# Vintage tube amp hum



## WannabeGood (Oct 24, 2007)

Hello GC amp techs. 
Seeking help/information for a substantial hum problem in my Symphonic MA-7 tube amp. 
I acquired this amp about 10 years ago and I had minimal playing time when I got it. Has been unused/in storage til recently. 
Amp has a power transformer but only a 2-wire power cord. When I fired it up the amp has an annoying hum which is exasperated with the volume knob.
Is this a case of needing to replace the caps and would a grounded power cord help? Amp is early '60's vintage.
Lastly what would realistic value be on the amp? Physical condition is VG and mostly original.
I made a new handle for it and extended the speakers leads to the Jensen 6" speaker. There is one on Reverb (lesser condition IMHO) priced at $550.
Any and all comments/suggestions/opinions appreciated.

Regards,


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## Paul Running (Apr 12, 2020)

Has an isolation transformer been installed?


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## WannabeGood (Oct 24, 2007)

Only has the power transformer and output (speaker) transformer. I'll try to post the wiring schematic later.

Regards,


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## Griff (Sep 7, 2016)

Just out of curiousity, have you tried flipping the 2 prong power cord around?


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## WannabeGood (Oct 24, 2007)

Griff said:


> Just out of curiousity, have you tried flipping the 2 prong power cord around?


Doh...........no I haven't. Wil try that shortly.

Regards,


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## WannabeGood (Oct 24, 2007)

WannabeGood said:


> Doh...........no I haven't. Will try that shortly.
> 
> Regards,
> 
> ...


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## WannabeGood (Oct 24, 2007)

WannabeGood said:


> Hello GC amp techs.
> Seeking help/information for a substantial hum problem in my Symphonic MA-7 tube amp.
> I acquired this amp about 10 years ago and I had minimal playing time when I got it. Has been unused/in storage til recently.
> Amp has a power transformer but only a 2-wire power cord. When I fired it up the amp has an annoying hum which is exasperated with the volume knob.
> ...


Edit.


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## Paul Running (Apr 12, 2020)

If that was my amp, I would replace the half-wave rectifier with a full-wave bridge, that would reduce some hum.
Modify the front-end to a typical front-end. Move the volume control to, after the first stage, preferably between the first and second stage.
Installing cathode by-pass caps in the preamp will reduce more hum and extend the gain; it will add more distortion which should not be an issue if you prefer added distortion


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## WannabeGood (Oct 24, 2007)

Paul Running said:


> If that was my amp, I would replace the half-wave rectifier with a full-wave bridge, that would reduce some hum.


Did I mention I'm electronically challenged?

Regards,


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## Latole (Aug 11, 2020)

Hum because.
Heater wiring are not insulated from ground.
Heater must be flaoting

And need a 3 prog power cord
Input jack with shorting contact to ground may help to keep amp quiet , only when no guitar plugged.

50L6 power tube have a maximum output power of 3.8 watts

This amp with some mod is a very good amp. I have 2 of them
Value on the market ? $200 ? Less ?


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## Latole (Aug 11, 2020)

Paul Running said:


> If that was my amp, I would replace the half-wave rectifier with a full-wave bridge, that would reduce some hum.


No


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## WCGill (Mar 27, 2009)

Unused, storage? Sounds like filter caps to me.


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## Latole (Aug 11, 2020)

Filter could be bad too
Fist, amp should have the right heater wiring. It have not. And 3 prong ac cord

Filter cap easy to check; read AC ( yes AC not DC) if more than 1 volt, replace filter caps


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## silvertonebetty (Jan 4, 2015)

+1 filter caps and three prong cords. Years ago my Mesa would pick up the radio and would buzz when an text message would come I.
My fender does the same . The closer any electronics are near it , the more it picks up the interference and buzzes.


And an old worker from long and McQuades told me

“ oldamps can pickup noises due to filter caps and the different wiring in your house”

i don’t know if it’s true or not but the cord is a safety thing so you don’t get blown through a wall.

have you went through a wall before? I went through a door once it hurts, but I learn double check that the door is open before jumping on your bed lol.
the Best part of that story is almost 20 years later , they still haven’t fix the door frame lol 😂


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## Robhotdad (Oct 27, 2016)

Filter caps. An amp has to be played otherwise the caps go bad sooner. As for the filament grounding, take a look at an old Marshall 100 Watt or a JCM 800. The 6.3 Vac secondary has a grounded center tap. That 6.3 Vac secondary is not floating in those amps.


