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Post subject: Re: '68 Deluxe Reverb problems...
Posted: Mon Mar 10, 2014 9:13 am
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BMW2002Ti wrote:
Inverse summation, for resistors in parallel. (1/R,sum = 1/R,1 + 1/R,2 ... +1/R,n)

For 820-ohms plus 1800-ohms, I get a resultant 563-ohms. Less resistance = greater GNFB voltage. Or "more" feedback. Now, just how this effects the tone is not quite that simple. This is somewhat system dependent. Best to do empirical testing. Try it, and make notes of the change (if any) to the tone.

The result maybe a more sterile sound, heftier bass, or no effect at all. Be careful with modifying these amps with circuit boards. Try to keep the tracings and pads intact.

Good luck!


He is looking at 1.5k in parallel with 1.8k which gives approximately 820. (818.181818...)

Art


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Post subject: Re: '68 Deluxe Reverb problems...
Posted: Mon Mar 10, 2014 9:14 am
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Retroverbial wrote:
Art1 wrote:
Both of these are blackface, but the Deluxe Reverb didn't change much over the years until CBS started screwing with master volumes in the mid 70's.


It's also 820 ohms in the pull-boost versions of the late '70s.

Arjay


Thanks Arjay! I hadn't researched that far. :oops:

Art


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Post subject: Re: '68 Deluxe Reverb problems...
Posted: Mon Mar 10, 2014 9:23 am
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BMW2002Ti wrote:
Inverse summation, for resistors in parallel. (1/R,sum = 1/R,1 + 1/R,2 ... +1/R,n)

For 820-ohms plus 1800-ohms, I get a resultant 563-ohms. Less resistance = greater GNFB voltage. Or "more" feedback. Now, just how this effects the tone is not quite that simple. This is somewhat system dependent. Best to do empirical testing. Try it, and make notes of the change (if any) to the tone.

The result maybe a more sterile sound, heftier bass, or no effect at all. Be careful with modifying these amps with circuit boards. Try to keep the tracings and pads intact.


Thanks BMW. I was using this calculator to get 818 ohms:
Image

t-luxe says the resistor is 1.5k. So it would be 1.5k (68 Custom value) + X = 820 ohms (vintage RI value), which is where I got the 1.8k value. My hope would be to reduce noise in the 68 custom amp.

Art1 wrote:
So two 1.8k would give you 900 ohms. Much better to replace with an 820 ohm, but why? You will be affecting the "Vintage" as well as the "custom" channel. Unpredictable results on the tone. You will also be killing your warranty.


t-luxe said it was 1.5k on the CDR so I picked a value that would get it close to the 820 ohms. Why? To reduce noise. That's the main topic of the thread ("found it had a lot of background noise (compared to my '65 DRRI)") and my original question. If I could piggyback a resistor onto the existing one, that is a simple mod, is very reversible, and could reduce the noise to an acceptable level. It's also something I could temporarily clip in to try (but would need to order the right value).

Ultimately if new tubes and a NFB resistor tweak won't tame the noise then I could just return the CDR and get a Mojotone kit or Vintage Sound 22. But it sounds good otherwise so I was just exploring the options and trying to help the OP.


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Post subject: Re: '68 Deluxe Reverb problems...
Posted: Mon Mar 10, 2014 9:59 am
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Wow, I don't know how he came up with the idea that decreasing the feedback resistance in GNFB circuit would decrease the noise. Yes, you increase the GNFB voltage, but this does not necessarily equal decrease background noise. GFNB is used to keep the amp in a linear, non-distorted state. Not reduce noise.

There are so many other factors that cause baseline hiss and hum, GNFB is a minor part. Noisy pre-amp tubes, poor grounding scheme, poor contact with jacks and sockets, poor PSU filtering, poor wiring layout, inadequate shielding of wires, esp input wiring from the input jacks --- these should all be ruled-out first. As they are a good order of magnitude above the GNFB cricuit, in terms of sources of noise.

In general, baseline hiss and hum which does not vary with volume control settings, is a PSU issue. Hum or noise that is volume control dependent most prolly lies in the gain stage (pre-amp stage).

Would you happen to have a sound bite of this hiss?

Thanks!


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Post subject: Re: '68 Deluxe Reverb problems...
Posted: Mon Mar 10, 2014 10:15 am
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BMW2002Ti wrote:
Wow, I don't know how he came up with the idea that decreasing the feedback resistance in GNFB circuit would decrease the noise. Yes, you increase the GNFB voltage, but this does not necessarily equal decrease background noise. GFNB is used to keep the amp in a linear, non-distorted state. Not reduce noise.


I don't want to put words in t-luxe's fingers (?). T-luxe only commented on the value. The reducing noise thing came from what I read elsewhere where someone said the lack of NFB contributed to noise. Since that was one of the few tweaks to the 68 custom circuit, and the 68 custom is noisier than the DRRI I was asking if putting the NFB value back to DRRI levels would help. I definitely agree lead dress, grounding issues, and tubes all play a role.


