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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 Posted: 22 Jan 2019, 04:34

Joined: 17 Jan 2018, 16:13
Posts: 32
ILoveHiFi wrote:
So the lm317 is set to a constant current source with about 1.25v accross 15r resulting in v=ir 83.33mA of total current draw.
You want both valves to draw same current so the voltage accross 1R resistor (TP1 TP2 measuring point) should be 41-42mV (1R*0.5*83.33mA) = (1R*41.665mA).

Ultamatley voltage accross TP1 TP2 should be same as TP2 TP3 as same current.
The amp should warm up say about 30 minutes after cold start its fully warm and you adjust so tubes are same current. This way afterlong operating hours or burn in both tubes have similar current.

Hi,
Let me ensure I understood you properly.
So should I measure voltage across TP1-TP2, then across TP3-TP4 and both should be 41mV (adjusted by rheostat)?
Second question, sorry maybe stupid, but I'm not professional electrician... To which TP should be connected multimeter "+" and "-"? I'm asking, because depending of this I read positive or negative voltage. I assume bias 41mV should be positive?

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 Posted: 22 Jan 2019, 05:59

Joined: 23 Feb 2017, 02:02
Posts: 553
+ and - dosen't matter as your only sensing the current in both output tubes.
Unless you have to determine direction of current in this case direction dosen't matter.

You want to adjust potentiometer(rheostat) in either direction untill voltage accross tp1 tp2 equals to voltage accross tp3 tp4

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 Posted: 22 Jan 2019, 06:28

Joined: 17 Jan 2018, 16:13
Posts: 32
ILoveHiFi wrote:
+ and - dosen't matter as your only sensing the current in both output tubes.
Unless you have to determine direction of current in this case direction dosen't matter.

You want to adjust potentiometer(rheostat) in either direction untill voltage accross tp1 tp2 equals to voltage accross tp3 tp4

Does it mean I need two multimeters simultanously, one across TP1 TP2 and second for TP3 TP4, right?

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 Posted: 22 Jan 2019, 09:02
 Project Author

Joined: 04 Jun 2008, 20:59
Posts: 4032
Location: Arizona, USA
Hi. you don't need two meters, but they are helpful if both are equally accurate. Just add both initial values together to get the actual current (the calculated values are usually not exact). The divide by two and set one side to that value and check the other afterwards. Make slight adjustments as needed. +/- about 2-3% difference is what you seek. Up to about 5-8% still good, above that an adjustment is indicated. After the initial power on recheck the settings after an hour or so. New tubes and components will drift a bit at the start. I like to recheck new builds about a week to a month later. Then only about every six months or so. I find if they are close at the end of week's use they are likely to be fine for a long time. I test my amps with a diy dual meter. I got two of the \$10 digital 0-1 volt meters that run on 9 volt batteries. You need two separate batteries as there is no common point between the two circuits under test. Just a DPDT switch to turn the meters on and off and some jacks for probes. The batteries last forever, mine have lots of use (I am always building and testing amps) ....roughly 7 years now.

Good listening
Bruce

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 Posted: 22 Jan 2019, 09:33

Joined: 17 Jan 2018, 16:13
Posts: 32
gofar99 wrote:
Hi. you don't need two meters, but they are helpful if both are equally accurate. Just add both initial values together to get the actual current (the calculated values are usually not exact). The divide by two and set one side to that value and check the other afterwards. Make slight adjustments as needed. +/- about 2-3% difference is what you seek. Up to about 5-8% still good, above that an adjustment is indicated. After the initial power on recheck the settings after an hour or so. New tubes and components will drift a bit at the start. I like to recheck new builds about a week to a month later. Then only about every six months or so. I find if they are close at the end of week's use they are likely to be fine for a long time. I test my amps with a diy dual meter. I got two of the \$10 digital 0-1 volt meters that run on 9 volt batteries. You need two separate batteries as there is no common point between the two circuits under test. Just a DPDT switch to turn the meters on and off and some jacks for probes. The batteries last forever, mine have lots of use (I am always building and testing amps) ....roughly 7 years now.

Good listening
Bruce

Hi Bruce,
Thank you for detailed answer and dividing tip, so obvious I should have hit upon this:)
Actually I even have two meters, but different brands, so gonna try both methods.
What doesn't help, I have 50R rheostat as I couldn't find here in Poland 25R with sufficient power rating. And this mine is really sensitive for even gentle touch, but I hope I will succeed with the adjustment.

