AC15/AA964 Princeton/Tweed Hybrid

[dwinstonwood]

Yes, the bulb was glowing, but not too bright.

Plugged straight into the wall, I measured the voltage across the shared 150 Ohm cathode resistor at 12.4V. The bias calculator spit out this data:

Total Cathode Current =   82.67 mA
Total Plate Current =   78.12 mA (Cathode Current minus approximate Screen Current of 5.5%)
Plate Current per Tube =   39.06 mA
Plate Dissipation per Tube =   12.34 Watts
Plate Dissipation per Tube % =   102.86 %

They're revving pretty good. I'll plug in a guitar.

[dwinstonwood]

I had to solder up a longer speaker cable.

It sounds really good! There's a big range in tone with the switch. From super bright to pretty warm. I need to look at the switch and write down what capacitance each one is.

It's definitely a very different sound than my 6G3 Deluxe with 6V6s. It's too soon to give a good description, but maybe more raw and bright for sure. It's what I was hoping for. It's why I built it, for the AC15 sound.

But, there are a couple of issues. I'm getting loud pops when I turn the tone switch. Not so with the volume turned down. I'm sure I bought a make-before-break switch. Do I still need to add resistors?
Also, some tone setting are very quiet and others create some hiss. Maybe that's just the nature of the smaller caps letting it through?

I'm calling this project a success! Here are the schematic and layout files.

[sluckey]

I have a small pop on both of my amps. Not really objectional. Might be in church. Resistors will help. Maybe look closely at your switch while slowly operating it. Can you see the wiper blade making before breaking?

[dwinstonwood]

I'll take a look. I also realized that I didn't really ground the switch. I wasn't sure exactly how ground it. But, it is making contact with the chassis.

[sluckey]

No reason to ground the switch.

[dwinstonwood]

Thanks. I'll give some 5M1 1/4W resistors a try. From what I've read the pop is from the caps discharging (which didn't occur to me), so the bleeder resistors make sense. It looks like Matchless uses them, too.

I thought I'd swap in a 160 Ohm cathode resistor for fun; that should drop my plate dissipation down around 95%-ish. And, since I've never bought a single NOS tube before, this amp might be a good excuse to try them. Some of the NOS Russian 6n14n's are relatively inexpensive.

[sluckey]

AES has some Amperex Bugle Boys for $150 each. I like the $11 JJs just fine.   

[dwinstonwood]

Just for the record, the 9-pin tube voltages:

EF86 pin 6 plate: 87.1VDC

12AX7 pin 1 plate: 151.5VDC
12AX7 pin 6 plate: 203.1VDC

I'll go ahead and add the voltages to the schematic for my future reference.

[dwinstonwood]

AES has some Amperex Bugle Boys for $150 each. I like the $11 JJs just fine.   

Yikes! That's eccentric rock star territory. $280 on an Alnico Blue or Gold would be money better spent, IMO.
The Reflektor 6n14n's I was looking into are $22 at KCA. But, yeah, I've bought JJ's for my other amps and I think they sound great.

[HotBluePlates]

AES has some Amperex Bugle Boys for $150 each.

$190.  But they're out of stock.  They do have ribbed-plate Telefunkens for $300/ea if you really need to buy-it-now.   

Fortunately, I've gotten the old-tube hookup a few times in the recent past, and might be set for life.  Sometimes it really helps to be able to buy in bulk when you stumble into a good opportunity.

Just for the record, the 9-pin tube voltages:

EF86 pin 6 plate: 87.1VDC
...

EF86 screen voltage matters.  A Lot.

If you get into a mood to tinker, or don't feel you have enough grit & grind from the EF86, try using a 3MΩ pot from B+ to ground with the wiper to the screen (probably with the 0.1µF bypass cap from wiper to ground).  One end of the pot will clean up the EF86, the other will make it dirtier.

[dwinstonwood]

Thank you HotBluePlates!

So, is this what you mean? Or, did I understand it all wrong? BTW, your suggestion sheds some light on the AC4's use of a 5M6 screen resistor, which I never understood until now.

What special precautions would need to be taken wiring up a pot with 226VDC B+ feeding into it?

Thanks.

[dwinstonwood]

I think this project is finished. I changed the 150 Ohm cathode resistor to a 160, and added 4.7M resistors to the tone selector.

It no longer pops when turning the tone switch, and my EL84's are running at 11.78 watts - 98% dissipation. I'm happy with all of the voltages, and the Allen iron.

Plate Inner EL84 = 315.8VDC
Plate Outer EL84 = 315.7VDC

160 Ohm Cathode Resistor = 12.62VDC

Total Cathode Current =   78.88 mA
Total Plate Current =   74.54 mA (Cathode Current minus approximate Screen Current of 5.5%)
Plate Current per Tube =   37.27 mA
Plate Dissipation per Tube =   11.78 Watts
Plate Dissipation per Tube % =   98.14 %

It sounds really good, with a wide range of tone settings. Thanks sluckey for recommending the selector switch!

I updated the schematic and layout, if anyone's interested. I included an option for a grid stopper for the PI (I didn't add it), since there seems to be a general dislike for Cathodyne phase inverter distortion. I won't really know what I think until I get the amp somewhere where I can crank it into distortion. Maybe this weekend.

Thanks everyone for all the help!

[HotBluePlates]

So, is this what you mean? ...

Yes, at least as a setup for testing.

What special precautions would need to be taken wiring up a pot with 226VDC B+ feeding into it?

Use an average large-size pot found in amplifier volume controls.  If it were going to be a permanent arrangement, we'd think harder about it.  But my suggestion was only to enable you to listen & fine-tube the pentode.

[dwinstonwood]

Thanks HotBluePlates. After I posted my reply, I suspected that it was meant as temporary means of determining the best resistor value. One thing I wasn't clear on was which way it changes things: does a higher value (above 1M) make the amp cleaner or dirtier? I'll try to read up on it. I understand the purpose of the screen, I'm just not clear yet on how it's voltage affects the clean/dirty sound.
Thanks!

[dwinstonwood]

A knock at the door... the cabinet arrived.

I went with a 10" Celestion Creamback. I'm still waiting on the faceplate material; here's the design I drew up. I'll laser etch it at work.

It's almost done.

[dwinstonwood]

Well, I just got the faceplate and backplate engraved. So, this project is finally "complete."
The next time I build an amp I'll make the faceplates early on, and attach them at the beginning... I had to de-solder the fuse holder and fiddle with that troublesome Fender pilot light bracket. But, no major deal.

I'm still messing around with the EF86 voltages. Right now I have a 330K plate resistor and a 2.2M screen resistor (DC30-ish). I might end up going back to the Vox 220K and 1M resistors.

This is a very light amp. The Celestion Creamback magnet is only 14ozs., and the GCD cabinet is lightweight pine. The amp is also compact (6G2 build format), but plenty loud for me!

[dwinstonwood]

I've been really happy with the tone(s) of this amp. But, I've always had a louder than preferable hum with the volume up past halfway. This didn't change whether a cable was plugged in or not.
For some reason I built the amp with one four-section cap can - 20/20/10/10uF - to do all of the filtering, using one ground. I wondered if that was the source of the hum. So, today I split the filtering in two: an F&T 16/16uF can for the power section, and a separate 8/8uF dual-cap for the preamp section. Well, that got rid of the hum. I forgot to take pictures, but here's a layout showing the new filtering with separate grounding points.
I was glad that after thinking about the problem for awhile I was able to finally see what was causing it. I know the single cap can with one ground was a mistake none of you all would have made! But, I learned something the hard way in the process.