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Post subject: Re: '68 Deluxe Reverb problems...
Posted: Mon Mar 10, 2014 4:56 pm
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polishbroadcast wrote:
BMW2002Ti wrote:
Wow, I don't know how he came up with the idea that decreasing the feedback resistance in GNFB circuit would decrease the noise. Yes, you increase the GNFB voltage, but this does not necessarily equal decrease background noise. GFNB is used to keep the amp in a linear, non-distorted state. Not reduce noise.


I don't want to put words in t-luxe's fingers (?). T-luxe only commented on the value. The reducing noise thing came from what I read elsewhere where someone said the lack of NFB contributed to noise. Since that was one of the few tweaks to the 68 custom circuit, and the 68 custom is noisier than the DRRI I was asking if putting the NFB value back to DRRI levels would help. I definitely agree lead dress, grounding issues, and tubes all play a role.


The guy that said that about the NFB (Blue Strat) is also currently a suspended member in that forum. Those guys actually prefer digital reverb to spring? To each his/her own I guess... :roll:

Art


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Post subject: Re: '68 Deluxe Reverb problems...
Posted: Mon Mar 10, 2014 5:38 pm
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@BMW: Thanks. Yes I have some clips but will record something and compare it to my Sweet Spot so you have a relative reference. There is a low hum from the reverb (expected) and a good amount of hiss at idle even with the volume at 0. My 71 DR had this too but the CDR is far worse. I suspect some of it is from the tubes but not all as I swapped in some relatively new ones.

Art1 wrote:
The guy that said that about the NFB (Blue Strat) is also currently a suspended member in that forum. Those guys actually prefer digital reverb to spring? To each his/her own I guess... :roll:

I'm not sure what either of those things have to do with the topic.

Here are other references of using NFB in relation to noise:
Quote:
http://www.thegearpage.net/board/showthread.php?p=9869084#post9869084
"To further reduce the hiss found on the Lux and increase your headroom you must add a NFB loop to the Lux."
Quote:
http://www.thegearpage.net/board/showthread.php?p=7993340#post7993340
"I just disconnected the NFB on my tweed champ. I did not like this mod. It did make the overdrive tone sound more raw. But my amp also got a lot noisier and the slight added rawness was not worth it."
Quote:
http://www.talkingelectronics.com/Download%20eBooks/Principles%20of%20electronics/CH-13.pdf
pg 336: "The principle of feedback is probably as old as the invention of first machine but it is only some 50 years ago that feedback has come into use in connection with electronic circuits. It has been found very useful in reducing noise in amplifiers and making amplifier operation stable."
Quote:
http://www.thegearpage.net/board/showthread.php?p=17423406#post17423406
"they've reduced the negative feedback loop so noise floor will go up automatically. I haven't seen a schematic yet but if they've reduced it enough to where the amp will break up a lot sooner then noise floor will go probably up significantly"

It could very well be this won't work on the Custom Deluxe Reverb, but that's what I'm trying to find out. If I had a resistor with the right value I would be trying it!


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Post subject: Re: '68 Deluxe Reverb problems...
Posted: Mon Mar 10, 2014 6:13 pm
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http://www.radioshack.com/product/index.jsp?productId=12554272&filterName=Type&filterValue=1%26amp%3B%23047%3B2-watt+resistors

http://www.radioshack.com/product/index.jsp?productId=12447359&filterName=Type&filterValue=1%26amp%3B%23047%3B2-watt+resistors

Take your pick. Pretty sure there is a local store. Although it says "web only" many of the stores have a small stock of the odd items.

Art


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Post subject: Re: '68 Deluxe Reverb problems...
Posted: Mon Mar 10, 2014 8:07 pm
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I'm still a little suspicious of this claim about noise and GNFB. Onset of OD and clipping, yeah. But general noise at low levels.... hmmm....

BTW... It's the ratio of the GNFB voltage to the grounding voltage that is important (it's a voltage divide network). Or the ratio of the GNFB resistor to the resistance-to-ground at the divide meeting point, that is important in most Fender amps. The traditional blackface DR used a 820-ohm GNFB resistor tied to a 47-ohm grounding resistor. Most BF 6L6GC amps use a 820/100-ohm voltage divide. So, simply changing one resistor may not accomplish what you desire (whatever that is) --- unless you know what is the other (grounding) resistor's value.

http://www.webphix.com/schematic%20heav ... _schem.pdf


In other words, the overall effect is a bit more complicated than simply one resistor. A good tech paper on this part of the amp's circuit was written by Randall Aiken. You can see some thought has gone into designing GNFB.


http://www.aikenamps.com/index.php/desi ... e-feedback


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Post subject: Re: '68 Deluxe Reverb problems...
Posted: Mon Mar 10, 2014 9:33 pm
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BMW2002Ti wrote:
may not accomplish what you desire (whatever that is)

Lowering idle noise.

