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Hoffman Amps Forum image Author Topic: MOD KIT NOISES?  (Read 8778 times)

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Offline jcr

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MOD KIT NOISES?
« on: April 10, 2011, 05:33:34 pm »
I used the search feature and I read "How to trace out a circuit problem." I just thought I would see if someone had a idea of what may be wrong and I could get a head start. Off to trace the problem.

Regardless of whether or not the guitar is plugged in at about half volume and more the amp has an oscillation noise like a helicopter. Other than that it works good.
Any ideas?
I included a schematic.

Thanks in advance.
« Last Edit: April 10, 2011, 05:51:49 pm by jcr »

Offline RicharD

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Re: MOD KIT NOISES?
« Reply #1 on: April 10, 2011, 06:33:37 pm »
If turning the volume up causes motorboating, I'd suspect a leaky cap or miswire in the tone stack.  Measure for DC on the secondary sides of the tone stack cap.  It should be zero volts DC.

Offline sluckey

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Re: MOD KIT NOISES?
« Reply #2 on: April 10, 2011, 07:04:27 pm »
It's a bad idea to share cathodes on consecutive gain stages. Try using a separate resistor/cap for V1A and V1B.
A schematic, layout, and hi-rez pics are very useful for troubleshooting your amp. Don't wait to be asked. JUST DO IT!

Offline octal

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Re: MOD KIT NOISES?
« Reply #3 on: April 10, 2011, 07:50:00 pm »
It's a bad idea to share cathodes on consecutive gain stages. Try using a separate resistor/cap for V1A and V1B.


+1. That is very poor design, just asking for instability. Double the value of the cathode resistor and separate 'em with individual bypass caps.

You could also have a grounding or layout issue.

Offline PRR

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Re: MOD KIT NOISES?
« Reply #4 on: April 10, 2011, 07:59:17 pm »
> about half volume and more

How can they sell that? When tone and Volume loss is less than first stage gain (plus a fudge) it is nearly sure to motorboat. With Fenderish tonestack, the shared cathode cap has to be enormous to shift the common-cathode bypass effect far below the tonestack passband. Agree: use TWO 1.5K cathode resistors and TWO 22u caps.

Offline RicharD

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Re: MOD KIT NOISES?
« Reply #5 on: April 10, 2011, 08:06:12 pm »
I don't like to trash talk other people's designs but......  They put the filter caps on top of the chassis.  Extremely dangerous IMO.

Offline jcr

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Re: MOD KIT NOISES?
« Reply #6 on: April 10, 2011, 08:19:50 pm »
I fixed it!!! I had a resistor leg touching just a bit that I couldn't see until I traced the circuit and used a magnifier.
I am attaching a pic of the tone stack that I am using.
Can you use paint and modify it to show me what you are talking about replacing?

I don't know how they sell it? I am still trying to understand a lot of what you guys talk about here. I have the Gerald Weber DVD set and Understanding Tube Amp Books.
I built an STF Champ head a year or so ago. I bought this because one I wanted a basic clean amp that I could use pedals with and two it was inexpensive $395.00 plus S&H.
Eventually I would like to build a few amps and maybe even sell a few, just to support the hobby them but for now I am just learning.
I would really like to learn repair.


Thanks, I appreciate all of the help you fellows give here.
 

Offline jcr

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Re: MOD KIT NOISES?
« Reply #7 on: April 10, 2011, 08:32:13 pm »
UHHHHHG ! Although the touching resistor wasn't good it is still doing it... :cussing:

Offline sluckey

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Re: MOD KIT NOISES?
« Reply #8 on: April 10, 2011, 09:40:34 pm »
...
A schematic, layout, and hi-rez pics are very useful for troubleshooting your amp. Don't wait to be asked. JUST DO IT!

Offline DummyLoad

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Re: MOD KIT NOISES?
« Reply #9 on: April 10, 2011, 10:40:38 pm »
use a separate resistor and cap... or make 22uF bypass cap HUGE - like hundreds of uF.

don't like to belittle anyone's work, but what's copyrightable on that?  - except the lousy design of the first two stages and slight change of tone stack values, everything else smells of normal channel of AB763 - single 1N4007s in PS are iffy design, 41uF is pretty low budget for SS supply, so is the 22uF cap that shorts to gnd via 1ohm every time the stdby switch is toggled on. on the upside, they did spec some nice iron.

