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What voltage too hi for reverb trannie and 12AT7 tube

11/3/2003 4:27 PM
Major Pain What voltage too hi for reverb trannie and 12AT7 tube
Well Ive taken a 5010 Marshall SS combo and gutted it and installed Tubes and trannies converting the amp to a 50 watt tube amp using a Fender twin circuit but the probelm Ive encountered is that the hi DC voltage is at 500 and I have to still install the reverb trannie so my question is what is the highest limit the reverb trannie and 12AT7 tube can take before distruction?  
Thanks M.P.
11/4/2003 4:00 PM
Man o'Blues
Fenders went as high as 450V with a paralleled 12AT7, using a 2.2K shared cathode resistor. I'd think that's as high as you'd want to go, but you might get away with 500V and a higher valued Rk to keep dissipation within the tube's limit.  
I'd prefer to decouple the rev. driver supply after the screen supply node, using a dropping resistor (and cap) to get the plates at around 400-420V. 500V's just feels too hot for me to risk it.
11/4/2003 6:24 PM
Major Pain
Thanks for the info MoB as thats how I feel that exact same way. Once had a Fender twin that would eat up 12AT7's because they were ran so hi on the DC voltage at the reverb driver (460vdc).  
I can always add that decoupling to bring the voltage down a bit.  
The PT im using is a hammond 273BX for this build and that PT is puting out to much voltage, especially cause its rated at 115vac (input primary) and my line voltage is 123vac. I might change out the PT to a lower voltage PT as well. The hammond has a little to hi voltage secondary for this build. Thanks again  
11/5/2003 5:52 PM
andy fuchs

Change the reverb driver to a triode connected EL-84 with the fender reverb transformer. It will give you mucho kick-ass reverb drive. :) You can also add a 10-K 5-W resistor to the B+ line feeding the 'former to cut down the plate voltage too.
11/6/2003 1:45 PM
Major Pain Re: Another reverb problem???
Well I have to ask a question to the guru amp builders here, In a past project I had taken a bassman head and installed into a 1x12 cab and then added another 12AT7 to this amp and then rebuilt the circuit to a fender AB763 style and added reverb and installed the reverb pot on the back of the chassis. That project came out great and was a good sounding amp. Loud and clean like a twin but in a 1x12 50 watt version. and smooth reverb  
Now Ive done the same thing but in a Marshall chassis in a 1x12 cab and the reverb is too hot and too loud in the circuit. I did use the smaller 9 "  
8AB2A1B reverb tank and now im wondering if this tank is the culpret as in other project the bassman conversion I used the standard 4AB3C1b tank and that reverb was perfect sounding in that bassman 1x12 amp setup.  
Can any one chime in on this problem. Reason I used the shorter tank was to keep it away from a lowly installed speaker in that 1x12 cab. I wasn't sure how close a reverb pan could get to a speaker magent without having problems.  
11/6/2003 9:23 PM
Bruce /Mission Amps

No... you have to describe how you mixed the reverb and dry signal back in... this is the secret to a good sounding tweed Bassman/Marshall with reverb topology!!  
11/7/2003 1:18 AM
Major Pain
Hey Bruce,  
What I did was gut a Marshall 5010 1x12 SS amp and build in the empty chassis a fender twin circuit minus the normal channel and tremolo. The reverb is mixed with the 3.3meg/10pf cap exactly like a AB763 schem. You know the standard fender reverb topology.  
I done this before with a bassman head in a 1x12 cab and it worked perfect then. I used the regular 16" tank for the bassman build with excellent results.  
In the Marshall chassis build I used the smaller 9 " 8AB2A1B reverb tank instead of the regular 16" 4AB3C1b tank so Im not sure if thats why I got to much reverb in this build. Ive searched this down but haven't found why the reverb is so thick. I know im missing something here so I will look again tonight.  
Does the two different tanks create this sort of problem? This is the first time ive used the shorter 8AB2A1B tank before.  
Im kinda not sure what a safe distance for the speaker magnet the longer 16' tank would be safe to use so thats why I used the 9" tank instead.  
M.P. 8^/
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