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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: 03 Sep 2017, 19:42 
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gofar99 wrote:
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


Looking forward to this. As a noob looking to build an amp for the first time without a full kit, all the variations are a bit hard to follow. If I don't blow something up I'll be shocked.


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PostPosted: 03 Sep 2017, 19:59 
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Hi, Probably. Disconnect it and see if it stops (it should). If it does then there are two choices. Swap the anode and screen wires from the two tubes if using U/L connections. Just the Anode wire if using pentode mode (or really triode mode). The reason it might not squeal in triode now and does in pentode is related to the gain of the circuit. With so little NFB it is possible for the amp to function with it reversed in triode mode as the overall gain is lower.

Good listening
Bruce

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PostPosted: 03 Sep 2017, 21:08 
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Some more info. I disconnected the feedback loop and put it on leads so that I could switch it around. The way that the feedback was previously attached, to the negative output of the speaker terminal, it squeals when in pentode mode. In pentode mode with the positive terminal attached, the connection causes a hum/buzz sound in both channels. In triode mode, attaching feedback to either the positive or negative terminal of the speaker causes a hum/buzz sound similar to the one that happens in pentode mode when feedback is connected to the positive terminal. With feedback disconnected, the amp sounds very nice in triode mode. In triode mode the amp sounds very much like a SET amp. Pentode mode with no feedback sounds harsh and screechy, but with much more volume. Pentode mode is not really useable. Any advice that would get pentode mode working properly with feedback is very much appreciated.


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PostPosted: 03 Sep 2017, 21:20 
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I don't have a UL connection on my OPTs, so I can only switch between triode or pentode mode. Is it necessary to swap the wires going to the tube and OPT, or is it sufficient to swap the terminal that the feedback is attached to? This is my first build of an amplifier with feedback, and I actually posted a few pages back in this thread about connecting feedback to the triode only mode that I had this amp in previously. I thought that I did it right that time, but maybe I didn't.


Last edited by tizman on 04 Sep 2017, 01:14, edited 1 time in total.

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PostPosted: 03 Sep 2017, 21:42 
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Hi, It sounds like you are not grounding the other lead of the output transformer. When you switch the NFB at the speaker side of the output transformer, you also need to switch the ground . Thus the ground becomes the positive output lead and the 8 ohm the ground. The which ever lead is being used as the gound lead must go to the signal ground of the amp. If not it will do some of the things you indicate. It can not float.

Good listening
Bruce

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PostPosted: 03 Sep 2017, 23:14 
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Do you mean that the negative speaker terminal should be connected to ground? I am so used to building single ended amps that it never occurs to me to ground speaker terminals.


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PostPosted: 03 Sep 2017, 23:37 
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I tried grounding the negative terminal and it fixed the problem. Both pentode and triode work now, and both sound good. I need to pay closer attention to schematics. I missed the connection of the negative terminal to ground. Thanks Bruce!


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PostPosted: 04 Sep 2017, 10:15 
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Hi, Don't feel bad. This is a fairly common thing. 8-)

Good listening
Bruce

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PostPosted: 09 Sep 2017, 10:52 
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Hi everyone. Finely got my podwatt finished. Problem is no sound, nothing, not even any hiss from the speakers. Valve (tube..... from UK!) heaters glowing, getting similar voltages at points that Iv'e read on other posts. Don't have an ocilloscope so limited in diagnosing.. Everything is new. One mistake I made was the connections on the LM317 reg, I shorted connections A and O, (soldered together and earthed through the 15R resistor), would this have blown the reg? if so would this give no output? Iv'e been using my phone for the music source. Any help appreciated!


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PostPosted: 09 Sep 2017, 12:04 
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deadgroovy wrote:
I shorted connections A and O, (soldered together and earthed through the 15R resistor), would this have blown the reg?
This would have turned the regulator into an "infinite" current source (1.25v/0Ω). It probably didn't like that. But before you replace it, with everything properly wired, measure the cathode voltages of the output tubes. If they are way off (i.e. near zero or very high) then the regulator is probably blown. Luckily it's a not very expensive part.

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