CRATE VC-3112 4xEL84 to 2xEL84 mod?

[Lostfollicles]

Sorry guy's I put this on Guitar amps first instead of mods and tweeks!

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

[rudy kenyon]

I just viewed your query today.
Keep in mind that the plate load will also change. Using a tube specs manual, the EL84/6BQ5 in a P-P pair
set-up should drive a transformer with a 4k/4kOhm primary, or close to that. But the cathode bias resistor
(R22, a 120Ohm/5W) will have to increase in value to around 220Ohm/5W. The screen bias resistors R24
and R25, both 100Ohm/2W, could be left alone.
[R46 and R47 do not apply to tube bias -- the circuit they are for is the op-amp reverb, so no worries, eh?]
Nothing else in the would change. But make certain the output transformer is compliant to the 4k/4k or
near-so. Then remove two tubes.
See Ya

[zoranrataj2000]

You should change bias resistor and speaker to higher impendance ( now 8ohm - become 16ohm, for example).The problem is if you wish to switch.Everything else is ok.
BUT if you wish to switch betwen 2 or 4 EL84 you should remove R43 and C29 with bridge ( 0 ohm resistor or 10 ohm 5 watt-you can testing bias current) and you should between R41-R42 put negative voltage for fixed bias.Than is eazy.Just pull 2 ouside or inside tubes out and change speaker with 16 ohm(or use extern box)

[olddawg]

You should change bias resistor and speaker to higher impendance ( now 8ohm - become 16ohm, for example).The problem is if you wish to switch.Everything else is ok.
BUT if you wish to switch betwen 2 or 4 EL84 you should remove R43 and C29 with bridge ( 0 ohm resistor or 10 ohm 5 watt-you can testing bias current) and you should between R41-R42 put negative voltage for fixed bias.Than is eazy.Just pull 2 ouside or inside tubes out and change speaker with 16 ohm(or use extern box)

If he is just trying to "tame the beast" , instead of replacing the speaker couldn't he just put an 8" resistor in series with the existing one. Another thing to consider is just buying a Weber Mini Mass attenuator. They are around $100. I use one all of the time. Might work better and much less trouble.