Naylor SD60 Build Issue

[Decko]

Hi Folks,

Running into some issues with my recent build.

I've been trouble shooting for last few hours but cannot find the issue.

Th SD60 is running 5881 power tubes and 12ax7s on preamp.

Symptoms are:

Lacks power
Flubby bass
Strange harmonics at the end go the notes.
Distorts at low volumes.

I checked my solder joints.
My electrolytics caps are JJs 100uM and seem fine.
Checked voltages.

Could you point me in the right direction?

See picture. Note the AC filters are removed.

Thanks
Danny

[Tillydog]

My guess would be that it's oscillating. You could confirm that if you have access to a 'scope.

The only thing that jumps out at me from the photo is the way the OT secondary connections thread through the connections to the PI. I would try and separate these as much as you can (run the OT wires further to the left, under the centre of the board & shorten / re-route the blue grid wire to the PI, maybe - definitely don't have one looping around the other).


Have you tried it with the NFB disconnected?

My £0.02...

[Decko]

Well, I did some tweaking around the PI.

No real difference.

The amp volume will swell in and out.

When picking a note the amp responds to attack almost percussive but the note sounds terrible.

Could this be a bad transformer?

Danny

[Leo_Gnardo]

Well, I did some tweaking around the PI.

No real difference.

The amp volume will swell in and out.

When picking a note the amp responds to attack almost percussive but the note sounds terrible.

Could this be a bad transformer?

Danny

I'm thinkin' Tilly's right - make no conclusion until you have had a look at your amp's output with an oscilloscope. You'll be looking for ultrasonic oscillation - "fuzzy" bits riding on your signal, or maybe even a rail-to-rail ultrasonic signal with your audio trying to break through.

This sort of thing will play hob with your signal in unpredictable and unmusical ways.

I doubt it's your iron.

One way of quelling ultrasonics in the output stage is to place a capacitor typical values 100 pF to 1000 pF (1 nF or .001 uF) between the plates of the output drive tube. Start with the larger end of the selection, and if it works, reduce until the problem starts to come back, then back up one value, and install that. If you don't have access to a scope try it anyway. The worst you could do is waste 15-20 minutes.

[Decko]

I will say that I was hasty with this build.

I have an oscope. But I have never used it so it will take some time to understand how to set it up.

For now I will try the caps on the output.

Is oscillation a result of poor wiring? What is the root cause of oscillation?

Thanks,
Danny

[Tillydog]

Is oscillation a result of poor wiring? What is the root cause of oscillation?

Try with the NFB disconnected - it could just be that you need to reverse the OT primaries, but lifting the NFB wire from the PI will tell you that in the time it takes for the soldering iron to warm up.

Otherwise, layout & lead-dress, yes. It's analogous with feedback between an amp and a guitar: Something that can pick up signals is too close to something that gives them out.

I would definitely move the OT wires away from the PI - they are looped through each other in your photo which will act like a (very inefficient) transformer and feed signal back from the OT wires to the PI.

[Reeltarded]

Shield. Grid stops. people here are going to disagree with me to an extent, but you need shield through the third stage and grid stoppers on 1,2, and 3. BIG ones.

I have on a similar gainy amp 220k, 470k, 470k. 120k of the middle stopper was to snub highend because the previous stage is crazy.

Shielded wire between each stage. make sure the amp has it's bottom plate on before even thinking about quiet.

As Tilly said untwine the PI stuff and do the shields then start throwing stoppers on.

[Leo_Gnardo]

Is oscillation a result of poor wiring?

Sometimes but not always.

What is the root cause of oscillation?

positive feedback loop. Maybe not at "all" frequencies, but just like a mic in a PA system, the racket starts at the frequency that finds its way around the loop easiest.

I'm sure you, and all of us, have at one time or another swapped the output transformer primary in an amp that has a negative feedback loop. By that reversal the loop is turned into a positive feedback loop and results in any number of noises, a helicopter-like wup wup wup, midrange hooowwwwl, or treble squeeeek. All to be avoided. It's possible to have most of the audio spectrum covered OK in an NFB amp but still have some hi frequency that defies the formula - and there's your parasitic oscillation.

PFB can be used to advantage, for instance in creating a Wien-bridge oscillator. Lo frequency ones are even used in guitar amps to create vibrato circuits.

And then there's one of my favorite electronic terms, the Barkhausen Criteria. Sounds like a spy thriller don't it? Barkhausen is a special case sort of, and gets things oscillating just fine... It will take more of an engineer than me to explain. Professor Manning? Can you take over from here?

[martin manning]

Hey, it's been a good long while... Long story short: loop gain of 1 and phase shift of an integer times pi.

[Reeltarded]

Do the shielded wire thing and add grid stops before getting all 'xotic.. The original amp has some. Find out what those are supposed to be then we can ask Martin to explain Gugleneschteznelendalschlatengucenfusel.

[Decko]

Thanks folks for chiming in.

I feel more hopeful...

The feedback loop was never hooked up. Interestingly, on initial start up the amp was performing with some intermittent oscillation. Then I blew one of 5881 power tubes. I mistakenly thought that I had a problem with the power transformer. So I bought another one from passfan. It was not the problem.

I look forward to trying the shields, grid stoppers, OT secondaries..etc.

Need to get some shielded wire.

Get back to y'all real soon!

Thanks Danny

[sepulchre]

. . . then we can ask Martin to explain Gugleneschteznelendalschlatengucenfusel.

I should know better than to read one of your posts while having lunch. I dang near choked!

[dcribbs1412]

I did this type build a while back
very cool circuit...not sure any of this info will help but here's link to build thread
https://tubeamparchive.com/viewtopic.ph ... or&start=0
I have some schems if needed

Darin

[Decko]

Hi Darin,

I would like to see the schematic for the power tube section. What component values did you use for the following compliment:

EL34's
Kt66
5881/ 6l6

I am using a full wave rectifier while the original uses the full wave bridge.

I did this type build a while back
very cool circuit...not sure any of this info will help but here's link to build thread
https://tubeamparchive.com/viewtopic.ph ... or&start=0
I have some schems if needed

Darin

[dcribbs1412]

Hi Decko
I'm running some old RCA 6L6gc's
not tried 34's or 66's yet.
Here's some info...rhinson a member here is very knowledgeable about this circuit
Hope this helps

Darin