Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dscottguitars on December 22, 2013, 04:29:39 pm
-
Hi, I built this dual/stereo EL84 amp and have a hum problem. I've tried 12AV7's in the preamp first and a 12AT7 in the phase inverter but too much gain so I changed them to 12AU7's and 12AV7-phase inverter. No change in the hum with different tubes. I tried different values in the negative feedback loop- no change. Disconnected the center tap in the 6.3v section-no change. In the picture you can see two 'star' grounding points that go to the main ground. The only part that is not equal is the input grounds and shielding from the input lines go to one side only.
The EL84/6BQ5's are Russian equivalents with higher plate voltage ratings.
I pull the phase inverters and the hum stops. I pull the first preamp tube, no change. BUT if I pull the second preamp tube the hum gets much louder.
Any suggestions??
This is a christmas present for my daughter to be a stereo/guitar/PA amp and hoped to take it to her Tuesday...
BTW, it really sounds great except for the hum in between songs.
Thanks,
Daniel
-
Does the hum increase if you turn more volume?
/Leevi
-
Not when I have signal going into it. As of now I plugged my computer into the RCA jacks. When I turn it up the hum does not get louder and the music over compensates for it. But, without any signal, it gets louder with the volume control.
-
You are using RCA input jacks.
The circuit is open if you don't plug anything there which
will cause hum since the input is not closed. In the guitar jack the input is connected to the ground
if the guitar is not plugged.
/Leevi
-
All the input jacks go to a switch-4 pole 5 positions so the only one connected is the one chosen on the switch. The 1/4" jack is a switch jack to ground if nothing is plugged in and the hum is the same there too.
Thanks...
-
Not when I have signal going into it. As of now I plugged my computer into the RCA jacks. When I turn it up the hum does not get louder and the music over compensates for it. But, without any signal, it gets louder with the volume control.
If more volume= same hum AND no PI tube = no hum ;
Look your hum is coming AFTER the volume and BEFORE Phase Inverter tube. Is there a long wire ( ? ) to the grid of PI tube need to be shielded ? Or something else act like antenna ?
The 47K from the volume wiper must be solder ON the 12AU7 grid or if long wire , put shielded wire . Same for capacitor to PI tube grid .
It any case , NO long wire to tube's grids.
-
The grid wires are short-less than an inch or maybe 1.5" in one case.
After a bit more research pulling all the tubes including the rectifier I hear a vibrating hum coming from the power transformer. It's very faint but I can see how that would amplify into the speakers. It is an old Motorola transformer from a 3 channel stereo, one for each side and a bass channel.
The hum does get louder with the volume control but I don't notice it when music is playing and it's still there with the PI tubes pulled out just not nearly as loud.
Is there anything about my power supply chain that could be the cause? I tried to do two separate supplies for each side but now I wonder if that is okay. I'm wondering about the first filtering with the two 47uF caps and the resistor in between them.
-
Just as a check do you have possibility to replace the 200 Ohm resistor between A and B with a choke.
If you don't have choke a primary of another PT could be used in order to see how the hum level will change.
/Leevi
-
The grid wires are short-less than an inch or maybe 1.5" in one case.
After a bit more research pulling all the tubes including the rectifier I hear a vibrating hum coming from the power transformer. It's very faint but I can see how that would amplify into the speakers. It is an old Motorola transformer from a 3 channel stereo, one for each side and a bass channel.
The hum does get louder with the volume control but I don't notice it when music is playing and it's still there with the PI tubes pulled out just not nearly as loud.
Is there anything about my power supply chain that could be the cause? I tried to do two separate supplies for each side but now I wonder if that is okay. I'm wondering about the first filtering with the two 47uF caps and the resistor in between them.
1 to 1.5 inches is sometimes too long if wiring is not down to the chassis and wiring must not be run in parallel, many close together.
If hum get louder with volume , noise is pickup by the circuit before the volume . Check improper wiring from the inputs jacks to the volume .
All your wires from the RCA jack to the switch and to the first tube's grid must be short ( 1/2 inches ) or shielded.
What is those two input /output black plastic guitar jacks ? Are they connected to the first tube ?
IMO, How this amplifier is built has all the chance to have problems with hum.
-
" I hear a vibrating hum coming from the power transformer."
Try lifting the transformer off the chassis and see if the hum goes away. Perhaps put something like tissue paper under the tranny so you don't have to hold it in hand.
