Let's Optimize/Tweak the Bobcat (6AQ5 5E2 Princeton-ish thing, sorta)

[mountainhick]

The other thread got sidetracked with the naming thing etc, I am calling it Bobcat.

Let's bring it back to tech talk.

I've been playing with it and it needs work. I expected as much, a few things are experimental. Basically, it has a good clean sound and a little useful range with breakup and distortion. But...

First off the circuit:

I don't know what's up with DIY creator, I can't even edit this further. It just totally locks up. There are a few minor inconsistancies and the power section isn't drawn. Has SS bridge rectifier and filament supply has artificial elevated CT with 100 ohm resistors.

The input has two jacks, typical Fender style Hi/Low with a single 1M grid leak and two 68K, for 68 K or 34 K in parallel.

Power transformer is ANTEK toroid 05T200
Output Transformer is EDCOR XPP-10-8-10 10 Watt 10K primary, 8 ohm secondary.

Filaments 3.14 and 3.16

First time using Robrob's bias calulator, does this look all right and good? If this calculator is for one tube, this is very cold. If it is for 2 tubes, would be 9.2 watt, 77%, which looks good. Which is right?

6AQ5 specs:

 

[mountainhick]

When I run Uncle Doug calcs:

Voltage drop 18.8 / cathode resistance 470= .041 / 2 = 20ma tube,

Can I let this current cat out of the bag by reducing the Rk?

 

[printer2]

When I run Uncle Doug calcs:

Voltage drop 18.8 / cathode resistance 470= .041 / 2 = 20ma tube,

Can I let this current cat out of the bag by reducing the Rk?

Yes. Using the simulator agrees with your numbers. But you are at a lower voltage and can increase current up to 45 mA (each tube).

Universal Loadline Calculator

This calculator allows you to trace the anode characteristics graph, and to compute and tune loadline and operating point of various vacuum tubes.

www.vtadiy.com

 

[mountainhick]

Yes. Using the simulator agrees with your numbers. But you are at a lower voltage and can increase current up to 45 mA (each tube).

Universal Loadline Calculator

This calculator allows you to trace the anode characteristics graph, and to compute and tune loadline and operating point of various vacuum tubes.

www.vtadiy.com

Thanks! I have never understood how to use the load line calculator. EDIT: OK I do see how adjusting the idle current moves the operating point. So then it is OK to run idle right at the dashed line?

Do I enter the Plate-Cathode voltage like rob's calculator or the Plate supply Voltage at the OT leads?

How from that do I cipher the Rk value? It seems if I try using Uncle doug's methodolgy i run into a problem in that changing the Rk, also changes the voltage drop. I don;t know how to reconcile that without just trying different resistor values, but don't want to redplate the tubes.

EDIT: I have 1K 5W, parallel would be 320ohm... SAFE enough to take measurements?

 

[printer2]

You can run up until the dashed line in cathode bias with most tubes as dissipation is reduced when you pass a signal. The 6AQ5 is a little more touchy as they do not have all that much glass surface area to dissipate heat, also if some are a little misaligned you can get a hot spot and melt a hole in the glass. I probably would go up to 90% (actually I have a lot of them, I might not be as timid), check to see if you can see any glowing on the plates in a dark room. The plate-cathode voltage, the calculator does not know if it is fixed or cathode bias. The Rk value, you use the Grid Bias Voltage and then use Ohm's Law to calculate the resistance, in P-P you will use double the current value. Changing the current draw will change the voltage drop (the power supply has resistance so more current draw the greater the voltage drop), that is what makes the real world fun. 320R would be safe, you can probably go 220R

 

[mountainhick]

You can run up until the dashed line in cathode bias with most tubes as dissipation is reduced when you pass a signal. The 6AQ5 is a little more touchy as they do not have all that much glass surface area to dissipate heat, also if some are a little misaligned you can get a hot spot and melt a hole in the glass. I probably would go up to 90% (actually I have a lot of them, I might not be as timid), check to see if you can see any glowing on the plates in a dark room. The plate-cathode voltage, the calculator does not know if it is fixed or cathode bias. The Rk value, you use the Grid Bias Voltage and then use Ohm's Law to calculate the resistance, in P-P you will use double the current value. Changing the current draw will change the voltage drop (the power supply has resistance so more current draw the greater the voltage drop), that is what makes the real world fun. 320R would be safe, you can probably go 220R

Waiting for dark!

 

[printer2]

Waiting for dark!

You never hid under the blankets as a kid?

 

[Peegoo]

@mountainhick This will make the amp sound the best:

CLICK HERE

 

[mountainhick]

You never hid under the blankets as a kid?

No blankets in the workroom.

Dusk now, Paralleling 1K is fine, feels a bit more alive. No red plating. Definitely going hotter. I have 330ohm 5W, and 250 ohm 10W resistors, I'll try the 250 just to see...

Next issue, too much bass/transient sensitivity. speaker cone excursion. Reduce 100uf power bypass cap?

 

[mountainhick]

@mountainhick This will make the amp sound the best:

CLICK HERE

NICE!

 

[printer2]

No blankets in the workroom.

Dusk now, Paralleling 1K is fine, feels a bit more alive. No red plating. Definitely going hotter. I have 330ohm 5W, and 250 ohm 10W resistors, I'll try the 250 just to see...

Next issue, too much bass/transient sensitivity. speaker cone excursion. Reduce 100uf power bypass cap?

Give it a try. Or try 22's instead of the 47's.

 

[mountainhick]

@printer2

In the load line calculator is the reactive impedance for a tube or pair. i.e. if set to PP, do i use the 10K OT primary impedance?

