6072a and 12au7 substitutes

[emrr]

The 1M is in theory good practice....in practice it has not been included in 99.99% of professional tube amplifiers made since the dawn of time. 

 

[BluegrassDan]

I see. Does it matter that I'm using a shorting stepped gain switch?

Would 2.2 meg be appropriate?

 

[ruffrecords]

I see. Does it matter that I'm using a shorting stepped gain switch?

Would 2.2 meg be appropriate?

Most tubes have a specification for a minimum value. As long as it is 10 times the pot value (1Meg) it will be fine. There is no advantage in making it bigger.

Cheers

Ian

 

[ruffrecords]

The 1M is in theory good practice....in practice it has not been included in 99.99% of professional tube amplifiers made since the dawn of time.

Do you mean (pre) amplifiers? The vast majority of those don't have a built in level control; it is all done externally e.g V72 et al.

Cheers

ian

Cheers

Ian

 

[emrr]

Preamplifiers, amplifiers, limiters, you name it. 

Altec/Collins/Gates/General Electric/Langevin/McCurdy/Presto/RCA/Western Electric:
anything with built in volume control which also provides grid resistance.

 

[BluegrassDan]

Okay. So where I'm at now, in hopes of snappier transients, is to remove C8 (did this months ago), remove grid stoppers, add 1 Meg resistor between attenuator and grid, perhaps mess around with C1, and experiment with different output transformers.

Lots of soup for one cauldron.

 

[emrr]

am I reading that schematic incorrectly, but it looks like the second grid (V1 1/2) has no DC bias?

It does, look gain.  Strangely drawn. Shield includes bottom of pot and ground. 

C1 isn't going to do any more for you than it already is, outside of any miracles associated with it being a high bandwidth extremely low ESR type or having parallel polyprop cap that does something measurable. 

Additional filter capacitance may help.  It may not. 

Transformers are going to be most of the action.  You already have a Jensen input.  You can go to their most expensive most linear input transformer with step-up, and get a little more, but people will complain about low gain. 

Output appears to be Edcor. 

Your EQ additions may be de-linearizing. 

I believe your build appears to be headed for short run production status, based on the panel design threads at realgearonline.

http://realgearonline.com/thread/7047/tube-preamp-prototype-2
http://realgearonline.com/attachment/download/5369
http://realgearonline.com/thread/7064/latest-front-panel-design-tube

 

[BluegrassDan]

I believe your build appears to be headed for short run production status, based on the panel design threads at realgearonline?

I have had Dan Deurloo build me ten cases with faceplates in total. Three or four for myself and the remaining cases for some musician/engineer friends of mine who are interested. Might end up keeping a couple at university studio where I teach.

I only wish I had the years of expertise everyone on this forum has. I know the subtle sonic differences that I like, but know just enough to paint-by-number.

 

[Matador]

It does, look gain.  Strangely drawn. Shield includes bottom of pot and ground. 

Ahh yes, thank you.  That is strangely drawn indeed!

If memory serves, the plate-to-grid feedback cap in some microphones was made with two short coils of insulated wire wrapped around each other, which gave 2-4pF.  You could wind tighter or looser and 'tune' the feedback.

 

[emrr]

Yes,  I have some pfd caps wound this way in power amps. 

 

[bluebird]

Transformers are going to be most of the action.  You already have a Jensen input.  You can go to their most expensive most linear input transformer with step-up, and get a little more, but people will complain about low gain. 

Exactly...I've come to find that the input transformer and it interfacing to the first stage is 90% of a pre amp sound.  Tube or solid state. Unless something in the design is wrong. The Jensen doesn't have a dual primary, but if you use an input transformer with a dual primary you can give people a choice of series parallel with a switch, usually labeled 150/600 ohm. The series position (600ohm) will definitely give people a more "open" sound but less gain of course.

For instance if you used a Lundahl LL1578 You could employ the series parallel switch.
http://www.lundahl.se/wp-content/uploads/datasheets/1578_8xl.pdf

Seems you already have your panels made so I guess its a moot point.

