[SonicTrance]

Anyone know how to remove the low/high gain switch?

Since using this switch I noticed that the resistor right next to it, the 390K resistor which reads 10K on low gain, has gone off so I checked the switch itself and found one of its resistors is not right probably causing an abnormally high reading on the 390K resistor.

Hm, it should read about 80k on low gain as it then has a 100k in parallell with it. And obviously 390k on high gain. As for removing the switch you can just desolder it and then use fixed resistors.

 

[klnglim]

Anyone know how to remove the low/high gain switch?

Since using this switch I noticed that the resistor right next to it, the 390K resistor which reads 10K on low gain, has gone off so I checked the switch itself and found one of its resistors is not right probably causing an abnormally high reading on the 390K resistor.

May be the resistor and the switch is parallel connection, the switch itself has resistance, that's why u will see the different reading in between high and low gain, but 390K is too high.I also notice my MK8se resistor 470K around that area all have to replace the new one, u need to desolder the resistor to read the right value. The switch is very easy to take off if u don't like it

Just finished fix all the damage parts, now need some times to wait for some replacement component to arrive

 

[SonicTrance]

Looking at those pics the feedback resistors for the mk8 are 100k and 68k. So 100k for high gain (low NFB) and 40k for low gain (high NFB). That’s a fair bit more NFB than the mk6’s 390k and 80k.

 

[klnglim]

Looking at those pics the feedback resistors for the mk8 are 100k and 68k. So 100k for high gain (low NFB) and 40k for low gain (high NFB). That’s a fair bit more NFB than the mk6’s 390k and 80k.

However after solder back 100k on PCB, the reading isn't 100k anymore from DMM and will have little bit differential of value as well if switch between high & low gain. (The rest of the resistor's value remain unchange even after solder back on PCB)

 

[baronbeehive]

However after solder back 100k on PCB, the reading isn't 100k anymore from DMM and will have little bit differential of value as well if switch between high & low gain. (The rest of the resistor's value remain unchange even after solder back on PCB)

Correct, sorry if I mislead you when I gave you those resistor readings, they were the measured values, the board values are sometimes different as you say.

Glad to see you are making progress with the repairs, looks like you've got those eyelets in and traces fixed nicely!

 

[baronbeehive]

Looking at those pics the feedback resistors for the mk8 are 100k and 68k. So 100k for high gain (low NFB) and 40k for low gain (high NFB). That’s a fair bit more NFB than the mk6’s 390k and 80k.

My 390K resistors are in fact 360K but are quite a bit lower than that when measured out of the board. The 390k and 80K in parallel give the 66K that I see in the 80K resistor on high gain but the 390K is connected to the gain switch because it reads 27K in high gain. In low gain the readings for those 2 resistors are both 10K. The 2 resistors in the gain switch appear to be both 100k, (from readings), but the faulty one causing the bad reading in the 390K resistor reads 125K.

Thanks guys for the info on the gain switch, I thought it might clip in like the opamps. I believe Maxx said that those switches were suspect and I agree after what I've found after changing from the gain.

All this has put back my listening tests with the different anode/cathode/gain settings. However after trying high gain I agree with Maxx that high gain sounds slightly better so I will be using that setting from now on. The volume knob settings are still useable, you only have to turn down a couple of notches to get the same level as on low gain.... bloodhawk will not notice as he likes extreme volume levels lol.

I've take inspiration from the hard work of kinglim and bloodhawk and decided it was time to renew the PSU resistors, diodes and transistors..... (I had to do this anyway because I stupidly shorted out the amp when taking voltage readings with a shaky hand..... I should have taken Maxx's advice and inhaled the solder smoke to steady myself LOL!

Edit: can you correct a possible misconception, is going from 100K to, say, 50K lower, or higher, I might be using the wrong terminology.

 

[baronbeehive]

Sorry, double post.

 

[klnglim]

Correct, sorry if I mislead you when I gave you those resistor readings, they were the measured values, the board values are sometimes different as you say.

Glad to see you are making progress with the repairs, looks like you've got those eyelets in and traces fixed nicely!

As myself just have a knowledge of the basic electronic and never study deeply, so need some times to understand everything inside the amp.
It's part of my hobby, I really enjoy to spend lot of times to learn and is great experience for self improvement.
Anyway, thanks for all the members from this mods forum for their wonderful sharing experience.
I also plan to build another P2P MK8se as I have all the spare component left behind

 

[SonicTrance]

Edit: can you correct a possible misconception, is going from 100K to, say, 50K lower, or higher, I might be using the wrong terminology.

