Did you try reversing the OPT primary leads for the channel that was oscillating before? Was it the right?
jeff
jeff
The amplifier schematic you posted is either an original circuit, or a modified circuit of the original circuit.
Those amplifiers with 6V6 or EL84, or 6BQ5 . . . are time tested and proven (except for some modifications the show up on some schematics).
Those amplifiers, with good parts, proper connections, matched output tubes, and attention to details will have much flatter frequency response into an 8 Ohm load resistor . . .
Versus the frequency response of 90% of the loudspeakers in home stereo installations.
Of course, most loudspeakers have impedance versus frequency that anything and everything larger and smaller than 8 Ohms.
If in order to get bass you can easily hear (or even feel), then if. you have to turn up the volume until the amplifier clips, you will get so much harmonic distortion that your ears will begin to hurt.
Questions to answer are:
What output transformers, give specs, model, and manufacturer . . . are you using?
What loudspeaker model, specs, and manufacturer, are you using?
Oh . . . you never reported the resistance of the primary windings, plate to center tap to plate; and UL tap to center tap, to UL tap.
Why not do that . . . Because . . .
If by mistake you connected the output tube plates to the UL taps . . .
You will get no Bass, low gain and low output power before clipping, and even before clipping it will have more high frequency output versus bass and low midrange frequencies.
With good output transformers, you should be able to make this amplifier sound really good on many many loudspeaker models, but not all loudspeaker models.
Once you get it working well, then you can try Triode wiring the output tubes if you want to (lower power, but in some cases less distortion, and higher damping factor, especially if you remove the global negative feedback).
Those amplifiers with 6V6 or EL84, or 6BQ5 . . . are time tested and proven (except for some modifications the show up on some schematics).
Those amplifiers, with good parts, proper connections, matched output tubes, and attention to details will have much flatter frequency response into an 8 Ohm load resistor . . .
Versus the frequency response of 90% of the loudspeakers in home stereo installations.
Of course, most loudspeakers have impedance versus frequency that anything and everything larger and smaller than 8 Ohms.
If in order to get bass you can easily hear (or even feel), then if. you have to turn up the volume until the amplifier clips, you will get so much harmonic distortion that your ears will begin to hurt.
Questions to answer are:
What output transformers, give specs, model, and manufacturer . . . are you using?
What loudspeaker model, specs, and manufacturer, are you using?
Oh . . . you never reported the resistance of the primary windings, plate to center tap to plate; and UL tap to center tap, to UL tap.
Why not do that . . . Because . . .
If by mistake you connected the output tube plates to the UL taps . . .
You will get no Bass, low gain and low output power before clipping, and even before clipping it will have more high frequency output versus bass and low midrange frequencies.
With good output transformers, you should be able to make this amplifier sound really good on many many loudspeaker models, but not all loudspeaker models.
Once you get it working well, then you can try Triode wiring the output tubes if you want to (lower power, but in some cases less distortion, and higher damping factor, especially if you remove the global negative feedback).
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Polk Monitor 60 speakers, 8 ohm, bought 2010
Edcor CXPP10-600-8K 10W with 4&8ohm outputs
I had oscillation earlier, after wiring the 0ohm to ground, and I swapped the P1/SG1&P2/SG2 leads going into the PCBs, stopping that awful noise.
Edcor CXPP10-600-8K 10W with 4&8ohm outputs
I had oscillation earlier, after wiring the 0ohm to ground, and I swapped the P1/SG1&P2/SG2 leads going into the PCBs, stopping that awful noise.
dubadub,
Thanks!
Ooops, I re-edited my post # 43, right after you posted # 44.
You might want to re-read my post # 43.
And, try connecting those Polk monitor 60 8 Ohm speakers on your amplifier's 4 Ohm tap.
I suspect that the parallel woofers minimum impedance goes down to 5, 4, or even 3 Ohms at low bass frequencies.
Take an Ohmmeter and measure the DC resistance of the speaker (might be very revealing).
Sometimes the manufacturers mix up the output transformer lead colors (seldom, but true).
That is a good reason to verify with an Ohmmeter.
Thanks!
Ooops, I re-edited my post # 43, right after you posted # 44.
You might want to re-read my post # 43.
And, try connecting those Polk monitor 60 8 Ohm speakers on your amplifier's 4 Ohm tap.
I suspect that the parallel woofers minimum impedance goes down to 5, 4, or even 3 Ohms at low bass frequencies.
Take an Ohmmeter and measure the DC resistance of the speaker (might be very revealing).
