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Little Dot Tube Amps: Vacuum Tube Rolling Guide - Page 338

post #5056 of 8460
Quote:
Originally Posted by mordy View Post
 

Thanks for the answers to my questions - I'll play it safe and just use one tube at a time.


Here is a 5A module from China with LED voltmeter.

http://www.ebay.com/itm/NEW-DC-DC-Step-Down-Power-Module-4V-38V-to-1-25V-36V-5A-Voltmeter-Adjustable/331064390176


This should power anything you can roll with a decent source such as a laptop power supply which are usually 19V / 4.75A

post #5057 of 8460

HI TD,

 

Thanks, looks like a good unit. The problem I have with my unit is that it seems that the LED voltage readout is stepped, and I cannot get it to read exactly 6.3V as an example. Either it reads 6.36V or 6.43V.

 

However, within that 6.36V range I can still turn the set screw and lower/increase the voltage as measured with an external voltmeter. After some fiddling, I am able to set it to 6.3V (or 6.29-6.31V). Sometimes there is a little drift, and when rechecking, the voltage is a little higher or lower, but negligible.

 

My eyesight isn't what it used to be, and sometimes it is hard for me to turn the little set screw with a jewelers flat blade screwdriver.

Any suggestion how to attach a turning knob to the little brass set screw? (It takes quite a few turns to go from 6.3V to 12.6V.)

 

Based on the picture, it seems that you need to attach a heat sink for higher power ratings. How do you attach it?


Edited by mordy - 2/16/14 at 9:25am
post #5058 of 8460
Quote:
Originally Posted by mordy View Post
 

HI TD,

 

Thanks, looks like a good unit. The problem I have with my unit is that it seems that the LED voltage readout is stepped, and I cannot get it to read exactly 6.3V as an example. Either it reads 6.36V or 6.43V.

 

However, within that 6.36V range I can still turn the set screw and lower/increase the voltage as measured with an external voltmeter. After some fiddling, I am able to set it to 6.3V (or 6.29-6.31V). Sometimes there is a little drift, and when rechecking, the voltage is a little higher or lower, but negligible.

 

My eyesight isn't what it used to be, and sometimes it is hard for me to turn the little set screw with a jewelers flat blade screwdriver.

Any suggestion how to attach a turning knob to the little brass set screw? (It takes quite a few turns to go from 6.3V to 12.6V.)

 

Delivering exactly 6.3 volts is not required. Anything between 6.2 and 6.4 is more than good enough. And I am quite sure that any setting between +/- 5% is also fine.

 

And I too have been trying to figure out what I could attach to that tiny little brass screw to make it easily to turn with fingers only. I am hoping that someone out there with a clever solution will please share. :)

post #5059 of 8460
Yes as gibosi say anything within 5% would be fine, there is no accuracy on a $10 Chi-nee LED readout regulator. biggrin.gif

Measure with a meter and if it is close enough to 6.3 or 12 then all is good.

Options for the little pot...
What I would do if I had one was measure the resistance of the pot in both 6.3 & 12 V setting (measure with power off after the setting is set and it might have to come off the board) then install a switch with a resistor that would give the appropriate value to toggle between 12 & 6.3 volts. The pot could stay in series with the resistor and the switch would cut it in or bypass it. If you have worked on circuit boards beforre this would be an easy mod.

You could also find a normal sized pot with the same value as the board one, remove the board one and run wires to the larger pot plus a way to mout the larger pot or let it hang. smily_headphones1.gif

Epoxy a piece of plastic to the little screw top, like a piece of pen tube or QTip stick etc...
post #5060 of 8460

Thanks Gibosi and TrollDragon,

 

Well, well TD,  you forgot about my secret weapon, the 8.4V tubes. Anyhow, the toggle switch solution is too complicated for me. However, a little plastic extension from the innards of a ball point pen might do the trick....

post #5061 of 8460

If you google the code on your Pot you can get the resistance value of the pot Ex.mine is BOURNE 3296 W 203 so it is 10R-2M Pot.

post #5062 of 8460
Quote:
Originally Posted by i luvmusic 2 View Post

If you google the code on your Pot you can get the resistance value of the pot Ex.mine is BOURNE 3296 W 203 so it is 10R-2M Pot.
The 203 shows that it is a 20K Ohm pot, since it is a 3 leg pot you would have to see if all 3 legs are hooked up in the circuit. Pins 1&3 give the whole 20K of resistance and pin 2 is the adjustable value. If you turn it so the resistance between pins 1&2 is 5K ohm the remaining resistance between pins 2&3 would be 15K. If they are using the pot as a voltage divider then there would be some calculations involved to bypass it with a switch. Probably easier to obtain a normal sized 20K pot and wire it in place of the little one.
post #5063 of 8460
Quote:
Originally Posted by TrollDragon View Post


The 203 shows that it is a 20K Ohm pot, since it is a 3 leg pot you would have to see if all 3 legs are hooked up in the circuit. Pins 1&3 give the whole 20K of resistance and pin 2 is the adjustable value. If you turn it so the resistance between pins 1&2 is 5K ohm the remaining resistance between pins 2&3 would be 15K. If they are using the pot as a voltage divider then there would be some calculations involved to bypass it with a switch. Probably easier to obtain a normal sized 20K pot and wire it in place of the little one.

