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Old 25th Nov 2022, 2:43 pm   #1
Diabolical Artificer
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Default Output transformer HT connection.

Imagine a valve amplifier, push pull AB1 stage, 10 - 20w OP Power supply is a SS bridge rectified with a CLC filter supply, where is it best to connect the output transformer center tap, and why?

Andy.
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Old 25th Nov 2022, 2:59 pm   #2
Robert Gribnau
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Default Re: Output transformer HT connection.

I prefer after the choke, so at the second capacitor, provided the dc-resistance of the choke is low (something like 60 Ohm).

In class AB1 the current consumption at full power is higher than under no-signal condition. Because of this any resistance in the power supply creates some sag, although with music/speech the sag will be low anyway. The dc-resistance of the choke will create some extra sag, but I find the advantage of much less ripple worth it. I know that in push-pull power supply ripple gets cancelled, but only totally when the output valves are exactly matched.
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Old 25th Nov 2022, 3:06 pm   #3
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Default Re: Output transformer HT connection.

Assuming the filter choke is rated to carry the amp's full HT current and the second filter capacitor (i.e. the first of the 'smoothing' capacitors) has a high enough value to supply any audio AC drawn from it, then you might as well connect the OP transformer centre-tap to that second filter capacitor.

If the output valves are tetrodes or pentodes, the amp is not conventionally ultralinear and the choke and/or second filter cap is a bit weedy then you might do better to connect the OP tranny's CT to the first filter cap (the 'reservoir') and use the choke and second filter cap to supply just the output valves' screen grids. If the quiescent DC currents through the two halves of the transformer primary are sufficiently well-balanced then the action of the transformer will null out hum due to the 100Hz HT ripple. It won't null out the ripple's 100Hz modulation of the output stage gain though, which will lead to +/-100Hz sidebands on the audio signal.

EDIT: Crossed with Robert, who makes some of the same points.

Cheers,

GJ
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Old 25th Nov 2022, 3:14 pm   #4
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Default Re: Output transformer HT connection.

I'd connect it to the rectifier-side of the choke; the impedance is lower at that point so it will better be able to deliver current-peaks.

[my experience though is based on AM transmitter-modulators for voice applications, where you generally have some heavy signal-processing in the audio chain - clipping and compression - as well as deliberately rolling-off the audio response below 300Hz so any 50- or 100Hz hum is not noticeable].
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Old 25th Nov 2022, 3:36 pm   #5
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Default Re: Output transformer HT connection.

In addition to straight-forward hum, you can also get hum-modulation of the amplifier's gain resulting in 50/100Hz sidebands on all the spectral components of the music/speech being played. This gets frequency-doubled by the push-pull circuit, rather than cancelled.

David
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Old 25th Nov 2022, 3:53 pm   #6
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Default Re: Output transformer HT connection.

Interesting, thanks Does it make a difference if the supply is valve rectified and if the stage is AB2? I looked at various vintage valve amps and there seems to be no consensus. The Quad 11 takes the OPT CT at C1, or the reservoir cap, the Mullard 5-10 & 20 after the choke, the Leak 20 after a dropping resistor as it uses a CRC filter.

I assume also the only applicable parameters we are interested as far the supply is concerned,is it's ability to provide a low ripple HT and to provide sufficient current at high OP levels and peaks like bass drum beats etc.

Andy.
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Old 25th Nov 2022, 4:20 pm   #7
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Default Re: Output transformer HT connection.

That's a really interesting question for which I have no answer, although my preference would be after the choke provided it has the right parameters to support the full current required.

I've just looked at over 10 different circuits and I agree there seems to be no general rule of thumb. For instance Marshall and Fender use both methods in amplifiers of a similar output, and as you say the Quad II takes it from the reservoir while the Radford STA25 is after the choke. I guess there maybe someone more wise than me who knows the answer....
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Old 25th Nov 2022, 4:27 pm   #8
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Default Re: Output transformer HT connection.

It seems that most guitar amps connect the output stage HT before the choke and then feed the rest of the circuitry after the choke, so the choke needs only to be rated at 100mA, but hi-fi amps usually connect after the choke.

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Old 25th Nov 2022, 4:36 pm   #9
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Default Re: Output transformer HT connection.

In guitar amps distortion is a 'feature' whereas in hi-fi amps it's a problem, as is hum.

The Quad II is an unusual case as it's 'sort of' ultralinear, only with the common part of the winding actually implemented as a completely separate winding and moved into the valve's cathode circuit. The principles of push-pull hum cancellation and good screen voltage stability still apply though.

It's always possible to make the supply impedance as low as is necessary at the second cap, as long as that cap is large enough and the choke is too. But large things cost money, so the answer to the original question will depend, in part at least, on whether cost is an issue. In the real world of commercial kit it nearly always is.

