1957 Decca "The Douglas" Distortion

[Hi-Fi-Mogul]

Experiencing a distortion problem in this
PP 50C5, 12AX7 mono amp.

Re-cap all done, selenium rectifier to silicon diode.
Changer & motor cleaned, new P-226 cart, new idler.

When playing the Beach Boys' "Surfin USA",
there is harsh distortion around particular
vocals (med to higher frequencies).

At moderate or higher volume, quite a bit.

At low volume, no distortion.

Bass frequencies don't seem affected.

I swapped out the 12AX7 as a starter, no change.

[electra225]

My first thought was needle or cartridge, but I see you have those covered. And I assume you have checked tracking and tone arm weight. You don't mention element voltages. Make sure those are correct. Have you measured the value of the cathode bias resistor, R13? Have you checked the resistors in the tone control circuits, R2, R4, R5? Try subbing the output tubes to 50EH5 if you have some. I see you subbed the 12AX7. What is B+ since you changed the rectifier?

[hermitcrab]

My first thought is a bum cartridge or as stated the tracking weight if too heavy can cause distortion ...also did you place a resistor in series with the new rectifier diode? Last P-226 I got one side was definitely lower in volume to the other side...I buy the bampa carts on amazon... same cartridge as the P-168 but only 11 bucks

[Hi-Fi-Mogul]

I will consider changing the cartridge,
but it is one that I used in another changer,
and played fine. Tracking is wnl of the cart.

Banpa carts have no bracket for V-M 1/2 inch mounts.

There is an anomaly with V's on Grids 1
of the 50C5's, I believe.

Here are the readings:

R13 (100K)= 109 K
R2 (100K)= 97 K
R4 (100K)= 102 K
R5 (100K)= 93 K

B+ Voltage= 132 V

50C5(A)
Plate Pin 7= 128 V
Cathode Pin 1 = 10.5 V (9.2 V)
Grid 1 Pin 2 = Positive 0.5 V and rising.
Grid 2 Pin 6 = 128 V

50C5(B)
Plate Pin 7 = 128 V
Cathode Pin 1 = 10.6 V (9.2V)
Grid 1 Pin 2 = Positive 0.5 V and rising.
Grid 2 Pin 6 = 128 V

12AX7 Plate(A) Pin 1= 80 V (80 V)
Plate(B) Pin 6= 82 V (80 V)

[electra225]

The value of R13 is 100 ohms, not 100K ohms. 109 ohms is at the top of that value. The schematic gives cathode voltage as 9.2 volts. If what you are calling Grid 1 is actually the cathode, you should investigate the discrepancy in those voltages. I believe I'd go over the chassis component by component to find a resistor out of range. Can you feed the output from another cartridge in to see if there is a difference?

[Hi-Fi-Mogul]

Yes Greg, R13 is 100 ohms and reads 109.

R7 10Meg ohms on Pin 2 of 12AX7
is elevated at 13.8 Meg ohms.

I'll look around for a 10 Meg replacement
for R7.

Counterpart R11 10Meg ohms on Pin 7 of 12AX7
is 10.9 Meg ohms.

And all other resistors are wnl.

My understanding from reading a 50C5 data sheet,
is that in this pentode, Grid 1 (pins 2 or 5) should
be a negative voltage reading.

[electra225]

The schematic gives 9.2 volts on the cathodes of the 50C5 output tubes. If the bias voltage is not right, the tubes get driven under or over their limits. You want the cathode more positive than the grid in this circuit. Make sure B- is still floating and not grounded to the chassis. C5 is the capacitor that keeps B- floating. R13 should be right on the money if you can get it there. Any resistors that have drifted high should be replaced.

[Hi-Fi-Mogul]

The distortion is still in the amp after resistors replaced.
I also subbed in a BSR changer to test if the V-M
changer cartridge was bad, and distortion remained.

R7 10 Meg replaced.
R13 100 ohms replaced with a 99.3 Ohm resistor
R8 reads 51 K ohms within 10% for 47 K ohms

Rechecked all other resistors and WNL.

C5 0.1 mF reads 0.103 mF and solder connections are
good to chassis and B minus.

Voltages after resistor changes are pretty much
the same.
The 50C5 cathode voltage dropped 0.3 volts to 10.2 (schematic= 9.5 Volts)

Please note that the 50C5 control grids voltage
is still positive.

I asked before, shouldn't it be a negative V ?
I'm measuring with probes on Pin 2 of 50C5 and B minus.

Thanks !

[electra225]

Remember, you will have the signal appearing on the grid, Pin 2. That will be AC voltage. You will get a small voltage on the grid. It should be less positive than the cathode. The last I knew, you only had .5 volt positive on the cathode. How did we get to almost 10 volts on the cathode? Have you subbed the output tubes? Gas or grid emissions are a possibility with a 50C5 due to how hot they get. How positive is the grid? What voltages do you see on the grid, Pin 2 of the output tubes? Is the distortion there when the unit is first turned on, or does it appear and get worse as it warms up?

