M
miggyi
New member
I been wanting to mod my 5150 II amp with 6550 tubes. A couple of people have told me it is possible but I just want to make sure...So what do ya think?
glpg80
Well-known member
its possible but not without some major/minor changes, all depends on your experience.
A: 6550 tubes wont fit. you will need to modify the area above the tubes where the vent is to get them in/out safely and have a little room to breathe. i've heard people say they do fit but you have to install them from the inner set first and work your way out.
next, screen grid resistors need to be doubled. i think in 5150IIs if i recall the schematic correctly they are 2.2K, they need to be 5.6K.
next, you will need to do the bias mod, even on the newer models. the newer models you will need to replace the bias resistor with one that is lower in value, otherwise you wont have enough swing to bias the 6550's correctly.
after that, you will be set. i believe the stock PT's can pull enough current without doing any harm with 6550's.
oh, and welcome to the forum
A: 6550 tubes wont fit. you will need to modify the area above the tubes where the vent is to get them in/out safely and have a little room to breathe. i've heard people say they do fit but you have to install them from the inner set first and work your way out.
next, screen grid resistors need to be doubled. i think in 5150IIs if i recall the schematic correctly they are 2.2K, they need to be 5.6K.
next, you will need to do the bias mod, even on the newer models. the newer models you will need to replace the bias resistor with one that is lower in value, otherwise you wont have enough swing to bias the 6550's correctly.
after that, you will be set. i believe the stock PT's can pull enough current without doing any harm with 6550's.
oh, and welcome to the forum
eternal_idol
New member
if i remember correctly, they wiring from the transformer to the power tubes can't handle the current for el34 or 6550s.
this might have only been in the first series of 5150, wait for JerryP to chime in... he knows them amps better then Peavy!
Brae
this might have only been in the first series of 5150, wait for JerryP to chime in... he knows them amps better then Peavy!
Brae
M
miggyi
New member
Thank You!
How do I go about upgrading the screen grid resistors?
How do I go about upgrading the screen grid resistors?
glpg80
Well-known member
eternal_idol":3itdxafj said:
you mean the PT wires for the voltages? the 5150's have PCBs
glpg80
Well-known member
miggyi":mazk17ll said:
if you dont know what a screen grid resistor is, or what it does, please take your amplifier to a certified tech and have them do these upgrades for you.
you have to worry about the wattage of the resistor, not just the values. and the voltages you are playing around are very high.
the filter caps hold their charge for days even though 5150's have self-draining resistors. plus if you install something incorrectly, you can almost guarantee you will be causing more harm than good to yourself and your 5150.
M
miggyi
New member
glpg80":3h55c4rv said:
I know people who know a lot about it. I just wanted to know personally.
eternal_idol
New member
glpg80":1xife0jk said:eternal_idol":1xife0jk said:
you mean the PT wires for the voltages? the 5150's have PCBs
I'm scartchingmy brain here... somethign about the rainbow grouped wires of one of the transformers.
been a long day its late..
glpg80
Well-known member
eternal_idol":34lqdcor said:glpg80":34lqdcor said:eternal_idol":34lqdcor said:
you mean the PT wires for the voltages? the 5150's have PCBs
I'm scartchingmy brain here... somethign about the rainbow grouped wires of one of the transformers.
been a long day its late..
extremely understandable
others will pitch in im sure. the PT on mine has been pretty stoute. it survived a bias runnaway from a shorted diode, and gets pounded at band practice biased at 35mA for 6L6's. i figure 10 more wouldnt do it any more harm than what mine has gone through. the voltages are usually around 450 for the plates as well, so its not too low.
D-Rock
Well-known member
Not sure about 6550's, but Bob at Eurotubes has put KT88's in a 5150 with only a simple bias adjustment. Maybe look into that.
