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Building my first PP vacuum amplifier

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Building my first PP vacuum amplifier

Post by Miguel López » Thu Mar 05, 2015 8:52 pm

Hi all

The title of this thread express my intention to build a vacuum push-pull amplifier, more specifically, a mono Williamson type amplifier. I'm just in the study phase as I have never built something like that, so I will not begin to cut metal very soon.

I want to use a couple of 807s (run at 550V HT) as output valves in class AB, avoiding the grid voltage to go more positive than the cathode voltage (not sure if this is AB1 or AB2).

It is not my intention to built some beast which can deliver more than 20W.

My first question here is not related with the output stage but with the preamplifier and phase splitter. I've been looking at the circuit at the National Valve Museum which uses a pentode and a triode as phase splitter and an EF86 as preamplifier.

Image

http://www.r-type.org/articles/art-097.htm

I have a couple of EF86 (sent to me by forum members) and several Soviet 6F1P triode-pentode valves. 6F1P is very similar to ECF80. Not sure if it would be suitable for this circuit.

I would like to hear your opinions regarding this. Will be worth to try this circuit?
Should I try anything more simple like a common triode-triode long-tail or a concertina?

 
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Re: Building my first PP vacuum amplifier

Post by Terrykc » Thu Mar 05, 2015 9:49 pm

Miguel, do you have an ECC83/12AX7 double triode available?

If you have, you might consider building one of the Mullard amplifier designs.

Here is the 20W amplifier: http://www.r-type.org/articles/art-003d.htm

and the 10W version: http://www.r-type.org/articles/art-003e.htm

The front end is identical in both versions and there is a lot of information about different output stage configurations which might help you adapt the design for an 807 version.

If you don't want to exceed 20 Watts, you may be able to bring the HT down considerably. As you will see, the EL34 requires a much lower HT to deliver excellent performance and I don't think that smoothing capacitors for a 550V HT rail are going to be easy to find anywhere!

 
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Re: Building my first PP vacuum amplifier

Post by PYE625 » Thu Mar 05, 2015 10:18 pm

Hi Miguel,

I agree with Terrykc.
It would be safer and easier to start with a good old standard, like the EL84 Mullard 10watt circuit.
Here is a useful site with amplifier projects..
http://diyaudioprojects.com/
It could be very expensive to obtain suitable GOOD transformers for a Williamson amplifier and high voltage components tricky to source.
Then of course there is a bigger risk to health with such a high HT ! :ccf

PS, the concertina or split-load phase splitter is excellent. Virtually any twin triode valve can be used for this.

 
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Re: Building my first PP vacuum amplifier

Post by Refugee » Fri Mar 06, 2015 1:31 am

Beware of heater cathode voltage with split load phase splitters :cch
I have a home HIFI based on adapted cinema amplifiers.
These have split load phase splitters using ECC33s and the H-K voltage is rated at 100v and the main HT is 415V and even after the filter resistors it is over 350V. A third of this is heater cathode voltage in a split load phase splitter. It is too much and there is a little bit of AC getting onto the audio.
This was not a problem when the things had to blast audio at an acoustic well of people at high volume just to be heard.
I must get those octal plugs and fit something with a higher H-K rating :aac

 
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Re: Building my first PP vacuum amplifier

Post by PYE625 » Fri Mar 06, 2015 8:34 am

Hi,
That's very true about the heater-cathode voltage. I forgot about that.
I built my own amp using an ECC81 driving a pair of EL84 to give 10 watts.
I managed to keep the cathode voltage at 75 volts by DC coupling the first stage to the split-load driver.
A fairly high anode resistor, (390k and a 3.9k cathode resistor) was fine.
I got 0.1% THD at full power of 10 W, and something like 0.02% at 1 W output.
I bought a pair of output transformers from Sowter....U064 type, rather pricey but good quality.
I find it drives my Quad ESL57's perfectly with a detailed and lifelike sound.
I wish I could post the circuit on here, but not sure how, as it is hand drawn.
There must be a way to scan it, will look into it.
Must take some pic's of it too :qq1

 
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Re: Building my first PP vacuum amplifier

Post by Refugee » Fri Mar 06, 2015 3:37 pm

Hand drawn circuits can be done with an ordinary digital camera that is set to macro.
Use the crop function in the editor to remove the edges before resizing to bring it down to 800 X 600 or 1024 X 768.
It should then fit on the forum.

