SLO-50 build and small issues

[Spencerleehorton]

Hi All,

Me again!!!
Have built another SLO-50 and it’s not sounding quite right compared to my other built if I detail what I’ve done hopefully can fault find and sort.
I’ve have built with pcbs bought from the web along with chassis and tried to be accurate to original as possible.
Even though the pcb had 5 x led switches I’ve taken out the 5th one which is a popular mod but isn’t original.
View attachment IMG_0636.jpeg

Here is a picture of the build.
Hopefully a bit neater than last one!!
I’m using KT77 output tubes, with 1k instead of 470R and 5.6k rather than 2.2k, I’m thinking these are the wrong values and lead to the tonestack problem?
Tonally the tonestack doesn’t seem to sound the same as my other build and overall the amp sounds darker, so wanted to find out why?
Also when plugged into fx loop when I unplug input there is a big screeching tone?
Could be that I haven’t wired the input jack correctly, I’m sure this is an easy fix and the switched tip of jack needs to go down to ground!!?

 

[Spencerleehorton]

I’ll change the grid stoppers for 2.2k and the screens to 470 ohm and re test

 

[soulsonic]

I would keep the screen resistors at 1K. It shouldn't be doing anything to make it sound darker. The grid stopper resistors maybe can make it sound darker, but I expect they would need to be a higher value.

I don't think those components would be having any affect on the tone stack at all, considering that the phase inverter isolates the tone stack from those components.

When I build my SLOs, I used relays for channel switching. Optos don't pop, but they do drift and stop working correctly after a time, so I'm not a fan of them.

The switch for the tip of the input jack definitely needs to go to ground to keep it from being noisy when nothing is plugged in. I don't know about the FX loop problem.

 

[Spencerleehorton]

This details it as darkening the sound if they are higher.

As it is with the 1k and the 5.6k it’s setup for EL34s so I will put a pair of them in and see if the overall tonestack works better.
Then change to 470 and 2.2k to test the KT77s

 

[soulsonic]

Consult AI at your own risk, man.

 

[Spencerleehorton]

KT77 are seen as 6L6s and this is the SLO50 schematic

 

[soulsonic]

I built my first SLO with EL34s. I used 1K screen resistors. And if you're using KT77s, or pretty much anything not 6L6s, you'd better stick with them unless you want fried tubes.
I used 2.2K grid stoppers.

What output transformer are you using? Because the impedance of the output transformer can have a big effect on the behavior of the output tubes if you're using anything other than what was intended for it - ie; if you're using an OT intended for a pair of 6L6, but you have a pair of KT77 in there instead.

But this focus on the output section feels like a distraction. If the tone controls aren't behaving correctly, that's probably nothing to do with the output tubes. If the FX loop is being weird, also nothing to do with the output section.

 

[Spencerleehorton]

The fx loop is fine, it’s just the input jack which needs the tip link to gnd on the switch side.
I’ve put in EL34s and the amp sounds much better, how it should or close to it.
Yes the output txf came out of an amp using 6L6s so I was under the impression that KT77 are sort of in between EL34 and 6L6s so to go for values somewhere in between?

 

[Spencerleehorton]

If the KT77 is truly in-between the EL34 and the 6L6 I would expect values of 4.2k and 750R?
But I’ll try what you have done and change the 5.6k to 2.2k and see how it sounds with the KT77

 

[Spencerleehorton]

i think im just not liking the KT77 output tubes, they sound much better now with 2.2k as grid stoppers bias at 34mv but the EL34s sound much better to me now, sound wise my previous build kicks this ones ass but noise floor wise this second build is much better!!!

 

[merlin]

The 2.2k grid stoppers are a small generic value that has no effect in the audio range. You don't need to change them for different tubes.

 

[Matador]

they sound much better now with 2.2k as grid stoppers bias at 34mv

This is surprising to me, as the effect of 1.5K versus 2.2K (should) be well above the audio band.

 

[bjosephs]

When people refer to KT77 as being “in between” EL34 and 6L6 they mean in terms of tone. It doesn’t apply at all to how we pick the right stopper values. The grid stoppers aren’t critical as Merlin states above and the screen grid resistors are a function of screen current draw.

 

[Spencerleehorton]

Ah ok then I need to check tonestack and other areas for errors.