--DL

Offline PRR

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Re: MOD KIT NOISES?
« Reply #10 on: April 10, 2011, 11:11:10 pm »
> what's copyrightable on that?

The drawing.

Offline DummyLoad

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Re: MOD KIT NOISES?
« Reply #11 on: April 11, 2011, 12:14:40 am »
> what's copyrightable on that?

The drawing.

so then i can copyright anything i draw - even if it's patterned from another design? i don't doubt your response, but that just doesn't seem right.


Offline VMS

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Re: MOD KIT NOISES?
« Reply #12 on: April 11, 2011, 04:18:25 am »
What is the idea on the POWER switch?

Offline jcr

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Re: MOD KIT NOISES?
« Reply #13 on: April 11, 2011, 07:18:01 am »
Thanks for the drawing Sluckey.
Wednesday when I get back I am off to Radio Shack to get the parts to redo this section.
Hopefully this will fix it. If not I'll continue to look. The impression I am getting here is that the mod of two 22uf caps and two 1.5k resistors would be a definite improvement to the circuit even if motor-boating wasn't an issue.
I did trace the circuit layout and all of the connections appear to be correct. I don't have a pdf of the layout or I would let you see that.

I noticed some other issues in question here, e.g. Dummyload speaking of 41 uf in the power supply and single 1n4007s. After I get the motor-boating issue under control I would like to look into some improvements and hear some suggestions in these areas as well.

Thanks again

Offline alerich

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Re: MOD KIT NOISES?
« Reply #14 on: April 11, 2011, 08:22:54 am »
I bought a Pepco (Pine Electronics) Riviera 725 head to use as a mod platform. Sort of a 60's Canadian Fender Deluxe. Two 12AX7's and 2 6V6's. Anyway, both gain stages and the PI were powered off the same PS node. lt didn't motorboat but probably only because it was not a real high gain amp. Really iffy design. I have read that this company sold stencil type amps that were the Canadian equivalent of the inexpensive Montgomery Wards style guitar amp sold here in the States way back when.

The real mystery was that the power rail had an unused node right before this 4-banger. I originally thought that perhaps this amp was mis-wired and that the other node should have been for the PI but I eventually found another hand drawn schematic online of the same amp - same power supply wiring.
Some of the most amazing music in history was made with equipment that's not as good as what you own right now.

Offline frus

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Re: MOD KIT NOISES?
« Reply #15 on: April 11, 2011, 08:56:54 am »
What is the idea on the POWER switch?

when you switch the amp off, it connects the resistor to the power supply cap, so the cap is drained

Offline RicharD

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Re: MOD KIT NOISES?
« Reply #16 on: April 11, 2011, 10:27:25 am »
A 1 ohm drain resistor?  Wire that switch wrong and you're in for a surprise.

Offline jeff

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Re: MOD KIT NOISES?
« Reply #17 on: April 11, 2011, 11:07:34 am »
My 2 cents: I don't like having the bias supply only off the wiper. I much prefer the marshall type where it's wired as a variable resistor. If the wiper losses contact for some reason- no bias, With a varible resistor set up, if it losses contact you still have bias voltage. I know most Fenders are like this but I always feel if something can go wrong it will.
Just my opinion.

Offline jhadhar65

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Re: MOD KIT NOISES?
« Reply #18 on: April 11, 2011, 12:36:52 pm »
>so then i can copyright anything i draw - even if it's patterned from another design?

You can 'register' it with the USPTO (or your country's equivalent)... copyright is automatic.  Patent infringement based on the design and how you market it is another matter.  The drawing itself is not the circuit.

Offline jcr

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Re: MOD KIT NOISES?
« Reply #19 on: April 11, 2011, 02:14:35 pm »
I had sent MOD support the problem when I posted it on this board as well. I just got a response and guess what it was.

If you have thoroughly double-checked your connections and your voltages match up closely with the troubleshooting guide, then you should be able to eliminate the problem by splitting up the V1 preamp tube's shared cathode (i.e. both triodes of the 12AX7 are biased using the same cathode resistor and bypass capacitor).  The shared cathode can cause a low frequency oscillation at high volume + high bass settings in some amps.   

