Hi all
I recently started on my first build of an amplifier.
The power transformer and output transformers were sourced from an old Philips radiogram. They were designed for the EZ80/81 + ECL86 combo.
So anyway, I found this circuit diagram on r-type, and it seems to be quite solid. So I redrew it on EasyEDA, and went to build the components.
It was built on a contactless PCB, with point to point wiring. I had opted to build one channel for now, to see if it works 100%.
So it works! Except for one small issue...
When I power it on, the DC voltage (measured from Pin 3 of the EZ81) rises to about 310V, and then it sounds perfect! But then it drops to about 280V (which according to the ECL86 datasheet is still very good), and it starts to distort... very badly.
I checked multiple times, and I don't think I made any error.
Regarding the issue... it's not unstable. The whole time the amp seems fine. When it starts to distort, there is still sound coming through, and when it's distorting it does not get any worse.
Attached is a PDF of the schematic I drew (one channel).
Here's a link to the r-type circuit...
Three Watt Economical Stereophonic Amplifier
I recently started on my first build of an amplifier.
The power transformer and output transformers were sourced from an old Philips radiogram. They were designed for the EZ80/81 + ECL86 combo.
So anyway, I found this circuit diagram on r-type, and it seems to be quite solid. So I redrew it on EasyEDA, and went to build the components.
It was built on a contactless PCB, with point to point wiring. I had opted to build one channel for now, to see if it works 100%.
So it works! Except for one small issue...
When I power it on, the DC voltage (measured from Pin 3 of the EZ81) rises to about 310V, and then it sounds perfect! But then it drops to about 280V (which according to the ECL86 datasheet is still very good), and it starts to distort... very badly.
I checked multiple times, and I don't think I made any error.
Regarding the issue... it's not unstable. The whole time the amp seems fine. When it starts to distort, there is still sound coming through, and when it's distorting it does not get any worse.
Attached is a PDF of the schematic I drew (one channel).
Here's a link to the r-type circuit...
Three Watt Economical Stereophonic Amplifier
No idea if it explains what you are experiencing but the value of the grid resistor of the first stage is too high (10M). Since the bias for this stage is derived by cathode bias, the grid resistor should be no higher than 1M, and for lower noise choose 50K or 100K.
A value of 10M is used when there is no cathode resistor, unlike in your amplifier. The bias for the tube is than derived by grid current flowing through the 10M grid resistor.
Mona beat me to it.
A value of 10M is used when there is no cathode resistor, unlike in your amplifier. The bias for the tube is than derived by grid current flowing through the 10M grid resistor.
Mona beat me to it.
I think your feedback should go to the other side of the OPT secondary. You are shorting out the cathode bias on the driver.
Thanks for all the replies so far! I'll fix R1 asap.
Please see the attached image for the OPT pinout. Please note it is DC resistance that is measured.
The primary (pin 1 & 2) in the image are connected like they are in the circuit diagram.
Pin 8 is connected to loudspeaker positive. Pin 7 is connected to loudspeaker negative and common ground. Pin 6 is connected to the feedback loop.
Apologies for not including this in the circuit diagram.
EDIT: Pins 3 and 4 are currently unused. I'm really not sure what it was used for.
Please see the attached image for the OPT pinout. Please note it is DC resistance that is measured.
The primary (pin 1 & 2) in the image are connected like they are in the circuit diagram.
Pin 8 is connected to loudspeaker positive. Pin 7 is connected to loudspeaker negative and common ground. Pin 6 is connected to the feedback loop.
Apologies for not including this in the circuit diagram.
EDIT: Pins 3 and 4 are currently unused. I'm really not sure what it was used for.
Attachments
Last edited:
The 4K7 is according to the Mullard amplifier (see post #1). But maybe you are right when this amplifier would be used with line level input (instead of a crystal phono catridge or something like that).
Last edited:
Line level is indeed the usage.
What would be the recommended changes I'd need to make, should I prefer a line level input signal?
What would be the recommended changes I'd need to make, should I prefer a line level input signal?
