Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on May 06, 2013, 02:54:53 pm
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I have met this issue earlier and noticed it again. Should the circuit of the channel 2 be as it has been drawn
in the schematic below. The channel has a huge amplification if I use these values. The 470k resistor there doesn't damp
the signal enough. The vol pot is totally unusable since already a little opening causes a very loud signal.
http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/newamps/matchless_dc30_old.pdf (http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/newamps/matchless_dc30_old.pdf)
/Leevi
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I added that DC30 EF86 preamp to my Matchless Lightning and I don't have the gain problem you describe. The only significant difference between mine and yours is that I used a 220K plate resistor and you used a 330K plate resistor. Yours will have more gain based on that larger resistor.
The way that 470K is connected will not knock the signal down very much. If using a smaller plate resistor doesn't get an acceptable amplification factor, consider moving the 470K. Put it between the tone switch common and the top of the volume pot. That will definitely knock the signal down.
Is this a stock DC30 or did you build this amp?
Here's my schematic for you to compare...
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Are you sure you aren't accidentally using a linear taper pot for the volume?
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Is this a stock DC30 or did you build this amp?
Thanks for the schematic sluckey. This is my build.
Are you sure you aren't accidentally using a linear taper pot for the volume?
It's audio pot, I even changed it since I thought there is some fault in the first one I used.
I think a correct ground resistor after the pot should be about 10k in order to get the volume under control.
/Leevi
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Excellent schematics here. is there any reason why most every schematic that i see with an EF86 has the rotary switch with various caps instead of a tone control network like Fenders and Marshalls? is there too much signal loss using it with an EF86? Thanks, JMac
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I think a correct ground resistor after the pot should be about 10k in order to get the volume under control.
That would be like having a 10k grid reference resistor for a 2nd gain stage to get the 1st stage's volume under control. That 10k is a very heavy load to the preceding stage.
Sluckey already provided the two "organic" ways to tame the volume: reduce EF86 gain by changing the plate load, or move the resistor to increase the voltage division of the pot (and create some volume loss even at maximum volume control setting).
... The channel has a huge amplification if I use these values. The 470k resistor there doesn't damp the signal enough. The vol pot is totally unusable since already a little opening causes a very loud signal.
Does the amp continue to get louder/more distorted as you continue to turn up the volume control? If yes, does that mean you just don't get a clean quiet sound at minimum volume setting?
I ask because this amp is meant to be played loud and distorted. It's not a bedroom practice amp, and the point of the EF86 channel is to be raucous. I'm thinking the amp may be working as intended.
It is possible that your volume control doesn't have a smooth audio taper especially at the low end. They're expensive, but when I've used Bourns conductive plastic pots, they've had a smooth taper at all settings. But I also wonder about people who build a big amp and want to play it just above zero... :dontknow:
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I think a correct ground resistor after the pot should be about 10k in order to get the volume under control.
That doesn't make sense to me. Why use such a large plate resistor that will give you more signal volume and then choke it by brute force loading with such a small resistor?
My volume control behaves just as I expected it on my modified Lightning. BTW, adding the EF86 preamp to the Lightning pretty much makes it a DC30, but with only a 15 watt output. Compare the schematics.
Is it possible that you have a wiring error? Have you tried a 220K plate resistor to see if that gets what you want?
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is there too much signal loss using it with an EF86?
Yes, even a two pots tone stack is challenging. A modified Baxandall stack has worked best IMO.
Sluckey already provided the two "organic" ways to tame the volume: reduce EF86 gain by changing the plate load, or move the resistor to increase the voltage division of the pot (and create some volume loss even at maximum volume control setting).
I tried Sluckeys way and it helped little bit but not remarkable.
Does the amp continue to get louder/more distorted as you continue to turn up the volume control? If yes, does that mean you just don't get a clean quiet sound at minimum volume setting?
It continues to get louder but what is weird the signal is very clean even with higher volume.
Finally it starts to oscillate since the sound is so loud.
It's not a bedroom practice amp
That's really true.
Is it possible that you have a wiring error?
