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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: 01 Aug 2018, 16:01 
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Hi, I used the lower power setting (like for EL34s) mostly for sound quality as I don't need the higher power output. Yes I used a single LM317. An easy load for it. Since all 4 amps (two are the ones from the 2008 project) use SMPS for the heater power there was no issue. They are rated at 5 amps / 12 VDC. For transformer applications I would want to be sure there was enough. The best combinations are for AC operation about 4 amps /12 V, 8 amps / 6V or for typical full wave rectification and capacitor filtering 10.9 to 11.2 VAC at 4 amps. This usually works out to an almost perfect 12VDC. The commercial kits using KT120s use 10.9.

The SMPS came from Amazon and were cheap. They have plastic cases that I fastened to the chassis sides with high strength double sided mounting tape. Great stuff for mounting components like big caps as well. I would not use it to hold parts that generate heat though.

EDIT: the heater current for a KT88 is about 1.8 amps and the current for a KT120 is just under 2. It is likely that any version of the amps using KT88s would be OK with KT120s. I would expect that the loading on the LM317 if you run them like KT88s ought to be fine as well as the B+ and current are not changed. In the KT120 amps the B+ is about the same, but the current is much higher and needs two LM317 to be safe.

Good listening
Bruce

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PostPosted: 01 Aug 2018, 18:15 
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Hi Bruce,

By “lower power setting” to you mean that you simply leave S1 open in the bias circuit? Even for the KT120’s?

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PostPosted: 01 Aug 2018, 20:11 
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Hi, Correct, I'm using the 62 ma per tube setting, not the 90ma one. Oddly, the actual power output is more than the EL34s and KT77s can deliver and only slightly lower than the KT88s would at the higher power. I was measuring a clean 18-19 wrms. Since I only need about 5 this is a bit of overkill. It seems that the KT120s are more efficient at 62 ma than I would expect. A nice bonus. It also means that any of you that have some of the marginal KT120s mentioned in another thread could most likely use them now.

Good listening
Bruce

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PostPosted: 01 Aug 2018, 22:06 
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Was the higher power setting a little less clean?


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PostPosted: 02 Aug 2018, 05:57 
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I was going to say....I thought the later (newer) TS, KT120's were red plating in these amps.

If someone could direct me to (link) the issue, and whether or not its been rectified (lol), i'd be much obliged.

I'd love to give them a try......


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PostPosted: 09 Aug 2018, 06:52 
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Hi Bruce,

Have you experienced any red playing with the Tung Sol KT120’s as Johnnycamp mentioned?

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PostPosted: 09 Aug 2018, 11:02 
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Hi, Yes. There is a whole thread on this topic from about 8-12 months back. It seems that there were a number of tubes that did this. The early ones didn't and apparently the newest ones don't either. Some were red plating at as low as 40-50% dissipation. They would not be suitable in the amps. If you get some like that just keep sending them back until you get some that don't. I don't have exact dates for the problem groups, but it seems that ones made before 2015 and since late 2017 are OK. On the plus side, if you already have some of them that can't go back you can use them in the low power mode (62ma each) in the KT77/KT88 versions of the amps. The output power is about 18 wrms then. I'm doing that in my system now as I don't require much power to drive my speakers. They sound great doing that.

Good listening
Bruce

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PostPosted: 18 Aug 2018, 12:26 
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Hi, would there be any benefit to running a regulated heater supply in this amp using LM317’s or similar?

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PostPosted: 19 Aug 2018, 09:07 
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Hi. A qualified probably not. Tubes with cathodes as opposed to bare heaters are relatively insensitive to issues in that circuit. Small deviation from the 6-6.3 volts are well tolerated. The heater circuit is already elevated above ground by a significant level. This will tend to mask any hum or noise there as well. DC regulated heaters are unlikely to cause any problems, but will add complexity. In the amps I have that derive DC heater voltage from the main transformer I do not regulate it. As long as it is in the correct range it is fine. In some build I used SMPS as a matter of ease in the build. They are DC (and BTW need to be of the isolated secondary type). Additionally, while I can measure slight differences in these amps when run with AC heaters, the level is so low that the use of AC heaters is really a good option. At about -90dbv a single db or so is not audible.

Good listening
Bruce

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PostPosted: 19 Aug 2018, 15:22 
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Thanks Bruce,

If I use the DC filament as is to supply the power tubes, would a regulator be the best way to drop the voltage to the SRPP driver? Or would a 20R resistor be just as good? I’m planning on using a 6SL7 because of availability.

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