If you like you can go the whole hog and make the HT supply actively regulated, reducing its impedance to a very low value indeed. I have a pair of Quad IIs modified by (sadly, the recently departed) Glenn Croft where this was done, albeit only for the supply to the KT66 screen grids and the EF86s. I'm not sure whether the effort is justified by any improvement, but it's an interesting example nonetheless.

Cheers,

GJ
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Old 25th Nov 2022, 4:46 pm   #10
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Default Re: Output transformer HT connection.

If you're looking at Class-AB2 or Class-B, the 'traditional' approach back in the days of valve rectifiers with limited peak-current handling ability along with expensive electrolytic capacitors wass the 'choke input' filter and maybe a swinging-choke.

These days we have solid-state rectifiers that can happily handle recurrent peak currents of 10A or more, and it's not a problem to deploy hundreds of uF of capacitance.

Personally, I consider chokes as an anachronism - even when designing RF linear amplifiers in the 80s I just went with a well-sized transformer [some of these were rated at 2500V/1A] and lots of capacitance so the surge-impedance of the supply as seen by the amplifier was low and any sort of voltage-sag on peaks was minimized.

I always pondered what the designers of 50s/60s audio-amplifiers would have done if they had been able to use low-forward-resistance/high-peak-current solid-state rectifiers and had access to cheap high-capacitance electrolytics.
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Old 25th Nov 2022, 5:30 pm   #11
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Default Re: Output transformer HT connection.

One point which hasn't been covered is that feeding the anodes after the choke will reduce the smoothing effect of the latter due to the (considerable) extra dc current through
it, reducing it's inductance, so poorer smoothing for the earlier stages. Life is one big compromise!! Plus ca change etc.
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Old 25th Nov 2022, 6:08 pm   #12
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Default Re: Output transformer HT connection.

Good point. But that drawback can be addressed by using a LC or RC filter between the power stage and the rest of the amplifier.

In a push-pull amplifier I built, with 2 x EL34 in ultra-lineair configuration, and with cathode feedback, I used power supplies (one per channel) with 5 uF-2 H/20 Ohm-50uf-2 H/20 Ohm-200 uF per channel. An extra RC filter separates the power stage from the preceding stage. It sounds very good to me.
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Old 25th Nov 2022, 6:19 pm   #13
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Default Re: Output transformer HT connection.

It's a bit of a self-defeating question!

I'd go after the choke - the HT is smoother, and won't modulate the audio.

As for not quite so good regulation, and inductance 'sag' due to increased current through it - well, you just use (or make) an uprated choke for it!
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Old 25th Nov 2022, 6:50 pm   #14
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Default Re: Output transformer HT connection.

Quote:
Originally Posted by G6Tanuki View Post
Personally, I consider chokes as an anachronism - even when designing RF linear amplifiers in the 80s I just went with a well-sized transformer [some of these were rated at 2500V/1A] and lots of capacitance so the surge-impedance of the supply as seen by the amplifier was low and any sort of voltage-sag on peaks was minimized.
But then isn't the mains-transformer/rectifier/big-capacitor an anachronism too?

For a one-off, home-built item, yes, I'd do it. But at that sort of power level, 2.5kW, the power companies want a decent power factor - which means a switching front-end.

It is of course possible to achieve a fairly good (90%) power factor with a choke-input filter - the current waveform approaches a square-wave with a large inductance, so maybe it's the big capacitor that's the anachronism!
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Old 26th Nov 2022, 12:47 pm   #15
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Default Re: Output transformer HT connection.

Hello,

I recently built a Marshall JTM45/Fender 5F6 amplifier, using KT66ís and GZ34 the efforts of which, can be seen post #7 onwards in the post below...

https://www.vintage-radio.net/forum/...d.php?t=195449

This was made with a salvaged a mains transformer from a basket case Jason amplifier and a choke and output transformer from a Selmer Organ and it was these that decided where I took the HT feed to the output transformer. The reservoir and filter capacitors are 50uF.

The choke has a DC resistance of 193Ω and inductance of 10H and is meaty enough to carry the HT current to the output transformer, however, seeing the output of the amplifier was in the mid 20 watts the current being drawn would cause a voltage drop over the choke, so in order to preserve output power, I took the feed before the choke. The Jason mains transformer is 320-0-320V and is possibly 10-20V below what Iíd have desired. But when salvaging you donít have the choice and you incorporate what you have in the toy box.

If I were using Ďsayí a 50Ω choke I may well have taken it after the choke, but with 193Ω I decided to take it before the choke. So sometimes, the available parts can drive decisions as regards the circuit design. I suppose the ideal transformer would have been the Radiospares Heavy Duty 350-0-350V half shroud drop-through or similar.