Please keep us posted if you need further assistance.

Thanks,

Kits


 :l2:

You fellows are good, maybe you should offer kits.

Oh well, if it wasn't giving a problem I wouldn't be learning anything.

Thanks again, I'll update when I get to have a go at repairing it.

Offline alerich

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Re: MOD KIT NOISES?
« Reply #20 on: April 11, 2011, 08:14:24 pm »
A 1 ohm drain resistor?  Wire that switch wrong and you're in for a surprise.

Even if you wire it correctly - isn't that side of the switch gonna arc like crazy each time you shut it off? Reminds me of the time the "smart guy" in electronics class showed us how to discharge power supply caps with a screwdriver blade. I think they're still peeling him off the ceiling.

Edited to add: Not to mention that it already has 220K bleeders on the PS caps. Yeesh - that's scary.
« Last Edit: April 11, 2011, 08:18:27 pm by alerich »
Some of the most amazing music in history was made with equipment that's not as good as what you own right now.

Offline jcr

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Re: MOD KIT NOISES?
« Reply #21 on: April 12, 2011, 06:13:13 am »
I see no evidence of arcing when turning the switch off. Should the 220k bleeder caps values be changed? If so what value? What value should the 1 ohm drain resistor be? What about the 41 uf in the power supply and single 1n4007s? should they be changed? Changing the bias to a variable resistor? I am getting a lot of greatly appreciated critiques of the circuit, however if solutions aren't spelled out it isn't of any use to me. Maybe there isn't a solution for some of the problems without a major rework, but if the amp ends up sounding as good as it does now without the motor boating then it will be what I am looking for. One day as my knowledge and budget grows I would like to build one of the Allen amps or maybe a Mojo Kit, but this is what I can afford right now so I plan on making it work. So keep the suggestions coming if you see something I can improve within reason I'll be trying it.

Thanks again.   

Offline sluckey

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Re: MOD KIT NOISES?
« Reply #22 on: April 12, 2011, 07:07:22 am »
Quote
Should the 220k bleeder caps values be changed? If so what value?
No. Those resistors serve two purposes. They equalize the charge across those 80uF caps and they bleed all the filter caps when the power is off.

Quote
What value should the 1 ohm drain resistor be?
Completely remove the 1 ohm resistor. That's just a bad idea. The 220K bleeder resistors do the same thing (only slower) without putting any stress on caps or switch.

Quote
What about the 41 uf in the power supply and single 1n4007s? should they be changed?
Two 80uF caps in series is 40uF, not 41uF. The comment was based on the fact that with silicon rectifiers you 'could' use much higher values for better filtering. However, IMO, better filtering is not needed. A 1N4007 is rated at 1000PIV. Using a single diode with that PT exceeds the 1000PIV rating. Use two diodes in series in each leg (total of 4 diodes) to safely provide a 2000PIV rating.

Quote
Changing the bias to a variable resistor?
That's just a personal preference (mine too). Your circuit is the same as a typical Fender circuit and has been used for 60 years with little or no problems. Leave it as is.

Quote
I am getting a lot of greatly appreciated critiques of the circuit, however if solutions aren't spelled out it isn't of any use to me. Maybe there isn't a solution for some of the problems without a major rework, but if the amp ends up sounding as good as it does now without the motor boating then it will be what I am looking for.
IMO, this amp has several poorly planned ideas. Your motorboating problem may be a result of one of them. Have you fixed that yet?
A schematic, layout, and hi-rez pics are very useful for troubleshooting your amp. Don't wait to be asked. JUST DO IT!

Offline VMS

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Re: MOD KIT NOISES?
« Reply #23 on: April 12, 2011, 08:16:27 am »
Because the plate voltage is taken from node B, I think there is enough filtering there.

This is how I would modify the power supply:
(...and maybe move the fuse other side of the switch)


Offline alerich

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Re: MOD KIT NOISES?
« Reply #24 on: April 12, 2011, 08:46:35 am »
jcr, I wasn't trying to make light of your amp and hope my comments didn't offend you. sluckey is spot on about removing the 1 ohm resistor. It's almost like putting a dead short across the main filter caps every time you shut the amp off. It will definitely discharge the caps but it's not a good idea. Better to remove it.

