I'm looking for some advise on how to pinpoint a problem with my AC30/4 Build. The Build has an ef86 Channel, A Normal Channel, a Master Volume ala Matchless, a 1/2 Power Switch ala Matchless. It's based on the Layout from Hoffman [IMG:772:434]http://i1200.photobucket.com/albums/bb3 ... 5ad697.png[/img]
I made a new Layout incorporating the additions mentioned above , this is the layout that I followed: [IMG
Upon Startup everything sounded great... Big, Bold, Punchy & Loud but as I played on it for about 5 minutes it began to lose volume and present an unpleasant grainy unmusical distortion on both channels... at the same time the 62 OHM el84 Cathode Resistor began to smoke
The really strange thing is that the B+ Voltage starts out at around 250VDC (418VDC with the power tubes removed) and will remain static until you begin to play hard and turn the volume up... then the B+ begins to rise as you play all the way up to 400V+ during which the sound degrades, volume drops and what sounds like crossover distortion takes over. I shut it down, cool things off for a minute or two and the cycle repeats every time...
While the Voltage is rising you can stop playing and the voltage will stay put for a moment and then begin to slowly drop but not all the way back down to it's original 250Volts.
Part of what appears so strange is that the 300vac-0-300vac remains fairly consistent while the DC HT fluctuates drastically.
The OT is for a jtm-45 and has only slightly less resistance than the AC30 OT (3.4k CT vs 3.7k CT respectively) again, i'm hoping to dial all of that in after I get the voltage stable. it should all come together when the el84's are properly biased. I have a 50w 10K Variable resistor coming which will allow me to easily find the sweet spot for the cathode resistor. I'll use a fixed resistor after I find the right value.
I can somewhat regulate the B+ by throttling the Master Volume. I can keep the voltage and bias stable at around 1/2 volume but as soon as I reach a certain threshold the B+ rises and the bias drops until there's very little volume and tons of nasty distortion.
What I've done so Far...
1) Inspected for wiring flaws, shorts, incorrect components Layout etc...
2) Replaced EL84's with a new set (no change)
3) Moved the HT from the Center Tap of 5V heater winding to pin 8 on 5AR4 (the Hammond 272JX has a center tap for the 5V winding... I thought I'd make use of it to reduce noise on the HT) but...(no change)
4) Adjusted values of EL84 Cathode Resistor from 400ohm all the way down to 50ohm. The starting point of the voltage changes accordingly but the voltage swing and resultant volume drop & blocking distortion persists. Currently a new 62ohm resistor resides.
5) Changed Speaker load from 8 to 16ohm (no change)
6) Used a Variac to regulate VAC to 275vac-O-275vac (optimum voltage for this build) *** heater voltages were reduced slightly as well so this test is not perfect...but (no change)
7) Replaced rectifier with another 5AR4, a 5U4GB and a 5Y3GT, voltage starting point changes but fluctuation persists.
8 ) Temporarily Replaced OT-45 with an AC30-OT (no Change)
9) Jumped out 1/2 power switch to check for current loss (no change)
10) Temporarily bypassed Choke (no change)
11) Temporarily bypassed Cathode Cap on EL84's (no change)
I'm not sure what to try next as my hacking skills are limited. I've built many amps and had success in troubleshooting new builds (sometimes with the help of this forum). The over-all the wiring seems correct to the Layout.
I'd appreciate any help at all from this forum. It would be awesome if someone could verify my layout as well. You guys have been invaluable to me in the past
![Smile :)](./images/smilies/icon_smile.gif)
Thanks!
Bluto