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Old 30th Mar 2020, 12:09 am   #1
AdrianH
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Default OK need assistance or clarification of output transformer values

I am new to actually making something with valves.

I am looking to make a simple audio amp from some 6021W valves and have found a few circuits to get me on my way. But I would like to work a few things out in my head better.

The valve is a sub miniature double triode (ECC70), now from the data ra is typically 6K5 Ohms per triode.
I intend to use two valves in push pull output, that is, the two triodes per valve in parallel acting as one valve. So I was initially thinking that the ra will be halved per valve to 3250 Ohms, but then thinking why, the anode voltage drop would be double if going through a single anode load, so perhaps the ra will stay at 6500Ohms, then I just get confused and head hurts. Probably over thinking it, just build it and see if it works!

Next question, I will be trying to get a Hammond 125B output transformer, the static anode current per triode will be around 6.5mA so 26mA typical in the transformer. and if I drive an 8 Ohm speaker think I would be using output pins 4&6 giving an input impedance of 12K8 Ohms end to end of the transformer.

Have I lost it completely or am I on the correct lines?

TIA

Adrian

I need to do something whilst on lock-down.
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Old 30th Mar 2020, 12:40 am   #2
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Default Re: OK need assistance or clarification of output transformer values

When you parallel two valves, be they triodes, tetrodes or pentodes the impedance will be divided by 2. So your first assumption is correct, it will become 3250 ohms Ra.
Your choice of Hammond 125B will work OK. . Current in the transformer will be HALF your 26 mA as there will be 13mA per side. The total flowing into the centre tap will be 26 mA, but that splits in half and 13 mA goes to each output valve.

Here is some reading for you, it is explained fairly well I believe.

http://www.r-type.org/articles/art-010d.htm

Joe
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Old 30th Mar 2020, 7:09 am   #3
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Default Re: OK need assistance or clarification of output transformer values

You need to draw a load line really, though it is possible to read off the value's off a datasheet and use those. First off it matters if your running the amp in SE ( single ended) or PP (push pull), what your HT is and also in what class. In class A SE you use the formula HT squared/ max power OP to find the rough ideal Z pri, more info here - http://www.valvewizard.co.uk/se.html

For PP amps are usually in class AB, so two load lines are drawn, I usually draw the B line first, you'll see why in a sec. In class B one valve will conduct for 1/2 the time and see half the Z pri (Z ratio is the sq of turns ratio) so we draw a line for 1/4 the Z pri, EG a shallower load line. In class A it will see 1/2 Z pri ( two valves yes? ). After you draw the class A line, you slide it to the left a bit, maintaining it's angle until it transects the B line, where it transects is up to you really bearing in mind how much power OP you want and at what point you want the OP to run in class B. more info here - http://www.valvewizard.co.uk/pp.html

Now, for two valves in parallel, we halve the Z pri as we have twice as much current, same voltage HT/B+. Usually triodes need a lower Z pri than pentodes/beam tetrodes I think, though not 100% on that as haven't designed any amps around them. After drawing load lines and doing a few calcs, knock up your circuit on the bench and measure it, this will teach you more in 5 mins than hours of reading.

Check out the Radio Designers Handbook, which is the bible for valve amp design, but it can be a bit heavy reading for a beginner. However there are loads of free PDF's online concerning valve design, Valve Amplifiers 2nd ed Morgan Jones, a few books by Norman Crowhurst etc, etc.I'm also doing a series of vids on Youtube about the subject, but struggling to make good interesting informative video's.

Andy.
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Old 30th Mar 2020, 9:50 am   #4
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Default Re: OK need assistance or clarification of output transformer values

OK many thanks for your responses. The circuits I am following are called the Powerman (and derivatives) have been small low power practice amps for guitars, but the circuit seems to be simple enough.

I was expecting to see some form of proper 180 phase splitter driving the finals, so not sure if these are in ClassA or AB. Thanks for the reading material I will look at all that definitely.

I must get the output transformer ordered as I suspect it will take some time to arrive. I have copied the circuit below, there are a few versions around of it and all rights stay with the original authors.

TA

Adrian
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Old 30th Mar 2020, 10:16 am   #5
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Default Re: OK need assistance or clarification of output transformer values

Quote:
Originally Posted by AdrianH View Post
...
I was expecting to see some form of proper 180 phase splitter driving the finals, so not sure if these are in ClassA or AB. Thanks for the reading material I will look at all that definitely.
That method is called various things like self inverting push pull (SIPP) or self splitting push pull (SSPP). The grid of the lower valve(s) is grounded and the full swing of the signal is applied to the upper grid. It has to operate in class A.

