Chasing Hum... - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 16th September 2009, 04:10 PM   #1
phil_2 is offline phil_2  United Kingdom
diyAudio Member
 
Join Date: Jul 2008
Default Chasing Hum...

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 :

Design using EL42 output pentodes

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.
  Reply With Quote
Old 16th September 2009, 11:29 PM   #2
rman is offline rman  Canada
diyAudio Member
 
rman's Avatar
 
Join Date: Oct 2007
Hi.

I would think that since the volume went down that it is correct now. Negative feedback will decrease the gain. You must have had positive feedback before.

Cheers.
Rolf.
  Reply With Quote
Old 20th October 2009, 07:52 AM   #3
Rispoli is offline Rispoli  United States
diyAudio Member
 
Join Date: May 2006
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 by Rispoli; 20th October 2009 at 07:55 AM. Reason: typo
  Reply With Quote
Old 20th October 2009, 11:19 AM   #4
phil_2 is offline phil_2  United Kingdom
diyAudio Member
 
Join Date: Jul 2008
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.
  Reply With Quote
Old 20th October 2009, 04:27 PM   #5
Rispoli is offline Rispoli  United States
diyAudio Member
 
Join Date: May 2006
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
  Reply With Quote
Old 20th October 2009, 04:36 PM   #6
phil_2 is offline phil_2  United Kingdom
diyAudio Member
 
Join Date: Jul 2008
Follow the link in my opening post above, you will see how others helped me on here with my own project; perhaps some of the design obsticles/decisions will be relevent to you. Good luck
  Reply With Quote
Old 20th October 2009, 07:02 PM   #7
diyAudio Member
 
richwalters's Avatar
 
Join Date: Sep 2003
Location: Alps:Tube amp designs over 150W, SMPS guru.
Phil_2.. Are you trying chasing two problems ? Does the hum you hear actually 100Hz modulation (higher pitch) i.e from PSU ripple or bog standard dull 50Hz from valve heaters etc ?

richy
  Reply With Quote
Old 21st October 2009, 09:19 AM   #8
phil_2 is offline phil_2  United Kingdom
diyAudio Member
 
Join Date: Jul 2008
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.
  Reply With Quote
Old 21st October 2009, 09:36 AM   #9
EVO1340 is offline EVO1340  United Kingdom
diyAudio Member
 
Join Date: Feb 2008
Default 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
  Reply With Quote
Old 21st October 2009, 02:07 PM   #10
phil_2 is offline phil_2  United Kingdom
diyAudio Member
 
Join Date: Jul 2008
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Chasing my Simple SE problem... Zap Tubes / Valves 9 15th October 2008 03:53 AM
Chasing hum in a new tube amp bwarden Tubes / Valves 21 15th June 2007 05:22 PM


New To Site? Need Help?

All times are GMT. The time now is 05:47 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2