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ForeWatt Tube SRPP Preamplifier - Support / Update Thread
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=2448
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Author:  ForsbackaKalle [ 16 Aug 2018, 02:50 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Thanks Bruce. Its JJ Electronics ECC802S without gold pins. I will get rid of the cathode caps. I will also try 47-50uF cap from heater circuit. From the positive or negative 6,3V DC? What volt rating will be suitable for this cap?
And yes, you are maybe right about the sensivity in my power amp. I will say it’s quite sensitive. I will laborate with this to see what’s suitable. But the sound from the whole system is better with this preamp installed than if i go directly to the DAC (Rega DAC) but then a little bit noisier at low level. So, it’s worth some extra trial and error.
Many thanks for your help and Best Regards // Kalle G

Author:  ForsbackaKalle [ 22 Aug 2018, 09:57 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Yes, Bruce was right. I padded down the final amp and now everything is working like a charm. I used a potentiometer and my ears to pad it down to a reasonable level and change for fixed resistors afterwards. I have learned a lot (about hum, noise and ground loops) when I had this sensitive amp. And now I think everything in my rig works better because I fixed things that was in the noise flor before. And when I put my ear to the speaker i can hardly hear anything.
So, thanks for the help and now its listening time :)

Author:  maxst67 [ 22 Aug 2018, 14:58 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Hello everyone I joined the forum because intrigued by this preamp, the "Forewatt" and I noted with pleasure the great availability of Bruce to answer all the questions and especially the great feedback of all those who built it.
Since my English is not the best and with the Google translator it is not always clear, reading the 61 pages is very difficult.
I asked politely if it was possible to have a summary with the last optimized scheme of the Forewatt in its best configuration.
thank you all

Author:  Diyengineer [ 23 Aug 2018, 19:42 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

I seem to have a recurring issue with my Forewatt , I’ve had 2 LR8N’s fail after about 12 hours of listening. Both were the left channel. When failing, the left channel would start to motorboat.

I’m using fixed resistors on both in place of the trimpot and they were both set to 213V.

Not sure what would cause this? Is there a better or heavier duty version of the LR8N? I’m using Microchip Technology LR8N3-G.

Thank you

Author:  Diyengineer [ 24 Aug 2018, 07:17 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

I notice the K4 version has about 3 times the power dissipation of the N3, would going to that be beneficial at all?

Author:  gofar99 [ 25 Aug 2018, 14:55 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Hi, Sounds like a bad ground issue. I have not had any do what you have had happen, but it seems likely that there is an intermittent ground. Could be a bad connection in the two resistors that make up the adjust settings or a capacitor there. I would initially re-solder the connections. Another possibility is a momentary open or short on the load side. This can cause failure instantly. The K4 is fine and I use them a lot of the time. I solder them to what is know as a "surf board". Then stick that into a socket or solder it directly to the main board. The Kind that has a lot of pins and pads is perfect for this. Just cut off about 4 pins worth.

Good listening
Bruce

Author:  Diyengineer [ 26 Aug 2018, 06:28 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Thanks Bruce,

Just for my knowledge, does the fact that the K4 has a higher power dissipation rating mean that it can handle a higher load? I notice it has the same 20ma output as the others. Not sure what the power dissipation means?

Thanks

Author:  gofar99 [ 26 Aug 2018, 09:12 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Hi, My understanding is that it allows for a greater input to output voltage not more current. Thermal load is approximately figured by the product of the current in amps times the voltage. So the K4 allows for a greater voltage drop at a given current. This is handy as the internal circuitry of the LR8 senses if it thinks it will over heat and will partially shut down. It goes to what it figures is a safe current and will not reset until the power is totally removed. The K4 has a greater margin and is less likely to have this happen. I typically use the LR8N3 at about 10ma or less and run the K4s up to about 15 ma. I set the voltage drops such that the dissipation is about 1/2 or less of the spec. It makes for stable operation and long life of the component. Remember that the K4 needs to have the heat tab soldered to something that will dissipate heat to achieve the ratings. I estimate that without a heat sink they are only good for about one watt.

Good listening
Bruce

Author:  Diyengineer [ 26 Aug 2018, 11:47 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

By the way, I’m willing to bet that you’re right about a bad ground or a bad joint in that portion of the circuit. When I measured the voltage out of the regulator on the faulty side, both times it measured about 200V instead of the 215V.

I ordered all new parts for that portion and will replace them. Hopefully that’ll fix it.

Thanks

Author:  Trackhappy [ 24 Sep 2018, 18:22 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

So my Forewatt is going to be a little heavy.... :)
The correct power transformer is US$17 from Edcor, but 8 week lead time and US$67 postage to me in Australia. :(
So I just got hold of a used transformer that has 6 windings:
6.3V @1.4A, 6.3V@4.5A, 6.3V@4.5A, 6.3V@4.5A (Battery charger anyone???)
289V@200mA - Hmmm power amp of some sort?
207V@125mA - Oh yeah Baby - Forewatt material????
5Kg of iron for a forewatt??? My favourite saying is "When needs must, the Devil drives".

So I calculate that the 207V will give me approx 291VDC after full wave rectification under load. Most likely more under such a light load. So a few extra ohms in the filter chain, or maybe a tube rectifier to drop some volts? Which tube rectifier?

Any other issues using this transformer? It is a fully sealed tin from a nice piece of HP equipment of some sort. Should be quiet at least.

Thanks,
Glenn.

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