The Red Light District - another PP EL84 amp

[SY]

70mA isn't too bad, especially if the screen voltage is kept reasonably low (275 or so). If everything works, let the tubes break in for a couple of days and then we can try to bring it up to 80 and see if it sounds better to you. It might mean changing a resistor in the regulator, depending on how "hot" your EL84 are.

If you lose one tube, replace them as a pair. The output tubes really should be well-matched for optimum performance.

[woodturner-fran]

I bought a set of 8 - russian types from ebay. When the whole thing settles down I'll invest in a better set. But for now they will do to at least see how it performs. No, they aren;t matched, but for checking out they'll be Ok for now I hope.


That purchase of some telefunkens or the like will have to wait a little!

My B+ to the EL84s is 315-320V. The screens are currently set at 292V I think. The range on the regulators will go a little higher and down to about 230V or so (that was off load, but in testing the reg stayed up fairly well with a 30K load)

I'm using 7 lines of 6 LEDs in with a 4.75R resistor at the top of the string.

So apart from doing some tidying up on the wiring and tying stuff down I have to wait on the new valves to arrive.

Thanks for all the help SY, and I have to mention Shoog and jkeny for mucho help.

Fran

[woodturner-fran]

SY, should I consider dropping the size of the anode load to maybe 120K to up the anode voltage a bit? Or is 100-105V ok to live with?

Fran

[Shoog]

Fran,
You said that one of the valves had gone down. This usually happens when the grid earth reference goes open. The valve then passes excessive current and the grid starts to draw excessive current - hence the grid stopper will tend to fry.

My advise is that you really should replace the grid leak and the grid stopper and possibly rewire the whole socket to be on the safe side. Do not place anything brand new into that socket until you have done this.

Shoog

[SY]

Originally posted by woodturner-fran
SY, should I consider dropping the size of the anode load to maybe 120K to up the anode voltage a bit? Or is 100-105V ok to live with?

Fran

I'd see about getting the supply voltage a little higher. Dropping the load will only increase distortion.

[woodturner-fran]

Shoog: did that - checked out all the connections (I was assuming it was my fault - maybe it still is)! I let that one channel run on its own for a few hours last night and it seems stable.



SY: Supply is already at 350ish - thats B+ at the top of the valve. Varies from 345-355 or so depending on the supply voltage in.


Fran

[SY]

Higher would be better for the driver stage supply. Remind me, what transformer did you use?

[woodturner-fran]

Its only a simple 230:230 isolation transformer 20VA - pulled. Goes to bridge rectifier, 100uF, choke, 100uF. I think the choke is pretty much as per the PS schematic. Voltage here is often 240V which I think is why I get it a bit higher. I did measure the off load AC voltage of the Tx and remember thinking it was high might have been 255-260 or so.

I originally had a telefuken one bought from ebay, but you may remember back up a bit that shoog identified it with the problems I was having. So its scrappped.

Fran

[SY]

You might want to rethink that transformer. One neat trick is to connect another transformer as a boost. You need probably another 40V or so of DC, so a 24V transformer could easily be used as a boost to get you to the 400V or so you want for the driver stage. It needn't be very large, the current requirements are minimal. My local Radio Shack has 24V 500mA jobs for under $10; you should be able to find something similar locally.

[woodturner-fran]

I would have to check: but I think I have a small 12-0-12 Tx at home - might be only 300mA though. I take I connect the primaries in parallel, but the secondaries in series? Is that right?

And wasn't there something about connecting them in the right phase or you would end up with less voltage rather than more?

Fran