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Hiwatt/Ampeg Clone
Hiwatt/Ampeg Clone
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Old 14th June 2018, 12:15 PM   #41
blackwhaleamp is offline blackwhaleamp  Belgium
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Quote:
Originally Posted by gingertube View Post
...
Have you gone over the power rail bypass caps. A dodgy joint at either end of a single bypass cap could also cause that POC POC POC from the "motor boating" form of oscillation via feedback on the power rail.
Wow, thank for the tips. I will try your solution. Someone also suggested me that the issue might be motor boating. I change the 12AU7 stage to another cap, and the POC POC improved a lot. However, it keeps coming from time to time

Quote:
Originally Posted by gingertube View Post
P.S. Bothering with this cause I like your design, something I might build for myself "one day".
The difficult part was to find/make the toroÔdale. Those are hand made, and I am afraid that this could be another source for the issue. I don't have a machine to build those kind of transformers.

You can find how to make those here

Bass preamplifier in the Ampeg SVT tradition

Quote:
Originally Posted by gingertube View Post
The output impedance from the 12AU7 cathodes is going to be really low, approaching 1/gm, say some thing like 100 Ohms, so try a 220 Ohm series "build out" resistor in series with the feed to C14, C16. Not 100% sure of that 100 Ohms output impedance "butt pluck / err. educated estimate from insufficient data" - so try resistors up to not more than 1K5. If that doesn't fix, higher values wont either.
I put a 100 Ohm resistor between the C14/C16 and the Cathode and it fix the issue

Sometimes the POC POC POC comes back , but not very frequent. I have no more ideas about what to do. In any case, I'll deliver the amplifier as is. There is something that triggers that noise, and I have no idea what could it be.

Thanks for the support
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Old Yesterday, 08:32 AM   #42
blackwhaleamp is offline blackwhaleamp  Belgium
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Default Yet another issue with Hiwatt replica

Hi

Here I come again asking for help for this Hiwatt replica that I build few months ago. There are the two threads that I received plenty of feedback to finalize the amp.

http://www.diyaudio.com/forums/instr...t-ampeg-4.html

http://www.diyaudio.com/forums/instr...att-ampeg.html

Few months ago I received the amplifier back because for some reason it starts to make a PUK PUK PUK noise. I added a video that shows the noise.

After looking for more info on the internet, I found out that it could be motor boating due to a defective decoupling cap on the power supply. I changed one of the two 100+100 uF cap. Maybe I need to change the second

I not sure if anyone can advise me what could I do.

Maybe changing the power supply itself, like adding another rectification stage.

Maybe someone has a layout of an original Hiwatt, it could be that I have a ground loop somewhere and it would be good to see how a real Hiwatt is cabled inside

Thank a lot for your help, this is the last step to complete this amplifier
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File Type: png Replica-Circuit.png (179.9 KB, 33 views)
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File Type: mp4 VID_20180717_094718.mp4 (3.64 MB, 1 views)
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Old Yesterday, 09:17 AM   #43
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Motor-boating is caused by instability. Fit grid stoppers for parasitic noise and decoupling for motor-boating, also layout is extremely important.
I have a pdf of the layout but it is too large to upload.
I will pm it to you.

Try here;
Attached Files
File Type: pdf HiWatt layout-02.pdf (194.0 KB, 13 views)
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Old Yesterday, 10:10 AM   #44
avtech23 is offline avtech23  Australia
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Hiwatt/Ampeg Clone
Not sure if it is just the recording audio, but it sounds like something arcing(?).

Have you checked all your solder connections are good? No accidental shorts? Resistors close to the valves all correct values?
A really thorough visual inspection should be your first step.

Check the continuity of the output transformer. I've heard of motorboating being caused by open circuit OPTs before, as well as defective cap.
Being built a few months ago, I assume the parts (capacitors and valves) are from good stock? Have a good look at the capacitors - any leakage?

Does it happen at all settings (volume etc)?

If competent around high voltage, check the behaviour of the voltages when the amp is making the noise - is there an area of the circuit where the voltage is dipping? Try to narrow down the search area.
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Old Yesterday, 10:30 AM   #45
jazbo8 is offline jazbo8
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Hiwatt/Ampeg Clone
@blackwhaleamp - It's against the Forum Rules to start multiple threads on the same topic, as a result, your threads have been merged.
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