I still have the amplifier on the bench, and I decided to do a couple of experiments/tests with the HT feed to the output transformer before or after the choke.

With the feed taken before the choke I got HT before = 363V, HT after = 355V and power output = 29.4 Watts

With the feed taken before the choke I got HT before = 369V, HT after = 341V and power output = 25 Watts

There is a loss of 4.4 Watts with the feed taken after the choke.

The other thing with the feed taken before the choke, is Ďrippleí present on the clipping, whether this is a problem or not under Ďnormal just under clipí use, Iím not entirely sure, and will probably depend on the user/musician, however I remember when I was working with the guitarist Robin Trower, he noticed a difference between amplifiers at full power clipping (overload) in the USA and in UK and Europe. I put this down to the HT ripple in the amplifier being at 120Hz in the USA and 100Hz in the UK and Europe, this ripple Ďbeatí with the played note of the guitar to give a slightly different sound. These were the Marshall 100Watt Super Lead/Bass 'Plexi' amplifiers, where the HT to output transformer came before the choke.

I've attached a couple of pictures of clipping of the output signal with the HT feed to the output transformer taken before and after the choke. The ripple can be seen on the clip before, and clean after the choke.

Lastly, I agree with Aub as it depends on whether itís a guitar amplifier or hi-fi amplifier, and generally with guitar amplifier it is taken form the rectifier/reservoir capacitor before the choke, however there is the odd amplifier like the Selmer amplifiers where itís taken after the choke. Mind you, adding some resistance (using a choke) in the HT feed to the output transformer will cause more sagging of the HT thus giving a different sound, more so at overload when the current demand is at maximum.

Crikey, waffling on now!

Terry.
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Old 26th Nov 2022, 2:35 pm   #16
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Default Re: Output transformer HT connection.

Oops

Should have been:

With the feed taken before the choke I got HT before = 363V, HT after = 355V and power output = 29.4 Watts

With the feed taken after the choke I got HT before = 369V, HT after = 341V and power output = 25 Watts

Terry.
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Old 26th Nov 2022, 8:31 pm   #17
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Default Re: Output transformer HT connection.

Losing 4 or 5 watts in 30 watts wont be noticable amount of volume. Its always better to go after the choke IF you can get a nice low resistance choke. Many amplifiers used a swinging choke which still works extremely well and sag doesnt become noticable untill your pushing full power.
Maybe playing death thrash it would be a problem, ahh maybe not?? . If Andy is building Hi-Fi its better after the choke. If its just for raw grunt before the choke gives a bit more power and much less sag.

I guess Terry its a "Bridge of Sighs "

Joe


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Old 26th Nov 2022, 8:49 pm   #18
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Default Re: Output transformer HT connection.

Hi Terry

When the anodes are fed before the choke, the ripple on clipping could be reduced by extra smoothing for the screen grids, which are more sensitive than anodes to ripple, say around 50-100 ohm and 32uf. Not possible if an ultra-linear output transformer is used of course.

Lester
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Old 26th Nov 2022, 9:11 pm   #19
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Default Re: Output transformer HT connection.

Joe,

I was Too Rolling StonedÖ

I know Robinís kid brother very well and played in band with him in the late 70's and 80's [he was the best man at my wedding] and I got to know and subsequently work with Robin from there, I also worked in the local Music shop, Tim Gentle Music, in Leigh in the late 70ís to Early 80ís with one of Robins later (early 80ís to the early 90ís) Bass players Dave Bronze. I fixed amps and Dave sorted the guitars. Dave is now one of the top-drawer Bass players.

Lester, the screen grids on the KT66's are fed via a 1K5 with 32uF decoupling. There are two 100Ω stopper resistors close to the screen grid connections on the valve bases.

Terry

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I guess Terry its a "Bridge of Sighs "
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Old 27th Nov 2022, 9:51 am   #20
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Default Re: Output transformer HT connection.

Hello,

Been thinking and I could use silicon rectifiers as sensibly suggested by G6Tanuki and Iíd more than make up for the voltage lost across the choke Ė if I were to take the HT feed to the output transformer after the choke Ė however, in keeping with the spirit of the build I wanted to use valve rectification.

Iím doing another two or even possibly three amplifier builds, and one or even two of these will use silicon rectification, which again is driven by the transformers and chokes in the toy box.

Looking back, Iím sorry for rambling on about times of yore and music shops and musicians etc., but I was in a retrospective mood yesterday. Maybe starting the day listening to Tony Backburn and Sound of the 60ís didnít help here

Terry
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