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Old 13th Oct 2017, 11:38 pm   #21
Radio Wrangler
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Default Re: Golden Ear Laboratory Amplifier Build

Argus got in while I was typing and broke off to eat. The comment he made on oscilloscope style design....

David
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Old 14th Oct 2017, 12:23 am   #22
Argus25
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Default Re: Golden Ear Laboratory Amplifier Build

At least in this case it is not a bad idea to use the design of a precision lab instrument, like an oscilloscope vertical amplifier, as an audio amplifier. I have seen much less impressive ideas from the audio field.

The design of this sort of amplifier of course was intended for good waveform reproduction on a CRT's screen, especially for square and other wave forms, where you want to see the original wave, free from amplitude or phase distortions. So its not a bad ethos for audio too.

The effort that went into these scope amplifiers focused on the knowledge that if the group phase delay for all of the signals in the bandwidth of interest was the same, and the amplitude of all individual frequencies that were being amplified was the same, then there will be no (or very minimal) harmonic distortion. So "distortion free" or high fidelity amplification is just another way of saying that an amplifier is free from errors in amplitude and phase.

To make this clear for the OP I have attached a page about it.

The only thing in this application that to some extent sabotages the notion of doing it for audio, as Radio Wrangler pointed out; "The output transformer had to go", is the phase shift due to the transformer's low frequency response limitations and resonances in the output transformer. That can, as noted, cause instability when the negative feedback is applied.

One thing that might help if there is low frequency oscillation with the feedback applied is to make sure the time constant of the few grid coupling caps that are there is large and go to higher value capacitors to help maintain the LF response because the cumulative delays due to RC coupling add up and the negative feedback at low frequencies can go positive and your amp could oscillate.
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Old 15th Oct 2017, 12:22 pm   #23
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Default Re: Golden Ear Laboratory Amplifier Build

The oscilloscope origin would explain it. The Decca Decola is another differential design but is AC coupled and no -ve feedback to worry about. It also uses a matching transformer to keep it differential all the way from the pickup.
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Old 13th Feb 2018, 2:31 am   #24
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Default Re: Golden Ear Laboratory Amplifier Build

A little further progress has been made! See photo.

Below is a copy of an email Andy, (Diabolical_Artificer)and I have been having.
It explains the basics of my layout for the power supply involved.
As always, I stand ready to be shot down for bad design/new ideas/ different views. and general comment.

So the crop circles are the power transformer positions, i.e. two toroids.
The big holes are for two by F&T dual 100 uF @ 500 volt capacitors.
Square layout of four small holes and two large holes are for HT chokes.
Its all dual mono
Back panel outside ends are for two standard circular connectors for the umbilicals to the main amp unit.
Im still humming and haring about wheather to put bias regulator bottles on the power supply chassis or the main amp chassis hhmm hmm ....
Bias is supplied by two el cheapo 240 volt to 18 volt power transformers from China.
They run backwards of course, being fed by a separate 12 volt supply into the 18 volt secondaries, and I obtain about 160 volts or so from the primaries
as I need feed a 105 volt OB2. Thats then fed to the driver and output stages. ( DC coupled) via a 10K pot.
There are also two bias supplies.
Heaters are supplied by 12.6 volt winding on the mains toroids. One for each channel.
So back to the circular connectors. They carry HT, main earth , which is also return for the HT supply. bias, either AC 12 volts to feed the transformers in the main amp chassis,
or negative 105 volts to the bias control pots, and 12.6 volts heater supply.
Connectors are 7 pin and are laid out in two pins, three pins and two pins.
I figure the 3 centre pins to be ground ( and something of a screen) the two top pins to be heater supply with the remaining two pins to be bias, OR 12 volts to feed the bias transformers.
Putting 425-450 volts on one pin and minus 105 volts on adjacent pin strikes me as a bit stoopid!!, as the connectors are rated 500 volts and 5 amps per pin, BUT I imagine that they mean
generally NOT to push the insulation to breakdown point.