[Hi-Fi-Mogul]

Remember, you will have the signal appearing on the grid, Pin 2. That will be AC voltage. You will get a small voltage on the grid. It should be less positive than the cathode.

The last I knew, you only had .5 volt positive on the cathode.
How did we get to almost 10 volts on the cathode?

Have you subbed the output tubes?
Gas or grid emissions are a possibility with a 50C5 due to how hot they get. How positive is the grid?

What voltages do you see on the grid, Pin 2 of the output tubes?

Is the distortion there when the unit is first turned on, or does it appear and get worse as it warms up?

The cathode voltage I posted earlier was 10.5 Volts.
Today's is 10.2 Volts after resistors replaced.

I have not subbed output tubes. I did test them and they
showed no defects.
I have some NIB 50C5's that I will try tomorrow.

50C5 Pin 2 control grid voltage over a 20 minute
period rose to 1.6 V, then dropped to 1.35 V,
then seemed to stabilize at 1.6 V.
So, as you said it should be, it is less positive than the cathode V.

Distortion happens as soon as tubes function,
no delay, stays at the same level throughout
playing a record.

[electra225]

Grid emissions may not show on a tube tester. The grid will get more positive as the unit warms up if a tube or tubes have grid emissions. It never hurts to sub the 50C5's to eliminate them as a source of distortion. The schematic shows 2uf non-polarized caps as crossover caps. Have you changed or checked those? Have you verified lead dress in the chassis? And, could you have a speaker acting up at higher volume? Do you have speakers you could sub to eliminate them as a problem? I don't think I'd worry about the bass speaker, just the treble.

[Hi-Fi-Mogul]

Grid emissions may not show on a tube tester. The grid will get more positive as the unit warms up if a tube or tubes have grid emissions.
It never hurts to sub the 50C5's to eliminate them as a source of distortion.
The schematic shows 2uf non-polarized caps as crossover caps. Have you changed or checked those? Have you verified lead dress in the chassis?
And, could you have a speaker acting up at higher volume? Do you have speakers you could sub to eliminate them as a problem? I don't think I'd worry about the bass speaker, just the treble.

Great suggestions.

The 2 mF cap was replaced by someone with a
2.2 mF film cap.
I'll check it.

The chassis is cramped (as usual) and I'll take a photo
for your review of potential problematic lead dress areas.

The distortion does occur at higher volume.
You read my mind about speakers.
I was thinking today that maybe the
answer lies in a bad speaker.
I'm a bit confused by the layout of the output
transformer secondary.

Dave1138 made some nice plug-in connections for
the cabinet speakers, so they are easy to unplug
from the amp.
I have some Panasonic Thruster stereo speakers I could try if
you would advise me on where to place some alligator
leads on the secondary of the OPT.
Thank you.

[electra225]

I'm not all that experienced on three- channel stereos. IT looks to me like the secondary of the audio output transformer is center-tapped. The treble speakers connect from "B" on the schematic, the center tap, to "C" on the schematic, one leg of the output transformer secondary. If you have a regular film cap in the crossover network, instead of a non-polarized electrolytic, that may not work very well. You should correct that so it will be like it's supposed to be. Isn't there a plug or connection where the speakers are connected? Unplug the speakers and connect the test leads where the original speaker was connected. Replace the crossover cap before you test the speakers.

[Hi-Fi-Mogul]

I'm not all that experienced on three- channel stereos.

If you have a regular film cap in the crossover network, instead of a non-polarized electrolytic, that may not work very well. You should correct that so it will be like it's supposed to be.

Isn't there a plug or connection where the speakers are connected? Unplug the speakers and connect the test leads where the original speaker was connected. Replace the crossover cap before you test the speakers.

Greg,
I wanted to let you know this amp is a mono push-pull one.

I found a 2.2 mF NP e-lytic cap that I can sub for the film capacitor.
I can't visualize the numerous OPT wiring on its terminal strip,
until that cap is removed.

BTW, do you have any idea what R15 100K and R16 4.7K
are doing on the OPT secondary ?

R16 is out of spec, so I'll replace both of them.

After these new parts are in, I'll retest the cabinet
speaker system.

Today I moved the Decca cabinet indoors,
due to 90 degree heat and voracious mosquitos again.
That should make things a bit easier to do.

And I still need to sub out the 50C5s, too.

[electra225]

I misspoke. Of course it's mono. Old age, Mr. Mogul, old age is the only excuse I have.......

The schematic shows you should have 4K ohms impedence across the primary of the output transformer, terminals "D" to "E". You should have 3.2 ohms across the secondary, terminals "A" to "C". The speakers are connected "B" and "C" for the treble speakers and "A" and "C" for the bass speakers. R 15, 100k ohms and R16 4.7K ohms are across the speakers for tone compensation, would be my best guess.