Hum help

[dscottguitars]

I was going for the same size transformer for the holes that are drilled, and price.  And, the one I have is I is 620v so the one I chose is only 40 more.  I'll do what you said about the resistance but I know nothing about any of what you said.  Thanks...

[1rebmem]

As far as the capacitance the 5U4 is seeing, nobody said there was a max, not in this thread.
I did point out what the data sheet say's.....40uF. And then I realized (and stated) that since your amp was obviously working, though with a bit of hum,  I doubted it was due to the 94uF.

I wouldn't concern myself with it because you said the thing is making good sound, just with hum.

If your new trans gives a bit high of B+ you can add some zener diodes to lower it. We can help you with that if needed.
I did the same thing on one amp, bought a PT that fit my existing chassis hole but used zener's to drop the B+.

[1rebmem]

To lower your B+, here is what I've done several times because it works and is cheap and easy, though others
will have other ideas. Google it.

Use some of these:  http://www.ebay.com/itm/151188819243?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

Use this calculator:  https://taweber.powweb.com/store/vdump.htm
Assuming you drop 40v and your amp is 40 watts = 6.4 watts

You install several series strung zener diodes in your PT secondary CT grounded leg.
Three of these 16v 5W zeners soldered in series between the CT wire and ground (diode cathodes to ground) will reduce all B+ voltages by about 35-40v.  It won't be 48 volts (3x16v). Solder them onto a solder strip without trimming any of their lead length to allow for heat dissipation.

Three in series leaves plenty of margin and gets the drop you desire if I calculate correct ?????  Check me......

[sluckey]

Be smart when using zeners (or resistors) in the HT center tap. If you have a negative bias supply that gets it's AC from a tap on the HT winding such as a Fender AB763, and you put a zener or dropping resistor in the CT, the negative bias voltage will be affected also.

[dscottguitars]

in the past I have put a low power switch in other amps by switching a 500 to 750 25W resistor from the standby to the first filter cap.  I've not heard of using the center tap.  Is there anything wrong with what I did?  Would a resistor on the center tap be better?

And, I also had to switch in different load resistors in the bias circuit to adjust for that.

[1rebmem]

Be smart when using zeners (or resistors) in the HT center tap. If you have a negative bias supply that gets it's AC from a tap on the HT winding such as a Fender AB763, and you put a zener or dropping resistor in the CT, the negative bias voltage will be affected also.

Good to post that, thanks.
I did check his schematic first to see if it was cathode biased. 

[DummyLoad]

First thing I noticed was that the noise level went down progressively as I turned up the volume from about 1/8 up to about 2/3 the way up and then got loud again going all the way up.  That seemed really strange.

maybe it's a ground loop between the source (in this case the PC?) and your amp?

if you're using a 3 wire power cord temporarily lift the ground with an adapter 3wire to 2wire adapter. if that does help, then you could lift your preamp circuits off chassis ground. have a look at a dynaco ST-70 for a visual on how-to - there are 2 x 10ohm resistors under the isolated RCA input jacks. link to schematic below...

http://diyaudioprojects.com/Schematics/images/Dynaco-ST70-Tube-Amp-Schematic.png

alternatively, you can use a diode arrangement as shown in the link below. scroll down about halfway for a schematic...

http://www.pmillett.com/dcpp.htm

--pete





 

[dscottguitars]

I decided to check my guitar amp and that transformer makes a slight humming noise too.  Is that normal???

This amp does not have a loud hum through the speakers though.  It has some low level buzz at full volume, but it's a very high gain amp.  The sound is just like the transformer noise but the resistor hiss is louder.

[dscottguitars]

Getting very frustrated!  Moving the volume did not fix that hum like I thought.  It still gets quieter up to 2/3 the way and then gets louder.  

