Nobsound 6p1 Amp flaws/mods

[StepheK]

Now that I see the circuits, it's still a cascode but it doesn't resolve the problem this amp inherently has. It's designed as a grid leak biased cascode while works good in a guitar amp, where you are looking for distortion, but it doesn't work well for hifi use. my mod it turning it into a fixed bias upper triode design, which amplifies with a much cleaner signal. I would run the driver with my mod and remove that other stuff, just my honest opinion.

Using separate power tube resistors is a good idea, but depending on their value, will make the power transformer run hot, as you found out. I was possibly being overly cautious and didn't want folks killing their power transformers

You can read more here http://www.valvewizard.co.uk/cascode.html

 

[Dr_Mick51]

Now that I see the circuits, it's still a cascode but it doesn't resolve the problem this amp inherently has. It's designed as a grid leak biased cascode while works good in a guitar amp, where you are looking for distortion, but it doesn't work well for hifi use. my mod it turning it into a fixed bias upper triode design, which amplifies with a much cleaner signal. I would run the driver with my mod and remove that other stuff, just my honest opinion.

Using separate power tube resistors is a good idea, but depending on their value, will make the power transformer run hot, as you found out. I was possibly being overly cautious and didn't want folks killing their power transformers

You can read more here http://www.valvewizard.co.uk/cascode.html

It is my understanding the rectifier tube has a maximum 120mA current capacity. If you split the cathode resistor on the output tubes , they could be drawing more current than the rectifier is capable. Unless you double the resistor value on each tube.
Last time I checked the voltage across the cathode resistor it was 14volts, and with a 270ohm resistor the current is 14/270=52mA, times 2, plus the current used for the driver tube(1.5mA each) it gives 107mA, So we still have some bandwidth.

 

[Dr_Mick51]

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.

Can you tell us what values for the cathode resistors and bypass capacitors on the output tubes you used?

It's a good idea to use a fan to cool down the transformers and tubes as well.

 

[maddmaxxdrum]

Now that I see the circuits, it's still a cascode but it doesn't resolve the problem this amp inherently has. It's designed as a grid leak biased cascode while works good in a guitar amp, where you are looking for distortion, but it doesn't work well for hifi use. my mod it turning it into a fixed bias upper triode design, which amplifies with a much cleaner signal. I would run the driver with my mod and remove that other stuff, just my honest opinion.

Using separate power tube resistors is a good idea, but depending on their value, will make the power transformer run hot, as you found out. I was possibly being overly cautious and didn't want folks killing their power transformers

You can read more here http://www.valvewizard.co.uk/cascode.html

Hi
Ok thx
I also did all your other mods.
Swapped the 150uF to 22uF cap after rectifier and also changed the second cap because the original one looked a little bubbled on top, didn’t want to take a chance) the one I put in is higher in capacitance(270uF, hope that’s ok?) but it’s what I had in stock... also added the 470k bleeder resistor, I added a EMI/RFI filter on the 120v input.
Thermistor after the power switch.
Changed the output transformers with some Japanese ones I had from another project. Same specs but 10watts.
Put in some Panasonic FC, Elna Silmic II electrolytics. Mundorf aluminum oil and obbligato decoupling caps. Dario Miniwatt GZ32/5V4G rectifier. Added a remote volume control circuit with a Alps 100k stereo motorized pot…

sounds great

y’a for the power tube individual bias resistors i used 540k per tube.and the bypass caps are 220uF..
Instead of the 270k shared resistor.
So far no problems.

thx

 

[Dr_Mick51]

Hi
Ok thx
I also did all your other mods.
Swapped the 150uF to 22uF cap after rectifier and also changed the second cap because the original one looked a little bubbled on top, didn’t want to take a chance) the one I put in is higher in capacitance(270uF, hope that’s ok?) but it’s what I had in stock... also added the 470k bleeder resistor, I added a EMI/RFI filter on the 120v input.
Thermistor after the power switch.
Changed the output transformers with some Japanese ones I had from another project. Same specs but 10watts.
Put in some Panasonic FC, Elna Silmic II electrolytics. Mundorf aluminum oil and obbligato decoupling caps. Dario Miniwatt GZ32/5V4G rectifier. Added a remote volume control circuit with a Alps 100k stereo motorized pot…

sounds great

y’a for the power tube individual bias resistors i used 540k per tube.and the bypass caps are 220uF..
Instead of the 270k shared resistor.
So far no problems.

thx

Ok. That's what I thought about the cathode resistors, two 540ohms instead of 270hms.

Nice mods you have done in there. Try playing with bypass capacitors also, they add something special to the high end and details.

You should add some pictures.

 

[maddmaxxdrum]

Ok. That's what I thought about the cathode resistors, two 540ohms instead of 270hms.

