Traynor Biasing Questions and Pulling 2 tubes

[martin manning]

I was referring to the current limiting via the 10R and 10V zener. That should limit screen current to about 1.5A, but it could be lowered with lower Vz or a larger resistor. When driving the power tubes to 0V Vg the screens will draw a lot of current, especially so if the load line is far below the knee. I’m wondering what the sound is like if that current is limited by the FET as opposed to screen resistors as in e.g. JTM45 and TW Express, where the load line is well below the knee due to the 6k6 (or 8k) primary. The FET is kind of like a variable screen resistor, so it may change the sound a bit. Might be a good way to protect the screens in those amps.

[bcmatt]

Ok, so I was trying to find the point where if I lowered the bias too much that it would introduce crossover distortion. However, even at 0mA, the clean tone was still clean, and the overdriven tone didn't reveal any nasty distortion. It would go up to about 5mA when playing chords but settle at 0mA when idle. I found at about 25mA, I had decent clean headroom still and it would be still uncomfortable to turn up the master volume all the way... but I would definitely ride it higher than if the bias was at 38mA where I had it yesterday. There are subtle differences in the feel... and I think it does feel more stiff as I lower the bias.... but I am confused about not hitting that "notch" that Aiken refers to. Why can I bias as cold as I want to and seem to only be limiting the overall power without really any repercussions?

Honestly, even 0mA is enough volume for me regularly, but for versatility sake, and the volume controls not being too overly sensitive, somewhere in the 20-30mA seems more ideal. But that seems pretty cold at probably 30-40% max dissipation for a 35W Kt-88.

[martin manning]

You can see what crossover distortion looks like and the effect on harmonic distortion is with reduced bias current here: https://www.aikenamps.com/index.php/wha ... distortion
In theory you would have to bias just below the point where plate current reaches zero to get it, but the load line is really an elliptical path, which probably makes the notch less visible and audible. I'm betting if you looked at it with a clean sine input to the power amp and a scope, you would see and hear it. Lowering bias below zero plate current will actually increase the power amp's headroom since there is more voltage swing available (to Vg = 0, where grid current limiting begins), but the output volume for a given input signal level is reduced. I don't think the FET screen supply is making any difference wrt the usual arrangement. Screen voltage is still going to be proportional to plate voltage (unless the current limiting feature of the FET supply comes into play). I would be inclined to bias at ~60% using cathode current as usual.

[GerryJ]

Just curious on measuring 0 mA bias, technique-wise. I use a bias probe, which for EL34 tubes takes pin 8- the cathode- measures the current, & then returns it to pin 8. So I'm measuring cathode current, and adjusting the bias pot (for control grid- pin 5) accordingly. If you're measuring cathode current, 0 mA would by definition be silence, so I'm sure you're not measuring cathode current as your bias metric. But I'm confused as what mA source you Are measuring (not a tech, thanks).

[martin manning]

If you're measuring cathode current, 0 mA would by definition be silence, so I'm sure you're not measuring cathode current as your bias metric. But I'm confused as what mA source you Are measuring (not a tech, thanks).

If you reduce the grid bias voltage (make it more negative) to the point where cathode current is zero at zero signal, you will still get sound when a signal is applied. The signal voltage brings the grid voltage up, and causes plate (and screen) current to flow. This is a Class B amplifier, where each side of the push-pull stage conducts for exactly half of the cycle. See: http://www.valvewizard.co.uk/pp.html

[bcmatt]

I did bring the bias up to about 55 or 60%... I forget now.... and I do feel like it improved the tone. I haven't played it in the last few months because of travels, and using other amps primarily, so I'll be looking into it again soon and seeing how it's working there.

[bcmatt]

I'm becoming quite happy with my other YGL-3a that was completely gutted and converted to an SSS... I'm feeling like this YGL-3a finally deserves a proper conversion to an ODS circuit.

I'm thinking I'd like to keep it running these 2 KT-88s. It seems to be working well, with this VVR for the screen supply.

However, I think I am missing out without fulling embracing some ODS preamp and perhaps a more classic overdrive circuit.
I've been trying to figure out if a 102 or a 124 would be more appropriate. Do you think one would work better with KT-88s over the other?
Since it would run just 2 power tubes, I think I could move the heatsink for the Mosfet could move to under the chassis where the near power tube sockets currently are. That's also right where the can blows.

ygl guts.jpeg

Also, I could easily have 7 preamp tubes without drilling any new holes.
So, I'm wresting with the idea of a 102 with 3-tube SSS-style reverb as well as a built-in d-later. I wonder if it's too ambitious to tackle it all together at once.
If D-laters are so important, why is it not super common to basically always build them in to the amps? Why did Dumble never seem to do that? Trying to keep the amp more compact?

[martin manning]

I'm becoming quite happy with my other YGL-3a that was completely gutted and converted to an SSS... I'm feeling like this YGL-3a finally deserves a proper conversion to an ODS circuit.

...I think I am missing out without fulling embracing some ODS preamp and perhaps a more classic overdrive circuit.
I've been trying to figure out if a 102 or a 124 would be more appropriate. Do you think one would work better with KT-88s over the other?

Since KT8K8 are beam tubes like 6L6, and the power amp is a clean Hi-Fi type, I don't think it would matter.

So, I'm wresting with the idea of a 102 with 3-tube SSS-style reverb as well as a built-in d-later. I wonder if it's too ambitious to tackle it all together at once.
If D-laters are so important, why is it not super common to basically always build them in to the amps? Why did Dumble never seem to do that? Trying to keep the amp more compact?