Sometimes the manufacturers mix up the output transformer lead colors (seldom, but true).
That is a good reason to verify with an Ohmmeter.
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Not familiar with this OT, but it's possible it's too small for good bass response.
Bass response depends on primary inductance which in turn depends on core size and number of primary turns.
What data is available?
I measured the b+ to plate and got 204-174 ohms on one pcb and 190-180 ohms on the other, and the screens were all around 75 ohms.
the Edcor CXPP10-8K are rated at 10w and 20Hz-20kHz <1dBu
the Edcor CXPP10-8K are rated at 10w and 20Hz-20kHz <1dBu
Rumbling? . . .
Good Bass?
Bad Bass?
Bass oscillations?
UL taps of 40% x 174 to 204 Ohms = 69.6 to 81.6 Ohms.
Sounds like the transformer P and UL wires are properly identified.
After the amplifier is powered down . . . Do not forget to disconnect the loudspeaker and measure the DCR with the Ohmmeter.
Just nice to know what the number is.
Then do not forget to re-connect the speakers.
Good Bass?
Bad Bass?
Bass oscillations?
UL taps of 40% x 174 to 204 Ohms = 69.6 to 81.6 Ohms.
Sounds like the transformer P and UL wires are properly identified.
After the amplifier is powered down . . . Do not forget to disconnect the loudspeaker and measure the DCR with the Ohmmeter.
Just nice to know what the number is.
Then do not forget to re-connect the speakers.
Whoops, I mean, the correct amount of bass. Sounds right. I need to measure all my speakers' ohm readings? This is the first amp I've built with 4&8 ohm taps, now I gotta go back to all those other amps and solder the 4 ohm wires to the + lug and give a listen?
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CXPP10-600-8K is a 8K:600 ohm, are you sure that's the model you are using? That would explain why you have no bass.
jcalverez,
You made the same mistake I made.
At the Edcor web site:
The 600 Ohm secondary is a 'Pull Down Entry Field'.
Click there and you can Select the 4 & 8 Ohm secondary model of that transformer.
You made the same mistake I made.
At the Edcor web site:
The 600 Ohm secondary is a 'Pull Down Entry Field'.
Click there and you can Select the 4 & 8 Ohm secondary model of that transformer.
I'd be kicking myself if opting for the $20 cheaper 10w model over the 25w cost me good bass. A friend is about to build the same circuit with the 25w, we'll see how it's different. Thanks guys
The low frequency (bass) of a push pull transformer is always better if the push and pull tubes quiescent DC currents are equal.
The Polk speakers measure 5.5 and 5.6 ohms.
My regular amp is a SE amp with 6B4G output tubes, and I've always used these speakers with the 8-ohm tap connected. I'm going to have to put the amp on the bench to bring the 4-ohm wires to the output terminals to see how they sound.
Interesting, I also have a pair of KRK 7000B that sounded like crap with that amp, I figured they'd measure closer 4 ohms, but no, they measured 10 ohms 🤔 but sound great with a SE EL84 amp w 8-ohm outputs
My regular amp is a SE amp with 6B4G output tubes, and I've always used these speakers with the 8-ohm tap connected. I'm going to have to put the amp on the bench to bring the 4-ohm wires to the output terminals to see how they sound.
Interesting, I also have a pair of KRK 7000B that sounded like crap with that amp, I figured they'd measure closer 4 ohms, but no, they measured 10 ohms 🤔 but sound great with a SE EL84 amp w 8-ohm outputs
Managed to hook this cheap little 'scope up to the amp across an 8-ohm 10w resistor, one across each output but just measuring one channel. Using a signal generator on my phone, I got this:
a nice clean some wave at the highest level output I could get from the phone's headphone jack, with the amp's pot wide open. I switched over to a square wave, and the amp, or maybe the resistors, started chirping. Here's that wave:
so now I need to adjust the Fb phase cap(?) to get that wave square-shaped? I think that's what someone was saying earlier.
Is a 8-ohm resistor alone a good dummy load, should there be a cap in there?
Now that I can put this DSO138 scope on the amp, how do I measure the overall frequency response?
Thanks!
a nice clean some wave at the highest level output I could get from the phone's headphone jack, with the amp's pot wide open. I switched over to a square wave, and the amp, or maybe the resistors, started chirping. Here's that wave:
Is a 8-ohm resistor alone a good dummy load, should there be a cap in there?
Now that I can put this DSO138 scope on the amp, how do I measure the overall frequency response?
Thanks!