AO-35 PT question

[joesatch]

Powered up with a variac to 120v without any tubes. The two red wires are reading 618VAC on the socket. Is that normal?

[sluckey]

Schematic calls for 580VAC with tubes, so 618VAC seems reasonable with no tubes.

[joesatch]

got it together. bias resistor is 130R. just fire it up? i've never built a cathode biased amp not even sure what to measure

[sluckey]

Measure voltages and compare to schematic. Why didn't you use a 150Ω cathode resistor?

[joesatch]

i have one but it's only 5 watts. the 130 is 7 watts

[sluckey]

150Ω 5W is called for on the schematic and is plenty big enough. With 10.4V across the resistor it will only be dissipating 0.72W.

[joesatch]

i can swap it out. i went by the notes which actually calls for a 10 watt 130R

[sluckey]

130Ω is probably called for more often than 150Ω in EL84 cat biased amps. DrZ used 130Ω. Many people up that value to 150 just to ease the strain on the EL84s a bit.

[joesatch]

ya i put the 150. brought it up on a variac, massive high pitched squeal.  i do hear the guitar very briefly. shouldnt be the OT leads reversed as those i never unsoldered from factory although i may swap them anyway as this is consistent with that issue

[shooter]

squeal there with NO guitar plugged in?

[joesatch]

squeal there with NO guitar plugged in?

most definitely

[sluckey]

That doesn't sound like NFB squeal to me. But, rather than swap the OT primary leads, just disconnect the NFB wire from the speaker jack to eliminate that possibility. Leave it disconnected until you find the issue.

[shooter]

double check tube pin wiring, had a screamer once, i mis-wired screens as plate
had problems with a batch of those "square" caps, when they seen the soldering iron get near them they died from fright, or heat, wound up cutting them all out, the black 500pF micas also.


as long as the tubes aren't arcing, glowing and VDC readings are close, you should be able to use a DL for speaker, straight to the wall for power, then a scope should get you close.  Check the power rail with meter on AC to see if there's bi AC riding the DCv

[joesatch]

swapped the grids and we're in business. This thing's a little gain monster. The treble pot behaves weirdly. It's reversed but when turning all the way down the bass comes in. I guess i may hardwire it with a value and try the cut control in it's place. A power tube might be bad there, one of them looks burnt

[shooter]

looks like a PA tube, left one arc's at time ~~17 seconds??

[labb]

Think the folks on the Marshall 18 watt forum are using 180 ohm for the cathode resistor. Last 18 watt type I built the JJ EL84’s red plated with a 150 ohm resistor. Ever other 18 watters I built I used the 150 ohm. Not sure what was going on there. 344 vdc on the plates.

[PRR]

The size of the cathode resistor is related to plate and screen voltages. You can't just copy-over values.

[joesatch]

What should the mains fuse be 1A slow blow or 2A slow blow?  i had a 1A fast acting and it blew no issues since putting the 1A slo blow.  Amp sounds incredible.


$15 box will do!


https://www.target.com/p/3-compartment-light-wood-crate-natural-brightroom-8482/-/A-83378260#lnk=sametab

[shooter]

the original schematic show a 2A SB, i'd leave the 1A SB in n play hardish for a "full set" n see

[sluckey]

the original schematic show a 2A SB, i'd leave the 1A SB in n play hardish for a "full set" n see

...and carry a bag full of 2A SLO for spares just in case.