On the motorboating try installing separate cathode resistors and bypass caps on the cathodes of the two V1 nodes as suggested first and see if that helps. If it is still motorboating then creating one more power supply node (another filter cap and dropping resistor) to supply each V1 plate separately may help. Try the cathodes first - no point in getting ahead of ourselves.



Some of the most amazing music in history was made with equipment that's not as good as what you own right now.

Offline jcr

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Re: MOD KIT NOISES?
« Reply #25 on: April 12, 2011, 01:59:02 pm »
No really I don't think any of you are making light of the amp and no offense was taken. I know that I have a relatively inexpensive amp kit although it seems the parts are of reasonable quality and may just be the design that is cheap. I really do appreciate the critiques of the circuit. I realize some are just personal preference and some are things that need serious consideration. I hope to get the parts this week. I think the shack will have most of what I need.

1. I plan to install separate cathode resistors and bypass caps on the two V1 nodes.
2. remove the 1 ohm resistors.
3. Take a closer look at the power switch mod and get a little more info on why and how this will benefit the situation as opposed to what I have now.

Thanks, I appreciate it.

Offline PRR

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Re: MOD KIT NOISES?
« Reply #26 on: April 13, 2011, 12:24:04 am »
> i can copyright anything i draw - ....

ANYthing you CREATE is copyright at birth.

> even if it's patterned from another design?

Yes, even if you plagiarize, YOUR pen-strokes and layout proportions are YOUR creation, and your copyright. This may do you no good, since you may not be able to sell copies without an objection from the work yours is based on.

I draw Mickey Mouse. My specific drawing and exact (xerox, photo) copies are my creation. Now you copy my drawing. If you use "any" creativity, you got your own copyright. If you make a literal copy of my work, I can call "infringement!" on you. But when we go to court and word gets around. Mickey's lawyers jump on me and you, and neither of us will be making copies of any Mickey.

Actually that's a bad example. Mickey would have to claim trademark infringement (unless I zeroxed him).

(That gets into the hard-cold facts of copyright law: the bigger mouse gets the cheese.)


Oddly, circuits can not be copyright. There may be heavy creativity, but also nearly everything has been done before. I feel this is a murky area which should be handled better, but nobody has agitated to get it changed.

A xerox copy of a schematic is copyright infringement.

A re-drawing of a schematic is not infringement.

A novel circuit with a description of benefits (and the fee) may be patented. Many audio patents could be argued and invalidated, but it is usually easier to evade (do it similar but different from the patent claims). Probably less than 1% of audio patents sell enough to cover the patent fees.

> that just doesn't seem right.

What is "right"? It's not in the Ten Commandments (or whatever stones you accept as authority). "We" (especially the bigger mice) decide what is "right".

Thomas Jefferson was opposed to patents, at first. Partly because english kings would grant letters of patent on stupid things like candy confections. Later Jefferson realized that protected public disclosure of socially useful inventions was better than inventors keeping secrets because that's the only way they could profit.

Charles Dickens wrote stuff, was paid on publication. Most stuff published is drek, but his stuff got re-printed a LOT without payment. He died poor and his widow was broke. Someone said "this is not right" and got copyright laws enacted. At first for limited time, but Mickey and friends agitated for more liberal terms. Anything copyright today will probably be copyright forever, though Moore's Law may undermine the idea of "copy".

US trademark law is such a mess, proof that "right" is a very fluid concept. In the US, trademark was mostly state law but global trade agreements may force some rationalization.



> Completely remove the 1 ohm resistor

I wonder if it is already "removed"? 400V, 1 ohm, 400 Amps.... 160,000 Watts. Much less after only 1/25 of a millisecond, but no sane-size resistor has surge rating that large.

« Last Edit: April 13, 2011, 12:26:43 am by PRR »

Offline jcr

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Re: MOD KIT NOISES?
« Reply #27 on: April 13, 2011, 06:06:32 pm »
Hooray !!! The cathode cap and resistor stopped the motor boating. Sounds great.
Now that 1 ohm resistor. I just need to replace them with a wire right?
The 1 ohm resistors on V3  and V4 are connected to pin 1 & 8 on each and then to a ground lug.

