DTBs 6B4G

Dave the bass · Post by **Dave the bass** » Tue Jul 24, 2007 10:15 am

Many thanks for all the advice my learned fellows.

Ok, the 'pre-heating' idea. I currently switch the heaters on for a a while before the HT on 6EM7. Yes, there's a thump upon switch on but that does save my precious lovelies (does Gollum imppression!) RCA 6EM7 valves from arcing on power up.

I've experimented with Andrew I's 1 Meg snubber resistor idea, I've seen that used on guitar FX pedal switching in the past to stop switching thumps. It didn't make much difference in my experiments on 6EM7.

I like Nicks idea of Pre-loading the HT with a high power resistor and then switching the HT over to the amp after warm-up.

I might give that a go on SE Heaven too before I apply it to DHTski.

I haven't got any Valve rectifiers or a 5 volt supply for the heaters but if DHTski is so good it becomes a keeper I suppose I could invest and go the whole hog and get rid of the SS rectification.

DTB

Andrew · Post by **Andrew** » Tue Jul 24, 2007 12:35 pm

How about an X-Class cap across the AC mains switch?

cheers,

-- Andrew

Dave the bass · Post by **Dave the bass** » Tue Jul 24, 2007 2:01 pm

Just spent this dinner time playing with 6EM7. The 'thump' comes from the application of HT to the amp. I've got 2 x 10K 50W in parallel acting as a PSU HT dummy load to pull the HT down whilst the Valve heats up. A 5K ohm load pulls the rail down to 370-ish volts, I'm feeding a SPDT standby switch to switch the o/p from the PSU to either the dummy load resistor or the amp (after the heaters have been on for about 20 seconds). I've tried AndrewI's 1 meg snubber resistor idea again but nope, I still get a thump but at least the Valves aren't arcing on power up.

Andrew wrote:How about an X-Class cap across the AC mains switch?

cheers,

-- Andrew

I don't think it'd help TBH Andrew, it's not a mains switched induced thump I believe, I could be wrong.

DTB

Will · Post by **Will** » Wed Jul 25, 2007 9:49 am

Dave
I can send you some 350v clamps they will stop the HT rising above 350v on switch on.

Dave the bass · Post by **Dave the bass** » Wed Jul 25, 2007 10:22 am

Will wrote:Dave
I can send you some 350v clamps they will stop the HT rising above 350v on switch on.

Ta for the offer Will, that may save a few of my precious lovelies. What do they look like,not like this I know...

We may have some knocking about in condemned equipment here I could womble. I remember seeing voltage clamp diodes in TV's many years ago, I think they were there to limit either the EHT or flyback, can't remember, it was a long time ago.

DHTski update.

HT section is up and running, 412 VDC off load.

Next up is C3g heaters which will be 6.3v AC but with one leg tied to the HT as advised. Is there a preffered voltage the heaters should sit above the HT or is it 'just above'-ish. I think WD88 had it's heaters sitting on 46V DC.

DTB (Junior Amp builder, 3rd Class)

Will · Post by **Will** » Wed Jul 25, 2007 10:51 am

PM your address to find out.

Dave the bass · Post by **Dave the bass** » Wed Jul 25, 2007 10:57 am

Will wrote:PM your address to find out.

Just have, many thanks for the offer Will.

Let me know if you need swappsies or summink.

DTB

Post by **Nick** » Wed Jul 25, 2007 11:24 am

I think WD88 had it's heaters sitting on 46V DC.

Yes, thats a good sort of voltage, if you have any cathodes at elevated voltages you end up having to find a value that sutes them all, but in this case 40ish volts will be fine.

As it happens the voltage can be +40 or -40 it still works the same, I wil see if I can find the paper that explains why.

Will:

I can send you some 350v clamps they will stop the HT rising above 350v on switch on.

They sould like interesting/useful things, I haven't encountered them before, any more details? (please).

Dave the bass · Post by **Dave the bass** » Wed Jul 25, 2007 12:16 pm

Nick wrote:
Yes, thats a good sort of voltage, if you have any cathodes at elevated voltages you end up having to find a value that sutes them all, but in this case 40ish volts will be fine.

As it happens the voltage can be +40 or -40 it still works the same, I wil see if I can find the paper that explains why.

Thanks NickG, and a 10uF cap on the lower leg of the PD too.

Looking next at the CCR cct. Is it wise to heatsink the LM338's?

DTB

simon · Post by **simon** » Wed Jul 25, 2007 12:32 pm

Dave the bass wrote:Thanks NickG, and a 10uF cap on the lower leg of the PD too.

I saw Nick suggest this earlier in the thread. Just wondering how you choose the value of the cap - I've used a similar arrangement on my headphone and phono amps but only used 0.1uF (cos that's what Broskie showed

).

Simon (Junior amp builder, no class at all

)

Dave the bass · Post by **Dave the bass** » Wed Jul 25, 2007 12:38 pm

simon wrote: Simon (Junior amp builder, no class at all )

You got loadsa class Simon, you had those turbo-nutter-XR3i-Ghia-GTI 300B's last time i saw ya.

Now thats class innit

DTB

Post by **Nick** » Wed Jul 25, 2007 12:57 pm

I saw Nick suggest this earlier in the thread. Just wondering how you choose the value of the cap - I've used a similar arrangement on my headphone and phono amps but only used 0.1uF

Well. MJ talks about needing a low impedance supply for this, but when I have done it, I have found it doesn;t matter that much, the small cap is probably better as the voltage will not be delayed as the cap charges via the resistor.

In this case you don't need to use the cap to smooth the voltage, as the point of the rasised voltage is it make sthe valve immune to induced hum. personally I think the cap is more important to remove any induced hash from the power supply, as I find that spikes will influence the valve even if hum doesn't.

So by that I would say a small cap would be better, as the ESR and inductance of the electrolytic would reduce its effectivness at HF.

I have to admit, I normally just use whats at hand and if there are problems look a bit harder at it then.

Dave the bass · Post by **Dave the bass** » Wed Jul 25, 2007 3:50 pm

Komrade DHTski update.

Productive break this afternoon.

PD installed (1M + 150K) gives about 38V with the HT pulled down to 300V with a bunch of WW resistors acting as a dummy load. 100V 10uF cap fitted across lower leg as described before. 6.3v AC heaters now sit on a 38v DC level.

2 off CC heater regs next up. Just made up a little bracket to mount the 2 multi turn pots on (Steve the Shadow would be proud of this, I've even cut the sides straight and screwed it on instead of using double sided tape

)

Next up make 2 more bridges using the 8 remaining 1N5820's -> 10,000uF 16V caps -> LM338

Should the Regs be mounted on a heatsink? I've found a nice couple of tiddlers going begging in an old bit of kit that could be wombled.

Looking good, might even be ready for WF3.

DTB

Post by **Nick** » Wed Jul 25, 2007 4:12 pm

Should the Regs be mounted on a heatsink

Yes, I use a bit of ali sheet, but they do have to work hard. If they stop passing current, then they have probbably shutdown so need more heatsinking.

Dave the bass · Post by **Dave the bass** » Wed Jul 25, 2007 4:27 pm

Nick wrote: Yes, I use a bit of ali sheet, but they do have to work hard. If they stop passing current, then they have probbably shutdown so need more heatsinking.

Thanks Nick, I guessed they might but just wanted to check.

DTB

audio-talk

DTBs 6B4G

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