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PostPosted: 09 Sep 2015, 18:36 
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Suncalc wrote:
Chris;

This is a simple issue to fix. The volume control on the output works as a variable voltage divider. You just need to change the range over which the potentiometer acts. Here's how you do it.

First, determine how much attenuation is needed. Typical log taper volume pots represent about -20dBv at the half way point. You indicate that about a quarter turn is took much so that's about -30dBv. You want some headroom, so lets shoot for -20dBv. -20dBv = 10^(-20/20) = 0.1 so this means that the existing pot needs to be about 10% of the total. The volume pot on the 4S is 250KΩ so you need a series impedance in line with the input to the volume control of 9*250kΩ or about 2.25MΩ. There is only one problem, this will radically increase the output impedance go the preamp; up to about 225kΩ. This is too much.

The best way to handle this situation is to move the pot to the input of the stage and put the series resistance there. Then the input impedance is high (which you want) and the output impedance is low (which is also what you want). This is actually a simple rewiring to perform but will require a new pot. I would use a 25KΩ audio pot on the front end with a 250kΩ series resistance with the input. Then just replace the 250kΩ pot on the output with a simple resistor to ground.

This results with a -21dBv pad in the system (which with a 12AU7 in the preamp makes the whole thing about unity gain) and gives you a significantly lower output impedance which should also drive most solid state gear as well.

Make sense?

When using my re-visited two stage tube preamp and one of my Class Ds I can barely turn the pot on for sufficient volume. The cheap pot favours the right channel at this level. I padded down the inputs where the pot also is but now, based on the above, realise it was no where near enough. This helps me too.

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PostPosted: 10 Sep 2015, 07:38 
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I am considering building this preamp after building the 6EM7 amplifier. What reasonable price tubes would anyone suggest for to start off with? I want some tubes that sound decent to start with and then maybe do some rolling later.


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PostPosted: 10 Sep 2015, 07:53 
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wdecho wrote:
I am considering building this preamp after building the 6EM7 amplifier. What reasonable price tubes would anyone suggest for to start off with? I want some tubes that sound decent to start with and then maybe do some rolling later.

I have used and are using some NOS JAN-GE 12AU7s. Not expensive and good sounding.

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PostPosted: 15 Sep 2015, 09:16 
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This preamp looks simple enough that I'm thinking of building it.
Translating a schematic to practical wiring is usually a challenge for me but I
think I have a fair grasp of the 4S preamp from the documentation with photos as
well as the comments posted here.

Can someone tell me if I've got this right -
1. The heater wires from the transformer goes to pins 4 & 5 on the 6CAW rectifier,
then the 100R resistors from these two pins go to the common signal/power ground,
then again from these same pins 4 & 5 two wires connect to the heater pins 4 & 5
on the 12A*7 tube ?

2. Nothing is connected to pin 9 on the 12A*7 tube as there is no heater center
tap from the transformer ?

Thanks for any comments.


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PostPosted: 15 Sep 2015, 20:41 
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harrydutt wrote:
Can someone tell me if I've got this right -

1. The heater wires from the transformer goes to pins 4 & 5 on the 6CAW rectifier, then the 100R resistors from these two pins go to the common signal/power ground, then again from these same pins 4 & 5 two wires connect to the heater pins 4 & 5 on the 12A*7 tube ?

2. Nothing is connected to pin 9 on the 12A*7 tube as there is no heater center tap from the transformer ?
Almost.

The wiring for the 6CA4 heater is correct as it has a 6.3v heater. However, the 12A*7 tubes have two elements each with a 6.3v heater. In the tube these are wired in series with the central connection tied to pin 9. This means that a 12.6v heater supply is wired to pins 4 & 5 with nothing on pin 9. However, a 6.3v heater supply needs to put the heaters in parallel so they each get 6.3v. This is done by bridging pins 4&5 and then connecting the 6.3v heater supply between pins 9 and 4/5. See the attached picture to see what I mean.
Attachment:
4S Heater Wiring.jpg
Does this help?


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PostPosted: 16 Sep 2015, 02:37 
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What resistors did you use here Matt? Vishay?

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PostPosted: 16 Sep 2015, 06:31 
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Suncalc wrote:
harrydutt wrote:
Can someone tell me if I've got this right -

1. The heater wires from the transformer goes to pins 4 & 5 on the 6CAW rectifier, then the 100R resistors from these two pins go to the common signal/power ground, then again from these same pins 4 & 5 two wires connect to the heater pins 4 & 5 on the 12A*7 tube ?

2. Nothing is connected to pin 9 on the 12A*7 tube as there is no heater center tap from the transformer ?
Almost.

The wiring for the 6CA4 heater is correct as it has a 6.3v heater. However, the 12A*7 tubes have two elements each with a 6.3v heater. In the tube these are wired in series with the central connection tied to pin 9. This means that a 12.6v heater supply is wired to pins 4 & 5 with nothing on pin 9. However, a 6.3v heater supply needs to put the heaters in parallel so they each get 6.3v. This is done by bridging pins 4&5 and then connecting the 6.3v heater supply between pins 9 and 4/5. See the attached picture to see what I mean.
Attachment:
4S Heater Wiring.jpg
Does this help?


Thank you Matt, This helps greatly especially with your labelled picture.


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PostPosted: 17 Sep 2015, 06:49 
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I have received all my parts to start building this preamplifier. A newbie question. Why do the filament wires go to ground through the 100R resistors? If I had to guess I would say to help prevent hum.


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PostPosted: 17 Sep 2015, 09:29 
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On transformers without a center tap on the filament winding, this provides a centered ground reference for the heaters. It's an anti-hum trick. Most effective in broadband amplifiers.

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PostPosted: 18 Sep 2015, 15:08 
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Joined: 10 Sep 2015, 13:36
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If I decide to try out the 33uF bypass cap in the preamp section what type should I get or will any
type work ?


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