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## Latole (Aug 11, 2020)

Isolating the heater wires from the chassis is the first thing to do with the 3-legged AC wire.

The filter cap is easily checked if it is the cause of the noise or not as I mentioned.
It is alway good to replace a old one if you have to work in the amp.
After those work, amp will be good for another 50 years


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## tomee2 (Feb 27, 2017)

Latole said:


> Hum because.
> Heater wiring are not insulated from ground.
> Heater must be flaoting
> 
> ...


Why does the schematic ground the heater? Mistake and it shouldn't be?


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## Latole (Aug 11, 2020)

More easy building, cost cutting or builders don't no know how to build amp ?


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## Lincoln (Jun 2, 2008)

tomee2 said:


> Why does the schematic ground the heater? Mistake and it shouldn't be?


old way of doing things. Even Fender in 50's grounded one leg of the heaters. I've seen old transformers with only one 6.3V heater wire. No choice but to ground the other side.


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## tomee2 (Feb 27, 2017)

Lincoln said:


> old way of doing things. Even Fender in 50's grounded one leg of the heaters. I've seen old transformers with only one 6.3V heater wire. No choice but to ground the other side.


Is this related to having plugs that you flipped around until the hum was lower, which was also the orientation to not get a shock if you touched another item plugged into the same outlet?


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## Lincoln (Jun 2, 2008)

tomee2 said:


> Is this related to having plugs that you flipped around until the hum was lower, which was also the orientation to not get a shock if you touched another item plugged into the same outlet?


I "think" that was more to do with which primary wire was hot and which one was neutral at the time. (the plug-in) 
Remember when amps all had a grounding switch? That was its purpose, flipping the hot and neutral legs.

Pre-1954, buildings were wired with 2 conductor wire (after that they used 3 conductor wire). When there was only 2 conductors and no big/little on the 2 prong plugs, I don't think electricians paid any attention to which side of a receptacle was hot and which was neutral.


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## Latole (Aug 11, 2020)

Lincoln said:


> old way of doing things. Even Fender in 50's grounded one leg of the heaters. I've seen old transformers with only one 6.3V heater wire. No choice but to ground the other side.



Transformer have always 2 heater wires. I mod many of these kind of amps.
OP schematic show 2 wires


Old Fender amp with one heater wire at ground , see PT with 2 wires.

View attachment 407372


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## dtsaudio (Apr 15, 2009)

Lincoln said:


> old way of doing things. Even Fender in 50's grounded one leg of the heaters. I've seen old transformers with only one 6.3V heater wire. No choice but to ground the other side.


Heaters need to referenced to something. Usually it is ground. Grounding one leg or using a center tap can amount to the same thing. Lifting that ground connection will make hum worse not better.
Your problem is quite likely the filter caps. This is an old amp, that's been sitting. Re-cap it.
Install a three prong power cord as well and remove the .05ufd cap on the transformer primary too.


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## Latole (Aug 11, 2020)

I answer on the wrong forum
Sorry


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## Latole (Aug 11, 2020)

dtsaudio said:


> Your problem is quite likely the filter caps.


You can't know before testing as I wrote.


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## tomee2 (Feb 27, 2017)

Latole said:


> Transformer have always 2 heater wires. I mod many of these kind of amps.
> OP schematic show 2 wires
> 
> 
> ...


Ok, so in this case.. you could put the 6.3V across the heaters, then add a pair of 100ohm resistors and put where they meet back to ground? No.. the heaters are in series here? I gues it depends on if that's a 12vac or 6.3vac winding?


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## Paul Running (Apr 12, 2020)

The way the secondary is configured for the mains transformer, it would be very difficult to balance the filament circuits...it is a tapped secondary, where the plate winding shares the filament winding. The mains transformer should have a separate winding for the filaments; this would allow for the implementation of a full-wave bridge rectifier and an easy method to balance the filaments. IMO anybody who tries to sell this amp as is, would be subject to a liability law suit...a law suit amp, unlike a law suit guitar.


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## dtsaudio (Apr 15, 2009)

Latole said:


> Add two 100 ohms resistor to ground, one from each leg.
> A very common mod
> 
> View attachment 407377





tomee2 said:


> Ok, so in this case.. you could put the 6.3V across the heaters, then add a pair of 100ohm resistors and put where they meet back to ground? No.. the heaters are in series here? I gues it depends on if that's a 12vac or 6.3vac winding?


