Frustration with hum and noise in guitar tube amp - 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 22nd May 2006, 03:22 PM   #1
diyAudio Member
 
Join Date: Apr 2006
Location: Utica
Default Frustration with hum and noise in guitar tube amp

Hi!

With some great help from this forum (here’s a link to my previous post Help with guitar tube amp mods I did a few mods to a tube amp head which is almost identical in appearance, layout and design to the Fender Blackface Tremolux Head. The mods were aimed to turn one of the amp’s channels into “overdrive channel” and to reduce hum and noise. I tried almost every hum and noise reducing technique, of which I was able to find information in this forum and the net, but with little success. The unchanged channel’s noise remained the same but is bearable. The “new” overdrive channel’s hum and noise are excessive and this is causing a great discomfort to my poor tone-loving soul. . I’m almost out of ideas. Please HELP if you can.

This experiment I did may be a clue for identifying the hum and noise source:
1. When I unplug the amp’s cable from the wall socket, it continues to play for about 6-7 seconds without ANY hum or noise whatsoever.
2. When I switch off both the power and the standby switches, the amp continues to play for about 6-7 seconds, but the hum and noise remain (decrease along with the signal).
3. When I switch-off the “standby” switch only (which cuts the HV supply to all tubes but leaves on the heating and the 50VDC power tubes bias) the amp plays only for less than a second (it’s more like a fart) and I can’t tell if there is noise or not. I have no explanation why in the above described cases the amp plays for a few seconds and in this case it goes quiet almost immediately.

Here are some other clues and notes:
• When I turn the “power” switch on (tubes heating and 50V bias only are "on") I can hear some low freq hum in the speaker. It’s almost non-audible and I really have to be very close to hear it, but it’s there. The PT is making some noise itself, but again, I have to practically put my ear on it to hear it.
• When all volume pots (a have a sep. volume pot for each channel and a master volume) are turned town, the amp is quiet, to the exception of the hum described in the previous paragraph but a bit louder.
• Even when the guitar is unplugged (thus the input is grounded), turning up any volume pots still produces quite hum and noise, more or less, depending on the stage for which the pot is used. The noise is not the “hiss” usually associated with pots, but an ugly mix of hum and noise,
• Plugging a guitar in does of course increase noise a lot (all else being the same) but not all that much, leading me to believe that the problem is not the guitar’s pickups, wiring or cable (one of my guitars has EMG 81 which is super quiet active bridge humbacker and the other has a passive humbacker which is as quiet as the EMG).
• By playing with the pots and by grounding the signal path here and there, I came to believe that each gain stage adds its share to the overall noise, the input stages (the first half of ECC83 and the also the guitar pickup – which could be viewed as a gain stage too) being more sensitive and contributing more noise than the latter stages.

Here’s a description of what I’ve tried so far with minimum to none results in terms of hum and noise reduction:
• Tried switching around and replacing all preamp tubes – no change at all
• Star grounded the whole amp, except for two of the power tubes grounds which are connected to the chassis - no noticable improvement
• Tried to use as short leads for parts as possible and ended up with some sort of ptp wiring - no improvement
• Used shielded wire for the first gain stage to the isolated input jack, to the overdrive pot and back, as well as to the tone stacks and back. The shields are connected only to one point - the ground for the corresponding section, usually the PS filter cap ground - no improvement
• Replaced all plate, grid, and cathode resistors in the preamp, as well as all tone stacks resistors with 0,6W Metal Film. I’m not sure of their voltage rating but the seller “assured” it’s sufficient, plus I’ve used the same resistors before in very quiet DIY tube preamps running at appx. 300V versus about 365V to 425V in my current project. So I wonder, what happens if the resistors in a tube amp are not rated for its voltages, can it be the cause of excessive hum and noise? (hmmmm – the amp was noisy with the old resistors so probably this is not it)
• For DC blocking, cathode bypass and signal attenuator caps in the preamp sections, I used WIMA MKP1/630V polypropylene caps - no improvement
• I made no changes or part replacements in the phase inverter and power amp sections
• Replaced the PS filter caps – no improvement
• Added in series a total of six 2k7 wirewound resistors and also new 33uF/450V filter caps decoupling the PS of each preamp gain stage - no improvement
• Tried placing very large resistors (39k to 68k) in the PS filtering RC network. No results despite the voltage drops.
• Disconnected the choke which was used for B+ to the OT and used it for B+ for the preamp section. No result, so I connected it back as it was.
• Tried DC bias of the 6.3V/AC filament supply – removed it, as it had no effect.
• I tried heating the first two tubes (four gain stages) with regulated 6.3V/DC (used a separate transformer). There was some improvement, but so small that I decided not to try providing regulated DC heating for the remaining tubes as it seems unpracticable.

So, I’m down to the following few mods I haven’t tried yet and most of which are associated with buying hart to find and expensive parts.
1. Put 10ohm/1W resistor across each rectifier diode – somebody in the forum suggested these will take care of voltage peaks from PS filter caps.
2. Get a suitable choke that can handle and filter the B+ for all tubes
3. Buy a 1:1 transformer to help clean up whatever is it that comes from the electricity grid and dares plaguing my sound. .
4. Try to heat all tubes with a car battery and if this helps, get a separate transformer and make a regulated 6,3 or 12,6 VDC for all tubes’ heating.
5. Regulate also the HV. (I suspect this may ruin the sound of the amp)

What do you think of these “last options”? Any other ideas!

