With no tube in place, pin 1 should show the full B+ for that stage; you've clearly got a problem there. Pin 6 looks OK, that's showing full B+.
Hi again,
I was talking to jkeny tonight and he reminded me of something I had noticed back up the thread a bit - the fact that the circuit ground and earth were at different potential.
So I went back out and had another try. I disconnected the ground breaker, and when I powered it up, I could measure 60V between circuit ground and mains earth.
So he probably very wisely told me to post my PSU.
So heres the link to the schematic of the original:
PSU link
Now mine is a little different.
My B+1 is supplied by a Tx with a single secondary (ie not centre tapped) and I am using it pretty much exactly as per the B+2 shown in the schematic. So its approx 250VAC out of Tx, through rectifer, then 100uF cap, choke, 100uF cap and theres your 330VDC.
The 2 power supply negatives are tied together and then go to the star point. The B+2 neg goes to B+1 pos via a 100uF cap, again as per the schematic.
SY, is this OK?
I think jkeny might be right - I need to solve that problem first and then go from there.
Fran
I was talking to jkeny tonight and he reminded me of something I had noticed back up the thread a bit - the fact that the circuit ground and earth were at different potential.
So I went back out and had another try. I disconnected the ground breaker, and when I powered it up, I could measure 60V between circuit ground and mains earth.
So he probably very wisely told me to post my PSU.
So heres the link to the schematic of the original:
PSU link
Now mine is a little different.
My B+1 is supplied by a Tx with a single secondary (ie not centre tapped) and I am using it pretty much exactly as per the B+2 shown in the schematic. So its approx 250VAC out of Tx, through rectifer, then 100uF cap, choke, 100uF cap and theres your 330VDC.
The 2 power supply negatives are tied together and then go to the star point. The B+2 neg goes to B+1 pos via a 100uF cap, again as per the schematic.
SY, is this OK?
I think jkeny might be right - I need to solve that problem first and then go from there.
Fran
OK,
I think my problem is to do with the 2 transformers. If I disconnect the primary of the Tx giving me the B+1, but leave the B+2 connected, I get my 350V at pin 1.
If I reconnect the second Tx primary (so I have heaters - thinking I would test it with valves in place) but disconnect the secondary for the B+1, I get 30V between chassis and ground - about half of what I had earlier, but the voltages are screwy then.
Somehow, that second transformer is causing the problem, or at least powering the 2 of them together is.
Would it be an idea to put in some diodes between the 2 like the ground breaker?
Fran
I think my problem is to do with the 2 transformers. If I disconnect the primary of the Tx giving me the B+1, but leave the B+2 connected, I get my 350V at pin 1.
If I reconnect the second Tx primary (so I have heaters - thinking I would test it with valves in place) but disconnect the secondary for the B+1, I get 30V between chassis and ground - about half of what I had earlier, but the voltages are screwy then.
Somehow, that second transformer is causing the problem, or at least powering the 2 of them together is.
Would it be an idea to put in some diodes between the 2 like the ground breaker?
Fran
I think I might see your problem. Where there's a crossover jump past a line, there's no connection. Where there's no crossover jump (i.e., the lines simply cross), there's a connection. I think you missed the connection of the two minus rails together and to ground (the second 100u cap's negative terminal connects to the ground rail, it doesn't jump over it).
SY, yes I do need to go back and look at that bit again. That could well be the issue, although I did bring all the grounds back to a star, so they should be connected there.
Shoog - yes, there is a buzz from the telefunken Tx - much like the one I sent on to you, its faint but there. Its the one that gives me the 350V and thats the only secondary on it I'm using (ie not using heaters from it).
More tomorrow night!
Fran
Shoog - yes, there is a buzz from the telefunken Tx - much like the one I sent on to you, its faint but there. Its the one that gives me the 350V and thats the only secondary on it I'm using (ie not using heaters from it).
More tomorrow night!
Fran
That sounds suspicious.
I know thats a single winding secondary, and they are prone to buzzing - but your load should only be 6mA or so. It shouldn't really be humming. I think your power supply might easily be the cause of your trouble. One thing to try might be to disconnect the Telfunken and run the front end off the other supply. See if that improves matters and then you have narrowed down your search.
Shoog
I know thats a single winding secondary, and they are prone to buzzing - but your load should only be 6mA or so. It shouldn't really be humming. I think your power supply might easily be the cause of your trouble. One thing to try might be to disconnect the Telfunken and run the front end off the other supply. See if that improves matters and then you have narrowed down your search.
