Questions about rehabbing an old PP 6L6 mono amp with phono pre

Fran, hi-fi OT's typically have primary winding self resonance out beyond 50kHz, and may have a sizeable shunt P-CT capacitance of circa 500pF due to interleaving. If you doubled the shunt capacitance by adding 500pF in parallel, then the simplistic resonance would reduce to about 70% (eg. ~35kHz).

Driving the 500pF primary shunt capacitance with pentode mode and about 30kohm plate resistance would give a simplistic low pass filter corner of about 11kHz. Doubling that shunt capacitance, by adding another 500pF, would lower the corner frequency to about 6kHz.

A not-so-hifi OT may have a somewhat lower shunt capacitance, and a lower self resonance.
 
Thank you for the explanation. That all figures given the past role of this amp coupled with a film projector of the era. It'll be interesting to hear how it sounds with acoustic recordings.
 
Hi guys,

So I have this amp put back together, and fired it up for the first time, yesterday. It puts out nothing except hum. I've checked, checked and rechecked all the connections, component values, and wiring against my extensive notes, images, and of course the schematic. I've found no errors. The only thing I did (other than eliminate the front-end stuff as suggested in the earliest posts), is relocate one single ground at one of the multicap ground lugs (the others are all common ground at the PS caps negative side) to chassis in the PS circuit. I even jumpered it to the common ground of the PS caps, to see if it made any difference - it did not. All components were replaced with equal values in their original places.

No input is connected, and the output is connected to an 8 ohm speaker. Tubes have all been checked. The original 5Y3 was bad, and I swapped in a good 5U4 I have, just for testing.

I checked the PT secondaries (NO TUBES INSTALLED), and found the following (just trying to provide as much info as I can for the moment):

Mains in: 122 VAC
Heaters (6l6's and 12AX7's): 7.14V
Rectifier:
Lug 4 = 370 VAC
Lug 6 = 370 VAC
Lug 2 = 5.7V
Lug 8 = 5.7V

I checked the wiring of the PS capps about a dozen times, and didn't find anything obvious to my eyes. The only things that've changed were in the front end, so I'm wondering if I eliminated something that maybe I shouldn't have.

The hum produced reminds me of input shorted to ground - 60Hz. The Volume control affects the loudness, from nothing to very loud. The Treble control affects the tone of the hum, but doesn't eliminate it. The Phono pot doesn't change the hum at all, and neither does the Bass pot.

My concern is what the elimination of C20 (20uF/50V), and R36 (33K) from one of the 6l6 cathodes to ground might have on the other 6l6 cathode to RC filter (C13 - 50uF/50V & R29 - 225/5W) to ground, if any?

Thanks for your help.

-Fran

Schematic Input Stage C20 R36 circled.JPG

Schematic Output cathodes to ground stage.JPG
 
Does it still do the same thing with something connected to the input? Hum with an open input connection is not uncommon.
 
My concern is what the elimination of C20 (20uF/50V), and R36 (33K) from one of the 6l6 cathodes to ground might have on the other 6l6 cathode to RC filter (C13 - 50uF/50V & R29 - 225/5W) to ground, if any?
The 6L6 cathodes are (or should be) linked.

Your description of the symptom points to something very wrong in the phono input stage. Are the DC operating voltages correct there? Is the B+ to that stage free of AC? Do the controls affect the pitch of the noise, or just its tone and amplitude? Does it go quiet if that stage's grid is grounded?
 
if you ground pin 2 of the 12ax7, does it stop?
Yes it does. There are 2 separate hums, apparently. Doing this eliminates the most prominent (and MUCH louder) of the 2.

With pin 2 grounded, the background hum is more in line with what I expected to hear, I think.

Fran
 
The 6L6 cathodes are (or should be) linked.

Your description of the symptom points to something very wrong in the phono input stage. Are the DC operating voltages correct there? Is the B+ to that stage free of AC? Do the controls affect the pitch of the noise, or just its tone and amplitude? Does it go quiet if that stage's grid is grounded?

The cathodes are linked, just one of the RC circuits was removed from the front end.

I haven't yet checked the DC voltages with the tubes in, and powered up, for fear of damaging something.

I have no o'scope so I can't check for AC present were it shouldn't. Or at least I don't know how to otherwise.

Thanks!
 
Great, thanks. I'll fire it up a little later and check the voltages.

I appreciate the help! This is a great learning experience.
 
I should've asked which 12ax7? I jumped the 2nd one.

The first one. It seems like at least the second half of that first tube onward works if the volume control changes the hum level. I'm thinking the issue is somewhere around the first half of the first 12AX7, which is basically the input stuff that got changed. If grounding the first grid of that first tube shuts it up, then the problem has to be between the grid and the input jack.

Slight hum with the second 12ax7 grounded may be poorly matched output tubes. Push-pull amps need the output tubes somewhat equal in current draw to make them decently quiet. Major mismatches will make it hum because the power supply filtering at that first stage just isn't all that good. Also, if R31 is bad or really out of adjustment, that can make it tend to hum more too. Once you get it making sound, tweak that to get it as quiet as possible. Usually thats somewhere close to the middle of travel.
 
