My first experience with electronic tubes - Page 5 - UK Vintage Radio Repair and Restoration Discussion Forum

Miguel Lopez · 10th Apr 2012, 9:52 pm

Here a picture of a brother of the "donnor" of the cabinet. The equipment bellow is similar to the one from which I retrieved the cabinet. It is a TESLA BM546 Digital Generator. Saddly, it was only the cabinet that was still available when I knew about the dumpping. I retrieved several cabinets like this one.

Miguel Lopez · 16th Apr 2012, 1:37 pm

I worked in the amp this weekend, mounting valves and checking bias.

The right channel is OK, with all valves mounted, good bias, good plate current, almost no hum.

The left channel has a little (but bothering) noise. I think it is a parasit coupling between both EL84 (6P14P) that make them oscillate, because when I remove any of them, the other works OK, with perfect bias.

I saw the noise in the scope and it looks as the heart signals that appears in a heart monitor (Hope it won't be alive

) Really bothering noise when heard in a speaker. The frequency of that signal is about 200 to 300 Hz

Another curious thing is that when I measure any point of the circuit with the voltmeter, the noise disappears and you see a flat line in the scope. If I replace the scope by a resistor of the same value of the internal resistance of the voltmeter, the noise remains.

Any idea about this.

16th Apr 2012, 4:03 pm

Try series grid resistors right next to the valve sockets (AKA stoppers), a few 10's of k ohms is about right, nothing critical. They stop the valves acting as transit time oscillators where the time it takes the electrons to travel from elecrtode to electrode causing a sufficient phase shift to cause oscillation.

Miguel Lopez · 23rd Apr 2012, 4:23 pm

Quote:

Originally Posted by merlinmaxwell

Try series grid resistors right next to the valve sockets

That's OK. The amplifier is now working GREAT!!!! I had it for several hours this Sunday playing music.

It's amazing, zero hum, very clean sound. And the magic eye is wonderful.

Thanks to all of you for helping me to achieve this result. I'm very happy with the sound of the amplifier, and being this my first work with electron tubes, I consider it a milestone in my electronics career

Miguel Lopez · 24th Apr 2012, 8:54 pm

Sorry, I forgot this in the last post, the resistance value I sued to solve the problem of oscilation was 33K. That's was really magical as the bothering sound disappeared inmediately.

I will show the final schematic of the amplifier soon. It differs something from the original intended. I'm just waiting for scanner time to scan it a post it here.

Miguel Lopez · 26th Apr 2012, 9:13 pm

This is the final schematic of my amplifier. Not too orthodox but it works. This is the schematic for one channel. Multiply it by two and then the amp is complete. The diode in the power supply is not a half wave rectifier, but an inter-channel decoupling diode. The other channel has one similar.

You can compare with the intended original in post #1 of this thread.

Refugee · 27th Apr 2012, 1:15 am

The next bit of learning is to convert one of the triode circuits into a phase splitter provided the transformers are pretty well matched. As they have there own cathode resistors it should cope with non matched valves better. The secondary of one transformer would have to be reversed to complete making it push pull. The other triode would amplify and feed the phase splitter.
As it is it could be converted to sterio pretty quickly too.

Miguel Lopez · 27th Apr 2012, 1:20 pm

I understand well how to connect the phase splitter, but I don't understand pretty well how to convert it in push-pull. Pentodes are not matched and they have some differences between them.

And on the other hand, does not one transformer load the other if I invert phases?

I will apretiate any comment regarding this.

I also understand how to convert it to stereo but there's no need as the one in the schematic is just one channel. The amplifier has another circuit similar to that for the other channel. It is doubled in the amp.

Refugee · 27th Apr 2012, 1:55 pm

The push pull drive is anti phase to start with. The output transformers are wound in anti phase when made. It would be interesting to try two small transformers wired like this. If it fails there will be no signal on the speaker so no burning.

Refugee · 27th Apr 2012, 1:57 pm

The matching is less of an issue with separate cathode resistors.

Miguel Lopez · 27th Apr 2012, 2:02 pm

Quote:

Originally Posted by Refugee

The matching is less of an issue with separate cathode resistors.

