Most of the Fender Tweed circuits I've looked at just have the signal going straight from the jack/resistor to the grid of the first preamp tube.
Looking at a Premier (76) amp schematic, I notice that there are coupling caps before the grid. Is this because the Premier is using grid-leak, or for some other reason?
I'm doing a re-build, and I'm wondering whether to 'keep' the input caps (i.e. replace with new caps) or go to a Fender style 'straight-in' input. The preamp tube will be cathode biased (Fender 5G9 style) in the rebuild.
Thanks.
Looking at a Premier (76) amp schematic, I notice that there are coupling caps before the grid. Is this because the Premier is using grid-leak, or for some other reason?
I'm doing a re-build, and I'm wondering whether to 'keep' the input caps (i.e. replace with new caps) or go to a Fender style 'straight-in' input. The preamp tube will be cathode biased (Fender 5G9 style) in the rebuild.
Thanks.
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Yes, the Premier is using grid leak bias. The tip-off is the size of the grid resistor, usually 2.2-3.3 Meg. ohm and no cathode resistor. They did away with grid leak sometime in the 50's because it didn't generate very much bias voltage which created distortion. Once this stage distorts, there is no going back. The signal cannot be cleaned up. The Fender style input is much more versatile for setting the bias voltage because you can choose the value of cathode resistor and also gives you the option of using a bypass capacitor. This gives you the option of what you want to get out of your input stage. It just depends on what you want the amp to do. If you want more gain out of the preamp as it is drawn, put a 12AX7 there or a 5751.
Sorry, I forgot about addressing the cap on the Premier input. If grid leak is used, the cap is always there. But just because the cap is there doesn't always mean grid leak is used. An example is the Peavey Classic 30 and 50 and their EVH 5150, at least the first versions did. I don't know if the newer versions do. I'm sure there are others.
Thanks, BT!
I'll check out those Peavey schematics (I was never much of a Peavey fan, but every circuit has something to teach me!- lots to learn).
I'll check out those Peavey schematics (I was never much of a Peavey fan, but every circuit has something to teach me!- lots to learn).
The best resource for schemos I have found is Doug Hoffman's site. He had a HUGE library. Tube amp parts, Guitar amp parts, Tube amp for guitar It's the third link under the scrolling pics of parts.
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In the old tweed days, all anyone ever plugged into their amp was their guitar. No need for a input cap. yes, grid leak bias stages needed the cap, but modern amp usually include the input cap to block any DC offset from any effect pedals and such that may be out front.
Glad to hear from you Enzo. Thanks for that bit of info. One more piece of the amp puzzle added to the big picture.
One thing about input caps....
There are many times that the circuit boards can have DC leakage currents flowing through them fiber boards and phenolic boards, especially when the plate voltage turrets/eyelets are in close proximity to the eyelets for the input signal... This can manifest itself into the volume pots on the guitar making all the scratchy sounds...as well as CryBaby wha pedal with a scratchy pots...
I had seen many fustrated players respraying thier pots 1000 times and eventually changing them out only to freak out from the scratchy-pot syndrome not going away...
An input cap can prevent this issue... or just floating the wires off the eyelets..
A series resisitor right at the tube socket GRID pin can help reduce RF , when it works in conjunction with the Cmiller capacitance of the pre-amp tube... Typically 68K is common to keep the first order POLE above the guitar band roughly 16KHz.. so this is 45 degree phase shift at 16kHz, so there will be some amount of phase shift in the upper guitar range...
There are many times that the circuit boards can have DC leakage currents flowing through them fiber boards and phenolic boards, especially when the plate voltage turrets/eyelets are in close proximity to the eyelets for the input signal... This can manifest itself into the volume pots on the guitar making all the scratchy sounds...as well as CryBaby wha pedal with a scratchy pots...
I had seen many fustrated players respraying thier pots 1000 times and eventually changing them out only to freak out from the scratchy-pot syndrome not going away...
An input cap can prevent this issue... or just floating the wires off the eyelets..
A series resisitor right at the tube socket GRID pin can help reduce RF , when it works in conjunction with the Cmiller capacitance of the pre-amp tube... Typically 68K is common to keep the first order POLE above the guitar band roughly 16KHz.. so this is 45 degree phase shift at 16kHz, so there will be some amount of phase shift in the upper guitar range...
Thanks for those tips.
Those older eyelet boards can be a problem - a number of years ago I offered to repair a Bassman head which ended up with all kinds of oscillation and 'ticking' problems which I finally solved by making a new eyelet board...what a pain. Humid climate and an old eyelet board...yuk. That was the end of my 'fix stuff for other people' career!😀
Besides grid leak bias being a distortion mechanism that I wouldn't want, some topologies allow DC to be put on your guitar pickups, which over time could vary the strength of the pickup magnets. I could be wrong about this, but I wouldn't want to take the chance. It's hard for me to imagine why anyone would ever want to use grid-leak bias.
A much better idea is to put a 10K in series at the input, then a 330p cap to ground, forming a passive Rf filter, which would then feed into a 1M to gnd, and then a 5K series grid R right at the input tube socket. Don't do the usual passive mixer with the 68K R's since that increases hiss noise.
A much better idea is to put a 10K in series at the input, then a 330p cap to ground, forming a passive Rf filter, which would then feed into a 1M to gnd, and then a 5K series grid R right at the input tube socket. Don't do the usual passive mixer with the 68K R's since that increases hiss noise.
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