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Old 8th March 2009, 02:26 PM   #1
ccschua is offline ccschua  Malaysia
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Default tube amp coupling cap - audiospace

I am using the Audiospace DU2.8i. This is an integrated tube amp in push pull AB1 with about 15W output max.

The preamp is
1x12AX7
2x12AU7

the power side is 4 x 6V6.

Stock tubes are all audiospace (which means it is Shuguang). I have replaced the stock tubes to

1x12AX7 -> Mullard reissue
2x12AU7 -> CV 4003

4 x 6V6 -> Tungsol Reissue 6V6GT.

The sound already has some improvement.

Next, I found the amp uses 4x0.22uF and 2 x0.1uF coupling cap (audiospace white)

If I replace theses cap to Mundorf Silver/Oil, will I get better improvement.

Also the unit uses 'auto biasing circuit' and the unit is from China which is rated at 220V ac. If I use it in 240V british, is the biasing properly / automatically done ?

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Old 23rd March 2009, 04:01 PM   #2
ccschua is offline ccschua  Malaysia
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I really hope someone could give me an advice.

Running the tube amp at 240V instead of at rated 220V will it shorten the tube life ?

Will I get better sound If I install a step down trany from 240 to 220V ?
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Old 23rd March 2009, 05:06 PM   #3
rknize is offline rknize  United States
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Quote:
Originally posted by ccschua
Running the tube amp at 240V instead of at rated 220V will it shorten the tube life ?

Will I get better sound If I install a step down trany from 240 to 220V ?
You are running it at less than 10% above the intended voltage. That maybe translates to 8% increase on the B+. The tubes are likely to be OK, but we could tell you more if you took some measurements. Is there a schematic for this amp? I don't see any bias pots, so I'm guessing it's "auto" biased with cathode resistors. The best thing would be to measure the voltage at the plates and also measure the voltage across the cathode bias resistors. Then we could tell you if you are pushing the 6V6s too hard. A lot of 6V6 PP designs, especially guitar amp, seem to like to run them at much higher-than-intended plate voltages. It usually caused problems when cheap current-production tubes from 5 or so years ago, but now they are much improved. The fact that it worked for a while (how long?) on the original Chinese tubes is probably a good sign.

I would be a bit more worried about your power transformer. Does it run hot? If you can put your hand on it and leave it there for a few seconds after several hours of playing, it is probably fine. If it is too hot to keep your hand on, then it might not live terribly long.

As far as the sound...well that is an interesting question. The 6V6GT PP "Thomas" amp I have runs the 6V6s at a rather high potential. I remember when I was learning about tube designs I was trying to figure out the operating points of this amp. I couldn't get a good idea about the output tubes because they were operating "off the chart" of any of the tube manuals I had.
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Old 24th March 2009, 11:39 AM   #4
ccschua is offline ccschua  Malaysia
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Hi,

can u show me how to measure the plate voltage ? Do I just turn on power supply without input signal and measure it (with respect to ground) ?

it is indeed auto bias. I dont have a schematic. the closest I find is this one (it is kt88)

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Old 24th March 2009, 02:17 PM   #5
ccschua is offline ccschua  Malaysia
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sorry. I think the nearest schematic is

Click the image to open in full size.

It looks like most transformer come with a tapping. Can i tap it lower ? Anyway, I will measure B+.
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Old 24th March 2009, 03:02 PM   #6
rknize is offline rknize  United States
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Yeah, the way to measure it is to connect the meter first and then turn on the amp with no input signal. Leave the speakers connected too (or put dummy loads on).

The schematic doesn't seem to match some of the component values I am seeing in the picture, but my guess is that those 680 ohm power resistors right between the tube sockets are probably the cathode resistors. If B1+ is 300V and those 680 ohms are the cathode resistors (I can't see what tube pins the traces go to), then they are not pushing those tubes very hard at all. It would be useful to measure the voltage across one of them, though.
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