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Super simple single stage tube preamp
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=3266
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Author:  dtsup1 [ 25 Jun 2011, 01:10 ]
Post subject:  Re: Super simple single stage tube preamp

Excellent explanation Matt.

Author:  mwhouston [ 25 Jun 2011, 03:33 ]
Post subject:  Re: Super simple single stage tube preamp

Matt: All good.

Author:  mwhouston [ 25 Jun 2011, 23:50 ]
Post subject:  Re: Super simple single stage tube preamp

Here is the MkII version of the 4S preamp. As metioned earlier the plate load resistor was raised and the cathode R lowered. Matt has calced. the distortion to 0.2% much lower than the Earlier verion.

I used the beaten copper Epox Enamel finish with some black knobs this time. Not as pretty as the last one but with two inputs. Much better construction inside and WBT silver solder used again. Wire warp wire links the cct. brd. to the pot and RACs etc. Alps Blue velvet 100K pot on the output and additional tar/Al dampening in the case. No PCB and no shieled wire. Fat silicon feet hold this little gem off the ground.
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Author:  mwhouston [ 29 Jun 2011, 08:29 ]
Post subject:  Re: Super simple single stage tube preamp

Matt: Though a new tube in the 4S MkII may give me the sound I found in the first build by altering Rp and Rc should I have changed the Rfb also? What about halving the FB.

Also with the build with the new values what if I chop Rfb out altogether. Will I get more gain? How different will it perform? I do need some more gain.

Ta mate.

Author:  Suncalc [ 29 Jun 2011, 22:50 ]
Post subject:  Re: Super simple single stage tube preamp

mwhouston wrote:
Though a new tube in the 4S MkII may give me the sound I found in the first build by altering Rp and Rc should I have changed the Rfb also? What about halving the FB.
Actually the changes to Rp and Rc have very little impact on the level of feed back in this design. There is no real need to mess with it when shifting the bias point by this small an amount.

mwhouston wrote:
Also with the build with the new values what if I chop Rfb out altogether. Will I get more gain? How different will it perform? I do need some more gain.
Actually I would get rid of the grid direct FB all together. This would simplify the circuit and yield more gain. Here is what I would do.

1. Remove the 330kΩ feedback resistor. This change alone will result in 19dB of additional gain. And distortion should be around 1.6% with an output swing of 86vp-p. Since the distortion is virtually all 2nd harmonic and directly proportional to gain, smaller output swing means smaller distortion. (e.g. at an output swing of 10vp-p, distortion should be about 0.2%.) However with no feedback the original sound of the stage will be gone.

so...

2. Remove the 220µf bypass capacitor. This will provide enough feedback to reduce the additional gain to about 16dBv and should restore the majority of the "tight" sound that was present in the original stage. It will also reduce the overall distortion by about 30% (1.6% to 1.1% at an output swing of 86vp-p).

3. I would also reduce the grid stopper to about 10kΩ (from the current 100kΩ). This will be necessary to restore the high end gain that will be lost due to the Miller capacitance.

This will leave you with a much simpler stage that should have excellent performance and the tight sound of the original. You have just a 470kΩ grid resistor, a 10kΩ grid stopper, an 800Ω cathode resistor, and a 28kΩ plate load. Leave the output cap at 0.47µf and replace the 470k bleeder with a 100kΩ pot as an output volume control. Gain will be ~19dBv (-8.9), distortion 1.1% worst case, and low end rolloff will be set exclusively by the output capacitor.

That is how I would do it. Simple. ;)

Author:  mwhouston [ 30 Jun 2011, 07:08 ]
Post subject:  Re: Super simple single stage tube preamp

Matt: With blindfold on I will follow your suggestions (hard to use a soldering iron like this though). I received the Golden Dragon 12AU7 tube today. It has only a few hours on it but has made a big improvement. I'm listen to the Silver Dragon with EH 300B Golds. Goods sound, natural without the extreme smooth those TJ Full Music 300B/SE brought.

I am thinking of buying Qians one good 300B/SE to make a SE guitar amp with. I have a new Jensen 12" guitar speaker (C12N) with ceramic magnet (about 92 plus db efficient) to go with it. Matt, one guess who will be doing the design work??? I need a two stage preamp and driver section. We need to make 100mV 10W!! How about 12AX7 or 6N1P three stage guitar amp with a $325 300B carbon plate doing the grunt work. I like.

I may just build another preamp from scratch with your tweaks. See how we go.

Enjoying the Russian Silver Dragon right now.

Author:  mwhouston [ 04 Jul 2011, 06:20 ]
Post subject:  Re: Super simple single stage tube preamp

I put the 4S MKII through the CRO:

All with a 27K load with volume at max.
Gain: 1Vpp input with 2.72VP-P out about +4.35db gain
-3db points <10hz to 152Khz
Better than 20Vp-p output

You see from the figures above the gain is slightly higher up from 3db and even greater upper frequency cut off. SQR waves were excellent right up to 20Khz and down to 100hz. Pleasing results.

Author:  Suncalc [ 04 Jul 2011, 12:40 ]
Post subject:  Re: Super simple single stage tube preamp

Actually, as this is a voltage amplifier, the gain is technically 8.7dBv but I won't quibble over trifles.

Sound like the stage is well behaved. Still sounding good?

Author:  cheap-Jack [ 04 Jul 2011, 14:22 ]
Post subject:  Re: Super simple single stage tube preamp

Hi.
Suncalc wrote:
... an 800Ω cathode resistor, and a 28kΩ plate load.

What will be the cathode bias voltage?

c-J

Author:  mwhouston [ 04 Jul 2011, 16:59 ]
Post subject:  Re: Super simple single stage tube preamp

Suncalc wrote:
Actually, as this is a voltage amplifier, the gain is technically 8.7dBv but I won't quibble over trifles.

Sound like the stage is well behaved. Still sounding good?

Sounds good with the new tube. I chose a Golden Dragon 12AU7. Nothing special just new. Probably needs a few hours of burn in yet.

Yes silly me 20log not 10log. This preamp will drive me SET to full power with the new tube in so it stays. In the past I have used two stage tube preamps that had 9db gain and they could do the same thing. A gain of 8 is close enough.

I still may build your design for this preamp Matt. I have to make a simular PS for a friend who I sold a two stage tube preamp to with a SMPS. Then I can look at the 807 amp.

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