Hi, some of you may remember the thread started by me about a year ago about designing and building a push-pull stereo amp from a pair of old mono record player amps, using EL42's as the output tubes, with a lot of help from you guys. Full story here :
http://www.diyaudio.com/forums/showthread.php?t=128662
Anyway, the amp sounds lovely and for the first few months it became my main amplifier, but there is however a slight problem with it, which made me go back to SS for a while, and thats the hum. I chased it and chased it, both channels had a 50Hz hum which was constant at any volume, although you only really noticed if listening at low levels. In the end I gave up and went back to my old amp.
But recently I decided to have one last shot at getting rid of the hum, I tried everything I could think of, moving the PT off the chassis, moving the choke, improving the grounding, (its star grounded) running the small signal tube heaters off a battery... nothing worked.
Until today, I tried something just on a whim, reversed the phase of the sheilded feedback cable that connects to the speaker output of one channel. hum has dropped considerably, overall volume has dropped too, you have to turn the put up more to get the same volume.
So my question I guess is more about feedback than hum, was it wrongly phased before or is it wrongly phased now? I'm amazed the amp would work at all if the feedback loops were the reverse of what they should have been.
how do you make sure you connect the feedback phase correctly?
Thanks for your time,
Phil.
http://www.diyaudio.com/forums/showthread.php?t=128662
Anyway, the amp sounds lovely and for the first few months it became my main amplifier, but there is however a slight problem with it, which made me go back to SS for a while, and thats the hum. I chased it and chased it, both channels had a 50Hz hum which was constant at any volume, although you only really noticed if listening at low levels. In the end I gave up and went back to my old amp.
But recently I decided to have one last shot at getting rid of the hum, I tried everything I could think of, moving the PT off the chassis, moving the choke, improving the grounding, (its star grounded) running the small signal tube heaters off a battery... nothing worked.
Until today, I tried something just on a whim, reversed the phase of the sheilded feedback cable that connects to the speaker output of one channel. hum has dropped considerably, overall volume has dropped too, you have to turn the put up more to get the same volume.
So my question I guess is more about feedback than hum, was it wrongly phased before or is it wrongly phased now? I'm amazed the amp would work at all if the feedback loops were the reverse of what they should have been.
how do you make sure you connect the feedback phase correctly?
Thanks for your time,
Phil.
Hi Phil, This sounds like my exact scenario. I am using a pair of Stromberg Carlson 6v6 monos that were basically a 1 input integrated for an old turntable unit. (Not sure, but probably ceramic cart.). I have replaced all resistors and all caps and have overall good sound, but there is still a low level hum (disappears from about 12" away) when the volume is off. I am trying to clean up the hum as well as reduce the gain. I have replaced the input tube from 6av6 to 6at6 (100mu to 70mu), which helped to lower the gain a bit. Still looking to lower the gain more. Could you please explain in more detail how to change the phase of the speaker output. Also, any other ideas to reduce the gain, increase/decrease certain resistors, etc. There is a 6av6 (now 6at6) followed by a 12AX7 (now 6829 for less gain) and 2 6v6GT outputs. There are treble, base, and volume pots. The volume dial needs only an 1/8-1/4 turn to approach distortion levels? I have other S/C stuff that sounds very good. These amps have potted Output Trannies and have potential to sound quite good. I also want to remove the tone controls, but do not know how at this point. I am running a CD player into the input(s), which I'm sure does not need the amount of gain that the old record player needed. Any advice would be appreciated. Ralph
Last edited:
Hi Rispoli,
Unfortunately I'm not qualified to suggest the best way to reduce the gain by the amount you need. My turntable amps pretty much had to be completely redesigned to be suitable for line-level inputs. Fortunately there are some very knowledgeable and helpful people on here who can help you out though.
As for your question on feedback, if your amp uses global feedback then it’s simply a case of changing the polarity of the two wires that connect to the output terminals of your output transformer around, try both ways, one should give you less gain than the other, which is the correct way. Chances are, if your tone controls are working then the feedback loop is correctly wired already and you are looking in the wrong place to reduce gain.
Regarding your hum issue, have you tried using larger caps in the power supply? Check the data sheet for your rectification valves and use the maximum sized cap recommended, no more or you will damage the valves. Alternatively if you have chokes in the power supply then you can increase the size of the capacitor that is placed after the choke to a much bigger value without damaging your rectifier valves.
hope that helps.
Phil.
Unfortunately I'm not qualified to suggest the best way to reduce the gain by the amount you need. My turntable amps pretty much had to be completely redesigned to be suitable for line-level inputs. Fortunately there are some very knowledgeable and helpful people on here who can help you out though.
As for your question on feedback, if your amp uses global feedback then it’s simply a case of changing the polarity of the two wires that connect to the output terminals of your output transformer around, try both ways, one should give you less gain than the other, which is the correct way. Chances are, if your tone controls are working then the feedback loop is correctly wired already and you are looking in the wrong place to reduce gain.
Regarding your hum issue, have you tried using larger caps in the power supply? Check the data sheet for your rectification valves and use the maximum sized cap recommended, no more or you will damage the valves. Alternatively if you have chokes in the power supply then you can increase the size of the capacitor that is placed after the choke to a much bigger value without damaging your rectifier valves.
hope that helps.
