Vox AC30/4 EF86 & Normal Preamp Channels- Bias Issue

[Bluto]

Hi,

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: [IMG731]http://i1200.photobucket.com/albums/bb3 ... 4390ef.png[/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

Thanks!

Bluto

[xtian]

The really strange thing is that the B+ Voltage starts out at around 250VDC

Welcome, and thanks for all the details, but I've got to stop you at the top. This sounds WAY low for B+, but as you say, 400+ with power tubes removed. Sounds like the power section is drawing WAY too much current, sucking your B+ down. Smoking your cat R is another sign. If everything was known to be working properly before (obviously not because this is a new build but just saying) I would suspect a bad power tube.

[Bluto]

I would suspect a bad power tube.

Thanks xtian! Appreciate the feedback.

Yea, I suspected that maybe I had a bad tube but I replaced the el84's with a known good set and nothing changed... I'm beginning to get the feeling that I've done something dumb and that I'm going to be embarrassed when I finally get to the bottom of this, but right now I'm willing to take that chance

Bluto

[JazzGuitarGimp]

Hi Bluto,

You didn't mention the standby switch - did you try bypassing it? Also when bypassing the choke, it's a good idea to disconnect both of the choke's wires from the circuit, just in case there is some weird leakage from its winding to the core (and therefore, chassis ground).

Good luck to ya!
Lou

[Bluto]

You didn't mention the standby switch - did you try bypassing it? .....Also when bypassing the choke, it's a good idea to disconnect both of the choke's wires from the circuit, just in case there is some weird leakage from its winding to the core (and therefore, chassis ground).

Thanks Lou,

I'll try both of those things this evening. Didn't even think that the Standby would be an issue but you're right... I've had them fail right out of the box before. I went to a high quality carling 20amp switch a while back but there still could be a problem with this one causing current draw.

I didn't disconnect the choke... just jumped it quickly to see if it could have been bogging down the B+. I'll cobble in another one tonight just in case.

Thanks Again,

Bluto

[Bluto]

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.

I've attached a Voltage Chart to this post. It may be helpful. [IMG:791:1024]http://i1200.photobucket.com/albums/bb3 ... d1d399.png[/img]

[Stevem]

Something is very odd, a 5y3 recto should have gave up with the current draw of this amp if it's burning up cathode resistors no?

Anyway, is your cathode bypass cap going bad, leaky or shorted, I would lift but out if the circuit for now.

[Bluto]

a 5y3 recto should have gave up with the current draw of this amp if it's burning up cathode resistors no? ----- is your cathode bypass cap going bad, leaky or shorted?

Hey Steve, Thanks for the reply!

Yes, I thought that the 5y3 should have at least minimized the runaway current. It did reduce the voltage temporarily but then after pushing the preamp past a certain point the voltage rose to where blocking distortion took over. When the voltage rises the bias falls.

I've lifted the cathode cap and there was no change in the behavior of the runaway voltage.

Something is really strange here...

Bluto

[Masco]

Even though it didn't make a change, to me it looks like your 62R/68R cathode switch is wired incorrectly. Maybe I can't read your drawing???
Make sure your 220k's on the EL84 input grids are grounded well.

[Bluto]

it looks like your 62R/68R cathode switch is wired incorrectly------- Make sure your 220k's on the EL84 input grids are grounded well.

Thanks Masco!

The 1/2 power switch is ok... had to stare at it for awhile to convince myself but it tests correct with the meter. The switch in the 1/2 power mode runs the two resistors in series, doubling the resistance for the 2 inside EL84s.

Oh and by the way... You Nailed it with the 220k's!! RE-flowed the ground and now it works great.

BTW... any connection to the Masco Amplifier Co? Brilliant Amps!


THANK YOU VERY MUCH!

Bluto

[Masco]

You're welcome.
No affiliation w/ Masco amps, it's just a nickname my friends call me from my last name Romasco. I have my own JMJ amplifiers.
I hope you didn't kill any of your EL84's.

[Bluto]

I have my own JMJ amplifiers-------
I hope you didn't kill any of your EL84's.

Wow, That's great... I'd love to hear clips of your amps sometime. I went to your website & I'm intrigued by your JMJ30. That's exactly what I had in mind to build for myself next.

The kid that I helped with this amp is going to be stoked, it sounds really good but i still need to dial a few things in.

Yeah, the el84s took a beating during this round of troubleshooting but I used an old set so as not to be too costly if I roasted any Actually they still SOUND pretty good, just not sure how much life will be left on them.