So in order for me to take a more accurate measurement on the pot i need to removed it from the PCB right?I turned the adjusting screw all the way up and measured it with my Multimeter and the reading is 11+K(Pin 1 to 3).Good thing Mordy mentioned about running 2 adapter from the same PS/Converter because i've been running both  LD 1 and LD MK III 6SL7/6SN7 adapter from the same PS/Regulator at the same time to burn-in the 2 tubes(6SN7) that i received and it works but when i switch one of the tube to 6SL7 it HUM(both amp),Do you think the HUM was cause by the PS/Regulator?Running 2 6SL7 at the same time is no HUM.Thank you for posting the ebay link i get to buy some parts that i needed to build a inclosed PS/regulator with LED(Votls and Amp. reading) Display i need to inclosed all the exposed electrical contacts(spiacially the VECTOR ADAPTER)i just got Shocked not long ago by those Vector adapter they can be hazardous.Need to take a measurement on top of LD 1 and LD MK III so i can buy or make a box/case to put the tubes on top so all the tube adapters are all inclosed.

post #5064 of 8460
Heater voltages should be biased over cathode voltage. So between heater + and - is 6.3v, but minus side from amplifier zero should be more than cathode voltage. Otherwise with some tubes cathode starts to draw electrons from filament.

Do not know is it even possible to raise voltages above cathode with those heater power supplys...

Example with my diy amps heater voltages are biased about 30v above amps zero.
post #5065 of 8460
Quote:
Originally Posted by mordy View Post
 

Based on the picture, it seems that you need to attach a heat sink for higher power ratings. How do you attach it?

The heat sink has a thermally conductive sticky pad on the back.

post #5066 of 8460
Quote:
Originally Posted by i luvmusic 2 View Post
 

So in order for me to take a more accurate measurement on the pot i need to removed it from the PCB right?I turned the adjusting screw all the way up and measured it with my Multimeter and the reading is 11+K(Pin 1 to 3).Good thing Mordy mentioned about running 2 adapter from the same PS/Converter because i've been running both  LD 1 and LD MK III 6SL7/6SN7 adapter from the same PS/Regulator at the same time to burn-in the 2 tubes(6SN7) that i received and it works but when i switch one of the tube to 6SL7 it HUM(both amp),Do you think the HUM was cause by the PS/Regulator?Running 2 6SL7 at the same time is no HUM.Thank you for posting the ebay link i get to buy some parts that i needed to build a inclosed PS/regulator with LED(Votls and Amp. reading) Display i need to inclosed all the exposed electrical contacts(spiacially the VECTOR ADAPTER)i just got Shocked not long ago by those Vector adapter they can be hazardous.Need to take a measurement on top of LD 1 and LD MK III so i can buy or make a box/case to put the tubes on top so all the tube adapters are all inclosed.

After looking at the datasheet for the TI chip http://www.ti.com/lit/ds/symlink/lm2596.pdf bottom of page 12 provides resistor and inductor calculations for variable voltage, this pot or resistor has to have short leads and be close to the chip. This will eliminate the use of an external regular size pot unless it can be connected with very short leads.

 

The number from your pot shows it to be 20K, if you only measure 11K then there are other board components affecting the measurement.

 

Sounds like if the tube hums in both amps then it is probably the tube itself.

post #5067 of 8460
Quote:
Originally Posted by Artsi View Post

Heater voltages should be biased over cathode voltage. So between heater + and - is 6.3v, but minus side from amplifier zero should be more than cathode voltage. Otherwise with some tubes cathode starts to draw electrons from filament.

Do not know is it even possible to raise voltages above cathode with those heater power supplys...

Example with my diy amps heater voltages are biased about 30v above amps zero.

The built in driver heater supply (AC) is already DC Biased.

Connection at the top.

 

With a pure DC heater supply I don't think the same thing applies but I am not 100% positive?


Edited by TrollDragon - 2/17/14 at 7:36am
post #5068 of 8460

Would this work as a driver for the MK IV with octal adapter and power supply?

 

http://www.ebay.com/itm/Vintage-TUNG-SOL-JAN-CTL-12SA7GTY-Tubes-/261179404620

post #5069 of 8460
Quote:
Originally Posted by mab1376 View Post
 

Would this work as a driver for the MK IV with octal adapter and power supply?

 

http://www.ebay.com/itm/Vintage-TUNG-SOL-JAN-CTL-12SA7GTY-Tubes-/261179404620

 

This is a heptode with 5 grids.

 

http://www.radiomuseum.org/tubes/tube_12sa7gt.html

 

http://www.r-type.org/pdfs/12sa7.pdf

 

If you wanted to use this in your LD, it would be best to have an octal breadboard socket to facilitate strapping the extra grids to the plate and/or cathode, that is, to convert it to a triode. So it could be done. However, in my opinion, it probably wouldn't be worth the time....

post #5070 of 8460

Hi,

 

Bought a grab bag of ten 6SN7 tubes. One turned out to be a re-branded Japanese made tube. I have trouble finding information about Japanese octal tubes. Here are a couple of pictures. Can anybody tell me who made it and when?

 

 

 

 

There is no information on the base. Brand Channel Master 6SN7GTB. Only inscription is J90 and Japan.

 

Another question about the breadboard adapter: When I set the voltage regulator to 6.3V the voltage taken at the octal breadboard screw terminals for the heater wires read 6.10V. (The terminals at the voltage regulator read 6.3V). In order to get 6.3V at the socket on the breadboard I have to increase the voltage regulator to 6.49V.

 

My other breadboard does not have this problem, and I get the same reading from the breadboard as from the voltage regulator.

 

The octal breadboard needed little wires soldered in since it had provisions for inserting resistors. Could the little wires cause this?

 

PS: Listening to the Channel Master octal tube. First impression very open and forward with wide sound stage with great width and height; punchy bass. Delicious....


Edited by mordy - 2/17/14 at 4:30pm
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