Phil.
Thanks Phil, Did someone else do the redesign/rebuild or do you have some insight into some of the steps taken? I have done quite a few rebuilds and tweeks, but not a redesign at this point (still learning). I am also looking to go from phono to line level input and maybe get rid of the tone controls. thanks, Ralph
Interesting point, thanks richy. The hum I was experiencing was of the low frequency 50Hz variety, however, adding extra capacitance to the PSU definitely helped. There is a small amount of hum remaining with my amp, but it’s much better than it was before I corrected the feedback polarity and added extra caps in the PSU. The remaining hum may be due to heater supply, but the heater wiring is all tightly twisted pair so I don't see how I can improve on this much. I may try powering all heaters from a battery, just to eliminate the possibility of heater hum; however the hum is barely noticeable now so I can live with it.
Regards, Phil.
Regards, Phil.
Hum in tube amp
Hi,
Most times the hum is coming from, as previously stated by others, from possible grounding problems. However, in your case, the hum is constant, and does not indicate it is caused in the feedback loop. In my experience, hum can be eliminated by using DC instead of AC for your heater supply. Wire the heater filaments in series, this helps as well. Ensure you use a regulator with plenty of buffer capacity for the heater. My own tube amps have the heaters in series, and run on 12 Volts DC, (4700 uF, 7812 regulator): no audible hum whatsoever. If you don't have the space to get a DC regulated supply in your casing, there is another possible solution: balancing the heater AC feed with a wirewound pot of 2KOhm, if I remember correctly.
Hope this helps,
Arthur
Hi,
Most times the hum is coming from, as previously stated by others, from possible grounding problems. However, in your case, the hum is constant, and does not indicate it is caused in the feedback loop. In my experience, hum can be eliminated by using DC instead of AC for your heater supply. Wire the heater filaments in series, this helps as well. Ensure you use a regulator with plenty of buffer capacity for the heater. My own tube amps have the heaters in series, and run on 12 Volts DC, (4700 uF, 7812 regulator): no audible hum whatsoever. If you don't have the space to get a DC regulated supply in your casing, there is another possible solution: balancing the heater AC feed with a wirewound pot of 2KOhm, if I remember correctly.
Hope this helps,
Arthur
Thanks for the tip EVO1340, I have considered using DC for the heaters, but the existing PT's are only supplying 6.3V AC, I figured I would have to use a separate transformer with a higher voltage due to the losses of rectification and regulation, which I don't have space for. One possibility though, been as I have separate power transformers for each amplifier, would be to connect the two 6.3V heater supplies in series, but then I'm not sure if connecting the secondary windings of two separate transformers together is advised...? in any case I'll try a battery first, just to make sure it is actually the heaters that are the problem.
By the way I have tried the heater ‘balancing’ method you mention, I have also tried ‘floating’ the heater supply on a higher voltage as I was advised to try in my previous thread, neither of these really helped in my case.
I’m going away now for a few days, but when I get back I might try the battery and report back…
Regards, Phil.
By the way I have tried the heater ‘balancing’ method you mention, I have also tried ‘floating’ the heater supply on a higher voltage as I was advised to try in my previous thread, neither of these really helped in my case.
I’m going away now for a few days, but when I get back I might try the battery and report back…
Regards, Phil.
Hello All,
Too much hum. A funny thing happened on the way to Burning Amp. I took all the parts and components that were cobbled together on the bench top for my project amp and screwed down the transformers power, filament, output with the voltage regulators to a piece of plywood. Things went from silent to too much hum in about 15 minutes.
The short story about the solution is this. Keep the line voltage (120 volt 60Hz here in California) away from the output transformers. Regulators do not help when there is 60 Hz induced in the output transformers.
DT
All just for fun!
Too much hum. A funny thing happened on the way to Burning Amp. I took all the parts and components that were cobbled together on the bench top for my project amp and screwed down the transformers power, filament, output with the voltage regulators to a piece of plywood. Things went from silent to too much hum in about 15 minutes.
The short story about the solution is this. Keep the line voltage (120 volt 60Hz here in California) away from the output transformers. Regulators do not help when there is 60 Hz induced in the output transformers.
DT
All just for fun!
Phil,
If you're sure you're battling 50 Hz hum rather than 100 Hz hum, there are a couple of options for where it can originate from:
- EM coupling with mains transformer.
- Capacitive coupling from heater to cathode.
- Crappy ground (increasing the EM coupling).
- Dead or incorrectly wired rectifier.
If you really are hearing 50 Hz hum (not 100 Hz hum) and adding reservoir capacitance to the power supply lowers the hum, I'd look at the rectifier. I assume it's the "standard" double rectified supply topology you're using. I.e. center tap of transformer grounded, the two "outer" taps connected to a twin diode rectifier. If this is the case, you should have 100 Hz ripple - not 50 Hz ripple... But if one of the diodes is dead, you'll get 50 Hz ripple. Another way to get 50 Hz hum is if the transformer is wired incorrectly -- an issue if you have two separate windings and are connecting them as one winding with a center tap. On a correctly wired double (or bridge) rectifier you should have ripple at 2x the mains frequency, so 100 Hz in your case. I'd look at the ripple with an oscilloscope and verify its frequency.
