and from here, I think there's something weird about your bias circuit.
for mine, I followed the advice of 6A3summer, cathode bias resistance of 370ohm(4 or 5 W) per pair of 6v6 + 220uf / 50v bypass cap and I added a DC balance potentiometer per pair of 6v6
for mine, I followed the advice of 6A3summer, cathode bias resistance of 370ohm(4 or 5 W) per pair of 6v6 + 220uf / 50v bypass cap and I added a DC balance potentiometer per pair of 6v6
I cracked, I bought another one like new exit of the factory , I'll try a quartet of 6p3S-e on it to see what happens
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I know this is an old topic but , bandit88, did you finally solved the problem? What was the solution for you? Got the same problem on this exact same beautiful amp and I'm trying to solve it too.
Thanks a lot
Thanks a lot
Hi nonolagratte
what have you already done with your A300, where are you in your restoration process?
if you want ,send me a pm,in french it's more easy 😉
I have one on my work table right now
Hello,
I private message you the other day but get no answer. Hope you receive it well. Thanks a lot anyway !! Have a good day 😀
Reviving this old topic, as i have not found a clear answer yet to the solution. I also have a buzz that comes from the speakers, that changes with the volume. Nothing i have tried until now changed anything.
My A300 is as follows:
Do not know how it sounded when i bought it, because i did not have the courage to start it up, due to the caps being old.
My A300 is as follows:
- one of the previous owners took it to a technician that was supposed to update the power supply, change the capacitors and tube sockets from V1 to V5. Other mods are also possible (for example the RIAA/NARTB switch has nothing connected to 2 of the pins - those located towards the middle of the chassis), but it is hard for me to tell
- Has been recaped by me, while also changing the diodes, and placing a choke instead of the resistor in the power supply.
- Replaced all tubes
Do not know how it sounded when i bought it, because i did not have the courage to start it up, due to the caps being old.
If it changes with the volume the hum source is before the volume control.
Is this a power amp with the level control at the input?
Does it still hum if you remove the input (from a preamp?) and short the input?
Jan
Is this a power amp with the level control at the input?
Does it still hum if you remove the input (from a preamp?) and short the input?
Jan
The "loudness" control is right after the input switch. Being an integrated amp, the only posibility is to try it with no signal thru the input. Tried it by switching to an unused input, but the buzzing remained.
My problem might differ from those before, but i am willing to try all i can before admitting defeat.
My problem might differ from those before, but i am willing to try all i can before admitting defeat.
You mean the level or volume control? A loudness control is a different control.
If you switch to an unused input, did you try to short that input?
That would give an indication as to the source.
Jan
If you switch to an unused input, did you try to short that input?
That would give an indication as to the source.
Jan
I think it is the volume control, but it is labeled as loudness (hence the "" in my previous post)
I did not know this technique until now, shorting that input. Basicly all i have to do is take an rca cable, cut in in two pieces, and connect center to shield. Will give it a go as soon as i am home.
Thanks
Robert
I did not know this technique until now, shorting that input. Basicly all i have to do is take an rca cable, cut in in two pieces, and connect center to shield. Will give it a go as soon as i am home.
Thanks
Robert
That's interesting. Loudness has a very strict defined meaning, it is a frequency response adjustment with volume, because we are less sensitive to lows and highs at lower level.
The input short is a test to see if it is something that comes from outside, or whether it is internal to the amp. If you cut a cable for this, cut it close to the plug to keep the cable length as short as possible. Just take any old worthless cable you have laying around.
Jan
The input short is a test to see if it is something that comes from outside, or whether it is internal to the amp. If you cut a cable for this, cut it close to the plug to keep the cable length as short as possible. Just take any old worthless cable you have laying around.
Jan
an important thing on the A300/A500 is the state of the connection between the front panel (block with potentiometers) and the chassis, I often have them buzz because of that (oxidation between the two) and now I solder a small piece of braid to desolder between the two each time I have one on the table.