Have a look for Oddwatt or Poddwatt to get some ideas of operation?
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Old 30th Mar 2020, 5:29 pm   #6
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Default Re: OK need assistance or clarification of output transformer values

Isn't it just a variation on a long tailed pair differential amplifier?

Move it back a stage and you get one of the classic phase splitter circuits.
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Old 30th Mar 2020, 6:44 pm   #7
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Default Re: OK need assistance or clarification of output transformer values

In times-past I recall a 1950s US 'Civil Air Patrol' AM-airband transmitter by Bendix that used two pairs of 6L6 in this configuration, driven from the output of a carbon-microphone transformer; it was known as a "See-Saw" output-stage because the push-pull pair's currents pivoted up and down about the common cathode-resistor.

Four 6L6, in a 2+2 configuration, 350V on the anodes, could deliver 60 Watts of audio, enough to fully-modulate a 100-Watt VHF airband transmitter.

I'm kinda tempted to re-engineer this using a couple of 6BW6 as the output-bottles; with 320V HT I reckon it would be good for 20 Watts output.
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Old 30th Mar 2020, 7:28 pm   #8
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Default Re: OK need assistance or clarification of output transformer values

Some 1940s/50s Ferguson radios/radiograms used this type of circuit. I think the 401rg was one.
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Old 30th Mar 2020, 8:53 pm   #9
AdrianH
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Default Re: OK need assistance or clarification of output transformer values

I did a search looking at Oddwatt and one of the first responses came back with this page:-
https://diyaudioprojects.com/Tubes/K...Pull-Tube-Amp/

There is a circuit towards the bottom of the page using KT77 valves as the output. The interesting thing is that Cathode bias is done using a LM317 regulator IC and the Valve cathodes connect to the input of the 317. Adjust is connected to ground with a fixed resistor on it's output. The accompanying text describes it as a constant current source so the cathode current of both devices is limited to a set amount. So if one valve takes more anode current it will force the other valve off to compensate, as in a long tailed pair as described by Herald1360. At least that is how my brain considers it to be working, correct or not.

I used to use power NPN transistors with a bias resistor and a couple of diodes on the base junction and a series emitter resistor at one time to provide a similar constant current source for charging Ni-cads.

Anyway, the idea does seem to be used in lots of projects.

Awaiting transformer and some potentiometers, think I should have ordered some turret board as well!

Adrian.
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Old 31st Mar 2020, 9:24 pm   #10
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Default Re: OK need assistance or clarification of output transformer values

I know this will be a ripple on the pond to many, but to me resurrecting old and forgotten concepts (to me) is a milestone.

I have started to build my little amp and it is taking absolutely ages, working with the charts for the valves, plotting the load lines, then seeing what value resistors I need and then seeing what is the closest I have, checking wattage levels etc, then trying to build it up on strip board getting the lens out to check for shorts, etc.

The diagram I showed prior uses a supply transformer which is 250-0-250 Volts and I will be running this little amp at 150 Volts from a buck style filament and HT PSU.

Have a problem with caps, do not have any with suitable voltage ratings, all the ones I stripped out of the old cold war receiver leaked, I thought I had one which when tested was 3M4 Ohm might have been OK, but it caused an excessive anode current, which as I sit here typing this, should have seen it as it was connected to a 470K grid resistor, from 100 Volts off the first anode.

But still I am progressing slowly, the single double triode glows, the cathode and anode volts are within a few volts of calculated, so the project has started well.

Does anyone know anyone who sells kits of say 250 Volt rating capacitors from 0.022uF to 22uF, or will it be a case of individual part numbers.

Adrian
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Old 31st Mar 2020, 11:18 pm   #11
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Default Re: OK need assistance or clarification of output transformer values

Hi Adrian, e bay should do, but if there is the odd radio/TV repair shop in your area he may sell you a couple of caps for not much.
A pity Golbourne is not on or you could have got a good selection from there

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Old 31st Mar 2020, 11:50 pm   #12
AdrianH
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Default Re: OK need assistance or clarification of output transformer values

Hello Ed.

Just spent an hour going through CPC in Preston, ordering some caps, just getting a mixture from0.022uF to 22uF and the odd value in the middle and a speaker for when the amp is near completion. My parts boxes are all low voltage so a bit useless for this.

It is a shame about the shows, both vintage and amateur radio and everything really being stopped, but if needs must. Golbourne would have been handy for me not many miles away.

Adrian
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Old 1st Apr 2020, 9:20 pm   #13
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Default Re: OK need assistance or clarification of output transformer values

Hi Adrian, watch out for it in early November (hopefully)

Ed
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Old 1st Apr 2020, 9:31 pm   #14
AdrianH
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Default Re: OK need assistance or clarification of output transformer values

Will do ED, do I need to join a club or something?