Reducing EM coupling between the transformer and sensitive circuitry is best done by increasing the distance between the circuitry and the interference source. Using twisted pairs and differential signalling between stages helps too.
You might be able to limit the coupling between the cathode and heater by applying a positive DC bias on the heater with respect to the cathode. There's more information on Valve Wizard's page.
Also, watch your screen supplies if you're using pentodes. Any hum on those will make it to the output without much attenuation.
~Tom
If you're sure you're battling 50 Hz hum rather than 100 Hz hum, there are a couple of options for where it can originate from:
- EM coupling with mains transformer.
- Capacitive coupling from heater to cathode.
- Crappy ground (increasing the EM coupling).
- Dead or incorrectly wired rectifier.
If you really are hearing 50 Hz hum (not 100 Hz hum) and adding reservoir capacitance to the power supply lowers the hum, I'd look at the rectifier. I assume it's the "standard" double rectified supply topology you're using. I.e. center tap of transformer grounded, the two "outer" taps connected to a twin diode rectifier. If this is the case, you should have 100 Hz ripple - not 50 Hz ripple... But if one of the diodes is dead, you'll get 50 Hz ripple. Another way to get 50 Hz hum is if the transformer is wired incorrectly -- an issue if you have two separate windings and are connecting them as one winding with a center tap. On a correctly wired double (or bridge) rectifier you should have ripple at 2x the mains frequency, so 100 Hz in your case. I'd look at the ripple with an oscilloscope and verify its frequency.
Reducing EM coupling between the transformer and sensitive circuitry is best done by increasing the distance between the circuitry and the interference source. Using twisted pairs and differential signalling between stages helps too.
You might be able to limit the coupling between the cathode and heater by applying a positive DC bias on the heater with respect to the cathode. There's more information on Valve Wizard's page.
Also, watch your screen supplies if you're using pentodes. Any hum on those will make it to the output without much attenuation.
~Tom
Your 6.3VAC heater will end up around 8.75VDC after FWB rectification and caps (1.4xVAC). You can then reduce voltage to 6.3VDC with a resistor after the smoothing cap. Your much better off with DC on heaters anyway...if you ask me...
Can you explain how you reversed polarity of feedback? On my tubeamp its one wire from 16 ohm tap to the cathode of the input tube, so where's the polarity??
Sounds like you disconnected the FB. You should get lower gain removing FB...correct? If you completely disconnect the FB, do you get the same effect?
My amp with no FB sounds flat, but hum/noise is reduced...you may just have to reduce the FB (larger FB resistor) to lower the gain and noise???
Can you explain how you reversed polarity of feedback? On my tubeamp its one wire from 16 ohm tap to the cathode of the input tube, so where's the polarity??
Sounds like you disconnected the FB. You should get lower gain removing FB...correct? If you completely disconnect the FB, do you get the same effect?
My amp with no FB sounds flat, but hum/noise is reduced...you may just have to reduce the FB (larger FB resistor) to lower the gain and noise???
Last edited:
Richy might be thinking what I am thinking...
It smells to me like your transformers are talking to each other.
Try unbolting your output transformer(s) and move it away from the power transformer and/or power supply choke if you have one.
Also try rotating the output transformer by 90 degrees.
I have played around with different resistors in the feedback loop and have lowered the hum to a tolerable level at this point, although one amp has a bit more hum than the other and all the same parts were replaced? I am now looking to do something with the tone controls. They work fine, but the base is over-emphasized. My guess is that it is compensation due to the phono source. I am looking to either bypass the tone controls or modify the bass pot to lower the bass level. I will search the threads to see if this scenario has been covered before. One additional question I have relates to a network circuit that runs between the 2 driver tubes and the 2 output tubes. It is a flat rectrangular piece resembling a ceramic disc capacitor, with multiple leads. Did they use this just for ease of installation (it is a tight fit- the amps are small)? Also, might this be where the over-emphasized base is originated? PS: this forum is great, I am slowly but surely learning, and having a great time tweaking and listening.....
An added thought from DT, if I recall correctly you stated that you have a single transformer that serves both B+ and heaters from separate secondarys. Especially if you are now regulating the heater supply there may be common mode hum/buzz being induced into the High voltage winding by the large current spikes in the heater winging. Regulating the heater voltage will increase the load and spikes in the transformer.
One more possible solution to consider is a separate filament transformer.
DT
All just for fun!
One more possible solution to consider is a separate filament transformer.
DT
All just for fun!
Going back to the schematic on 1st page , I noticed the feedback loop with tone pot and series cap isn't quite so straight-forward as it makes the nfb loop frequency selective, a common technique used on budget equipment. So whatever the resistor changes, becareful as one is also changing the circuit response.So becareful with resistor changes.
Generally, during the 1950's+60's, circuits of this type wern't considered HiFi standard, but the ones I've heard gave a suprisingly high standard of sound.
richy
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.