1959slp build, crackle issue

[jelle]

What is the bias current in mA per EL34? And what Output transformer? Primary Z...1.7K?

[overtone]

Worth nothing that when I try playing it cranked (through the attenuator) I can hear the power tubes cackling from the heat....despite my bias being set fairly low (think)

to Iommic_Doom:
I'd like to repeat Jelle's question to you.
As Miles already mentioned, there is a bit too much speculation at the moment. Stand back and start with the basics.
I have a speculation, but first give us the answers to Jelle's questions please.

Best, tony

[Iommic_Doom]

The Primary impedance of the OT on the spec sheet is 1.8k. It's the mojotone/heyboer "silver bells" "800/plexi style" transformer.

By bias current, you mean the reading at pin 5? -36v, and I remember flipping the meter to mA, and getting 36mA, but it jumps around between 34-38.


Last time I played the amp, I rolled the power tubes and replaced one of the svetlanas that was a bit mismatched with a JJ...despite being different from the other tubes, I now got uniform readings across all four power tubes.

I also did a few more mods...
Upped the 47k NFB resistor to 120k. This resulted in greater stability in the presence circuit. No more buzzing when turned up. Doesnt matter though, because turns out I don't need the presence knob turned up at all when playing with the band. I might just replace it with a fixed resistor and turn that knob into something else.


Fizz cap up to 100pf. No more nasty fizz....nice and smooth.

.68uf cathode bypass on cathode of v2. Better grit, shittier cleans.

upped slope resistor from 33k to 80k. The tone stack now actually does something. With the 33k, the bass was too low, the highs too high, and the mids were weird. Now it sounds proper.

Nos Sylvania 12ax7 in V1.
JJ 12ax7s V2
Nos JAN 12at7 in V3.
+silicon Tube dampers on all pre tubes


All of this resulted in greater stability, clearer sound, better distortion...and no blown fuses!

I just got back from practice. The amp was running for close to 6 hours with channels bridged, both gains dimed or close to it, sometimes with a pretty serious boost (super hard on), and no problems at all. Sounded great, the band loves it. I have quite a collection few Matamps, Sunns, oranges, fender bassmans of all types (no actual marshalls though)...this thing blows them all away.

Thanks!

[jelle]

Ok, so so far we have 450V B+, a primary load of 1.8K for a quad of EL34's, and an unknown bias current. That last bit is a recipe for trouble.

You describe using the 'Negative Grid voltage method'. See: http://www.aikenamps.com/Biasing.html Big no no.

I would advise the cathode current method as described in the link above. This will make your amp much more stable and reliable. Have fun and rock on!

Jelle

[Reeltarded]

Gooder!

Try somewhere in between with the slope. You are leaving some goodies on the table @80k. I use variable 33k-80k and except for big clean the sweep is usually right around 47k give or take a teeny. Worthy switch or knob. Best!

[Cliff Schecht]

I'm with Vibrato, it looks like you sneezed inside of your amp a few times!