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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: 09 Jul 2017, 01:04 
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Here's a photo. The leads for feedback are visible next to the speaker jacks. The OPTs are labeled TO2-17.


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PostPosted: 09 Jul 2017, 09:50 
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Hi, I believe I understand you used triode mode as U/L was not available. I would actually probably use a pentode mode as I have not found the triode mode to be particularly better and it does alter the gain sufficiently that the NFB is nearly non-functional. A little electronics 101....NFB can be used on any mode of operation and class A can benefit from it as well as the others. It does several things when used judiciously. First it can reduce distortion, I don't use it for that. Numerous companies have huge amounts of it (20 or more db) to fix their amps distortion. I figure it is better to build without distortion in the first place. Second it can allow for tube and component aging. This I do consider, but only to a small degree. Next it can be used for frequency shaping which ties in my case to stability. All the Poddwatts are by nature of design very wide band. It varies with the individual sizes and tube choices but most can easily reach into the 70-80KHZ range with sufficient gain and output to be susceptible to EMI and "ringing" (possibly even oscillation depending on the build layout). The sources of the potential instability is the interaction of the amps with the speakers. Some really wild crossovers can introduce enough phase shifts as to potentially cause problems. I have not found any yet, but I decided to eliminate the chance. The NFB in the U/L version is set for approximately 2.5-3.0 db with a desired frequency to act at about 30-35KHZ. This is just enough to eliminate the possibility of instability and noise pick up in that region. It reduces the gain by a like amount (think tube aging protection again) and reduces the distortion very slightly. The amps will run fine without it. If I were to redo the amp I would use the schematic of the Mini Blocks or the 2C. I have built a version that uses DC on the heaters, but it really was not that much better than the ones with AC there. I believe that Edcor (I have no financial interest with them except as a supplier of transformers) has a kit of transformers for the Dual Mono Block version at the end of their listings. These are the ones used in the my commercial kits. The output trannies are also listed now as the CXPP10-MS-10K. They were designed to my specs for this amp. A bit more costly than some others (notably the GXPP10-6-10K) but have exceptional performance in these amps. The kit as I recall saves about 10% over the individual parts. An alternative is to use a single supply and just get the outputs. Lots of choices...that is what makes DIY fun. I attached a few of the alternatives. You can sort of modularly combine the power supplies with active parts. AC heaters with regulated DC bias with 2C amp, DC heaters with amp...and so on. The very best is the DC heaters with regulated bias mono block configuration with LR8 regulated B+ to the driver tubes. It is about 4 db quieter than the next best (AC heaters, regulated bias....LR8 B+). It is a bit quieter than the AC heaters standard (unregulated) heater bias with LR8 B+. Even the version with AC heaters, standard heater bias and unregulated B+ is really quiet. The changes in the amp circuit itself should be done to all versions IMO.

Good listening
Bruce
Attachment:
DMB Power Supply DC heaters July 2017 Update.png
Attachment:
DMB Power Supply AC heaters with regulated bias July 2017 update.png
Attachment:
PoddWatt_2C Consolidated Schematic Feb 2 2014B checked 2107.jpg


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PostPosted: 09 Jul 2017, 12:22 
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Hi Everyone, Since there are so many variations of the Poddwatt (and the Mini Blocks) I will be putting together a set of schematics for the various ones with an explanation of what is different and how much. Just give me a few days to prepare them. All the versions are really over achievers, some just a bit more so. To some extent cost is a factor, but any of them will provide great sound. I used the lowliest one in my office (when I worked for a company) for nearly 7 years before I upgraded it. Lovely sound feeding a pair of vintage KG4 floor standers (not the newer KG4s).

Good listening
Bruce

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PostPosted: 09 Jul 2017, 17:14 
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Thank you Bruce for the quick and thorough response. I realized last night that I made an error in wiring the NFB portion of the circuit. I forgot to wire the part that went from the resistor/capacitor to the tube. Which is why there was no discernible difference no matter how I connected it. Once fixed, attaching the NFB to the negative speaker terminal produced the best sound. With respect to running the circuit in pentode mode vs triode, what does that look like? Right now I have a 100 Ohm resistor between pins 9 and 7 of the power tubes. Is it enough to simply remove these resistors, or do I need to modify the circuit further? Thanks again!


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PostPosted: 16 Aug 2017, 23:38 
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Hi Bruce. I was hoping to see what you thought would be the best way to pentode connect your circuit as I have no UL tap on the OPTs I have installed, and I'm finding my amp could use a bit more power in the set up and room it has been moved to. No pressure, of course....


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PostPosted: 17 Aug 2017, 08:55 
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Hi hooking them up that way makes the tubes run in triode mode. Less power output, but perhaps nicer sound. It also will reduce the gain of the amplifier and the NFB loop may not have enough to work with thus the only slight change in sound. The best way (IMO...YMMV) to use the tubes in pentode mode is to hook the screens (pin 9) on the tubes in pairs (each channel separate) together. Attach a 1 uf capacitor from there to the ground, put a 1 watt or larger resistor of around 2K from the pins to the the place you are feeding in the B+ to the output transformers. The values are a bit fluid and several combinations will work. Larger caps are fine, and the actual value of the resistor can vary from around 1K to perhaps as much as 10K. It will alter the gain in the larger values. While it is hard to predict accurately the change from what you have now to a pentode mode should about double the output power. Remember thought that the change in volume will not double...it is a logarithmic ratio and will be less.

Good listening
Bruce

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PostPosted: 01 Sep 2017, 16:52 
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Thanks for your response, and sorry for my delayed one. I just got back to building after the summer break, and would like to add a triode/pentode switch to my amp. I have attached a photo of a hand drawn schematic based on the recommendations in your last post. It doesn't show the switch or the existing triode connection for the sake of keeping the drawing simple. Is this drawn correctly? The additional circuitry's connection to the B+, and its 10uF cap is my primary concern. Thanks again for your support of your design.


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PostPosted: 01 Sep 2017, 20:32 
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Hi, That is how I would do it.

Good listening
Bruce

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PostPosted: 01 Sep 2017, 22:25 
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Thanks Bruce. I'll do it tonight.


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PostPosted: 03 Sep 2017, 17:05 
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Hi Bruce. I modified the amp as per the schematic of my earlier post. Triode mode works as it did before, but with the switch in pentode mode, the amp makes a loud squealling sound. Is this an improperly connected feedback loop?


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