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Re: (2)6V6 Class'A' Champ


 :
10/3/1997 3:54 AM
Dave Harris Re: (2)6V6 Class'A' Champ
Duh ! I confused 8" with 8 ohm.  
 
If your Celestion is 4 ohm and you use the Mojo 4 ohm Champ OT then you will get roughly the same power not less. It is not worth doing all the work to end up with the same power so make sure you can get the right OT.  
 
Dave
 
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10/3/1997 7:10 AM
Mike Tremante

Dave,  
 
The Mojo tansformer is the OT. What I'm concerned with is the fact that the 6V6s get their B+ through the OT, which could burn this one up. I have a couple of Mojo Deluxe OTs, but their primary impedence would be that for 2-6V6s running in Class 'AB', which you said is around 8K. But it has windings for 2 output tubes so I suspect it could better handle the current. The PT is a Fender from a SF Princeton Reverb, which shouldn't have a problem with current or filament supply as it ran 2-6V6s in Class 'AB' and 4-12aXs, and here it would be running 2-6V6s in class 'A' and 1-12AX7. I don't know the exact numbers but it seems to me it should come close to evening out.  
My Celestion is 4 ohm, as is the Mojo Champ OT. The Princeton OT os 8 ohm. I have a couple of 8 ohm 10s around, I suppose I could cut the baffle for one, but I would rather not. I wonder if I could use 2 champ OTs. Hmmm.... Would the secondarys be OK paralleled? And what about he B+.  
 
Mike
 
10/3/1997 8:50 AM
Steve M.

Mike,  
 
Your idea about paralleling two output sections might work. I've seen this done on a few few vitage amps. I know Marshall did this on there first few 100 watt amps, they paralleled two 50 watt OTs to get 100 watts. These were push pull amps, but I believe it would work for SE amps as well.  
 
Instead of paralleling two 6V6 tubes into a 2.5K ohm primary, you could add another 6V6 and 5K ohm primary OT in parallel with the existing output section. One consideration would be connecting the secondaries. One way would be to connect the two 4 ohm secondaries in series to get an 8 ohm load (if you paralled them you would get a 2 ohm load). Or you could connect OT to its own speaker, but then you'd have to get a cab to house both speakers, or you could add and extension speaker. There's a lot to consider here.  
 
hope this helps,  
 
Steve
 
10/3/1997 9:00 AM
Dave Harris

Mike,  
 
Double the current may burn out the OT. It could also saturate the core and the impedance match is wrong you would need a 2 ohm speaker. The Deluxe OT is push pull so the winding only has the current of one 6V6 in each half and the impedance is wrong so you have the same problem and it is not designed for single ended use. If you use two Champ transformers in parallel (pri and sec) the speaker will have to be 2 ohms. The only way I think you can do it is to run the primaries in parallel and the secs. in series into 8 ohms.  
 
The ideal solution is a single ended 10W 3500/4 ohm OT that will take the current of 2 x 6V6. Anyone ?  
 
Dave
 
10/3/1997 9:44 AM
Steve M.

Dave,  
 
AES" target="_blank">http://www.tubesandmore.com/">AES has a few trannies that are close to this. Hammond and One Electron both make transformers that have a 2.5K to 3K primary and multiple secondary taps, but they are both about $120 US. For one thing, they are hifi OTs and are rated at 25w and 12w respectively Also, for that much you could buy another used SF champ :) There is also a "universal" tranny listed that comes with a table for calculating impedence ratios. It can either be used for SE or PP operation. It is about $33 US.  
 
BTW, I thought the primary impedence for two 6V6s in parallel would be 2500 ohm, not 3500, am I missing something?  
 
respectfully,  
 
Steve
 
10/3/1997 11:06 AM
Mike Tremante

I have a couple of Champ OTs around, and a couple of Fender reverb trannys that appear to be the same as the SF champ. I think I'll try the 2-OT configuration first, with the primaries in parallel and the secondaries in series for 8 ohms to the speaker. That would be less strain on the amp the 2 ohms, right? The idea of a seperate speaker for each OT is a good one, I'll have to think about it. If I could use my mediocre woodworking skills to build a cabinet, I also have a Kendrick 8" 4ohm speaker that I could use on the smaller Fender OT. I've seen the AES catalog and I've seen all the ones mentioned as candidates, and the P-T125E or the UBT1 or 2 looked like they might work. But I'll work with what I have first, at least until something goes up in smoke. I think the PT is my main concern at this time. Does anyone know of a way to measure the primary on an OT?  
 
I'm a little foggy about Steves suggestion for adding a 5K OT in parallel. Are you assuming the the present installed OT is 2.5K, because all I know is that it's a Mojo Champ. The idea of 2 speakers, one for each OT is a real good one, but I don't understand the mismatch in primaries.  
 
There is a very lot to cosider here...  
 
 
Mike
 
10/3/1997 1:00 PM
Steve M.

Mike:  
 
I believe the stock Champ OT is 5K ohm to 4 ohm. This is for one 6V6. If you add another *tube* in parallel (using only one OT), the impedence of the OT would need to be 2.5K ohms. It works like speakers: two 8 ohm speakers in parallel are a 4 ohm load. This is only for adding a *tube*. If you add a completely new power section (in this case a 6V6 and a 5K ohm OT)in parallel you now have two complete output sections running in parallel. At this point, you can either run the secondaries in series for 8 ohms (or parallel for a 2 ohm load, but this will not give you optimal power unless connected to a 2 ohm speaker, when was the last time you saw one of those for guitar? ). The other option is to use each output section to power a 4 ohm load, i.e one 4 ohm speaker for each power section.  
 
Regarding testing OTs check out this website:  
Testing" target="_blank">http://www.flash.net/~billhar/outimp.htm">Testing Transformer Impedence  
 
Dave Harris did a pretty good job explaing this on this board, but this site has pictures for those like me with a strong back and a weak mind :)  
 
Hope this clears things up a bit,  
 
Steve
 

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