Very interesting, as I intend to improve this amplifier in the future with several valves sent to me by Jeremy (aka Pamphonica) throught Andy (aka Dr Wobble) in order to make a PP.
I have not tested those valves yet (just the heaters) but surely they will not be very matched. I will wind the thansformer (I've winded transformers for transistor PP), that is not a problem, but I was worry about the matching.

Refugee · 27th Apr 2012, 2:13 pm

The matching only makes a difference to the final sound quality and will not matter for testing.
Your home made test set can be used for matching using common valves from the old soviet TVs. Frame output ones are best.

Miguel Lopez · 27th Apr 2012, 2:20 pm

Oh sorry, I think I missed something here. I didn't get your point. Please could you explain more. I apologise for my missunderstanding.

The test I want to perform is:
1-) To test the valves emission (to see if they are usable)
2-) To draw a line with different grid voltages vs plate current to see if valves respond similarly (this is what I've understood for matching)

Please, give me light.

Refugee · 27th Apr 2012, 2:26 pm

That is spot on. If they turn out to have different pin outs you just change the wiring to suit.

Miguel Lopez · 27th Apr 2012, 2:33 pm

Still in the clouds.

I think we are talking about differents things. Please, remember I'm a Spanish speaker, I think I'm missunderstanding what you talk. Please, be more explicit.

Refugee · 27th Apr 2012, 4:37 pm

What i was saying is that you are doing what the shops do to select matched valves.
It looks like your best chance of finding valves is from old TVs as it looks like there are quite lot of them being broken down for spare parts.
You will have little trouble changing the wiring on the valve sockets if the pins are in different places on the valves you find.
If you wind your own transformer it is worth making a bobbin with two windings side by side and a spacer in between with the speaker winding over the top of both of these.
I have a push pull amp and it has 4 windings in the primary.
This is a high quality part and has the speaker winding in between two pairs of primary windings.
They are wired to make the normal 3 terminal anode pins like an ordinary P/P amplifier.
A high quality part like this will be more than can be made at home as all the turns would have to be counted and written down and then have the ends joined after all the work is finished.
The cheep ones are just wound as one long winding with a tap half way and the second half has slightly more DC resistance than the first half. This type is often found in things like record players. It does not have to be that well matched unless you want a studio quality amplifier.
With the parts i have seen in your photos it looks like you have a matched set of frame scan transformers that could be used to make two push pull amplifiers.
I am fairly sure that they can be wired so that they work like an ordinary push pull transformer.

Miguel Lopez · 27th Apr 2012, 8:11 pm

Quote:

Originally Posted by Refugee

It looks like your best chance of finding valves is from old TVs

Yes, and several sent by some forum members (bless you all

)

Quote:

Originally Posted by Refugee

as it looks like there are quite lot of them

Nop. Nowadays they are very strange and uncommon.

Quote:

Originally Posted by Refugee

You will have little trouble changing the wiring on the valve sockets if the pins are in different places on the valves you find.

That's right. No problem at all to change wiring. I planned the amp with that perspective in mind.

Quote:

Originally Posted by Refugee

If you wind your own transformer it is worth making a bobbin with two windings side by side and a spacer in between with the speaker winding over the top of both of these.

Yep, I will wind it but not by now. I've got some experience winding power transformers, and I'm studying about output transformers to do the job. Thanks for your advices, they are very valuable for me. Now I will enjoy the amp, and I will be making some tests to improve it with bigger valves and bigger output transformers.

Quote:

Originally Posted by Refugee

It does not have to be that well matched unless you want a studio quality amplifier.

Nop, maybe in ten years I'll be able to do that

, this is my first valve work, and I'm very pleased with the result.

Refugee · 27th Apr 2012, 9:06 pm

I wonder if anyone has a couple of output transformers that are the same so that we can try the two transformer push pull i was thinking of.
I was thinking of trying it with a couple of 9 volt 2.2 VAs with small signal valves to test the principal as i have a number of these that are new old stock that are water damaged so that the shielding has come off.

glowinganode · 27th Apr 2012, 11:13 pm

I think the two secondaries would have to be wired in parallel to make a push pull transformer from two seperate transformers. They need to be magnetically coupled some how.
Rob.

julie_m · 28th Apr 2012, 8:22 am

Do the primaries need to be magnetically coupled?

