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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: 07 Nov 2019, 13:50 
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Joined: 21 Sep 2014, 12:07
Posts: 33
Hello Bruce,
I'm getting ready to build and expand on your "Funkywatt" design. I see on the schematic that the current needs to be set at 41.5mA per tube. You don't show them in the schematic, but can I put 1-ohm, 1% resistors between the cathodes and the LM317 for measurement like in the Oddwatt amps?
My plan is to use 6Q7GTs in place of the 12AX7s, and 2-6F5GTs for a bass and treble control. I think I'm going to call it the "5150", which is the police code for an involuntary 72-hour hold in a psychiatric facility :).


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PostPosted: 07 Nov 2019, 14:43 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, if you use the 832s they have a tied together internal cathode arrangement and the 1 ohm resistors have to be in the anode circuit so you can tell which section is getting what current. For any tubes with separate cathodes or any separate tubes the 1 ohms could and should go in the cathode circuit, it is much safer as the voltages there are typically in the 20-40 volt range. In the anode circuit you have the full B+ and measuring it requires some care. The sound of the amps is ok but the output power is relatively small compared to typical power tubes (KT77-88 etc) . Its claim to fame IMO is its cool looks. 8-)

Good listening
Bruce

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PostPosted: 12 Dec 2019, 12:36 
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Joined: 21 Sep 2014, 12:07
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Bruce, Thanks for the answer, it's as I suspected. I'm finally getting back to this :). I bought a "break before make" 5-position rotary switch and a 0-100mv panel meter. The switch will only see millivolts accross it, and there will be no cross-shorting between the anode circuits. Am I on the right track? I had thought about putting capacitors accross the switch poles as well to prevent arcing, but I doube that will be an issue.


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PostPosted: 04 Jan 2020, 14:54 
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Joined: 21 Sep 2014, 12:07
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Hello Bruce, and Happy New Year!
I've got my amp built. As usual though, I've run in to an issue I can't seem to figure out. My power supply is identical to yours, although I did use a 5 watt, 1K resistor in place of the 1 watt. I'm having a severe voltage drop which, after 2 days of trial and error I have found is happening when the screen grid of the 832A is connected to the B+ via the 8.2K resistor. Having only the 832A tubes plugged in (actually with only one) my B+ drops from 265V open circuit to 185 volts! The current through the 8.2K resistor is 4.5-5 mA (the datasheet calls for 2mA). The current seems to be flowing back through the cathode/LM317, when I disconnect the cathode from the circuit the current ceases. I have checked all of the "dumb" stuff, the tube is wired correctly. I tried a new LM317 with the identical result, and 3 different 832A NOS tubes, same thing. The moment I make the connection to the screen grid, poof!, 80 volts vanish. The 8.2K has even become discolored from so much current (a 1/2 watter). It's obvious why you can't use the UL taps as the screen grids are tied internally. But looking at the datasheet for the 832A I see where the screens are tied to the cathode via an internal cap. Checking the tubes with an ohmmeter reveals no shorts, so I am completely stumped as to what is causing the current draw and the resulting voltage drop. Either I have a whole batch of bad 832As (not likely) or there is something staring me right in the face that I just can't see. The CT of the OPT is connected to the 225V B+, with an 8.2K resistor going from the B+ to the screens on pin 3. I'm using a 1.1uF poly cap across the 8.2K, which is rated at 350vac. Your schematic shows a second capacitor at the OPT center tap with no value, but I'm quite sure that is irrelevant in this case. Any thoughts? Prayers? Thanks for your help.
Cliff


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PostPosted: 05 Jan 2020, 18:06 
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Hi, Not a severe drop but about what is expected. The current listed for the 832 is somewhat ambiguous. As an RF tube it can do as much as 3.5 watts screen dissipation. How it actually performs as an audio tube is kind of up for grabs. At 4 ma and about 175 volts (voltage on cathodes need to be subtracted from the screen and anode ones) the screen dissipation is about 0.75 watts. This should be fine for the type of circuit used. I selected the resistor to keep the voltage at about 175 or so and really didn't worry about the resultant current. Thus 4 ma or so at 185 volts is probably the same range as mine were. I would use more than a 1/2 watt resistor there...minimum of 1 watt. Yes current from the screens exits via the cathode. (that is if you use old school current flow of positive charges) The capacitor at the output transformer CT is in the PS and I like to use a 10uf poly there to minimize the impedance to ground. Remember the PS is actually 1/2 of the audio circuit. Yes this tube is not great for U/L use without putting resistors in series with the taps.

Good listening
Bruce

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PostPosted: 06 Jan 2020, 15:33 
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I'll be ordering some 8.2K, 1 and 2 watt resistors. Murphy's Law dictates that even though you may have 10,000 parts, the particular one you need isn't among them! But having spent a third day pounding my skull against a rock, I found the problem. A discontinuity in the power ground from one wire I forgot to add! I'm ready to hook it up to speakers as I write this. I'll report back with pictures when I work out all of the bugs (I'm sure I'll find a couple more). Thanks for getting back to me so quickly.


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PostPosted: 07 Jan 2020, 21:23 
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:D :D I have at least 10,000 parts and still have to order some. The 8.2K is not that critical a value...pretty much anything from about 6.8K to 10K is fine. Only rather marginal changes in gain. The design is funky thus the name and while it works reasonably well it will not out perform other amps designed with tubes like the EL34, KT77 etc. I tried a push-pull parallel version to get more power but the difficulty in getting each tube section to behave at the same time was not worth it. The Funkywatt is definitely a eye catcher though.

Good listening
Bruce

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