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B7 10k:10k volume control

64.254.110.201
Posted on September 17, 2009 at 23:00:34
DanK tubes
Audiophile

Posts: 132
Location: California
Joined: November 9, 2004
 I've seen the B7 10k:10k transformer used to provide a constant 10k load for a volume control. Mike has posted this up before (see link below). This looks like a nice passive volume control. However, most tube amps have a grid leak resistor at the input. So the pot, as shown in the schematic, is in parallel with the grid resistor. For this case, is it best to use a linear pot and a grid leak resistor in a "law fake" configuration? If anyone has built built this can the comment on pot/resistor choices? Thanks.
Dan

What is a good mid-priced volume control these days?, posted on September 18, 2009 at 20:42:49
VoltSecond
Audiophile

Posts: 2423
Location: Arizona
Joined: October 14, 2000
 What is a good mid-priced volume control these days?

Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.

. . .Oh!. . .Remember: Modifying things voids their warranty.
RE: What is a good mid-priced volume control these days?, posted on October 6, 2009 at 12:25:06
amioutaline?
Audiophile

Posts: 309
Location: Great Smoky Mountains
Joined: August 29, 2008
 I bought 2, then 2 more. Not a Goldpoint but as tuneful as a TKD and unlike the PEC, doesn't require a balance control.

Soldering the wires to the terminals that also hold SMD Rs requires a certain amount of Blind Faith.

And then there is K&K audio's shunt attenuator for 159., posted on September 19, 2009 at 12:00:55
Bas Horneman
Distributor or Rep

Posts: 3549
Joined: March 28, 2001
 Got it but still haven't implemented it yet. It's earmarked to go just after the MQ B7 10k:10k


To infinity and beyond!!!
RE: What is a good mid-priced volume control these days?, posted on September 18, 2009 at 21:29:17
Steve Eddy
Manufacturer

Posts: 9588
Location: Northern California
Joined: April 5, 2000
 This looks promising if they don't end up being vaporware (no one has received any yet).

Khozmo High Quality Audio & Industrial Attenuators









se
RE: What is a good mid-priced volume control these days?, posted on September 18, 2009 at 21:10:45
mqracing
Manufacturer

Posts: 3314
Joined: June 29, 2000
Contributor
  Since: May 7, 2004
 with a devilish smile might I recommend...

The Penny & Giles RF-15 at $250 each.

Unless you want to step up to the Alps RK-50 at $500 each.

For mere mortals the PEC's usually come recommended by lots of folks
and there is the old standby Alps RK-27.

cheers,

MSL





Builder of MagneQuest™ & Peerless™ transformers since 1989
RE: B7 10k:10k volume control, posted on September 18, 2009 at 11:34:17
Caucasian Blackplate
Industry Professional

Posts: 4445
Location: Seattke
Joined: June 18, 2004
 There is no reason to use a grid leak resistor in this type of configuration. (Especially with the 10K pot and the 10K transformer)

RE: B7 10k:10k volume control, posted on September 18, 2009 at 13:25:48
Steve Eddy
Manufacturer

Posts: 9588
Location: Northern California
Joined: April 5, 2000
There is no reason to use a grid leak resistor in this type of configuration. (Especially with the 10K pot and the 10K transformer)

Some would prefer to keep it in the event that the wiper fails on the pot.

se
RE: B7 10k:10k volume control, posted on September 18, 2009 at 13:27:58
Caucasian Blackplate
Industry Professional

Posts: 4445
Location: Seattke
Joined: June 18, 2004
 The secondary of the transformer would also have to open up...

RE: B7 10k:10k volume control, posted on September 18, 2009 at 14:23:35
Steve Eddy
Manufacturer

Posts: 9588
Location: Northern California
Joined: April 5, 2000
The secondary of the transformer would also have to open up...

No it wouldn't.

If the wiper goes, the grid's all on its own. No other path to ground.



se
RE: B7 10k:10k volume control, posted on September 18, 2009 at 14:28:50
Caucasian Blackplate
Industry Professional

Posts: 4445
Location: Seattke
Joined: June 18, 2004
 Oops, you are quite correct!

RE: B7 10k:10k volume control, posted on September 18, 2009 at 09:52:53
Steve Eddy
Manufacturer

Posts: 9588
Location: Northern California
Joined: April 5, 2000
I've seen the B7 10k:10k transformer used to provide a constant 10k load for a volume control.

That's not what it does. Although they're often specified using an impedance ratio, transformers don't have any intrinsic impedance of their own. They simply reflect impedances as the square of the turns ratio. A 10k:10k transformer has a turns ratio of 1:1, so the impedances will be reflected without any transformation (save for winding resistances).

In other words, the 10k load gets reflected to the primary as 10k so that the source sees a 10k load, and the source impedance gets reflected to the 10k load as whatever the source impedance is (not accounting for the winding resistances of the transformer).

The main benefit of the transformer is ground isolation.

However, most tube amps have a grid leak resistor at the input. So the pot, as shown in the schematic, is in parallel with the grid resistor. For this case, is it best to use a linear pot and a grid leak resistor in a "law fake" configuration?

No need for that. The grid resistor is likely to be on the order of 100k or more. If you're going to be using a 10k pot, that 100k or more won't have any appreciable effect on anything.

se
RE: B7 10k:10k volume control, posted on September 18, 2009 at 12:15:25
DanK tubes
Audiophile

Posts: 132
Location: California
Joined: November 9, 2004
 Hi Steve,
Thanks for the comments. My first thought was a resistor in parallel with a log pot would not really have log response. I guess I should have thought it through more before posting. I'll just plan on keeping that grid resistor (if I use one) much much larger than a 10k pot.
Dan

RE: B7 10k:10k volume control, posted on September 18, 2009 at 13:27:40
Steve Eddy
Manufacturer

Posts: 9588
Location: Northern California
Joined: April 5, 2000
Thanks for the comments. My first thought was a resistor in parallel with a log pot would not really have log response. I guess I should have thought it through more before posting. I'll just plan on keeping that grid resistor (if I use one) much much larger than a 10k pot.

Yeah. If it's significantly larger than 10k, it won't really matter. Also, it's only across the portion between the wiper and the bottom of the pot. So unless you've got the volume turned all the way up, it'll be even less relevant.

se
link corrected above (nt), posted on September 18, 2009 at 08:57:38
DanK tubes
Audiophile

Posts: 132
Location: California
Joined: November 9, 2004
 nt

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