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Post subject: Hot rod deluxe problem
Posted: Sun Jul 14, 2013 4:33 pm
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Hi, I was reading through some mods that were recommended by justin holton on his site about hot rods and came across a mod to remove some of the cathode bypass caps to increase the headroom. I decided to try it so I clipped C1 to lower the clean gain and C8 in the drive channel. I decided to replace C1 back into the board but now the amp is not producing sound, even after it was removed again. I previously biased the tubes to about 82 but the multimeter showed only about 46, and the tubes glowed dimly. It doesn't appear that I severed any other connections in the amp but theres obviously a problem in the preamp. Any suggestions? Id contact justin himself but his email was removed from the site. Thanks.


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Post subject: Re: Hot rod deluxe problem
Posted: Sun Jul 14, 2013 7:14 pm
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jazguitar,

It never fails to amuse me, what 'tips" are on the Net. The function of the bypass cap of V1 and all other gain stage tubes (the 9-pin minis) is to increase bass frequencies, by lowering the effective "elbow" or cutoff frequency. By removing them, you decrease the bias point of the tube and raise the cutoff frequency. To do this mod properly, you need to change the cathode tied resistor's value & maybe even the anode plate voltages. Then, check how changing one tube's idle point effects the other tubes response in the pre-amp. You'll definitely lower the voltage swing (removing the bypass cap). This may have an adverse effect on the pre-amp's overall gain.

Now... changing the gain stage bias point (12AX7 or 12AT7) should have little effect on the output tubes' current draw at idle (6L6GCs) or their idle point --- at least in all the Fender amps I've worked on. There are some complex hi-fi amps -- like Audio Research or McIntosh -- that have a 12AT7 or 6CG7 acting as a servo, which helps control the bias of the output tubes during operation. Never seen this design in a guitar amp.

You must have altered the 6L6GC bias point, separate from removing 12AX7 bypass caps. Maybe, go back over ALL of the mods you did to the amp.

Good luck!

One other note: those bypass caps have polarity. Sometimes the cap companies mark the negative end. Sometimes, the positive. If reversed, you effectively short out the cathode-to-ground point. Which may lead to a damaged tube. And no sound.



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


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Post subject: Re: Hot rod deluxe problem
Posted: Sun Jul 14, 2013 8:32 pm
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Thanks for the help. I dont think that any other mods would have changed the power amp because I biased right before trying to replace C1. Is there any way you could show me a picture of what each side of the board should look like in that area? Im not nearly as good with diagrams as I should be...


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Post subject: Re: Hot rod deluxe problem
Posted: Sun Jul 14, 2013 9:27 pm
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Check the bottom of the link I posted. It has the circuit board layout.


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Post subject: Re: Hot rod deluxe problem
Posted: Sun Jul 14, 2013 11:35 pm
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jazguitar wrote:
...I decided to replace C1 back into the board but now the amp is not producing sound...


Did you do this without removing the circuit board? If you removed the board, did you reconnect the green ground wire on the right? Maybe you accidentally unplugged another connection on the board? Maybe you disturbed another joint near C1, or broke a connection or wire on one of the ribbon cables.

Lots of possibilities, look at all of these things, but obviously something happened when you tried to replace C1.

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Post subject: Re: Hot rod deluxe problem
Posted: Mon Jul 15, 2013 4:02 am
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Image

Heres the board from the back. I did remove the board but reconnected the ground. However, I think the problem lies with the third wire from the ribbon, which is now not connected to anything. Should it be wired to the resistor located above it? Any other obvious problems I missed?


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Post subject: Re: Hot rod deluxe problem
Posted: Mon Jul 15, 2013 5:09 am
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jazguitar, your photo shows definite tracing damage on the board. You are going to have to look at the layout and solder jumpers to fix this issue.

BTW... I do not desolder & pull components out of the board. esp thin, flimsy cicruit boards. I cut the component leads very close to the body of the component (like a cap). This way the leads are left soldered to the board.

Then, I can twist the new component's wire onto this leftover lead. I call it "pigtailing." Solder the leads together & add heat-shrink insulation to help secure the pigtailed leads.

This way you don't heat the board and risk damage when pulling parts off of it.


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Post subject: Re: Hot rod deluxe problem
Posted: Mon Jul 15, 2013 7:21 am
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BMW2002Ti wrote:
jazguitar, your photo shows definite tracing damage on the board. You are going to have to look at the layout and solder jumpers to fix this issue.

BTW... I do not desolder & pull components out of the board. esp thin, flimsy cicruit boards. I cut the component leads very close to the body of the component (like a cap). This way the leads are left soldered to the board.

Then, I can twist the new component's wire onto this leftover lead. I call it "pigtailing." Solder the leads together & add heat-shrink insulation to help secure the pigtailed leads.

This way you don't heat the board and risk damage when pulling parts off of it.


+1 If the lead is long enough, you can use a small alligator clip as a heat sink to help protect the board from excessive heat damage too.

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Post subject: Re: Hot rod deluxe problem
Posted: Mon Jul 15, 2013 11:37 am
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That will rarely happen if you use proper technique and proper tools. :idea:

Looks like you used the gorilla technique to remove that cap. :lol:

I have used a Weller soldering iron and a cheap Radio Shack de-solder iron with the built in bulb, for years without having any pads lifting. I sometimes use solder wick but it can lift pads like that as it has to be on there for a longer time. I recently got one of these (pic below) just to try, and it is surprisingly very good. I found one on Amazon for about $192.00 including shipping/tax. For temperature, 350C for soldering, and 400C for de-soldering makes for a quick and neat job, no trace damage. I've used Xytronic stations quite a bit previously, also a very good station and they do a great job, but cost a bit more. Yes, the resistor above the ribbon cable connects to the ribbon cable connection at the point where the cap lead was.

Image

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Post subject: Re: Hot rod deluxe problem
Posted: Mon Jul 15, 2013 4:25 pm
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Shimmy,

Like you stated, I find that most boo-boo's happen on circuit boards, where the original component is inadequately desoldered, before removal. Nothing really beats a good sucker, in terms of timely effectiveness. I do a lot of these amps --- so the Hakko 808 de-soldering gun is worth the $$$.

But, a manual desolder tool is certainly better than nothing.

http://www.amazon.com/Aven-17537-Desold ... esoldering


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Post subject: Re: Hot rod deluxe problem
Posted: Mon Jul 15, 2013 8:13 pm
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I read a lot of good reviews about the Hakko, and almost got one at Fry's, but I really wanted a station with solder and de-solder in one unit. The Weller stations are like $1200 or more, and good used ones are rare and still in the $600 range, so I tried the Aoyue and am pleased with it so far. I still want a Weller though, and might get one anyway.

But you are correct, my cheap Radio Shack de-soldering iron isn't half bad, and beats solder wick most of the time.

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