AD30HTC - HT fuse blows near max vol

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BlkNOrngBlood
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AD30HTC - HT fuse blows near max vol

Post by BlkNOrngBlood » Fri Aug 24, 2018 6:40 pm

I have a 2002 AD30HTC. Keeps blowing HT fuse but ONLY when playing through and ONLY when really turned up. No problem if idling or with gain way down. I can operate for hours and hours below 3/4 volume. Above that, HT fuse fizzles out. I sort of ignored it as it sounded great even with this problem and it met my volume needs. It seems to have gotten a little worse in that it blows at lower levels now than the first time. I tried about 4 new rectifier tubes (JJs and Tube Store), 5 sets of power tubes (original tubes, used JJs, new Tube Store Preferred, old EHX, new EHX, Yugo Ei) , and 3 sets of pre-amp tubes (ran British Mullards for past 10 years, original Golden Dragons, others). Same problem. Always with T500mA fuses.

All DC voltages are within tolerances on the schematics while idling, except the supply to the first gain stage is a few percent low. Plate voltage is within 0.5V side to side. Some carbon trails around a few socket pins. Lacquer on OT has seem some heat. Caps look good. I haven't looked at much more yet. I have eliminated shorted tubes unless there is a dynamic aspect that only happens happens near max vol. I am looking for some pointers, thoughts, diagnostics for this dynamic issue, or any related experience.

This amp is the best money I have ever spent in my life and I am hurting right now.
Thank you,

Jondog
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Re: AD30HTC - HT fuse blows near max vol

Post by Jondog » Fri Aug 24, 2018 9:36 pm

Check the values of the bias and screen resistors maybe?
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Les Paul Lover
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Re: AD30HTC - HT fuse blows near max vol

Post by Les Paul Lover » Fri Aug 24, 2018 10:06 pm

If the issue is getting worse over time, it's likely due to a part degrading.

You have tried all the valves..... which really are the only user serviceable parts.

My guess would be one of the electrolytic capacitor in the power supply / filtering drifting out slowly, but that's difficult.

Since that amp is now 16yo, it might be worth considering replacing them all, as it's something you should do in the next 5 years or so.

I've got an AD15 1 or 2 years older than yours, and and high volume high gain squeal has developped, so I'm heading towards a cap job too there.

If that doesn't sort it out, you at least have carried out the preventative maintenance required for another 15/20 years.
Ant

Orange Gear: RV50 MKI, R30, AD15, AD5, PPC212
Past Orange: Orange AD30TC Combo, Tiny Terror

Guitars: Gibson Les Paul Standard Faded, Vigier Expert Retro 54, Gibson SG 70s Tribute, Aria Pro II RS X80, G&L ASAT Special Tribute

bclaire
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Re: AD30HTC - HT fuse blows near max vol

Post by bclaire » Sun Aug 26, 2018 11:45 pm

Les Paul Lover wrote:
Fri Aug 24, 2018 10:06 pm
I've got an AD15 1 or 2 years older than yours, and high volume high gain squeal has developed, so I'm heading towards a cap job too there.
Typically, I find that high gain squeal is actually a bad preamp tube.

Les Paul Lover
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Re: AD30HTC - HT fuse blows near max vol

Post by Les Paul Lover » Mon Aug 27, 2018 9:22 am

bclaire wrote:
Sun Aug 26, 2018 11:45 pm
Les Paul Lover wrote:
Fri Aug 24, 2018 10:06 pm
I've got an AD15 1 or 2 years older than yours, and high volume high gain squeal has developed, so I'm heading towards a cap job too there.
Typically, I find that high gain squeal is actually a bad preamp tube.

Absolutely.
But I've tried 12 different preamp tunes there, as I've used my AD15 to test a bunch of 12AX7/ECC83 I've bought recently. So I've tried JJ, Harma (selected EH I believe), Sovtek, NOS RFT and NOS tungsram.

The squeal stays there, but is worse with the JJ and worst with the Harma.

Those preamp valves are silent in other amps too.
Ant

Orange Gear: RV50 MKI, R30, AD15, AD5, PPC212
Past Orange: Orange AD30TC Combo, Tiny Terror

Guitars: Gibson Les Paul Standard Faded, Vigier Expert Retro 54, Gibson SG 70s Tribute, Aria Pro II RS X80, G&L ASAT Special Tribute

bclaire
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Re: AD30HTC - HT fuse blows near max vol

Post by bclaire » Mon Aug 27, 2018 11:19 am

Wow... that's not good.

Les Paul Lover
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Re: AD30HTC - HT fuse blows near max vol

Post by Les Paul Lover » Mon Aug 27, 2018 1:13 pm

bclaire wrote:
Mon Aug 27, 2018 11:19 am
Wow... that's not good.

To be honest, it only happens with the volume maxed out and the gain beyond 75%.

Since I never play it that way, it isn't an issue for me, and I could do nothing and still use it perfectly happily. :)
Ant

Orange Gear: RV50 MKI, R30, AD15, AD5, PPC212
Past Orange: Orange AD30TC Combo, Tiny Terror

Guitars: Gibson Les Paul Standard Faded, Vigier Expert Retro 54, Gibson SG 70s Tribute, Aria Pro II RS X80, G&L ASAT Special Tribute

fiveightandten
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Re: AD30HTC - HT fuse blows near max vol

Post by fiveightandten » Thu Aug 30, 2018 12:14 am

Carbon trailing means it's arcing externally over the sockets. That's probably why the fuse is popping. The trick is finding why its arcing. Is that on every EL-84, or just one (or some)? Which pins?

Did you clean the carbon off when you tried new power tubes? It will arc easier over the socket with carbon there.


-What kind of dissipation are the tubes running at, at idle? Ie. is the bias within spec for the tubes (12-14W or so)?

Dissipation wattage = Corrected plate voltage x current draw
Current draw: Pin 3 voltage divided by cathode resistor value
Corrected Plate voltage: (Pin 7 voltage minus Pin 3 voltage)

-Have you measured what they're drawing at high volume? I know this is tough to catch on a meter, but to at least see what the spikes look like.

-Have you measured the output of the amp to see what it's producing when it blows the fuse? Just curious.

I'd check out the output transformer to rule out a short or burned winding. See here for some simple checks:
http://www.geofex.com/ampdbug/outtrans.htm


If all of that checks out, you can spend more time trouble shooting, or you can replace all the screen grid, control grid, and cathode resistors in the power amp, which is likely to solve the issue (if you only see arcing on one socket, I'd just do that resistor set). Depending on how bad the sockets look, you may want to replace those. I'd certainly clean the carbon off the board. This is a delicate process, be careful with the traces, they're delicate.

-Nick
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'71 GRO100 || '96 OR-80 || AD30 || '64 AC-50 || AC-30TBX || Hiwatt DR504 || HI-TONE HT30
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bclaire
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Re: AD30HTC - HT fuse blows near max vol

Post by bclaire » Tue Sep 04, 2018 2:02 pm

Just thinking about this. Have a tech check for oxidation on the tube sockets and clean and tighten those...

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