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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: 13 Aug 2009, 23:48 
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Yes, the voltages there were under load, and verified using two multimeters (i am less trusting now). The mains doesn't drop at all under load, and (yesterday at least) was stable 248v at all power points.

Bearing in mind you mention elsewhere that the heater bias should not be more than 1/3 of the SRPP anode voltage, if I increase the second 47k only, the heater bias will be significantly more than 1/3. Therefore, I suppose increasing the first 47k resistor is the way to go.

To get 225v on the SRPP I guess I would have about 280-300v on the first 50uF cap, which would lead to a heater voltage bias between 90 and 100v, significantly above 1/3 of 225.

Can you confirm the intended voltage range on the heater bias? You say 1/3 of the 240v SRPP anode voltage, but the 220k/100k divider is wired to the top of the first 50uF cap, not the second, and will always be significantly above 1/3.

Cheers


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PostPosted: 14 Aug 2009, 10:35 
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Hi, The target voltage on the SRPP heaters is 1/3, however the ratings (probably rather conservative, but I would not push it too far) is 100 for most of the possible triodes for the circuit. so 90-100 is fine. The two voltages that are key are the H-C on the lower tube section as the cathode is nearly at zero and the H-C on the upper one that is at about 1/2 the anode voltage. It is a juggling act. My sense is that the one on the lower one is the one to watch. Thus the 1/3 setting. mine run about 80 on the heaters with the components I used. If you find yours much over that, then I would adjust the divider to get it back. FYI, I tried having them float entirely free of the PS and ended up with too much noise in the circuit. Another not so obvious feature of the components selected for the PS is the fairly long time constant for charging the last two sections of the filters. It is intentionally so, to allow the voltage to build up on the SRPP slowly. With a SS power supply it could (if not so designed) charge up the filters quickly and exceed the voltages that way as well.

Good listening
Bruce

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PostPosted: 15 Aug 2009, 03:23 
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Thanks. I have changed the first 47k to a 68k, and now my voltags are 235 (top anode) and 92.2 (heater bias) on one amp, and 245.5 and 93.7 on the other.

The AC in at the time of measuring was 253v. I've been monitoring the mains over the last couple of days and and at one point it was up at 258!

Still no idea where the discrepency between the two amps comes from, but I'm happy that they are both now within spec, and should remain so, even if my mains goes back to 240v.

So everything looks great now, thanks for all your help :thumbsup:


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PostPosted: 15 Aug 2009, 11:15 
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Hi, I'm glad all is OK now. Where are you that the Mains fluctuate so much? If you have any sensitive equipment I would really consider a power conditioner of some sort. A thing to consider with the oddwatt style of amp is that with some combinations of tubes if you vary the heater voltage a lot the idle point will shift and need re balancing.

Good listening
Bruce

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PostPosted: 17 Aug 2009, 01:38 
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Hi Bruce, I'm in Malaysia and have a new 3 tower 50 story condo under construction next door, which could be to blame. As I'm using the original design with the regulated SMPS for the heaters instead of the transformers, I shouldn't have any fluctuations in the heaters. I've looked into the idea of a power conditioner/regulator/ups etc. before, but they're expensive, and I don't think I have anything too sensitive (nothing that can't be replaced anyway). I do get through an enormous number of incandescent lightbulbs though (love my dimmer too much to go green I'm afraid)...


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PostPosted: 17 Aug 2009, 19:59 
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Hi , That explains it. The use of an SMPS for the heaters is an idea I like a lot. I frequently do that in my stuff. The main reason for the conventional supply in the projects is so that folks can build them without having problems getting good SMPS. Some are not very reliable and some fail to start with the huge resistive load that cold tubes put on them. With the SMPS supplying the heaters, you should be OK. If you want to insure that there will be mo problems with the SRPP stage, you can run B+ as low as 200 on them and it will not affect the sound. Below that you would need to change the cathode resistors on them to function correctly.

Good listening
Bruce

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PostPosted: 31 Aug 2009, 16:06 
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Hi Everyone, It's been some time since I updated this thread. Three things new (one or more might be good for a new thread). I had the opportunity to try the amps (with blue glass JJ KT88s) on some Martin Logan Vistas (ESL hybrids) and was so impressed that the Vistas are now in my listening room. (I am a bit poorer as well) The combination really sings. BTW the ML dealer wants to carry the kits as he was impressed as well. Synergy is everything. For all the folks that think you need 100s of watts to fill a room, the combination was tested in a room about 35 feet wide, 30 feet deep and 15 high. Loudness was not an issue. The MLs are 90 db at 2.83 volts, they are 4 ohm speakers. Stability was considered as the MLs drop to 1.2 ohms at 20K HZ. I didn't hear any evidence of problems. I can't say the amps will work on all systems, but this combo is a winner. My follow on project is done as well. The smaller stereo amp is being written up and I'll get it to the main Project Site shortly. I also completed a companion tube line stage preamp for the Odd Blocks. It tests well and sounds better. That will be my next write up.

Good listening
Bruce

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PostPosted: 01 Sep 2009, 06:37 
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Great news and congratulations! I look forward to the write ups.

Also, I would be interested in a comparison between the blue JJs and Gold Lion KT88s.

Enjoy the new speakers. Those are really great.


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PostPosted: 01 Sep 2009, 11:03 
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Hi, Thanks, My Gold Lions are out on loan with a pair of amps right now so I can't compare them on the MLs. But with the previous speakers I was able to do so. The two sounded a lot a like. The Gold Lions seemed to have a stronger bass than the JJ Blues. This is a good thing with some systems. With mine it was super on some music and IMHO a bit too much on other. Great if you like rock music. The JJs seem a little more forward in the mid range than the GLs. Both have rather nice top ends with the edge going slightly to the GLs. Generally, either tube will give a great performance and both behave properly in the circuit. If you have just tuned in, I was unable to keep EH KT88s balanced. I suspect it is some thermal effect and really no fault of theirs. They just don't work. Lots of others do so it really is no big deal. I expect to try the KT77s in the amps this weekend and see how they sound on the Martin Logans. I have more power than I need with the KT88s. The blue glass is really cool looking though. I believe that JJ has other blue glass tubes and may get some. BTW in the past with the "old" speakers the what I considered to be the most articulate sound was from the JJ KT77s. Not as much bottom end, but really sweet mids and top. Like I said some where else, don't ever listen to a tube amp om Martin Logans. You will most likely be cursed for decades (I was).

Good listening
Bruce

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PostPosted: 19 Sep 2009, 17:04 
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Hi Everyone, A project update. If you built the amps with the 6NO30, you can tune out. If you used the transistor circuit that we had in the early stages. Listen in. The circuit was voltage sensitive and variations in your AC mains could cause problems. These only occurred in the amps that did not use a 12 volt SMPS for the heaters. The circuit would either trip early or in some cases when the voltage was low not trip at all. I will be posting a cheap fix. It uses a LM555, an additional rectifier (any type or voltage), a .2uf cap (any kind), a 100K resistor and an elecrolytic cap of about 470uf (16 volts is fine). You sharp folks out there can probably figure out how it works. The whole thing is about postage stamp size and makes the delay very precise. Me, I much like the 6NO30 circuit. There is a certain elegance about using a thermal delay tube in a modern circuit.

The new smaller amp is done and I will be starting a thread on it. The same for the tube line stage preamp. The projects will be ready for posting shortly.

Good listening
Bruce

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