Removing Two Power Tubes in the AD140HTC???

andrew darby · Post by **andrew darby** » Tue Dec 20, 2005 1:17 am

Is it possible to remove the middle two EL34 tubes in an AD140HTC, and then re-bias the amp to 70 watts? If so what do I re-bias it too?

VintageJon · Post by **VintageJon** » Tue Dec 20, 2005 4:35 am

Actually you don't need to rebias it, BUT YOU MUST DOUBLE THE SPEAKER LOAD IMPEDANCE!

andrew darby · Post by **andrew darby** » Tue Dec 20, 2005 6:04 am

Why must you double the speaker load impedance? And if I did that, what would the new load impedance have to be?

pwar66 · Post by **pwar66** » Tue Dec 20, 2005 3:51 pm

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by andrew darby
 Why must you double the speaker load impedance? And if I did that, what would the new load impedance have to be?
<hr height="1" noshade id="quote"></blockquote id="quote">

I did this last week. the day before a gig I had a power valve go - no.4 on the right hand side looking at the back of the amp. I spoke to Andy at Orange and he said to remove it and it's opposite partner - No.1 and then plug my 16Ohm Orange 4x12 into one of the 8 Ohm speaker outputs. Worked like a dream. A tiny bit quiter, but not really noticeable to be honest , certainly not at the level I usually play at (just over 8 on master, gain on full) All the real power on the AD140 comes between 9 & 10 and so I really didn't notice that much of a difference. A bit less Bass, but I normally dial the bass down to around 5, so all I did was dial the bass back up again.

I havn't got my new Power valves yet so havn't been able to do a direct comparison with/without, but It certainly worked for me on the night!

I was going to ask anyone wether this would have any serious effect on the amp if I keep it like this?

andrew darby · Post by **andrew darby** » Tue Dec 20, 2005 4:08 pm

Well I only have an 8 ohm mono cabinet, how do I double the speaker load impedance if the Orange only has 1x16 ohm speaker output and 2x8 ohm speaker output?

pwar66 · Post by **pwar66** » Tue Dec 20, 2005 4:35 pm

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by andrew darby
 Well I only have an 8 ohm mono cabinet, how do I double the speaker load impedance if the Orange only has 1x16 ohm speaker output and 2x8 ohm speaker output?
<hr height="1" noshade id="quote"></blockquote id="quote">

to be honest - dunno. I must admit I don't really know how Ohm and impedence work.

Andy H. · Post by **Andy H.** » Tue Dec 20, 2005 8:03 pm

<blockquote id="quote">quote:<hr height="1" noshade id="quote">(pwar66)
I did this last week. the day before a gig I had a power valve go - no.4 on the right hand side looking at the back of the amp. I spoke to Andy at Orange and he said to remove it and it's opposite partner - No.1 and then plug my 16Ohm Orange 4x12 into one of the 8 Ohm speaker outputs. Worked like a dream...

...I was going to ask anyone wether this would have any serious effect on the amp if I keep it like this?

<hr height="1" noshade id="quote"></blockquote id="quote">No, that should be fine.

Andy.

pwar66 · Post by **pwar66** » Wed Dec 21, 2005 12:58 pm

Cheers Andy

VintageJon · Post by **VintageJon** » Thu Dec 22, 2005 5:42 am

Forgot to mention that you won't have much volume reduction as it'll only be 3Db- and this is what we are taught to understand as the MINIMUM the human ear can discern as a difference in volume level.

Forget "power tube break-up". By the time the power tubes break-up you'll be deaf and yer ears will bleed!

-Jon

VintageJon · Post by **VintageJon** » Thu Dec 22, 2005 5:49 am

Well, "let's get technical":

You have to double the speaker load as the Output Transformer interrupts the Plate Impedance of the output tubes in relation to the Turns Ratio of the OutPut Transformer to the Speaker Impedance. Thus in removing two of the four output tubes you double the Output Impedance of the Primary so you must double the impedance of the Secondary...

(It's about Turns Ratio...)

-Jon

Removing Two Power Tubes in the AD140HTC???

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