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

[mountainhick]

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.

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.

Cool! More goosing to do...

 

[mountainhick]

More questions, refer to numbers below.

I haven't added the 6.3V to the HV yet, but have updated the V1 Cathode scheme. It is definitely more useful.

Not feeling the love yet, it feels a little blunt/dull. Not lacking in treble per se, just not very sensitive. Feels like it needs to open up. I dig in playing and it still doesn't really sing.

The voltages are lower with the 250ohm Cathode resistor even though wall VAC is a volt higher today. More sag I guess.

1- Is this all the result of inadequate voltage? I am fine ordering another ANTEK at 220/240 if that would help.

2- This has 5E2 type tone circuit but with 500K Tone pot, would 1M brighten it? It doesn't seem like a very useful range, just run it at max with both single coils and humbuckers.

3- I am also thinking about converting to 5F2A tone circuit instead Plusses/minuses?

I might try a different speaker as well.

4- I rewired MV before FX, no difference. Without this turing to full blown discussion about buffers, anything wrong with this one's placement in the circuit?

I've had it with DIYLC. The newly downloaded version is just locking up repeatedly.

So back to pencil and paper. Here's what I have today:

 

[mountainhick]

Having a convo with myself I guess.

Today I will try:

-A different speaker

-Adding the 6.3V to the HV,

-I might try a smaller dropping resistor value to B+3 for higher voltage to preamp and PI. (I will probably also order an ANTEK 240/220V PT)

-Removing the 100K grid stopper to V2

-Change to 5F2A type tone circuit

-Try different tone pot value(s)

I have no idea about the FX loop issue. Should I move it to V1A's output?

 

[mountainhick]

Having a convo with myself I guess.

Today I will try:

-A different speaker

-Adding the 6.3V to the HV,

-I might try a smaller dropping resistor value to B+3 for higher voltage to preamp and PI. (I will probably also order an ANTEK 240/220V PT)

-Removing the 100K grid stopper to V2

-Change to 5F2A type tone circuit

-Try different tone pot value(s)

I have no idea about the FX loop issue. Should I move it to V1A's output?

Changed to 5F2A tone circuit and dropped the 100K grid stopper to V1B... Really good progress! feels more alive and responsive, and working well with humbuckers on low input, single coils on high as they are supposed to.

Distortion is better. Not quite as sensitive transition from clean to breakup as I like, but getting better. I think a light push from OD pedal would light it up.

The three cathode selections are significantly different in character, and all are useful. Have to spend some time playing to find out how I want to tweak these. I have a couple ideas to optimize each position, including bright caps and potentially V1A coupling cap switch. These can also be integrated into the 3 position rotary switch, but i may want to keep them independent.

Can someone please tell me how changing tone pot value would change treble? I understand a simple low pass RC filter, but don't understand with the caps on either side of the pot. And I can't run the Duncan TMB sim, I don't have Windows.

Here it is now: Mods in blue

 

[printer2]

Changed to 5F2A tone circuit and dropped the 100K grid stopper to V1B... Really good progress! feels more alive and responsive, and working well with humbuckers on low input, single coils on high as they are supposed to.

Distortion is better. Not quite as sensitive transition from clean to breakup as I like, but getting better. I think a light push from OD pedal would light it up.

The three cathode selections are significantly different in character, and all are useful. Have to spend some time playing to find out how I want to tweak these. I have a couple ideas to optimize each position, including bright caps and potentially V1A coupling cap switch. These can also be integrated into the 3 position rotary switch, but i may want to keep them independent.

Can someone please tell me how changing tone pot value would change treble? I understand a simple low pass RC filter, but don't understand with the caps on either side of the pot. And I can't run the Duncan TMB sim, I don't have Windows.

Here it is now: Mods in blue

View attachment 1080401

Even the web version?

TSC in the web

 

[mountainhick]

Even the web version?

TSC in the web

You mean Duncan? did I miss something?

 

[printer2]

@2L man , can you look at the FX in the above schematic. Do you think a 1M grid leak on the input to the 6AV6 might help the FX loop problem?

You mean Duncan? did I miss something?

What you do in the privacy of your own home, may be better if I did not answer that question. Does the online version work for you?

 

[Lowerleftcoast]

Can someone please tell me how changing tone pot value would change treble?

