AD30 voltages: Head vs combo and PT variance

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fiveightandten
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AD30 voltages: Head vs combo and PT variance

Post by fiveightandten » Mon Mar 12, 2018 1:03 am

Hopefully there's someone around here who has torn into their AD30, as I have a few questions you may be able to answer by taking some readings and poking around.

I have 2 AD30s:
1) 2006 2x12 combo
2) 2009 Head

They sound markedly different, which I had always attributed to the fact that the head has a Mercury Magnetics OT. Though, the head has always had MUCH more pre-amp gain than the combo. The OT should have no bearing on this, so I decided to poke around and see if I could figure out why...

Well, I found out why. The voltages in these 2 amps are wildly different. That's one oddity. Another is that the schematic lists different voltages for the late B+ rail stages, with higher voltages listed for the head. However, there are no component differences for the voltage dropping resistors. Furthermore, I can't even find 2 voltage dropping resistors in the amp! I should mention that I metered out every component in each amp. They're all right on spec.

First, the voltages in my amps (118VAC at the wall):

PT secondary winding:
Spec- 260VAC
Combo- 302VAC
Head- 267VAC
(We start out with the head on spec and the combo way over)

TP5 (rectifier output):
Spec- 355V
Combo- 377V
Head- 320V
(After the rectifier, the head is under and the combo is over)

TP6 (after 680 ohm dropping R)
Spec- 340V
Combo- 363V
Head- 303V
(Again, the head is under and the combo is over)

TP7 (after 22K dropping R, which I can't find in the amp)
Spec- 255V combo / 270V head
Combo- 276V
Head- 231V
(This is where is gets weird, as the schematic has different specs for head and combo, and mine are basically reversed, with the head still under spec)

TP8 (after 2nd 22K dropping R, which I also can't find in the amp)
Spec - 220V combo / 235V head
Combo- 231V
Head- 195V
(We end here with the combo at head spec, and the head below spec for either)


As you can see, the head runs much lower than the combo. This is out of the same mains outlet at the same time. I've even swapped tubes to see if there's an appreciable difference, and there is not. One of these PT's just plain puts out higher voltages than the other. The annoying part is that neither one of these amps is really on spec with the schematic.

My questions:
1) What voltages are you seeing in your AD30?
2) Are the head and the combo actually supposed to differ? And if so, how would they differ if the voltage dropping resistors are the same in both amps?
3) Are head and combo spec'd with the same PT?
4) Where are the 22K 1W B+ voltage dropping resistors in the amp? I can't find them anywhere! Haha

Schematics:
Power Supply - http://www.orangefieldguide.com/OFG_SCH ... AD30_2.pdf
Main Board - http://www.orangefieldguide.com/OFG_SCH ... inpcb1.pdf
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'71 GRO100 || '96 OR-80 || AD30 || '64 AC-50 || AC-30TBX || Hiwatt DR504 || HI-TONE HT30
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Mr Mustard
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Re: AD30 voltages: Head vs combo and PT variance

Post by Mr Mustard » Mon Mar 12, 2018 4:21 am

Interesting that the combo runs high at the PT but is within tolerance at the test points, while the head is okay at the PT but below spec at the test points. Same thing even if you adjust the TP specs to account for the differences at the PT.

Makes me want to open up my AD15 and check it.
Mr. Mustard

tolm
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Re: AD30 voltages: Head vs combo and PT variance

Post by tolm » Mon Mar 12, 2018 4:18 pm

Was the mercury OT from the factory? Just thinking - if it was fitted after market some other things might have been changed (intentionally or not!) at the same time?

I have a 2004 AD30HTC but I’m afraid I have no idea how to measure any of that without killing myself! ;)
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fiveightandten
Orange Master
Posts: 3845
Joined: Sat Apr 05, 2003 6:18 pm
Location: Connecticut, USA

Re: AD30 voltages: Head vs combo and PT variance

Post by fiveightandten » Tue Mar 13, 2018 2:57 am

Mr Mustard wrote:
Mon Mar 12, 2018 4:21 am
Interesting that the combo runs high at the PT but is within tolerance at the test points, while the head is okay at the PT but below spec at the test points. Same thing even if you adjust the TP specs to account for the differences at the PT.

Makes me want to open up my AD15 and check it.
It is odd. This is why I want to find these 22K B+ rail resistors...but I don't see them anywhere! It's curious that they spec different voltages for the first pre-amp stages (head vs combo), but don't spec different values for the dropping resistors. And I can't find them in the amp to see if they're actually different.

The oddest part is the huge variance in what the PT's are putting out. And it's just the HT winding. The heater windings are spot on and even from amp to amp, within a few hundredths of a volt of one another.
tolm wrote:
Mon Mar 12, 2018 4:18 pm
Was the mercury OT from the factory? Just thinking - if it was fitted after market some other things might have been changed (intentionally or not!) at the same time?

I have a 2004 AD30HTC but I’m afraid I have no idea how to measure any of that without killing myself! ;)
The Mercury OT is an after market upgrade. But it shouldn't have any notable affect on the B+ voltage at idle. The power transformers themselves are putting out much different secondary voltage. All of the PT connections are correct, and the voltage switches are in the right positions. Heater voltage readings verify that the PT's are seeing the correct input voltage. And the components in the amp are all really close to each other (tight to spec.).

The variance really does seem to be just be the HT winding on these PT's. But I'm surprised at such a wide variance between amps, and I'm curious about what the intended difference between head and combo is. We're talking a 57V variance at the EL84 plates. That's huge for 2 amps that are supposed to be the same. A sample size of 2 is quite small. But it makes me wonder what the tolerance is for the windings on these PTs.
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'71 GRO100 || '96 OR-80 || AD30 || '64 AC-50 || AC-30TBX || Hiwatt DR504 || HI-TONE HT30
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