Look at the schematic. One tube has a 50V filament on has 35V filament and one has a 12V filament. They must be in series, and that resistor mod will not work. Leave them wired as is.



Latole said:


> You can't know before testing as I wrote.


You have a 60 year old humming amp. What do you think the problem is?


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## tomee2 (Feb 27, 2017)

dtsaudio said:


> Look at the schematic. One tube has a 50V filament on has 35V filament and one has a 12V filament. They must be in series, and that resistor mod will not work. Leave them wired as is......


My comment was for a different schematic, champ 5c1, that was posted that had 2 6v tubes.


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## Latole (Aug 11, 2020)

tomee2 said:


> Ok, so in this case.. you could put the 6.3V across the heaters, then add a pair of 100ohm resistors and put where they meet back to ground? No.. the heaters are in series here? I gues it depends on if that's a 12vac or 6.3vac winding?


I answer on the wrong forum
Sorry


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## Latole (Aug 11, 2020)

Latole said:


> I answer on the wrong forum
> Sorry



Update.


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## dtsaudio (Apr 15, 2009)

Paul Running said:


> The way the secondary is configured for the mains transformer, it would be very difficult to balance the filament circuits...it is a tapped secondary, where the plate winding shares the filament winding. The mains transformer should have a separate winding for the filaments; this would allow for the implementation of a full-wave bridge rectifier and an easy method to balance the filaments. IMO anybody who tries to sell this amp as is, would be subject to a liability law suit...a law suit amp, unlike a law suit guitar.
> View attachment 407395


While this is certainly not an ideal way to wire the filaments, fused and grounded properly it should be relatively safe.
You'd need to add a 48V transformer to do what you suggest, but this will also remove the filaments and their resistance from the high voltage, raising B+ quite a bit. 
Notice the fuse is wired incorrectly by today's standards too.
One mod that could be done is replace the 56 ohm resistor with a choke.


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## Paul Running (Apr 12, 2020)




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## Lincoln (Jun 2, 2008)

dtsaudio said:


> Heaters need to referenced to something. Usually it is ground. Grounding one leg or using a center tap can amount to the same thing. Lifting that ground connection will make hum worse not better.
> Your problem is quite likely the filter caps. This is an old amp, that's been sitting. Re-cap it.
> Install a three prong power cord as well and remove the .05ufd cap on the transformer primary too.





dtsaudio said:


> Look at the schematic. One tube has a 50V filament on has 35V filament and one has a 12V filament. They must be in series, and that resistor mod will not work. Leave them wired as is.
> 
> 
> You have a 60 year old humming amp. What do you think the problem is?


You can bet it didn't leave the factory humming hard. What changed? Filter caps went for sheet of course. 

Kind of an interesting design though, for me at least. First time I've seen radio tubes used with a power transformer. They must have been paying people to take massive lots of radio tubes at one point?


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## diyfabtone (Mar 9, 2016)

I'm confused: Do you have a M-7 or a M-1 ... totally different schematics. The M-1 is single-ended and will be prone to hum unless you take extra steps to correct the 'flaws'. The M-7 schematic is strange as the output tranny has no centre-tap and it's push-pull.


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## Paul Running (Apr 12, 2020)

diyfabtone said:


> Do you have a M-7 or a M-1


The posted schematic shows MA-1.


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## WannabeGood (Oct 24, 2007)

diyfabtone said:


> I'm confused: Do you have a M-7 or a M-1 ... totally different schematics. The M-1 is single-ended and will be prone to hum unless you take extra steps to correct the 'flaws'. The M-7 schematic is strange as the output tranny has no centre-tap and it's push-pull.


Let me try to clarify.
1) The schematic posted is what is mounted inside the amp cab.
2) The following photos are the actual amp in question.
I will post gut shots later if that might help.
I presume you know Symphonic amps and will correct my inaccuracies.

Regards,


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## WannabeGood (Oct 24, 2007)

My thanks to those that provided comment and some insight as to what might need to be done to quiet the hum problem my Symphonic amp has. The suggested modifications, sans the three wire cord replacement, is outside of my skill set and wheel house so I've resolved myself to find a service tech to remedy the problem. Any volunteers? 

Regards,


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