Express PI blocking distortion?

[sonofmickel]

Fixed: see next post...Check your power tubes!

Completed an Express homebuild.
I am getting some crunchy breakup when I dig in the some power chords.
All my voltages look good except the PI.

My voltages on the PI:
198.7, 32.9, 35.6 - 199.5, 32.7, 35.6
from the voltage chart it should be more like:
190, 28, 42 - 193, 27, 42

Pins 2-3 and 7-8 voltage ratio is what I am looking at.
On the chart they are approximately 14 volts difference.
On my build they are only 2.5-3 volts of a differential.
Is there anything to the voltage difference here?
Could that be an indicator of what I perceive to be blocking distortion?
Am I overloading the PI somehow?

Build is stock Express, no mods except for power string and transformers.
My b+ is 450 (Marshall PT), a few cement resistors to drop voltage to 308 for the PI,10k dropper for V2 and 10k dropper for V1.
OT is 50 watt Classictone 40-18025 Plexi 3.6k.
Running 6l6GC for now until I change the Bias resistor to bring it in range for EL34's and 6V6s.
So yeah...no mods.

Cabs used are a 1x12 Blankenship openback with Mesa Black Shadow and a pine 2x12 closed back with Century Neo 100's.
Cabs work great with other amps.
When I turn the Express to thremo-nuclear, 2-3 o'clock on the volume, the amp starts to compress a bit and I don't notice the distortion so much(saturation seems to compress a bit as well). I only tested at that volume for a minute though...neighbors.
This is NOT an apartment amp, nope, never.

Sounds like the crispiest touch sensitive amp I have ever played. VERY LOUD AMP!
Low hum, regular hiss, and can turn everything up near 10 before I get some high oscillation (I'll tackle that next).
Guitar pots are very interactive. Never played such an alive amp!

Anyway, just wondering if the PI voltages reveal anything.

[sonofmickel]

Fixed: I was using some beater 6l6GC tubes to test the amp.
Changed bias resistor for EL34 and 6V6s tubes.
JJ 6V6s = sweet singing, "soft" saturation. A little too saturated for me though, guitar didn't clean up enough, kind of "wooly" in the bootom.
EL34B's bring the pain, waaay too loud but crisp vintage gain and cleaned up much better than the 6V6s tubes.

Bias differential is still 3 volts, must be the tube.

I will say that this particular amp build immediately sounded better the longer I let the amp "burn in".
Could literally hear the amp smoothing itself hour after hour.
Noticeable difference over approximately 10 hours of bench time(with the old 6L6GC tubes)

Now, need to build an attenuator of some sorts.

[wpaulvogel]

Had you used a proper 6.6k output transformer, it would be even better. I wouldn’t doubt your amp is making 50-55 watts clean rms and 95-100 watts full power. An Express makes 32-35 watts rms and 65-70 watts full power.

[sonofmickel]

6.6k OT...that would be better with the 6v6s too...
What differences could I expect?

All I think I would want is a bit softer attack.
I've got some NOS preamp 12at7 tubes and a JJ ECC832(12ax7 first triode and 12au7 second triode), so that would flip in circuit to 12au7 input, 12ax7 eq recovery...might be cool.
Might try out 150k-220k plates...I'll research.

Best 6.6k OT?
Classictone?

[wpaulvogel]

Pacific
I have one spare
I can sell it

[sonofmickel]

Amp is done for me, keeping the 50 watt 3.4k Classictone Plexi OT.

Tried some mods last night and this morning. Ended up going back to stock on everything except for grid stoppers 33k input and 820 ohm after tone stack(those made the amp completely stable at with everything on 10).

Mods tried:
Changed 150K voltage dropper to 47k and 68k, sound was cool but lost the early onset of gain.
Back to 150k.

Different bypass caps on V1a and b. Tried 4.7uf, 5uf, 6.6uf, .68uf on both.
The small caps were just too thin sounding. Really cut some bass and allowed everything on 10 but the sound was too shrill.
Back to stock. Mine are Sprague 30D 25uf 25vdc.

Preamp tube swaps
12AT7, ECC81, Preamp and PI.
Sound was actually more controlled but lost the "snarl"? Still a very bright amp but lost the controlled feedback.
ECC832 in V1 was not very good, seemed to lose the complete character of the amp.
All tubes back to 12AX7/ECC83s.

Power Amp tubes:
EL34's are the ticket, clean to mean, it's all there.
6V6S are cool too, might experiment with these more as volume was much lower to get the gain going. Still was a bit mushy though.

