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Building my first PP vacuum amplifier

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Re: Building my first PP vacuum amplifier

Post by Miguel López » Mon Feb 15, 2016 4:32 pm

Herald1360 wrote:Interesting octal sockets, there- never seen anything like them.

Well Chris, those octals came to me from the UK, donated by a forum member, not sure if it was Andy, Jeremy, Jim, Howard or yourself. I have received so many helps from all of you that there is a point in which I lose the track. Bellow I show a picture of those sockets while still at the UK at Andy's backyard. Maybe Andy could remember who sent them to him. The PY33 and the PL36 were also sent by forum members. Anyway, thank to all of you, as all of you are part of this project too. :aad

Herald1360 wrote:Would those white wrapped transformers be home brewed?

Yes, they are. The mains transformer (at the center) is winded on an old GDR core, and the output is winded on a Soviet TCA-70 core. The choke is original and it is fron Czechoslovakia.

Herald1360 wrote:Are you running the PL36s as triodes or pentodes?

Pentodes
Attachments
Valve bases.jpg

 
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Re: Building my first PP vacuum amplifier

Post by Terrykc » Tue Feb 16, 2016 12:15 pm

Ah! All is clear now that you've posted that picture of the valve holders in their original state! They are perfectly normal but are missing the usual metal mounting plate.

You have made some specially shaped plates to engage in the groove which would normally hold that plate and clamp the holder to the chassis.

That is what gives them their unique appearance!

Ten out of ten for ingenuity, Miguel!

 
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Re: Building my first PP vacuum amplifier

Post by Miguel López » Tue Feb 16, 2016 2:34 pm

Terry wrote:You have made some specially shaped plates to engage in the groove which would normally hold that plate and clamp the holder to the chassis.

Hi Terry, thanks for your compliment.
Here in Cuba we say: "We must plow with the oxes that we have", so we must find a solution for the problem, cause the problem will not solve itself.

Here some pictures of the layout. Not finished yet, but I'm on my way.
Attachments
DSCN0999.JPG
DSCN1004.JPG
DSCN1005.JPG
DSCN1008.JPG

 
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Re: Building my first PP vacuum amplifier

Post by Miguel López » Mon Feb 22, 2016 5:07 pm

An update of my advances.
Attachments
DSCN1140.JPG
DSCN1143.JPG
DSCN1144.JPG

 
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Re: Building my first PP vacuum amplifier

Post by Miguel López » Mon Feb 29, 2016 3:00 pm

I had had a problem in the amplifier, althought I don't want to call it a set back properly, for reasons that I will expose forward. The thing is that the PL36 oscillates as hell once connected to the phase splitter. Without connect them to the previous stage, they work fine, no noise, a nice flat line in the secondary of the OPT. When you connect one of the PL36 to the phase splitter, it is good too. No noise and you can see the 1kHz sine wave in the secondary.

But when I connect the second PL36, oscillation starts and I have found no way to stop it. I did everything at hand to try to stop oscillations (change grid stoppers values, swap valves, tried diferent PL36).

Then I thought that If I can not solve that problem, I can go on a tangent. So, I mounted an output stage in the HTBB with a couple of EL84, and it worked fine connected to the phase splitter of the amplifier. I connected a speaker in the secondary and a radio in the amplifier input, and voilá!!!, the nice music appeared undistorted in the speaker. As conclusions:

1-) I have to learn more about PP outputs.
2-) I will use another valve type for the output in this project. Not sure if EL84, a pair of EL84 in parallel, or a couple of Russian 6V6 that I have. Have to make some more tests

 
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Re: Building my first PP vacuum amplifier

Post by Miguel López » Mon Feb 29, 2016 6:51 pm

Question:

I've been looking at some 6V6GT schematics for ideas, and I have noticed that the screen is often connected directly to HT+

Why?

 
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Re: Building my first PP vacuum amplifier

Post by Refugee » Mon Feb 29, 2016 7:09 pm

Have you got a decoupling capacitor and dropper resistor between the PL36 output stage and the phase splitter?
Most amps have this extra circuitry.

 
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Re: Building my first PP vacuum amplifier

Post by Miguel López » Mon Feb 29, 2016 7:56 pm

Refugee wrote:Have you got a decoupling capacitor

Yes. I tried from 100nF to 2,2nF. Same results.

