Hum in Motorola HS-711 amp

ToxicTele

ToxicTele

AK Subscriber
Subscriber
Sweet little amp, 4 6BQ5's power tubes and a couple of 12AX's for the pre. I have recapped all except the ceramics. The amp sounds sweet and pure except for a slight hum when no music is playing. Question - could this be caused by old, mismatched power tubes or the ceramic caps? Or by both? I also changed any resistors that did not fall within specs.
 
Mismatched output tubes can cause hum in PP amps,,, I have found hum to be in the input jacks and leads more often then not in a freshly recapped amp...
 
All "hum" is not created equal, you need to quantify the frequency of the hum. Do you have 60 or 120hz hum? How many db is the hum, voltage. When you shunt the inputs dose the hum go away or reduced.
 
If the filter and coupling caps have been done, it is likely a grounding problem, or a bad tube(in one channel)
 
Thank you all. I have new tubes on the way, supposedly matched quad set. I did move the ground from under the multi-cap to one end when I replaced the multi cap with individual caps. I'll run a ground from the end to the original and see if that helps. Other than the hum issue, this is one solid little amp. I have 4 RCA's, one maggie, and two of these HS711's and they are by far my favorite.
 
Thank you RWood,,how do I identify high voltage secondary center tap? I did change the filter cap ground.
 
It will be one of the PT wires soldered to the chassis, and it is often Red w Yellow line.

With the unit powered down and drained, use your ohm meter and measure between the two (usually red) High Voltage secondary wires (pins 4 and 6 of an octal rectifier tube) and note the resistance. For example, lets say it is 200 ohms. The resistance from either of those red leads to your center tap will be approx half that reading, 100 ohms in our example.

But you will need to temporarily lift the lead you suspect to be the centertap from ground to verify this.

Or you could return your filter cap ground back to its original location. Or if you have 2-3 wires from the PT that are grounded in the same place, ground your reservoir cap there.

Long description, but it is fairly easy to try a few things, and it won’t require any new parts.
 
Glad to help.

If yours looks like this HS711B that I found in another AK post, then the original CT wire was grounded right at the cap can. Does yours look like this?

IMG_3691.jpg
 
Yes that's it. I soldered a terminal strip and remounted the caps on the side wall (where the yellow arrow starts) and in doing so moved the ground. I will run a ground wire back to the original ground and see if that clears the hum. Thank you very much!
 
RWood said:
Make sure that the high voltage secondary center tap wire is grounded to the chassis where the first filter cap is grounded. They ride together.

Here is a good read about grounding from Merlin Blencowe:

http://www.valvewizard.co.uk/Grounding.pdf
Click to expand...
Just read and re-read this chapter,,, very informative, but it raises a couple thoughts... If/when a can cap is connect to PT CT, at PS end of an amp, and the input jack is normally as far away as possible and mounted to the chassis,,, it seems that would be a ground loop? Anything I scratch build has a bus bar for grounding and usually starts on a PT bolt,,which is where the line in ground is also connected and terminates on the input jack grd lug... So its also tied to the chassis at that point... Chapter 15.7 states "to ground it only at the input jack"... All the amps are quiet, but it seems I need to go back thru them and disconnect the PS end of the bus,, but then it will have to be soldered to the input jack/chassis termination, to ensure it can't get loose? I guess mounting isolated input jacks would be wrong, then? I have done that to eliminate hum... specially on Pre amps...
Going to take an amp to the shop and see if/what difference it makes!!!! Thanks for the link!
 
Hey John,

I believe there are multiple "best practices" for where and how to ground components. One that everyone seems to agree on, and is borne out in the pic of the RCA above is that the rectifier ground (center tap in this case) and the first filter cap need to be joined together ...... wherever they are ultimately connected to the chassis. I included Merlin's article to show some background on why this is important, but not to imply that all amps should only use these techniques. In the guitar amp world, though, with high gain input stages, grounding technique does become more important - and noticeable - than in more traditional low gain circuits.

Like you, I personally like one ground buss connected in one place to the chassis, and have tried it connected at either the input jack or at the power supply end. It's kind of amusing that in the amp pictured above, made by RCA who literally wrote the book on audio circuits, there are least nine different connections to the chassis..... and yet it got RCA's stamp of approval and probably sounded great. Can it be improved upon? I would say "maybe" and that empirical evidence carries more weight than somebody's book learnin'. So please let us know what you find out when you try some different things!
 
Hey Dick,,, took the 5C1 down yesterday after reading the article,,, It has a split chassis, with controls at top and chassis sitting upright in the bottom... I built it with CT, bus, PS caps and line tap all connected to to the same lug on the PT... It was dead quiet, as I used all shielded cables from chassis to pots and jacks on one side of panel, line leads to the other... The bus terminates on a lug strip and the cable shields to the pot/jacks started there...

For S/Gs I cut the bus end from the PT lug bonded the line grounds from control panel to PT lug, thus leaving the bus connected to chassis only at the input end ... No change in noise/hum, its still quiet...
Its been my limited, humble experience that different methods work on different amps/circuits etc... Since I know it is well bonded to AC in,, and well grounded to the CT, main DC supply lug... I'm good with it!!!!
Thanks again for sharing that link, I should probably find the book!!!!
Hope the OP finds this pertinent to the thread, rather than derailing...
 
No worries about derailing. It's all relevant and grounded in practical solutions. The amp shown above is a Motorola, just wanted to clarify. I do have 3 RCA amps and the ground on those is no where near as straightforward as the Motorola. RCA on certain models used a funky ground wire that runs the length of the chassis and connected at each end.
 
Yep,,, That's the ground bus,,, I have a RCA movie/PA amp I rebuilt that also has one, but not all the grounds terminate on it...
 
Just want to give RWood a shout out! I moved the caps ground back to the CT ground and the amp is dead silent (well, almost dead silent). I must add that these motorola amps are very sweet sounding. And Ziggy played guitar.
 
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