You are thinking SSS clean section feeding a 102 overdrive section? Where would you take the reverb from? What about the preamp feedback loops? What about the filter section?
I recall mixed reviews on built-in vs. stand-alone D'lator. Some flexibility is lost with the built-in arrangement. It certainly would be a very complex build with all that on one chassis.

[bcmatt]

Since KT8K8 are beam tubes like 6L6, and the power amp is a clean Hi-Fi type, I don't think it would matter.

That's what I figured. Mostly it's something like the 183 or HRM that get a little different. I'm getting a little mixed up keeping track of all the different models and trying to hear the differences. I do gravitate toward single-coils, which I hear people say they like the low-plate iterations for... but I have a hard time understanding the explanation as to the reasoning. I also end up playing more medium-to-low volumes, and I've also read that the 102 doesn't require being so loud to let its overdrive shine compared to other models...

You are thinking SSS clean section feeding a 102 overdrive section? Where would you take the reverb from? What about the preamp feedback loops? What about the filter section?
I recall mixed reviews on built-in vs. stand-alone D'lator. Some flexibility is lost with the built-in arrangement. It certainly would be a very complex build with all that on one chassis.

No, I was thinking of the classic 102 for clean and overdrive (well, maybe the updated skyline version).
It's just that with all the real-estate of this giant 26" x 8" chassis and possible 4 other preamp tubes available as well as the 12 knobs, 2 switches and 4 inputs on the front, there's the temptation to put them to use.

However, I recognize that my skills and understanding are (while much improved since I started building myself amps 17 years ago), are nowhere near the level to ever consider doing this as a profession. (I have the greatest respect for those who do, but I have no interest in ever doing this for more than fun). I am strictly a hobbyist and someone who builds myself amps that I could otherwise not afford. I completely depend on the help from ampgarage to ensure I get a successful amp that I'm pleased with actually up and running safely.

I recognize that even trying to do an amp conversion is asking for more complication in itself. There are always hurdles that arise from adapting transformers designed for a different amp. However, I can't resist the financial efficiency of giving new life to an amp that is just not able to compete for playing time with my others. I would say the other YGL -> SSS was an outstanding success. It was a complete gut and re-build... and required some innovation from yourself, Martin, to design the CF Follower power supply. Then I spent a couple weeks recently tweaking it to really optimize it for the right feel. But I think it really arrived at something special now.

I'm thinking that perhaps I should resist the urge to use all the tubes and knobs at first. Maybe I should concentrate on just doing a straightforward 102 with KT-88s. I could leave lots of space if I get brave later to add in a d-later.

But yes, about the filtering... should I try to keep the YGL power section and filtering to start? Or would it make more sense to clean it out right away, buy a choke, and build the typical 102 filtering? I suppose if I want an active loop, I need to factor in that extra bit of B+ supply.
Here is a YGL Schematic again:

traynor_mkiii_ygl3_3a_b.pdf

I see GASTan has a thread about a 102 with a buffered loop. Perhaps, that is similar to what I hope to do... I need to study it a bit more and sort out which documents are actually 102 related. His project might be similar to what I want to end up with. But I see he referenced some layout with an active loop.. Looks to be a 183 HRM:

183 plus loop.jpg

I think if I put my loop send and return controls on the front panel, that would actually populate much of my holes...

[bcmatt]

I want to forget about adding reverb for now... but am still obsessing about trying to build in a Dumbleator.

I am maybe redoing someone's work, but I haven't found it exactly... so I messed around with Paint to theorize how one would add a dumbleator to a 102 circuit:

102 with Dumbleator Theorized.png

I imagine the B+ dropping resisters would require some messing with to land the plate voltages appropriately. But I wonder if I am even on the right track. Thoughts anyone?

I wanted to spit this out before I play with my current Traynor Power amp situation with the the 2x KT-88s in the same schematic. The way I currently have the schematic may actually be useful to someone else.

Edit: (I added the FX Loop IN/OUT switch)

[bcmatt]

Just looking at the actual power section of this YGL:

YGL with KT88 screen VVR.png

[bcmatt]

Since this is going to be some sort of ODS conversion, I think it would be appropriate that I start a thread with my plan in the Dumble Discussion. This is beginning to become the wrong section for such talk.

[martin manning]

Just looking at the actual power section of this YGL:
YGL with KT88 screen VVR.png

Pretty similar to ODS. A branch from B+2 before the VVR could be used to supply the D'lator... I guess that's what you're thinking, and that's a proven approach.

[bcmatt]

Pretty similar to ODS. A branch from B+2 before the VVR could be used to supply the D'lator... I guess that's what you're thinking, and that's a proven approach.

Since I'm not really sure what model I should use for a 2x KT88 ODS power section. I'll start by making a schematic with the current power section married to the ODS.

I just checked the current bias.

With 470V on the plates of the 2 KT88s
the VVR is helping give 300V on the Screens . That is 2 x 240 ohm 10W power resisters in Series before the VVR circuit, and 22 ohm screen resisters on the two sockets I'm using.
They are dissipating about 40mA (38 and 40), so it's set to about 53% for 35W KT88s.

Do you know how they might have arrived at the screen stopper value of 22R? Most schematics of this model have 47R on those two sockets. My schematic glued under the hood actually has is marked as 22R from the factory, but it is sort of bold like they are pointing out that they had changed it. This is a later model (I think about 76 or 77).

[martin manning]

Do you know how they might have arrived at the screen stopper value of 22R? Most schematics of this model have 47R on those two sockets. My schematic glued under the hood actually has is marked as 22R from the factory, but it is sort of bold like they are pointing out that they had changed it. This is a later model (I think about 76 or 77).

I don't know. It just needs to be big enough to prevent oscillation. It's hard to imagine 22 vs. 47 ohms would make much difference.