Nobsound 6p1 Amp flaws/mods

[Edutin]

Hello @StepheK I am getting ready to do the modifications you have recommended for this amp but I would also like to enable the 4 Ohms taps to use it with my 4 Ohms speakers (Klipsch KG 2 FB). I opened the amp and there are two "yellow" cables that are not connected, are those the ones for the 4 Ohms taps? and if so, where do they go? Please help!
Also, after the mods are complete, how bad would it be if I connect those speakers to the 8 Ohms taps? Thank you

 

[Dr_Mick51]

Hello @StepheK I am getting ready to do the modifications you have recommended for this amp but I would also like to enable the 4 Ohms taps to use it with my 4 Ohms speakers (Klipsch KG 2 FB). I opened the amp and there are two "yellow" cables that are not connected, are those the ones for the 4 Ohms taps? and if so, where do they go? Please help!
Also, after the mods are complete, how bad would it be if I connect those speakers to the 8 Ohms taps? Thank you

It is my understanding that the amplifier design is Parallel Single Ended. But the output transformers are 5kOhm impedance which are ideal for a Single Ended design. So in this case the 4ohms taps are not recommended because even if you use 4ohms speakers , for the tubes to see the right impedance on the OT the 8ohm taps are required.
It is something like that. @StepheK can elaborate more on the explanation.

 

[Edutin]

Thank you @Dr_Mick51 for the quick response. So it should work with the 4ohms speakers as it is? Sorry, this is my first tube amp.

 

[FlaCharlie]

I would also like to enable the 4 Ohms taps to use it with my 4 Ohms speakers (Klipsch KG 2 FB). . . Also, after the mods are complete, how bad would it be if I connect those speakers to the 8 Ohms taps?

It is my understanding that the amplifier design is Parallel Single Ended. But the output transformers are 5kOhm impedance which are ideal for a Single Ended design. So in this case the 4ohms taps are not recommended because even if you use 4ohms speakers , for the tubes to see the right impedance on the OT the 8ohm taps are required.

I don't think she ever actually measured the OTs, perhaps she will either confirm this or correct me. As I recall, she speculated that the OTs in the amp looked exactly like OTs being sold as having 5k primaries.

Dr Mick's comments are correct.

The tubes, when used in SE configuration, have an ideal load impedance of 5k. When used in PSE, parallel SE, the ideal load impedance would be half that, 2.5k.

That said, the actual load used by manufacturers has always varied quite a bit and using the exact "ideal" load isn't absolutely necessary. And, in the real world, the load varies as music is playing because the actual impedance of a speaker changes with frequency. The "nominal impedance" of a speaker is just that.

If we assume that the OT primary is 5k with an 8 ohm load, then if you use a 4 ohm speaker on the 8 ohm tap it effectively cuts the primary impedance in half which, technically, should work better than using 8 ohm speakers.

If 4 ohm speakers were used on the 4 ohm tap, the primary impedance would be 5k. If 8 ohm speakers are used on the 4 ohm tap, the primary impedance effectively doubles to 10k.

 

[Edutin]

@FlaCharlie Thank you for the additional information! I appreciate the group's knowledge and advise.

 

[mudkicker78]

Hello @StepheK I am getting ready to do the modifications you have recommended for this amp but I would also like to enable the 4 Ohms taps to use it with my 4 Ohms speakers (Klipsch KG 2 FB). I opened the amp and there are two "yellow" cables that are not connected, are those the ones for the 4 Ohms taps? and if so, where do they go? Please help!
Also, after the mods are complete, how bad would it be if I connect those speakers to the 8 Ohms taps? Thank you

Honestly I don't think you need to change to the 4 ohm tap. I have been running a set of Wharfedale D310/320 speakers that are 4 ohm (yes I have both sets). They don't stress the amplifier and I think that has more to do with the fact you are not reaching power requirements the speakers ask for. Having said that, they are 87db sensitive rating and easily hit 98db in my listening area, which for me is more than enough.

 

[Edutin]

I completed the mods and the amp was working great for about a week until last night. The sound cut off for a few seconds then it came back but at a lower volume. Now I have to set the volume almost to the max and the sound is a bit distorted. Also, the tubes are dim, not glowing as bright as they used to. What could have gone wrong? and how do I fix it? any suggestions please?

 

[StepheK]

I completed the mods and the amp was working great for about a week until last night. The sound cut off for a few seconds then it came back but at a lower volume. Now I have to set the volume almost to the max and the sound is a bit distorted. Also, the tubes are dim, not glowing as bright as they used to. What could have gone wrong? and how do I fix it? any suggestions please?

My guess would be a solder joint failed. Make sure there is no voltage inside, then check every joint carefully.

 

[Edutin]

Thank you @StepheK, I will check the joints later today when the caps are fully drained.

 

[Edutin]

@StepheK Your knowledge is greatly appreciated, I checked and resoldered all the joints I worked on when doing the modification and the amp is back to normal! Moving on to the next mod! Thank you!!!!

 

[maddmaxxdrum]

Also, I'd be concerned about drawing any more ma through the output tubes. They are being run just a little over 1/2 power as designed and even at that the power transformer runs pretty hot.

