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 Post subject: Danelectro guitar amp
PostPosted: Dec Wed 20, 2017 12:57 am 
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Location: Chicago, IL
Hi! I got a 1950's Danelectro "special" guitar amp working the other day. I had it for quite awhile, and have recapped it. I replaced the electrolytics with new ones equivalent to the original capacitance values, as well as the bypass cap and all other caps with the right values. I left the .02 tone cap and the other.02 caps that are in the tremolo circuit alone, as they don't have any voltage across them. I changed one .02 in that circuit that has about 100 volts or so on it with a .02 cap. I also removed the "death cap" that was from the AC line to ground. I did a nice job recapping, and put shrink wrap on al my heads of the new parts. The amp plays fine through all inputs and the tremolo works. My question is that there is a noticeable hum in the speaker with the volume all the way down and nothing connected to the input. The shield under the chassis is installed and all connections are secure. I'm trying to figure out if this hum is normal or not for one of these. Should it be dead quiet, or is a small hum acceptable?

The chassis is NOT grounded, and yes I know I should ground it, and I will in the future. I tried connected a ground wire to the chassis and it made no difference. I'm trying to figure out where the hum is coming from. I tested all tubes and none show any H-K leakage. I wonder if it could be lead dress related? But that doesn't make much sense as I didn't disturb any wires. Thanks for your help! Schematic here: http://el34world.com/charts/Schematics/ ... pecial.pdf


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 Post subject: Re: Danelectro guitar amp
PostPosted: Dec Wed 20, 2017 1:55 am 
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Location: Silver Spring MD, USA
Lots of thoughts:

A small hum may be normal for your amp. Is it bothersome when playing? If not, I wouldn't worry about it. But if it's maddening, read on.

Does it hum with a guitar plugged in and the volume turned down? If the hum goes down with a guitar plugged it, the hum may be because the hot side of the input jack(s) isn't grounded. Looking at the schematic it appears that it uses a 2 contact jack rather than a shorting jack a la Fender. Sometimes that can cause hum even with the volume down, not always likely but worth checking. Just ground the hot side to the chassis, easy check.

To see if lead dress is an issue, use a chopstick to move leads while the amp is idling and see if the hum level changes. Especially play with the grid leads (see below).

Other suspects beyond that: grounds. Old amps are notorious for ground loops and just bad grounds in general. A lot of times I will redo grounds to follow a star ground scheme.

How long are the leads going from the input(s) to the tone, then the volume control and from the volume to the input cap and the grid of the 6SJ7? If the leads are relatively long, and not shielded, that is another source of hum and noise. Same for the leads from the preamp tube to the grids of the power tubes.

Finally, how are the filaments wired? If one side uses the chassis ground as a return, that can cause hum too.

-Kevin

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 Post subject: Re: Danelectro guitar amp
PostPosted: Dec Wed 20, 2017 2:19 am 
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Joined: Nov Mon 06, 2017 7:45 pm
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Location: Sherwood, Oregon
This probably has nothing to do with the hum
I'd put a Y2 safety cap in there where you removed the "death cap" (line to ground).
Plus, I don't trust those 6X5 rectifier tubes, especially if they're mounted sideways (not sure how yours is). They'll short out and fry the power transformer. Lots of people replace these with a couple diodes and a dropping resistor. You can also buy plug in replacements. Of course there's people who would poo poo everything I said.


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 Post subject: Re: Danelectro guitar amp
PostPosted: Dec Wed 20, 2017 5:35 am 
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I agree with Kevin. The hum you describe is probably normal for that circuit, as you haven't made any changes to it. The hum lets you know the amp is ready to rock. :lol:
Usually, the wiring for the tube heaters is installed first and all wires are routed close to the chassis. Keep the other leads away from those wires, especially around the input and first gain stage.
If plugging the guitar in lowers the hum level, you can assume that a grounding jack will help, easy fix.

You probably don't want to change the amp circuit too much but if you wanted to: that circuit is a bit archaic, as the weak signal from the guitar pickup goes through the tone and volume circuits before being amplified. Noise in the front end is amplified along with the guitar input. If you look at a late 50's Fender circuit, the 5E3 Deluxe, you see that the jack connects directly with a resistor to the grid of the first gain stage. Then the amplified signal goes to volume and tone controls, where the stronger signal drowns out the noise.

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 Post subject: Re: Danelectro guitar amp
PostPosted: Dec Thu 21, 2017 7:39 am 
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Location: Spring Hill, FL
Make sure the filament wires are twisted and as stated, close to the chassis, and away from other wires as much as possible. Particularly the line from the input jack to the first tube. You could change the line to shielded cable, but that also may not be the cause of the hum.

But also as stated, it could be a normal amount of hum. Can you shoot a video of it, and post it maybe on YouTube and link here? That might help everyone here determine if maybe it's just a normal amount of hum or something more.

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 Post subject: Re: Danelectro guitar amp
PostPosted: Dec Fri 22, 2017 5:35 pm 
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Location: Powell River BC Canada
Did you recap with attention to the outer foil ?

Also is the contact on the foot switch jack closing properly. Pull the 6SJ7 out of
the vibrato circuit and see if hum goes away.

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 Post subject: Re: Danelectro guitar amp
PostPosted: Dec Sun 24, 2017 10:34 pm 
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Joined: Apr Thu 14, 2016 8:25 pm
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Location: pensacola fl
Also try this for a test. Disconnect guitar and turn volume all the way down and unplug any foot switch. Now be very careful with this part. With clip leads add additional filter capacitors across the capacitors in the power supply. Here are some test values to try.
v1 60uf
v2 40uf
v3 20uf
of course at the same or higher voltage rating as you have in there. You can then unhook them one at a time to see the results. The v2 and V3 are feeding lower level circuits and therefore more sensitive to hum intrusion. The ripple at v1 will be fairly high due to the small value you have there but increasing this one may change the compression you want so to compensate increasing the other 2 may help you out. If you like it install the capacitor values that serve your needs. By the way be careful of the capacitors you disconnected make sure you discharge them through a resistor of about 10k 2 watts for safety. You do not want a hand full or hell the next time you grab the capacitor.


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 Post subject: Re: Danelectro guitar amp
PostPosted: Dec Mon 25, 2017 3:54 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Gainesville, Florida
that was my thinking. original power supply filters may be small allowing ripple that would be normal for this design. the rectifier may not be able to handle a lot of start up current from larger capacitors. I measure across each filter capacitor with an AC meter to see how much is passing


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