Does this project make sense? (I think it does)

[dwinstonwood]

I've been thinking about the Express's high-pass filter before the third triode with its .002uF coupling cap and 150K resistor. From what I've been reading this is an important part of the preamp's character.

So, I'm wondering if the third circuit (on the right) in this drawing will maybe preserve more of that high-pass filter's "quality" than the middle Musing circuit. Or, is there a better way to wire the gain pot?

Thanks for any input!

[dwinstonwood]

The -3dB cutoff point with the Express's .002uF cap/150K resistor is about 581 Hz.

A 750pF cap and 500K I think works out to about a 425 Hz cutoff point.

That's a bit more low end, but close, if I'm doing this right.

[dwinstonwood]

Well, I've come full circle again. 

It's back to basically the AA864/Musing design, but with a few of the Valve Wizard's recommended anti-blocking distortion tweaks in the PI, and his use of a warmer-biased third gain stage.

Suggestions/criticisms welcome!

[sluckey]

[dwinstonwood]

Well, since I added back the global negative feedback, I would have to answer yes to Billy.

But, I do seem to brainstorm on this forum. Which is good, because a fair number of my "ideas" are solidly based on a lack of knowledge.

[dwinstonwood]

Here are my drawings so far.

Don't think of them as final.

Will I run into any trouble grounding the heater artificial center tap on the board along with the power and bias grounds as shown in the layout?

Thanks

[bmccowan]

Which is good, because a fair number of my "ideas" are solidly based on a lack of knowledge.

Very funny 
What I like is that you spend a bunch of time thinking about your builds and asking questions. It's the Socratic method of amp building. I should do more of that. I tend to just jump right in - build it - and then start making changes.

[sluckey]

Will I run into any trouble grounding the heater artificial center tap on the board along with the power and bias grounds as shown in the layout?

no. You're getting pretty good with that layout program.

[dwinstonwood]

sluckey, it's getting maybe a bit obsessive. I created my own terminal lug drawing. 

[sluckey]

Hahaha. I totally get it!

[dwinstonwood]

I've finally gathered all the parts for this project. One bit of good news is that JJ tubes - from Slovakia - are reappearing as "in stock" at some of the online merchants I've visited. The prices are somewhat higher, but not ridiculous (I got a matched pair of JJ 6V6's for $48).

So, I've reverted back to the cathode biased version. And, I'm going to try the elevated heater method that pdf64 suggested.

I'm sure there are mistakes in my drawings.

[dwinstonwood]

Seriously, this time. The noodling is done. This is how I'm building it:

[sluckey]

Looks like a plan. 

[dwinstonwood]

Thanks. Yeah, this should give me a pretty interesting range. With the TMB and NFB switched out it won't really be a 5E3, but it should lean in that direction. And, having the gain control on the third triode is a nice extra.

[dwinstonwood]

Some progress.

[sluckey]

 

[66Strat]

Some progress.

What chassis did you use?

[dwinstonwood]

Thanks!

66Strat, I used Hoffman's Stout chassis.

[dwinstonwood]

New summer work schedule = less time during the week to do amp stuff.

I did finish and install the board, and I mounted the odd bits like the terminal strips and cap can clamp. So, hardware is done.

You can see all of the diode rectifier and fixed bias holes in the board that I filled in with epoxy.

Tomorrow I'll install the cap can and solder up all of the PT and the OPT wires.

I should finish before the first day of summer. 

[dwinstonwood]

The soldering is finished. I'll trace over everything one more time, and dim-bulb test it in the morning.

[66Strat]

Nice work.

[dwinstonwood]

I still get shaky and nervous whenever I fire up a new project for the first time.

OK, so Antique Electronic Supply does a good job of matching tubes:

JJ 6V6S #1 - 323.4VDC plate to cathode
JJ 6V6S #2 - 323.2VDC plate to cathode

Voltage across 270 Ohm cathode resistor - 20.05VDC

Here's a screen shot from Rob's calculator. To be safe, I aimed low and landed low. 

80.7% PD with an average of 11.3W per tube.
35.1mA average per tube.

I need to breathe for awhile and I'll plug in my guitar.

Edited to correct: The cathode resistor actually measures 266.6 Ohms. So, PD is really 82.1% at 11.5W per tube average, and plate current is 35.6mA average per tube. At any rate, I can switch to a 250 Ohm later on, or two parallel 470 Ohm 5W's for about 235 Ohms to up the PD, if I want.