Shoog
OK,
Well I think I might have made some progress, but its not right yet. I think my problems are all down to the power supply. Someway that I don't understand, the 2 are interacting and thats whats causing the problems.
First of all, here is a rough schematic of what I have (I've omitted the heater drawings just for clarity):
When the amp is wired that way, I am getting the low voltage at the plate of the driver just as before. If I separate the 2 supplies, so that the grounds are not connected, the plate voltage rises, but only to about 105V, the cathode of the phase splitter is about 2V higher at 107V and anode of phase splitter is at 250V. The driver anode voltage is still a bit low, which needs to be sorted, but the whole picture looks better.
Thats one channel. The other channel is similar. I get 101V at driver plate, but 112V at splitter cathode and 350V (full B+) at anode of phase splitter.
The overall picture is better, but its still not right.
My schematic shown above differs from the original in that the PSU for the B+2 that SY used has a centre tap and ?half wave? rectification.
So can anyone shed anymore light?
Shoog and Jkeny - emails sent!
Fran
Well I think I might have made some progress, but its not right yet. I think my problems are all down to the power supply. Someway that I don't understand, the 2 are interacting and thats whats causing the problems.
First of all, here is a rough schematic of what I have (I've omitted the heater drawings just for clarity):
An externally hosted image should be here but it was not working when we last tested it.
When the amp is wired that way, I am getting the low voltage at the plate of the driver just as before. If I separate the 2 supplies, so that the grounds are not connected, the plate voltage rises, but only to about 105V, the cathode of the phase splitter is about 2V higher at 107V and anode of phase splitter is at 250V. The driver anode voltage is still a bit low, which needs to be sorted, but the whole picture looks better.
Thats one channel. The other channel is similar. I get 101V at driver plate, but 112V at splitter cathode and 350V (full B+) at anode of phase splitter.
The overall picture is better, but its still not right.
My schematic shown above differs from the original in that the PSU for the B+2 that SY used has a centre tap and ?half wave? rectification.
So can anyone shed anymore light?
Shoog and Jkeny - emails sent!
Fran
105 volts is a little low, but not at all unreasonable. Yes, it does seem down to the power supplies. Have you confirmed that their commons really are common?
No SY, it only works like this when I separate the 2 GNDs from the 2 supplies.
Is the difference in the second channel I talked about there way too much? ie the 102 on the driver anode and say 10-12V higher on the splitter cathode?
************************
I couldn't resist sticking in the EL84s. The LEDs all light up - after about 20 secs I'd say, but the bias current only gets to about 40mV. Screen voltage is too low I think. But I only quickly tried that on one channel.
Fran
Is the difference in the second channel I talked about there way too much? ie the 102 on the driver anode and say 10-12V higher on the splitter cathode?
************************
I couldn't resist sticking in the EL84s. The LEDs all light up - after about 20 secs I'd say, but the bias current only gets to about 40mV. Screen voltage is too low I think. But I only quickly tried that on one channel.
Fran
The second channel is a problem. For whatever reason, the splitter is running in cutoff. Check the resistance to ground from the splitter cathode (power off, please!).
OK!!
here we go for tonights installment:
Good progress! RE: the other channel - it would have been helpful if I had reconnected the grounds! It was an easy miss though cos the wires were braided together and they were sitting right underneath where the GND bus was.
So measurements:
*************************
Channel 1:
12AT7:
to GND:
pin 1 - 101
pin 6 - 254
pin 8 - 104
EL84a:
pin 3 - 11.2
pin 7 - 332
pin 9 - 245
EL84b:
pin 3 - 11.2
pin 7 - 328.8
pin 9 - 246
*************************
Channel 2:
12AT7:
to GND:
pin 1 - 105
pin 6 - 258
pin 8 - 108
EL84a:
pin 3 - 10.9
pin 7 - 333
pin 9 - 252
EL84b:
pin 3 - 10.9
pin 7 - 332.5
pin 9 - 252
LEDs all light up OK. When I first wired it I had oscillation on one channel, but reversing the OTx leads fixed that. I also measured for AC on the speaker output terminals and got 0.001 on both, good sign I hope.
*****************
So now to the next problem 🙄
I am trying to measure the mV across the 1R at the bottom of the LED string, only getting 33mV - And as you will have seen the screen voltage is a lot lower than 290V. When I try to adjust the regulators, there is a very small range of adjustment. From fully in on the pot to fully out (say, 20 turns), theres only a change in the screen voltage of about 5-7V or so.
I built my regulators using the PCB layout that was posted on the last page - and a post r so after that, the components I used. Could someone take a look and see if I have it right?
