300B PSE Hum !!! - World-Designs-Forum

soulminer · #1 27th June 2007, 10:06 PM

This is a new thread to try and keep things in their correct place.

As mentioned in previous thread, the WAD 300B monoblocs have a hum on the outputs that cannot be "hum bucked" out. With the 6AU6 and ECC82 removed, the hum is all but gone and measures 0.001V on the DVM. The system is silent. The hum buckers don't do anything at all with the ECC82 removed. There is literally no output at all, which I assume is due to the lack of any voltage on the grids of the 300Bs. It is the same with just the 6AU6 and 300Bs fitted. Nothing until the ECC82 is refitted.

If I remove just the 6AU6 the hum is still there and cannot be bucked out. It is no worse but no better which makes me think that the 6AU6 is not effecting the circuit. I could be wrong.

I just wish I had a scope to see what is going on.

The ECC82 is AC heated and that heater wiring comes off the ECC82 B9A base to the rectifier pack to feed the DC heaters of the 6AU6.

The circuit is standard apart from some recently added PSU bypass caps and some Mills cathode resistors on the 300Bs.

What could be causing it? Any ideas?

david counter · #2 27th June 2007, 10:12 PM

have you ever tried different ECC82s?

just a thought

soulminer · #3 27th June 2007, 10:19 PM

Yes. Several types. EIs, RCAs, Sylvanias, Tungsram. They all have the same results. The only thing that does reduce the hum is the use of the TJ meshplates. Could it be the 300Bs themselves I wonder? This is why I thought it may have been the DHT heaters. I noticed that the EH filaments start to glow at a different rate from one pair to the other. I got them from China-arts ages ago.

I will fit the TJs in now and see what the hum levels are. I will also swap the EHs across the amps and see if the hum follows the valves.

soulminer · #4 27th June 2007, 10:55 PM

I replaced the EHs with TJs in the right amp that was giving me 10mV hum and the hum could be bucked down to about 1-2 mV. Good enough for me.

I replaced the EHs with the same TJs in the left amp that was giving me 7mV of hum and the hum could be bucked down to 1mV. Good enough for me.

I swapped the EHs from one amp to the other, and the amp that was at 10mV is now down to 7mV and the other one is up from 6mV to 10mV.

I have left the TJs in the right amp and as the valves warm up to normal temperature (I will give them, say, 15 mins) I am already seeing the hum rise. It is now at 4mV after only 3 or 4 mins. I will see what it reads after 15 mins and see whether or not I can buck them down again.

5 mins in and they are at 5mV and rising.

soulminer · #5 27th June 2007, 11:09 PM

They seem to have settled at about 4-5mV. They are not changing any more. Both amps are warm and the TJs are now hot. The other amp with the EHs still fitted is settled at 10mV.

Does this mean that the 300Bs are being overdriven or are the EHs faulty or just poorly made? This is not right surely? A 300B, regardless of brand, should exhibit the same properties more or less shouldn't it?

The amps are sounding fine. They don't seem to complain and work superbly regardless of this slight background hum. Odd!!

Should I start checking circuit voltages or is this all normal and I am simply being plagued by a combination of "PSE DHTs on AC heaters" syndrome and Cheapo 300Bs ??

Greg. · #6 27th June 2007, 11:57 PM

Hi Lee,

Ignorant contribution here, but what watts are you dissipating on the EH's? They don't like to be driven near to the 40W spec. I've got mine down to 30W or there abouts with no plate glow and all is well. I know we have different amps but we are working with the same valve. My experience with EH300B is that you get plate glow soon above 30W. That may be significant or it might be a total red herring.

Best wishes,

Greg

soulminer · #7 28th June 2007, 12:13 AM

Hi Greg.

I am not sure what the dissipation is in this application. I will calculate this based on my amps' setup and report back. Maybe the valves are being thrashed and this is causing hum. Not sure about plate glowing either. Not noticed any, but why would the TJs be fine and not the EHs?

Cheers,

Greg. · #8 28th June 2007, 12:54 AM

Hi Lee,

That is an interesting point. The sellers of TJ's, because they are mesh plate recommend very low dissipation, mabe 18 to 20W to ensure longevity of valve life. Personally I'm not happy with that for a valve designed to operate at 40W and therefore my reason for not going there. As ever, this still maybe a red herring but I would advocate you firing up your amps in a very dark room and seeing if you have cherry red plate glow even in a small section of the plate. If you have, adjust your cathode bias/HT to reduce the voltage and when you no longer have glow, see what the effect is on sound.

Best wishes,

Greg

soulminer · #9 28th June 2007, 01:41 AM

Not sure if this is right, but here goes:

Plate dissipation = plate voltage * plate current

plate voltage = HT - cathode voltage = 425v - 40v = 385v

plate current = cathode voltage/cathode resistance = 40v / 470R = 85mA

therefore dissipation = 385 * 0.085 = 32.8 watts.

This is all theoretical BTW. Maybe someone could put me right if I am wrong, but it looks sort of OK. I will measure some real life voltages tomorrow and re-calculate if what I have calculated here is about right.

It seems well within specification for a 300B which is 40W max plate dissipation, and considering that 300Bs like to run hot (at about 85-90%) then 32 watts is only about 80%. It's actually running below the sweet spot. So why should the EHs start packing up at anything above 30W? Is there something wrong here or what? Would the overloading of the plate cause hum if that were the case? What would altering the B+ or cathode resistance do to the sound of these amps? Not sure I like the sound of de-tuning an amp from its sweet spot to cater for possibly dodgy valves that start overheating at only 75% of their supposed maximum dissipation.

Has anyone here other than Greg and I noticed any problems with EH GGs?

soulminer · #10 28th June 2007, 02:22 AM

Final post before bed !!

Just powered the amps up in inky black darkness. There are only bits glowing that should be glowing. The heaters. No sign of any glowing plates at all. I left them heat up for a full 15 minutes too. If the calculations are correct in the previous post (or thereabouts) then I am happy that at only 80% ish and with no glow, the 300Bs are happy in the circuit.

This just leaves the original problem of the hum. I will perform a couple of final tests tomorrow of a TJ and an EH per amplifier and see what that does to the hum, and also running the amps with a single 300B fitted (TJ and EH) and see what that does to the hum. I will also measure the true voltages and see if I can find any anomalies anywhere else. I am happy if I can get the hum to within specification (4mV) as I can with the TJs, but I am not happy with twice or even close to three times this specified figure with the EHs.

It will be a shame if the EHs turn out to be poor quality, seconds, rejects or faulty though. False economy.

I always wondered why, when I got two matched pairs of EH GG, the heaters filaments of one matched pair took a good three or four seconds longer to come to full brightness than the other matched pair, regardless of which amp or socket they were fitted in, whereas the TJs all powered up at a similar rate and I had no problems with them until they blew that is. Poor quality, seconds, rejects or faulty ones again maybe? It does make one wonder how these eBay sellers can sell two of these valves for the RRP of one and still make a profit. Where are they getting them from? On the other hand, are DHTs such as 300Bs naturally so fragile? Maybe I am expecting too much from them?? Nobody else seems to have had a problem with EH GG 300Bs except maybe Greg.... Do they ???