The treble side turned *up* is very much like a treble bleed. As the volume is turned up the treble bleed is less effective. I would think 470pF would be enough but you can take it higher. Cap values add when in parallel so it is easy to clip in another cap, say 250pF, and have a listen. Solder it in if you like it.

Looking at the coupling cap values and the RC filters they create it appears there is a lot of bass in this amplifier circuit. That can make it seem treble weak. Maybe getting rid of some bass will help.

Do you notice a difference in sound/function when the 560k grid leak resistors are paralleled with ~560k resistors (= 280k-ish)?

 

[mountainhick]

What you do in the privacy of your own home, may be better if I did not answer that question. Does the online version work for you?

I am utterly confused. What?

TSC

That link has no web based calulator present.

Sorry the 2Lman part was attached to that message, was supposed to be a different post. I will edit.

 

[mountainhick]

The treble side turned *up* is very much like a treble bleed. As the volume is turned up the treble bleed is less effective. I would think 470pF would be enough but you can take it higher. Cap values add when in parallel so it is easy to clip in another cap, say 250pF, and have a listen. Solder it in if you like it.

Looking at the coupling cap values and the RC filters they create it appears there is a lot of bass in this amplifier circuit. That can make it seem treble weak. Maybe getting rid of some bass will help.

Do you notice a difference in sound/function when the 560k grid leak resistors are paralleled with ~560k resistors (= 280k-ish)?

Thanks for that! Interesting, I will try it!

I wonder why the 560Ks were chosen. Now that you mention it, I see these are typically 220K in the similar Fender circuits. The way Terry drew it was .047s and the 560Ks.

My question about the tone stack was about tone pot value? The Princetons have 250K and 1M. This one sits in the middle.

 

[Lowerleftcoast]

My question about the tone stack was about tone pot value?

I'd lean toward the 1M to keep the trebles. 470k should be OK though. I wouldn't use 250k in this circuit.

 

[mountainhick]

I'd lean toward the 1M to keep the trebles. 470k should be OK though. I wouldn't use 250k in this circuit.

Righto, that's what I was thinking. The 1M would block more to ground.

 

[Mexitele Blues]

I am utterly confused. What?

TSC

That link has no web based calulator present.

Direct link to 5F2a:

TSC in the web

 

[mountainhick]

Direct link to 5F2a:

TSC in the web

DOH! Thank you. I will get so much use out of this.

And thanks @printer2 . Sorry for being half brain dead today. Yesterday was better.

So just a bit between the pot values.

 

[Mexitele Blues]

Can someone please tell me how changing tone pot value would change treble?

My Gibsonette copy, with more or less the same preamp and a 1M tone pot, is exceptionally bright. Effective knob travel is 8:00 to 1:00. I play it at 9-10:00.

 

[Commodore 64]

Have you tried bypassing the FX loop?

Have you tried a bypass cap of some sort on V1B?

 

[mountainhick]

Have you tried bypassing the FX loop?

Have you tried a bypass cap of some sort on V1B?

Thanks,

Bypassing FX works fine. Seems like load/impedance mismatch with pedals.

No V1B bypass cap as of yet. Not sure I will need to try one.

A lot of brightness now with single coils.

 

[printer2]

DOH! Thank you. I will get so much use out of this.

And thanks @printer2 . Sorry for being half brain dead today. Yesterday was better.

So just a bit between the pot values.

I could have explained it but thought you playing around would be much more clear. Glad it worked for you.

 

[mountainhick]

I could have explained it but thought you playing around would be much more clear. Glad it worked for you.

Yeah, no problem, I just didn't realize the links in the top menu bar were the business

 

[mountainhick]

I added in the extra 6.3V to the HV, and put a Caline pure sky with moderate gain and bass trimmed in front with my Tele this morning...

GOOD LORD! With a little kick up front the Bobcat is FEROCIOUS! Perhaps it is just destined to shine with a pedal or two.

Need to spend some time with it with the other guitars

I will continue to trim bass in the amp itself until I can run it flat out without the extra bass resonances. I am half way thinking about a bass control. And seriously thinking about that V1A coupling cap switch for the 0.68/3K cathode setting

I will also replace the PT with a 240/220 version