• 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.

Boyuu EL34 A9 Tube Amp

Too low of a bleeder resistance discharges faster, but wastes power, and adds up heat in the amp enclosure.
Good advice!
Usually I use 50-100k bleeder, and yes, I check with a meter also.

I also have one of these 'Sparkpen'; it works.
Hot Sale Sparkpen Capacitor Discharge Pen +LED Light & Sound 3RD Gen Upgrade New 849724047317 | eBay

Bleeder resistor calculator from Welwyn via Digikey:
https://www.digikey.com/-/media/Images/Supplier%20Centers/T/TT%20Electronics%20Welwyn/tt-electronics-capacitor-discharge-calculator.xls?la=en&ts=452828e7-5a3d-47a6-af87-1bece327a585

Online calculator:
Capacitor charge and discharge calculator | MustCalculate
 

I have one of these (linked below) - I quite like it. Not as good as a dedicated Oscilloscope nor as multi-functional as above but for a hobbyist who only uses it only once in a while it certainly meets my needs.

Hantek PC Based USB Digital Storage Oscilloscope 6022BE, 20Mhz Bandwidth: Amazon.ca: Tools & Home Improvement
 
I have one of these (linked below) - I quite like it. Not as good as a dedicated Oscilloscope nor as multi-functional as above but for a hobbyist who only uses it only once in a while it certainly meets my needs.
We discussed this model. You should have bought at least 6022bl this with the analyzer.
INSTRUSTAR software is better than Hantec.
 

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navydiver,

It is nice to have a capacitor discharge unit.

But it seems that so many schematics of the power supply in this thread do Not have any bleeder resistor across the B+. (post # 720 for example)

If you ever forget to use the capacitor discharge unit, and you do not have a bleeder resistor across the B+, you may be surprised (shocked).

Even though all my amplifiers have a bleeder resistor, after I turn off my amp, I always check the B+ with a meter before doing any other probing around, or modifications. Most bleeders do take some time to discharge the caps to a safe voltage. And what if the bleeder resistor fails, or there is a bad connection?

Too low of a bleeder resistance discharges faster, but wastes power, and adds up heat in the amp enclosure.

You should make things as simple as possible, but no simpler - Albert Einstein

I hear you 6A3sUMMER. I have been using my DIY Cap Discharge device (CDD) across B+ and the largest accessible filter cap since the CDD build - works fab. It has two leads that plug directly into my multi-meter to monitor the voltage dropping in real time as it happens. It has two LEDs to light up when discharging (wired backwards from each other using resistors and zener diodes so you don't really have to worry about + or - arrangement of probes) and of course the probes and tester leads are rated to 1000V 20Amp for high capacity bleed down (also with alligator clip end options needed for one handed work inside the amp cse). Thing works very well for my needs. I do also note that when the amp is powered off, music still plays through the speakers until the caps have significantly discharged. I of course don't trust this for obvious reasons and use my device each time along with unplugging the unit....

Cap_Discharge.jpg
 
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So if the a9 has an put sensitivity of 750mv and my CD player outputs 2 volts, would this be the reason for Distortion at higher volumes?
that over 750mV will be fed to the amplifier causing signal distortion.
Be sure that the sensitivity of the input amplifier circuit must match the level of the output signal of the head unit. Input sensitivity is adjustable in the amplifier and sets the magnification factor, but high input sensitivity can lead to large signal distortions.
 
I do also note that when the amp is powered off, music still plays through the speakers until the caps have significantly discharged.

If you take out the rectifier tube and power up the amp with speakers attached, do you still hear the hum?

I hear you 6A3sUMMER....snip.... Thing works very well for my needs.
I agree with 6A3sUmmer that a bleeder resistor should be added to the B+ supply, if one isn't already fitted. You should be able to find spots for connection of the bleeder resistor on the PCB.
 
A little excess gain in the amplifier is why we have volume controls.

Many CD players (with the DAC at full scale) have 2Vrms out.

Suppose you set the amplifier gain so that it is just below clipping (or just below high distortion) when the CD player puts out 2Vrms (DAC at full scale). But when you play a typical Chesky Records CD, the loudest section of the recording is -10dB below full scale. Now, your 10 Watt amplifier can only put out 1 Watt when listening to that CD (so you have to turn the gain up, but you can only do that if you have the excess gain available).
 
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All
Great success. Explained herein: YouTube

I am sure there is room for tweaking once I get to know this A9 a bit better. Got some listening to do to find any warts but for now am very happy with outcome!
Cheers and thanks all!
TMF

Another question I have:YouTube

Short and to the point, anyone know where I can get a cage for this amp (see linked video)?
 
A little excess gain in the amplifier is why we have volume controls.

Many CD players (with the DAC at full scale) have 2Vrms out
I tried to explain clearly, but you persistently with one friend drank beer when I explained.
I do not urge everyone to make a divider, but in my case it is necessary.
Conclusion from my DAC 2.5V, send such a signal to your amplifier (750mv), then you will understand.


navydiver
BOYUU EL34 tube protection grill cover Reisong BOYUU A10 EL34 tube protection grill cover-in Amplifier from Consumer Electronics on Aliexpress.com | Alibaba Group
 

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Short and to the point, anyone know where I can get a cage for this amp (see linked video)?
If you're concerned about safety, and possibly fingers getting in the wrong places (kids, adults moving the amp or dusting), a (perforated?) bottom cover/panel could go on your list. Pretty easy DIY with a piece of Al from MetalSupermarket or something steel from old commercial gear. Al is easier to cut...

It's baffling to me how many DIY-type units come without bottom covers (Bottlehead Crack Headphone amp was the same @ $400+...)

Good work in finding the wiring error/errors and getting the amp running properly!
:up:
 
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