Regarding the output transformer, I've always had a love hate relationship with Edcor's. Love them because they are cheap and get the job done, but hate them for any serious fidelity work.  I believe they use the minimal amount of wire/wraps for the minimal amount of inductance to get by. All on a steel core. Since there is no DC in the output and its a step down I really don't know to what extent its affecting the over all sound. Like I said before you can just try it with no output transformer at all and if you like the way that sounds but need it to be balanced, get a Lundahl...
Its all project specific but my general transformer rules are:

Edcor to mess around
Jensen for a middle of the road nice sound
Lundahl for a transparent "nothing there" sound
Vintage UTC or Triad for the best sound  ;D ;D ;D

 

[Bill Wilson]

Glad to see you are sticking w 6072/12AY7 on the input. It was specifically designed by GE as a low noise mike preamp tube. Most do not realize that GE manufactured consoles & other broadcast equipment. The difference between 12AU7 & 12BH7 is more than a difference in current draw. 12BH7 was used to drive the output pair in Mac Intosh  MC 30, 40 & I believe MC 76 & Altec 458 program amplifier. The 12BH7 is basically equivalent to 6CG7; the difference between them is the filament voltage. Altec used 6CG7's as the line amplifier stage in the 1567 mixer. I would encourage you to look at the schematic of this unit.

In order to make the 6027 stages as quiet as possible, eliminate C1 & C3 cathode bypass capacitors. They are an old trick to squeeze more gain for the stage. When you increase gain you also raise the noise level. It is for this reason I recommend eliminating them. Manley does not use them in any of their circuits. Also the plate resistor of the 1st. stage should be 100-150k; you want the plate res of the second stage to be 69k or so, as you need more signal swing from the second stage. Take a look in Mixers & Monitor Systems on page 4 near the bottom at "My Location Recording Mixer". This is a 6 mike input w L-C-R switches on each input w stereo out. The mixer was dead quiet & used for recording Master tapes for several record cos., in the 60s. The output stage is a cathode follower, that will deliver approx. 4v RMS. It fed the top of the 100k record level control on my Ampex 351-2 studio recorder. If I were trying to feed a 600 ohm input it would have been a different circuit w output transformer.
The Hamptone output is a bit different; not quite a White Cathode Follower.
Feel free to contact me via P.M. for further information.

Bill Wilson

 

[ruffrecords]

Mixers & Monitor Systems ???  cannot find this anywhere. Do you have a link to it?

Cheers

Ian

 

[My3gger]

Exactly...I've come to find that the input transformer and it interfacing to the first stage is 90% of a pre amp sound.  Tube or solid state. Unless something in the design is wrong. The Jensen doesn't have a dual primary, but if you use an input transformer with a dual primary you can give people a choice of series parallel with a switch, usually labeled 150/600 ohm. The series position (600ohm) will definitely give people a more "open" sound but less gain of course.

For instance if you used a Lundahl LL1578 You could employ the series parallel switch.
http://www.lundahl.se/wp-content/uploads/datasheets/1578_8xl.pdf

Seems you already have your panels made so I guess its a moot point.

Regarding the output transformer, I've always had a love hate relationship with Edcor's. Love them because they are cheap and get the job done, but hate them for any serious fidelity work.  I believe they use the minimal amount of wire/wraps for the minimal amount of inductance to get by. All on a steel core. Since there is no DC in the output and its a step down I really don't know to what extent its affecting the over all sound. Like I said before you can just try it with no output transformer at all and if you like the way that sounds but need it to be balanced, get a Lundahl...
Its all project specific but my general transformer rules are:

Edcor to mess around
Jensen for a middle of the road nice sound
Lundahl for a transparent "nothing there" sound
Vintage UTC or Triad for the best sound  ;D ;D ;D

My experiences with Edcor and some other iron core transformers are very similar. Effect seems more pronounced with DC coupled designs, cap coupled a bit less.
Cinemag is very flexible when asked for different cores, windings can be similar to vintage stuff too. It just takes some time and got more expensive getting them to Europe.

 

[ruffrecords]

In order to make the 6027 stages as quiet as possible, eliminate C1 & C3 cathode bypass capacitors. They are an old trick to squeeze more gain for the stage. When you increase gain you also raise the noise level. It is for this reason I recommend eliminating them.
Bill Wilson

Noise is determined by the first stage so eliminating C3 will not affect the noise. Eliminating C1 will actually worsen the signal to noise ratio because the cathode bias resistor is now an additional noise source in series with the input so it actually worsens the signal to noise ratio.

Of course, if you increase the gain, you amplify the noise more so the noise at the output is bigger but so is the signal.

Cheers

Ian