Refugee wrote:dropper resistor

Not sure about this. I connected the decoupling capacitor to a 220K resistor, which other end was connected to bias. The joint between these cap and resistor was connected to the grid via a grid stopper. I tried grid stoppers from 1K to 33K. Same results.

 
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Re: Building my first PP vacuum amplifier

Post by Herald1360 » Fri Mar 04, 2016 12:05 am

That sounds more like the grid coupling than a decoupling circuit. The decoupling will be a resistor in series with the HT line feeding the phase splitter valve(s) and a capacitor to 0V on the phase splitter side, usually a moderate sized electrolytic- 10uF or so. In extreme cases adding 100n in parallel to bypass RF as well may help. This stops any voltage changes on the HT caused by varying output stage HT current from affecting the phase splitter stage. Since any such feedback getting through will be positive for one of the output valves, oscillation may result.

Those bare octal holders look like some I have, but missing a sort of crinkly spring washer that would retain them from underneath when the holder is dropped into the chassis through the right size hole from above. Mine are black not brown though so I don't think they came from me.

 
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Re: Building my first PP vacuum amplifier

Post by Miguel López » Mon Mar 07, 2016 8:16 pm

I'm really puzzled with the amplifier.

I spent the whole weekend working on it and the nasty oscillation did not stop. I tried diferent configurations, including several phase splitters and outputs. The fact is that if I include three stages, it oscillates, and I don't know why. With the output and the phase splitter only it works fine, but there is not enough gain, then I included another stage and it oscillates.

Oscillation occurs without signal, so it is inherent to the circuit, I guess. I tried several pre-amplifier and phase splitter configurations, including a concertina, and it oscillates all the time, when the whole circuit is connected together.

The pre-amplifier and the phase splitter alone do not oscillate (two stages). The problem is when I connect three stages. I really don't know what is happening there.

Maybe if I connect the negative feedback it would stop oscillating.

Help!!!! I need somebody!!!!

 
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Re: Building my first PP vacuum amplifier

Post by Terrykc » Wed Mar 09, 2016 12:00 pm

Miguel López wrote:Maybe if I connect the negative feedback it would stop oscillating.
I would hope so!

If it is stable with the feedback connected, try reducing the feedback until it is just oscillating or on the point of oscillating.

Then investigate to see if you can identify a particularly sensitive area, perhaps by moving wires or components or from hand capacity effects. I'm not quite sure how this will help but at least it should point you in the right direction.

Some further thoughts. Two stages are stable but three are not. This implies that there might be capacitive feedback between two wires or adjacent components on your tag boards connected in such a way that the feedback reverses when the third stage is connected from negative to positive. 

Could a high resistance ground connection be responsible for the unwanted feedback?

On the subject of grounding, it is good practice in such amplifiers to have everything connected to a common ground bus bar which is itself only connected to the chassis at the input.

Quote from the book "Mullard Circuits for Audio Amplifiers":

"In audio equipment, it is advisable to have only one direct connection to the chassis, and this should be made close to the input socket. With a number of connections, eddy currents can be set up in the chassis and hum voltages can be induced in the sensitive sections of the equipment by the magnetic fields associated with these currents. Normally, a common earth lead or busbar is recommended, and the centre spigots of the valveholders often serve as convenient points for securing this lead."

You can download a high quality scan of the book here but beware, it is a 50MB download! There are individual links to several of the designs in the second paragraph of this document which are displayed as web pages.

 
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Re: Building my first PP vacuum amplifier

Post by Miguel López » Wed May 04, 2016 9:24 pm

Hi, I'm here again

I have been away from the forum due to work. On the other hand I had nothing to share on this thread as the amplifier is not finished and I decided to give myself a break, in order to learn more about this issue. I wasn't able to supress the oscillation, and this project has taken me a lot of time (more than foreseen), so I want to make a stop to go on with some other projects. I will resume this one, but not soon (maybe next year). Mods please, do not close the thread if possible.

I will open other threads soon, to share my other projects.

All the best from sunny Cuba

 
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Re: Building my first PP vacuum amplifier

Post by Katie Bush » Wed May 04, 2016 9:40 pm

Hi Miguel,

Threads are very rarely closed unless requested by the 'owner' so please don't worry.. :aad

It's good to know you're still with us. :aag

Marion

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