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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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PostPosted: 08 Nov 2017, 05:56 
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Joined: 01 Apr 2017, 19:56
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Bruce, thanks for the answer.
What I meant about using the ECC802 was not as a substitute for the Oddblock SRPP stage. In fact, I have implemented it successfully with a 5751, 1200R cathode resistors and 300V supply.
What I am trying to do is to build a Forewatt (which uses the ECC802) into the same chassis as the Oddblock and therefore using the same 300V supply.
Since the original Forewatt considers a 215 V supply and 470R cathode resistors, looking on the ECC802 curves I have concluded that, to keep the same 4,5 mA bias current with the 300V supply, the resistors would be something close to 1000R. Is this correct? Thanks again.


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PostPosted: 08 Nov 2017, 08:58 
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Location: Arizona, USA
Hi, Based on testing in my shop, I would use either 820 or 910. 1000 would certainly work, but not my first choice. With 820 R I get a value of 5.6ma, right in the zone.

Good listening
Bruce

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PostPosted: 08 Nov 2017, 09:14 
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Excellent, Bruce! Thanks again.

About the KT88 bias, have you tested the oddblocks with a cathode resistor in place of the CCS? Say a 200 ohm 10W resistor?

Or have you even tried fixed bias? -40v on the 220k resistors (instead of ground) and grounding the center tap of the 25R pot?

I wonder how the amp would sound like that. I'm considering testing, do you think it would work?


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PostPosted: 08 Nov 2017, 10:15 
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victorzwk wrote:
About the KT88 bias, have you tested the oddblocks with a cathode resistor in place of the CCS? Say a 200 ohm 10W resistor?
It won't work in that way.
victorzwk wrote:
Or have you even tried fixed bias? -40v on the 220k resistors (instead of ground) and grounding the center tap of the 25R pot?
It won't work in that way either.
CCS in the "tail" works here as phase splitter and bias circuit. While you can use a resistor or fixed bias for biasing you cannot use a push-pull stage without phase splitter.
On the other hand - there is a way to shift SIPP into the first stage (doubles the driver tubes) and use any biasing layout you want in the power stage. There are a lot of amplifiers built that way.


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PostPosted: 08 Nov 2017, 11:42 
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Hi, Actually the design will sort of work with a fixed resistor. Not real well, but it will function. The original application for this type of stage was about 1948. It was designed as a cheap power amp that avoided the inverter stage. Not nearly hi-fi by modern standards but it was louder than a SE stage. Fixed bias per se will not work though. You can use a small (5-10VDC) amount of positive DC on each grid and if you choose make it adjustable (see attached schematic of a prototype amp I am working on now) to eliminate the 25R cathode pot. The cathode method is cheap, effective and trouble free. The adding voltage to the grids can be a good thing in small amps like EL84s. You can use it to raise the cathode voltage a few volts and eliminate issues with the LM317 going too low and getting non-linear. The problem is that if you increase the voltage on the grids it also does the same to the cathodes. This in turn puts more stress on the LM317. In small amps this it is OK as the cathode voltage without the increase is often in the 10 volt range. In the large amps the nominal voltage is often close to 40 volts. If you bump it up too much you will exceed the rating for the LM317 and cause failure. True you could use a different CCS...buy why make life complicated? Additionally every volt of increase on the cathode is one less for output power....perhaps not a big deal, but a negative aspect.

Good listening
Bruce
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PostPosted: 08 Nov 2017, 14:12 
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Joined: 29 Jun 2016, 15:59
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Guys, started ordering parts. But, need some confirmation before starting building the amps. Thanks for your support.

1) The wires for the DC supplied heaters do not have to be twisted as in AC supplied heaters, correct?
2) Using the recommended XPWR264 power tranny, what is the voltage (approx.) at the output of the B+ Full Bridge rectification?
3) The Power Supply LED I plan to use has a built-in current limiting resistor. Thus, the 680R resistor is not needed, correct?
4) Having a hard time finding shielded cable for the input signal. Any recommendations?
5) To control the input volume, I would like to replace the 100K input resistor with a 100K pot. I assume that this is OK.

Thanks,

Danny


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PostPosted: 08 Nov 2017, 14:58 
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Hi Danny,

1. ..the wires do not need to be twisted.
2. ..under load it will vary depending on your AC mains voltage. Figure it will be between 425 (worst case for low voltage) to 460 for slighlty higher than normal AC mains. Unloaded it will go over 500, often to as much as 550. Capacitors with 500 volt working ratings are fine, just don't run the PS without a load. OK to test it, but keep it reasonably short time. Most filter caps will tolerate up to 25% or even more overvoltage for a while.
3... .no need for the resistor
4.... I like Mogami 2333. Very easy to use and inexpensive. Several sources. Other cables will work as well.
5. ...That will work fine, I have done it in the past when I wanted to run the amps without a preamp.

Good listening
Bruce

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PostPosted: 08 Nov 2017, 16:43 
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Bruce,

Thanks.


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PostPosted: 08 Nov 2017, 19:31 
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Bruce, thanks for the answers!

Have you ever considered substituting the LM317 for a MOSFET CCS?

I have measured my build and found that low frequencies (under 30 Hz) makes the LM317 distort the signal. Also, when overdriven, the distortion is quite nasty (probably because the LM317 shuts momentairly down).


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PostPosted: 09 Nov 2017, 02:58 
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Joined: 24 Oct 2010, 07:05
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victorzwk wrote:
Have you ever considered substituting the LM317 for a MOSFET CCS?
Several of my friends did it following my slightly more complex than MOSFET layout (opamp+{cascode of BJT+MOSFET}). You can also use a DN2540 or other depleted mode MOSFETs in pairs. The main problem here is the need for some biasing to allow several volts across CCS, but it's doable.
There is also a simple mod that almost eliminating the need of the 25Ohm pot (use its own CCS for each tube and connect the power tube cathodes together by a small resistor of 10-15Ohm value). The mod also helps with heat problems and allows the usage of lower current devices.
victorzwk wrote:
I have measured my build and found that low frequencies (under 30 Hz) makes the LM317 distort the signal. Also, when overdriven, the distortion is quite nasty (probably because the LM317 shuts momentairly down).
When overdriven - yes, agree, but any CCS has its own compliance voltage at which it will distort severely.
Regarding low frequencies distortion: in my case the problem has not occured. Most probably it is the output transformer problem, not LM317.


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