Expectations Too High ??

[Blind Lemon]

Just thinking out loud and wondering if my expectations are too high?

I have built two 6V6 Plexis and have not been satisfied with either. And please don't think this is an indictment or criticism of any one layout, it's not. More a questioning of my own expectations.

So what are my expectations?

That this pre section should drive the 6V6 tubes into submission. I would have thought that this preamp section, made to drive big bottles would drive the hell out of a pair of 6V6 power tubes. I finished this amp and it's the same as I remember the other one I built some years back. Even put in the hot switch after the fact, pre amp distortion went up, not my deal. Changed it back.

In between the 2 builds I used basically the same circuit using cathode Biased EL84s. It screamed, drove the power tubes more than I thought it would. Loud, overdriven and would hold a note fooorrrr eevvvvverrrr. NO PEDALS NEEDED. The tone was good but a Train Wreck Rocket is more versatile, you had a hard time getting clean out of the EL84 version.

So whats my latest build, about the same as the first: sluckey layout, Weber Deluxe PT, Triode TF110-4816UL 6.6K (clipped UL leads), Fender Deluxe choke.

The amp shows no signs of having a problem. I guess I'm just looking more for thoughts of those that have built and may have another opinion.

Again, not that the amp sounds bad, just didn't meet my expections.

I'll post a few shots.

BL

[Blind Lemon]

Build tread:

http://el34world.com/Forum/index.php?topic=21957.msg233517#msg233517

[SILVERGUN]

Using sluckey's schematic as a reference, this is what I'd try to address your concerns:
- Change the grid leak resistors on the 6V6s to 270K
- Lower the dropping resistor value between nodes B-C to raise the voltage to the PI (maybe 4.7K, as a big jump from 22K, then maybe put the 22K between C-D, if needed)
- Change the PI to 12AT7, because PI distortion sounds a lot like additional preamp distortion, and the AT7 can help clean that up by swinging a bigger signal before clipping (change the PI shared cathode resistor to 820 ohm in the process, if needed)

That stuff first, then I would consider:
- Raising the value of the coupling caps after the PI to .05
- Experiment with NFB values

[92Volts]

I don't think 6V6s are much easier to drive than EL34s or 6L6s. EL84s are especially sensitive, needing 1/2 or 1/3 bias and audio signal voltages as most other power tubes (including 6V6) in similar circuits.

Are you using this schematic? http://el34world.com/Hoffman/files/Hoffman_Plexi_6V6-V2.pdf

I noticed voltage C is low due to the 20k filtering resistor. Is your problem that you get preamp distortion before you reach full power? Or that you never get distortion at all?

Because you could lower the value of R39 to send more voltage to the preamp, getting you poweramp distortion with less or no preamp distortion.

If you don't have enough gain to get overdrive at all, that's a different question. You'd need to rework the front end of the preamp, as not much gain is obviously "wasted" in that circuit. Or use a clean boost pedal.

[jjasilli]

I think it's not your expectations, but rather your plate voltage which is too high.    This is resulting in a way high bias voltage -- which may be correct for tube operation at your off-the-chart plate voltage -- but causes too much headroom with your input signal level. 


Tube charts show a pair of PP 6V6's typically run from 250 - 360 plate volts, with G1 Bias from -15 -to- -22.5V.  Your schematic shows 394V with -36V bias!  I think this is defeating your signal from overdriving the power tubes.  If so, the solution is to lower plate & screen voltages, along with bias voltage to spec levels.


Alternatively, at existing voltages, you might try large screen resistors in the 1.5K to 2.2K range or more, to force the power tubes into submission under hi signal conditions.

[Blind Lemon]

Voltages:

[92Volts]

EDIT: Just saw your voltage chart. 20v on Pin 8 of the 6V6s, or 20mV? 20v sounds unlikely but R33 and R34 (1 ohm bias-measuring resistors) would need to be the wrong value for that to be possible.

If your transformer can handle 3A @ 5v, using a 5U4 rectifier will lower B+ a little.

400v isn't crazy for 6V6s, old Fenders were over 400v (I've seen reports of 450+). Of course, I don't know of a 6V6 Marshall to compare to. I do know Marshall plate voltages on EL34s are higher than this, and bias voltage with EL34s under those conditions would have been similar or larger than needed in this amp.

If you decide to lower plate voltage, decrease power supply dropping resistors to keep the preamp from going lower. I'm not sure this is "too low" (it's obviously sounded good to multiple people) but any lower has to start cutting into clean headroom.

