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ForeWatt Tube SRPP Preamplifier - Support / Update Thread
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Author:  connemaraguitars [ 28 Dec 2010, 11:46 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Thread

Hello, Mr. Mazz,
I'd be very interested in a couple of your PIO caps. PM me with cost and shipping...
And thanks!
Lofton

Author:  mazzas [ 28 Dec 2010, 13:38 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Thread

I've got 1.0uF that I assume you want and I've also got some of the 0.33uF K40Y-9 PIO caps that I intend to use to build a pair of Bruce's KT88 monoblocks. Let me know if you want those, too. Include destination zip code (if domestic).
Attachment:
IMG_20101228_132412.jpg

Author:  phildas [ 24 Feb 2011, 13:34 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Hi! Newbie here again. Can anyone upload a PCB layout? Thanks.

Author:  gofar99 [ 02 Mar 2011, 23:06 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Hi All, an update. If you need more gain.... I rewired one of my Forewatts to use 6N1P tubes. :) Not too difficult as the changes are minor. The heater pins need to be changed and pin 9 needs to go to a ground. The circuit changes are really simple. Change all 4 cathode resistors to 200 ohms. The net result is a preamp that sounds as good as the original and has about twice the gain. I have not had a chance to bench test it, but my ears :headphones: tell me it has the same response, S/N and distortion levels. Stay tuned for data. BTW I tried 6DJ8s and E88CCs in it and they did not sound as good as new production Sovtek 6N1Ps.

Good listening
Bruce

Author:  poty [ 03 Mar 2011, 16:19 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

GREAT!!! The tube is much more cheaper and easier to get (in Russia - surely) and it seems then you can drop the heater voltage floating. Hope this lowers the noise floor some dB more!
It seems I'm doing the same things that you suggests, but you are significally faster (in your last post in OddWatt forum you have mentioned almost all the changes I'm trying with help of my breadboard :) ). Now I'm more assured in the new design.

Author:  gofar99 [ 03 Mar 2011, 17:36 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Hi Poty, Nice to know that others are working on similar projects. I still keep the heater DC bias on the heaters. With a B+ on the top tube section of about 215-225 it is too close to the heater cathode breakdown rating of the tubes. Most claim that they can handle 200 new, Unfortunately they all seem to fall to about 100 with age. I have had occasion to have wonderful spark and light shows in a few in the early days of some projects because I exceeded the 100 mark. :firefighter:

Good listening
Bruce

Author:  poty [ 04 Mar 2011, 04:20 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

gofar99 wrote:
I still keep the heater DC bias on the heaters. With a B+ on the top tube section of about 215-225 it is too close to the heater cathode breakdown rating of the tubes. Most claim that they can handle 200 new, Unfortunately they all seem to fall to about 100 with age. I have had occasion to have wonderful spark and light shows in a few in the early days of some projects because I exceeded the 100 mark. :firefighter:

I'm not sure about the new tubes, but "original" ones 6N1P (together with -E and -EB modifications) have 250V cathode-heater rating. I have L1-3 tube tester with the test card for the tube - it states that even at the end of life it should be 250V (that voltage is applied to the tube during testing procedure) and if the leakage current is more than 15 microamperes (12 microamperes in case of -E and -EB mods) the tube is considered faulty.
But I agree, that if there is an "universal" approach - then the heaters floating should be saved.

Author:  Lazvtech [ 07 Mar 2011, 14:35 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Thanks Bruce, for this nice project.
I built one of this and I'm very happy with the results. I will post some pictures when I get the time.
Is there a simple way to reduce the gain of this preamp. I am using it in between my iPhone and power amp HK AVR-254. I am using 2 12au7a and 2 12volt transformers back to back with a voltage doubler. I found the schematic for the power supply from other projects on the web.
Thanks again,
Laz.

Author:  gofar99 [ 07 Mar 2011, 16:28 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Hi Laz, I'm glad you like the project. There are really two ways that can reduce the gain. Both are quite similar. Put a voltage divider on the input or output side of the preamp. I would recommend the input side as it will have minimal effect on the preamp. The easiest is to put a 100K resistor in series with the input. Next I would try a pair of 100K resistors. Tap the signal at the junction of them (one resistor to the source and one to the ground). There might be a slight high frequncy shift depending on the impedance of your source. If that happens a small bypass capacitor on the input resistor could be used to make a high pass filter and bring it back into line. You would have to experiment a bit to get the right value. I would try a 100pf and work in what ever direction gives the best results. The alternative is to put a voltage divider in the output. I believe I would use a 10K from the existing output to a 1-5K (going to the ground) and tap off the junction. This has one advantage, it will cut the noise level as well. The disadvantges are, it requires the preamp to deliver a higher level of output signal and while there is a lot available it isn't infinite and you could get into levels where distortion becomes an issue. It also will alter the output impedance and might have some effect on high frequencies like the divider on the input side could do.

BTW, if you are using the cathode bypass capacitor in your build - removing it will cut the gain by about 25%.


Good listening
Bruce

Author:  Lazvtech [ 09 Mar 2011, 11:39 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Thanks, Bruce for the quick reply.
I have tried padding the output but I don't like the way it sounds. Do you think switching to a cathode follower circuit will reduce the overall gain of the preamp. I have been looking for a single stage preamp schematic but I can't find one. Thanks for taking the time to help others.

Laz

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