Stereomour puzzles

[observer]

Hi

I recently received and assembled my Stereomour and am running out of ideas in debugging process.  The left channel inexplicably has lower gain (about 4 times lower - do not remember the exact voltages) than the right one.  I will post voltages etc. but all resistance checks and DC voltages are normal.  I did swap the tubes (no effect) so it must be the wiring or other components of that chanel. 

The problem is confined to the triode output current loop - when 250mV of 1000Hz sine wave is on the grid of the first tube the signal level on the grids of the output triodes is the same in both channels.  The amplitude of the signal on the anodes of the output tubes is, however, very different.

I re-soldered cathode hum balance network joints (no effect) and suspected a dry joint or a flaw with the cathode bypass capacitor (although in theory the way OPT is connected in Stereomour neither cathode nor B+ bypass caps should have much impact) - replaced in both channells with 470uFx100V I had to hand with no effect.  The next thing I will try is swapping B+ connections of the chokes and/or re-solder the B+ bypass caps.

Any ideas or past experiences with similar problems would be greatly appreciated.  I understand it may still be just a dry joint somewhere - when I get back to my work bench (table) next weekend I will go through all of them.  But then again, DC conditions are normal, which suggests either a missing bypass capacitor or a flaw with the choke/OPT.  The OPT seems to have been configured correctly.

Another observation that I found interesting.   I configiured the Stereomour as a 45 amp.  When I first put in the stock 2A3, the right channel had audible hum.  When I replaced 2A3s with 45s, the hum was not audible.  The puzzle is that in both cases the hum level was balanced down to 1.8mV.  My scope is a very simple handheld one and the lowest resolution is 5mV/division but as far as I could see, the hum with 2A3 was predominantly 100Hz, the hum with 45 was predominantly 50Hz (and at 1.8mV is far less audible on my full range Fostex horns).  I did swap the tubes and it was not specific to a particular 2A3 (out of the two stock ones I have).  Tapping the tube (or indeed any surface/point of the chassis) with a stock 2A3 in the socket resulted in massive hum level fluctuations (to +20mV) settling down to 1.8mV.  With 45s tapping results in the hum level increasing by just a few mV before settling back to 1.8mV.

[Jim R.]

Did you swap the 2a3s from one side to the other or try another set entirely?  Sounds like you possibly have a defective set of 2a3s there, but the iron and solder joints are probably the next things I'd consider.

Do the static measurements of the windings of both transformers look about the same?  Any shorts to the frames?

Just things to consider next time you're working on it.

Good luck,

Jim

[observer]

Many thanks for quick responses - much appreciated.

I did swap the tubes - no effect/change so I cleared 2A3 from suspicion...  Same situation with the pair of 45s (also swapped).
The filament voltage is 2.45 - 2.5V on both tubes.

[Doc B.]

Measure the resistance of the grid resistors on the outputs tubes, and check theose connections.

[Paul Birkeland]

Since you have the equipment to do so, with the amplifier off, I would pass a few volts of 1khz AC through the primary of each OT, then measure the voltage across the secondary.

This can also be something as simple as a bad joint between the OT and the speaker jacks. 

[2wo]

I don't know how many taps are on this output transformer. Is it possible, it is connected wrong?...John

[Paul Joppa]

The Stereomour can be wired for 2, 4, 8, or 16 ohms. It's not taps, it's a change in series/parallel arrangements of the various secondary portions. It would be easy to confuse that part, I'm afraid. Checking the resistance, and the voltage at the input and output of the transformer (as suggested by others) is a good place to start.

[Doc B.]

Didn't the OP say he checked the output level at the plate of the 2A3, and not the OT secondary?

[Paul Birkeland]

Didn't the OP say he checked the output level at the plate of the 2A3, and not the OT secondary?

Yes, I apparently can't read.  An un-bypassed cathode resistor would possibly cause similar issues, that might be the next place to look...