I have 4 A300s now and on two of them I grafted the A500 bias balancing circuit (a 100R pot and a 22ohm resistor between each pair of 6v6 7193)
I have 4 A300s now and on two of them I grafted the A500 bias balancing circuit (a 100R pot and a 22ohm resistor between each pair of 6v6 7193)
Refer to the first post. This pot does both. IAC, audio maker terminology is not enforced by any language authority.
Troubleshooting normally requires isolating the different parts of the amplifier.
Turn the balance control clockwise all the way, and notice if the hum is reduced in one channel's loudspeaker.
Now turn the balance control counter-clockwise all the way, and notice if the hum is reduced in the other channel's loudspeaker.
If the hum is the same, in both channels no matter where the balance control is turned, then the hum is from the stages that follow the balance control.
Find that out before proceeding further.
If the hum is from the stages after the balance control, the first place I would look is the filament to cathode leakage of the concertina phase splitter tubes. I think the concertina phase splitter was a 12AU7 or equivalent or similar. Not all tubes are created equal, some 12AU7 have more filament to cathode leakage than others. You might try the JJ 12AU7 from eurotubes.com, they extensively re-test the tubes they get from JJ in Slovakia. The JJ 12AU7/ECC82 uses spiral wound filaments.
Regarding modifying the output tube's cathode circuit,
If I remember correctly, the HK A300 uses the self biasing cathode current of all 4 output tubes to run the series string of the filaments of the two 12AX7 phono preamp tubes. The output tube's cathodes are at about 24VDC, and the output tubes grids are at about 12VDC.
Changing the cathode circuit is more complex than you might think.
Turn the balance control clockwise all the way, and notice if the hum is reduced in one channel's loudspeaker.
Now turn the balance control counter-clockwise all the way, and notice if the hum is reduced in the other channel's loudspeaker.
If the hum is the same, in both channels no matter where the balance control is turned, then the hum is from the stages that follow the balance control.
Find that out before proceeding further.
If the hum is from the stages after the balance control, the first place I would look is the filament to cathode leakage of the concertina phase splitter tubes. I think the concertina phase splitter was a 12AU7 or equivalent or similar. Not all tubes are created equal, some 12AU7 have more filament to cathode leakage than others. You might try the JJ 12AU7 from eurotubes.com, they extensively re-test the tubes they get from JJ in Slovakia. The JJ 12AU7/ECC82 uses spiral wound filaments.
Regarding modifying the output tube's cathode circuit,
If I remember correctly, the HK A300 uses the self biasing cathode current of all 4 output tubes to run the series string of the filaments of the two 12AX7 phono preamp tubes. The output tube's cathodes are at about 24VDC, and the output tubes grids are at about 12VDC.
Changing the cathode circuit is more complex than you might think.
Ahhh yes, I see. Combined level/loudness, with the loudness being defeatable. Neat.
BTW, another diagnostic possibility: use the Tape Out as an input from another preamp, to see if you still have the hum/buzz. If not, the hum/buzz comes from the stuff to the left of the volume pot.
Jan
Yes!! That happened to an A500 integrated amp I had before. Make sure the bottom cover is conductive to the screws and other metal parts. Sandpaper rust or dirt layer in touch with the chassis.
Thank you all for the feedback, i have added some more tasks on the to-do/check list - it is getting long due to time checking things.
Currently one of the tube red plates randomly so i have a few more things to do/check.
For those who have worked on one of these amps, what more would you do to it, even if only as a precaution?
Currently it is recapped, and am planing on updating the phono stage (and removing the riaa/nartb switch), changing the 470k ohm resistors in the power stage, as one appears to be malfunctioning.
Currently one of the tube red plates randomly so i have a few more things to do/check.
For those who have worked on one of these amps, what more would you do to it, even if only as a precaution?
Currently it is recapped, and am planing on updating the phono stage (and removing the riaa/nartb switch), changing the 470k ohm resistors in the power stage, as one appears to be malfunctioning.