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Marshall 50W amp/preamp project

5/10/1998 4:29 AM
Paz
Marshall 50W amp/preamp project
I'm pretty new to valves so this forum has been useful the last few weeks as I have been bulding my first amp. Thanks to everyone for sharing information so freely.  
 
However, I still have a couple of design problems which I hope someone can help with.  
 
1. TWIN INPUTS  
I used the Marshall Master Channel Preamp and 50W power amp circuit diagrams given on Ampage as a starting point as it had 2 inputs. However, the clean channel is massively quieter than the other, and to get any half decent volume I have to crank everything up to 10, by which time there is distortion creeping in. I am currently switching between two 1M pots (just after the first 12AX7) to act as a channel gain for each input, so I can access both channels on 1 jack socket. Could I change the value of whichever controls the clean channel for more (clean!) gain? Could I have switchable biasing for some of the triode stages in the preamp?  
 
2. TRANSFORMER VOLTS  
My transformer secondary for the power valves measures about 6.8V open circuit, and drops to 6.3/6.4 under load. Whilst this is ok if all the filaments are there, is it asking for trouble if any of them blows thus allowing the voltage to rise?  
 
3. SPEAKER HUM  
This is a weird one - I have a (what sounds like) 60Hz hum from the speaker when the amp is on, even if the HT is turned off and there are no power valves in their sockets! How can this be?? Nothing in the debugging FAQ seems to cover this scenario. It doesn't change when the EL34s and HT are added, but does disappear if I shunt either of the anodes to CT on the output trannie (as for measuring bias current). Any ideas?  
 
Regards,  
 
- Paul  
Leeds, UK  
 
P.S. If you're in the UK and want a good quality custom transformer, I have found a place in Bradford which will make a 300V @ 250mA/6.3V @ 4A plus another low voltage secondary for just 30 + VAT. Makes me wish I hadn't bothered building mine myself...

 
Replies:
liam My AC30 had a similar hum. I knew ... -- 5/10/1998 7:21 AM
liam I just noticed, you've got the HT t... -- 5/10/1998 7:23 AM
HM PazCould it be an ot imbala... -- 5/10/1998 7:34 AM
Jim S. I wouldn't worry about the filament... -- 5/10/1998 10:47 PM