Changing MV55 to 16 ohms output

[nyrocker]

Hello,

I am picking up an MV55 and would like to convert it to 16 ohms output. This is usually a pretty straightforward change, but I would like to find a schematic or receive some direction from someone who has done this modification. I could not find anything in a search so I thought I would post this and ask for help.

Thank you in advance!

[admin]

First, welcome to CJO. Great to have you with us. I too am a MV-55 owner. Really great amp.

As for the rewiring question. I don't have a specific reference for the wiring diagram but in our schematic database we do have some speaker resistance diagrams for the Premier 11-12 series as well as the MV-60 (you can find that in the "troubleshooting" section of the forum, it's the top sticky post). It's a good chance that the wire coloring is the same, but again, not 100% sure.

Out of curiosity, what kind of speakers are you using that require 16 ohm output?

[nyrocker]

Thank you! I'll hopefully be able to determine if the color codes are the same. My speakers are Musical Affairs single drivers using French Phy-Hp drivers in beautiful spruce semi-open baffle cabinets. They're 15 ohm with 97dB sensitivity. I'll be running the MV55 in triode.

[AnotherJohnson]

In many units the transformer output wiring schematic is inside by the transformer. You might pm solderslinger or Roberto. I think one of them said once that a resistor value needs to also be changed, but I may remember this wrong.

[AnotherJohnson]

Out of curiosity, what kind of speakers are you using that require 16 ohm output?

The classic case is the Rogers LS3/5A. This was a standard monitor and many copied it. It is inefficient, but it was well thought of back in the day.

The other classic case is that someone intends to wire two pair of speakers and their plan is to put the two rights in series and the two lefts also in series.

There are other examples. It is not common in the present day.

[AnotherJohnson]

FWIW, the goal of impedance matching is to facilitate transfer of power from amp to speakers. A 16 ohm speaker can be run on a 4 ohm tap, or vise versa. Since most speakers have dips, the former is probably a better plan than the latter. But YMMV. Your ears, your system.

As far as I know, the only taboo with a tube amp with output transformer load matching is the YOU NEVER WANT TO RUN THE AMP WITHOUT A LOAD ATTACHED.

On the work bench I have a pair of 16 ohm power resistors wired in parallel to produce an 8 ohm load. Or I hook up the PSB Alpha A/Vs which are nominally 6 ohms. You will damage your CJ tube amp if you turn it on without a load attached.

[nyrocker]

The PHY-HPs have a response curve that lives well above 15 ohms, which is the lowest point. I've run them off of 8 ohm taps but 16 has always been better. Even Quad IIs, which many feel run LS3/5As beautifully, sounded underpowered on my speakers. I may bite the bullet and send it into CJ but I want to take a look underneath with my local tech first. Thanks to all for your replies!

[AnotherJohnson]

There are a lot of aftermarket drivers out there with nominal 15 or 16 Ohm ratings that can be used in DIY projects.

In guitar cabinets two 16 Ohm speakers in parallel become an 8 Ohm load. Two 8 Ohm in parallel become 4, but then wired in series with another similar pair jump the load back to 8.

All in good fun.

[SolderSlinger]

The wiring diagram in the CJO schematics folder for a Premier 11 is a good reference. Need to change the output transformer connection and the feedback resistor.

Have fun!