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4x 6V6 for 40watts ??


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6/23/2005 12:26 PM
Derek
4x 6V6 for 40watts ??
Hi folks,  
 
My favourite amp is my original '65 BFDR, but I usually need 40 watts. I have a '66 BFPR for this, but want to retire both amps to the living room, as they are worth a few bucks and near mint.  
 
I am thinking of building a 4x 6V6 amp, with two tubes switchable out for 20/40 Watt option. I have tried the triode mode on dual 6L6 for power reduction, but the loss of high frequency is no good for regular gigging.  
 
Anyone tried this configuration? What transformers were used? Any comments?  
 
Thanks,  
Derek
 
6/23/2005 1:51 PM
Darin

You can use 4x 6V6 in fixed bias using SS rectification to get the 40watts. Then you can add a switch that puts the grids to groud on one set of 6V6s. This will be like pulling a set of power tubes. Any set of properly rated trannies should work fine.
 
6/30/2005 11:00 AM
Balijukka
It will still leave the DC going through the tubes and the OT. and heater current. Might be good or bad, don't really know. If you cut the cathodes away, then the dc is away, heaters still are on.  
Jukka
 
6/23/2005 2:37 PM
Shea

Derek:  
 
I'm in the process of planning out a four 6V6 amp using a Heyboer version of a Fender 022798 power transformer, in a standup configuration, and a 018343 output transformer with 8, 4, and 2-ohm secondaries (I had to ask them to add the 4 and 2-ohm taps; other suppliers' versions of the 018343 already have them).  
 
Basically, any of the output trannies that Fender used in their 40-watt, 2 x 6L6 amps in the blackface and silverface years ought to work, if it has the right turns ratio for your purposes (i.e., if it is intended for the speaker impedance that you'll be using). I think the power trannies used in Bandmaster Reverbs, Vibroluxes, and Pro Reverbs should work too. The B+ voltage may end up being a little too high for some makes of 6V6s, but there are ways to drop it if necessary.  
 
I'm using a Weber 6M100 (100-watt Marshall) chassis so I can use a tube rectifier. I'll put the socket for the rectifier in the hole at the rear left corner of the chassis, where a cap can normally goes. But I had to look around for an octal socket that was big enough to cover that hole, because it's bigger than a typical hole for an octal socket. I found a big enough socket at Triode Electronics, but I think Antique Electronic Supply carries the same one.  
 
If you don't mind top mounted controls, then you could use a tweed twin chassis and power transformer.  
 
Keep in mind I haven't built this yet, so I can't promise that I won't run into some unforeseen bugs as a result of using this combination of chassis and trannies. But I can't think of any yet, other than having to drill extra holes to mount the trannies, and maybe having to drop the B+ voltage if it turns out to be too high for 6V6s.  
 
Shea
 
6/23/2005 4:17 PM
Chris@CMWamps

Hi Derek  
 
The switching-option suggested above is just excellent 'cause you don't have to worry about impedance switching.  
For a 4 ohms load a blackface Bassman OT would be a good option, otherwise (and also) check the various Fender style transformers and chokes offerd by http://www.MercuryMagnetics.com . Talk/email with Paul if you do want a custom option suited for your needs and sound-wishes.  
 
Good luck/6V6s rock!  
 
Chris  
Love, peace & loudness  
http://www.CMWamps.com
 
6/27/2005 10:37 AM
Dave H

quote:
"The switching-option suggested above is just excellent 'cause you don't have to worry about impedance switching."
 
 
Grounding the grids of two power tubes is a good idea because the plate voltage won’t go up unlike pulling the tubes or disconnecting the cathodes but I think the impedance still need to be switched to keep the load the same for the driving 6V6s.  
 
Dave
 
6/27/2005 10:51 AM
Shea

If it's a fixed bias amp, then those tubes would get pretty hot and wear out rather quickly if their grids were grounded. You'd have to apply a negative voltage from sufficiently low impedance to be able to swamp out the signal coming from the pi. Maybe just shorting across the grid bias resistors would do it, but there'd have to be a separate grid bias resistor and blocking cap for each power tube.  
 
Shea
 

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