[dwinstonwood]

Everything works as intended. Next, I'll hook it up to the 12" speaker in my 6G3 cabinet. The little 8" test speaker sitting on the desk doesn't tell too much.

I thought I might get pops when I flipped the tone stack and NFB switches, but no noise.

There's an expected jump in gain when I switch off the TMB.

I ended up using a JJ medium gain 12AX7 at the input, a regular 12AX7 in the second position, and a 12AT7 for the PI.

There is a little bit of hiss, but not bad. There's literally no hum. I'm guessing that grounding the heaters at the power tube cathodes helped with that. Thanks to PDF64 for that suggestion. Anyway, I'm not going to tweak anything for now. I just need to play through it.

Thanks for all the guidance and encouragement everyone!

[Lectroid]

I need to breathe for awhile and I'll plug in my guitar.

   

[tubeswell]

There's an expected jump in gain when I switch off the TMB. Anyway, I'm not going to tweak anything for now.

Surely not ‘unexpected’? TMB tone stacks are significantly lossy, so if you use a TS lift, you’ll get a significant jump in gain.

[dwinstonwood]

Surely not ‘unexpected’? TMB tone stacks are significantly lossy, so if you use a TS lift, you’ll get a significant jump in gain.

Yes, I did mean "expected." I knew the gain would jump up. The next bit about tweaking was unrelated to that, but I see how it looks like it was.

I replaced the gain pot return wire (to the 220k/220k divider) with a shielded wire and the hiss dropped quite a bit.

It also sounds a lot better through the Celestion Creamback in my AC15 cab.

[sluckey]

I like the looks of a big ole amp in a tiny chassis. 

[dwinstonwood]

Thanks tubeswell. Yeah, it is basically like a boost switch. You can tell the difference without the bright cap, but there's still plenty of treble by using the guitar's tone control. IOW, it doesn't turn into a dark amp when you flip the TMB off. You just sort of lose the bright cap part, and it sounds more full.

Thanks sluckey for your help and support with this project. Some of the soldering was a challenge in the confined areas. But, I agree, it's cool having six tubes on a 12" chassis.

So, here's a question for anyone. I'm at about 82% dissipation at 11.5W per tube. How would bringing the dissipation up closer to 100% change the tone? I haven't had time to play the amp enough yet, and it sounds good now. But, it seems like most people prefer to run cathode biased amps near the limit.

[shooter]

closer to 100% change the tone?

SE tone comes mostly from the pre since they're basically Class A - Amplify what comes in without change.


80 - 100 changes the "operating point" - Vk.  it should give a little more headroom.


I would play it til Your fingers bleed, then make a change and evaluate.  It's more about your happy than the tubes.  If your happy has tubes melting at 6mths, then tubes melt at 6mths.
don't forget to use ANY and ALL pedals while evaluating.

[dwinstonwood]

Thanks shooter.

You're right. I need to just play it. There are four different ways to run this amp: tone stack with or without NFB, and tone stack bypassed with or without NFB. So, it will take some playing to really check out all the differences. Plus, I can swap around preamp tubes - try a 12AY7 for the input, 12AX7 for the PI, etc.

[sluckey]

I can see the value of using a switch to lift the tonestack, but I wanted to throw out this idea that I sometimes use. I replace the 25K mid pot with a 250K-A pot (10% taper). The pot acts as a standard mid pot for the first half of rotation and acts as a variable tonestack lift (raw) for the rest of the rotation.

[dwinstonwood]

sluckey, that's without doubt a more efficient and useful solution. I think I'll keep the switch for the bright cap, and swap in a 250K-A pot. Having those two things independent of each other is even more versatile.
Thanks!

[dwinstonwood]

I'm lovin' this amp. Here's a pic of my "home rig" that lets me push things into overdrive through the 12" WGS G12C/S in my 6G3 cab. It can do clean/bright AB763, and some kind of meaner raw gain thing. And whatever in-between. At slightly bigger than a lunchbox - thanks Mr. Hoffman for your chassis.
(I'll post my final drawings with all the voltages when I get a chance to measure them)

[dwinstonwood]

Here are my drawings with the DC voltages entered on the schematic. With all the help I get here I like to share them since only a part of it is really my work.

Thanks again to everyone. This is a great place to build amp projects!

[mk_716]

Great job, man, thanks for sharing this!
m.

[dwinstonwood]

So, I'm dropping 28.8 volts across a 1K 3W resistor in the B+ right before my choke (which feeds the 6V6 screens).

