• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

OPT Transformer Question

hp4766

Member
2020-02-20 12:11 am
Hi all, new to this forum. I was wondering if 2 OPT Transformer of the same brand can be wound opposite or the wires can be opposite. OK, So I ordered 2 OPT from Edcor and after finishing my build (Pete Millet's 20w Engineer Amp), fired it up with no unusual noise, no smoke etc. So everything's fine, voltage measurements, bias etc, until I hooked up a pair of speaker and cranked up the volume slowly ( I added a Pot to control the volume). About halfway I started to get distortion from one of the channels (like oscillating noise, and speaker vibrating uncontrollably. Immediately turn volume down and it goes away. Check everything over and seems to be fine. After a couple of e-mail exchanges with Pete, he suggested that I reverse the plate wires on the channel that causing the distortion and sure enough it solved the problem. My question is, did I do something wrong on the board for it to be reversed or did Edcor sent me a OPT wired wrong? Amp is running great, but I want to know what happened if anyone can shed some light on this. Thanks
 

6A3sUMMER

Member
2016-06-07 6:50 am
Are you using global negative feedback?

Are you using any negative feedback from the secondary tap?

How about a schematic?
Just give us a link to Pete Millet's 20 Watt Engineer Amp (I am lazy).

Now that you have reversed one transformer, how is the bass?
Is there less bass, or more bass if you connect one loudspeaker in reverse (for example connect speaker hot to amp common, and speaker common to amp 8 Ohm tap)?

Also, since the oscillation depends on the setting of the volume control, that may be either a ground loop, or where a signal wire is near a higher signal ledvel wire or connection in the amp (early stage signal wire and later stage signal wire proximity).
 
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if the amplifier had a general positive feedback loop, it would eventually cause it to go into a state of oscillation, flipping the primary or secondary winding causes the incoming feedback signal to reverse and change this from positive feedback to negative feedback thus causing the amplifier "work" in a more stable way, its also needed to say that this is a crude trick, your Opt could be miss wired
 

6A3sUMMER

Member
2016-06-07 6:50 am
1. Yes, the amp has global negative feedback.
Mis-wiring either the primary or the secondary could cause an oscillation.

2. Mis-wiring which input tube plate, connects to which coupling cap, that connects to which output tube control grid . . . can cause oscillation (a swap of those signals).

3. Mis-wiring which input tube control grid gets the input signal, and which input tube control grid gets the global negative feedback can cause oscillation.

4. Check your wiring on the volume control.
They should be wired the same on both channels, including the input jacks and grounds.
A ground loop can cause a problem.
 
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hp4766

Member
2020-02-20 12:11 am
Thanks 6A3summer. That is the only changed I made to the amp, adding the volume control. It has been running great with the plate on one channel reversed. Would it be OK to leave as is as I don't want to tear into again and maybe messed up something else LOL. Thanks for your help.
 

6A3sUMMER

Member
2016-06-07 6:50 am
If you have a 2 channel scope, you need to check one channel at a time; to see if the signal at the input versus the signal at the output is in phase, or if it is out of phase.

If one channel in to out phase is different than the other channel, you need to fix that.

Otherwise, you will get bass cancellation in mono and stereo recording playback.
And the music will seem to be beyond the speakers, and not between the speakers.
A good test for this is what I mentioned earlier . . . just reverse the wires on one speaker.
Be careful, do not change speaker wiring while the amp is on.

Happy testing, and happy listening.
 

6A3sUMMER

Member
2016-06-07 6:50 am
The problem with phase reversal is that it can occur at several places:

Input connector and negative feedback connections, versus the input tubes grids.

Input tube plates, versus coupling caps and output tube grids.

Output tube plates, and output transformer primary connections.

Output transformers secondary connections.

Any reversal of output transformers internal windings, or any reversal of how the internal windings connect to the color of the external transformer wires connected to them.

Wiring connections to the ECB / PCB

Only a very intensive investigation can prove what is the culprit that causes the phase reversal.