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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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It is currently 20 Apr 2019, 08:18

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PostPosted: 14 Mar 2019, 20:38 
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Joined: 11 Nov 2013, 22:35
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That is what I thought. I don't plan on changing anymore resistors.

Any further changes I am considering are small.

Suncalc,
Any chance you will revisit your build and optimize it? Or suggestions for things to try? Or concerns or disagreement with any changes I have tried so far?

Other feedback appreciated as well

Thanks


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PostPosted: 30 Mar 2019, 20:13 
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Joined: 11 Nov 2013, 22:35
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Help! Ground loop! I think.
I continue to enjoy listening to this amp and keep trying small stuff.
With the designed cascode two stage driver I can get a gain of ~94. As a result I am running the volume pot at 9 o'clock at most and that is pretty loud. I figured without the cathode bias capacitor on the first stage I would still get a gain of ~49 which would be more than enough with my CD player putting out 2Vrms. So I removed the 22uF bypass capacitors from the first stage. Two solder joints at the tubes and two snips at the ground. I did this so I would not mess with the existing ground. But now I have a very obvious 120hz hum. Before the change I had a slight 120hz hum. It was noticeable if you were about 1 foot from the (97db) speaker. Now the 120hz hum is filling the room. Annoying.

I have had the amp upside down twice to look, but I am not seeing anything else that I changed or moved. Would the bypass capacitors minimize/prevent a ground loop? Is there something else I should have accounted for? I can always put them back in to see if the hum goes away, but with the bypass caps removed I still have plenty of volume. Now I can raise the volume pot to about 11.

I thought this change was harmless, but obviously I am missing something.

Help appreciated.

thanks,


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PostPosted: 30 Mar 2019, 22:01 
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By removing the bypass cap you have reduced the Ripple Rejection of the stage. Obviously the ripple on the B+ is marginal. Compensate with additional filtering on the driver B+.

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PostPosted: 01 Apr 2019, 09:55 
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Joined: 11 Nov 2013, 22:35
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Suncalc,
That was it. I added 33uF on the 2nd cap, but no noticeable effect. Then I added small chokes (Hammond 156M, 3H) on the 2nd stage of the driver ps and that dropped the hum by 10dB. However it is still noticeable. I like having the lower gain, but I can't say that I like the sound better. I am planning on putting the bypass capacitors back in but leave the chokes.

Wondering about a single stage driver with a gain about 50. A 12AT7 perhaps.

Feedback appreciated

Thanks again


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PostPosted: 14 Apr 2019, 08:42 
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Beast update
As mentioned in the last post I put the bypass caps back in the first stage of the driver. Now it is dead quiet. No hum from the speakers (97dB). Only noticeable hum is from the power strip.

Next update
Although I like the sound of the higher amps in the power stage, I thought I might be saturating my OPT's during some loud passages. I changed the Rk's on the output tube from 920 ohm to 1000 ohm. With the 920 ohm Rk, I was getting 86Vdc at the cathode, 94mA, with a plate to cathode of 255Vdc, 341Vdc at the plate. With the 1000 ohm Rk it is 88Vdc at the cathode, 89mA, with a plate to cathode of 259Vdc and 348 Vdc at the plate. The change is subtle, but not worse so I left it to live with it for a little bit to see if I changed my mind.

Last update and question,
I have had only one 6336 tube for this build, so I bought two more to have 3 to play with. My first tube has been a Tung-Sol 6336A. My two new ones are Cetron 6336B's. The first impression with the 1st 6336B was WOW. It is more dynamic and more musical than my 6336A. Even my wife made a "that sounds good, much better than before" comment. I was curious what else changed, so I took some measurements. With the 6336B, I have 80Vdc at the cathode, 81mA, with a plate to cathode of 276Vdc, and 356Vdc at the plate. It is obviously different. The other 6336B does not sound as good, so I will put those in at some point and get measurements for another data point. I found a Glass Audio article for a 6336 push pull amp and his distortion measurements for the Tung-Sol 6336A were better than the Cetron 6336B. Not sure if I have found a better bias point or a better bias point for this particular tube. Regardless, it sounds good, so I will live with it a while and just listen to music.

Question,

The amp has 2.5K OPT's and if I had 250Vdc plate to cathode and 100mA's that works out to be right on the load line. If I understand correctly. My current setup is 276Vdc plate to cathode and 81mA's. That is 3407 ohms. In a world were parts are free would it be better to have a 3.5K OPT? Just trying to understand the relationship of things.

feedback appreciated.
thanks,


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PostPosted: 14 Apr 2019, 13:45 
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ToddD wrote:
The amp has 2.5K OPT's and if I had 250Vdc plate to cathode and 100mA's that works out to be right on the load line. If I understand correctly. My current setup is 276Vdc plate to cathode and 81mA's. That is 3407 ohms. In a world were parts are free would it be better to have a 3.5K OPT?
No.

I believe you are confusing the bias point with the load. The fact that the voltage and current of the DC bias point reflects any particular resistance may be interesting, but it has nothing to do with the AC load. The AC load line goes through the bias point in a single ended topology regardless of value. The DC load line of the power stage goes through the bias point even though it's on the order of 94Ω.

A 3.5kΩ transformer would result in lower power and lower distortion. I never looked specifically at the 3.5kΩ. All my design variations were run with the 1.25kΩ, 1.7kΩ, and 2.5kΩ outputs. I didn't really see the need in going higher after I went to the 2.5kΩ. Note that the plate resistance at my bias point is only 400Ω. This would lead many to try very low AC plate loads (i.e. 800Ω). However, what I found is that if you want to get the power output up, with reasonable level of distortion, in a 6336 you really need to get up around 2.5kΩ.

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PostPosted: 14 Apr 2019, 16:48 
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Joined: 23 Feb 2017, 02:02
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Quote:
In a world were parts are free would it be better to have a 3.5K OPT?

You need matching for voltage supply used, for the valves and scehamtic used.
Theres no one answer if 3.5K OPT is better as it depends on the three values.
If you match these three values then you can find the OPT impedance to allow for maximum power output.


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PostPosted: 15 Apr 2019, 12:28 
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Joined: 11 Nov 2013, 22:35
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Suncalc,
Yes I confused bias and load. I am not a morning person and the numbers kind of work.
Thanks for answering my ignorant question.


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