Questions:
If you remove the feed back wire from '6' on the transformer how does it sound?
What voltage do you measure on the cathode pin 7?
What voltage do you measure on the screen pin 3?
Did you use new resistors and caps or salvaged ones from the 'gram?
Do you have the schematic of the original Philips or model number?
If you remove the feed back wire from '6' on the transformer how does it sound?
What voltage do you measure on the cathode pin 7?
What voltage do you measure on the screen pin 3?
Did you use new resistors and caps or salvaged ones from the 'gram?
Do you have the schematic of the original Philips or model number?
1. I don't know. Will check.
2. Same as 1.
3. Same as 1.
4. I purchased brand new components from RS. I got high end caps from the likes of Nichicon, Rubycon, Panasonic, WIMA, etc...
5. Yes I do. Please see attached
Do note the schematic is 99% complete. There are some issues with the tone control which is placed in the feedback loop. Other than that, it's a 1:1 copy of the original.
This was drawn by me before I made the move to draw things neatly, so please excuse the lack of proper component placement. Sorry.
2. Same as 1.
3. Same as 1.
4. I purchased brand new components from RS. I got high end caps from the likes of Nichicon, Rubycon, Panasonic, WIMA, etc...
5. Yes I do. Please see attached
Do note the schematic is 99% complete. There are some issues with the tone control which is placed in the feedback loop. Other than that, it's a 1:1 copy of the original.
This was drawn by me before I made the move to draw things neatly, so please excuse the lack of proper component placement. Sorry.
Do you know the type of Philips radiogram the OPT came from?
Big chance that winding 3-4 is an anti-hum provision. Philips used that a lot. You can just leave it unused.
As for the secondary: It could be suited for cathode feedback applied to the power stage, with 7 earthed and 6 going to the cathode resistor of the power stage. The resistance you measured (33.9 Ohm) fits this. I think that in your amplifier it's better to take the negative feedback from 8 and leave 7 earthed (so 6 would be not connected to anything). The speaker still stays connected between 8 and 7. If this arrangement leads to oscillating, then reverse the earthing and the feedback connection (or reverse 1 and 2).
Attached an example of an OPT with cathode feedback and negative feedback over the whole amplifier (I made it some time ago with a Philips OPT; the anti-hum provision is also used in it).
As for the 4K7: Just try it first with the 4K7. If you experience problems (at higher input levels) than maybe it's wise to make some changes.
Attachments
Last edited:
Arh I see (now I've deciphered the 'original' schematic in #11).
Typical Philips, the feedback winding from pin 6 of the OPTX is part of the tone control network and works back to pin 2 the cathode of the triode section. No point trying to use that with the R-type schematic.
Reproduce the original circuit with just 4 connections to the output transformer. Primary on 1 and 2 and outputs from 7 and 8. If you keep 7 as the ground, make sure 1 goes to the pentode section anode (pin 6). You will have to 'work out' the feed back resistor values later. That will need to come off 8 of the output transformer eventually.
As has been mentioned, the 3 and 4 winding originally fed the screens and triode section, best left alone for now...
Typical Philips, the feedback winding from pin 6 of the OPTX is part of the tone control network and works back to pin 2 the cathode of the triode section. No point trying to use that with the R-type schematic.
Reproduce the original circuit with just 4 connections to the output transformer. Primary on 1 and 2 and outputs from 7 and 8. If you keep 7 as the ground, make sure 1 goes to the pentode section anode (pin 6). You will have to 'work out' the feed back resistor values later. That will need to come off 8 of the output transformer eventually.
As has been mentioned, the 3 and 4 winding originally fed the screens and triode section, best left alone for now...
@PFL200 and @Alan4411 thanks!
Please see the updated diagram. I believe this perfectly follows your suggestions.
I'll test it in a few minutes, and update everyone on my findings.
Please see the updated diagram. I believe this perfectly follows your suggestions.
I'll test it in a few minutes, and update everyone on my findings.
I have swapped R1 with a 100k unit.
