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Old 21st May 2008, 07:15 PM   #1
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Question Beginner WLTM fixer of 300B hum...

Hello all!

I've lurked here for a while, ever since starting building my 5687/ECC83/ECC82 pre-amp last year. I have since made a 6SN7GT/300B power amp too and am now at the "troubleshooting" stage without knowing very much about how the valve circuits work.
I have a basic bit from making them with my retired electrical engineer grandfather who made amps in the 50s, however he's at the other end of the country and I couldn't test it with him.

The basic problem is that the 300B amp hums...I'm making some Fostex full-range speakers using the FE207E so as it's audible now it will become far more so when they're finished. Not having an oscilloscope, I played my bass guitar while listening to the hum and have decided it's around a bottom G#, or about 50Hz

Attached are the diagrams for the PSU and the amplifier circuit.

It hums at an audible level - with some old 93dB/W/m Technics from my dad's HiFi it's clearly heard as a low tone from across my room - instantly noticeable as having been there if it's switched off, if you get my drift.

Measuring AC across the speaker terminals gave 46mV across one and 17mV across the other, however this has levelled off to around 20mV on both today. Part of the problem may be that I yesterday noticed one of the 6SN7GTs 'jangles' when it's moved, indicating something pinging around inside - I guessed this was what caused the intermittent crackle too. However it hummed before this so I don't suppose it is making much difference. (Don't worry - I am getting a replacement!)

Things I've tried:

Checking grounding here there and everywhere. Not to any great extent, you understand, just that everything that is grounded has 0 ohms between it and ground. An earthing diagram is attached.

I've tried it with no valves in (weirdly there's still a small hum, even when B+ is disconnected) and with valves in then after the caps are charged I removed the B+ to see if it was high voltage in the output transformer, this actually increased hum so there doesn't seem to a be any induced stuff there.

Swapping tubes, but as it's equal on both channels I had not many hopes for this. With no 6SN7s plugged in the hum's basically the same, so I think I've ruled out the input stage as it's also the same with shorted phono plugs and all valves plugged in.

I wondered whether the 300Bs were running at the wrong values as I metered 460V the other day, but I expect I had the meter (borrowed) set wrongly as I've been happily listening on low efficiency speakers for a few months now with no strange glows or weirdness. It sounds (or sounded) great with a Rega p3 on some B&W speakers (I tried some last week at a shop) but I was too far from the speakers to notice the hum. Oh, and the 6SN7 wasn't broken at that point.


What I'd be very grateful for any of you chaps (and chapesses!) to do is have a look at the circuit and see if there's anything I ought to be fiddling with and explain why it's been suggested so I can understand what's going on...I'd quite like to know how to troubleshoot myself and gain some experience of what should be where and why. I've got a physics A-level so I'm up to speed on most of the laws/rules and not a total layman. Also could someone give step by step instructions on which pins to meter on a 300B and what sort of values I should be looking at?

Thank you very much for reading and I look forward to any replies...

I might treat myself to some cautious optimism, having seen what geniuses lurk on these fora...
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File Type: jpg 300b psu.jpg (78.8 KB, 348 views)
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Old 21st May 2008, 08:53 PM   #2
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Here's the audio circuit.
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File Type: jpg 300b circuit.jpg (72.8 KB, 321 views)
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Old 21st May 2008, 10:23 PM   #3
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Is there a way to put more than one file in a post?

Here's a photo of the wiring (I do feel ashamed at its untidiness now I've seen what some people can do) With the grounding points marked. I can post a picture of the layout of the components "above ground" as it were if that will help anyone relate it to the schematic.

Oops...resized to fit here it's a bit small. The top right is "Mains + HT", top right is "Output transformers" and bottom is "Signal and input".

Thanks for looking!
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Old 21st May 2008, 10:56 PM   #4
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Assuming the green dots are your ground points, this is your problem. Or at least a big part of it. You have grounds at three different locations on the chassis causing ground loops between them. You need to use a single point (star) ground or a heavy ground buss that's grounded to the chassis at only one point. That point is usually near the center of the chassis, but may need to be found experimently by moving it from one place to another with the use of a buss.

Start by lifting all three of those points and connecting them to the chassis at one central location near the middle.

As far as I know it's one picture at a time using the site as a host.

Victor
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Old 22nd May 2008, 12:07 AM   #5
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I agree with the above. I would do a ground bus or one big star ground and see if that helps with the hum. Even if that is not the problem it is still a good practice.

Just out of interest how big is your choke?

Have you tried a hum pot on your 300B?

Good luck with your build and solving your hum problem!

Cheers

James
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Old 22nd May 2008, 04:14 PM   #6
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Did you say that you still have hum without the valves fitted to the chassis...?

If so, it could be induced hum from mains Tx to O/P Tx....

Normally the mains and o/p transformers are mounted at right angles to each other, so as to reduce to a minimum the possibility that the magnetic fields from the mains Tx are not induced into the O/P Tx....

'Star' grounding is best, with the chassis grounded at one point only--Often, this is done at the Input sockets or at the first gain stage....
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Old 22nd May 2008, 04:17 PM   #7
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Once you have sorted out the grounding issues, you should also take a look at the way you are heating your 300B. I have not had acceptable results with AC heating and high efficiency speakers (97dB/1W and 103dB/1W) with any dht running on more than 2.5VAC filaments.