 
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Re: Building my first PP vacuum amplifier

Post by Terrykc » Fri Mar 06, 2015 4:25 pm

If you have access to a scanner, use that and set it for a black and white scan. If you use a camera, the background will always come out as graduations of grey, so no matter how much the file is compressed, the majority of the file size will consist of compressed shades of grey with very little data actually devoted to the black lines (and even they might be shades of dark grey!) which comprise the image you want to portray.

A decent scan, on the other hand, should keep the paper uniformly white - which then compresses beautifully - and black. Save the scan as a .png file - never, ever a jpeg!

Open your file in Paint and crop any unnecessary borders and re save as a .png file. At the bottom of the screen you will see the size of the image and the size of the file. If the file is less than 120KB you can upload it to the forum. If it is too big - but not much too big - try resaving it as a .gif file and check the size again. If it is still too big, you will need to attack it a different way.

Re-open the original .png file and look at the size in pixels. If it really is big, try resizing the image. Don't go over board! If you resize by 50% you will reduce both height and width and end up with a file only a quarter of the size, which may be too small! Use Control + Z to undo the resizing and try again. When you are happy, save it again as a .png but make sure you give it a new name - just stick a '2' on the end, for example. What is the file size now? Still too big? Try it as a .gif.

Don't forget that if your schematic is really big it might be much simpler to cut it in two at some convenient point. Your main aim must always be clarity.

Here is a recent example of what can be done: viewtopic.php?f=20&t=10637&p=111105#p111105

Note that the drawing is 1000 x 1414 pixels big!

 
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Re: Building my first PP vacuum amplifier

Post by Miguel López » Fri Mar 06, 2015 4:34 pm

Thank you very much for all your comments. I think I did not explain myself well.

My purpose is to build a PP amplifier using the 807 tubes. That's why the very high HT. On the other hand I'm thinking to use the triode-pentode phase splitter as a challenge and the way to learn something new. If it is too complicated I will desist, and I will try a simple triode-triode phase splitter.

Refugee wrote:Beware of heater cathode voltage with split load phase splitters

My intention is to run the 807 on fixed bias mode with negative voltage supply. Heater-to-cathode voltage limit on the 807 is 135V according to datasheet, while its maximun grid voltage is about 75 V so I see no problem on this. I will also use a separate heater winding for the 807 so I will be able to adjust the heater-to-cathode voltage.
I would like you to explain me what you mean with split load phase splitter. Sorry mate, I got lost there.

PYE625 wrote:It would be safer and easier to start with a good old standard, like the EL84 Mullard 10watt circuit.

Safer and easier....possibly, but then where is the fun and the challenge. :bba

PYE625 wrote:It could be very expensive to obtain suitable GOOD transformers for a Williamson amplifier

I have a Hammond transformer delivering 365-0-365 V AC and two separate heater windings. I will use a center tapped rectifier with two 1N4007 in series on both ends. My intention is to wind the OPT myself, using a Soviet TCA-70 core.

PYE625 wrote:Then of course there is a bigger risk to health with such a high HT !

I can assure you that I'm aware of that. I worked with 5,5 kV while building my scope, I receive a nasty electric shock from time to time in my bench, so I will take care :thumb: Thanks for your concern.

Terry wrote:I don't think that smoothing capacitors for a 550V HT rail are going to be easy to find anywhere!

Well Terry, remember that my case is special. I do not even think on any project if I'm not sure to have all of the required components since the beginning. I have several 220uF @ 500V electrolytics. I will use them in series with resistors in parallel in order to get 110uF on both legs of a Pi filter.

Terry wrote:If you have, you might consider building one of the Mullard amplifier designs.

