Crate VC-3112 Combo 4 EL84 To 2 EL84 conversion?

[Lostfollicles]

Hi Guys' Can anyone throw some light on the very dark subject of changing a 4x EL84 amp to a 2xEl84 amp using a switch but also removing one pair inner/outer of the four?

I cannot afford a new amp and just want to tame the beast down a bit!

I just want to say I have tried the amp with two valves removed but of course the valves were red plating "a little bit" and although she sounded great I decided to leave the four in for now for piece of mind!

Looking at the differences between the VC-1512 and the VC-3112 schematics (See attached) it looks like on my VC-3112 I will have to add a 60 ohm resistor in series to R43 with a switch across the added resistor, also add a 220 ohm resistor in parallel with R52 with a switch in series with the added resistor to enable the specs to come close to the VC-1512.

Would I also have to change the relevant grid resistors to 10K (depending on valve pair used?) R45/R44/R46/R47 or would these be OK to leave as 2.7K? I understand why I need to change the bias settings for two EL84's but what difference would changing these make?

I also understand that the VC-3112 output transformer would be looking at a change of impedance and that I should change the speaker for a 16 ohm type (No 4 ohm tap) but would I really hear the difference?

As I said earlier she sounded great I just didn't want her to burn up!?

Any help guy's would be very appreciated!

Regards

Lostfollicles

[Pedro Vecino]

For a half power reduction you only need to disconnect the cathodes in the inactive pair of EL84 while incorporating a resistor of 82 Ohm /6 watts in series with the existing cathode resistor. You can do selectable via a switch.
This operation I have done previously and the amp sounds a little darker with less electrical character.
Regards

[Jazz P Bass]

Another method is to simply disconnect the screens from the unused tubes.
That way does not mess with the power supply.

[Lostfollicles]

Thanks Pedro Vecino and Jazz P Bass

I appreciate your ideas, food for thought!

For my needs though I really just want to use the amp as a 15 watter! I want to remove the unwanted tubes, the only reason I want to install the switch "Apart from the electronic needs of the amp" Is to make life easier if at a later stage I ever need the 30 watts again or if I want to sell it!

If I do change the Grid/Screen resistors from 2.7k to 10k will this lower the gain?

Thanks again for your help guy's

Lostfollicles

https://soundcloud.com/lostfollicles

[Pedro Vecino]

The value of these resistors is not related to the gain. Are basically to prevent oscillations. You will not notice differences in sound changing these resistors from 10K to 2K7.

[Lostfollicles]

The value of these resistors is not related to the gain. Are basically to prevent oscillations. You will not notice differences in sound changing these resistors from 10K to 2K7.

Thank you Pedro that's all I needed to know I can now start doing the tweek now with some confidence!

Regards

Lostfollicles