In a standard push-pull output transformer, the magnetic coupling allows the DC components to cancel one another out. Single-ended output transformers are already designed to tolerate DC in the primary.

10th Apr 2012, 9:52 pm	#81
Miguel Lopez Heptode Join Date: Dec 2011 Location: Mayabeque, Cuba Posts: 617	Re: My first experience with electronic tubes Here a picture of a brother of the "donnor" of the cabinet. The equipment bellow is similar to the one from which I retrieved the cabinet. It is a TESLA BM546 Digital Generator. Saddly, it was only the cabinet that was still available when I knew about the dumpping. I retrieved several cabinets like this one. Attached Thumbnails __________________ When electrons move, things happen. There's nothing you can do that can't be done.

16th Apr 2012, 1:37 pm	#82
Miguel Lopez Heptode Join Date: Dec 2011 Location: Mayabeque, Cuba Posts: 617	Re: My first experience with electronic tubes I worked in the amp this weekend, mounting valves and checking bias. The right channel is OK, with all valves mounted, good bias, good plate current, almost no hum. The left channel has a little (but bothering) noise. I think it is a parasit coupling between both EL84 (6P14P) that make them oscillate, because when I remove any of them, the other works OK, with perfect bias. I saw the noise in the scope and it looks as the heart signals that appears in a heart monitor (Hope it won't be alive ) Really bothering noise when heard in a speaker. The frequency of that signal is about 200 to 300 Hz Another curious thing is that when I measure any point of the circuit with the voltmeter, the noise disappears and you see a flat line in the scope. If I replace the scope by a resistor of the same value of the internal resistance of the voltmeter, the noise remains. Any idea about this. __________________ When electrons move, things happen. There's nothing you can do that can't be done.

16th Apr 2012, 4:03 pm	#83
Guest Guest Posts: n/a	Re: My first experience with electronic tubes Try series grid resistors right next to the valve sockets (AKA stoppers), a few 10's of k ohms is about right, nothing critical. They stop the valves acting as transit time oscillators where the time it takes the electrons to travel from elecrtode to electrode causing a sufficient phase shift to cause oscillation.

24th Apr 2012, 8:54 pm	#85
Miguel Lopez Heptode Join Date: Dec 2011 Location: Mayabeque, Cuba Posts: 617	Re: My first experience with electronic tubes Sorry, I forgot this in the last post, the resistance value I sued to solve the problem of oscilation was 33K. That's was really magical as the bothering sound disappeared inmediately. I will show the final schematic of the amplifier soon. It differs something from the original intended. I'm just waiting for scanner time to scan it a post it here. __________________ When electrons move, things happen. There's nothing you can do that can't be done.

26th Apr 2012, 9:13 pm	#86
Miguel Lopez Heptode Join Date: Dec 2011 Location: Mayabeque, Cuba Posts: 617	Re: My first experience with electronic tubes This is the final schematic of my amplifier. Not too orthodox but it works. This is the schematic for one channel. Multiply it by two and then the amp is complete. The diode in the power supply is not a half wave rectifier, but an inter-channel decoupling diode. The other channel has one similar. You can compare with the intended original in post #1 of this thread. Attached Thumbnails __________________ When electrons move, things happen. There's nothing you can do that can't be done.

27th Apr 2012, 1:15 am	#87
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: My first experience with electronic tubes The next bit of learning is to convert one of the triode circuits into a phase splitter provided the transformers are pretty well matched. As they have there own cathode resistors it should cope with non matched valves better. The secondary of one transformer would have to be reversed to complete making it push pull. The other triode would amplify and feed the phase splitter. As it is it could be converted to sterio pretty quickly too. Last edited by Refugee; 27th Apr 2012 at 1:21 am.

27th Apr 2012, 1:20 pm	#88
Miguel Lopez Heptode Join Date: Dec 2011 Location: Mayabeque, Cuba Posts: 617	Re: My first experience with electronic tubes I understand well how to connect the phase splitter, but I don't understand pretty well how to convert it in push-pull. Pentodes are not matched and they have some differences between them. And on the other hand, does not one transformer load the other if I invert phases? I will apretiate any comment regarding this. I also understand how to convert it to stereo but there's no need as the one in the schematic is just one channel. The amplifier has another circuit similar to that for the other channel. It is doubled in the amp. __________________ When electrons move, things happen. There's nothing you can do that can't be done.

