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Zen Kit 1 - general questions (Read 763 times)
Wharfcreek
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Zen Kit 1 - general questions
12/11/19 at 15:21:06
 
Hello Forum, new guy here.   Just registered, and I'm here as a result of attempting to resolve some problems with a Zen Kit build that a friend of mine did and turned over to me to 'de-bug'.

I found a thread in the Forum that addressed one issue I had, which was what I believed to be an error on the Schematic reflected as being pins 6 and 8 of the power tube where it should be pins 7 and 9.   In believing this to be 'resolved', I'm moving on to another part of this, which is also on the power tube, but related to pins 1 and 3.   My Schematic reflects a .1 cap across those two pins.  The PC board has a location for this cap, but the board is designed for the 6P15P tube, which connects Pin 6 and pin 1 internally.   So, the board's connection of this .1 cap goes between pins 1 and 6. When using a 6BQ5, there is no 'internal' connection between pin 1 and pin 6, so the connection for this cap isn't made.   I'm just wondering what affect that has on the circuit, and if anyone has added a 'jumper' between pins 6 and 1 in order to complete this part of the circuit?

Next, I'm curious about the pair of 330K resistors that are on this board, but not shown on the schematic?  Can anyone shed any light on what these are for/about...... and why they're on the PC board but NOT on the schematic?   Should I remove them / leave them in?

Finally, I'm dealing with a bit of a 'hum' issue with this amp.  In attempting to diagnose it's cause, I've determined that if the 6DJ8 tube is removed, the hum goes away.   When 'in' the amp, the volume control doesn't affect the hum level any.   My 'rookie' belief about this is that this indicates that the hum is not in the section of the power supply that affects the 'output' tubes, nor is it being generated from within the output tubes.   I also believe this indicates it's not on the 'input' side of the AF tube (6DG8), since the Volume Control has no affect on the noise.   This leaves me with either a 'filtering' issue as related to the AF tube, or some form of 'bleed' or 'inducted' noise getting to the PC board.   This amp has NOT had the film cap updates to the PS filter caps as indicated on the Schematic.   I'm wondering if that could be the cause?  Has anyone else encountered this issue?  Amp does have the 100 ohm 'virtual center tap' installed across the filament supply leads.

If anyone can address any of the above issues for me, I'd really appreciate the help!

Many thanks!!         Tom D.
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Radarek
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Re: Zen Kit 1 - general questions
Reply #1 - 12/11/19 at 20:28:10
 
Hello Tom

I'm rookie too, but also building this kit at the same time as you. About 0.1 cap for the power tubes. This mode will work only with 6p16p/SV83 tubes and is inactive with El84/6p14p/6bq5 tubes. If you going to stick with el84 family no caps is needed either jumpers, the board is designed in the way that you can switch between el84 and sv83 on the spot without any other action required but to get best from sv83 0.1 caps have to be in . That's my understanding, could be wrong. For the hum ... make sure you have this amp connected to the same outlet as your source that will get rid of ground loop. Check if all the grounds are done within the amp/ shielding on the signals cables within amp soldered only on one side etc...

Regards

Darius
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Wharfcreek
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Re: Zen Kit 1 - general questions
Reply #2 - 12/12/19 at 16:30:44
 
Radarek, thanks for your response.  I'm going to order a set of the 6P15Ps just to see if they make any difference in the function of the amp.  However, I hold little hope that they'll affect this 'hum' problem any.   As I indicated, I've not yet added the film caps to the electrolytic filter caps.   I plan to do that as well.  In the end, I fear the hum problem may be a result of the build layout the amp's owner/constructor chose for this project.  He actually has the position of the PC board 180 around from the location of the Power Transformer.  So, a lot of long leads flopping around, and as much as I've attempted to clean them up and locate them as harmlessly as possible, it's still not an ideal situation; both from the PT side of the build, as well as the Output Transformer side.   The Plate leads are also too long.  

Anyway, do you have any knowledge of the two 330K resistors that are on the board, but NOT on the Schematic?

Thanks,

Tom D.
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Denizen
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Re: Zen Kit 1 - general questions
Reply #3 - 12/13/19 at 20:59:33
 
Yes, you'll need the 330k ohm resistors, but first a little flow charting might be helpful. the signal comes into the input tube on pins 2(r.ch) and pin 7(l.ch), output is taken off pins 1 and 6, flows through the .1 coupling caps, it then goes through a 330 ohm resistor to pin 2 of the output tube, this is where the 330k ohm comes in. In old school parlance this was known as a grid return resistor it feeds a portion of the signal back into audio ground grid. This stabilizes the coupling cap. So at some point on the circuit board the coupling cap, the 330 ohm input resistor and the 330k ohm resistor should join together and the opposite end of the 330k ohm resistor should terminate to the ground. The other .1 cap between pins 1 and 3 are the CCE mod.
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fidelity71
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Re: Zen Kit 1 - general questions
Reply #4 - 10/22/20 at 03:45:37
 
Hi Denizen,
after reading your flow chart, thanks, what in your opinion would the lowest uf coupling cap? 3.3 uf is listed by steve, to low and bass roll off would occur, going the other way would 4.7uf be to high?

Thanks again
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Denizen
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Posts: 34
Re: Zen Kit 1 - general questions
Reply #5 - 10/22/20 at 21:03:55
 
The 3.3 and the 0.1 caps in parallel are bypass caps, you'll notice they are grounded. They filter out noise from the D.C. bus. The .1 caps are the correct coupling caps, they couple one amplifier stage to the next.
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