300B PSE Hum !!!

[NickG]

Ok, I am not sure which thread this should be posted on. You seem to have to issues (at least), with the EH's the hum can't be reduced below a level, and then rises. With the TJ's they can go down lower, but still rise.

So it seems you have two things, the amount of hum that can be reduced at turn on, and what the hum does after a while.

The suggestion of altering the heater bias, was to try and affect the rise in hum, as it still seems to happen with both types of 300b's.

Yes, standing the heater off at +20 will increase the ecc82 from 35 to 55v, but what I was trying to alter was what the input valve saw on its heater, thats the one that will have the most effect, as its got the highest gain.

[soulminer]

Hi all.

New test results this morning with amps powered up from cold.

Both amps in the same configuration as they were from last night (hot) but cold after sitting for 10 hours. The left amp with the EHs measured 9mV hum at power up and the right amp with the TJs measured 4mV hum from power up. In other words, the hum is only increasing by 1 or 2 mV on the amps with the EHs fitted and not at all with the TJs. That extra 1 or 2mV must be enough to start getting audible from the sitting position.

Fitted a TJ and a EH to each amp. This brings the overall hum down to 4-5mV. Almost there!! With judicial selection of which EH used with each TJ I can get the hum down to 4mV dead and it stays like that.

It makes no difference which socket the EH goes in, and some of the EHs combined with a TJ managed to take the hum up to 6 or 7mV. The odd thing is that the EHs that provide the least hum are a single one from each matched pair. It's as though they were never matched at all, because if one is low, then surely the other should be just as low if they were matched.

This may seem like a stupid question to those in the know, but why is there only one hum bucker pot on one 300B? What would happen if a second one were fitted to the other 300B? Or doesn't it work like that? If I leave just one 300B in (the one with the hum bucker) I can buck the hum down to 1mV. If I leave the other 300B in (the one without the hum bucker) it stays at 4 or 5mV regardless of position of hum bucker. Would fitting a second hum bucker to the other 300B remove this last bit of hum too? If I fit the EH that previously measured 10 or 11mV in the socket with the hum bucker, I can buck that down to 5 or 6 mV. This still indicates to me that there is something odd with these EH GGs, but at least with a second hum bucker I could remove a touch more hum, maybe??

Nick.

You are right. I did my sums incorrectly. Placing the CT of the Fil tx at +20v above ground would indeed create a potential difference of 55v. I am not sure this is a problem any more though is it? Doesn't the fact that I can get the TJs down to 1 - 2mV on their own and 4mV as a pair mean that the rest of the circuit is just fine?

Sorry to be a pest about all this, but it has often made me wonder why two valves from different manufacturers should cause different levels of hum, and I was thinking there was something wrong with the amplifier rather than the valves themselves.

So the question is.... Why only one hum bucker pot on the WAD 300B PSE, and would fitting a second make for a quieter amplifier?

Thanks everyone,

[soulminer]

Just looking at this picture of John Caswell's 300B PSE, it appears that he has used double hum buckers. Maybe he can comment on why he did this (same reasons as I am discovering now? - hum). Can I just copy the circuit on the other hum bucker pot and simply add another ?

[david counter]

I notice that the eminent Dr uses EH golden grids, perhaps it would interesting to hear his views on them,

[soulminer]

Originally Posted by david counter

I notice that the eminent Dr uses EH golden grids, perhaps it would interesting to hear his views on them,

My thoughts precisely David, in both cases. EH GGs and double hum buckers.

[Dik Dolan]

I've been using EH GGs in my Glasshouse 300 set since I built it in December 2005. No problems, no glowing plates, no hum. I like 'em so much I bought a spare pair (at £86 for the pair from a local supplier, bargain)

[Ali Tait]

I used to have a pair of these.As I recall, the minimum quoted hum in the instructions for this circuit was 4-5 mV,so you will be struggling to get below this with the circuit as standard! You could try dc heaters if the hum is an annoyance.
Regards,Ali

[soulminer]

Nice one Dik.

Glad to hear yours are fine. I wonder if I have bought a "duff" set?

Hi Ali.

They have been on all day again, and with the TJ/EH combo fitted in each amp, right amp is settled at 3mV and the left is at 4-5mV. This is almost acceptable.

Still waiting to hear from Dr John (or anyone else) if a second hum bucker is a worthwhile addition. I am happy with 3-4mV per channel. I think if I fit another hum bucker to each amp and get a second pair of TJs (in the future) then I should be able to get this kind of figure and they should be near silent.

Cheers,

[Greg.]

Hi Lee,

Because I and others with the WAD 300B PP suffered plate glow with EH 300B goldgrids, that is why I posted on this. There was detailed discussion on the forum at the time that might still be searched out. I also suspected the the cheap Hong-Kong supplier was selling duff valves and this led to a significant email conversation with him who rigerously insisted his valves were perfect factory production and furthermore stated that the matching was factory based from factory testing and not his own work. You should see that he majors on sales of other products and these valves are just something extra to him. At the end I concluded that he is a reliable seller with good access to the factory allowing very affordable sales. I'm satisfied that he is not selling duff products and certainly my EH 300B goldgrids have performed wonderfully for me, subject to the need to reduce plate dissipation. Having said that, we may be the culprits because we uprated our PS diodes which led to a higher HT from original and some of us did stuff to compensate there. Don't get paranoid about the quality of your EH 300B's. They're probably fine.

Best wishes,

Greg

[Ali Tait]

Hi Lee,
I reckon that's as good as you'll get with a stock build.