The several 3mm small holes on the back panel hold a BIG 2HP rated ( for inductive loads) contactor rated 25 amps at 250 volts, with a cosine of .4
and the remaining holes are for the 24 volts inrush limiter relay.


The four holes on top level of back panel are for input and output fuses for the HT supplies, and the large square hole is for a fused IEC connector for mains supply.
The outside will be painted 50% flat black, and the inside will be very light gray matt, so this old fart can read resistor colours.
The bottom panel is yet to be made, but is 1.6mm punched steel as well as the chassis, which is called "zincalum" in australia. Its zinc and aluminium electroplated onto the steel surface
and is bullet proof, rust and corrosion wise.
Topping it off on the bottom will be four 1" rubber door stops screwed onto the base as feet.

Its a standard construction technique of mine. All internal wiring will be laced with British Admiralty waxed linen thread from WWII.
Any other holes I may require for tagstrips or terminals will be drilled as needed with a hand drill.

I dont know if you play with power toroids, but the reason for my crop circles, ESPECIALLY on the inside of the torus, is for cooling air!!
The top washer that holds the whole kit and kaboodle down is drilled with matching holes, as are the neoprene washers underneath the traffo,
and underneath the hold down washer. In fact I make a LARGE hole in the centre of the neoprene washer underneath the transfomer,
that insulates the transformer from the chassis, but doesnt block the vent holes. Why the big rave ? your toroid will be AT LEAST 20 degrees cooler.
Ohh!!! and I never buy or make toroids with a cast in resin centre for single bolt hold downs, for exactly the above reason.

Another update when I proceed some.

Regards
Joe
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Old 15th Feb 2018, 10:38 pm   #25
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Default Re: Golden Ear Laboratory Amplifier Build

Interesting project to follow.
Do you know there was a follow-up on the Marshall articles in Radio Electronics ?
An important one at that.
Here it is and it concerns a stability issue of the input-circuit that you should maybe consider taking into account.
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Old 15th Feb 2018, 11:34 pm   #26
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Default Re: Golden Ear Laboratory Amplifier Build

I thank you very much for that tip!!! I have searched the web, and all the magazine articles I could find on Joseph Marshalls designs, but didnt pick that one up.
I have used Marshalls origional modification that uses two OA2 regulators, and if I still have problems I will look into the feedback mod you describe.
I also was wondering if modern components might also help? I have most of the components ( passives, anyway) in precision versions as they are so cheap and freely available today. For instance the balance pots I have obtained are all 3 watt wirewound ten turn units, and resistors are .5% tolerance 2 watt units, so initial balance should be easy. I have used different coloured 4mm test sockets on all the points where measurements are taken to perform balancing and I have in the back of my head an idea using different coloured test wires that plug in directly, a switch to select each function, and a handful of resistors all contained in a box, so that I can balance "anytime" so to speak. The pots all have 1/8" shafts or 3mm with a screwdriver slot. This also means I dont have to cart the monster around and remove covers to perform the work.
Yep !!! its taken me half my lifetime, but I do try to work smarter these days. I have redrawn the circuit as well ( its actually to "fix" any errors, in my drawing, hence the pencil scribbles where I need move bits n pieces ) to make it easier to understand.

Joe
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Old 6th Mar 2018, 3:38 am   #27
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Default Re: Golden Ear Laboratory Amplifier Build

So I have made up some chassis!! and this DID take some chassis bashing .

You must remember EVERY hole started as 3 millimetre.
The "front" view is self explanatory, with the power supply on the left, and amplifier proper on the right. Front panel of power supply has on/off switch and pilot lamp ( a lovely big glass lens type from Belling and Lee, NEON of course, there is no 6 trillion candle power blue leds here !!! ).

Amplifier has output level meters, scavenged from Japanese sand amp I wrote off years ago as "not worth it".

