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Old 10th November 2003, 03:01 AM   #11
JDeV is offline JDeV  South Africa
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Default Re: 6072A Line-Amp

Quote:
Originally posted by JDeV

I just completed this one and got some final measurements and some questions. I used a nice solid Rhodesian Rose Wood for chassis with stainless steel top and copper cladding on inside. Alps Blue pot and Holco resistors. Input and output wires are some left overs from "Van Der Hull" and "Nordost" interconnects. Main trafo and separate heater PSU trafo are in separate box , connected with 8 core umbilical to pre-amp box. Still using mixed brand 6072's , but are very pleased with sound.

For more nice projects and good tube stuff, visit Dick's site here: Thanx for all of this Dick

http://www.triodedick.com/
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Old 10th November 2003, 06:51 PM   #12
JDeV is offline JDeV  South Africa
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Quote:
Originally posted by EC8010
Is the oscillation leaving the pre-amplifier? (DC couple the oscilloscope)
Is the heater supply stable?
Is the HT supply stable? (AC couple the oscilloscope)

As analog_sa says power supply decoupling close to the valves and star earthing usually avoids most problems.

Some more info:
I did notice that low F oscillation is already present just after 10uF cap on HT supply. Still don't know why.

I use a good star ground layout to start with, so that is o.k.
I did couple of things today which all helped to reduce the problem, but not solve it.
1) Fit 1uf OSCON on tube bases over dc heater supply.
2) Rerouted rectifier tube supply and HT.
3) Changed 33uF supply caps to 120uF - is this O.K. - not too much for valves?
4) Fit "silent" HT trafo - not making any vibrating noises.
5) Fit 600ohm resistors on 230-0-230 supply lines to reduce HT supply down to 290V. (Was 315V dc with "silent" trafo.)

The output signal is now very stable with small 50Hz AC component and very small low F oscillation. Can not hear or see it at all on SS amp (unless putting ear right inside speaker almost), but Mullard's are still not happy with it. I guess I should now start to work on them, or what do you think - Frank

Questions:
1] The EZ81 is making a physical vibrating sound, is it normal? Can hear it from about 1/2m away when no other noise in room.
2] Can I use DC for EZ81 heater supply, or is it bad practice for rectifier tubes?
3] How much capacitance can you put on supply for 6072A?

Thanx for all advise up till now
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Old 10th November 2003, 07:44 PM   #13
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Hi,

Quote:
1] The EZ81 is making a physical vibrating sound, is it normal? Can hear it from about 1/2m away when no other noise in room.
Definetely NOT normal. Have you tried another EZ81?

Quote:
2] Can I use DC for EZ81 heater supply, or is it bad practice for rectifier tubes?
You can, it's not bad practice but it's absolutely unnecessary.

Quote:
3] How much capacitance can you put on supply for 6072A?
As much as you like but that's not going to solve anything.

Quote:
I guess I should now start to work on them, or what do you think - Frank
The only thing I can think of right now is that I find it odd, really odd....

Cheers,
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Old 12th November 2003, 01:33 AM   #14
EC8010 is offline EC8010  United Kingdom
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Quote:
1] The EZ81 is making a physical vibrating sound, is it normal? Can hear it from about 1/2m away when no other noise in room.
2] Can I use DC for EZ81 heater supply, or is it bad practice for rectifier tubes?
3] How much capacitance can you put on supply for 6072A?
Making an EZ81 buzz is amazing! Mind you, I've never loaded one with 120uF. The manufacturer's maximum is 50uF.

Of course you can use DC heaters on an EZ81. Can't think why anyone would want to, mind.

Two thoughts:

You have an HT supply that splits between channels. I wonder if the motorboating (oh yes, that's what it is) is circulating between the channels. Try removing one 6072 and see if the other channel suddenly behaves itself.

Another possibility is that the heater/cathode insulation on your EZ81 is poor. If you have another, or even an EZ80 (perfectly adequate for your circuit), give it a go.
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Old 12th November 2003, 01:56 AM   #15
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Hi,

Quote:
Making an EZ81 buzz is amazing! Mind you, I've never loaded one with 120uF.
And they're not.
Loaded with a 120µF cap that is, that cap is after the choke.

Quote:
I wonder if the motorboating (oh yes, that's what it is) is circulating between the channels.
Possible, but I still suspect a dodgy rectumfrier....

Cheers,
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Old 12th November 2003, 02:57 AM   #16
JDeV is offline JDeV  South Africa
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That EZ81 is much quiter now, can hear it still but with ear right next to it.

This whole LF oscillating thing, could it be because of impedance differances: 6072a --> Mullard5-20 ?

I remember that the "Improved Audionote M7" (As per Robert Ang - http://www.audio-genesis.com/casanova.html )
was stated to have an output impedance of 425ohm, what the input impedance of Mullard5-20 is , I don't know, neither how to measure or calculate it.

How do you measure/calculate these impedances?
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Old 12th November 2003, 03:10 AM   #17
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by JDeV
This whole LF oscillating thing, could it be because of impedance differences: 6072a --> Mullard5-20?
No. If you can see something going on at the HT, then the problem is firmly within the line stage.

The input impedance of a Mullard 5-20 is 1M//<10pF. In transistor terms, that's an open-circuit.
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Old 12th November 2003, 11:36 AM   #18
DrG is offline DrG  South Africa
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Love the layout and especially the woodwork, JDeV.
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Old 17th November 2003, 04:05 PM   #19
JDeV is offline JDeV  South Africa
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Default LF oscil. progress

I got hold of scope and actually traced LF origin back to B+. It starts just after 10H choke (where I pick it up for 1st time). In meantime I moved chokes also to seperate box and increased 33uF caps to 470uF. I also replaced EZ81 with normal diodes. This all helped alot , but there is still small amount of LF oscil. Something else that I noticed now, is that there are more hiss, I can actually hear from my listening chair when vol. is turned right down. Also when preamp POT is only 25% open, Mullard is already at max. ouput. How can I reduce sensitivity (gain) ?

Except for these little pains, this preamp really sounds amazing, both with SS amp and valve amp. Can recommend definately.
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Old 17th November 2003, 05:36 PM   #20
EC8010 is offline EC8010  United Kingdom
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Default Far too much gain

The sensitivity of the Mullard 5-20 is 220mV for full power. A few quick calculations suggest that the pre-amplifier has a gain of about 18, resulting in a sensitivity of 12mV for full power at the volume control. Even particularly weedy tuners produce 100mV, so it's no wonder you have hiss and overload once the volume control reaches 10 o'clock.

You just don't need that gain stage. All you need is the cathode follower to buffer the volume control. To do that, you need to remove the first stage, and insert a 2k2 resistor between the cathode and the existing 100k cathode resistor, then take a 1M resistor from this junction to the grid. (And add a 1k grid-stopper to the cathode follower to prevent RF oscillation.) Finally, connect the grid to the volume control via a 10n 400V capacitor. This will get rid of all your superfluous gain, and will probably sound even better. It certainly won't hiss any more.
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