I have considered them. If I do not succeed with the triode-pentode circuit, I will try that one.

 
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Re: Building my first PP vacuum amplifier

Post by Terrykc » Fri Mar 06, 2015 5:34 pm

Miguel López wrote:... I would like you to explain me what you mean with split load phase splitter. Sorry mate, I got lost there ...

It is a simple form of phase splitter which only requires one valve or transistor.

It is sometimes known as a concertina phase splitter - see the circuit here: http://www.r-type.org/articles/art-010p.htm

Essentially, the load resistor is split into two equal halves, one in the anode circuit and the other in the cathode circuit.

As a result, the valve cathode always sits at half the HT voltage, so it is important to ensure that the heater cathode rating is adequate.

 
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Re: Building my first PP vacuum amplifier

Post by Miguel López » Fri Mar 06, 2015 6:08 pm

I know the concertina, but I didn't know it is also called split load phase splitter

Thanks :thumb

 
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Re: Building my first PP vacuum amplifier

Post by PYE625 » Fri Mar 06, 2015 8:09 pm

Thanks refugee and terrykc, I will give it a try.

Miguel, I think it will be a good and interesting project. You certainly know what you are doing.
If you can wind your own output transformer, you have my admiration !! I wouldn't dare to tackle that myself. :ccf
Just be sure you wind the output transformer EXACTLY in accordance with the specified constraints of Mr Williamson, and you should be fine.

The Williamson is a bench-mark design by a true pioneer in audio.
Well worthy to build one and to experiment with various configurations of stages.
The thing to bear in mind is that the concertina phase splitter cannot provide enough voltage to drive the output tubes directly (807, KT66 etc), but it is fine with higher gain output tubes such as the EL84.

I am quite excited for you and look forward to hear how you get along with the project. :aad

 
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Re: Building my first PP vacuum amplifier

Post by PYE625 » Fri Mar 06, 2015 8:50 pm

Well here is my attempt at getting a schematic on here and it's unreadable.
I simply can't do it properly, sorry.
I have to reduce it so much so the forum can accept it, that it is just too small !
I've spent a couple of hours trying and pulling my hair out now :ccb
Attachments
001.png
Last edited by PYE625 on Fri Mar 06, 2015 9:21 pm, edited 2 times in total.

 
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Re: Building my first PP vacuum amplifier

Post by Ed Dinning » Fri Mar 06, 2015 9:02 pm

Hi Miguel, 807's make great output valves. Geloso used them for many post war amps and they were very robust.
Fixed bias is probably the best way to go, but still earth one side of the heaters, use a centre tap or a "humbucking" pot.
Ultralinear connection of 807's works well, either with 20% or 43% screen taps. If you are winding your own transformer then the simplest way to go is with 4 primary sections and 3 secondary sections interleaved with the primaries.

Good Luck, Ed

 
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Re: Building my first PP vacuum amplifier

Post by valvekits » Fri Mar 06, 2015 9:18 pm

Hi Miguel
I once saw a Williamson amplifier in the Science museum in London. I just stared at it through the glass cabinet for a really long time. I felt kind of sad; I really would have liked to listen to it, instead of just looking at it.
Maybe that is an affliction that we vintage enthusiasts suffer.

If you can wind a transformer then that will be quite a feat in itself and I wish you well.
There is a site that sometimes get’s mentioned on here, it has some amplifier calculators and you can experiment by substituting various values into the calculators. I have used it for designing a phase splitter, just click on the "Launch me" to change the values and see the results. In the end I actually ended up with stock values. Some handy tutorials on there as well.

http://www.ampbooks.com/mobile/amplifie ... iled-pair/

Eddie

 
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Re: Building my first PP vacuum amplifier

Post by PYE625 » Fri Mar 06, 2015 11:58 pm

Well, I have managed to unscramble my brain and managed to re-size the scan of one of my amplifier circuits.
Try clicking on the pic. this time !! :qq1

There is nothing original here, just my adoption of various standard circuits that seem to work well for me.
Attachments
rsz_006.jpg
rsz_007.jpg

 
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Re: Building my first PP vacuum amplifier

Post by PYE625 » Sat Mar 07, 2015 11:25 am

Just for interest, here is a circuit of a Williamson amplifier that I built some years back.
Guess you could say it's my interpretation of the Williamson design scaled down to suit the components I had available at the time!
Attachments
rsz_001.jpg

 
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Re: Building my first PP vacuum amplifier

Post by Terrykc » Sat Mar 07, 2015 5:32 pm

PYE625 wrote:Well, I have managed to unscramble my brain and managed to re-size the scan of one of my amplifier circuits.
Try clicking on the pic. this time !!