What is worth doing it changing up the bias setup on the upper section of the driver tube cascode from "auto" to fixed. That makes a very visible on a scope, and audible improvement in headroom and distortion. This only requires moving two 1M resistors and adding a couple of 2M ones.

hi luv your work. Thx for sharing. I have a question
I did the mod to the driver tube but it was a different method. Some one had helped me with that in another forum. He called it a self bias resistor. Here’s what he said: “there needs to be a self bias resistor in the top triode of the cascode amplifier (between the cathode and junction of the bottom of the 1Meg grid resistor and plate of the bottom triode) other wise there will be clipping.
His method adds a 2.2k resistor and 100uf cap in parallel in the top triode. Is this the same concept as your 2M ohm resistors?

Thx

 

[Dr_Mick51]

hi luv your work. Thx for sharing. I have a question
I did the mod to the driver tube but it was a different method. Some one had helped me with that in another forum. He called it a self bias resistor. Here’s what he said: “there needs to be a self bias resistor in the top triode of the cascode amplifier (between the cathode and junction of the bottom of the 1Meg grid resistor and plate of the bottom triode) other wise there will be clipping.
His method adds a 2.2k resistor and 100uf cap in parallel in the top triode. Is this the same concept as your 2M ohm resistors?

Thx

It looks like self-biased parallel triode, each triode with its own cathode resistor and bypass capacitor. That's a different mod. Do you have a schematics for the mod?

 

[StepheK]

It looks like self-biased parallel triode, each triode with its own cathode resistor and bypass capacitor. That's a different mod. Do you have a schematics for the mod?

Right, that is no longer a cascode and honestly, this amp doesn't need a cascode'd 6SN7 to drive a 6P1 tube.

 

[Dr_Mick51]

Right, that is no longer a cascode and honestly, this amp doesn't need a cascode'd 6SN7 to drive a 6P1 tube.

I saw that for your Reisong A12 mods you replaced a parallel triode with a single hard-driven triode. Is it possible to do the same on this amp?. One single triode driving the signal to output tubes.
And the same for the output tubes. Instead of two tubes in parallel can we use one tube only?. This reduces the chances of unbalances/differences on each output tube. I really don't care much about power because I know I only need 50mW for each channel and that's enough for me. So, even if power is reduced it won't affect me much.

 

[maddmaxxdrum]

It looks like self-biased parallel triode, each triode with its own cathode resistor and bypass capacitor. That's a different mod. Do you have a schematics for the mod?

Hi, here is the schematic.in the cascode section, circled in red, u can see the 2.2k/100uF network on top triode added. He also added some 1K resistor in a few spots. Something about parasitic noise.
And in the power tubes section he added a separate self bias resistor and bypass cap for each tube. But I think u were saying this is not good due to more current draw?

 

[maddmaxxdrum]

Right, that is no longer a cascode and honestly, this amp doesn't need a cascode'd 6SN7 to drive a 6P1 tube.

Hi, here is is explanation for the cascode mod

 

[StepheK]

Hi, here is the schematic.in the cascode section, circled in red, u can see the 2.2k/100uF network on top triode added. He also added some 1K resistor in a few spots. Something about parasitic noise.
And in the power tubes section he added a separate self bias resistor and bypass cap for each tube. But I think u were saying this is not good due to more current draw?

I am planning on doing a "round 2" on this amp and plan to try a single triode driver and a single output tube wired as a pentode.

 

[maddmaxxdrum]

Right, that is no longer a cascode and honestly, this amp doesn't need a cascode'd 6SN7 to drive a 6P1 tube.

Hi. So are you saying it’s ok to leave this mod ? Is having both your driver mod using 2Meg resistors and this mod ok?
Or just revert to original and add only your driver mod with the 2Meg resistors?

 

[Dr_Mick51]

Hi. So are you saying it’s ok to leave this mod ? Is having both your driver mod using 2Meg resistors and this mod ok?
Or just revert to original and add only your driver mod with the 2Meg resistors?

Do you have access to an oscilloscope? even a cheap one($50) from Amazon will allow you to see the signal. If the signal is not distorted at maximum volume then you should be ok. You don't want to amplify a distorted signal on the output tubes.

You also want to pay attention to the signal Gain. How much AC voltage you get from the drivers for each AC Volt from the input signal

 

[maddmaxxdrum]

Do you have access to an oscilloscope? even a cheap one($50) from Amazon will allow you to see the signal. If the signal is not distorted at maximum volume then you should be ok. You don't want to amplify a distorted signal on the output tubes.

You also want to pay attention to the signal Gain. How much AC voltage you get from the drivers for each AC Volt from the input signal

Hi, no I don’t have one. My main dilemma here is I had followed a mod in another forum which was adding the 2.2k/100uF network to the upper triode of the driver tube. The poster’s explanation for this is in the pic attached…But then I saw StepheK’s 2Meg resistor mod to the driver tube and was wondering which is more suited for this circuit, or if both together are ok. And also about the power tube individual self bias resistor/cap mod. StepheK had mentioned this was not good because it will increase milliamp draw on the power transformer. But I did that mod. Not knowing this. I only saw StepheK’s videos after…I’ve run the amp for hrs and hrs. No issue. So I wonder if it really is an issue.
The transformer does get hot. I’m running it at 110vac from APC line-R. i have a little fan right above the amp so it helps.