Well, so far I've found one cap way out of spec. The first 1000pF measures on my Fluke 87 at 1.07 uF! It's got pretty short leads, I'm thinking I got it too hot. I'll replace it in the morning - with clips on the leads this time. It's tough relearning little tricks I knew, and have forgotten in 40+ years.

Thanks again for all of the help.
 
Hi guys, I checked all the voltages at schematic test points, all are higher than spec, I expected that because of the higher than original mains voltage, but many seem WAY of of wack to me. Remember, I'm using a 5u4 in place of the specified 5Y3 - just for testing. If I can get this mess figured out, I'll spring for a 5Y3.

I also tested all the ceramic caps I bought, and they're ALL out of spec. Grrr. I can't believe the majority was from getting them too hot from soldering. I know how to solder, been doing it since about 1964 with my Dad. I even worked at RF Communications for about 1-1/2 years with him on the massive Marine Corp project that put them on the map. BUT, there are some terminals that have a lot going on, so maybe I am to blame. All the resistors I checked are perfect. I'm including a better, more legible schematic with the values plugged in, I hope AK doesn't downsize it too much.

If there's a way to just simply upload the image full-sized here, I'd like to know how. We don't have this problem at the Talking Machine Forum.

EDIT:
Full-sized images at Image Shack, here: http://imageshack.com/a/img922/3563/QdQdBx.jpg
http://imageshack.com/a/img923/6992/2Ckbec.jpg


Anyhoo, here's the list of voltages as measured with my Fluke 87:

TEST VOLTAGES 4-23-17.jpg

Revised Schematic Clean With Values Legal size.JPG
 
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Voltages appear fine in general - the output stage tubes are idling a bit hot (19.6W each from cathode current, if balanced).

Each V1 triode should have equivalent cathode and anode voltages. Can you swap around 12AX7 and see if the same voltages. Perhaps R7 - not too sure why its that high.

Are you able to measure anode voltages on the triodes as well?

Can you recheck cathode (pin 8) AC voltages on the 6L6 - is than mV or V ?

Did you measure capacitance on new caps first? Your meter may be the issue for capacitance.

Perhaps best to not run the amp for too long during testing, as the 5V heater current is 3A.
 
Voltages appear fine in general - the output stage tubes are idling a bit hot (19.6W each from cathode current, if balanced).

Each V1 triode should have equivalent cathode and anode voltages. Can you swap around 12AX7 and see if the same voltages. Perhaps R7 - not too sure why its that high.

Are you able to measure anode voltages on the triodes as well?
The measured values are:
V1 Anodes
Pin 1: 237.5 mVAC; 203.7 VDC
Pin 6: 8.5mVAC; 200.0 VDC

V2 Anodes

Pin 1: 40 mVAC; 241.5 VDC
Pin 6: 40 mVAC; 199 VDC

AFTER SWAPPING AROUND V1 & V2

V1 Anodes

Pin 1: 344 mVAC; 203.2 VDC
Pin 6: 11.3 mVAC; 200.3 VDC

V2 Anodes
Pin 1: 43 mVAC; 243.7 VDC
Pin 6: 38.4 mVAC; 197.8 VDC

Can you recheck cathode (pin 8) AC voltages on the 6L6 - is than mV or V ?
That should have been millivolts, I rechecked them both, and both are 11.0 mVAC. Geez, I don't know what I did earlier with that. :dunno:

Did you measure capacitance on new caps first? Your meter may be the issue for capacitance.
I did not, and I'm regretting it. As for the Fluke, I measured some other new unused caps I have in my parts stash that measured at their correct values. HOWEVER, I can't get any capacitance value for the polarized electrolytics (the meter reads "0.L"), so I don't know if I'm using the meter correctly on those. I tried swapping the neg and pos leads around, too, same result.

Perhaps best to not run the amp for too long during testing, as the 5V heater current is 3A.
Thanks, that's what I was thinking.
 
V1 pin 1 anode has hum or noise on it. I think it is a vestige of the original design related to photo tube connection. I can't see a good reason for the grid stopper R7 to be 3.3Meg, and suggest you replace it or bypass it with a grid stopper at the socket terminal of say 10k. Normally an input source would have a low impedance - if the source isn't connected, then the input should be shorted, otherwise the effective grid leak resistance will generate noise. Better to play with required frequency response later in the circuitry.

If your heater voltage is 7.14V, then one would expect 15% higher circuit voltages, and the likelihood of short tube life - so you will need to work on a practical solution, eg. a variac or primary side buck transformer or ...
 
Thank you for the analysis. I'm going to have to think about this amp for a little while. I was hoping to leave the circuit more or less original, just for fun.

Best,
Fran
 
The 5U4 has less voltage drop, so the B+ will be high. Something else to be aware of though, a 5U4 wants 3 amps of heater supply where a 5Y3 wants 2. Your transformer will not appreciate this.
 
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