I’m also currently tracing and drawing the complete schematic of the amp, but will probably not finish it until next week. I’ll post it, if anybody at all responds to my call for help and needs the schematic. Here’s a link to the schematic and layout of the Fender Tremolux which is almost the same amp http://www.schematicheaven.com/fende...b763_schem.pdf

Thanks & sorry for the post being so long
  Reply With Quote
Old 22nd May 2006, 04:17 PM   #2
teemuk is offline teemuk  Finland
diyAudio Member
 
Join Date: Nov 2004
Location: Suomi, Finland
Sounds like the source of hum is somewhere in the early stages of the preamp since you can control it's amplitude with gain/volume potentiometers. Then it is just gradually amplified throughout the various gain stages. This you have actually already figured out. Unplugging the amp will break the mains ground path - leaving it plugged in to the mains but powered off may not - this is the case at least with SPST mains switch. This might explain why no noise occures when you pull out the mains plug. This leaves me wonder what way the chassis grounded, does the amp have a three-prong coord?

A photograph showing the layout/wire dressing might be helpful. Here's few things you might want to investigate: Insulating jacks from the chassis - or are they already - are there some high current wires running near the preamp sections, the mains/chassis grounding scheme, use of separate audio/power ground paths.

Teemu K
  Reply With Quote
Old 22nd May 2006, 04:46 PM   #3
kwantam is offline kwantam  United States
diyAudio Member
 
kwantam's Avatar
 
Join Date: Jan 2006
Location: usa
Default Re: Frustration with hum and noise in guitar tube amp

Quote:
1. Put 10ohm/1W resistor across each rectifier diode – somebody in the forum suggested these will take care of voltage peaks from PS filter caps.
This will do nothing useful and will likely blow something up, quite possibly your power transformer. Don't shunt the diodes!

Quote:
2. Get a suitable choke that can handle and filter the B+ for all tubes
Possibly. If you do this, put another cap on the other side of the choke (shunt cap - series choke - shunt cap) of at least as large a value as the one on the input side. For that matter, you could just beef up the cap on the input side wthout adding a choke.

Quote:
3. Buy a 1:1 transformer to help clean up whatever is it that comes from the electricity grid and dares plaguing my sound. J.
Expensive and unlikely to work.

Quote:
4. Try to heat all tubes with a car battery and if this helps, get a separate transformer and make a regulated 6,3 or 12,6 VDC for all tubes’ heating.
Maybe, but try other things first. This is a pretty drastic step.

Quote:
5. Regulate also the HV. (I suspect this may ruin the sound of the amp)
It probably won't ruin the sound of the amp. Your supplies want to be stable. However, it is a lot of work. Going with a passive filter (as in option 2 above) is easier, and you can probably get almost the same improvement.

One other thing you should try: make sure that the power transformers are not coupling flux into the output transformers. Try to keep them far away from each other, or at the very least be sure that the fields are perpendicular to each other (that will reduce their coupling as much as possible). To do this: locate the axis along which the windings in the power transformer are wound, and the same for the output transformers. Make sure that these axes do not line up! So, if you have two output transformers and one power transformer that must live near each other:

- | -

will be much quieter than

- - -

Good luck!

-kwantam
  Reply With Quote
Old 22nd May 2006, 05:11 PM   #4
testlab is offline testlab  United States
diyAudio Member
 
testlab's Avatar
 
Join Date: Dec 2005
Location: Grand Rapids MI
Do your preamp tubes have shields on them? You can also try a heavier ac line cord.
__________________
It's not whether you win or lose, it's how you lay the blame.
  Reply With Quote
Old 22nd May 2006, 06:46 PM   #5
poobah is offline poobah  United States
diyAudio Member
 
poobah's Avatar
 
Join Date: Nov 2005
voivodata,

Your test, "pulling the plug", says something.

How is the AC wiring run through the chassis? The problem could be this simple. Magnetic or electrostatic fields from the AC lines could be coupling to your preamp circuits.

Both the LINE and the NEUTRAL entering your chassis should be tighly twisted together. The pair should be routed to the switch, and that pair should follow through to the transformer. The entire path should be kept well away from any of the ampification circuitry, especially the inputs and related tubes.

Can you post a photo?



  Reply With Quote
Old 22nd May 2006, 08:10 PM   #6
diyAudio Member
 
Join Date: Jan 2002
Location: nowhere
I see you have 50V for biasing the EL34s and the heaters. Does that mean the heater bias is negative? It must be positive or it will be worse than no bias!
About the list of mods you havent done yet, certainly do not do step 1! Such resistors will render the rectifiers useless and let the whole AC through. They are probably ment to go in series and not parallel? Actually I dont think any of those steps are any good. Regulating the HV will likely kill the tone, unless you only play jazz or speed metal. Im outta clues, just wanted to mention that heater bias must be positive. I think poobah has a good point about wiring.
Oh, the amp farts for only a brief moment when removing HV and not the heater probably because with heaters on the tubes suck all the juice out of the reservoir faster than when the heaters are shut off since without heaters they lose emission. My guess anyways.
  Reply With Quote
Old 22nd May 2006, 09:16 PM   #7
diyAudio Member
 
Join Date: Apr 2006
Location: Utica
Thank you all for your time!


