Bringing A 1965 JMI Vox AC30/6 Up To My Standards - Part 3

[@tedmich]

ALWAYS use the hole! Its the basis for our success as a species!

[@goodun2974]

My dad, the son of Greek immigrants, told me of visiting a place called Triipi, translated as "hole", where legend has it that ancient Spartans would purportedly toss babies who were sickly or deformed. Sometimes using the hole might be detrimental to our survival as individuals! 😉

[@terrydemol5354]

As usual, good to see VHQ tech work, there are a few of us left that still care :) WRT AC30's running OP tubes at 14 W +, a few OP tube options not so commonly known: 6P14P-EV, 5000 hr Russian NOS EL84 rated at 14W, V robust construction. Sound great, available at reasonable prices. 7189A, still available, reportedly just a re-labelled 6P14P-EV, no experience with them. EL84L NOS Siemens, higher plate V / dissipation, crazy prices = audiophile fodder :).

[@BradsGuitarGarage]

I have a pair of NOS Hitachi 7189A's still in the cello-wrap. I'm saving them for a special occasion!

[@carlBigSpoonjohnson]

I’m shocked that you don’t have wayyyyyyy more subscribers Damn it man! You have very good content. 🤘😎🤘cheers 🍻

[@halvach1998]

Yr strat sounds nice through that amp

[@YeatzeeGuitar]

Wish you showed some of the work in action, but looks great!

[@BradsGuitarGarage]

You can catch that on the livestreams, mate. The public attention span is waaaay too short for that on the edited videos!

[@YeatzeeGuitar]

@@BradsGuitarGarage I want to see it on the vox with those fragile tag boards

[@andrewhorne7443]

Hi Brad, Glad you're back on VOX amps again as painful as they can be (I refer to your Dallas era ac30 videos - still great vids though). Could you please talk more about the bias cooling issue - A) the affect that raising the bias resistor has on PD &/or other potential badness & B) raising screen grids and grid stoppers. Lyal talked about a 68 or 75 ohm Bias resistor and taking the pin 1 grid stoppers from 1.5k to 3k and the pin 9 screen grids from 100R to 1k (as you did on this one) but he never discussed his final results and theories on this matter. Could you please discuss in part 4? You are the champion my friend.

[@BradsGuitarGarage]

The changed value of the cathode resistor simply shifts the bias point of the EL84's. The higher screen grid resistor value just limits the screen current under high output. I may do a video on the two in the future if I can find the time.

[@andrewhorne7443]

@@BradsGuitarGarage Thanks Brad, when you mentioned that raising the Bias R can also raise the plate dissipation that concerned me for my amp (Dallas era ac30-new JJ's cherry plating problem) Did you end up with the original 50 ohm value in this one? I understand a video on this might be a bit dry for general consumption and is purely self interest on my part. love your work. Go Coffee!!

[@BradsGuitarGarage]

No, it's the opposite. Raising the value of the plate resistor will lower the dissipation, but that will lower the load on the power supply, causing the plate voltage to rise, possibly to levels dangerous to the lifespan of your EL84's.

[@BradsGuitarGarage]

In regard to the red-plate condition, check the control grid voltages on each pair to confirm your PI coupling caps aren't leaking first.

[@matthewf1979]

I totally agree with the piggyback-ing thing. Just install the right value in the first place. I would love to own a JMI ACXX someday. I’d probably buy someone’s hack attempt at a science fair project and restore it to factory spec, just for fun.

[@bradjackson8944]

That’s what she said

[@russellesimonetta9071]

Gird yer loins and dig in.