[Blind Lemon]

This a weber PT and I don't really want to take a chance on a 5U4. But I do have a weber copper cap in a 5Y3 I could play with. Yes the bias on the 6V6s was ~21mA.

BL

[tubenit]

This a weber PT and I don't really want to take a chance on a 5U4.

What about a 5V4 which like a 5U4 is approximately 1.2 x VAC but draws 2amps instead of the 5U4 drawing 3 amps.

With respect, Tubenit

[Blind Lemon]

Just looked and I got me a few of those. 

Thanks TN

[jjasilli]

400v isn't crazy for 6V6s,
N.B.: By "off the charts" I didn't mean crazy, just literally "off the charts":  400V is not on the 6V6 tube charts.  Actually, I have no problem with crazy, off the charts, or both.  DIY can open up these possibilities, and a reason why we're here (i.e., on this particular Forum).  But there are consequences.  The thing is, we can inadvertently drive ourselves to incompatible design goals.

If you want to go the lower B+ voltage route:  other than a different Rectifier tube, my formula is: a B+ dropping resistor and/or a 12V CT bucking tranny.  With a B+ dropping resistor you can drop up to 50 plate volts.  Maybe that alone will work for you, after re-biasing.  With a 12V bucking tranny - given your PT multiple of about 4 - you can drop another 48 B+ volts; or with the 6V CT you can drop 24V of B+. 

Also, though I love chokes, it's working against your design goal for PA overdrive.  The induction of the choke kills ripple, while its low DC resistance keeps screen voltage high under hi signal conditions.  High screen voltage keeps plate conductance high, resulting in higher power tube output, more headroom, and less power tube overdrive.  That's why I suggested the possibility of larger screen resistors.  Under high signal conditions, large value screen resistors will drop screen voltage compared to plate voltage, hence reducing plate conductance.  This is another route, within the power amp section, to earlier overdrive. 

In sum, the combination of i) high plate voltage, plus ii) a choke followed by small screen stopper resistors that keep screen voltage high under high signal conditions, is incompatible with the goal of easily overdriving these power tubes.

[VMS]

mhuss had that 5k/5W resistor in series with the choke so maybe you can try something similar to drop the screen voltages:


http://mhuss.com/Plexi6V6/Plexi6V6c.gif

[PRR]

You have over 400V on G2 and only 265V on the driver (287V-22V tail loss). This will "drive" but maybe not OVER-drive. If the driver runs on G2 voltage we can usually make the tube cry; you may be a hair short of that.

Tack one 5K resistor as shown. Expect roughly these voltages. Beat it. Do the 6V6 start to faint under heavy hammering?

I did not bother to "correct" the preamp voltages because IMHO they were on the low side of acceptability *depending what you want*. I think the higher voltages are quite fine for a "submission" amp.

[tubenit]

Have you thought at all about losing V1 12AX7 and putting a 5879 pentode in there for V1?   I think they have a reasonably FAT and chimey tone.

I don't think that will make the 6V6's sustain any more, but you may like the tone better with the 5879?    Just a thought since you don't seem very excited about the current tone.

with respect, Tubenit

[jjasilli]

Without radically changing the amp, there are two choices: increase signal voltage; or make the PA more susceptible to overdrive.  (I guess you could do a bit of both as a "3rd choice".)  Everyone focused on the former, so I emphasized the latter.  Either alternative (or both) can be achieved by simply swapping (and/or adding screen) resistors in the B+ supply w/o a radical re-design. 

[Blind Lemon]

Here is what I ended the day with:

A-437 =>413
B-435 =>390
C-287 =>285
D-246 =>265
E-229 =>247

Sustain is better with the Bright Volume @ 2:30 and the Normal at 11:30. (Les Paul)

My ears are done for the day.

BL

TN > I got one of your Carolina jobbies that been setting around not finished. It's next in line.

[DummyLoad]

B+ nodes C D and E are too low. amp will sound best with 350V+ at node C. 


--pete

[ac427v]

I used a Weber Deluxe PT on my 2-6V6 amp. It has two sets of secondaries. With the Red/white secondaries and a 5AR4 I get 360 volt B+. Is that an option with yours?
Craig

[ALBATROS1234]

sounds to me like you prefer the sound of an el84

[Blind Lemon]

Ding Ding Ding - Winner Winner Chicken Dinner

Mr. Cobra was right, dual secondarys on this PT.

I'm not sure this sounds what I had in my head, but I like it a darn site more to my liking.

These are the voltages I ended up with, I may still work on V1 a little.