I've seen mechanical feed back a couple times. The first time was a Fender Bandmaster head placed on top of a wooden box. The box vibrated and caused the mechanical feed back to appear in the amp.
The most recent incident was the Peavey Roadmaster. The cooling fan bearings are starting to go and it put a vibrational feed back into the circuit. I unplugged the fan and the howling stopped.
If you can find the Magnavox 9310 schematic, a cathodyne power stereo power amp, they have a hum control circuit. But that circuit is found in many other amps too.
Is the power transformer being overloaded?
Silverfox.
-
Check your heater wiring , On your sketch it look ok but did you build like that ?
-
Stratele: I don't see how my layout is bad. The inputs are on the right side, two shielded 4 conductor wires going to a rotary switch in the center. From there to the first tube next to switch right and left, then to the tone stack, volume to the next tube, then into the PI and up to the power tubes. No crossing over of signal wires, the power rail is clean and separated. The heater wires are the blue and yellow wires looping over the top of the sockets. The ground wires all go to one spot then to the main ground. Each channel is separate. The two plastic jacks are the speaker jacks. I don't think the hum is any part of that, but the power transformer making noise.
Silverfox, I'll try that but why would the transformer make noise in the first place, I've never heard it in other amps?
Thanks for all of your help...
-
dscottguitars,
It is not easy to judge an amplifier from a photo. It's not like having three dimension to the eyes. But the problem you described to me could be a building flaw .
I do not know either your competence in amplifiers, how thou hast built before it ?
-
Yes I understand a photo doesn't show everything. I have built about 6 amps from scratch and only one I couldn't get the hum out. But that one was too small for all the stuff I had in it and I ended up splitting the preamp and power amp into two units. I'm sure my layout is good and I follow Gerald Weber's book on advice for lead dress and Aiken amps pages for grounding also.
BTW: lifting the PT did not take away the noise. I also disconnected the wire from the 5v tap to the filter cap. It still makes noise.
Does anyone know about this noise issue in a power transformer? Does it mean it's bad? Can the wires from the plug into the wall make a difference as to which one goes to the on/off switch? I have the hot lead going to the fuse, then to the on/off switch, then to one transformer primary. The neutral lead is going directly to the other transformer primary wire.
I am thinking about buying a new transformer and replacing it later.
-
Yes I understand a photo doesn't show everything. I have built about 6 amps from scratch and only one I couldn't get the hum out. But that one was too small for all the stuff I had in it and I ended up splitting the preamp and power amp into two units. I'm sure my layout is good and I follow Gerald Weber's book on advice for lead dress and Aiken amps pages for grounding also.
Thank you for having clarified your experience in amplifier constuction, it helps to situate some who are talking about.
If you did not commit an error mounting perhaps the transformer which induces magnetic field
-
Does anyone know about this noise issue in a power transformer? Does it mean it's bad? Can the wires from the plug into the wall make a difference as to which one goes to the on/off switch? I have the hot lead going to the fuse, then to the on/off switch, then to one transformer primary. The neutral lead is going directly to the other transformer primary wire.
I am thinking about buying a new transformer and replacing it later.
About the noisy transformer ; you ask if could it be bad ? I only see that one time , on a Vox AC30 ,a vintage amp . If you don't like the noise , yes it is bad .
About the the power cord from the wall , any wire any where do not make noise . I like the way you do this wiring I do like that too ; hot on the fuse
-
About the noisy transformer ; you ask if could it be bad ? I only see that one time , on a Vox AC30 ,a vintage amp . If you don't like the noise , yes it is bad .
Yes, I agree that's bad, but I'm wondering if there is something I can do to fix it
I just plugged my guitar into it and WOW!! It really cranks and sounds awesome! Much louder than I expected. I only measured 5 watts with an 8 ohm resistor in one side.
Does anyone know what causes the PT to make noise?
-
Some transformers do hum, does not mean they are bad. Could be the source of your hum but hard to prove.
I suggest to power the 6.3 heaters off a 6v lantern battery. If the hum goes away then you know the source.
Some of your heater wires appear a bit close to signal wires/cps.
-
I'm somewhat confused about what you mean when you say the heater wires are close to signal wires because the pins for the heaters are very close to the pins that the signal wire is connected. There isn't any of these wires that are closer than the pins. Also, if that were the source of the hum then I would think that the hum volume would change when I move the heater wires around, yes?