[observer]

Many thanks for all suggestions.  The culprit was identified - a defect in the internal wiring of the output transformer.  Measured voltage ratio (with transformer configured for 8 Ohm load) is about 44 (about 21 for the other OPT in the right channel, which is working fine). 

I will request a replacement, in the meanwhile I re-configured the transformer for 16 Ohm, which yielded the ratio of 27 - not exactly a match but not as far from 21 in the other channel. 

I can listen now and I like what I hear although the stereoimage is of course skewed towards the corner of the room.  I will post again when I replace the OPT to confirm and close the topic.  I will open a new one for the listening impressions.       

[Jim R.]

Nice work!  It's really rare for a BH transformer to be defective, but it does happen every now and then.

Inn themeaan time, why not configure both sides for 16 ohms?

-- Jim

[observer]

Thanks again for the support.  I received a replacement output transformer (a quick shipment thanks to Eileen) and quickly soldered it in - it works now and I am enjoying the sound waiting for the Munforf SIO (parafeed) and Soviet FT-3 (interstage) to fully burn in.  Will post listening impressions in a week or so.

Another observation - I configured the amplifier to run 45s and the anode voltages were far too high with the stock value of the R in power supply (see below - I believe 45 plate to cathode max coltage is usually specified at 275V and I had 305).  I replaced the resistor with a 10W4.7K and 3W 3.3K that I had to hand in parallel  and at approx. 2K the voltages were what I would expect (285V plate to cathode at 33mA)- the sound and stereoimage definitely improved.  I did check the filament voltage, which remained at 2.5V.

   description   ref manual   R 1.2K   R2K

   B+                   350                   372           347
1   anode V2           340                   360           338
2   anode V1           230                   201           201

5   cathode V2   57                   56           53

11   cathode V2   57                   57           54

14   anode V1           230                   185           184
15   anode V2           340                   358           338

[Paul Joppa]

I would not call that "far" too high, myself. Tube specs are for nominal design purposes, and usually allow +/-10 percent for tolerances and power line variations.

Your power line is probably a little higher than the design value of 120 volts; most are. (I run about 123v at my home.) And your 45 is running a bit low on current, relative to the published curves (from 1932!). The combination puts it at the higher voltage that you are observing. The tube current may increase a bit, if these tubes are not broken in yet.

I did chose to run the 45 at its maximum condition. Your solution, to drop the voltage a bit further, is not a bad one. But check the voltage across the power supply resistor so you can calculate the dissipation - those resistors do dissipate a lot of heat. It's my preference to run resistors at no more than 1/3 of their rated power.

For anyone else reading this, if you want to operate more conservatively than the specs, I recommend sticking a 2A3 into the amp after setting it up for the 45.

[observer]

Paul, thanks - this makes sense.  I did check the dissipation and will replace the two parallel resistors once I can get a 2K resistor rated at 7W or 10W.  Now the two in parallel dissipate about 2.2W (33mA).  The 4.7K I was able to find at 10W, so no problem (it went into the space for 1.2K - same type and size) and 3.3K I was only able to find quickly at 3W.  It went into the space of 360 Ohm resistor on the PCB and I was careful to leave a bit of space between the body and the PCB.  It should dissipate 1.3W, which will do for now as it is rated for 3W.  I checked the temperature after a few hours (of course carefully and having switched off etc etc) - it is hot to a careful touch but not burning hot.

I am with you on the engineering choices as far as safety margin is concerned.  I actually wanted to put in KBG parafeed capacitors.  I read the trail on volatges and your remarks on high transients, which I agree with, and decided not to buy 600V rated caps. I got a pair of 2x2uF rated at 1000V but they are so large that there is no room - this will have to wait until I can make a custom chassis to play with filament heating etc.  Hence I had to do with Mundorfs rated at 1000 or 1200V.

My voltage is actually 240V 50Hz (UK) - I got the kit as soon as the new 240V power transformers were available.  The UK voltage is higher than the EU standard specified at 230V (actually they specify a range which covers 220V that used to be the standard in many countries and the UK 240V as no-one ever intended to re-build the supply grids to fit the new standard :-).