Hopefully not, I have doublechecked the wiring. I remember a corresponding case from past and I ended up to
replace the cap switch with a tone control that caused the necessary loss.
Have you tried a 220K plate resistor to see if that gets what you want?
Yes I tried see above.
Do you think that different EF86s can have different gain factors? I'm using Svetlana.
/Leevi
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Do you think that different EF86s can have different gain factors? I'm using Svetlana.
I don't know if it is the problem, but yes different EF86's could have differing gains.
In a triode, Mu is a relatively constant characteristic. Gain is pretty stable and predictable.
In a pentode, gain is Gm * plate load resistor. If you happen to have a tube with very, very much higher Gm than typical, it will also have more gain in a given circuit.
Gm increases as plate current increases. The fastest way to change idle plate current in a pentode (other than by changing control grid voltage) is to change screen voltage. Increase the screen resistor, and you should reduce idle current and knock down the pentode's gain some.
Still, the difference in your experience and Sluckey's with essentially the same circuit makes me wonder.
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What is your G2 voltage?
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Still, the difference in your experience and Sluckey's with essentially the same circuit makes me wonder.
Me too
The situation improved little bit when I got the Master and CUT installed. Now it can be somehow tested when
the volume is under control. But this is not the solution. By selecting a smaller coupling cap via the switch the volume is little bit lower.
One way to solve this is to decrease the 470k resistor and try in that way to find a suitable signal level.
What is your G2 voltage?
I'll come back to that after I'm back home from the job (European time).
/Leevi
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What is your G2 voltage?
Measured voltages:
B+: 340v
G2: 136v
Plate: 100v
Cathode: 2.8V
/Leevi
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Here's another amp that uses the same EF86 circuit that I put in the lightning. It has voltages on it that may be of some interest. I have no problems with the volume control on this one either. Notice there is no 470K on the volume pot wiper. This is a dirty sounding amp and it gets dirty very early. Think Neil Young Cinnamon Girl.
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Notice there is no 470K on the volume pot wiper.
Yes, but the Vol pot is 500k instead of 1M. Furthermore the PI is different.
Also the "D" voltage is much lower (268v) than in my case (340v).
Sluckey, do you have any voltage values for the circuit you posted in your first reply?
/Leevi
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What is your G2 voltage?
Measured voltages:
B+: 340v
G2: 136v
Plate: 100v
Cathode: 2.8V
Also the "D" voltage is much lower (268v) than in my case (340v).
This seems key.
The ratio of screen voltage to plate voltage is slightly higher in your case than in Sluckey's (probably due to the 330k plate load), but is reasonably close. The higher supply voltage yields a higher plate voltage, and may lead to a higher possible output voltage from the EF86.
So... perhaps feeding the EF86 a lower supply voltage would reduce output, without the need to chop it down afterwards. Again, this is another suggestion for how to reduce gain and/or output at the EF86, rather than loading it down after the fact.
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Sluckey, do you have any voltage values for the circuit you posted in your first reply?
The only voltages for my lightning are what you see on the schematic. I never recorded the pin voltages for the EF86.
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So... perhaps feeding the EF86 a lower supply voltage would reduce output
Yes, I think so
I correct my earlier information regarding the measured voltages: 340v is not B+ but supply voltage for EF86.
B+ is 376v.
I'm using now Sluckey's resistor values and have still that much higher voltages there.
One clarification is that I'm using the Matchless' type power supply which feeds the preamp with higher voltages.
/Leevi
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Are you able to record a sample? It still seems possible that your "too much gain" will be these other guys' "yeah that's what it sounds like." Or it will reveal that there is indeed something unexpected going on.
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Are you able to record a sample?
Unfortunately not now.
There is a demo in youtube of similar type of amp. There are probably same kind of issues
with the volume control in channel 2 since the guy plays through the low-sensitive input and the master
is all the time just a little bit open.
Matchless DC-30 Clone by CeriaTone (http://www.youtube.com/watch?v=3FC3G6rwHiw#ws)
/Leevi