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Super simple single stage tube preamp
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Author:  mwhouston [ 18 Jun 2011, 22:22 ]
Post subject:  Re: Super simple single stage tube preamp

dtsup1 wrote:
Mark, roughly how much did you charge for the completed preamp in case?
I am getting a small 6F3 tube amp (US $120) which outputs 2.5W into 8Ohms.
Judging from the specs, do you think a preamp is necessary?

Technical Specification of 6F3 mini tube amp:

Output power: 2.5W+2.5W@ 8 ohms
Frequency Response: 30Hz-40KHz (+/-1dB)
Input Sensitivity: 800mV
Residual Noise < 0.6mVac
Input AC Voltage: 100V-240V
Input Impedance: 10K ohms
Output Impedance (speaker output): 4/6/8/16 ohms
Output Transformer: Japan Z11 EI48*24
Power Consumption: 33W
Dimension: 132mmW x 122mmD (with RCA & knob) x 170mmH (with 6F3 tubes)
Weight: 1.8 KG
Input: RCA jacks
Output: Banana jacks
AC: Standard IEC with power switch and fuse (2A).
Attachment:
6F3-03 silver.jpg

You should not need a preamp with this amp. It looks good but I'm suspicious of its performance. The all up cost in the case, for the preamp I made, was $375 with a gold hand engraved "retro-thermionic..." plaque. The plaque was $25 so you can save $25 here. Parts come to $250.00.

Author:  Les [ 19 Jun 2011, 01:33 ]
Post subject:  Re: Super simple single stage tube preamp

Nice build, Mark! :thumbsup:
mwhouston wrote:
What is the best way to get the bias voltage down. How about a 30K plate resistor instead of the 20K one??

I think I would split the difference - increase plate resistor to ~25kR and decrease the cathode to ~820R. I believe in this way, it would do the least "damage" to the gain and keep the plate current in check. Increasing just plate resistor is also going to reduce plate current (and possibly your gain), while decreasing just the cathode resistance will increase plate current (and may not have any beneficial effect on gain). In either case, you will lower your cathode bias voltage. Adjust both resistances still gets the cathode bias on target, and we keep the gain and plate current healthy. Yes?

Author:  mwhouston [ 19 Jun 2011, 01:38 ]
Post subject:  Re: Super simple single stage tube preamp

Les: thanks.

Author:  cheap-Jack [ 19 Jun 2011, 11:05 ]
Post subject:  Re: Super simple single stage tube preamp

Hi.
mwhouston wrote:
What is the best way to get the bias voltage down. How about a 30K plate resistor instead of the 20K one??

Les wrote:
I think I would split the difference - increase plate resistor to ~25kR and decrease the cathode to ~820R. I believe in this way, it would do the least "damage" to the gain and keep the plate current in check

No need to blind guess. Bring up the Vp-Ip-Vg transfer curve chart of the tube: 12AU7 & reset the load line using 30K or whatever & THEN choose the right operation point at the curve chart most linear area.

The point I WANT to make is to get rid of the "urgly" cathode bias resistor/bypass cap with something else much much better sonically - battery - NMHi rechargeable battery. I am using such battery fixed bias & I am a very happy camper! :smoking:

c-J

Author:  mwhouston [ 19 Jun 2011, 16:37 ]
Post subject:  Re: Super simple single stage tube preamp

Cheap-Jack: Not sure I'm ready for the battery yet. I have seen leds used in the cathode cct. For now I'll stay with good old Rc.

Author:  Suncalc [ 19 Jun 2011, 21:30 ]
Post subject:  Re: Super simple single stage tube preamp

gofar99 wrote:
Hi again, just saw the bit about the bias voltage, I'm with C-J on this. My thinking is that 4.5 is a bit much for the application. I find that 12AU7s seem to run best with about 1.5 to 2.5 volts bias. It is a more linear operational point. For small signals (under about 0.1 volts output) it might not matter, for larger ones it probably will, but might not be audible.

I'm not sure I would be quite so quick to adjust the bias point downward. I performed a load line analysis of this stage and where the forward signal path distortion is significant (≈12% with a 3v peak AC input) the final distortion is much smaller. Look at the initial analysis...
Attachment:
SSSS LL Analysis.jpg

Although I did end up calculating a somewhat higher bias voltage with the 1.1kΩ cathode resistor, the gain and distortion end of the analysis should be generally applicable. This circuit has significant feedback which is going to significantly reduce both gain and distortion. My quick analysis shows that with the feedback and a bypassed cathode the distortion is only ≈1.6% with a 3v peak AC signal. With a 1.5v peak AC input the distortion is going to be more like ≈0.8%. This is actually a pretty clean little stage as designed.

