Of course I mean 'upsite down' electrically - not mechanically ...
It can be tested by swapping two wires.
Jan
So looking at your schematics it's not clear to me where your FB resistor is located? Normally off of secondary tap of OT back to first stage? Can you highlight the FB resistor and any comp capacitor going with it. I just got done building my first tube Amp and had to troubleshoot fb issues. I'm assuming the 8.2 ohm on the secondary is the speaker VC?
There is no feedback , 8.2ohm is the dummy load I have. 10 parallel 82 ohm 5w white coffin resistors
Another strange thing is that the frequency response is reasonable flat in triode mode , but is a slope with rising response in Ultralinaer..
I hurt my back and cannot do measurements of the Op tranny just now
Another strange thing is that the frequency response is reasonable flat in triode mode , but is a slope with rising response in Ultralinaer..
I hurt my back and cannot do measurements of the Op tranny just now
Frequency response Triode and UL. I do not have a picture of the oscilloscope but it resembles what is seen in post 485..also ocurs when the level goes above a certain limit
distortion, the crazy thing is that I do not hear any hum or distortion on either,
distortion, the crazy thing is that I do not hear any hum or distortion on either,
At listening level it loom looks this in triode
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I think so, via the chassis connection, Signal to chassis plat at input RCA, power to chassis at transformer? in my hand drawing . cannot see it clearly in photo?
Attachments
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There is feedback to G2 in UL mode.
A rising F-response with frequency points to instability.
The magnitude peak at high frequency has large phase shift and that is where it can oscillate.
Try to look at the freq response out to 100kHz or more, you can see the peak.
Jan
186 ohm total , 51ohm from screen tap to B+. Is this OK or reversed?
PSU grounded to chassis at capacitors
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You could try putting a grid-stop resistor in the grid connection of the 6550 power tube, maybe a 5k6 resistor might do the job, one other thing I would do is this, see how you have a wire going from the power supply node after the 100 Ohm resistor, to the output transformer, I would move that wire to the power supply node before the 100 Ohm resistor, and then run a second wire from the node after that 100 Ohm resistor to the 100 Ohm resistor on the 6550's screen grid, also, I would change that 100 Ohm screen grid resistor to something like 1k, hope that helps.
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Thanks , but not sure I want to do all that , would that 100 ohm tricks not
1. give me more power supply ripple on the B+ power side of the transfomer
2. Reduce /nullify the triode mode?
You comment made me see that I did not mark the existing grid stopper on pin 6 to 5 on 6550 in the schematic, thanks
1. give me more power supply ripple on the B+ power side of the transfomer
2. Reduce /nullify the triode mode?
You comment made me see that I did not mark the existing grid stopper on pin 6 to 5 on 6550 in the schematic, thanks
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If you have a look at the power supply schematic you drew, you've got two 100uF supply bypass capacitors with their + terminals connected to the two ends of an inductor, or choke, I'm assuming it's an iron-core inductor, that basically forms what's called a Pi-Filter, it's common in guitar amplifiers to have one end of the Pi-Filter going to the center-tap of the output transformer, and the other end of the Pi-Filter going to the screen grids of the power tubes, the inductor will smooth-out any ripple going to the screen grids, and will also stop high-frequency signals on the +HT supply getting into the screen grids, I don't know if many people are aware of this, but the screen-grid can also act as a signal-input as well as a screen grid, I've seen a few Marshall amp schematics where the screen grid resistor is 1k not 100 Ohms, but they are schematics for Class AB1 Push-Pull amps, connecting the screen grid to the node between the inductor and the 2k2 resistor will mean that the 6550 tube will be operating as a Pentode in Class A, there's nothing wrong at all about connecting the screen grid to the 6550's plate/anode via a screen grid resistor though.
10H should be more than enough inductance, especially with two 100uF filter caps, most Marshall amps use a couple of dual 50uF/500V can caps with a 5H inductor.
I should mention that I'm no expert when it comes to designing/building tube amps, I'm still learning it all myself.
One thing you can try, assuming you have access to a good oscilloscope, is to have a look at the +HT rails in your amp build with the scope to see if there are any signals that aren't being properly filtered out or bypassed to circuit ground.
So I got 27% UL tap, and grid stopper… . I can live with triode mode only for now I guess. I use it as a headphone amp. Load is a 200 ohm resistor in parallel with a 6/1040 ohm voltage divider give my a amp load of 160 ohms… so I limit the input voltage to not swing anode and screen voltage too high
Am I doing something stupid with this?
Am I doing something stupid with this?
1. In the wiring layout here, the G1 wiring runs parallel and very close to the anode wiring.
Coupling between A and G1 is the way to make an oscillator!
G1 wiring should be as short as possible, an far away from anode wiring.
2. G1 stopper lead can be shorter , and make the stopper more effective. Make it zero length!