Hello, this is my first post, please don't kick me.
This is a 1971 vintage Plush 1060 Bass amp. How exactly can I fine tune the bias of the 6l6 output stages. Obviously there is a variable pot between the plates, but doesn't that just adjust the balance between the two tubes? Should they be biased individually first?
2nd question, This old amp has a recording output. Is it practical to use this output in this day and age? I've completely rebuilt the amp, except this stage and not sure if it's worth while...
This is a 1971 vintage Plush 1060 Bass amp. How exactly can I fine tune the bias of the 6l6 output stages. Obviously there is a variable pot between the plates, but doesn't that just adjust the balance between the two tubes? Should they be biased individually first?
2nd question, This old amp has a recording output. Is it practical to use this output in this day and age? I've completely rebuilt the amp, except this stage and not sure if it's worth while...
Attachments
Yeah, where's the entire schematic?
If you don't want to record, you don't need that tube.
If you do, then you do.
If you don't want to record, you don't need that tube.
If you do, then you do.
The output tubes for each phase are in parallel, and cannot be individually adjusted.
So, how do I set them up right? I know how to do individuals tubes, but parallel I don't
And 2nd question, concerting the recording output? Can I actually use this as a recording out to modern electronics?
And 2nd question, concerting the recording output? Can I actually use this as a recording out to modern electronics?
The original instructions would have said to adjust for a certain DC grid voltage. Pretty crude.
There's no cathode resistor to actually measure the cathode current.
And the two tubes of each phase must be a matched pair.
The cathode follower can put out signal, but with higher distortion, into loads like 10k. Not ideal.
There's no cathode resistor to actually measure the cathode current.
And the two tubes of each phase must be a matched pair.
The cathode follower can put out signal, but with higher distortion, into loads like 10k. Not ideal.
There is no way to balance bias between the paralleled tubes in this case. You just need to have tubes that are matched well enough.
As far as the record feature goes, just make sure there is no offset voltage at the output. If the coupling cap is leaky or the 1M resistor to ground is open you could have enough voltage to break something.
As far as the record feature goes, just make sure there is no offset voltage at the output. If the coupling cap is leaky or the 1M resistor to ground is open you could have enough voltage to break something.
You can certainly adjust each tube's idle current with an additional small cathode resistor for each tube so that you create a quasi-hybrid biasing system. That will also allow you to monitor the currents from then on. You would choose a target bias current, whatever is recommended, then use trial and error with a bit of Ohm's law to pick a resistor value for each tube. The resistor values would need to be kept small enough so that the master bias control would still have enough voltage control range. But with two good tubes within 5% of each other you wouldn't need a lot of cathode biasing control. It's a bit of overkill for an instrument amp, though. A shared sense resistor for monitoring a pair would be good. Otherwise, I guess you now just set the bias voltage and if there is no hum then it's all good....yes?
Welcome to the Tubes / Valves threads of diyAudio (often striving for better sounding, and/or more accurate sounding vacuum tube HI Fi and Stereo amplifiers; Or getting the best sound from cheap components).
The Instruments and Amps threads of diyAudio strives for better sounding guitar amplifiers. Usually better as defined by guitar players, not by Hi Fi nuts.
The desired sound of a Hi Fi Stereo amplifier is often very different than the desired sound of a guitar amplifier.
The easiest way to make your Bass guitar amplifier work reliably and with long life . . .
is to purchase a very well matched Quad of 6L6 tubes (6L6GC is best for long life if the amplifier is driven hard into overload clipping).
. . . Or, if a little less power is OK, then converting from: Adjustable Fixed Bias; To: Individual Self Bias resistors and individual bypass caps across each resistor. That will allow for tubes that are not as closely matched, and essentially becomes "Plug and Play" (just plop in a set of 4 tubes, and you are ready to go). The conversion requires some knowledge of the B+ voltage, and of the grid bias voltage of the original circuit (Ask, it can be answered).
As to how to improve the sound, those guitar amplifier fans that read both the Tubes / Valves threads, and Instruments and Amps threads would be the best ones to answer.
It might be helpful to all if you tell us what the sound is like now, and what you want it to sound like when you modify it.
One example: the bass is very solid, you want it to sound more flabby and uncontrolled.
Another example: the bass is far too distorted at medium volume, you want it sound more smooth and warm there.
etc.
No target to hit, nothing gets done.
I hope that helps.
Have Fun!
The Instruments and Amps threads of diyAudio strives for better sounding guitar amplifiers. Usually better as defined by guitar players, not by Hi Fi nuts.
The desired sound of a Hi Fi Stereo amplifier is often very different than the desired sound of a guitar amplifier.
The easiest way to make your Bass guitar amplifier work reliably and with long life . . .
is to purchase a very well matched Quad of 6L6 tubes (6L6GC is best for long life if the amplifier is driven hard into overload clipping).
. . . Or, if a little less power is OK, then converting from: Adjustable Fixed Bias; To: Individual Self Bias resistors and individual bypass caps across each resistor. That will allow for tubes that are not as closely matched, and essentially becomes "Plug and Play" (just plop in a set of 4 tubes, and you are ready to go). The conversion requires some knowledge of the B+ voltage, and of the grid bias voltage of the original circuit (Ask, it can be answered).
As to how to improve the sound, those guitar amplifier fans that read both the Tubes / Valves threads, and Instruments and Amps threads would be the best ones to answer.
It might be helpful to all if you tell us what the sound is like now, and what you want it to sound like when you modify it.
One example: the bass is very solid, you want it to sound more flabby and uncontrolled.
Another example: the bass is far too distorted at medium volume, you want it sound more smooth and warm there.
etc.
No target to hit, nothing gets done.
I hope that helps.
Have Fun!
Sorry for not getting back. Daughter had a baby and I was helping to get the family settled. Food, baby sitting the older child.. etc
Sound is fine, just some background hum. Thought it was bias related, but turned out to be a wire that should have been shielded. Concerning the recording output. I measured DC offset and it's less than .003 volts. AC is zero. I'm going to plug it into my Tascam recorder to determine any output noise. Planning on sending this across country to my son, who plays bass. I think he will probably hope to use the DI output to Abelton. I bought this amp in 1970, played it a few years and put it in storage. However, 50+ years in storage is not good for a tube amp. Basically I had to replace most everything including the power cord and death cap.
Sound is fine, just some background hum. Thought it was bias related, but turned out to be a wire that should have been shielded. Concerning the recording output. I measured DC offset and it's less than .003 volts. AC is zero. I'm going to plug it into my Tascam recorder to determine any output noise. Planning on sending this across country to my son, who plays bass. I think he will probably hope to use the DI output to Abelton. I bought this amp in 1970, played it a few years and put it in storage. However, 50+ years in storage is not good for a tube amp. Basically I had to replace most everything including the power cord and death cap.
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