Nice mods you have done in there. Try playing with bypass capacitors also, they add something special to the high end and details.

You should add some pictures.

Hi
So this is the latest pic. I actually had changed the first cap after rectifier to 47uF seeing as the GZ32 can handle up to 60uF.

Also forgot to mention I changed all the resistors to Kiwame Carbon film, PRP metal film and TKD metal film
PSU caps are Rubycon.
I also tried something else.
12AU7 to 6SN7 adaptors. Popped in some ECC83 Mullards Blackburn. and 6AQ5 to 6P1 adaptors for power tubes. I used rca black plate 6005’s.

i attached some pics.
Ps; U don’t see the Obbligato decoupling caps because they are actually below the yellow pcb strip…
So I guess I will remove the 2.2k/100uF network from the the upper triode of driver tubes and will add @StepheK 2Meg resistor mod.

Also I might try some Sylvania 6F8G VT-99 as the driver tubes.
Basically an early 6SN7 with top cap. I need the adaptors.

 

[Dr_Mick51]

Hi
So this is the latest pic. I actually had changed the first cap after rectifier to 47uF seeing as the GZ32 can handle up to 60uF.

Also forgot to mention I changed all the resistors to Kiwame Carbon film, PRP metal film and TKD metal film
PSU caps are Rubycon.
I also tried something else.
12AU7 to 6SN7 adaptors. Popped in some ECC83 Mullards Blackburn. and 6AQ5 to 6P1 adaptors for power tubes. I used rca black plate 6005’s.

i attached some pics.
Ps; I don’t see the Obbligato decoupling caps because they are actually below the yellow pcb strip…
So I guess I will remove the 2.2k/100uF network from the the upper triode of driver tubes and will add @StepheK 2Meg resistor mod.

Also I might try some Sylvania 6F8G VT-99 as the driver tubes.
Basically an early 6SN7 with top cap. I need the adaptors.

Nice, a lot under the hood.
I had a hard time trying make the Miflex KPCU coupling caps to fit in there.

 

[maddmaxxdrum]

Ok. That's what I thought about the cathode resistors, two 540ohms instead of 270hms.

Nice mods you have done in there. Try playing with bypass capacitors also, they add something special to the high end and details.

You should add some pictures.

Ok. That's what I thought about the cathode resistors, two 540ohms instead of 270hms.

Nice mods you have done in there. Try playing with bypass capacitors also, they add something special to the high end and details.

You should add some pictures.

Hi just noticed I typed 540R resistors but it was 560R. I had some nice 5W wirewound mills resistors so I used those. Figured it was close enough…

 

[maddmaxxdrum]

Ok. That's what I thought about the cathode resistors, two 540ohms instead of 270hms.

Nice mods you have done in there. Try playing with bypass capacitors also, they add something special to the high end and details.

You should add some pictures.

hi i was looking at the bypass capacitors i used for the individual cathode resistors mod. i used different brands per tube. i didn't have 4 of the same when i did the mod. so basically left channel tube 1: 560R resistor bypassed by elna silmic ii 220uf cap, tube 2: 560R resistor bypassed by vishay 220uF cap. same for right channel.. is this ok? using different brand caps per tube pair? will different ESR's mess up anything.. seeing as they are paralleled?

 

[FlaCharlie]

hi i was looking at the bypass capacitors i used for the individual cathode resistors mod. i used a different brands per tube. i didn't have 4 of the same when i did the mod. so basically left channel tube 1: 560R resistor bypassed by elna silmic ii 220uf cap, tube 2: 560R resistor bypassed by vishay 220uF cap. same for right channel.. is this ok? using different brand caps per tube pair? will different ESR's mess up anything.. seeing as they are paralleled?

I wouldn't worry about it. Their specs are unlikely to be significantly different from each other. The main thing here is their capacitance value, if they measure about the same I doubt you would hear any difference.

 

[KaliMah]

Ok, so I know the argument against just replacing the power transformer is cost and size matching. Perfectly valid concerns. But if a fella felt it was worth the extra $$ above and beyond the APC regulator unit because he wanted one less unit taking up space on/under his desk, didn't want to make the bucking transformer which would be plugged in 24/7, and didn't mind fabbing up a metal plate to mount the transformer on (and cover the hole left by the horizontal style transformer) is there one out there that would be applicable?

For some reason I don't seem able to navigate the various websites out there to find a 250v/6.3v/5v of any type, let alone shop for best price/size configuration.

 

[maddmaxxdrum]

I wouldn't worry about it. Their specs are unlikely to be significantly different from each other. The main thing here is their capacitance value, if they measure about the same I doubt you would hear any difference.

ok, thx.

 

[WizardWG4]

Hammond 300 series may have something that would work.
http://www.hammondmfg.com/5cpwr.htm

You could just change to solid state rectification and drop the heater voltage with resistors or diodes.