Rear is as follows:
Power supply has fused IEC socket in centre
4 fuses two for left two for right channel, one for mains and one for HT per channel.
Two standard circular connectors for umbilical to left and right channels on the amp chassis.

Amp rear is as follows:
Bottom two holes are standard circular connectors from PSU.
Two output socket pairs ( in fact they are 19mm test terminals GOLD plated no less , as I could not get standard nickle plate variety), immediately behind the output transformer cutouts.
Two quick release fuse holders that are for setting/monitoring the output quad of 5B/254 cathode currents.

The multitude of holes on top of chassis are pretty self explanatory, and in any case will become self evident as the build progresses. N.B. the "farm circles" surrounding every valve is for convection cooling of all the glass on board.

Pic entitled "layout: is me trying to see logic into the layout, and planning wiring and component positions.

This layout took me several weeks of "thinking", and its wet season here so dragging my 60 kilogram drill press out the door every day was not possible.

I dont want to be a pest, BUT, if somebody will look at the circuit, specifically the 12BH7 drive to the 5B/254 grids, there is only ONE grid resistor, both in my redraw, and in the origional article writeup. Please correct me if I am wrong, but shouldnt it be that EACH G1 of the 5B/254 have a 1k2 resistor from the cathode of the 12BH7A, rather than one from the cathode pin of the 12BH7A to the "first" ( I would call it "the closest) grid and then hard wired to the second G1 of the second ( or furthest ) 5B/254?.
As I was taught many years ago, the resistor body should be "as close as practical" to the G1 pin on the valve socket.

So when it stops raining I will post some pics of the paint job, and the main hardware assembly, to be followed by my "build"



Cheers for now

Joe
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Old 7th Mar 2018, 4:01 am   #28
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Default Re: Golden Ear Laboratory Amplifier Build

It doesn't matter greatly that the grids of two valves are connected together and just the one 1k2 Grid stopper used. It just doubles the capacitance that the 1k2 feeds and determines the HF roll off. If you went to a separate Grid resistor for each valve, which might be a tad better as you can have each close to the pin on the valve socket, I would recommend increasing the value of each to 2k4.
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Old 7th Mar 2018, 4:37 am   #29
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Default Re: Golden Ear Laboratory Amplifier Build

Thank you Hugo,
As the value of the resistor is quite small ( even 2K4) I hadnt thought at all about the capacitance the 12BH7 has to drive. I really dont like "anti oscillatory" resistors at all, and in my circuit redraw I have deleted the plate resistors. I will retain the screen grid resistors though, and have even thought to increase them to something more like 470R.
I was thinking it will give a modicum of relief to the screen grids, which are not really rated to about 400 volts of HT, albeit in ultra liner mode. I feel that this amp will take a little ironing out to get it really working well.

Thanks for your input

Joe
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Old 8th Mar 2018, 8:16 am   #30
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Default Re: Golden Ear Laboratory Amplifier Build

I'd put a grid stopper on each OP valve, something like 1k to start with, 100r for the G2's.

One other thing I've noted Joe is the balancing pots as shown on the schematic are a bit iffy, if a wiper lifts..... I put 1M resistors from the wipers on my pots in parallel as a fail safe. Here's two way of doing it below, see att. The second way can use a double linear pot, one pot connected "backwards", that way you only have one screw to fiddle with to get good balance. Wouldn't bother with the second pot, it's fighting against the first, would use 10r or 15r sense resistors to ground instead.

Lastly you'll have to go through some valves to get reasonably matched OP valves; you can find valves that are matched as regards quiescent current, but they won't be matched at full or even half power. I found valves benefit from being run in for a while before matching.

Andy.
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Old 8th Mar 2018, 11:59 am   #31
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Default Re: Golden Ear Laboratory Amplifier Build

Nice work there Joe on the chassis. I bet that took a while at 3mm thickness and there are some rectangular slots that look more involved to produce so neatly. Looking forward to the next instalment!

