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Old 18th April 2012, 12:55 AM   #11
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One more message just to try this photo thing. Setup when I just fired up the preamp.Click the image to open in full size.
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Old 18th April 2012, 09:00 AM   #12
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by moparmike View Post
So you guys have an idea of my madness I got a flickr account. I hope that that the links show up right. These drawings I edited in paint and excel so they are kinda rough but are readable. They also came off the Fun With Tubes

Preamp2 | Flickr - Photo Sharing!

003 | Flickr - Photo Sharing!

Not really proud of the last link but my construction is getting better. Yes those are Octals I used 6SL7 instead of 12ax7. I have a thing for octals and the sockets are easier to wire at least for me. All the AC is in the little boxed in area. I didn't have the alps blue pot in this pic or my signal grounds isolated from the chassis. The RCA plugs I used do have insulation keeping them separated from the chassis. Thanks for the help so far. Oh and what value of pot should I be using for that preamp? I haven't had time to do any soldering tonight so the external pot will have to wait until tomorrow.
Well,

The number one question has got to be, why have you got a volume control on the input (Is this some kind of balance control?) and output of the preamp?
You only need one on the input and drive straight from the last tube to the power amp..
2nd question is why have you got a potential divider on the input to the preamp?

I would expect a balance / volume on the input and direct drive out of the last tube?

Just a thought...if the two pots on the input were volume individual left and right you would have balance and volume..or put bal/volume on the input..

Regards
M. Gregg
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Old 18th April 2012, 09:32 AM   #13
M Gregg is offline M Gregg  United Kingdom
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Here is some info..

Have a look at the volume and balance circuits..

Potentiometers (Beginners' Guide to Pots)

Or just adapt what you are using now..


Regards
M. Gregg
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What is the sound of one hand clapping?

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Old 18th April 2012, 12:33 PM   #14
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Yes that is a balance control right after the input and before the tubes. I would rather not have to adjust 2 volume pots at the same time. I listen to a lot of music in stereo so not having to adjust the balance several times to get it perfect is preferred. I have seen the dual pots with a split shaft that would work nice because I could adjust left and right and still turn the whole thing up and down or I hope that’s how it would work. I did read the website and have seen it before but the question that remains is how does a person decide what value of pot is appropriate for the unit? Tonight I should have some time and just for trial I will remove the balance circuit and put the volume pot in its place. I will let you know what I find out.
Mike
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Old 18th April 2012, 03:16 PM   #15
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by moparmike View Post
Yes that is a balance control right after the input and before the tubes. I would rather not have to adjust 2 volume pots at the same time. I listen to a lot of music in stereo so not having to adjust the balance several times to get it perfect is preferred. I have seen the dual pots with a split shaft that would work nice because I could adjust left and right and still turn the whole thing up and down or I hope that’s how it would work. I did read the website and have seen it before but the question that remains is how does a person decide what value of pot is appropriate for the unit? Tonight I should have some time and just for trial I will remove the balance circuit and put the volume pot in its place. I will let you know what I find out.
Mike
Ok,

This is what I would do..

remove the 500K pot from the output and fit a 100K resistor so the ouput cap is Grounded via the resistor..Take the output to the phono straight from the cap after the last tube.

Fit a 100K Log pot as the volume on the front end of the preamp with your balance control..

If you dont have a 100K log pot use a resistor to set the resistance taper to log as shown in the link.

As to what size pot do I use it depends on the application, however you won't go wrong to stick with 100K as volume others may use a different value. The idea is to not load the previous stage but still keep the connecting lead close to gnd with no signal. There is more such as the effect of the resistance in line with the tube grid. Etc. (Miller Capacitance)..

Regards
M. Gregg
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What is the sound of one hand clapping?

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Old 18th April 2012, 03:49 PM   #16
M Gregg is offline M Gregg  United Kingdom
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Link for Miller effect,

You have to remember that the top section of the volume control is in series with the grid..The lower the volume the more resistance is in series..

The Valve Wizard

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M. Gregg
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Old 18th April 2012, 10:59 PM   #17
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had this problem once....I used a cheater plug on the pre-amp and left the amp grounded and hum stopped....same senerio...hum was there even with pre amp of.
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Old 19th April 2012, 01:21 AM   #18
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So after a evening of soldering I did make the modifications M. Gregg recommended. The hum is still there. Even with the preamp unplugged. When I did turn it on the hum disappeared for a second and then came back at louder than ever. Plugged in my laptop and the amp doesn't hum and music sounds great. I even used a cheesy RCA to 3.5mm adapter in to the RCA cables that I have been using. I did make the flying leads pot and the amp stops humming at 450 ohms. One question I do have, isn't there a lot of stray capacitance to ground if I put one pot in line with another which would cause me to lose some high frequencies? What are you guys thinking that I should try next? Thank you for the help too.
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Old 19th April 2012, 02:08 AM   #19
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Update. Tonight I plugged in a cd player to the preamp and plugged it in to the aux port of my Bogen RP60 amp and guess what it works with out the hum. But when I do crank up the volume on the Bogen amp I do get a higher pitch hum like 120 hz but that can be fixed later. Probity due to the heater supply. This whole deal has me baffled and what I should do with the preamp and why it won't work with my amps.

Mike
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Old 19th April 2012, 08:44 AM   #20
M Gregg is offline M Gregg  United Kingdom
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Sounds like you have more than one fault,

This isn't magic even if it seems like it.. it can only be grounding or psu problems,

If it hums when unplugged then it could be that the chassis is now not at Gnd..you can prove this by disconnecting the supply to the mains Tx and plug back in..

The thing to do is work on the other hum that you have and some times it will effect the situation and give clues to further problems. Just don't give up.

The phono sockets should be isolated from the chassis and a star ground should be in place.. If the preamp works with another amp then you need to ask what is different with the grounding when linked to the different power amps. (Taking into account the difference on the input side of the power amps)

Never remove a safety mains chassis earth to get rid of hum...

Regards
M. Gregg
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What is the sound of one hand clapping?

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