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Old 16th November 2013, 11:24 PM   #1
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Default Conrad Johnson MV75 help needed

I am attempting to repair a torched MV75.
Can someone tell me what the voltages at the plate of V1 and V2 should be?
Thanks!

Jim
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Old 17th November 2013, 01:00 AM   #2
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Jim,

It's not like hints abound on the schematic. It looks like the bias supply is 51 V., BFD.

How much leeway do you have in bringing the "beast" back to life? I can't say that the OEM circuitry impresses me. Regulated bias and unregulated O/P tube B+ is a no/no. A "textbook" Mullard circuit has 1 fewer high pass pole in it, to cause LF instability. If the owner gives you carte blanche, in salvaging the situation, consider a CCS loaded 12AT7 section as the voltage amplifier and use Mullard topology. A 'T7 section sounds damned good with 200 to 220 V. on the plate and an IB of 3 mA. CCS load the LTP tail and force symmetry between the 2 sides.

NOS 6550As are scarce and expensive. The best current production tube in that category is the "reissue" Gold Lion KT88 made by New Sensor at Saratov, Russia. Take no prisoners and put 100 Ω safety resistors in the lines between the screen grids and the O/P trafo taps.
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Old 17th November 2013, 02:27 PM   #3
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Eli:

As long as it is functional and doesn't devour tubes, I have quite a bit of leeway. Tube suggestions appreciated; customer provided 4 new Electro-Harmonix KT88s.

I'm sure I can make it work - still, it would be nice to know the plate voltages before I get started.

Jim
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Old 17th November 2013, 02:41 PM   #4
GaryB is offline GaryB  United States
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Quote:
Originally Posted by dowdylama View Post
I'm sure I can make it work - still, it would be nice to know the plate voltages before I get started.
I've got a non-functioning MV75 that I'm either going to repair one of these days or just reuse the chassis and iron to make a new amp. Because of that I've kept my eye out for documentation about it. I found the following file on my hard drive with some voltages annotated. Hope it helps.
---Gary
p.s. I remember reading that the PC board can develop hairline cracks where the tubes are attached. Presumably this is from the heat of the tubes and the mechanical flexing of insert and removing tubes. These can lead to intermittent failures that are really hard to debug. Might be worth proactively adding some bridging/reinforcing wires from the socket to the traces.
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Old 17th November 2013, 02:51 PM   #5
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Thanks, Gary - that helps!

I may be interested in your MV75 if you want to get rid of it.

Jim
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Old 17th November 2013, 03:05 PM   #6
GaryB is offline GaryB  United States
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Quote:
Originally Posted by dowdylama View Post
Thanks, Gary - that helps!
Jim,
I found this as well. Might be useful as you try to restore things.
---Gary
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Old 17th November 2013, 03:15 PM   #7
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Quote:
Tube suggestions appreciated; customer provided 4 new Electro-Harmonix KT88s.
At 1/2 the price of GLs, the EH KT88 is an excellent value. The EH KT88 could use a bit more bass extension, but what bass it has is taut and tuneful. Bottom line, use 'em and enjoy.

I believe the A1 revision schematic Gary posted is a Mullard circuit.

Without going stark raving mad, a possible PSU enhancement lies in the B+ bridge rectifier. Replace the OEM diodes with snubbed UFnnnn parts on the ground side and 2X Cree 2 A./1200 PIV C4D02120A SiC Schottkys on the "hot" side. Both the UF4007 and the UF5408 are 1000 PIV, but the '4007 is 1 A. and the '5408 is 3 A. I suggest only 2 Schottkys, as the TO220 cases are quite bulky. Cylindrical UFnnnn parts are not as space hungry.
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