So progress, but more to go!
Fran
here we go for tonights installment:
Good progress! RE: the other channel - it would have been helpful if I had reconnected the grounds! It was an easy miss though cos the wires were braided together and they were sitting right underneath where the GND bus was.
So measurements:
*************************
Channel 1:
12AT7:
to GND:
pin 1 - 101
pin 6 - 254
pin 8 - 104
EL84a:
pin 3 - 11.2
pin 7 - 332
pin 9 - 245
EL84b:
pin 3 - 11.2
pin 7 - 328.8
pin 9 - 246
*************************
Channel 2:
12AT7:
to GND:
pin 1 - 105
pin 6 - 258
pin 8 - 108
EL84a:
pin 3 - 10.9
pin 7 - 333
pin 9 - 252
EL84b:
pin 3 - 10.9
pin 7 - 332.5
pin 9 - 252
LEDs all light up OK. When I first wired it I had oscillation on one channel, but reversing the OTx leads fixed that. I also measured for AC on the speaker output terminals and got 0.001 on both, good sign I hope.
*****************
So now to the next problem 🙄
I am trying to measure the mV across the 1R at the bottom of the LED string, only getting 33mV - And as you will have seen the screen voltage is a lot lower than 290V. When I try to adjust the regulators, there is a very small range of adjustment. From fully in on the pot to fully out (say, 20 turns), theres only a change in the screen voltage of about 5-7V or so.
I built my regulators using the PCB layout that was posted on the last page - and a post r so after that, the components I used. Could someone take a look and see if I have it right?
So progress, but more to go!
Fran
Make sure there's 1.25V between the 317's output and adjust pin. And that there's something like 10V from output to input of the 317. And, of course, make sure that your B+ to the regulator board isn't low.
OK, theres a problem there alright. Voltages between those pins are only 800mV and 300mV respectively. I'm wondering should I completely rebuild one. I've checked resistor values, even pulling some off the board to check. I noticed I did touch the side of the pot with the iron, but when I took it out and checked it resistance I had from 20K to 100K, so it must be OK.
How would I check the zener? Anyway of checking the 317 or the TIP50 with an ordinary DMM?
Fran
How would I check the zener? Anyway of checking the 317 or the TIP50 with an ordinary DMM?
Fran
I'd check the pass transistor and the 317 first. If you miswired something, they're toast.
No it musn't.
No it musn't.
OK,
I put back in another 317 and tip50, but the result is the same. I also put in another zener when I was at it.
I've just gone back and re-read the bit about the circuit board, and pinkmouse made a comment that he thought he used a different transistor. I wonder if thats whats wrong. Looks like I may need to order some more parts.
SY, I know regulating the screens is very much a part of this design and sonically its better, but while I get that stuff sorted, could I use a resistor to drop the voltage enough to see if the amp plays OK in other areas? If thats possible, what value R would you try first (~330V input)
Fran
I put back in another 317 and tip50, but the result is the same. I also put in another zener when I was at it.
I've just gone back and re-read the bit about the circuit board, and pinkmouse made a comment that he thought he used a different transistor. I wonder if thats whats wrong. Looks like I may need to order some more parts.
SY, I know regulating the screens is very much a part of this design and sonically its better, but while I get that stuff sorted, could I use a resistor to drop the voltage enough to see if the amp plays OK in other areas? If thats possible, what value R would you try first (~330V input)
Fran
Someone suggested to me via email that you may have burned your boards backwards. You might want to check that.
In general, if something causes a transistor or chip to pop, substituting another one will just cause more silicon-to-carbon transmutation. One needs to get to fundamental causes.
In general, if something causes a transistor or chip to pop, substituting another one will just cause more silicon-to-carbon transmutation. One needs to get to fundamental causes.
Thats all possible. When doing the boards I just copied the layout from pinkmouse right onto the copper and dropped in the components as per the list a post or so after the PCB layout.
If that was a mirror image or something then thats that.
I will order up some new parts I think and maybe just wire it P2P not on a PCB.
I will try subbing in a resistor for now just to see is the rest working. If I have screwed something else up elsewhere and need more components, I want to do all that in one order from mouser.
Fran
If that was a mirror image or something then thats that.
I will order up some new parts I think and maybe just wire it P2P not on a PCB.
I will try subbing in a resistor for now just to see is the rest working. If I have screwed something else up elsewhere and need more components, I want to do all that in one order from mouser.
Fran
Or you could burn the PCB right way round. But it's an easy hour wiring P2P on a couple of perf boards.