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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 16 Nov 2016, 10:49 
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Joined: 04 Aug 2016, 15:52
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I'm refurbishing an old tube amp and made some measuring, I have no experience in rectifying tubes so I it makes me wonder...
It does work and sounds great but needs some work done on it to make it electrical safe, that's why I'm doing this, it uses (ECC83 and 6V6Gt) tubes if anyone wonders.

The schematic looks a lot like the one in the picture, having 2x275VAC at 4-6 ( 5V at 2-8).
When measuring the B+ from the 5Y3GT i only get like 300VDC, I would have expected at least 380V ( maybe a little lower) ?

I even disconnected the cord from 8 and measured directly on 8 but still about 300V, is this normal for this tube or is it time to change it?
Maybe I'm totally wrong here but since the high voltage (B+) is directly connected to the modulated signal level, shouldn't a low B+ give lower amplifying?


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PostPosted: 16 Nov 2016, 11:22 
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Joined: 02 Mar 2009, 12:41
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Location: Vänersborg, Sweden
A wild guess that's not that crazy, you have a significat voltage drop over the rectifier tube. Not any 0,7V like across a normal sh*tty 1N4007.

I think this graph can shine a little (soft orange) light:
Image

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PostPosted: 16 Nov 2016, 17:17 
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Here's a trick. with the unit in operation, measure the voltage drop across R3. That voltage divided by the value of R3 is your total supply current. Multiply this number by 4 and then look at the voltage drop curve for the 5Y3 and find the voltage drop at this current (i.e. 4 times measured). Subtract this voltage number from 275*1.414 and you should get an answer fairly close (+/- 15%) to what your measuring at the B+ line.

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PostPosted: 17 Nov 2016, 13:38 
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Joined: 04 Aug 2016, 15:52
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Ok... Thanks... I dropped one of the 6V6Gt to the floor and broke the glass :-(
Have ordered new ones so I must wait a couple a days until I can test it, read somewhere that one shouldn't operate the 5Y3GT without any tubes in the sockets.


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PostPosted: 17 Nov 2016, 14:15 
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Pex wrote:
Ok... Thanks... I dropped one of the 6V6Gt to the floor and broke the glass :-(
Have ordered new ones so I must wait a couple a days until I can test it, read somewhere that one shouldn't operate the 5Y3GT without any tubes in the sockets.



No, one should not. B+ can spike, taking out the filter caps, or worse.

There will be a chorus of "sure you cans" from the great unwashed - but then, they have never seen the magic smoke escape thereby.


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PostPosted: 22 Nov 2016, 06:28 
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Nothing to gain by turning on an amp with no rec. tube. I usually allow about a 50V drop across a rec. tube. Its a rough figure but a start. Having a C-L-C-R-C filter network (as I usually do) will generally allow some tweaking of the HT. Varying the last filterlimiting resistor (under full load - all tubes in) will allow fine tuning of the HT.

I also like to use wire-wounds here as they are ceramic 5W or 10W or more (parallel them) and can take a lot of heat.

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