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Little Dot Tube Amps: Vacuum Tube Rolling Guide - Page 295

post #4411 of 7513

Continuing my exploration of my newly acquired stash of 6DJ8 derivatives. Have you ever heard of Coronado tubes? Neither have I, until now. Here are two action pictures of my 6BC8 tube:

 

 

Obviously, Coronado is a re-brander, but who made this tube? No information on the tube except the name, designation, and a yellow rectangle. Let's look at the box: In fine print it says Gamble-Skogmo Inc, Minneapolis, MN. (Artsi and Oskari know that Skogmo means wood marsh - there were many Swedes that settled in Minnesota.) The bottom inside flap says BC-646 with the number 37 underneath.  Perhaps this tube could have been made in June 1964. (1946?)

 

Next stop Google. Gamble-Skogmo was founded in the 20's selling cars and started to produce radios in 1935. This company encompassed many different businesses, stores and department chains and grew to be the 15th largest US retailer in the 70's with 4300 stores under many names and 26,000 employees. When one of the founders retired in the late 70's the company was sold. The new owners drove the company into bankruptcy and it went under with many divisions sold off.

 

For you designers out there, here is a picture of a beautiful 50s-60s Coronado tube radio:

 

http://img1.etsystatic.com/010/0/7321842/il_570xN.439082179_eaxm.jpg

 

If you live near Colorado Springs, this tube radio console is available for local pick up for $195.00:

 

http://www.ebay.com/itm/Vintage-Antique-Vacuum-Tube-CORONADO-Console-Radio-Professionally-Tested-WORKS-/301041001384?pt=Vintage_Electronics_R2&hash=item46177123a8

 

Back from our history detour to the the tube. It has the 6DJ8 signature sound of very wide sound stage, punchy bass (in this case extra punchy) and great treble with tons of detail. A light, quick and lively presentation. The treble is slightly bright, but this tube has not been burnt in yet.

The manufacturer? No clue, perhaps Raytheon. My only hunch is the yellow rectangle which reminds me of the yellow silk screening on some US Raytheon tubes.

Another winner.

post #4412 of 7513
Quote:
Originally Posted by Audiofanboy View Post
 

Very curious to know whether or not there's an audible difference with the external power supply and regulator!

 

From the beginning, my flying wire adapter has been plagued with random RF noise which I assumed was due to all the exposed wires. In fact, the noise would often increase or decrease as if I moved the amp around. And therefore, I had thought that the solution was shorter wire runs and perhaps some shielding. However, now that I have an external DC power supply for the heater filaments, the amp is dead quiet. To those who might be experiencing similar noises, I strongly recommend an external DC power supply to supply power to the heaters.  It is cheap and it works.

 

If you don't have a suitable wall wort, something like this is what you need:

(24V will allow the use of 6.3, 12.6 and 20 volt tubes)

 

http://www.ebay.com/itm/350833307159?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

 

And you will also need a voltage regulator:

(I chose this one because I wanted an LED readout and the vendor is located in the the US, ensuring fast delivery) 

 

http://www.ebay.com/itm/131062144956?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

 

I have the amp and wall wort plugged into a surge protector and use it to turn both the amp and wall wort on and off. Doing this means I don't have to worry about inadvertently leaving the heaters on when the amp is turned off. And further, I ran a small piece of wire from the negative out terminal on the voltage regulator to the closest LD chassis screw to provide a suitable ground path.

 

I wish to thank Ctritical Bill and TrollDragon for giving me a path to follow. I am learning a lot from you guys! :)

post #4413 of 7513

Hi Gibosi,

 

How do you regulate the voltage? By turning the little brass screw?

 

Does it matter how you connect the heater wires 3 and 4 on the Vector?

 

Would you mind taking a picture showing the heater wires and the grounding arrangement?

post #4414 of 7513
Quote:
Originally Posted by gibosi View Post

I wish to thank Ctritical Bill and TrollDragon for giving me a path to follow. I am learning a lot from you guys! :)

 

My pleasure, anytime.

:beerchug:

post #4415 of 7513
Quote:
Originally Posted by mordy View Post
 

Hi Gibosi,

 

How do you regulate the voltage? By turning the little brass screw?

 

Does it matter how you connect the heater wires 3 and 4 on the Vector?

 

Would you mind taking a picture showing the heater wires and the grounding arrangement?

 

Yes, you turn the little brass screw to adjust the voltage. And no, you do not connect this to the Vectors. This would likely be very bad. Instead, disconnect the wires currently connecting Vector pins 3 and 4 to the 9-pin socket pins 4 and 5. I put some electrical tape around the ends in case I want to use the LD's heaters again.

 

 

Run wires from the voltage regulator OUT to pins 4 and 5 on your 9-pin socket, and connect negative OUT to chassis screw.

 

post #4416 of 7513

Many thanks for the explanations and pictures - I think that I understand how to do it now.

 

May I suggest two small velcro self stick fasteners for the voltage regulator module instead of the rubber band - will make it stay in place and it can be removed easily.

 

Hope that the grounding will cure the hum I hear when I turn up the volume loud (higher than normal listening volume).