Yes, as you have discovered, it is important to use the 'Place in-line' option or you risk ending up with nothing more than a thumbnail!

Is that the only change you made? It is obvious from many posts on here that many people have problems producing clear illustrations and drawings and the Forum Guide is aimed at pictures, rather than drawings, which doesn't help them very much so, let them learn from your experiences!

For some strange reason I didn't see your clear drawing this morning - I must have forgotten to refresh the page which is odd, because I usually start off with View unread posts!

So, I tied to copy your original iteration in Paint. The circuit wasn't too much of a problem after I'd increased the size somewhat but, of course, I could read more than about four or five component values!

I was going to post it 'as is' when I spotted your nice clear replacement!

Anyway, it was an excuse to do something with the new symbol set I've been putting together - you are welcome to a copy if you want - so here goes:

10W_amp.png

By the way, is that 3.3ρF capacitor in the feedback loop correct?
______________________________________________________
Oops! There must have been a speck on my screen! I,ve looked again and it is 33ρF!

Now corrected - but it still sounds rather small ...

 
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Re: Building my first PP vacuum amplifier

Post by PYE625 » Sat Mar 07, 2015 6:29 pm

Hi Terry, That re-drawing is brilliant !! :aad

I selected the 33pf on the basis of minimum square wave ringing with the least roll-off.
It may well be too low, because the response is more or less flat from 20HZ to at least 70KHZ at 1 watt output !

The trimmer in the cathode circuit of the output stage is in fact a 2 watt cermet, and not 3 watt as shown, plus the two 47 ohm need not be 3 watt either!
I found that this method was good because it allows compensation of slight variations in cathode current between the EL84's.
In other words, DC balance can be achieved in the output TX.

Apologies Miguel for slightly drifting off your original topic, but I hope you find it interesting :)

Best regards,
Andrew.

 
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Re: Building my first PP vacuum amplifier

Post by Miguel López » Mon Mar 09, 2015 9:59 pm

Well, thank everybody for your comments and advices.

Now let me re-take the path of this "adventure". This week I tested the triode-pentode circuit, using an EF86 and one 6F1P as intended. It worked fine.

I got up to 100Vpk-pk ourput on the anodes of the phase splitter, with a 100mV pk-pk input signal.

Outputs were slighly diferent in amplitude, but I was expecting that. I will correct this adjusting the amplitude of one of the valves via a potentiometer in series in the anode. On the other hand, the couple of 807s that I have has diferent gain so I have to do that to correct this too.

I'm not interested in an ultralinear amplifier, as I intend to fix the screen at 300V via a resistive divider. I have seen this on several circuits.

Althought I know I would obtain the same amount of power with a less complicated circuit, I would like to expose the reasons why I want to use the 807:
1. To learn new things.
2. To face a new challenge.
3. To honor my friend Andy (aka Dr. Wobble), who has been my patron on all of this vacuum adventures.
4. To honor all of the other forum members who have contributed to this.
5. To have fun. :qq1 (don't worry, I have learned that high voltage on my fingers is not funny)

 
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Re: Building my first PP vacuum amplifier

Post by Miguel López » Mon Mar 16, 2015 9:04 pm

Thank you very much for all your kindness.

Saddly, I will have to "mothball" the PP amplifier project. I tested the 807s this weekend and one of them seems to be dead. I fixed-biased them with 280V on anode and -28V on grid. Cathode fixed to HT-, and Screen to anode via a 1K resistor. One of the 807s emitted 30mA with -24V on grid, so this seems to be OK. The other required -7.5V on grid to emit the same amount of current so I guess this one is "kaput".

