Questions on my Rev B

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Questions on my Rev B

Postby veg1515 » Tue Sep 02, 2008 10:34 pm

I have finally put everything together after some long delays . Thanks to all who have helped in the past.

I have a WAD El84 kits I am comparing this kit to. One thing I notice about the Rev B is the trannys get really hot and the volume max is about a 1/3 of what my Wad amp is. I think there is a problem with the amp .

Any suggestions on where to start.

(When biasing the amp I turned the pots all the way to one side and the numbers were still a little off)

I did the tube rec mod and I am using Ned's trannys
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Postby TomMcNally » Wed Sep 03, 2008 9:17 pm

I don't know what a WAD is, so I can't comment there.

Which transformers are you using specifically - the
Dynaclone type PA-774 power transformer ?

What are your AC (6.3 volt filament) readings, and
also are your DC test points checking out within
10% of the manual ?

The bias should be pretty close to the manual as
well ... how far off are your readings ?

How hot is hot ? Can't touch it at all ?
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Postby Ty_Bower » Thu Sep 04, 2008 6:40 am

Ned's power tranny, as well as any original PA774 power transformer, is likely to get very hot. You probably won't be comfortable keeping a finger on it for more than a second or two.

The output transformers should not be especially hot, unless they are unusually close to the power transformer or the tubes.

You mentioned you did the "tube rec mod"? I'm assuming that's a vacuum tube rectifier? From what I recall, Ned's power transformer didn't have a 5v winding for an octal base tube rectifier. Did you add another filament transformer?

I have a Rev B, and I couldn't get my idle currents to 35 mA, even with the bias pots turned all the way. I run them around 32 mA, and even then the bias pots are almost all the way to one side. However, you can't really use mine for comparison because my B+ voltages are rather high - over 400V.

Photos of the top and bottom of the amp would be helpful, as would voltage readings from the various checkpoints noted in the manual.
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hot stuff

Postby EWBrown » Thu Sep 04, 2008 2:25 pm

If you can't get the full 35 mA out of the rev B, try changing the four EL84 ohm cathode resistors to a lower value, if you used 560 ohms, try 510 or 470. ohms If you used 470 ohms, try 430 ohms. The lower the cathode resistance, the higher the cathode current will be, in a fairly inverse proportional relationship . Generally the pre-adjusted RK "bias" value should be around 400 ohms, for each tube.

In fact if you don't want to mess around with the bias current adjustability feature, just take out the "adjustment" components, and use 390 to 430 ohm, 2W, resistors, bypassed with the original 330 to470 uF / 35 VDC caps, from pin 3 directly to ground,, for each EL84.

The PA774s DO get hot, and the OPTs should not generate much of their own heat, but they can get warmed up from the radiated heat emanating from the nearby EL84s and power trannie.

HTH

/ed B
Real Radios Glow in the Dark
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