 

All the best,

post #4417 of 7513
Good point mordy, I would put something behind that regulator as the transformer case is just paint. If there are any component leads poking through the board they could scratch the paint and short out on the metal beneath.
post #4418 of 7513

Point taken. I have put a piece of felt fabric between the transformer and the voltage regulator for the time being.

post #4419 of 7513

I have made up some 9 pin adapters so that I can run the power tubes out of their sockets and can take some measurements.

So in the LD Mk111 with stock 6N6P power tubes and some Mullard EF95 driver tubes this is what I recorded.

 

LEFT

  Heater    6.43v

  Plate      105.8v   208.0v

  Cathode  2.16v    108.0v

 

RIGHT

  Heater    6.44v

  Plate      105.6v   207.9v

  Cathode  2.26v    108.0v

 

The heaters are a little high but within 5% of 6.3v so not bad. The 6N6P draws 750ma and the transformer is rated at 6.8v 2.5 amp so about what you might expect.

 

The second plate reading is the B+ value minus the plate resistor multiplied by the current which is an almost insignificant amount so we can safely say the B+ is 210v.

The cathode readings ( grid bias ) show a little inbalance between my tubes but otherwise good standard values.

 

Theses readings give a calculated value of 10ma through the power tubes at idle. This is where I have a problem. Looking at the datasheet for the 6N6P at 105v plate and 2.2v grid the tube should be giving about 20ma. That doesn't fit with my readings so either I have made a mistake or the tubes are worn and not producing much current.

Interestingly, if you look at the datasheet of the Electro Harmonix 6H30Pi at the same plate and grid values that tube produces about 60ma, quite a difference.

 

I don't know if any of this will be of interest to anyone but now I have an idea of what is going on with the power tubes I am going to try some different tubes that might be suitable.

 

{EDIT}

 

I tried some 6N5P as power tubes as I have some handy and they looked suitable from the datasheet. They work very well. Sound a liitle more detailed than the 6N6P and possibly a better soundstage too. I will keep them in as I like how they sound.

These have about 15ma through them and  I would have expected the B+ voltage to drop a little compared to the 6N6P if they were producing more current but the opposite is true. I have been basing my calculations on a cathode resistor value of 220 ohm from Trolldragon's schematic but the values I am getting suggest it may be more like 100 ohm. I have the LD mk111 with a ver. 4 board so maybe the values have changed. Anyway, with a cathode resistor value of 100 ohm the 6N6P is putting out just over 20ma which agrees with the datasheet so I am going with that value.


Edited by Ctritical Bill - 12/18/13 at 7:06pm
post #4420 of 7513

 

Ok, after much wait... I'm in... this rig is temporary... the aluminum enclosure for the tube adapter and more permanent connections to the little dot socket savers will be done soon... Starting with the Teslas!

post #4421 of 7513
Subbed.
Thanks Gibosi, looks like a lot of fun.
post #4422 of 7513

Well all the ceramic machined sockets have arrived from China...

 

Built a 6DJ8 to octal adapter so I could listen to the 6BQ7A's that MIKELAP sent me.

 

Nice tube I like it, thanks MIKELAP!

:beerchug:до свидания :beerchug:

post #4423 of 7513

RE: Little Dot 1+ op amps:

 

For some time, I have been running the MUSES 02 op amp in my LD 1+, but have been wondering about the MUSES 01, which some believe is even better, so decided to give it a try. 

 

The 01's stage seems a bit wider, but this is not to say that the stage on the 02 is narrow. The 01 has a bit more grain. It is not as fluid and liquid as the 02. The 01 has less bass presence than the 02, With less bass, the mids and treble are relatively brighter. And the 02, with more bass, is relatively blacker and darker.
 

With the Heerlen E188CC, which has that glorious wet mid range and great liquidity and flow characteristic of the Heerlen 6DJ8 family, putting the 01 in, there was less flow and the mid range was a bit drier. Putting the 02 back in, the flow came back and mid range was glorious again.

 

The MUSES 02 is back in to stay. :)

post #4424 of 7513

Hi Trolldragon,

 

 

До свидания!

 

 

If you like the 6BQ7, then I would recommend the 6BC8 aka 6BZ8 aka X155, which I feel is better sounding. The question is, are these tubes the equal of the 6N23P or dare I say the 6922/7308?

 

Let the comparisons begin! There is no doubt that the value is superior compared to the expensive Amperex tubes. I am eagerly waiting to hear from others who have tried these tubes, especially comparing them to the gold pin standard of the A tubes....

 

Meanwhile, I still have several more 6BC8 and 6BZ7 tubes to burn in, so I have to wait to do my shootout.

post #4425 of 7513
Quote:
Originally Posted by mordy View Post
 

Hi Trolldragon,

 

 

До свидания!

 

 

If you like the 6BQ7, then I would recommend the 6BC8 aka 6BZ8 aka X155, which I feel is better sounding. The question is, are these tubes the equal of the 6N23P or dare I say the 6922/7308?

 

Let the comparisons begin! There is no doubt that the value is superior compared to the expensive Amperex tubes. I am eagerly waiting to hear from others who have tried these tubes, especially comparing them to the gold pin standard of the A tubes....

 

Meanwhile, I still have several more 6BC8 and 6BZ7 tubes to burn in, so I have to wait to do my shootout.

Thanks mordy!

 

I will look into those, they are a very reasonable price.

I do have some tubes on the way though that should keep me busy for quite a while. :L3000:

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