Conclusion for me:
Great sounding amp(would like to try a 6.6k OT).
The amp is completely impractical for me, to get the "goods" I would need to move to a bunker style home or music studio.
I'm keeping the amp like it is for at least a few months, try some different pedals with it, try to get a hold of an attenuator and try to source a 6.6k OT when funds allow.
This amp project was made with found parts, some new, some used and is the most "alive" I've ever played, it truly is another instrument and should be approached as such.
I am still basically a beginner at building amps and I learned more about lead dress and component value choices than any other build, each component seemed to make a noticeable difference.

The Trainwreck Express is also an incredible litmus test on a guitar players ability, you are completely exposed, either a good player or a hack...I'm a good hack ; )

[Roe]

Amp is done for me, keeping the 50 watt 3.4k Classictone Plexi OT.

Tried some mods last night and this morning. Ended up going back to stock on everything except for grid stoppers 33k input and 820 ohm after tone stack(those made the amp completely stable at with everything on 10).

Mods tried:
Changed 150K voltage dropper to 47k and 68k, sound was cool but lost the early onset of gain.
Back to 150k.

Different bypass caps on V1a and b. Tried 4.7uf, 5uf, 6.6uf, .68uf on both.
The small caps were just too thin sounding. Really cut some bass and allowed everything on 10 but the sound was too shrill.
Back to stock. Mine are Sprague 30D 25uf 25vdc.

Preamp tube swaps
12AT7, ECC81, Preamp and PI.
Sound was actually more controlled but lost the "snarl"? Still a very bright amp but lost the controlled feedback.
ECC832 in V1 was not very good, seemed to lose the complete character of the amp.
All tubes back to 12AX7/ECC83s.

Power Amp tubes:
EL34's are the ticket, clean to mean, it's all there.
6V6S are cool too, might experiment with these more as volume was much lower to get the gain going. Still was a bit mushy though.

Conclusion for me:
Great sounding amp(would like to try a 6.6k OT).
The amp is completely impractical for me, to get the "goods" I would need to move to a bunker style home or music studio.
I'm keeping the amp like it is for at least a few months, try some different pedals with it, try to get a hold of an attenuator and try to source a 6.6k OT when funds allow.
This amp project was made with found parts, some new, some used and is the most "alive" I've ever played, it truly is another instrument and should be approached as such.
I am still basically a beginner at building amps and I learned more about lead dress and component value choices than any other build, each component seemed to make a noticeable difference.

The Trainwreck Express is also an incredible litmus test on a guitar players ability, you are completely exposed, either a good player or a hack...I'm a good hack ; )

Try a mismatched load, e.g. Running a 16 ohm cab from the 8 ohms secondary

[katopan]

Mismatching the load is a great idea. Running 3.4k means the screens won't be drawing as much current as the power stage goes into clipping. This will effect the clean to mean transition dynamics which are finely tuned in the design. If you can run you speaker cab into the lower output jack as Roe suggests, you'll be at 6.8k - you'll notice the amp feel will be even better than what you're experiencing now.

I was disappointed with 6V6s in my Express and run EL34s all the time. To me that is the tone I wanted from the amp after hearing sound clips.

Keep the amp, get an attenuator set up. I love playing my Express at home and use a reactive dummy load with a line out, which goes to an old SS bass amp combo (where I've replaced the speaker with a V30) for re-amping. I preferred this set up over passive attenuation driving direct to the speaker and allows me to play anything down to below TV volumes. It sounds fantastic. So true what you say about an Express being revealing of the player. There are days I love it and days I hate it, all depending on how I'm playing on the day. No other amp does that to the same degree.

[sonofmickel]

So... 8 ohm cabinet into 4 ohm OT jack will reflect 6.8k with EL34's? (mine has individual OT speaker outputs (4,8,16).

How about with 6V6S tubes, will they then "see" 6.8k?

How about speaker for my 1x12?
I have the Mesa Black Shadow in there now 8 ohm 90 watts but I have an 8 ohm V30 around here too.
Or...a 2x10 Deluxe Reverb with Weber 10A125 speakers(rated at 20 watts each).

[Roe]

So... 8 ohm cabinet into 4 ohm OT jack will reflect 6.8k with EL34's? (mine has individual OT speaker outputs (4,8,16).

How about with 6V6S tubes, will they then "see" 6.8k?

yes the reflected impedence will be 6k8 in both cases