It's not the way that I would have gone about designing a 12AU7 preamp stage but the final result is actually pretty good. :up:

Author:  mwhouston [ 20 Jun 2011, 07:58 ]
Post subject:  Re: Super simple single stage tube preamp

Yesterday I started my own (or one I hope to keep at least for a while) Super Simple Single Stage tube preamp (The 4S). I bought everything I needed at the one store (is that matching component retailers??). First thing I did was to drill and prime the caes. Its now ready for painting. Rather than watch the paint dry I started on the cct. brd. which I virtually had finished at 1:30 am this morning.

Tonight I finished the brd. and ran the trusty analogue meter over the dead then live unit. No 12V for the heaters! That's fine I left off one very short link (regulator to 10uf poly).

NOW please note I have increased Rp to 28K and reduced Rc to 800ohms. Matt thought the values were right as they were but I had bought the parts based on the suggestion to raise Rp and lower Rc.

Rp=28K up from 20K and Rc=800ohms down from 1.1K. What do you think Matt? I had built the 470k load R (output R) in before you told me it is not required Matt.

Here are the live measurements:
HT=280V
Plate to GRND voltage = 125V
Cathode drop (bias)=3.6V (down from 4.2)
Total current draw through both plates added together = 9ma (4.5mA through each plate).
Plate dissipated in each tube = (125 - 3.6) * 4.5mA = 100mW
Vf=12V

I did a much better job on the build (very neat) and added some better caps in places good caps should go. I also added some better snubbing caps. So all in all a much better build. I'm listening to "The 4S" now laying nude on my Oppo BDP-83SE. Nice.

Here is an interesting observation. With another tube (all the tubes I am using are very old 40 to 50 years) I was using I got two very different plate to GRND values (240V & 160V). On swapping the tube out I got 125V on both plates. I would say the tube has a real problem with one of the triodes. I'm listening to some old AWA Australian made 12AU7s which have been under a house for 40 years.

Big question (only I can answer) "what colour to spray the case"?

Author:  bda [ 20 Jun 2011, 08:17 ]
Post subject:  Re: Super simple single stage tube preamp

I don't know why anyone bothers with these low gain linestages , let alone ones with feedback . I personally find that an AVC switched volume control is all that's required , I have used Goldpoints , Alps pots , switched attenuators and also constant impedence attenuators and the Intact AVC blows them all away (with or without a gain stage the other side)

BDA

Author:  mwhouston [ 20 Jun 2011, 08:27 ]
Post subject:  Re: Super simple single stage tube preamp

bda wrote:
I don't know why anyone bothers with these low gain linestages , let alone ones with feedback . I personally find that an AVC switched volume control is all that's required , I have used Goldpoints , Alps pots , switched attenuators and also constant impedence attenuators and the Intact AVC blows them all away (with or without a gain stage the other side)

AVC ?? I give up what is it. Also I need gain because my 3W SET has very little and the CD player wont drive it loud enough. This little preamp can make it very loud with room to spare.

Author:  bda [ 20 Jun 2011, 10:57 ]
Post subject:  Re: Super simple single stage tube preamp

AVC = autoformer volume control . It uses a tapped choke to provide attenuation , similar in operation to a variac . In my opinion I have not found any form of resistive volume control to sound better than this $200 device . An AVC can also be configured to provide gain by connecting the input to one of the upper taps . A typical CD player output stage should have a low enough impedence to drive an AVC in this configuration . I am very surprised to hear you don't have sufficient gain of your SET amp for reasonable volume , it could be that your speakers are not a good match . With SET amps the envelope should be within the first 'magic watt' , with the rest just there for peaks . It's the 'chocolate sauce factor' as some put it , if you're pushing the amp harder or into clipping to get the volume you require or maybe it's designed for use with an active linestage ? I can only assume your speakers are sub 90dB/1watt if this is the case , I literally get blown out of the room even with a 1.5 watt per channel 4P1L SE amp with my 94dB/1 watt speakers . See intactaudio.com for details of their AVC product , there is also a forum over there for support

BDA

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