AC30 cracklin', popin' scratchin. Minimal volume

GrooVey

GrooVey

Well-Known Member
OK to the point... 1- would a scorched power tube (EL84) cause an AC30 to have a very weakened output? and crackly, scratchy...
2- Do I need to be concerned as to what caused this tube to toast?
3-Figured I replace all the tubes. 4 power 3 pre (I think)
4-AC30H Do I need to worry about Bias when installing new tubes?
5-And yea... I get it, but I'll ask anyway... Which brand tube to get? I'm Not chasing NOS.




Cracklin' n popin', not very loud. Similar to a bad cord/lead. But its not that. So I ignored it for a month or so. Till the volume has dropped so low its sounds anemic. Meaning it should be shaking the floors but I don't even have to wear ear plugs. Finally has its turn on the bench. Found this...

toasted EL84.jpg

It came from this slot: 4 power tubes (2nd slot from right.) Power-Power-Power-Power-Pre-Pre-Pre

(upcoming Rant) I can't upload this pic to save my life. And as there are so many variants of the AC30, I can't be certain I've found mine. To compare a diagram.
I can say that my tubes are accessible through a panel on the underside. Remember, this is just a head.

So. If I have my tubes presented, left to right, there are 4 power tubes on the right, then 3 pre amp tubes.


The tube in the second slot (from the right-an EL84) is the burnt tube. Every channel is affected. Even each 'high n low.'
While I had it open, and as I just got this thing last month, I figured I'd swap all the tubes. Hoping this burnt tube is the problem.


I don't know much about tubes. Or the workings of Amps. I know amps could benefit from a blast of air or a spritz of DeOxit. That's about it. I do know what the capacitors are and not to f with them.
 
GrooVey said:
1- would a scorched power tube (EL84) cause an AC30 to have a very weakened output? and crackly, scratchy.
Click to expand...
Yes, it would.
GrooVey said:
2- Do I need to be concerned as to what caused this tube to toast?
Click to expand...
Yes. Simply replacing the tubes may be ignoring an underlying issue…like a fried screen resistor…those little brown and/or blue capsules with the stripes. Also, looks like some cooking with the glass was going on…black carbon dust/scoring on the tube socket can cause further issues. That black stuff is conductive, and can cause shorts on the socket and its surrounding components.
GrooVey said:
3-Figured I replace all the tubes. 4 power 3 pre (I think)
Click to expand...
Yes. Maybe save the others(if they look good) for duty in a single EL84 amp…like a Valve Jr or sumpin.
GrooVey said:
Which brand tube to get?
Click to expand...
I’ve used these in the past…

www.tubedepot.com

Sovtek EL84M Power Vacuum Tube

A military-spec version of the standard Sovtek EL84 - the Sovtek EL84M's extended voltage tolerance - improved plate dissipation - and rugged construction make it equivalent to the RCA 7189. Chosen by Matchless for their designs - the EL84M also features higher transconductance - more power - and
www.tubedepot.com www.tubedepot.com

Edit: while I am comfortable doing my own thing with a soldering iron, and learning my own gear repairs…I’m not very good with communicating repair instructions.
 
Thank you @Thatbastarddon !

I'm now feeling more positive about this.

I'm comfortable with all things guitar but not amps. The most I feel comfortable doing inside the chassis is a dusting and a spritz of deOxit. So I'll do that while I have it on the bench.

Sovtek.. cool.

I'm leaving today for a week. It'll take the tubes that long to get here anyway.

Will report back.
 
GrooVey said:
4-AC30H Do I need to worry about Bias when installing new tubes?
Click to expand...
The Vox AC30 is cathode biased, and it’s setup to be plug and play. The amp has no bias adjustment features because it is not a fixed bias circuit with an adjustable trim pot. A good tube vendor will provide you with a quad set of properly matched EL84’s. If you want to go with a cheaper set of tubes, look into the JJ’s. The JJ’s are solid as long as the vendor has properly vetted them. I like doing business with these guys:
EL84, JJ Electronics, output pentode, Vacuum Tube | Antique Electronic Supply

GrooVey said:
I'm comfortable with all things guitar but not amps. The most I feel comfortable doing inside the chassis is a dusting and a spritz of deOxit. So I'll do that while I have it on the bench.
Click to expand...
Thatbastarddon mentioned “fried screen resistors”. Sometimes a tube can fail on its own and not contribute damage to other areas of the amp. Sometimes a failed power tube can damage its associated screen resistor. Sometimes a failed screen resistor can damage a good tube. Take a good look inside before you fire up the amp with a new set of tubes.
 
JJ tubes don't stand a chance in a Chinese VOX they red plate the bias is to hot what needs to be done is change the 10 watt 50 ohm resistor
to 10 watt 100 ohm resistor R119 on the Schematic
 
@67plexi , @syscokid , @Thatbastarddon

Ok. Read about a dozen articles. Learned a lot yesterday about AC 30s and tubes. AC30s run hot. AC30s eat tubes. Short plate, long plate, black plate, grey plate. Compiled a list of tube brands pros/cons.

I think I know this much
V1- pre amp EEC83/12ax7
V2- pre amp EEC83
V3 ?
V4 to V7- power EL84

A few questions
1. What does V3 do?
2. For V3 I see some
recommendations for a 12au7.
But EEC83 works too.
3. Here is my list of tubes. Some of
these tubes are proclaming they
can handle higher temps/more
volts. Seems like I should go
with one of these? Or are these
claims unfounded?

Mullard (new) sound good

Sovtek EL84M dependable, takes
the heat 148/4 burned in n
matched

Ei poor QC

JJ can't take the
heat, less $$$

EH 23$/tube

Prefered Series
7189 Platimum takes the heat

TungSol not for high
heat

TungSul 6BQ5 better for high
voltage 129$/4


As always I appreciate all your insight and suggestions.
 
If the claims to handle higher temps and more volts are valid, I'm leaning to one of these. For power tubes.

Sovtek EL84M
Prefered Series 7189 prefered
TungSul 6BQ5
 
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The Sovtek are rebranded 6N14N-EP were designed for Russian ICBM they can take the shock and higher plate voltages
20-25 years ago I could buy a case of 50 direct from Russia for $2.40 per tube plus $70.00 shipping they are 5,000 service life hour tube.
 
The Vox line in general is known to cathode bias the power tubes screaming hot. Take 67 Plexi's advice and change out the bias resistor or continually stay on the path of Yum Yum, Eat'um Up on your power tubes. Granted cooling down the bias a small tad might take some of that hot as "He77" tone off at the start, but it doesnt change the whole tone at all.
 
T-Rex said:
the bias resistor
Click to expand...
Last two days I've run the amp hard. Nearly dimed. A good six hours each day. Tuesday on the Top Boost channel, Wed on the normal channel. The amp never got hot. Just as warm as the others.

I'm not opposed to the resistor swap.

Will sit on this for a bit. This would be very involved for me. I know my way around a 335 cavity, strat pickguard and can handle a soldering iron.

But working on amps is new to me. I know what the capacitors are and how to discharge them. Or just stay away from them. It just looks awful crowded in the chassis.
 
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