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Old 2nd March 2010, 04:01 PM   #1
ryc989 is offline ryc989  United States
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Default Simple SE power transfomer question?

Hi,

I am just putting together everything for my Simple SE project and had a general question about the tube types that can be used. I see that most tubes (EL34, 6L6GC, KT88, 6550, ect..) are all in the same general B+ voltage ratings, but if I wanted to use a 6V6 output tube, could I do that using the Allied 6K7VG?? If so, would it be as simple as using a different rectifier tube?? I don't want to overly complicate things as this is my first build, but I am also trying to understand options. I know that I would need a different value for the cathode resistor, and the value would be based on desired output power and B+ voltage. Any help and would be appreciated, also what would be a good B+ voltage in this amp for a 6V6 tube??

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Old 2nd March 2010, 10:22 PM   #2
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...if I wanted to use a 6V6 output tube, could I do that using the Allied 6K7VG?
No. You'll end up with too high B+ voltage. Use the Allied 6K56VG instead.
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Old 2nd March 2010, 10:28 PM   #3
rknize is offline rknize  United States
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The 6V6 wants to see a higher primary impedance on the OPT as well, though it would probably do OK at 5k. Personally, I thought that trying to support the 6V6GT was not worth the trouble.
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Old 3rd March 2010, 12:05 AM   #4
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Any help and would be appreciated, also what would be a good B+ voltage in this amp for a 6V6 tube??
The 6V6 works best in triode with a B+ of 320 to 340 volts. This is a bit over the 315 volt spec, but you will lose a few volts in the cathode resistor and in the OPT. Many guitar amps ran the little 6V6 at plate voltages near 400 volts without exploding them, but this was in pentode mode. They will definitely not last long in triode or UL with the 6K7VG and even a dead rectifier tube.

I have a dedicated 6V6 Simple SE and it does use a 6K56VG, and Edcor XSE15-8-5K OPT's. The 6V6 does seem to work best with a 6K ohm load, but my speakers are well above 8 ohms for most of the audio range.
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Old 3rd March 2010, 04:46 PM   #5
ryc989 is offline ryc989  United States
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Hi,

I really appreciate everyone's input, I am trying to learn as I go along. I think for now I will stick with the higher B+ tube types and perhaps build a second Simple SE amp designed for the 6V6 at a later date. I have already purchased the bigger transformer and I am looking at output transformer options. I have all the other parts necessary to do the build.
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Old 7th March 2010, 06:16 PM   #6
ryc989 is offline ryc989  United States
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Hi,

I have a question about the values for the cathode resistors on the output tubes. In general it seems like the 560 ohm value is a 'middle of the road' value, but if I wanted to use a larger value to lower the Diss and Ik, how would I know if the value were too large?? And if I used a larger value resistor, would impact the tubes I could use?? I plan on using 6L6GC tubes to start with, but I would like to try different type output tubes to see which ones I prefer.
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Old 7th March 2010, 06:36 PM   #7
rknize is offline rknize  United States
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The chart on this page is meant to help you answer that very question:

Tubes & Transformers

It depends mainly on the following factors:

1) Tube choice
2) Actual B+ voltage
3) Desired idle current of the tube (Diss)
4) Primary impedance of the OPT (higher turns ratio has more windings and therefore higher DCR)

There is also some variance with the tubes themselves. I would buy a couple of different sets of resistors so that you can change it if need be. 560 is a good first choice. I'd pick up a set of 470 and 680 ohms as well. Some of the Chinese 6L6 tubes seem to run hot and need a larger resistor.

Avoid bending the leads on the back of the board when you solder them in. It make it easier to lift them out later.

Once you make you choice, you can look at other tube types in the chart to see how they will operate. The orange Diss numbers indicate that they will run hot, but within spec (some tubes can't handle these levels even though they are within published specs). The red numbers are over spec, but some tubes handle this fine without glowing. The other numbers run the tubes colder. You will notice that distortion goes down and dampening factor goes up as the tube is run hotter (last three columns). That is the trade off of tube life versus performance.

Last edited by rknize; 7th March 2010 at 06:39 PM.
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Old 7th March 2010, 06:42 PM   #8
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There is also some variance with the tubes themselves. I would buy a couple of different sets of resistors so that you can change it if need be. 560 is a good first choice. I'd pick up a set of 470 and 680 ohms as well. Some of the Chinese 6L6 tubes seem to run hot and need a larger resistor.
The coke bottles from AES sure do run hot! The EH 6l6gcs have no issues running with a 560ohm resistor in.
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Old 7th March 2010, 07:18 PM   #9
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In general it seems like the 560 ohm value is a 'middle of the road' value, but if I wanted to use a larger value to lower the Diss and Ik, how would I know if the value were too large?
I started with 560 ohm resistors, but decided my tubes were running too hot. I added another 250 ohms in series with the original 560 ohm resistors for a total of 810 ohms. This seems about right for most EL34/6L6GC type tubes.

The 560 ohm resistors were good for quality KT88 tubes. I can easily go back to 560 ohms just by putting a small shorting jumper around the extra 250 ohm resistor.

Click the image to open in full size.
http://i69.photobucket.com/albums/i4...E/P1100584.jpg
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Old 7th March 2010, 07:28 PM   #10
rknize is offline rknize  United States
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Or you can get all fancy-pants:

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