Choke question

[Fred_P]

Two month back when my Steromour kit arrived, upgading the OPTs (of the Stereomour) left me with a pair of extra BH OT-2s.
I wanted to use these in my S.E.X I. amp instead of the original line transformers,
but there was a problem: both the OT-2 and the PC-1 had exposed terminals.
To work around this issue, I purchased a pair of Hammond 157G chokes off ebay to replace the PC-1s.
The Hammonds have flying leads, so they could be mounted on top of the chassis plate,
while the OT-2s with the exposed terminals were mounted below.

The amp was running fine for around 6 weeks, and was unused for the last 2 weeks or so.
Now I turned it on again and noticed that one channel died completely.
Tracking down the issue led me to the chokes.
I do not have the equipment to measure inductance, but their DC resistance is way out of range.
The datasheet specifies 595 Ohms +/- 15%.
What I measure is 75 Ohms on one, and 23 Ohms on the other.

I probably did measure the DC resistance before installation when they were still new, but I can't remember for sure any more.

The possible scenarios I can think of include:
- Wrong choice of chokes.
       Based on the data sheet they look OK to be used instead of the PC-1s, but I might be wrong here.
- Wiring mistakes during the installation
       I just noticed, that I did not connect the primary of the OT-2s directly to ground, but through the secondaries.
       I created an autoformer by mistake, and although this may not be the only mistake I made, the sound was still OK for many weeks.
- Product quality issues with the chokes
       If only one of them was affected I would tend to agree, but since both have the same issue...

Worst case I can still go back to original configuration with the PC-1s, but it still bugs me what could go wrong to the Hammonds.
Any insight is much appreciated.

Fred

[Paul Birkeland]

Though I wouldn't be terribly excited about using Hammond chokes as plate chokes, it should at least function adequately.  It would help a lot to have voltage measurements from your amplifier, maybe just each pin on each octal socket is a good start. 

The SEX 2.0 is autoformer wired, were you intending to keep it that way?  I am guessing that you are not, since you were concerned about exposed terminals on the OT-2 (All of which would be at ground potential in the autoformer configuration).

Even not wired as an autoformer, if you keep the stock power supply, there will still be 0V on the output transformer.  You would have to further modify the amp to introduce positive cathode bias voltage onto the OT-2, and at that point we are still under 20V (in other words, exposed terminals on the top of the amp aren't as much of a concern with this design). 

Based on your description, I think you have a wiring error that will show up as presenting improper bias voltage to the output tubes.  I am guessing that they have drawn way too much current, which has damaged the chokes and basically shorted them out.  I have seen this happen once or twice from wiring errors on other parallel feed circuits, and the drastic reduction in DCR for the plate choke is a characteristic of this problem.

-PB

[Paul Joppa]

Just a note - while the OPT terminals are at 0 volts DC, they do have up to +/-200v peak AC when driven with a clipping-level signal. For that reason I do not recommend them on the top of the amp.

Other that that, I agree with PB's assessment. If you have not already done so, disconnect one wire of the choke from the rest of the circuitry and re-measure the DC resistance, to be sure you are measuring only the choke.

For what it's worth, I would normally choose a filter choke rated about 1.5 times the DC current for use as a plate choke - normal filter chokes are not designed to handle their rated DC AND a high voltage AC signal at deep-bass frequencies at the same time.

[Fred_P]

Thanks for the answers.

Currently the chokes are completely disconnected and removed from the amp, sitting lonely on my desk.
The DC resistance measurement is therefore accurate.

I will put them back into the amp along with the OT-2s in the original autoformer confing (which was by mistake)
and measure the DC voltages on the tube pins as suggested.

I will come back with the values then.

Fred

[Fred_P]

I put back the choke and the opt, both in the incorrect autoformer configuration as well as in the correct way, the primary return wire connected to ground.
(The actual difference in the wiring is where OT-2 terminal 10 is connected. I had mistakenly connected it to terminal 5 of the amp [where OT-2 terminal 7 was also connected] instead of terminal 10 of the amp [ground])

Anyway, here are the values:

terminal	unit	autoformer	correct grounding
A1	V	-14,1	-15,7
A2	V	369	367
A3	V	0	0
A4	V	0	0
A5	V	48,7	49,9
A6	V	2,31	2,42
A7	V	0	0
A8	V	5,6	5,75
I plate	mA	23,3	23,2
B1	V	-15,2	-15,3
B2	V	368	369
B3	V	0	0
B4	V	0	0
B5	V	52,6	53,2
B6	V	2,39	2,42
B7	V	0	0
B8	V	5,7	5,8
I plate	mA	22,8	22,9

No surprises I guess, except that the plate voltage of the gain stage is some 30% lower than it should be.
All the other voltages as well as the plate DC current are within range.

So I think it is safe to assume that the DC current could not have damaged the Hammonds.
The wiring mistake is irrelevant DCwise, and I do not see how it could have damaged the chokes ACwise.

Even what PJ mentioned about the choke rating is OK in this case.
The rated 40mA is 1.7 times of the measured 23mA DC.
So the headroom for the AC current must be sufficient.
(Btw. the PC1 is only rated 35mA.)

I am still kinda puzzled about this.

Fred

[Paul Joppa]

I'm going with a faulty hoke then - possibly mechanical damage to the winding, possibly internal.

All our plate chokes are rated for plate choke duty, with substantial AC voltage in addition to rated DC current.

[Fred_P]

I have just contacted Hammond about the issue, let's see what they say....

[Fred_P]

After some discussion with Hammond it turned out that after the outer layer of insulation was removed, and the saddle lifted up, the DC resistance was back to nominal. No idea how this could happen, or what the actual reason was, but the choke was damaged in the process and can't be used anymore.
This is obviously a product quality issue and sadly Hammond has no intentions to do anything about it.

Now I could put back the original SEX 2.0 OPT/Choke into my SEX amp leaving a pair of OT-2s from my Stereomour I unused.
Alternatively, if a pair of PC-3 was available for sale separately, I'd be glad to buy them to match the OT-2s.

Thanks

Fred