That equates to .0288A and therefore .839 watts.

Is a 3W resistor enough leeway, or should I go with a 1K 5W resistor?

Thanks!

[acheld]

I usually double the expected watts and use that as a rough guide to rating.   So in your case, I think a 3W resistor should be more than enough. 

[shooter]

a 1K 3W resistor in the B+ right before my choke

we used to call that a brother-in-law circuit 
the inductor should be good enough for the screen since inductors really hates changes in current, thus protecting G2 from large swings.  My sense is the resistor is just along for the ride, but hey, you helped out your sister. 

[dwinstonwood]

Thanks acheld, I suspected that it's fine.

Thanks shooter. I was sort of looking at the 5E3 power amp when I drew this project up. I guess I wanted to lower the screen voltage and benefit from using a choke. The amp works fine, so I'm just leaving both in place. 

[dwinstonwood]

I've been reading Merlin Blencowe's (a.k.a. Valve Wizard) new 2nd edition of Designing Power Supplies for Valve Amplifiers and I came across this section on suppressing the resonant peak of chokes.

He gives an equation for determining the damping resistor value.

The choke I'm using is 4.8H and 100Ω. The capacitor is 32uF. I'm no good at math, but I get 775Ω. I happen to have a 680/5W that would give a total resistance of about 780Ω. And, dropping down from 1K to 680 will also increase the B+ to the screens and preamp a little bit, too.

shooter, maybe that brother-in-law is not completely worthless. 

[dwinstonwood]

This morning I went back into this amp and made a few changes. It was already a good sounding amp, but now I think it's really good.

First, I went from the 270Ω cathode resistor to a 250Ω 1%, and a 620Ω 1% in front of the choke (using Valve Wizard's resonant peak suppression formula).
This biased the amp hotter at about 93%.

Then, I disconnected the tone stack bypass wiring, but left the Bright cap switchable.

Finally, I went with some TW Express resistor values on the three triodes:
V1 - 100K load/1.5K cathode
V2 - 150K load/2.7K cathode
V3 - 100K load/10K cathode

With the bright cap on, the volume up around 6, and the gain at around 3 it's bright (but, not piercing) and slightly crunchy.
With the bright cap off, the volume around 3, and the gain at 6 it's clean and warm - great for jazz chords.

I'm really happy with what this amp can do now. I think I've found the optimal values for what I want. As I've said before, I'm just borrowing the work others have done before.

Here are the latest drawings. I didn't take all the voltage readings, and the ones on the schematic were measured across the tubes, not to ground.

[dwinstonwood]

I played the amp for a good three hours tonight through the 12" WGS speaker in my 6G3 cabinet.
I put a BYOC Tube Screamer clone in front of it.
With the gain dimed and the volume around 9 o'clock it's aggressive and edgy, and does the live Midnight Rambler thing pretty good.
With the volume dimed and the gain around 9 o"clock it does that creamy Mick Ronson Moonage Daydream thing great.
In between, it's got a whole range of clean to crunchy sounds.
It's the best amp I've built, and the most versatile.

[dwinstonwood]

I needed something to work on, so I changed the preamp components in this amp back to Fender values.
I also took a bunch of voltage readings and added them to the schematic.

Then, I spent some time in PSUD2 creating a sim that is very close to the actual amp voltages. I did this mainly to get better at modeling power supplies.

The amp sounds better.

[dwinstonwood]

You're probably all tired of hearing about this project. 

I made a few more small changes (for the better, I think).

I'm now using 2.2k > 4.7K dropping resistors that brought the input tube B+ right near 300VDC.

The first two triodes are biased at -1.5V.

But, for the third gain stage I went from first using a cold clipper, to a center bias, to now using a warm bias with an 820 Ohm resistor and -1V on the grid. Of the three, I like this the best. My second gain/volume pot is connected to this grid, so I can crank the first volume pot (right after the tone stack) and then control how far into grid-current clipping I go with this stage. In this amp that sounds better than cold clipping into cutoff - at least to my ears. But, it's subjective and subtle.

The 6V6's are cathode biased with a B+ at around 350V, so it's sort of like a 5E3 power amp with an AB763 input and TMB tone stack, and an AA864 extra gain triode that's warm biased with a gain pot in front.

It's a simple amp that has a wide range of sounds with only a bright switch and an NFB bypass switch. With JJ tubes, Hammond iron and a Hoffman Stout chassis it was something I could afford to build.

Thanks for reading.