I moved one ground wire.  Looking at the picture only the right side is hooked up-power supply too and no tubes in the left side.  The green wires are ground.  The all connect to the point where the large white resistor is at-the cathode resistor and then to the main ground behind the filter caps.  I moved that wire to where the preamp cathode resistors are and no change.  Those are the ones with the black bypass caps above them and the small green 'jumper wire' going the the white resistor.  I also tried unhooking the input ground but no change there either.

[dscottguitars]

DL:  I don't have an adapter but I could remove the ground from the chassis, would that be okay?  Also, what is PC?  And I don't understand that dynaco schematic with the 10 ohm resistors either.  thanks...

[DummyLoad]

Not when I have signal going into it.  As of now I plugged my computer into the RCA jacks. +  Also, what is PC?


PC = personal computer.

I don't understand that dynaco schematic with the 10 ohm resistors either

the 10ohm resistors break potential ground loops with source equipment because the input stages common of the dyanco are not at "ground" level. input circuits of the dynaco have a better path to ground via source system through the interconnect cables shield wire(s) than through the 10ohm resistors. usually source equipment have 2 wire mains so single path to wall ground or floating source alleviates ground loop hum. some pro gear have a ground-lift switch to alleviate interconnect ground loops.

interconnected systems ground loops:
in an "ideal" system to system interconnection all systems share a single path to wall ground.

please note this is my feeble attempt to explain the phenomenon of ground loops, so hopefully folks here that have a better grasp of the subject will chime in and clarify.

could remove the ground from the chassis...

yes. though not permanently i hope! :-)

question:

i think leevi was eluding to this; with the volume(s) set at minimum and speakers connected, what is the level of hum when have the inputs to shorted to ground then turn up the volumes to midpoint and then to full up? do you still hear hum at any level? how much? less or more than when the amp is connected to a source?

in a nutshell; if you hear an abnormally high level of hum (subjective) with the inputs shorted and volume at midpoint, then the hum is probably build/layout related. if you hear an acceptable level of hum (again, subjective) with the inputs shorted and volume at midpoint, then hum is likely caused by a ground loop between the source equipment and the amp.

nothing stands out in the pic. can we assume the motola hi-fi you cannibalized PT from was quiet and you stole PS and output stage plan from that? if yes, then this should just work..and your frustration is understandable.

good luck and keep poking at it. you'll find it.

--pete

[dscottguitars]

Thanks...

First:  I'm using the 1/4" jack-shorted for now.  I only use the RCA's when I test music from my PC.

The volume at half then is at an acceptable level, but even better at 2/3-maybe not that much different.  It is noisier with the RCA cables, but I think it's cable noise and not the hum because the 1/4" jack is much quieter.  I have not tried to short the RCA jacks to make a comparison, but think it shouldn't matter.  So there is no source hooked up when I'm checking the hum level.

I never played the motorola amp before so I don't know if it was quiet.

The power supply is my own, not taken from any schematic.

I hope this narrows some things down a bit.

[dscottguitars]

I made this amp, one of my first builds and it was very quiet.  It is one channel, volume and tone and one preamp tube before the phase inverter.  It was an EL84 PP amp.  As you can see the wiring was quiet messy, but I was learning and used an RCA chassis.

[dscottguitars]

I'm returning to this thread because I have made some changes and now I am confused once again.

First, I'm not sure if it makes a difference but I did not have the bottom cover on the power transformer so I got one and put it on.  It did not seem to make a difference.

Second, I unhooked the middle tube connections and bypassed it.  That helped.

Third I accidentally switched the power leads B and C and that helped tremendously, although I don't know why.  The hum is very low at low volume but gets too loud with the volume up.  At least that's better than before where there was loud hum at low volume.  I noticed this when I was trying to use the second preamp tube and noticed those leads were backwards.  The same original hum came back so I back tracked to see what it was and that was it, the B and C high voltage points.

So, trying to find the hum problem I decided to unhook the negative feedback wires.  NO signal gets through!  Why??  This does not make any sense to me and makes me wonder if that area is where my problem lies although I don't know what to do.

Any thoughts??

Thanks...