BMW2002Ti wrote:
The traditional blackface DR used a 820-ohm GNFB resistor tied to a 47-ohm grounding resistor.

The 68 CDR changed that value to 1.5k, hence wondering if getting back to the DR spec would help. I recognize this isn't a modding community and most people don't have resistors laying around so I was just wondering if it would work.

It sounds noisy to me:
0:00-0:23 = Custom SFDR
0:23-0:46 = Sweet Spot
https://soundcloud.com/spiesteleviv/68- ... se/s-Kl814

The Sweet Spot isn't silent but it's about half the volume of noise and is a much more narrow frequency.


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Post subject: Re: '68 Deluxe Reverb problems...
Posted: Mon Mar 10, 2014 10:19 pm
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OK. It's worth a try. I suspect you won't gain too much with this change. As there are a number of portions of the amp that can induce noise. More than GNFB changes can attentuate. Modifying this part of the amp's circuit should not be look at as a panacea to cure noises generated within the amp. Also, GNFB changes doesn't come without a cost.

I think it's better to look for other areas responsible for the generation of noise and try to minimize them, before altering basic circuit designs.

BTW... your recording sounds like 120Hz hum, not random white noise. Like hiss. Hum is usually a product of inadequate filtering in the PSU, AC noise picked up by components & wiring in the amp, and/or poor grounding schemes or contacts. Maybe rearranging of the internal wiring, checking the grounding points, & eliminating ground loops are all that is need to reduce this problem. I would definitely rule this out, before altering any circuitry.

JMO. Good luck!


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Post subject: Re: '68 Deluxe Reverb problems...
Posted: Tue Mar 11, 2014 4:15 am
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BMW2002Ti wrote:
I'm still a little suspicious of this claim about noise and GNFB. Onset of OD and clipping, yeah. But general noise at low levels.... hmmm....

BTW... It's the ratio of the GNFB voltage to the grounding voltage that is important (it's a voltage divide network). Or the ratio of the GNFB resistor to the resistance-to-ground at the divide meeting point, that is important in most Fender amps. The traditional blackface DR used a 820-ohm GNFB resistor tied to a 47-ohm grounding resistor. Most BF 6L6GC amps use a 820/100-ohm voltage divide. So, simply changing one resistor may not accomplish what you desire (whatever that is) --- unless you know what is the other (grounding) resistor's value.

http://www.webphix.com/schematic%20heav ... _schem.pdf


In other words, the overall effect is a bit more complicated than simply one resistor. A good tech paper on this part of the amp's circuit was written by Randall Aiken. You can see some thought has gone into designing GNFB.


http://www.aikenamps.com/index.php/desi ... e-feedback


From what I can make out from the schematic, it's still tied to a 47 ohm grounding resistor.

Art


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Post subject: Re: '68 Deluxe Reverb problems...
Posted: Tue Mar 11, 2014 2:58 pm
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Thanks BMW2002Ti for your other suggestions. I've read all 6 pages of this thread—including tangents about how the amp is digital (?!) and suing Fender—but there were no practical suggestions on how to address the idle noise, but maybe because it's not widespread. Your brainstorming is helpful. I think it's both hiss (white noise) and hum.

BMW2002Ti wrote:
In general, baseline hiss and hum which does not vary with volume control settings, is a PSU issue.


That makes sense. That would be new tubes and new / better filter caps which goes into swapping components—something I don't want to do on a new amp. It's too much troubleshooting for a brand new amp so it's likely getting returned which is a real shame since the tone is outstanding. If I get my hands on new tubes and a 1.8k resistor I'll do a quick clip-in test. I can also check out the lead dress and make sure all the grounds are solid.

If other people are having noise issues and/or have suggestions please share.

Thanks.


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Post subject: Re: '68 Deluxe Reverb problems...
Posted: Tue Mar 11, 2014 3:17 pm
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polishbroadcast wrote:
...it's likely getting returned which is a real shame since the tone is outstanding.


I have high hopes for the forthcoming (?) Vibrolux Reverb re-issue. I just hope this hiss/hum syndrome isn't endemic throughout the entire silverface line.

:shock:

Arjay

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Post subject: Re: '68 Deluxe Reverb problems...
Posted: Wed Mar 12, 2014 5:55 am
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this is what bums me...when you see these issues, you don't have a way to know if this is a small percentage of amps having this problem, or a major flaw.

in my 30 plus years of sales, I have sold TV's and appliances (among other things). I can tell when a customer has read a consumer magazine or read online reviews by the questions they ask. I tell them to take them with a grain of salt, as typically reviews are written by people unhappy with their purchase. those who love their product tend not to post.

I will look at reviews when I get my next amp, but they won't make 100% of my decision.


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