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.
If you lose one tube, replace them as a pair. The output tubes really should be well-matched for optimum performance.
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
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
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
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
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: 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
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
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
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.
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
And wasn't there something about connecting them in the right phase or you would end up with less voltage rather than more?
Fran
OK, did that. B+ is now ~380-385VDC. Anode voltage is now up towards 115V, all the other voltages tie in as appropriate.
My bias: 70mV/70mA - I have 6 in each series with 10.2V and the screen is at 293. B+ to the EL84 is at 315-320VDC. Should I be dropping another resistor from the string to bring that up a bit more?
Fran
My bias: 70mV/70mA - I have 6 in each series with 10.2V and the screen is at 293. B+ to the EL84 is at 315-320VDC. Should I be dropping another resistor from the string to bring that up a bit more?
Fran
IT LIVES!!!!!
yes, the extra tubes I needed arrived today. Stuck 'em and fired her up and left it running for about 2 hours out in the workshop. Bias is stable at 70mV (70mA) and all seems present and correct.
Then I brought it in and hooked it up to the main system (various sources>>6GM8 aikido>>RLD>>Esl 57) - have to tell you it sounds bloody good! And thats only a quick listen so far! Soundstaging seems excellent, great depth. I will post more in time on this (after all thats why I built it!) but to start with, I'm very happy.
I am picking up a bit of buzz, not that intrusive, but naturally I would like it gone, but it might not be the amp - eg with the other amp I had hum in one channel from the pre. That hum doesn't seem to be as noticeable now. I also get the odd few clicks, like a bit of buzz now and again, but they aren't loud but you would hear them if no music was playing. More testing is needed here. There was some noise and activity in the room when I started her up so I need a more critical listen at a quieter time.
SY: something I noticed and meant to ask several times. When you turn it on from cold I notice the LED array get quite bright and if I measure I get bias of ~200mV. Now that starts to drop after about a minute or so and by 5 mins later its stable. Is that normal?
Fran
yes, the extra tubes I needed arrived today. Stuck 'em and fired her up and left it running for about 2 hours out in the workshop. Bias is stable at 70mV (70mA) and all seems present and correct.
Then I brought it in and hooked it up to the main system (various sources>>6GM8 aikido>>RLD>>Esl 57) - have to tell you it sounds bloody good! And thats only a quick listen so far! Soundstaging seems excellent, great depth. I will post more in time on this (after all thats why I built it!) but to start with, I'm very happy.
I am picking up a bit of buzz, not that intrusive, but naturally I would like it gone, but it might not be the amp - eg with the other amp I had hum in one channel from the pre. That hum doesn't seem to be as noticeable now. I also get the odd few clicks, like a bit of buzz now and again, but they aren't loud but you would hear them if no music was playing. More testing is needed here. There was some noise and activity in the room when I started her up so I need a more critical listen at a quieter time.
SY: something I noticed and meant to ask several times. When you turn it on from cold I notice the LED array get quite bright and if I measure I get bias of ~200mV. Now that starts to drop after about a minute or so and by 5 mins later its stable. Is that normal?
Fran
OK SY, will check that out - those are 270K I think?
(The schematic is outside in the workshop)
Fran
EDIT: yes, I did check, each one 270K to ground, so from the actual pin (2) to GND is 317K (ie 270K + 47K). across the 1R to GND is 1R
BTW, I don't know if this makes a difference, but I also noticed that my PS comes up a little slowly. At turn on the B+ is at say 280V and over the course of a few mins it rises to 320V and is then stable there.
Double edit: just in case it wasn't clear - at turn on, the LEDs are out, then after about 10-15secs they start to light up, get fairly bright (the high mV reading at this stage) and then go back down to the 70mV within 5 mins.
(The schematic is outside in the workshop)
Fran
EDIT: yes, I did check, each one 270K to ground, so from the actual pin (2) to GND is 317K (ie 270K + 47K). across the 1R to GND is 1R
BTW, I don't know if this makes a difference, but I also noticed that my PS comes up a little slowly. At turn on the B+ is at say 280V and over the course of a few mins it rises to 320V and is then stable there.
Double edit: just in case it wasn't clear - at turn on, the LEDs are out, then after about 10-15secs they start to light up, get fairly bright (the high mV reading at this stage) and then go back down to the 70mV within 5 mins.
Screen voltage is set at 293V and it never goes above that - it is below it if the B+ is below it naturally, but doesn't rise above it.
I can connect a scope - to the speaker outputs? What way should I connect it up? (tektronix 465) I have a signal generator as well if that would be any help.
Fran
I can connect a scope - to the speaker outputs? What way should I connect it up? (tektronix 465) I have a signal generator as well if that would be any help.
Fran
V
Vek
Some days ago I came across this:
http://klausmobile.narod.ru/testerfiles/heat_e.htm
(scroll down to see el84 warm-up)
Seems like the LEDs make this behaviour easily detectable ;-)
greets
http://klausmobile.narod.ru/testerfiles/heat_e.htm
(scroll down to see el84 warm-up)
Seems like the LEDs make this behaviour easily detectable ;-)
greets
Vek, that's exactly why I asked my last question. I'm wondering about gas if the tubes are some oddball Russian or Chinese jobs.
Fran, just connect between ground and the 16 ohm tap. No applied signal. If there's any high frequency oscillation, the scope will show it. If the trace just sort of bounces slowly once or twice, then settles, that's OK.
Fran, just connect between ground and the 16 ohm tap. No applied signal. If there's any high frequency oscillation, the scope will show it. If the trace just sort of bounces slowly once or twice, then settles, that's OK.