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Another paralleled to single push-pull EL84 mod.

Niquel

Member
2012-02-28 10:49 am
Hi there all,

I have a paralleled push-pull el84 guitar amp from a friend which is supposed to be 50-deafening-Watts so the friend asked me to modify it to be able to use it at lower levels.
After having taken a look to the schematic it seems 8 Ohm matched, fixed bias so the mod should be as simple as removing one of the output pairs to get half ouput power.
Its OT has only 8 ohms leads and the load (is a 2x12" combo) is 16ohm//16ohm and 30 W each.
If I pull an output pair I could use just one 12" speaker at 16Ohm to the 8Ohm output in order to keep its impedance matched. But, as it is a pitty not using the whole cabinet, would it be really bad for the output stage to use it complete and thus umatching the impedance?
I have read that many people do this, and even some people prefer its sound but it is not my amp and it is now working so I wouldn't like to burn it with experiments...
I would realy apreciate any suggestion.

Thank you.
 

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45

Member
2008-12-18 2:29 am
UK
It depends on the actual plate-to-plate impedance. If the original OT has 4K/8R for parallel operation (i.e. 8K for a single pair) you can disconnect one pair and use it. For guitar amps a signle pair with 4K should work fine. More likely you will not get exactly the same sound. It could be better....you just need to try.
Maybe you need to check the idle current is enough to avoid cross-over distortion.
 
The problem is the 2x12, not the parallel tubes. I have a 100 watt Fender Twin, with 4 5881s. I can pull two of the tubes for 50 watts, and it is a hardly noticeable change. I can flip a switch and take the 50 watts down to 15 or so in triode mode. Sill, a minor change in volume. Loud no matter what.

By disconnecting one speaker and the set of tubes, you can make a difference. The only pity about the 2nd disconnected speaker is that your friend has to lug it around from gig to gig.

Or he can sell it and buy a more appropriate amp.

Or tell your friend to just dial back the volume knob, though few guitar players can manage that. Surprising how many guitar players complain the amp is too loud when their fingers are inches away from at least two volume controls set to maximum.
 

45

Member
2008-12-18 2:29 am
UK
The problem is the 2x12, not the parallel tubes. I have a 100 watt Fender Twin, with 4 5881s. I can pull two of the tubes for 50 watts, and it is a hardly noticeable change. I can flip a switch and take the 50 watts down to 15 or so in triode mode. Sill, a minor change in volume. Loud no matter what.

That's true. A 18W EL84 PP + Eminence GB-12 will sound so loud even outdoor! You don't need anything else unless you are going to play in stadium....
12" cones usually give 96-102 dB SPL with just 1W.
 
A 1X12, on axis, is not that much quieter than a 2X12; what it is is a lot more directional at high frequencies. Which means a couple of people in each row are overwhelmed by the guitar too loud, while two seats off to the side they hear a sort of muffled approximation of the chord sequence.

Have you considered a resistive attenuator, which keeps the amp properly loaded while eating three quarters of the power, à la Power Soak? I used to get the big green wirewound resistors out of tube TVs, but I imagine this is no longer an option. There's a minor change in the amp's sound, due I imagine to seeing a resistive load rather than to rather strange impedance of a guitar speaker in a slotted back enclosure.

At one time I would have said "replace the speakers with Fane crescendos - reduce your level by 10dB at a stroke." Nowadays I don't know the names of the high power, low efficiency drivers. Anyway, that would change the sound a lot.