Yamaha T100C and T50C

[MKoenig]

I recently picked up a Yamaha T100C. 100 watts is a bit much for me, so I was wondering if there was an easy modification to convert the T100C into a T50C. The lower wattage would also help me because I need to replace the speaker and don't want to have to purchase an expensive high wattage speaker. Is it possible and practical to try to convert it?

[DonMoose]

The difference in loud between 50W and 100W (with the same speaker) is only 3dB - a 'barely noticeable difference.'

If you really want to, you can just pull two of the output tube (inner pair or outer pair of the quad). If might not be a bad idea to rebias. You'll also want to change your speaker impedance setting to match this doubled primary impedance ... but I don't remember which direction. I want to say for an 8-ohm speaker you want to set to 16.

As you probably know, these came from the factory with Celestion-branded speakers, but Weber makes a nice product.

Hope this helps!

[JazzGuitarGimp]

Hi Don, right concept, wrong direction. If you pull two tubes from a quad amp, you need to double the OT primary impedance to the tubes. To do this, plug a 4 ohm speaker into the amps 2 ohm output (if it has one), or 8 ohm speaker into the 4 ohm output, or a 16 ohm speaker into the 8 ohm output.

+1 on the rebias suggestion.

Agreed that you won't notice much difference between 50W and 100W, but it's easy and no-cost to try as long as you can match the speaker impedance as staed above. I wouldn't reccommend running the amp into the wrong speaker impedance though as it could possibly damage the outpot transformer and / or the output tubes.

Cheers,
Lou

[JerryFJA]

You'll have to change the main fuse to half its original value if you plan to run any amp with half the power tubes removed.
Jerry

[roberto]

The difference in loud between 50W and 100W (with the same speaker) is only 3dB - a 'barely noticeable difference.'

In this case the difference will be even less than that. I know, mathematically the difference is 3 dB, but what we perceive is the behaviour of the amp at transitories: in this case we'll have a 100W amp (I mean power supply and so on designed for 100W) that runs with half the foreseen tubes. But the feel is pretty the same as the 100W one.

I'd paradossally go on the opposite way:
increase the first two stages of caps to 220uFs instead of 82, substitute the 470R with a 3H 120mA choke, and modify the NFB from 47k to 82k, and add in series to this a fixed depth: 100 kOhm in parallel with 4,7nF.

The poweramp will blow out more harmonics and the amp will sound more "free", more easy to be played and less dependand on the volume you want to blew out of it.

[MKoenig]

Thank you everyone for all the info. I think I'm just going to buy a celestion G12K or G12T-100 and put it in there. The speaker that was in it was a JBL and could barely pick up the amp, it was so heavey, lol.

[DonMoose]

Thank you everyone for all the info. I think I'm just going to buy a celestion G12K or G12T-100 and put it in there. The speaker that was in it was a JBL and could barely pick up the amp, it was so heavey, lol.

The G12K-100 weighs about 4 pounds less than a JBL D120F. Remember, the transformers weigh almost as much (each) as a speaker. This is the drawback to a high-power combo format.