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Input transformer with or without grid leak resistor?

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Posted on August 15, 2012 at 09:23:44
vbenonisen
Audiophile

Posts: 56
Joined: November 20, 2010
 Hello!

I´m messing with my stargrounded 1H4G-46-300B single ended power monoblocks and for some reason they have a ground loop with the preamp. To solve this I´ve bought two Lundahl 1676 input transformers.

My question is - do I still need to use a grid leak resistor on the input tube or will the transformer take care of the "leak"?

Regards,
Vidar, Norway

 

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RE: Input transformer with or without grid leak resistor?, posted on August 15, 2012 at 11:07:19
Jim Womble
Audiophile

Posts: 194
Location: Tennessee
Joined: December 15, 2004
 The transformer will take care of the "leak", however, you may still wish to put a load across its secondary if you need to reflect a certain impedance load for a preamp, cdp, etc. Some folks have been concerned about ringing when running them unloaded, however, I use these transformers in two different amps and could never hear any improvement/or loss subjectively from the loading. In the power amps, I have 47k across each half of a center tapped secondary, in my line stage, I am using a shut stepped attenuator as a volume control, so the secondary is seeing a load as well. In both cases the amps are push/pull, and use the transformer as an input/phase splitter.

 

RE: Input transformer with or without grid leak resistor?, posted on August 15, 2012 at 12:16:17
vbenonisen
Audiophile

Posts: 56
Joined: November 20, 2010
 Thank you. Good to know.

V

 

You don't need either- it would be better if you fixed the ground loop., posted on August 15, 2012 at 14:06:39
Ralph
Manufacturer

Posts: 2188
Location: Minnesota
Joined: April 24, 2002
 Here's how:

The chassis should be at AC ground, that is, it is tied to the ground pin of the AC cord.

The circuit should not have contact with the chassis ground at any point.

A 100 ohm (or similar) resistor should be between the two. This way the circuit ground will float at the chassis potential. The preamp and amp should both be treated this way.

You will hear other improvements besides hum reduction- there will be less noise and a blacker background.

 

RE: You don't need either- it would be better if you fixed the ground loop., posted on August 15, 2012 at 15:11:18
vbenonisen
Audiophile

Posts: 56
Joined: November 20, 2010
 The thing here in Norway is that the AC wall sockets is not grounded in living rooms, bedrooms and other "dry" rooms, so all of the cabinets in my entire hifi system is in fact floating. Can that be a problem with your solution or will it work here as well?

Regards,
Vidar

 

Is it really grid leak bias?, posted on August 15, 2012 at 20:53:25
Triode_Kingdom
Audiophile

Posts: 2784
Location: Texas
Joined: September 24, 2006
 What's the input tube and what value is the grid resistor? If the stage is really grid leak biased, you can't connect the transformer directly to the grid. OTOH, if this is the typical grid resistor value, say between 220K and 1Meg, you should be OK substituting the transformer.


--------------------------
Buy Chinese. Bury freedom.

 

If all the AC lines are floating (have no ground), then no. nt, posted on August 16, 2012 at 07:55:22
Ralph
Manufacturer

Posts: 2188
Location: Minnesota
Joined: April 24, 2002
 -

 

RE: Is it really grid leak bias?, posted on August 16, 2012 at 11:29:25
Tre'
Industry Professional

Posts: 7965
Location: So. Cal.
Joined: February 9, 2002
 I think I know what you are getting at and I agree.

The term "grid leak resistor" should only be used when the tube is being biased using grid leakage current. Otherwise, as with a cathode biased stage where the grid resistor is just providing a DC ground reference for the grid, the grid resistor should be called just that, the "grid resistor".


"grid leak resistor

a very large resistor from the grid of a tube to ground, which is used to generate the bias voltage for the tube. See "grid leak biasing" for an explanation of how this works. This term is sometimes incorrectly used when referring to the grid-to-ground resistor in a cathode biased configuration, which is used to provide a DC ground reference for the grid circuit."

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

No, and it's a good point ..., posted on August 16, 2012 at 18:50:01
Naz
Audiophile

Posts: 1869
Location: Sydney
Joined: September 2, 2005
 that you've brought up. Some don't understand Grid Leak Bias and some of us (me included) have just become lazy.

I've always hated the term "grid leak" resistor because of this very point ... in 99% of cases it is just a ground ref R.

But like so many words it's become the accepted term because it's just too hard to have to explain whether it's the grid to ground or grid stopper you are talking about every time you make reference to it so it's just easier to call it the grid leak resistor.

Naz

 

RE: No, and it's a good point ..., posted on August 16, 2012 at 20:09:12
Triode_Kingdom
Audiophile

Posts: 2784
Location: Texas
Joined: September 24, 2006
 "it's just easier to call it the grid leak resistor"

No it's not, that's the wrong term, and it will also require clarification (as it did in this case). If the forum you're addressing doesn't understand the term "grid resistor," you just have to muddle through. These confusions are the result of a non-professional audience, and there aren't many shortcuts you can take when dealing with that.

--------------------------
Buy Chinese. Bury freedom.

 

RE: No, and it's a good point ..., posted on August 16, 2012 at 21:38:00
Naz
Audiophile

Posts: 1869
Location: Sydney
Joined: September 2, 2005
 I agree but it's such common practice ... gotta start somewhere I guess

Naz

 

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