Quote:
This leaves me wonder what way the chassis grounded, does the amp have a three-prong coord?
Teemuk, you hit the spot right there. I'm so embaressed to adimit how stupid I am. The amp has a two-line cord. After seeing your post I remebered reading somwhere that older amps are sensative to plug orienation. I reversed the plug, and half the noise was gone, just like that . The amp has a 0.015/1000V cap from the chassis to the appropriate cord line selected by some sort of a "ground orienatation" switch. I'm so happy. The amp still has hum and noise but now I have the feeling it is curable.
Quote:
Insulating jacks from the chassis - or are they already - are there some high current wires running near the preamp sections, the mains/chassis grounding scheme, use of separate audio/power ground paths.
All jacks are insulated. Other than heating, which is elevated there are no high current wires close to the preamp. I used two star ground points - one for the first PS filter cap, the OT, and PT center taps, and the other for the rest of the circuit. However, the audio/power ground paths are not totally separate. The local ground for each preamp stage is actually the ground lead of the PS filter cap for that particular stage. Is this wrong?



Quote:
One other thing you should try: make sure that the power transformers are not coupling flux into the output transformers. Try to keep them far away from each other, or at the very least be sure that the fields are perpendicular to each other
Kwantam,
The PT and OT are perpendicular and set exactly as you decribe is the right way. The PT however is bolted to the chassis which has a whole cut in it, so half of the PT is actually inside the chassis. I may try to come up with cover for it. Thanks for the other tips too. I've read only good things about chokes and this is what I'll go for next.



Quote:
Both the LINE and the NEUTRAL entering your chassis should be tighly twisted together. The pair should be routed to the switch, and that pair should follow through to the transformer. The entire path should be kept well away from any of the ampification circuitry, especially the inputs and related tubes.
Poobah,
Will do as you suggest. The AC is well away from input stages, but the "ground orientation" switch, the fuse, and power on switch, are right next to the power tube sockets.
Quote:
Can you post a photo?
.
Being a newbie, I have like 100k limit to attachments. I'll try to resample the pics but the quality will probably be awful.


Quote:
I see you have 50V for biasing the EL34s and the heaters. Does that mean the heater bias is negative? It must be positive or it will be worse than no bias!
It measures -50V, so it is positive - sorry about that I'm a new to tubes and not exert in electronics to begin with, which you can tell from my question about "rectifier diode shunting"



Quote:
Do your preamp tubes have shields on them?
No. Will have to buy some new sockets with shields.
  Reply With Quote
Old 22nd May 2006, 09:38 PM   #8
poobah is offline poobah  United States
diyAudio Member
 
poobah's Avatar
 
Join Date: Nov 2005
voivo,

Newbies and oldbies alike are limited to 100K. The trick is post your picture to an image-hosting-website (free), copy the URL and post that. your thumbnail will show in the post and then link the reader to image hosting site.

google "image shack"

You know... you should consider changing your input wiring to the new standard... it's about safety. A new cord, some "Y" caps and you're good to go. A ground is a nice thing with a metal chassis.



  Reply With Quote
Old 22nd May 2006, 09:46 PM   #9
diyAudio Member
 
Join Date: Feb 2006
Location: East central MN
^Agreed

Since you've already modded the amp quite a bit (lots of mods can bring down the price of vintage amps), replacing the power cable with a 3-prong, and maybe throwing a polarity switch in there too is a great place to start.

Funny, I've got almost no hum issues with my '64 Bogen using a 2-prong cord. I'm replacing mine with a computer-style socket so I can unplug the cable from the amp to move it, and change lengths if I need to. Just something to consider.

-Darren
  Reply With Quote
Old 22nd May 2006, 10:20 PM   #10
diyAudio Member
 
Join Date: Apr 2006
Location: Utica
Thanks for the tips

Here's the amp before mods
Click the image to open in full size.

...and after
Click the image to open in full size.

looks quite messy - used all sorts of colors for grounding wire, etc... The white things near the preamp are ceramic coated wirewound PS filtering resistors. I noticed I've cut out the input cable, but it's only a shielded wire going from pin 7 to the isolated input jack
  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
anyone with a low noise preamplifier circuit for guitar? winner Instruments and Amps 3 14th January 2009 06:51 AM
how to make a low noise guitar preamp? winner Instruments and Amps 4 8th January 2009 06:07 PM
noise after fixing guitar amp? rumpelstiltskin Solid State 4 26th April 2005 09:13 AM
Help - Extreme Guitar Problem (noise related) AlvinB Everything Else 10 3rd December 2003 03:36 AM


New To Site? Need Help?

All times are GMT. The time now is 09:04 PM.


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