A bit further on the simple amp today, awaiting delivery (sometime) of the transformer, volume control and a few HV caps.

Adrian
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Old 2nd Apr 2020, 8:48 am   #15
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Default Re: OK need assistance or clarification of output transformer values

Hi Adrian, all are welcome but you may need to be signed in on the door.

Interesting build up, but for long term use you may need to get PCB type valveholders that have long connecting "legs" or the board will get too hot

Ed
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Old 2nd Apr 2020, 9:11 am   #16
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Default Re: OK need assistance or clarification of output transformer values

Looks like your cracking on, never seen a HS for valves before. could have sent you some caps, looks like you're sorted.

Re CCS, a CCS presents a near infinate Z to AC so you get get better balance say on a LTP, i prefer using a MJE340, zener, few resistors to a LM317 based CCS as you can put a preset pot in the Re and tweek it to set V/I spot on.

Good work so far, remember to keep a good distance between Hi V potential tracks and watch out for ground loops.

Andy.
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Old 2nd Apr 2020, 9:19 am   #17
AdrianH
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Default Re: OK need assistance or clarification of output transformer values

OK ED thanks.

On the build, I am a bit restricted in options at present, the valves and the heat-sink hardware came from a Mil surplus unit, when I remove valves I can have short leads left.
But the box comes with 5 Triodes and 4 Pentodes, two snapped off wires in the removal process, but it is mainly a learning exercise for me, I doubt it will be used for long periods of time, but who knows.
The plan is a simple amp, then probably separate tuner if I can get the bits. I doubt if I would end up collecting lots of old vintage gear, just not the room to put them, but it keeps my grey cells occupied.

Hi Andy your post came whilst I was typing so a quick edit. Ground loops I will just have to see how it goes with the build. On the CCS this will just get a fixed resistor for the Cathode resistor, if I manage 1 Watt I will be pleased. The output transformer is still days off arriving yet, so waiting to see what the DC resistance is to work out the resistor for idle current. I have kept the HT low at 150 Volts so not that high, being careful of shorts between tracks and in the end the underside will get a coat of spray lacquer.

Adrian
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Old 6th Apr 2020, 9:56 pm   #18
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Default Re: OK need assistance or clarification of output transformer values

Well the line output transformer arrived today with a couple of volume pots, so it kept me busy for a little while, also building a speaker enclosure from a cardboard box as I have no wood to play with, so when restrictions get lifted I will be out to build a small wooden speaker/amp cabinet.
By the time that comes around I could well have a simple tone control included.

Not as powerful as thought driving the 4 Ohm speaker but loud enough for the miniature valve amp.

Total draw at 150 Volts is 25.4 mA, 6.3 Volt at 0.9 Amp so it is not the most efficient considering also using 12 Volt buck converters, but it has been fun to play and follow charts. Pentodes next!

Adrian
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Old 11th Apr 2020, 12:50 pm   #19
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Default Re: OK need assistance or clarification of output transformer values

Just an update and to ask for anyone a bit more knowledgable than me a few questions.

I found an old plywood board in the garage loft so used that to build a speaker enclosure with the small amp and buck PSU in the base, it is amazing how much better it sounds.

I have also tried to improve it from a small guitar amp that distorts very easily to something that is a bit less volatile by including some negative feedback from the speaker side of the transformer to the second stage cathode. It is never going to break any records but not that bad to my ears which are shot over ~6KHz.

I did some measurements using a radio test-set trying to figure the audio response, it was flat going up to 20KHz so added a 270pF cap on the second stage to bring the top end down and stop any oscillations that could occur with the neg feedback, or at least that was the idea after reading about phase shifts.

I am also trying to learn about Kicad, but that is another thing altogether. If someone would look over the pdf and see if there is anything that could cause an issue and why, I would like to benefit from you knowledge.

Granted I have few spare parts for this but it is fun learning and building.

Adrian
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Old 11th Apr 2020, 9:06 pm   #20
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Default Re: OK need assistance or clarification of output transformer values

C3 seems a bit OTT uF wise but doesn't really matter.

Not sure about that 270pF where it is, though. It'll roll off the feedback at HF but at the same time increase the gain of the second stage itself. To get a good handle on the feedback you really need to know what the open loop response is from the feedback insertion point to the output. That'll give an idea of how much NFB the circuit will stand and still be stable.

Can't comment on the circuit in detail other than to say it all looks reasonable in principle. As long as the first two stages are biassed to sit their anodes at a little above half their HT supply voltage and the output stages are running within their Pa ratings it should all be happy enough.
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