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4S Universal Tube Preamp Project (12A*7)
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Author:  mwhouston [ 07 Feb 2015, 22:55 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Thanks for the explanation. Never thought of a step attenuator. That would be a great solution on a number of levels and allow us to use your first design. Win, win.

The flip side of all of this is to use a 6SN7 cathode follower preamp\active attenuator. I love the 6SN7 as a driver tube and though I have never built one, I think they would make an excellent preamp tube.

Author:  allenb [ 08 Feb 2015, 14:39 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I've been searching today for an "affordable" stepped attenuator and it appears to be a real zoo out there! Do you know of any, whether it be a DIY version or already assembled that can be had for less than $50? In my travels today I happened upon the lightspeed attenuator using LDR's and looks much more affordable than other means but not sure if it would be a fit for the 4S Universal.

"The flip side of all of this is to use a 6SN7 cathode follower preamp\active attenuator"

Can you point me to where to look at examples/schematics of this?

Thanks,

Allen

Author:  Soundbrigade [ 08 Feb 2015, 14:44 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Check www.vt4c.com or www.analog-metric.com!

Author:  Suncalc [ 08 Feb 2015, 15:03 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

allenb wrote:
Do you know of any, whether it be a DIY version or already assembled that can be had for less than $50?
http://www.8audio-mall.com/volume-control/ $24 USD.

Author:  mwhouston [ 08 Feb 2015, 18:25 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Look around but I did find this http://glass-ware.stores.yahoo.net/acpcbocandus.html

Author:  allenb [ 08 Feb 2015, 22:10 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I think everything's looking good so far and appreciate everyone's help in getting this figured out!

Just to recap and make sure I'm following:

- I can use your first design of the three options for adding a cathode follower stage.
- Use a stepped attenuator to keep the noise down. (thanks for the links, prices look great)
- No need for bypass cap

Question: Should I use lower than a 100K ohm potentiometer with the amps having a 25K ohm input impedance or does the buffer make this value irrelevant?

Allen

Author:  allenb [ 09 Feb 2015, 09:04 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Quote:
Question: Should I use lower than a 100K ohm potentiometer with the amps having a 25K ohm input impedance or does the buffer make this value irrelevant?


Didn't mean to state "irrelevant" but meant to say, with the buffer in the circuit, the amp's input impedance being lower shouldn't require a lower R value to the pot?

One more while I'm here. I had read where the lower cost stepped attenuators were quite difficult to turn and were more switch-like. Does anyone have any experience with them and can tell me if I'll need a larger than typical control knob to turn it?

Allen

Author:  laurie54 [ 09 Feb 2015, 16:38 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Hello allend. I have been away so i am late to this thread, never the less if this helps I have the original project published by popular electronics from the 70's on the Universal Tiger by SWTP. It has a 2.2K in ser. with input to base of trans. Also from base of trans is a 20K in parallel with 220pF to gnd. Thus the 25K input figure. I have also Googled SWTP UT and the results show a modified input with 1Kr added before the trans base. I hope this helps.

Author:  allenb [ 09 Feb 2015, 19:07 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I sure appreciate you finding and sharing the SWTP tiger schematic! This is great to be able to know for sure the 25k ohm I was given earlier is valid.

Allen

Author:  Suncalc [ 10 Feb 2015, 23:07 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

allenb wrote:
I had read where the lower cost stepped attenuators were quite difficult to turn and were more switch-like. Does anyone have any experience with them and can tell me if I'll need a larger than typical control knob to turn it?
Yup; they take some torque to turn. If you look here (http://diyaudioprojects.com/Misc/DIY-Stepped-Attenuator/) you'll see that the one I used is fairly large (≈2-1/4"). The other thing that I have learned is to use some Loctite® on the set screw in the knob. Otherwise, the switch action tends to work the set screw loose over time (a relatively short time).

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