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Old 14th September 2004, 03:12 AM   #1
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Default Which trans wire is which?

The output transformers on this amp (please see the picture) have only two unmarked wires from each secondary side. How can I tell which is positive and which is negative so can connect them properly to the speaker output?
Thanks.
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Old 14th September 2004, 06:11 AM   #2
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If your amplifier uses no NFB off the speaker side of the tranny, polarity is irrelevant. If you have NFB and your circuit oscillates, switch polarity
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Old 14th September 2004, 01:36 PM   #3
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Quote:
Originally posted by Geek
If your amplifier uses no NFB off the speaker side of the tranny, polarity is irrelevant. If you have NFB and your circuit oscillates, switch polarity
I'm not sure it it uses negative feedback or not. It is a home-brew amp. So there is no schematic. The former owner attempted to recap it and to install new speaker binding posts and input jacks but gave up on the task and left lots of wires disconnected, components removed, etc.
Since polarity makes no difference if there is no NFB, maybe I should just wire it without NFB. What effect would this have on its performance?

And if I do wire in NFB, should the connection be made from the + binding post through a 5.6K resistor and a 390MMF cap to G2 of the EF86? (I'm assuming, due to its tube configuration, that the amp is based on the Mullard 5/20 design).
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Old 14th September 2004, 01:49 PM   #4
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Alright, now we need to know a bit more.

What tubes does the amp use? If a pentode like 6V6 for output, is there a resistor or jumper connecting pins 3 and 4? If so, it doesn't need NFB. If not, it probably does, or was designed to - is there anything connected to the speaker terminals besides the transformer wires? Say, a resistor with capacitor in parallel? That would be for NFB.

Easy enough anyway, as Geek says - switch polarity if it oscillates. If it oscillates either way.... you have worse problems

<Edit for an edit. Durn simulposters you!>

The exact NFB network required depends on overall gain, setup, desired NFB, etc. I'd need a circuit to say for sure.

Tim
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Old 14th September 2004, 02:35 PM   #5
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Quote:
Originally posted by Sch3mat1c
Alright, now we need to know a bit more.

What tubes does the amp use? If a pentode like 6V6 for output, is there a resistor or jumper connecting pins 3 and 4? If so, it doesn't need NFB. If not, it probably does, or was designed to - is there anything connected to the speaker terminals besides the transformer wires? Say, a resistor with capacitor in parallel? That would be for NFB.

Easy enough anyway, as Geek says - switch polarity if it oscillates. If it oscillates either way.... you have worse problems

<Edit for an edit. Durn simulposters you!>

The exact NFB network required depends on overall gain, setup, desired NFB, etc. I'd need a circuit to say for sure.

Tim
It uses (per channel) two EL34, one EF86, one 12AX7, and one GZ34. Please see the enclosed photo of the output tube wiring. I received the amp with a small box full of caps, resistors, that had been removed, so I'm not sure what (if anything) it had connected to the speaker terminals.
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Old 14th September 2004, 03:09 PM   #6
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If you have Mullard's schem,you can rebuild it as a Mullard 5/20!
About the OPT connections,I think it's not too difficult to understant the primary connections.
About the secondary ... where are the grey cables going??
Maybe these are for the speaker terminals.
So do as Geek says.
I think the other are connections to achieve 4 , 8 or 16ohms.
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Old 14th September 2004, 03:40 PM   #7
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The output (secondary sides) of the OPT have only two grey wires that presently are unconnected. So, these OPT do not have the usual 4-8-16 ohm taps. The Mullard 5/20 schematic says that R13 and C9 (feedback loop components) are to be "chosen to match speaker as follows: 3.75 ohm speaker - 3.9K and 470pF respectively; 7.5 ohm speaker - 5.6K and 330pF; 15 ohm speaker, 8.2K and 220pF.

The Mullard design shows C10 and C11 (EL34 control grid bypass caps) as 0.5F 350v; this amp has 0.1F 1000V oil cans.
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Old 15th September 2004, 09:13 PM   #8
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I have tried running the amp bypassing the feedback circuit (the 5.6K resistor and 390MMF cap for each channel) and the result is that it works - with no howling, oscillation, etc., but the gain is far to high and the sound even with the preamp volume control way down is a bit harsh.
The output transformers, with two leads coming out of the secondary side of each, are a mystery to me.
Would it make sense to try to run EL34 tubes in triode mode? As I understand it, that would decrease the gain. For such a set up would I simply bridge the G2 screen grids and the anodes? Would I keep the present 0.1MF 600v cathode bypass caps and the 470 ohm cathode bypass resistors or would I need to change the values?
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Old 15th September 2004, 11:23 PM   #9
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The links on the output transformer suggest it has several secondaries, connected in series. You could take 4/8/16 ohms taps off those junctions. I suspect it will be set up for 16 ohms, what impedance speakers are you using on it?

One secondary lead from the output transformer should be grounded, the other is the speaker + connection.

As previous posters have said, you should connect the feedbach resistor from the + speaker output, to the input tube, at the junction of the two cathode resistors. If it howls, reverse the output transformer secondary leads.

If I were you, I would study the Mullard 5-20 schematic and simply copy that. You will notice that the input tube has two cathode resistors, one of ~100 to ground, the other several Kohms to the tube. The junction of these is where you atach your FB resistor going to the + speaker terminal.

Quote:
Would it make sense to try to run EL34 tubes in triode mode? As I understand it, that would decrease the gain. For such a set up would I simply bridge the G2 screen grids and the anodes? Would I keep the present 0.1MF 600v cathode bypass caps and the 470 ohm cathode bypass resistors or would I need to change the values?
Triode mode will not decrease gain, just lower the power. Ie it will still be a small movement on the volume knob for loud output, but distort more easily. Triode mode is a 200 ohm resistor from screen (g2) to anode. I would be suprised that the el34s have 0.1 cathode caps, that sounds like a coupling cap to the grid (g1) to me.

BTW those are some mullard el34s in the background I think...
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Old 15th September 2004, 11:28 PM   #10
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Hi,

Quote:
BTW those are some mullard el34s I think...
It reads Philips Miniwatt through my glasses....Not that I wear any...

Why redo the circuit? It seems to be working O.K, the distortion without NFB is what you'd expect from an amp from that era, no?

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