Higher feedback resistance is lower NFB and lower feedback resistance is higher NFB. Its logical when you think about it If there’s lower resistance more of the signal gets fed back, hence higher NFB.

 

[baronbeehive]

I've desoldered the switch and am trying to figure it out.

Below are the connections to the switch which is ON-OFF.

My values for high gain for the 390K and 68K resistors are 27K and 68K, the values for low gain are 10K and 10K.

My problem is seeing how these values are arrived at from the connections below, can anybody see how to arrive at the above values which I believe are correct because they are the same for 3 of the 4 circuits, wheras the values for the 4th circuit are 37K and 68K!!!

There is also resistance in the switch position but it must be built into the board because I measured 100K from the 2 points on the board with the switch out.

I believe my problem with the 390K resistor reading is due to the switch not fully opening or closing properly, but until I know how to arrive at the correct values I can't bypass the switch with fixed resistors.

I'm just having trouble seeing what is in parallel to what here!

However I think with the switch out the connections are configured to high gain because on low gain the switch is on and without the switch in place it would be off.

 

[klnglim]

I've desoldered the switch and am trying to figure it out.

Below are the connections to the switch which is ON-OFF.

My values for high gain for the 390K and 68K resistors are 27K and 68K, the values for low gain are 10K and 10K.

My problem is seeing how these values are arrived at from the connections below, can anybody see how to arrive at the above values which I believe are correct because they are the same for 3 of the 4 circuits, wheras the values for the 4th circuit are 37K and 68K!!!

There is also resistance in the switch position but it must be built into the board because I measured 100K from the 2 points on the board with the switch out.



I believe my problem with the 390K resistor reading is due to the switch not fully opening or closing properly, but until I know how to arrive at the correct values I can't bypass the switch with fixed resistors.

I'm just having trouble seeing what is in parallel to what here!

However I think with the switch out the connections are configured to high gain because on low gain the switch is on and without the switch in place it would be off.

I hope below pictures might help soft your problem?
If need anymore info let me know, hope u can figure out the way

 

[SonicTrance]

My values for high gain for the 390K and 68K resistors are 27K and 68K, the values for low gain are 10K and 10K.

My amp uses 390k and 100k, not 68k like you say? 390k is for high gain and 390 parallell with 100 = 80k is for low gain. I have not measured these resistors when soldered on the board though.

I believe my problem with the 390K resistor reading is due to the switch not fully opening or closing properly, but until I know how to arrive at the correct values I can't bypass the switch with fixed resistors.

If you have a connection problem like that you'd hear it.

However I think with the switch out the connections are configured to high gain because on low gain the switch is on and without the switch in place it would be off.

Yes, just measure resistance from input tube grid to the output. Look at my schem here:

 

[klnglim]

My amp uses 390k and 100k, not 68k like you say? 390k is for high gain and 390 parallell with 100 = 80k is for low gain. I have not measured these resistors when soldered on the board though.


If you have a connection problem like that you'd hear it.


Yes, just measure resistance from input tube grid to the output. Look at my schem here:

May I know how many watts recommendation for the resistor to put there?

 

[baronbeehive]

Thanks guys! More of this in the morning.

Hey kinglim, that lighting idea is good, I was wondering if it might help all of us if you could take some good closeups of the board top and bottom to reveal the traces. This was a problem are for us at the start of this thread and with all the components in place it isn't easy to see them all.

Maybe Maxx would let you put them on the first page.

 

[MrCurwen]

May I know how many watts recommendation for the resistor to put there?

Let's say the signal swing at the junction of RL and R16 is 50VPP. This would mean the differential (meaning, actual voltage seen by the load) voltage across the load would be an astounding 100VPP, completely crushing your ears and burning to death your headphone speaker coils. So in other words, enough voltage.

This voltage is between the mentioned junction and circuit ground. The path to ground from that junction that goes thru the feedback resistor R16 is either 390k + 10k or 80k + 10k depending on the setting. So 400k or 90k.

Let's use Ohm's Law: 50V divided by 400k means current of 0.125 mA. Combined dissipation thru that path is then 0.125 mA times 50V which means 0.00625 W. Divide that by 400k and multiply by 390k and you get the dissipation for R16 at 390k as 0.006 Watts.

For the 90k scenario: 50 divided by 90k means current of 0.55 mA. Combined dissipation thru that path is then 0.55 mA times 50V which means 0.0275 W. Divide that by 90k and multiply by 80k and you get the dissipation for R16 at 80k as 0.0244 Watts.

In both cases you'll be more than fine with 0.25W resistors. However I recommend simply using 1W no-name carbon film resistors for all spots that dissipate under 0.75 W. That covers usually 95% of resistors inside any circuit. Why mess with different types.