Another thing: how would I hook up the battery? I thought the heater wire taps were AC and a battery is DC. Does that not make a difference?
thanks...
-
I just had a thought and I don't know how legitimate this is but: The three channel power transformer- What did you do with the unused power tap. The one that would have been used for a Bass channel. Is there some sort of impedance thing going on with a floating output that is causing the transformer to overload?? Does that tap have a CT?
Try to figure out what value resistor would load the third unused channel to stop it from just floating.
This may be a kooky idea so feel free to shoot it down.
Silverfox.
-
It didn't have an unused power tap. It was hooked up to a power supply rail that powered the tubes. Here is the Motorola schematic:
-
Another thing: how would I hook up the battery? I thought the heater wire taps were AC and a battery is DC. Does that not make a difference?
thanks...
For heater 6 V DC or AC are same. Proof , many hi-fi amp use DC on heater .
___________________________________________________
As I write before , if hum get lounder with volume. It come BEFORE the volume;
Just look your amp circuit from input jacks to the the volume pot .
1- Bad wiring
2- Noisy tube
3- Heater with no center tap or transformer induce hum or center tap defective
4- Filter caps/ DC supply who feed this circuit
You must find something there.
First and easy test to do ; You may do that before ?
1- Connect the end of the 48K ,4 watt to ground. The end from the input jack switch .If no hum , check input jack and switch. Hum comes from there.
-
About the noisy transformer ; you ask if could it be bad ? I only see that one time , on a Vox AC30 ,a vintage amp . If you don't like the noise , yes it is bad .
Yes, I agree that's bad, but I'm wondering if there is something I can do to fix it
If I knew, I would have written. What can make noise in a transformer? There is a blend of fine steel blade for all kinds of reasons probably can get a vibrate with the application of 60 cycles
-
I Also, if that were the source of the hum then I would think that the hum volume would change when I move the heater wires around, yes?
Anywhere you move heater wire you may be close to signal path .
Your heater wire are the yellow and the blue one , I'm right ?
What is the yellow one going from one tube to a double pot ? Bottom of the picture
-
@dscottguitars
Do you have it built as shown on your schem?
Something that caught my eyes: I don't know whether it's related to your hum issue, but looking to P.I. Output shows 220K after grid stoppers. IMHO, grid stoppers must be mounted directly on control grid pins, preferably touching it. Your drawing shows power tubes grid ground reference after grid stoppers. Again, not sure if it's related to your issue but some books suggests it's not the best practice.
Do you have a scope?
Hope this helps
Best Regards
R.
-
@dscottguitars
Do you have it built as shown on your schem? building error hapenned very often
Something that caught my eyes: I don't know whether it's related to your hum issue, but looking to P.I. Output shows 220K after grid stoppers. IMHO, grid stoppers must be mounted directly on control grid pins, preferably touching it. Your drawing shows power tubes grid ground reference after grid stoppers. Again, not sure if it's related to your issue but some books suggests it's not the best practice.
I agree , it not common to built amp like that . Where the schematic come from ?
Do you have a scope? And do you know how to use it ?
Hope this helps
Best Regards
R.
+1
-
Another thing: how would I hook up the battery? I thought the heater wire taps were AC and a battery is DC. Does that not make a difference?
thanks...
Probably one of the biggest sources of 60Hz hum is from the heaters. The best way to completely eliminate that cause is a 6v battery.
Powering the heaters from DC will be a definitive check. Moving wires may or may not identify heater hum as the source, then you are left wondering.
If you disconnect the 6.3v wires coming from the transformer, disconnect them from their first connection point and jumper in the 6V battery using alligator leads if you have some.
-
Also.....in your first post you said when you disconnected your heater center tap the hum did NOT get any louder.
It should have. Having that CT connected makes a HUGE difference in hum levels..........normally.
I think your problem is heater wiring related.
Being that you said the amp sounds good and you are happy with it other than the hum, I see no reason to dissect any other aspects of your build. I love the old school look!
BTW......I wire my amps like you. Star ground with one connection to chassis (other than AC connection ground).
Here is a 50 watt Marshall : http://s991.photobucket.com/user/1rebmem/media/DSC00444.jpg.html?sort=6&o=35 (http://s991.photobucket.com/user/1rebmem/media/DSC00444.jpg.html?sort=6&o=35)
Heater wires are red/black.
-
Rzenc: the 220K and 1.5K resistors are on the pins.