Now I'm thinking to use the PL36 or PL504 that forum members sent to me some time ago (via Andy), I tested a PL504 and with -28V on grid it emitted 120mA which is very good. As I have several valves of this type, I can use them without problem with spares.

I realized that I can not face a project without spares cause if some component fails (or I kill it), then the whole project (and work and time) get lost. I also have a couple of 6V6 which are good (I also tested them) but there is the same problem with spares. They are just two, so if any of them fails, well, the project is "kaput" too. I think that the PL504 or PL36 should be fine. I have to test the PL36 yet.

Then I'm thinking to use a couple of PY33 diodes (I have three in fact) that forum members sent to me some time ago (via Andy) in order to build my first all-vacuum amplifier. For this I will have to rewind and existing transformer as all of this valves requires 27V on heaters, so, it will takes a little more time, that's why I will make a stop by now.

This project is not ended but "mothballed", and I will resume it soon.

Bless you all.

 
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Re: Building my first PP vacuum amplifier

Post by PYE625 » Mon Mar 16, 2015 9:53 pm

Hello Miguel,

Sorry to hear you are mothballing it for now, but I hope soon you decide to experiment further. It is an interesting project.
(6V6 are also known as 6n6c as a Russian type and are pretty cheap on ebay)
(PY33 are a great high-current rectifier but are limited to about 250 vac input I think.)

Take care and best regards,
Andrew.

 
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Re: Building my first PP vacuum amplifier

Post by Miguel López » Wed Oct 28, 2015 2:18 pm

Hello

It is time to resume this project. In fact I've been working on the chassis for this amplifier. I have a doubt that I like to clear with your comments.

As the 807 is a very tall valve, and I want to put the amplifier inside a cabinet as I don't want such dangerous voltages and temperatures exposed (my wife's duster is very restless and audacious), so I thought to mount the valves horizontally and then the doubt arose.

In the datasheet by RCA (USA), it is said that the valve can be mounted on any position, but in the datasheet by STC (Australia) it is said the following:

Mounting: The valve should be mounted in equipment si that it operates in a vertical position with the base downward. In exceptional circumstances it is permisible to mount the valve horizontally but only in such a way that the plane of the majot axis of the control grid and screen are vertical. The plane of these grids is correct if Pin 3 is on the vertical diameter of the valve base.

Does anybody have any experience mounting this valve horizontally?

 
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Re: Building my first PP vacuum amplifier

Post by PYE625 » Wed Oct 28, 2015 8:28 pm

Hi,
I don't have first hand experience of these valves, but from the above statement, I would be inclined to follow the advice given from STC.
The position suggested would minimise the risk of a short occurring due to electrode sag when the valve is hot and after some period of use.

Happy Building!!

 
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Re: Building my first PP vacuum amplifier

Post by Miguel López » Fri Feb 12, 2016 3:57 pm

Hello everybody

I'm here again to resume this project. Have to say that now, thanks to forums friends, I have several 807, but as a man thinks as he lives, I have decided to desist of using them in this project, and now I have another project for them. I will use them in a stereo PP amplifier for the next year.

For this project I will use PL36 as output, with fixed bias, and PY33 diodes on the HT rectifier, and a 6AL5 to rectify the negative bias. This will be an all-vacuum amplifier, as I will not use a single solid-state device. the negative bias will also power a small relay which connect HT to the circuit and if the mains fails (usual in Cuba), it will bleed the HT capacitors to avoid a situation of having HT without bias, due to such mains fail.

I have built the chassis and I'm now in the wiring stage. Here some pictures.

DSCN0974.JPG


DSCN0981.JPG


DSCN0985.JPG

 
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Re: Building my first PP vacuum amplifier

Post by Herald1360 » Sat Feb 13, 2016 6:39 pm

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

Would those white wrapped transformers be home brewed?

Are you running the PL36s as triodes or pentodes?

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