Previously used was a 1M resistor. The radiogram used a 10M one, but I didn't have any.
Feedback loop was disconnected. Gain is significantly higher!! But it now has a lot of hum (the high frequency kind, not the usual 50/60Hz).
Will test shortly with the loop connected to pin 8 of the OPT as suggested.
Previously used was a 1M resistor. The radiogram used a 10M one, but I didn't have any.
Feedback loop was disconnected. Gain is significantly higher!! But it now has a lot of hum (the high frequency kind, not the usual 50/60Hz).
Will test shortly with the loop connected to pin 8 of the OPT as suggested.
Nearly, but not quite...
You cannot connect the transformer output directly to the feed back point on the triode cathode. That's 100%...
You need to chose a value that suits the output transformer and circuit design. Try a 20k ohm pot and reduce the resistance slowly. You need test gear to do it properly.
You might well find the limitations of the original transformer and how clever Philips were with their designs...
You cannot connect the transformer output directly to the feed back point on the triode cathode. That's 100%...
You need to chose a value that suits the output transformer and circuit design. Try a 20k ohm pot and reduce the resistance slowly. You need test gear to do it properly.
You might well find the limitations of the original transformer and how clever Philips were with their designs...
Last edited:
Measuring Pin 3 of EZ81 starts strong at 310V, but quickly drops to 280V
Measuring Pin 3 of ECL86 also starts well at 310V, but drops to 252V.
Measuring Pin 7 of ECL86 gives me a good 6.78V
6.78 V on the cathode means that the cathode current is 37.7 mA.
280 V on the plate means that V(a-c) is about 273 V
252 V on the screen means V(g2-c) is about 245 V
Estimating that from the 37.7 mA about 5 mA flows through g2, this would give:
Pa = 273 x 0.0327 = 8.93 Watt
Pg2 = 245 x 0.005 = 1.23 Watt
Although the anode voltage is a bit high, the values for plate and screen grid dissipation are under the limiting values in the datasheets for the ECL86.
So to me your results seem fine.
Addition: Its normal that the voltages on the anode and screengrid are higher at start up. The tube needs a little time to conduct fully so only than the current draw will be normal. This current creates voltage drops over several resistances in the amplifier. These voltage drops are lower at start up, hence the higher voltages.
280 V on the plate means that V(a-c) is about 273 V
252 V on the screen means V(g2-c) is about 245 V
Estimating that from the 37.7 mA about 5 mA flows through g2, this would give:
Pa = 273 x 0.0327 = 8.93 Watt
Pg2 = 245 x 0.005 = 1.23 Watt
Although the anode voltage is a bit high, the values for plate and screen grid dissipation are under the limiting values in the datasheets for the ECL86.
So to me your results seem fine.
Addition: Its normal that the voltages on the anode and screengrid are higher at start up. The tube needs a little time to conduct fully so only than the current draw will be normal. This current creates voltage drops over several resistances in the amplifier. These voltage drops are lower at start up, hence the higher voltages.
Last edited:
I agree and there will be more volts dropped when the other ECL86 is installed and drawing current.
@Zilch remember, at switch on the ECL86 is cold and is not drawing any current. As the EZ81 warms up and charges the reservoir caps there is little to no load on the power supply so you see a 'peak' voltage. As the pentode of the ECL86 warms up and draws current (the 38mA) that loads the power supply and drops the voltage to 'normal' steady state.
@Zilch remember, at switch on the ECL86 is cold and is not drawing any current. As the EZ81 warms up and charges the reservoir caps there is little to no load on the power supply so you see a 'peak' voltage. As the pentode of the ECL86 warms up and draws current (the 38mA) that loads the power supply and drops the voltage to 'normal' steady state.
The transformer looks like it is one with a tap which I think made part of the winding into a choke, as in this portion of the schema from a B5X23A.
EDIT: I see that there are comments for that now, but here is a bit if a schematic anyway.
EDIT: I see that there are comments for that now, but here is a bit if a schematic anyway.
Attachments
Last edited: