• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Scott 222B Hum

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Administrator
Joined 2004
Paid Member
Rectifier pins 4:326VAC and 6:332VAC
Which are the phono tubes, the 12AX7's?
If so, I did see a lot of ripple and hashing on some pins.
All the electrolytics are brand new, retaining the original values.

Which tubes and which pins?

Also as Frank has indicated the voltage difference between pins 4 & 6 of your 5AR4 is rather large - seems possible that there is a partial HV secondary short in that power transformer. Scott wound their own transformers and it is possible that there is a quality control issue, but unlikely.. If there is a problem with this transformer it should get very hot even with all of the tubes removed, if not it won't. Might not be a bad idea to measure the primary AC current with the transformer unloaded as described..
 
One thing that I just noticed is your oscilloscope waveform. It looks like alternate positive peaks are higher in amplitude. That could certainly indicate some kind of transformer imbalance on the B+ winding.
As kevinkr suggested, pull all of the tubes and measure the current on the primary of the transformer.
Leave the amp powered up for a couple of hours with all of the tubes pulled. The power transformer should be only slightly warmer than the ambient air.
 
Administrator
Joined 2004
Paid Member
I've run into Scott amps where the secondary windings were off by several volts, and the transformer in question was otherwise OK, but I'd be very suspicious of one where the difference was 6V or so..

If it doesn't get hot unloaded then it is probably OK.. A partial short will result in a very warm or hot (smelly) transformer after some period of time.
 
Administrator
Joined 2004
Paid Member
I pulled all the tubes and will run the amp for a few hours to see if it gets hot. It always ran slightly warm.
What amperage should I expect to see on the primary side?
Would you like another screenshot of the scope of a particular test?

If you can touch the transformer after a couple of hours and it is not over say 60C or so it is probably fine. A shorted winding should result in it getting hot enough over some period of time (shorter the more severe the short) that you can't comfortably touch it.

I would hope that the transformer excitation current with no load applied would be not too much over 100mA, a true power meter would be helpful, but you probably don't have access to said device.

Something to keep in mind is that these amps use the chassis for signal grounds, so it might make sense to double check the integrity of all chassis ground connections, and you might also consider busing them all together with heavy wire.. You should check each ground for a solid connection to the chassis - I've run into a few instances where the connection had developed highish resistance between the lug and chassis...
 
Administrator
Joined 2004
Paid Member
Hi Kevin,
Good advice. I was going to suggest pulling the rectifier and measuring the HV secondaries again. I haven't seen winding that far off before. Leaving it sitting there on with all tubes pulled is a good test for transformer quality.

Another source for hum is the grounding for the filter caps, and also the circuit ground connections. It isn't uncommon to see a grounded tag on a strip used for a ground point. If the screw or rivet becomes loose, that ground chassis connection becomes intermittent or just plain open. This can be difficult to find if you aren't aware of the possibility. The same happens with chassis connections on tube sockets.

For heater hum, assuming everything else is fine, try disconnecting the heater centre tap and bias the entire thing up about +30VDC. That will greatly reduce hum with AC heater power (which I like). You can do the same thing with DC heater power as long as that supply isn't shared with anything else.

BTW, Heyboer seems to be an excellent company.

-Chris
 
Great advice again fellas...thanks!

It ran for 3 hours sans tubes and the xfmr remained at ambient.

I previously went over all the grounds under a magnifier and all looked fine, but it won't hurt to study them once again.

This amp ran whisper quiet for 2 weeks after recapping before the humming began.

I also tested all the replacement caps and they're also good.

My secondary measurement was with the rectifier in place. I'll take another reading without it.

Again, I'd like to exhaust all other possibilites before purchasing a new transformer.

Could a faulty 6BL8 be injecting this noise down the line?

Chris, would you please detail the bias test? I'm soft of a newbie to tube audio.

Thanks...
Ken
 
Administrator
Joined 2004
Paid Member
Rectifier socket pin voltages without all tubes:
P2-5.5vac, P4-347vac, P6-347vac, P8-0v

I rechecked all the ground connections. No issues found.
Are we getting closer to the humming source?

Well I think your power transformer is OK, however I am less sure about your rectifier. Please measure the DCR of the HV secondary from center tap to pins 4 & 6 of your rectifier - if the resistance is not significantly different I would consider the purchase of a new rectifier tube.

I'm not sure whether we are closer to the source of the hum or not. You may need to literally trouble shoot stage by stage to determine the source of the problem. I would certainly take a look at the ripple at every single supply decoupling capacitor and see if it makes sense. The ripple level on the plate supply to the phono stage should be very low - probably on the order of mV..
 
Iam looking at my schematic, and remembering problems in servicing these. I have read ( but may not remember all that has been said so far...so please excuse).

