I want to add a single pot tone control, 6v6 grid stopper (15K) and master volume. Do the revisions to this 5F3 schematic look OK?
http://oi59.tinypic.com/23lzdjq.jpg
thanks!
http://oi59.tinypic.com/23lzdjq.jpg
thanks!
Tone control looks ok, but the grid stopper should be closest to the 6V6. You should connect the grid stopper between the centre wiper of the volume pot and the grid of the 6V6. Why so high (15k)?
Thanks for the reply.
Updated pic: http://oi59.tinypic.com/c2s1e.jpg
What do you suggest for the grid stopper value?
Also, is there an advantage to the filament wiring as shown, v.s. an artificial center tap (two 100ohm resistors) placed between the 6v secondary?
Updated pic: http://oi59.tinypic.com/c2s1e.jpg
What do you suggest for the grid stopper value?
Also, is there an advantage to the filament wiring as shown, v.s. an artificial center tap (two 100ohm resistors) placed between the 6v secondary?
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That 15k grid stopper value is probably a typo and should read 1.5k which is what I usually see being used. And it should be soldered right on the tube socket.
The heater wiring as shown in that schematic is a crude way of doing it, neatly twisted wires and a center tap whether it be artificial or not is the better option.
The heater wiring as shown in that schematic is a crude way of doing it, neatly twisted wires and a center tap whether it be artificial or not is the better option.
The 220K resistor (now in parallel with your 500K master volume) was there in the original circuit as a grid leak for the 6V6. The grid leak function is mainly being provided by your 500K pot in the new design. The new version will work OK, but it might be nicer to remove the 220K and reduce the master volume pot. to 220K or similar. Its just a minor suggestion - your circuit would be fine anyway.
Ok, thank you. For now I'll pull the 220K grid leak but I may leave the pot at 500K for the time being.
I also recall seeing 1.5K grid stoppers in most fender circuits, but those are almost exclusively push-pull. Does 1.5K still figure in for a single-ended design to stop oscillation?
Last question, what would be the tonal effect of pulling the 22K feedback resistor?
I also recall seeing 1.5K grid stoppers in most fender circuits, but those are almost exclusively push-pull. Does 1.5K still figure in for a single-ended design to stop oscillation?
Last question, what would be the tonal effect of pulling the 22K feedback resistor?
In the latest production run of Fender Champs there is no grid stopper on the 6V6.
I find that for these type of amps I like to run them without negative feedback, I find the sound to be more raw. Basically I turn the amp up to max volume and use my guitar's volume to control breakup, without feedback I get a better tonal range of breakup with my guitar's volume control. Remember when these amps were designed they didn't like distortion so the feedback was to keep the amp more stable and clean. If you aren't sure you can always make it switchable which I have done but like I said I end up leaving it with no feedback, but everyone is different.
I find that for these type of amps I like to run them without negative feedback, I find the sound to be more raw. Basically I turn the amp up to max volume and use my guitar's volume to control breakup, without feedback I get a better tonal range of breakup with my guitar's volume control. Remember when these amps were designed they didn't like distortion so the feedback was to keep the amp more stable and clean. If you aren't sure you can always make it switchable which I have done but like I said I end up leaving it with no feedback, but everyone is different.
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So up and running...
No feedback resistor, no grid leak resistor. 68uf bypass caps on both sides of the 12ax7 cathode. The tone control is more of a bass cut/boost.
Two problems: every single 6V6 I've tried (like...10 of them) are microphonic like crazy - is this is really bad string of tubes or can this be caused by the circuit? I also have a hum being introduced between the 12AX7 and 6V6 (Hum goes away if you pull the 6V6 only). 60 or 120Hz, not sure. Scope shows nothing on the input of the 6V6. There is a 50mV ripple on the screen (side note, worth adding a 470ohm screen resistor?)