Stratele: that yellow wire is also a heater twisted with the blue wire and they go around the rotary switch so they're not close to the signal wires.
rebmem: I am surprised that the center tap didn't change the hum. The motorola schematic show it connected to the cathode of the power tubes and I tried that too with no change. I really think it's the buzzing sound of the transformer.
I read in another forum issues with the transformer bolts not being tight enough and causing a buzzing sound. I'm going to try that and see what happens, next I'll get a 6v battery and then maybe a new transformer.
Thanks for all your input...
Daniel
-
Is there anything about my power supply chain that could be the cause? I tried to do two separate supplies for each side but now I wonder if that is okay. I'm wondering about the first filtering with the two 47uF caps and the resistor in between them.
I think separate B+ rails is fine. But think about that 47K resistor between the two 47µF caps. Both ends of the resistor connect to the same place. The resistor is totally shorted out and does absolutely nothing. This is not your hum problem but thought I'd mention it.
Now about the hum... Put a temporary ground strap (gator clip lead) between the junction of that 820Ω and 47KΩ in the NFB loop and ground. Do this on both channels. Does this help the hum issue?
-
Also.....in your first post you said when you disconnected your heater center tap the hum did NOT get any louder.
It should have. Having that CT connected makes a HUGE difference in hum levels..........normally.
I think your problem is heater wiring related.
Easy to check with AC voltmeter . One probe to ground , the other on one side of the heater . With center good you may read 3 volts . No center tap you'll read "0" .
And with ohmeter check DC resistance between each of heater wire from the transformer secondary . You must remove all tubes. Amp power OFF for the test
Being that you said the amp sounds good and you are happy with it other than the hum, I see no reason to dissect any other aspects of your build. I love the old school look!
Old shool must make hum ? I desagree
BTW......I wire my amps like you. Star ground with one connection to chassis (other than AC connection ground).
-
Stratele: that yellow wire is also a heater twisted with the blue wire and they go around the rotary switch so they're not close to the signal wires.
Daniel
What is the use of the rotary switch if is not to switch some signal input from input jacks ?
I wait for the answer
-
Thanks Sluckey, I didn't know if that would do what I thought it would and I'll remove it. Do you think I need both power supplies? I could just take out the one side.
Also, I did remove the NFB loop-no change and changed the load resistor to 2.2K and no change but I'll try grounding that junction. I'm going to Chicago and will take it apart when I get there.
Stratele: I did check voltage from the heater to the center tap and got about 3v. I also checked for continuity to make sure that it was a center tap and it checked out okay.
The rotary switch is to switch the inputs and the heater wires are along the front panel away from the switch. I didn't want them close to the switch or the inputs...
-
BTW: here is a final pic of the amp and speakers to with it. The amp cabinet is hickory and the speaker cabs are poplar with an 8" siesmic audio woofer and a piezo tweeter. The bass port slot at the bottom really works great, very clean and low bass.
-
Stratele: I did check voltage from the heater to the center tap and got about 3v. I also checked for continuity to make sure that it was a center tap and it checked out okay.
Good
The rotary switch is to switch the inputs and the heater wires are along the front panel away from the switch. I didn't want them close to the switch or the inputs...
Ok I understand , yellow is twisted with blue wire on the bottom of the amp and switch is far from the bottom
-
Wood Cabs; Very nice job , congrat
-
Man, those cabs are beautiful!
I love hickory, my kitchen cabinets are hickory.
Brad :bravo1:
-
stratele52.....
When I said: "Being that you said the amp sounds good and you are happy with it other than the hum, I see no reason to dissect any other aspects of your build. I love the old school look"
You replied: "Old shool must make hum ? I desagree"
I never meant to imply old school MUST hum. What I meant to say is that if the guy is happy with his build other than the hum then I won't comment (and hopefully others will follow) on things that don't cause hum. I also meant to compliment him on his old school build............
I further stated I build this style myself. I wouldn't if they ALL hummed!
Back to the issue:
dscottguitars..........Another thing of concern is the 5U4 is seeing 94uF of capacitance. That is double the 5U4 data sheet capacitance.
Also, does it hum in both left and right channels? I didn't see this question answered unless I missed it.
-
1rebmem,
English is not my mother tongue, I probably misinterpreted your words. Sorry .
-
I had the same situation. When I first fired it up it had a slight hum. The more I looked for it (longer) the worse it got until I noticed the Transformer vibrating and then began clicking too.