Offline RicharD

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Re: MOD KIT NOISES?
« Reply #28 on: April 13, 2011, 06:26:11 pm »
The 1 ohm resistors under the cathodes of the output tubes are fine.  Makes measuring bias current easy.  The 1 ohm resistor that's part of the standby switch should be removed.

Offline jcr

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Re: MOD KIT NOISES?
« Reply #29 on: April 13, 2011, 06:57:43 pm »
Oops :embarrassed: :, I misunderstood, off to take care of that.

Offline sluckey

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Re: MOD KIT NOISES?
« Reply #30 on: April 13, 2011, 07:08:45 pm »
Now that 1 ohm resistor. I just need to replace them with a wire right?
The 1 ohm resistors on V3  and V4 are connected to pin 1 & 8 on each and then to a ground lug.
WHAT! The only 1Ω resistor on that schematic is connected to the power switch. That's the only 1Ω resistor that's been mentioned 'til now. Clip it out. Don't replace it with anything! There ain't no 1Ω resistors on V3 and V4 (should be though. Just another example of the poor design of this amp.)

I can't believe the makers of this amp sold it as is and later advised you of the fix. They should have at least included an errata sheet with the kit, but preferrably just fix the documentation and the circuit. That's just poor cheap ass work. Who sells this stuff anyhow? Got a web address?
A schematic, layout, and hi-rez pics are very useful for troubleshooting your amp. Don't wait to be asked. JUST DO IT!

Offline Inca Roads

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Re: MOD KIT NOISES?
« Reply #31 on: April 13, 2011, 07:53:41 pm »

I can't believe the makers of this amp sold it as is and later advised you of the fix. They should have at least included an errata sheet with the kit, but preferrably just fix the documentation and the circuit. That's just poor cheap ass work. Who sells this stuff anyhow? Got a web address?


That kit is manufactured by CE dist and available through all of their retail outlets, which include AES, Amplifiedparts.com, and the site the OP purchased it from - modkitsdiy.com.  You would think given their experience that they could come up with something better.  I think that the 1 ohm resistor standby thing must be a misprint and maybe should have read 1K or something. 


JCR - glad to hear you got it working.  Now that you have an amp to tinker with, I suggest that you spend some time learning what makes it tick by continuing your amp education via this forum and reading some applicable books, like this one for example : http://www.diyguitarist.com/Misc/J_Darr.htm  so that when you are ready to tackle your next project you could source the parts yourself.  It's not hard to do, you will save a little money compared to buying a kit, and it's more fun I think.  Plus, if you are married you can trickle the parts in so it flies under the radar  :icon_biggrin: Doug has lots of great layouts and schematics and his customer service is second to none.

Offline jcr

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Re: MOD KIT NOISES?
« Reply #32 on: April 13, 2011, 08:09:13 pm »
It works great now !!! I appreciate the link to the site on learning more about amps. Yes, one would think that with law suits as rampant as they do more safety instructions would be included. It doesn't even tell you how to discharge the caps in the amp before attempting to work on it. I am glad I have the Weber videos to know how to do it.
The amp sounds great, I appreciate the help and I'll keep on learning.

Offline alerich

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Re: MOD KIT NOISES?
« Reply #33 on: April 13, 2011, 09:09:02 pm »
jcr - That was the intended purpose of that 1 ohm resistor that you (hopefully) removed from the one side of the power switch. The design was to discharge the power supply caps and while it did that - it did so in a very poorly designed way. A 1 ohm resistor is almost like a piece of straight wire. The Weber video probably does not instruct you to use a 1 ohm resistor. A larger resistor would have been more appropriate but since you know how from the Weber video it is not needed at all and should be left out. Never make things any more complicated than they need to be. Less things to go wrong.

Glad you solved your problem. I listened to the You Tube clip of that amp. Pretty nice sounding little rig. I am more of a higher gain guy myself but for a nice clean amp with a reasonable bite at higher volumes it sounded pretty good.
Some of the most amazing music in history was made with equipment that's not as good as what you own right now.