27th Apr 2012, 1:55 pm	#89
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: My first experience with electronic tubes The push pull drive is anti phase to start with. The output transformers are wound in anti phase when made. It would be interesting to try two small transformers wired like this. If it fails there will be no signal on the speaker so no burning.

27th Apr 2012, 1:57 pm	#90
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: My first experience with electronic tubes The matching is less of an issue with separate cathode resistors.

27th Apr 2012, 2:13 pm	#92
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: My first experience with electronic tubes The matching only makes a difference to the final sound quality and will not matter for testing. Your home made test set can be used for matching using common valves from the old soviet TVs. Frame output ones are best.

27th Apr 2012, 2:20 pm	#93
Miguel Lopez Heptode Join Date: Dec 2011 Location: Mayabeque, Cuba Posts: 617	Re: My first experience with electronic tubes Oh sorry, I think I missed something here. I didn't get your point. Please could you explain more. I apologise for my missunderstanding. The test I want to perform is: 1-) To test the valves emission (to see if they are usable) 2-) To draw a line with different grid voltages vs plate current to see if valves respond similarly (this is what I've understood for matching) Please, give me light. __________________ When electrons move, things happen. There's nothing you can do that can't be done.

27th Apr 2012, 2:26 pm	#94
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: My first experience with electronic tubes That is spot on. If they turn out to have different pin outs you just change the wiring to suit.

27th Apr 2012, 2:33 pm	#95
Miguel Lopez Heptode Join Date: Dec 2011 Location: Mayabeque, Cuba Posts: 617	Re: My first experience with electronic tubes Still in the clouds. I think we are talking about differents things. Please, remember I'm a Spanish speaker, I think I'm missunderstanding what you talk. Please, be more explicit. __________________ When electrons move, things happen. There's nothing you can do that can't be done.

27th Apr 2012, 4:37 pm	#96
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: My first experience with electronic tubes What i was saying is that you are doing what the shops do to select matched valves. It looks like your best chance of finding valves is from old TVs as it looks like there are quite lot of them being broken down for spare parts. You will have little trouble changing the wiring on the valve sockets if the pins are in different places on the valves you find. If you wind your own transformer it is worth making a bobbin with two windings side by side and a spacer in between with the speaker winding over the top of both of these. I have a push pull amp and it has 4 windings in the primary. This is a high quality part and has the speaker winding in between two pairs of primary windings. They are wired to make the normal 3 terminal anode pins like an ordinary P/P amplifier. A high quality part like this will be more than can be made at home as all the turns would have to be counted and written down and then have the ends joined after all the work is finished. The cheep ones are just wound as one long winding with a tap half way and the second half has slightly more DC resistance than the first half. This type is often found in things like record players. It does not have to be that well matched unless you want a studio quality amplifier. With the parts i have seen in your photos it looks like you have a matched set of frame scan transformers that could be used to make two push pull amplifiers. I am fairly sure that they can be wired so that they work like an ordinary push pull transformer.

27th Apr 2012, 9:06 pm	#98
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: My first experience with electronic tubes I wonder if anyone has a couple of output transformers that are the same so that we can try the two transformer push pull i was thinking of. I was thinking of trying it with a couple of 9 volt 2.2 VAs with small signal valves to test the principal as i have a number of these that are new old stock that are water damaged so that the shielding has come off.

27th Apr 2012, 11:13 pm	#99
glowinganode Octode Join Date: Apr 2009 Location: Aylesbury, Buckinghamshire, UK. Posts: 1,522	Re: My first experience with electronic tubes I think the two secondaries would have to be wired in parallel to make a push pull transformer from two seperate transformers. They need to be magnetically coupled some how. Rob. __________________ We have done so much, for so long, with so little, we are now qualified to do anything with nothing.

28th Apr 2012, 8:22 am	#100
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: My first experience with electronic tubes Do the primaries need to be magnetically coupled? In a standard push-pull output transformer, the magnetic coupling allows the DC components to cancel one another out. Single-ended output transformers are already designed to tolerate DC in the primary. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.