Questions:
are the 5ar4 tubes new? Both? Same brand? Have a tube tester and the both test good, close sides, no shorts or leakage?

First cap (20 at 450), what is the AC ripple? More than 1% or 4 volts?
Second power cap (20@450) what is is the AC ripple there? millivolts?

Have you checked the preamp tubes for short/leagage? Are any EI or Seimens or RFT?

This hum that appeared after a few weeks ( correct? ), how did it come on...slowly, all at once...is it there as soon as you turn on now or does it take a few minutes to appear?

Have you checked or rechecked C202 in the bias chain? Do you have 48ish volts at the common point for these caps....>>>>>(and these good) the cathode caps C127 and 128, 27 and 28?.

Have you double checked ( power off all caps discharged) all your grounds from one common point?

That you had this appear after some time without any warning, tells me it is not your transformer. You can check your windings ( unsolder the leads) against ground and or other windings if you are in doubt.
 
also...use your didital meter and see if you have the hum hz at the tape out. If you don't have a frequency function, the ac should be near zero if not zero after a few seconds.

If you need to check stages, disconnect the lead out from the loudness and try to input it to a different amp ( it is cap protected at that point so no DC will be sent to the second amp).
 
The HH Scott amps have chassis ground wires looped between terminal strips and through the common connections of the electrolytics. The wires provide a good ground connection that doesn't rely on the terminal strip rivets to provide stable connections to the chassis.
Is it possible that you have broken the ground wire circuit when you disconnected the original electrolytics?
Perhaps one of the riveted ground terminals has developed some resistance to the chassis.

Since you tested the amplifier using a 5U4 tube and observed the same hum, I doubt that the problem is the 5AR4 tube.

Whatever. It must be power supply related since you have hum in both channels.

I'm still a bit concerned that the B+ voltage is low. The old 6BQ5 tubes might be drawing excessive current. You may have a power supply problem that has become audible because one or more of the output tubes is drawing too much current.
 
Administrator
Joined 2004
Paid Member
I attempted to post the following last night, but Microsoft deemed I wasn't worthy. I did manage to save my post, so here it is. Please excuse material already covered from then by other members ...

Hi Ken,
If your transformer ran for three hours without warming up, I'd be a bit suspicious that the primary wasn't receiving power. All transformers have some energy lost through eddy currents in the core, so expect them to become a little warm when left running with no load on the secondaries. Just a little warm is normal for that test. Also, when running that test, measure the secondaries to the chassis again and write down the voltages please.

If a 6BL8 were to inject noise, I would expect it to be in the form of a 60 cycle sine wave. If you are getting a saw tooth at 120 Hz, that tells me you have a problem in a rectified power supply. I'd be more comfortable if you just followed the clues rather than jumped to conclusions. Let's forget about the 6BL8 for now and follow the clues.

From the looks of the schematic, your heaters are firmly grounded. The small signal tube heaters appear to be supplied by the cathode current from the four output tubes. This is supposed to supply an almost noise-free DC current. Therefore, no playing around with DC heater bias.

Two things come to mind now. Your low plate voltages can be caused by resistors that have gone up in value, or an output stage that is drawing more current than normal. Either can cause low plate voltage, and yours isn't extremely low anyway. Have you checked the resistances of the dropping resistors in the power supply yet? It's also a good idea to measure the voltage drop across each of the resistors and write everything down on paper as you make the measurements.

A bad output tube, either low or high plate current that doesn't match the other of the pair will create a hum on that channel. Make sure the various balance controls are near their center positions to start. Then perform the balancing adjustments per the manual. Once you have finished the various adjustments, listen for hum again.

-Chris
How's that? I put it in a quote to differentiate it from right now.

Hi Frank,
You're saying pretty much what I am - so I think you're on the right track! :) I still would like to consider that the older dropping resistors have shifted value. Something I see very commonly along with all the others (excepting ww types).

Hi Ken,
I guess luke-warm is ambient then, and your xformer is receiving power just fine. Your equal AC voltages pretty much clear your transformer. Please do check the resistances of your HV dropping resistors, then go back with all the tubes installed and measure your actual voltage drops please. It isn't easy to measure cathode current in this amp, so one idea may be to install 1R0 resistors in series with each output tube cathode, then measure the drops. These will not interfere with normal operation too much and allow you to easily determine what each output tube is conducting. You can also get a really good idea of balance between each pair as well.

-Chris
 
All; The apparent cause of the 120Hz hum was poor grounding. The exisiting ground connections were using light gauge wire, so I beefed up the grounds throughout the chassis and the humming disappeared. This 50 year old amplifier has be operating daily for 2 weeks now, without any issues. Thanks for all your great advice.
--Ken
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.