of how to 'feel' this.  I really can't give accurate guidance remotely at this point, not being able to see for myself.  

Are you listening to/for audio all the while?

 

[plumsolly]

Hey emrr - I totally understand. I went ahead and subbed in some paralleled old film caps from various places and the dc readings look better. Its not a great solution, however, because these caps are 40 years old, so I'm not sure about them either. My reading at c7/t4 is steady at less than a mV. My reading at c6/r13 is fluctuating a fair amount from about 17mv to -5mv hovering mostly around 5mv, but thats about what the meter reads when it isn't hooked up to anything. I am listening to audio the whole time - I have been running music through. I haven't noticed anything strange; it's bandwidth limited and there is a lot of hum, but it's sitting next to a variac.

Thanks and best, Ben

 

[emrr]

Those levels sound fine. 

 

[plumsolly]

Thanks emrr. So I realized a made a mistake with Power Transformer. Instead of getting a 6.3v CT filament winding, I got a 6.3v 0v 6.3 - so a 12.6v CT. That explains why I was getting signal at 25v on the variac. I realized my mistake before the filament got more than ~8.3v, so I didn't do any damage, but now I'm not sure how to proceed. I've got a second 6.3v filament winding with no CT and I've got a couple of rheostats from a tube power supply I could steal...

Thanks and best, Ben

 

[emrr]

Use a different filament transformer, and make an artificial center tap if you need.    either (2) 50-200 ohm resistors to ground or an equivalent wirewound pot.   

Or, use only half the existing winding and artificial tap that for the time being. 

 

[plumsolly]

I will create an artificial center tap on the other filament winding of this transformer. I flipped it into limiting mode during my tests today and the audio went away completely - also the V.I. meter didn't read any gain reduction with the input cranked. (The meter does work, though, which is good news) So I will have to figure out whats happening with that. Also, things got sorted out with broadcastdocuments; he gave me a refund and sent me a pdf of the manual and schematic. The pdf is a version I already have, but it is nice to have a schematic that is not split up. I'll edit my previous posts to indicate that the issue was resolved. I will resume work on this tomorrow after my western civ test (French Revolution-WWI)

Best, Ben

 

[emrr]

Pull the 6C8G and see if you get sound.  Then check the switch contacts. 

 

[plumsolly]

Alright, got some new caps today and the leakage at both junctions is down really low. Also got some 100r, 1w resistors and got an artificial center tap going. The good news: I'm getting some sound when its in bypass, and all currents read pretty close to perfect on the test meter, plate voltage is perfect, filament voltage is perfect. The bad news: The frequency response in bypass literally goes from something like -50db @ 20hz to 0db @ 20k in a straight, diagonal line. It looks like this '/' I get almost no sound when its not in bypass. Also, the output sees pretty low. On a side note, I just realized that when it's working, this will be perfect to drive my passive reverb box.  

**edit: I tried the pulling the 6C8g and no change. I also checked the switch and it seems ok.

Thanks and best, Ben

 

[emrr]

As bad as that sounds, I expect you need to confirm continuity for every connection on the bypass switch, and also everything around it. 

With it in bypass (off), jumper from output side of c6 to top of R31 output level pot, and see what changes.

 

[plumsolly]

Ok. I rechecked the bypass switch and its definitely good. I haven't yet traced all the leads coming off of it to make sure they go where they are supposed to go and make contact. I tried jumpering the output side of C6 to the top of R31, and all I got was a couple of db jump in level.

Thanks and best, Ben

 

[emrr]

inject signal at the various interstage spots and see if you can isolate a section.  Start with putting signal into pri of T4, and work backwards.