Original 300BPP hum fixed

[Steve Grimshaw]

Hi Richard, many thanks for this interesting post. Obviously as you say you wanted to do this for safety reasons, but the reduced hum puzzles me a little. I remember hearing your amp and am sure I cannot remember any noticeable hum. Is this something which has got worse as the valves have aged?

Steve Grimshaw

[Richard]

Hi Steve,

Your memory's fine, it didn't hum noticeably. It was always on the cusp, a presence in the room rather than a hum. With 5 of us there the rise in the ambient noise floor easily masked it but I was aware of it when alone.

After a quick comparison between my standard EH300Bs and Greg's GG versions we used his valves in my amp if you recall. I later bought a set of GG and over the past year they haven't needed adjusting.

When I posted this thread I'd only just done the earthing. Since, I've listened to more records and there is more than just an improvement in hum. Deep bass has improved, and possibly overall clarity to some degree but that's harder to say.

I do find this fascinating. Looking at the pic with the pink earth lines on it I'm reminded of a glass with 5 straws (don't ask why). All the straws are small pipes to a large reservoir.

If some of the straws were joined together before going into the glass, what would the effect be on the drinkers? Hence, priority/relative impedance is important I think. Give the biggest suckers (the speaker posts) the best route to the glass and they won't interfere with the others. And so on down the pecking order. I think it's Morgan Jones who explains that simply changing the order/distance of connections on the same earth buss bar can change the hum.

As the earth is half the signal I suppose it's not surprising it should affect sound quality.

Rich

[Steve Grimshaw]

Well that is very interesting Richard. I've never really considered the order/distance to the earthing bus bar before now, but this makes perfect sense and as you say there is some reason to think that the earthing arrangement could affect the sound. There has always been a slight hum on my amp audible at the listening position, so I think I will give this a try over the weekend if I can get the time (I'm busy at the moment building a theremin for my daughters band). I'll report back:-)

Steve Grimshaw

[Ianm2]

90% of problems in vavle amps are usually down to earthing, other 'build' issues aside like soldering.

don't forget layout and wire routing is critical, too.

couple of issues with the hum pot, its not my area, but over time, with the cathodes changing, it could perhaps give problems initially when 'balanced', it may be fine, but ageing?

be interesting to see what happens

however, ideally, I would like to do away with the pots.

I notice in the original tim de para design, there were pots on all the valves, on Andy groves rehash, he only provided half.

they aren't ideal, as everytime it begins to age, you have to take the bottom off, put your hands in 565 volts, and adjust.

its easier if they are on top, but that gives aesthetics problems

in a good push pull amp, there should be minimal hum so you can use ac heaters, recall that push pull/balanced/differential tends to cancel unwanted things like noise, distortion, and all that.
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chokes were used early on apparently as big psu caps weren't available, so they needed xtra filtering, and amps were primarily single ended, too.

so its something few have questioned, I have a feeling they ought to be redundant in pp, but I will of course revise if listening proves otherwise


that's why I am banging on about removing chokes in psu's, as there is less need for filtering, and may improve the transients in the psu, leading to perhaps better sound, its an easy mod, just replace with a resistor the same dc res., and you could then use bigger caps for more grunt.
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if hums still a problem, there is always dc heaters, but shouldn't be needed in a pp.

I have looked at the schematic again, there isn't much you can do to the circuitry to improve it, its just a 'simple' pp amp, with decent valve choice.

perhaps try removing the rectifier....heresy, and see if that improves, again using resistor to compensate for ht.

and I can't push enough experimenting with output transformers, i know the amp sounds great, but an amp of this ability deserves the very best, some old ones fit the bill, and some expensive modern ones.

interstages too, but its hassle.

[Steve Grimshaw]

Hi Ian

Yes, doing away with the pots altogether would be the way to go but at the moment they are quite critical, even changing over to a different make of valve necessitates a change in pot adjustment so no chance of a fixed resistor there as things are at the moment. I think all the original 300b clones have the pot adjustment on the top of the amp, mine certainly does and I seem to remember Richards, Gregs and Neals also are the same. It would be a hell of a hassle if the adjustment were inside the amp :-)

I did at one stage consider changing the interstage transformers for some really good ones but decided against it after some thought. Better interstage/op transformers, would no doubt be technically better but would they upset the balance of sound for which the WAD original 300b is so well regarded? You could end up remortgaging the house to afford that 6n silver wound transformer set only to find you preferred things as they were. On the other hand it could sound much better with the new trannys :-)

Steve Grimshaw

[NealG]

There's me all smug thinking how well my 300B is sounding when it goes and develops a loud hum! New valves and a fruitless session with the hum pots didn't make it go away.

I had a suspicion of what it was though. My amp is about 14 years old now and still running with the original PSU 'lytics....I'd purchased some Rifas last year but not bothered to fit them, too much hassle. Well it looked like I was being forced into doing something about it LOL!

The original 'lytics fit under the PSU board and are 40mm in height, I couldn't find direct replacements so went for the Rifa's at 50mm with the view to locate them elsewhere in the amp. They are 85c versions and the Rifa's 105c so they should last a bit longer.

Inspecting them I saw one had started to swell so out they came!

With everything back together hum had dropped to 1.2~1.5mv! Good hunch huh?!

What I was not prepared for though was the improvement in sound! It's amazing how you can get used to a sound and not notice a drop off in performance over time. What I can't work out is why these caps seem to have an influence on the sound, they are 'behind' the chokes and GZ37 so not really part of the signal path, C11 and C12 effectively short AC to ground in front of the chokes....anyhow they work and my amp is again sounding great!

[Steve Grimshaw]

Well done Neal! Just checked mine and found that I had put in the rifas to start with. So, if yours lasted 14 years rated at 85c I guess I can look forwards to 20 years or so with the rifas :-) As you say the rifas are 50mm height and fit under the power supply board. I did manage to fit them in situ under the board as normal but I remember I had to use a reduced height pcb standoff to get them in. They just fit with about 1mm clearance!

Steve Grimshaw

[Richard]

Yes well done Neal but oh bugger that probably means mine will go soon

[Richard]

Originally Posted by NealG

What I was not prepared for though was the improvement in sound! It's amazing how you can get used to a sound and not notice a drop off in performance over time. What I can't work out is why these caps seem to have an influence on the sound, they are 'behind' the chokes and GZ37 so not really part of the signal path, C11 and C12 effectively short AC to ground in front of the chokes....

Yes, strange how we look to the HT path for reasons but not the ground isn't it. You've improved the ground connection Try taking the big connections individually to the 0v point on the psu if they don't do that at the mo.

Rich

[NealG]

Yes, that would make sense Rich. I have all of the 'heavy' returns going to a new earth post on the chassis, I think I'll rework this as you suggest and take them all back to 0V on the PSU board.