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FDP Forum / Amp Mods, Repairs, and Projects / Converting 100 watt Ampeg to 50 watt?

Mike the marksman

Kansas City, MO

Dec 12th, 2018 07:07 AM   Edit   Profile  

I have the opportunity to buy an early 70s Ampeg VT-22 combo with a blown OT for cheap (combo version of the V4-2x12, 100 watts)

Would it be possible to replace the OT with a V2/VT-40 OT to convert it to 50 watts?

Contributing Member


Caught snipping the Bright Cap
Dec 12th, 2018 07:12 AM   Edit   Profile  

You'd pull two tubes, as well. I'd replace the OT with a suitable replacement and just pull two tubes to use it. Then you could restore it to its full glory if you ever decided to sell it.

Here's an article on the ins and outs. There's more to it than pulling two tubes. Which tubes to pull? Impedance? Jeff talks about it in the article.

External link

Mike the marksman

Kansas City, MO

Dec 12th, 2018 07:18 AM   Edit   Profile  

Right, I figured with a 50 watt OT then the extra two power tube sockets wouldn't be wired up anyway, am I correct on that?

Contributing Member

Results not typical.

Do not try this at home.
Dec 12th, 2018 09:13 AM   Edit   Profile  

Because it's a quad of tubes in push-pull, the wiring of the tube sockets has to be done properly, i.e., one tube on each side of the phase inverter.

If you can follow a schematic, you can solder, and you can work safely with circuits, it's easily within your abilities to do.



Dec 12th, 2018 09:47 AM   Edit   Profile  

may I ask you want to make the change? The difference in perceived volume between 50 and 100 Watts is barely noticeable. To halve the volume of a 100 Watt amplifier you'd need a 10 Watter.


Too Near Atlanta GA

Amp Tech Emeritus
Dec 12th, 2018 01:47 PM   Edit   Profile  

I was thinking along those same lines as SonicBlue...perhaps an inexpensive attenuator might be a viable option to get you where you want to go without doing a major mod to an amp only to find you still aren't there yet. :-)


Mike the marksman

Kansas City, MO

Dec 12th, 2018 02:44 PM   Edit   Profile  

"may I ask you want to make the change?"

The OT is already blown, that's why the amp is so cheap- if I have to replace an OT anyway why not replace it with a 50W OT, get two power tubes out of the equation and make the amp a little more useable? I'd basically be converting it into a VT-40 but with 212s instead of 410s.

I wouldn't be modding a fully functioning amplifier

(This message was last edited by Mike the marksman at 04:52 PM, Dec 12th, 2018)


Too Near Atlanta GA

Amp Tech Emeritus
Dec 12th, 2018 02:47 PM   Edit   Profile  

understandable...though, an unmodded,functioning VT-22 is worth $500-$600...modded, not so much.


Mike the marksman

Kansas City, MO

Dec 12th, 2018 02:55 PM   Edit   Profile  

(This message was last edited by Mike the marksman at 04:57 PM, Dec 12th, 2018)



Dec 13th, 2018 06:45 PM   Edit   Profile  

My $0.02 worth......

Put the amp back to stock with the correct OT and four power tubes and 100 watts.
If you want to run two tube after that, then do so.
Don't be surprised if it's still too loud, though.

Look into getting a power attenuator. IMO you'll get a more practical solution to your desires to quiet the amp down by doing this.


I believe that this amp and the V4 have Pre Out/Power In jacks.
Years and years and decades ago Ampeg actually offered a master volume control or something that was used externally.
I believe that it plugged into these jacks.
That gives me strong suspicions that it is nothing more than what we now refer to as a "volume box".
This is also being discussed right now in another topic somewhere on the FDP.

Shlock the monster


Mar 6th, 2019 12:38 PM   Edit   Profile  

I read once in an interview that Jim Marshall rejects this option by simply pulling out two tubes, mainly because the impedances don't fit anymore. It's not that easy that the impedances simply double (4 Ohms by 100 Wt = 8 Ohms by 50 Wt).
Better you use a power attenuator or an adaptor socket where you simply can fit in EL84s or 6V6 instead.

Moderator Emeritus
(with many stars)

When I was crazy...

I thought you were great.
Mar 6th, 2019 01:13 PM   Edit   Profile  

Get an OX Box.

Te 52

Laws of Physics

strictly enforced
Mar 6th, 2019 01:58 PM   Edit   Profile  

Bear in mind that halving the power will result in a 3 dB decrease in sound pressure. 3 dB is considered the smallest difference in loudness that is easily heard by most listeners, so it is only a slight decrease in volume.

6dB is heard as a significant decrease in loudness, but that requires cutting the power by a factor of 4, e.g., going from 100 watts to 25 watts.

FDP Forum / Amp Mods, Repairs, and Projects / Converting 100 watt Ampeg to 50 watt?

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