Sorry...I've been misleading...here is the freshest circuit...note the bridge rectifier, extra filter stage, 3 zeners to help bring the voltage down. There is 5V ripple is present on the first and second filter stage (OT primary), 50mV on the 6v6 screen. http://i60.tinypic.com/2eobp0o.gif
Voltages:
Rectifier: 327
First filter: 322 (5V ripple)
second filter (OT): 304 (5V ripple)
third filter (6v6 screen); 285 (50mV ripple)
fourth filter (pre's) 255
preamp plates; 170
No feedback resistor, no grid leak resistor. 68uf bypass caps on both sides of the 12ax7 cathode. The tone control is more of a bass cut/boost.
Two problems: every single 6V6 I've tried (like...10 of them) are microphonic like crazy - is this is really bad string of tubes or can this be caused by the circuit? I also have a hum being introduced between the 12AX7 and 6V6 (Hum goes away if you pull the 6V6 only). 60 or 120Hz, not sure. Scope shows nothing on the input of the 6V6. There is a 50mV ripple on the screen (side note, worth adding a 470ohm screen resistor?)
Sorry...I've been misleading...here is the freshest circuit...note the bridge rectifier, extra filter stage, 3 zeners to help bring the voltage down. There is 5V ripple is present on the first and second filter stage (OT primary), 50mV on the 6v6 screen. http://i60.tinypic.com/2eobp0o.gif
Voltages:
Rectifier: 327
First filter: 322 (5V ripple)
second filter (OT): 304 (5V ripple)
third filter (6v6 screen); 285 (50mV ripple)
fourth filter (pre's) 255
preamp plates; 170
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Move that 40uf cap to the bridge rectifier side of the 100ohm resistor. And then run the 3 zener's on the negative side of bridge rectifier to ground.
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Dropped the zenors complete:
First filter: 330
Second filter/OT: 325
Third filter/6v6 plate: 305
I think the ripple went down to 2V p-p at the first two filters.
This amp is a single input, with a 1/4" output (permanently 8ohm padded) for driving a speaker or another amp, plus a Leslie 147 output complete with speed control. Fired it up with the Leslie and I think the noise level is acceptable for now - though I'd like to improve in the future. Off to the gig for now! Thanks again for everyone's help 🙂
First filter: 330
Second filter/OT: 325
Third filter/6v6 plate: 305
I think the ripple went down to 2V p-p at the first two filters.
This amp is a single input, with a 1/4" output (permanently 8ohm padded) for driving a speaker or another amp, plus a Leslie 147 output complete with speed control. Fired it up with the Leslie and I think the noise level is acceptable for now - though I'd like to improve in the future. Off to the gig for now! Thanks again for everyone's help 🙂
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Can't quite figure out the quote thingy. Explanation: first 2 paragraph is intended as the quote and the next 2 are the replies.
Thanks! I did build it, solid cherry & walnut.
I'm almost certain the 6V6 is causing the noise - its pretty obvious when tapping each tube.
If you're referring to a simulated center tap for the filaments, I've got one in place. Sorry I should have changed that diagram.
I'm almost certain the 6V6 is causing the noise - its pretty obvious when tapping each tube.
If you're referring to a simulated center tap for the filaments, I've got one in place. Sorry I should have changed that diagram.
Adding a 500k back as a grid leak would make the overall grid leak 250k which is close to stock. I think a 6v6 is supposed to have 220k or so max for the grid leak (not sure) and I've never seen over 220k in that spot. Almost any component can be microphonic. I had a microphonic wire once but I've never had a microphonic 6v6.
Are you able to get distortion with the preamp volume maxed and the output pot set low?
Are you able to get distortion with the preamp volume maxed and the output pot set low?
The total grid circuit resistance can be up to 500K, when cathode biased, according to the data sheets. This total includes any grid stopper. I agree, around 220k is commonly used for the grid leak.
Driving my ss 100W yorkville keyboard amp, no distortion is possible. But I was able to get distortion through the Leslie. The original design of this amp was a Herzog clone - I wanted to get a really nice distortion but ended up with too many issues to do with oscillation. The master volume was on the OT secondary (which the Leslie never liked, I would have to run it wide-open).
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