With 250 ohm Rk I am getting 257V plate, 250 screen, 15V cathode. robrob's calculator says 60% @30ma/tube, so have gained 10ma from the starting point. Sounds very good and gets very loud, might not need to goose it further.

 

[SerpentRuss]

Here's a thought. Switch the 6AV6 to the dual envelope, common cathode 6J6 and drive your tubes with an LTP PI. That gives you a bit of gain in the PI instead of losing it there.

 

[mountainhick]

Here's a thought. Switch the 6AV6 to the dual envelope, common cathode 6J6 and drive your tubes with an LTP PI. That gives you a bit of gain in the PI instead of losing it there.

Thanks, I'll keep it in mind. Not a problem yet. It distorts, just the distortion isn't that good yet. Some tuning to do first.

I tried the FX loop as well, the pedal bogs it down way too much. I may need to add a tube for buffer, which can also factor in. And it's also about what connects to the loop. I'll be making a reverb where I can also incorporate the buffer if needed.

 

[mountainhick]

Help please,

Mods to schematic:

1: reduced the power cathode cap to 22uf,

2: coupling caps from PI now .022

Result, yes bass/ transient response in control. It feels less responsive as well.

V1A/B cathode switching, as expected, the 10k isn't useful, the other 2 settings are. I think I am going to retask the rotary switch to 3 cathode settings, 820R/250uf, 1.5 K 25uf, And 2.7k, 0.68uf.

Question 1: is there a reasonable way to add a switch to select between two V1A coupling cap values? I could potentially task one of the rotary switches parallel to the cathode switch function. Concerned about the DCV aspect of switching, and capacitive "pop".

Question 2: this schematic has the output from the cathode side of the PI coming off between the resistors, not right off the cathode of the tube. Seems I would get a gain bump straight off the cathode as output. Is there any problem doing that?

Question 3: would moving the load of the MV before the FX loop jacks instead of after affect the FX function? As is, a reverb pedal bogs it way down.

 

[2L man]

1.If you already use a rotary switch perhaps you could use more "throws". To reduce "pop" sound it is better to have lowest value capacitor and highest value resistor in permanent use and then switch connect resistors and capacitors parallel.

2. PI bottom output is took there and it comes the same with top output when over 56k resistors come about same voltage loss when signal pass PI. On triode its anode and cathode currents are almost the same. Obviously bias is less effected when 1k cathode resistor left out of signal output path? Sort of "local negative feedback effect"

When you did change cathode bypass capacitance lower it did increase local NFB to low frequency range.

3. Often there must be a "grid leak resistor" which keep control grid on potential where it is designed to stay. 1M MV pot does that. Effect connect to FX loop not necessarily do that?

 

[Lowerleftcoast]

The toroid PT has a 180V tap. Idk, if you are using 200V or the 180V tap. Try the 200V secondary if you are not already.

If you are not using both the 6.3V secondaries you can use one of them as extra windings on the 200V coil. The correct attachment will add 6V to the 200V. Reversing the wires will take 6V away.

Imo, it will be helpful to increase the plate voltage on the power tubes and... adjust the bias closer to 100%MPD.

 

[mountainhick]

1.If you already use a rotary switch perhaps you could use more "throws". To reduce "pop" sound it is better to have lowest value capacitor and highest value resistor in permanent use and then switch connect resistors and capacitors parallel.

I am OK for the cathode switching, no problem, but I am not sure how to do this for coupling caps. I just want to switch between 2 coupling cap values, there's no resistor involved (yet), but I think there needs to be one referenced to ground somewhere. I don't know where. before or after the cap, and what kind of value?

3. Often there must be a "grid leak resistor" which keep control grid on potential where it is designed to stay. 1M MV pot does that. Effect connect to FX loop not necessarily do that?

I thought so, sounds likely it is wrong load from pedal. I'll try switching so MV is first.

 

[mountainhick]

The toroid PT has a 180V tap. Idk, if you are using 200V or the 180V tap. Try the 200V secondary if you are not already.

If you are not using both the 6.3V secondaries you can use one of them as extra windings on the 200V coil. The correct attachment will add 6V to the 200V. Reversing the wires will take 6V away.

Imo, it will be helpful to increase the plate voltage on the power tubes and... adjust the bias closer to 100%MPD.

Ideally how high B+ would you run 6AQ5? I haven't rechecked yet, but it's around 255 I am using the 200V tap

Wiring the extra 6V, new concept for me. in parallel or series with HV?

 

[Lowerleftcoast]

Ideally how high V would you run 6AQ5?

Depending on how it sounds...
The GE datasheet says the *design center* is 250V on the plate. That is where you are running now. 265 - 18.8 = 246.2. Happy little tubes. ...but you are wanting more yummy sounds. Maybe a not so happy tube will get you there.

The TungSol datasheet *says* a Max of 275V. It would be near 300V (subtract cathode voltage) to be near 275V Max.

Idk which would sound better but a rule of thumb with SE amps is to run them at or over 100%MPD. The Plate voltage drops as the % goes up. I am concerned the voltage will drop below the *voltage sweet range* once the bias is in the *bias sweet range*. Idk this for sure with a 6AQ5. I am guessing.

To answer directly, I would consider as high as ~320B+ but for this application probably no more than ~300V B+, and that might be past the sweet range.

Wiring the extra 6V, new concept for me. in parallel or series with HV?

In series.

The secondary 200V winding will have thinner wire than the secondary 6.3V winding so there is no issue adding this extra length of wire to the 200V coil. Essentially this is what is done with a *bucking transformer*. Depending on which end of the coil is in series with the 200V secondary the voltage will *Buck* or *Boost* the ~6V. (It is only 6V so it is not a big deal but sometimes more is more,)

For safety, the windings should be on the same side of the PT. Iow, do not use a secondary winding to add/subtract from the primary winding.