See Dropping Heather Voltage section in Valve Wizard's site.
http://www.valvewizard.co.uk/heater.html

You can also change the filter choke and or the resistor in the C-L-C-R-C filter resistor(s) to drop the B+ to where the 6P1s need to be.

This guy addresses the B+ but I don't think he drops the filaments at least in this video.

 

[KaliMah]

I had scanned Edcore and Hammond, but their 250v transformers only appear to have the single output leads not both 5v and 6.3v. But being rather noobish, I can't be sure I wasn't just failing to enter the proper search string. Also looked at Paul Millet's PCB's but while I'm confident in my soldering abilities I'm not so confident in my ability to do the math and figure out which values of which components need to selected.

Was really hoping for something at little more along the R&R variety- de-solder half a dozen wires, replace them with the new ones, maybe trim a steel plate and drill a couple holes to adapt it. After a quick measurement of existing holes it looks like they designed the case to allow for the output transformers to be mounted at 90* from their current orientation without much fuss so that potentially frees up a little real estate if need be. Might just have to bit the bullet and have them make one.

 

[WizardWG4]

IMO a new transformer is just not worth the cost or trouble.

You may need to experiment with the dropping resistors to get the filaments and B+ where you need but still far cheaper and easier than changing the transformer.

 

[KaliMah]

That's the thing- I don't want to be messing around inside with it powered up checking voltages. I'll likely be forced to concede and do the APC unit, but ideally I don't want another device to have to deal with.

 

[WizardWG4]

Another thought. Looks like there's probably room under the chassis to install the bucking transformer.

https://robrobinette.com/5e3_Modifications.htm#Bucking_Transformer

 

[KaliMah]

Hmm, that idea has some wheels. Worst case scenario I print some taller feet for ground clearance.

 

[FlaCharlie]

That's the thing- I don't want to be messing around inside with it powered up checking voltages. I'll likely be forced to concede and do the APC unit, but ideally I don't want another device to have to deal with.

In terms of voltages, the main concern is that the heater voltages will be too high when the amp is powered directly from the wall. The easiest way to solve this is to use one of those power conditioners, like the APC, but this is also the most expensive solution.

If you go the APC route, or if you wire in a bucking transformer, all the voltages will be reduced. Nothing wrong with that approach, but the high voltage that supplies the tubes is not that much of a concern because the tubes are not being operated anywhere close to their maximum dissipation.

The heater voltage problem can also be easily and inexpensively solved by adding a pair of dropping resistors to each of the heater windings.

Determining the value of the resistors and confirming their effectiveness after installation does require that you measure the heater voltage before and after the resistors are installed. The voltages involved are ~5v on the rectifier and ~6.3v for the other tubes.

Wiring in a bucking transformer will also require you to measure voltages.

When measuring voltages there is no danger of being shocked as long as you're holding only the plastic covered portion of the probes. It's really no different than plugging something into the wall. Presumably you're able to accomplish this without touching the metal blades of the plug. You can, of course, also use probes that clip onto the measuring points while the amp is powered off and then turn the amp on and just observe the meter.

 

[Dr_Mick51]

In terms of voltages, the main concern is that the heater voltages will be too high when the amp is powered directly from the wall. The easiest way to solve this is to use one of those power conditioners, like the APC, but this is also the most expensive solution.

If you go the APC route, or if you wire in a bucking transformer, all the voltages will be reduced. Nothing wrong with that approach, but the high voltage that supplies the tubes is not that much of a concern because the tubes are not being operated anywhere close to their maximum dissipation.

The heater voltage problem can also be easily and inexpensively solved by adding a pair of dropping resistors to each of the heater windings.

Determining the value of the resistors and confirming their effectiveness after installation does require that you measure the heater voltage before and after the resistors are installed. The voltages involved are ~5v on the rectifier and ~6.3v for the other tubes.

Wiring in a bucking transformer will also require you to measure voltages.

When measuring voltages there is no danger of being shocked as long as you're holding only the plastic covered portion of the probes. It's really no different than plugging something into the wall. Presumably you're able to accomplish this without touching the metal blades of the plug. You can, of course, also use probes that clip onto the measuring points while the amp is powered off and then turn the amp on and just observe the meter.

Nice. So do we also need to measure the current flowing from the heater windings?. And the Voltage drop is calculated by V=IxR.
Knowing the heater voltages by measuring directly on the windings we can determine how much voltage drop we need on each one to reach 5v and 6.3v. For example if the measured voltage on the 6.3v winding is in fact 7v. And the current is 2A. If we know we need a 0.7v drop, then 0.7v/2A=0.35ohms resistor. Is that correct? The power dissipation of the resistor is P=I^2xR= 2x2x0.35=1.4W. We would need a 0.35ohm 5W resistor to be safe, right?