I simply pulled the tubes and replaced the filter caps. They were all new. QUIET AND ROCKIN NOW.
-
I had the same situation. When I first fired it up it had a slight hum. The more I looked for it (longer) the worse it got until I noticed the Transformer vibrating and then began clicking too.
I simply pulled the tubes and replaced the filter caps. They were all new. QUIET AND ROCKIN NOW.
Interesting Ed.
What led you to think a capacitor problem? I've had two builds with humming transformers, figured I had to live with it or replace them. Both of those tranformers were pretty old.
As far as Daniel's hum problem here, I figured the only problem with excessive capacitance on the 5U4 would have been potentially high current pulled through it? Which apparently isn't happening. Something to ponder.
It's not easy to prove transformer hum, not that I've found.
-
1rebmem,
English is not my mother tongue, I probably misinterpreted your words. Sorry .
Ah....OK. Thanks.
-
I will change the filter cap to only have one side, 47uF and 2-20uF. I guess my thought of a double rail B+ supply didn't work. But the transformer is still making a buzz sound without any of the filter caps hooked up. I am still thinking this is my hum problem, but will try it tomorrow with the correct filter cap capacitance to see. I don't have much computer access here at my daughter's so if I don't respond, that's why.
Thinking about the capacitance issue, I have used 2-220uF caps in series for my 6L6 guitar amps. A recommendation from Gerald Weber's book. That's 110uF total and to use a 5U4 rectifier without any problems.
Thanks again and Merry Christmas to all of you...
Daniel
-
Daniel,
I'm finding it hard to believe your parallel filter caps are causing your hum, but worth a try. I noticed the 2 parallel caps and realized it was a lot for a 5U4 so I mentioned it.
Here is an interesting discussion on transformer hum troubleshooting, in particular see the 3rd paragraph of the originator's post:
http://ampgarage.com/forum/viewtopic.php?t=17505&sid=ba9ad5772023677bf254f34509e65a3f (http://ampgarage.com/forum/viewtopic.php?t=17505&sid=ba9ad5772023677bf254f34509e65a3f)
-
Interesting.
Look at the very last post by mauiboy81 on page 3. That might be the problem, smaller core still with a pretty good amount of current, 300mA B+ and 7.5A filament.
Their copy's of the old Stancor PT's that Fisher was using. Maybe Stancor knew something about winding that smaller core PT that Heyboer is missing?
Brad :dontknow:
-
Interesting.
Look at the very last post by mauiboy81 on page 3. That might be the problem, smaller core still with a pretty good amount of current, 300mA B+ and 7.5A filament.
Their copy's of the old Stancor PT's that Fisher was using. Maybe Stancor knew something about winding that smaller core PT that Heyboer is missing?
Brad :dontknow:
Yeah that is interesting.
I believe that if it is transformer hum and is coming from the speakers then it is induced from the PT to the OT?
If true then the "headphone test" should prove it. You would pull the rectifier and both power tubes. Connect head phones to the speaker jack. Power on the amp so that the PT has primary power applied and listen to the head phones. Hum should be very faint if any at all.
Head phone sensitivity matters. I have one pair that I never hear hum but with the other I definitely can.
-
I'm somewhat confused about what you mean when you say the heater wires are close to signal wires because the pins for the heaters are very close to the pins that the signal wire is connected. There isn't any of these wires that are closer than the pins. Also, if that were the source of the hum then I would think that the hum volume would change when I move the heater wires around, yes?
Another thing: how would I hook up the battery? I thought the heater wire taps were AC and a battery is DC. Does that not make a difference?
thanks...
Daniel- I also wanted to address your point that the pins for the heaters are close to signal pins and none of your wires are even that close. The pins are only about 3/16 apart and obviously it isn't a hum issue. Chances increase for hum to be induced from the heater wires when they run parallel to signal wires or capacitor leads. So a one inch run of heater wiring, parallel and 3/4 inch away from a signal wire, may cause hum. Hard for us to see in that one pic, we weren't sure but that's why stratele52 and I mentioned it.
Two other things for your consideration:
Are your shielded wires only grounded at one end?
And: I read in Merlin's book that with 2 channel amps ground loop hum issues can be a problem if you connect both channels to chassis improperly. See: http://valvewizard2.webs.com/Grounding.pdf (http://valvewizard2.webs.com/Grounding.pdf)
Second half of page 14 and the sketch on 15. Note in the sketch how the 2 channels tie into the same star ground point which then goes to the chassis.