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 Posted: 22 Jan 2019, 18:46

Joined: 23 Feb 2017, 02:02
Posts: 553
You can try a 47R or 56R resistor in parallel with the 50r rheostat to drop it to 25r and help you get precise adjustments

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 Posted: 03 Feb 2019, 10:50

Joined: 03 Feb 2019, 10:21
Posts: 3
Location: Italy
gofar99 wrote:
Hi. you don't need two meters, but they are helpful if both are equally accurate. Just add both initial values together to get the actual current (the calculated values are usually not exact). The divide by two and set one side to that value and check the other afterwards. Make slight adjustments as needed. +/- about 2-3% difference is what you seek. Up to about 5-8% still good, above that an adjustment is indicated. After the initial power on recheck the settings after an hour or so. New tubes and components will drift a bit at the start. I like to recheck new builds about a week to a month later. Then only about every six months or so. I find if they are close at the end of week's use they are likely to be fine for a long time. I test my amps with a diy dual meter. I got two of the \$10 digital 0-1 volt meters that run on 9 volt batteries. You need two separate batteries as there is no common point between the two circuits under test. Just a DPDT switch to turn the meters on and off and some jacks for probes. The batteries last forever, mine have lots of use (I am always building and testing amps) ....roughly 7 years now.

Good listening
Bruce

Hello I' am new in the forum, and I am very pleased to be here.
I am trying to build the Poddwatt series 2 and I have some question to ask for.

Q1 The 10uF cap is both to the two channels and is only one? And there are preferences about it? I have chosen a Mundorf oil cap

Q2 Does it matter if I use a output transformer fixed to 4 ohm?

Q3 The heater circuit, pins 9 of the 5751 valves are floating? (it's the center of the heater)

Q4 The DC reference for the heater circuit can be taken from one of the two 200V regulator? Or I have to make a third regulator?

Q5 The SSL, what is purpose? I don't understand the purpose of 3 led in parallel, and is not recommended to add a backverse diode in parallel to each led? to reduce reverse current

Q6 For the power supply I am ordering a PCB, is recommended to do the same with the valve circuit?

Q7 The ground buses, I ground all the components of the power supply and the I connect it to the anti ground loop, I ground all the signal components, and then I tie them both together at the input connection?

Q8 As potentiometer a 100+100K by alps is good? (I don't know this brand)

Q9 The only connection that have to be made with shielded wire are, from jack to potentiometer (I don't need more inputs), and then to the potentiometer to the board?

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 Posted: 10 Feb 2019, 16:42

Joined: 17 Jan 2018, 16:13
Posts: 32
Unfortunately I'm back here... My amp was playing very well few weeks, but I decided to make some rebuilt.
Initially amplifing part was on PCBs on the chassis base and it needed long wires to connect to the tubes. So I decided to make it closer to the tubes, to reduce signal path length. Unluckly after switch on only right channel plays...
I checked all the wiring and it's OK. Measured voltages on LM317 and there is 14.4V on both channels. Something what worries me in the dead channel is over 40V across 1R resistor of the second EL84 tube in the pair.
Does it indicate tube failture?

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 Posted: 10 Feb 2019, 17:11
 Project Author

Joined: 04 Jun 2008, 20:59
Posts: 4032
Location: Arizona, USA
Hi, 40 volts ( ) or 40 millivolts? The one is fine the other is quite horrible. If it is 40mv, then that circuit is working OK. I suspect that is the case as the cathode to ground voltage is correct. So....you have a problem somewhere else. The tubes are probably just fine or the values would be off. They are durable and don't suddenly fail like this. You need to go through and check for some connection that is missing or has a bad solder joint. make sure you didn't accidently ground the output terminals or for that matter the input ones. Check to be sure the drive tube is functioning properly. Measure the voltage at the coupling capacitor on the driver tube side. It ought to be about 1/2 the B+ give or take up to 25%. If not then you have located the problem stage. If it is OK then you have some sort of signal issue. Either none going into the stage, none getting to the output stage or none gettin past the transformer.

Good listening
Bruce

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 Posted: 11 Feb 2019, 01:14

Joined: 17 Jan 2018, 16:13
Posts: 32
It is over 40V (not mV), indeed
I will check all the connections again, but a the moment have no idea what goes wrong.

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