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Old 9th Mar 2018, 12:22 am   #32
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Default Re: Golden Ear Laboratory Amplifier Build

Andy, thats why my question was asked!! I usually see one resistor per output bottle.

BUT at the same time with very neat layout and short wiring and specifically grid drive leads, they can be gotten rid of. I have built a very similar amplifier using 807's, and they were not needed, but then again that design was extended class A, one valve wired as a triode, and the other as a tetrode on each side of the centre tap. I don't think the pots will fail ( he said, and held it to the light) they are beautifully made 2 watts. Bias pot is 5 watts and the balance pot in the cathodes is huge 25 watts with a large carbon brush that operates on the winding ( see previous picture).

This is my first build using 5B/254 aka CV 428, so I am just treading gently. I have 50 of them, so I will be able to balance them I think. I get them for a few dollars each, and there are a few thousand available. I can also get 5B/255 aka CV391, which are "identical", but without topcap, the plate being taken to the valve base, namely pin 2.
They are more expensive however, as the guitar amp builders replace the famous RCA blackplates with them. ( RCA blackplates now fetch about $250 dollars each!!!!!! ).

Thanks for the compliment Al, what I meant about 3mm is the diameter of the hole I drill first as the pilot hole, I then scale up as I go. Chassis material is 1.6mm thick "zincalume" steel. Zincalume is electro plated mix of aluminium and zinc that can't be soldered. It also doesn't rust, even after years in the weather. I use a jig saw to cut the transformer drop throughs, a mm or so smaller than it needs be, and then file finish them. In fact I don't have drills the right size for valve bases either, so they are filed up to size too. I still have blisters healing on my fingers from the filing .

WE had 4" of rain overnight ( the first "wet season" rain in about 20 years) so painting will have to wait untill the 90% humidity goes. I will keep you all posted.

Regards, Joe.
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Old 10th Mar 2018, 10:13 pm   #33
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Default Re: Golden Ear Laboratory Amplifier Build

Joe,

Interesting the idea of putting resistors in series with the screen grid. I'm not 100% sure of all the effects of this, but depending on how large the R value, this could be three. (Maybe more could be added to the list ? )

1) It could lower the gain a little or signal level at the plate.

2) with the screen moving around with signal voltage, there will be Miller effects at the grid (g1) so the apparrent input capacitance of the valve will probably increase, lowering the frequency response at radio frequencies, maybe why it helps with HF stability.

3) maybe suppresses Barkhausen oscillations of electrons around the screen grid.

What other effects I wonder ?

In some vintage TV's I have seen using EL33 output tubes I have seen 47R resistors placed in series with the plate. Presumably this was to suppress oscillations due to the capacitance and the primary leakage reactance of the transformer with the secondary loaded by the speaker.

But with small class A output stages at least, I have never had one yet that has not been tamed with the screen bypassed to AC directly to the cathode with a combination of a large value for low audio frequencies and a good 0.1uF cap and a large enough series g1 resistor. I have not had one where the anode resistor is definitely required, though I have fitted one as a link across socket pins for wiring convenience, but it was all for radio, not HiFi.

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Old 26th Mar 2018, 2:57 am   #34
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Default Re: Golden Ear Laboratory Amplifier Build

A further question for the boffins,
Please see attached pencil scribble,

I have dual power supplies on a separate chassis to the power amp.
When the HT voltage ( about 425 volts) enters the amplifier chassis, I take it directly to another 100uF 500 volts F&T brand dual electrolytic. I hope that will "reduce" the impedance of the one metre umbilical power supply lead. This point is also connected directly to the output transformer centre tap. Also connected here, is a small choke: 15 henry, 70 mA, 500 ohm, that decouples the main PSU from the driver stages, i.e. 12BH7 cathode follower, driver, phase splitter, and balanced input. As I have TWO complete power supplies coming into the power amplifier chassis, this will of course be doubled up. Now, my probably silly question is this, Does it make any difference to the main 100uF, 500 volt caps if I run "cap 1" from left supply, and "cap 2" from the right supply? As per my sketch. Its maybe a silly question, but I have not seen this idea before.
WHY am I doing this, you ask?, It makes two complete wiring looms, one for left and one for right, EXCEPT for the main capacitors.
Thanks in advance for your patience!!