-
Daniel- I also wanted to address your point that the pins for the heaters are close to signal pins and none of your wires are even that close. The pins are only about 3/16 apart and obviously it isn't a hum issue. Chances increase for hum to be induced from the heater wires when they run parallel to signal wires or capacitor leads. So a one inch run of heater wiring, parallel and 3/4 inch away from a signal wire, may cause hum. Hard for us to see in that one pic, we weren't sure but that's why stratele52 and I mentioned it.
Two other things for your consideration:
Are your shielded wires only grounded at one end?
And: I read in Merlin's book that with 2 channel amps ground loop hum issues can be a problem if you connect both channels to chassis improperly. See: http://valvewizard2.webs.com/Grounding.pdf (http://valvewizard2.webs.com/Grounding.pdf)
Second half of page 14 and the sketch on 15. Note in the sketch how the 2 channels tie into the same star ground point which then goes to the chassis.
+1
-
Some progress:
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. I removed one channel completely to isolate what's going on. When I get the one side working I will hook up the other side.
I also tried tightening the transformer bolts and switching the primaries around with no change
I moved the volume control to the other side of the tube, see new drawing and now the hum is consistent. It is low at zero volume and progressively gets louder with the volume turned up. Before the noise was more of a loud hum at zero volume and when turning up it got softer but turned into more of a buzzing sound at loud volume. Why this happens, I don't know. But it makes me convinced the hum is in the power transformer and not the wiring.
And that's because the 'note' the transformer hums at when it is completely disconnected from any caps or rectifier tube is the same as what comes out of the speaker when all is hooked up.
I am going to buy a new transformer and try that out. If it does not help, I can use the transformer for my next project. At the moment, for how loud the amp needs to be for a stereo, the hum is minimal and tolerable. But if used as a guitar amp cranked the hum would be too loud. The one I'll get is here: http://taweber.powweb.com/store/022798sch.jpg (http://taweber.powweb.com/store/022798sch.jpg) I will use the 660v taps and may have to drop the voltage a bit.
What's the best way to drop the initial voltage for the B+ supply?
I looked at that grounding link and have been there before. I'm sure I've got the wiring under good control. I also checked for parallel links to the heater wires and there isn't any I can see. The input wires are shield grounded only on one side. If I move any of the signal wires and the heater wires apart from each other I should hear differences in the hum volume if that's where it is coming from but I don't hear any change.
This transformer is around 50 years old and may have had the noise all along or may be because of its age, I have no idea.
Thanks for all your help...
-
... I am going to buy a new transformer ... I will use the 660v taps and may have to drop the voltage a bit.
What's the best way to drop the initial voltage for the B+ supply? ...
Start with less a.c. voltage, so you don't have to find ways to lower it.
Really, if you're buying a new transformer, why not find a suitable transformer that gives you what you need? Worst-worst case, you could also call Edcor with your requirements and have them custom-wind a transformer to give exactly what you need.
330 * 1.414 = 466vdc, before the 5U4's voltage drop (but that still won't be anything like 100vdc). If you need 350vdc, it seems like it would be better to start with no more than 275vac.
... Another thing of concern is the 5U4 is seeing 94uF of capacitance. That is double the 5U4 data sheet capacitance. ...
Nowhere on the data sheet does it show a "maximum capacitance" even though many misconstrue the value listed under "Typical Operation" as a maximum. However, notice the line right below the capacitance value, "Total Plate-Supply Resistance per Plate".
If you suck more peak current through the rectifier (with More-C, Higher-V, or both), then you have to increase the supply impedance in the plate circuit of the 5U4 to keep it within its ratings.
What is this resistance? Measure the R of the PT secondary from CT to one end, and that is much of it. The R of the PT primary (referred to the secondary) gets added; however, you could skip this step and make sure just the secondary resistance is enough and be doubly safe.
If the R of the transformer is not enough, low-value resistors get added between the PT secondary and each plate to bring the total up to the needed value.
-
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...
-
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+.
-
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 (http://www.ebay.com/itm/151188819243?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649)
Use this calculator: https://taweber.powweb.com/store/vdump.htm (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......
-
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.
-
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.
-
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.
-
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 (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 (http://www.pmillett.com/dcpp.htm)
--pete
-
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.
-
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.
-
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...
-
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
-
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.
-
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.
-
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...