Monkeymatic Half Dome

[xtian]

Converting a Conn organ amp into a Fender Pro-Amp 5E3. Paralleled triodes > James tone stack > 100K/1K5 triode > Concertina PI > cathode biased 6L6GC pair, 5U4GB recto, 40/40/40uF filtering.

Going to start with no screen resistors and no gNFB, just like the schematic. Should be pretty lively!

About half done on the wiring.

Look at that little window on the front panel. Yup, backlit monkey logo. That's exactly what I was thinking, too.

[xtian]

I finished this amp within a day or two of first post, but it has trouble in the power section, I think. With only the 5U4G and pair of 6L6s installed, amp draws 50 watts (on my Kill A Watt line meter) during warmup, and then climbs quickly to 100 watts as power tubes come online. That's too much. I just had my 100watt Double Bassmaster operating at 65 watts.

Bias measures ok, about 340v plates, 360R cat resistor. Running about 70%, if I recall.

Have tried two different tube sets.

Amp plays, but loud buzzy hash makes it unlistenable. Preamp seems ok by itself...

Of course I have considered ground loops and misc circuit errors, but haven't found anything in several weeks of looking at this thing.

[xtian]

Aha! You have noticed the MOSFET B+ reducer in my photos. Yes, that's a clue. (I have used this gem in a hand full of other amps, and it runs with very little heat, even dropping 80v as I have it, here.) My MOSFET gets really hot, really fast. Painful to touch in 120 seconds or so.

I measured 97 watts (wall AC consumption) with power tubes only while running, and WITHOUT the MOSFET, I measured 104 watts--but that's with something like 440v on the plates, so expected.

I'm thinking the heaters draw the 50 watts of wall current I see when I flip switch to run. The the power tubes start to conduct, and I expect another 150mA or so supplied by the HT, so so so

Could the power tubes be oscillating? Can't see it on my scope.

[xtian]

Any ideas, team?

[xtian]

I need a whack upside the head.

I continue to be baffled by this amp. Same symptoms as described above. I have only the recto and power tubes installed. On power up, monitored wall wattage spikes briefly as reservoir fills, quickly drops to 45 watts as heaters warm up the cathodes, and after 10 seconds or so, current rises to ~100 watts as power tubes start to conduct.

The power tubes can't account for that doubling of consumed current. Plate current is 46mA each 6L6GC (412v plates, 34v shared cathode), screen current is 2.5mA.

Could the recto be faulty? I've tried several with same results.

[dorrisant]

That is weird... I'm not seeing anything out of the ordinary.

[dorrisant]

Have you tried a different OT?

[xtian]

Have you tried a different OT?

No. Thanks for the suggestion!

[Phil_S]

Bad rectifier? Short somewhere? Tried those power tubes in another amp?
What happens with only the rectifier installed?

[xtian]

Well, hell, I got it figured out. And I can't really explain how. Critical, though, was a ground loop that was giving me a lot of hash when turning the volume up past zero.

As I mentioned, the MOSFET B+ reducer was getting terribly hot, so I ditched it. Voltages are high, but within tolerance. I've got 490v plates, 43v cathode (488R). I'm pretty close to the 5E3 schematic, with just one input, and much stiffer filtering. Speaking of filtering, I've got a 100uF reservoir...and a 5R4WGA rectifier tube...that sounds bad. Guess I'll have to lighten the reservoir.

Sounds awesome, though. Solved my noise issues, so quiet at idle, reasonably spanking clean headroom for such a small OT, and great growling overdrive when turned up. LOUD!

[Leo_Gnardo]

Well, hell, I got it figured out.

That's good! I just found this thread, and I was getting worried your PT was fried. On rare occasion I find a power transformer that draws a lot of current, all by itself. Last time I was able to successfully sub in a transformer meant for a Super Reverb. I'm glad to see you've sorted yours out. Now you know one reason there's a limit on filter cappage with rectifier tubes, won't make that misteak again, right?

[xtian]

Now you know one reason there's a limit on filter cappage with rectifier tubes, won't make that misteak again, right?

No, I didn't blow out a recto tube--fortunately I noticed my mistake early.

Thanks, Leo. Thanks, guys!

[xtian]

She's baked! Spent days French polishing with blond shellac, alcohol and walnut oil. Enjoyed the process and the results very much. But, man, you've got to work for it. Wouldn't want to French polish a grand piano. I'm waiting a few days, then will buff with rottenstone and finish with paste wax.

Also got that laser etched, back-lit monkey in the control cable hole. Eep!

monkeymatic_halfdome1.jpg

monkeymatic_halfdome2.jpg

monkeymatic_halfdome3.jpg

monkeymatic_halfdome4.jpg

[sonicmojo]

Very cool! Might have to do something similar to my Webster Electric 81-15

[dorrisant]

So Beautiful.jpg