I heat my 300B using dc CCS built around LT1085 and the hum at the output of my amplifier is <500uVrms.. You might get away with a 100 ohm 5W pot in the filament circuit of each output tube. (Connect the pot across the filament winding and move the cathode bias resistor and bypass cap to the wiper, disconnect from the center tap. This will allow you to find the hum null point if it exists for each individual dht.)

Given that you are using cathode bias you can also employ a variation of the old WE hum canceling technique. Connect a film cap of ~1/5 the value of the cathode bypass cap from the B+ to the cathode reference point of each 300B. (CT of filament transformer or wiper of pot if you go that route.) This reduces ripple induced noise on the output and can be good for >20dB with care, but does not address the filament issue.

As you fix each issue you may find others get unmasked so don't give up.

460V B+ is not unreasonable since the amplifier is cathode biased, and depending on the tube op point the effective plate voltage is 70 - 100V lower than this. I run mine at 400V and 60 - 80mA typically.

Removing the driver tubes indicates that the problem is arising in the output stage, and may be filament or magnetic coupling related rather than anything else. (Filament coupling will usually have a pretty strong 2nd harmonic component to it: 100Hz for 50Hz mains, but I have had mains frequency hum from this source as well despite the theory.)

FWIW if you have the skills to build a 300B amplifier (and you clearly do) you ought to get an inexpensive scope and learn to use it - you will be amazed at how much easier this makes things..
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Old 22nd May 2008, 05:40 PM   #8
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HollowState: I did wonder whether there might be ground loops, but as I was given those grounding points on a diagram as the best way to do it I assumed they'd been tried and tested. Unfortunately I can't get in touch with the chap who supplies my bits (and the diagrams) as he's not answering his phone. Would it be all right if I simply unscrewed the bolts that are the grounding points for two circuit areas and simply brought a wire across from each of them to the remaining bolt? I assume if I found that that made a difference I'd wire it up permanently with some relatively heavy gauge copper?

JPeitzman: Hang on while I check my notes...my choke is a 5H 300mA. I haven't tried a hum pot for the simple reasons there wasn't one on the schematic and after I'd read about them I wasn't sure what value to use or how it was to be implemented However kevinkr's post seems to have sorted that one out. I should try the altered grounding first though? Thanks for the good wishes!

Alastair E: I have a small amount of hum when the speakers are connected, the thing's on with no valves and the B+ is disconnected from before the choke, so it hasn't even had a chance to reach the caps! I hoped that would indicate no hum from an induced emf of the B+ through the output transformer... The trannies are oriented at right angles to each other and as far away as the chassis allows. I attach a picture of the top of the chassis for your delectation.
I wondered whether speakers have some kind of "residual hum", if you like, from merely being connected, then thought what a silly idea that was if they have no power. Mind you, they are tiny Sony satellites from a home theatre system but if I can hear hum through those, I thought my poor soon-to-be Fostexes will be very upset. I don't see how they can have even the slightest hum when there's nothing going through it! It stops when the amp's turned off though. I thought it might be induced in the speaker wire from something, but moving it around didn't seem to help. Very weird!

kevinkr: I did wonder about DC heating, but thought I'd see what I can do before that as having to filter and rectify etc. seemed a last resort. If nothing else works, that's the road I have to take I suppose! Are the cathode bias resistors the 1k 50W ones I have connected to the heater centre taps? I shall be reading up on biasing this evening as I'm not sure what it's all about. To be honest I didn't know I was cathode biased or what!
For the WE suggestion I should put (if I'm looking at the right resistor/capacitor pair on my diagram) a 20F 110V (should the voltage rating be similar?) capacitor from the B+ of each 300B (left and right) to - if everything else in the circuit stayed the same - before the resistor and bypass capacitor or to ground?
Could you give me some short instructions on what pin to meter to where to get reliable values? Is the plate voltage measured from it to ground for example? I have a constant 5.15V on the heaters - is that too high?

Many thanks for all the replies - what a great bunch of helpful people. I shall try things in this order and get back to you:

Lifting the grounding to a single point, as close to 'star' as I can get.

See if I get better results with a heavy earth rail

Hum pot

kevinkr's WE technique

Unfortunately there are NO component shops in York, so everything has to be mail order, so the latter two will take a little longer. Disgraceful, eh?

Thanks again.
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Old 22nd May 2008, 05:40 PM   #9
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Oh, and Alastair...those gold valves look brilliant in your picture! What are they?

Is it just my eyes, or does your valve have two spikes on the top, kevinkr?
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Old 22nd May 2008, 09:26 PM   #10
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Yep, those are two spikes on top of that 6C33, there are also 3 spike versions floating around. I used to have both - now I have none..


WRT the WE configuration the 20uF - 22uF cap should be at least 400V rated. Don't use an electrolytic here, I would recommend a Clarity Cap SA type for this location..

The cap goes between B+ and the cathode common point. (Currently the center tap of the transformer.)

The improvement in sound quality (high frequency performance) was quite surprising in the 2A3 amplifier I built having this feature, and the reduction in output ripple was measurable.
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