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It is currently 19 Aug 2019, 18:58

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PostPosted: 18 Jun 2019, 17:49 
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Not being disrespectfull about the schematic.

But if you followed everything as excatly and went through few sets of valves already and dosen't work.
Then theres something inthernatly a bit wrong with the schematic


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PostPosted: 18 Jun 2019, 18:38 
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Namely in one channel higher tones are slighly quieter than in another. It is not very obvious, but noticerable.

It could be a wrong or damaged capacitor used or the output transformer has some prolbems.
This reduces the high freqency votlage because its being filtered out by the wrong capacitor size or damaged

However if the prolbem dissapears when you change the valves, then its something it do with the schematic


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PostPosted: 18 Jun 2019, 20:40 
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Hi, I don't get offended (you should have seen some of the really early comments about 10 years ago) but, the schematic is fine, there are 100"s of these amps both diy and commercial kits working just fine. Amps built according to the plans work without issues. There is a nearly 10 year long track record of it. That said....There are some things that can cause one channel to not match the other in response. The number one cause is mis-matched driver tubes. Variations between tubes of the same brand and lot are usually small, but not always so. If you have different brands that can easily cause response mis-matching. You didn't indicate...but are the output tubes the same for both channels. If not that can easily cause response issues as well. I have measured response variations as much as 3 db at 15K HZ from different brands. Surprisingly the worst deviations were with supposed to be high quality NOS ones. I have found that JJ and EH seem to be the most consistent in response for outputs and EH, JJ, Sovtek and some NOS ( like the Phillips one I use) are most consistent for the drivers . Second most common cause is incorrect resistance values in the grids of the output tubes. If one is way low it will alter the response significantly. A marginal solder connection there can do this as well. Coupling capacitors can cause the problem but it is not common. They would have to be way off value to cause a problem. The versions of the amp that use two coupling capacitors in the output grid circuit (one coupling and one to ground on the slave tube) do need to be the same type and same size. Some defective output transformers have been reported and this easily can cause the problem. I personally have not encountered any though. The part of the circuit with the LM317 is incapable of causing the frequency issue unless they are bogus parts and even then if the circuit works it is unlikely to do this. I have never heard of this occurring. So where to start. Beginning at square one and making no assumptions. Swap the driver tubes see if the problem follows it. BTW I hope you have previously tried swapping the speakers as they are prime suspects in any sort of response issue. Second, with the power off measure the resistance from the output tube grids to a signal ground. It doesn't matter if the tubes are in place or not as long as they have stopped conducting. The negative speaker terminal is a good ground to use. They should be equal in both tubes on each channel and the same on both channels. If all that stuff checks out next I would pull all the tubes and measure the resistance to ground for each pin and verify that the same pin on the alternate channel is the same within about 5-10%. If that is good then put the tubes back, flip the amp over and with the amp running, no input signal, with some load on it preferably resistive but speakers can be used measure the voltages on each corresponding pin to ground and compare the two channels. They should be close. Pay particular attention to the driver stage. But be aware that because of variations in tubes the mid point values can be as much as 10-15% different. Let us know what you find. If all these tests are OK then I would suspect something amiss in the output transformer.

Good listening
Bruce

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PostPosted: 19 Jun 2019, 13:41 
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Hi Bruce,

First of all, I will never question the schematics, as I'm not a professional electronic and additionally it was many times checked by the other users, as you mentioned. Apologies if you have understood me this way. I always appreciate support I found in this thread.
Answering your questions:
1) speaker issue eliminated;
2) drive tubes swapped - issue stays in the same channel;
3) attached my measurements. I was hard to measure resistance on some pins, as it was constantly swinging. In the table red marked differences between drive tubes voltages.
4) checked cathode resistors as suspected, but equal in both channels (same color coding).

Looking forward for your comments.
Regards
Michal

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2019-06-19 20_36_47-Window.jpg


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PostPosted: 19 Jun 2019, 14:04 
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Hi, Like I said no offense. Considering there is some confusion with the design I am preparing an overview with schematics. This design has been evolving over a period of 10 years and while all versions are excellent performers the newest ones are the best.

Looking at your voltage data I see that the right channel driver tube is not working correctly. Since the pins 1, 7 and 8 are at the same approximate voltage I suspect a number of possible causes. Carefully examine around pins 1, 6, 7, 8 for solder bridges, bad solder joints (re heat the connections), correct resistors of the right value in the right place and short circuits between the pins. Essentially this tube section is acting like it wasn't there. I am surprised that channel is working at all. The output tube sections look OK. Let us know what you find.

Good listening
Bruce

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PostPosted: 20 Jun 2019, 07:08 
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Hi Bruce,

thank you for the answer.
The differences around 100V between driver tubes are only because of my stupidity...
While in LEFT channel I disconnected NFB circuit only on transformer side, on RIGHT channel I also disconnected from pin 3 of driver tube.
When NFB is connected to pin 3 in both channels, voltages on pins 1,7, 8 are slightly above 100V in both tubes.
1) does above indicate that circuit works correctly?
2) does suspicion go now to coupling caps?
3) where properly disconnect NFB if I don't want to use it?


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PostPosted: 20 Jun 2019, 09:18 
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Hi, That is a good clue. What is happening is that the feedback loop is providing the path to ground for the tube through the output transformer. When it is disconnected there is no effective ground for that tube's cathode and the voltages will go way off. You need to look at the resistor connected from the feed back junction (the one with the electrolytic capacitor, two resistors and the feed for the NFB components) that goes to the ground. It will be something in the range of 82-150 ohms (varies on the date of the schematic you built it from). I suspect it is not actually grounded or there is a bad solder joint. During normal operation disconnecting the NFB will change the sound a bit and will alter the gain a bit, but not cause the voltage error you measured. Having the tube grounded through the feed back network and transformer will cause the frequency response to be off.

Good listening
Bruce

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PostPosted: 20 Jun 2019, 10:28 
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Hi,
I based on below schematics.
So if I don't want to use the feedback loop, will it be enough to leave the part I marked in red?

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Poddwatt DMB Main Circuit August 25, 2013.jpg


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PostPosted: 20 Jun 2019, 14:21 
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Hi, Leave the other parts in place. Just disconnect the end at the transformer. While the sound can be a bit different (slightly so) the actual amplifier performance is not as good. The main purpose of the NFB is to guard against above band oscillations and to some extent high frequency ringing that might be caused by them. The transformers have a resonant point in this circuit (all transformers will have one) at about 75KHZ. High frequency noise, crud that might get through the power supply from the AC mains and EMI can possibly excite the resonance and cause unusual sonic effects. If bad enough it can reduce the power output. There is a slight benefit in noise reduction (1-3 db) and distortion ( about 0.25 to 0.5%) when using the NFB, but that really is minor as both are quite good without it. I always use it my amps. If you prefer the sound without it fine. The negative effects are rather unlikely, but possible. You can always hook it back up.

Good listening
Bruce

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PostPosted: 20 Jun 2019, 14:51 
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Ok, I will do as you recommend. The aim of omitting feedback is higher gain, as my current speakers are not super efficient.
Back to the high frequency quietness in right channel, what should I check as next (when voltages are fine)?

Cheers


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