Offline jcr

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Re: MOD KIT NOISES?
« Reply #34 on: April 14, 2011, 06:25:04 am »
The only 1Ω resistor on that schematic is connected to the power switch. That's the only 1Ω resistor that's been mentioned 'til now. Clip it out. Don't replace it with anything! There ain't no 1Ω resistors on V3 and V4 (should be though. Just another example of the poor design of this amp.)

I was just reviewing the responses this morning and that's pretty bad isn't it. There actually is a 1 ohm resistor on both V3 and V4, but according on the schematic.

Offline jcr

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Re: MOD KIT NOISES?
« Reply #35 on: April 14, 2011, 06:36:29 am »
jcr - That was the intended purpose of that 1 ohm resistor that you (hopefully) removed from the one side of the power switch. The design was to discharge the power supply caps and while it did that - it did so in a very poorly designed way. A 1 ohm resistor is almost like a piece of straight wire. The Weber video probably does not instruct you to use a 1 ohm resistor. A larger resistor would have been more appropriate but since you know how from the Weber video it is not needed at all and should be left out. Never make things any more complicated than they need to be. Less things to go wrong.

Glad you solved your problem. I listened to the You Tube clip of that amp. Pretty nice sounding little rig. I am more of a higher gain guy myself but for a nice clean amp with a reasonable bite at higher volumes it sounded pretty good.


Weber video uses a jumper from v1 pin 1 to ground then wait a couple of minutes for it to drain and then check.

Although not high gain the supposed BF version I built sounds a lot different to me than the MOD DIY video. I realize that none will have the girth of actually being there because of recording and our computer speakers is present but the flavor and sound of the one I just finished sounds a lot more like this one http://www.youtube.com/watch?v=_N8wNmi9B40 when cranked. If I need more gain I use a pedal, but since I play oldies and country now instead of Maiden and Priest like I did in the old days this type amp works well. I can use a pedal and an eq if I want to go back or better yet, build another amp, lol.

Offline HotBluePlates

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Re: MOD KIT NOISES?
« Reply #36 on: April 14, 2011, 07:16:42 pm »

Weber video uses a jumper from v1 pin 1 to ground then wait a couple of minutes for it to drain and then check.


Yes, but...

What some don't realize at first is that the plate load resistor is in series with that jumper to ground (so see it as 100k), plus any dropping resistors in the power supply (maybe anything from 1k to 50k). So there is a largish resistance to ground, they're just using existing resistors in the circuit in a way that's not obvious at first.

Sluckey already told you right about this issue earlier: you already have bleeder resistors across your filter caps, which do the exact same thing (and with no effort on your part). The jumper is only needed if the amp in question had no bleeder resistance.

Offline jcr

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Re: MOD KIT NOISES?
« Reply #37 on: April 14, 2011, 07:31:55 pm »


Yes, but...

What some don't realize at first is that the plate load resistor is in series with that jumper to ground (so see it as 100k), plus any dropping resistors in the power supply (maybe anything from 1k to 50k). So there is a largish resistance to ground, they're just using existing resistors in the circuit in a way that's not obvious at first.

Sluckey already told you right about this issue earlier: you already have bleeder resistors across your filter caps, which do the exact same thing (and with no effort on your part). The jumper is only needed if the amp in question had no bleeder resistance.

I guess I just incorporated it into my method as a redundant safety precaution, but it is nice to know that it is already taken care of on this amp.

Offline RicharD

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Re: MOD KIT NOISES?
« Reply #38 on: April 14, 2011, 11:19:31 pm »
My theory behind they fast drain resistor is because they put the power supply caps on the outside of the chassis and they're hoping you'll shut the amp down before removing the cage.  I'm inclined to agree with PRR's math, that resistor will eventually open itself up.


Offline jcr

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Re: MOD DIY KIT NOISES and Other Issues Update
« Reply #39 on: August 19, 2016, 09:16:28 am »
Thanks to the suggestions and the instruction of the good folks on this forum this amps still performing well. Literally performing with our oldies band this weekend at Lake Lure, Dirty Dancing Festival. I reduced the negative feedback and use triode Yellow Jackets for lower stage volume most of the time. I get compliments on the sound quite often too. :icon_biggrin:

Thanks again

Offline PRR

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Re: MOD KIT NOISES?
« Reply #40 on: August 19, 2016, 02:19:56 pm »
Thanks for the happy follow-up.

 


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