Regards

Joe
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Old 26th Mar 2018, 4:32 pm   #35
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Default Re: Golden Ear Laboratory Amplifier Build

Hi Joe, I can't see it making any difference, but try adding a 1uF polyester or similar cap in parallel to the e-caps. This greatly reduces the RF impedance of the HT lines.

H&S would also suggest a bleed resistor across the caps.

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Old 26th Mar 2018, 7:07 pm   #36
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Default Re: Golden Ear Laboratory Amplifier Build

I think that the only difference wiring the capacitors as you've shown in the sketch will make will be to the cross-coupling between the channels because of signal currents flowing in what are now the common grounds. To be honest I wouldn't expect that effect to be very large.

The question of whether it's a good idea to put large HT smoothing capacitors at the amp end of the umbilical might depend on the rest of the power supply circuit. Specifically, when you call these 'the main capacitors' are there actually other capacitors back in the power supplies themselves and is there any impedance (a resistor or a choke) in the HT rail between those capacitors and the 100uF ones on the amp chassis ? The reason I ask is that if there is no impedance, and particularly if these really are the main capacitors because there are none back in the power supply, then you run the risk of large pulsed charging currents flowing along the umbilical cores. This can be a recipe for generating a lot of noise at the AC line frequency plus a very broad comb of harmonics. Even if most of the capacitance is back in the psu, significant pulsed currents can still flow into your 100uFs. Of course the choice and layout of the grounding scheme will be critical if you want low-noise operation. Normally in a 2-chassis scheme I think the standard approach is to do as much filtering as possible back in the psu chassis and to transport as near as possible constant DC voltages down the umbilical. The resistance of the umbilical cores should be of the order of an ohm or less. At bass frequencies, where you're likely to need the most power, the effective impedance of realistic capacitors is going to be a lot larger than that, whichever end of the umbilical you put them (100uF at 100 Hz has a reactance of 16ohms). So you might as well put them at the psu end where they won't generate a lot of charging noise.

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Old 27th Mar 2018, 1:32 am   #37
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Default Re: Golden Ear Laboratory Amplifier Build

Thanks for the answers guys,

I have made a couple more sketches showing what's in the power supply (without the inrush and primary circuits)and what's on the amplifier chassis.

REMEMBER !!

There are two identical circuits on the power supply chassis. There are of course two identical circuits on the amplifier chassis as it's a stereo system.

These additional drawings will make it a bit easier to visualize I hope.
My original question relates to the two capacitors in the top left of the amplifier chassis, so now you can see there are in fact 4 of the 100 uF 500 volt F&T capacitors, two on the power supply chassis, before and after the main filter choke, and the two under discussion on the amp chassis.

I have added the positive bias supply applied to the input valves, to reduce heater/cathode voltage, and I guess too, it will reduce hum to some degree.
The reason I have tied it to pin 9 ( centre tap of the heaters for each individual triode) is because it saves two resistors, and I figure that the two heater resistances within a single valve will be from the same production run of heater windings, and will be near as dammit identical, so the centre tap should approach the ideal.

I have another question though!! after reading up on gas regulators.
On the bias supply, which consists of the power components on the power supply chassis, and includes the OB2, then, via the umbilical, there is a 50 uF electrolytic cap connected to what is effectively the output of the OB2.
Isn't that capacitance going to cause problems with oscilations of the OB2?
or have I misunderstood what I have read about capacitors AFTER a gas regulator bottle?.

So off topic, but we had almost 500 mm of rain in 24 hours, so I still haven't painted the chassis. All up we have had almost half a metre of rain in 4 days.

Cheers

Joe
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