New Express build; first time post

[geetarpicker]

Now to really get into long shoot territory. C

Could you have some odd short between a board terminal (underneath the board) and wiring running underneath? Or possibly some odd short between a turret on the PS board and a power transformer mounting bolt? With the original style flea clips I used I had to be very careful with board placement and the transformer mounting bolts. Also if you are soldering a turret on top of the board the other side of the turret below can get pretty hot and if there are wires running close to it things may melt...etc.

Have you verified all the wires underneath the boards (that you can't see once it's done) are all correct?

You would think with an error of this sort the amp wouldn't work at all and/or blow fuses, but it's worth a thought.

[WhopperPlate]

I'm not sure of anything anymore. I'm still getting audible oscillations.

I don't know what I'm going to try next besides the power tubes I'm waiting for.

Gotta go to work!

Charlie

[geetarpicker]

You could even unscrew the boards and put some makeshift spacers under them to lift them up a tad more than normal. Also if you have a very small mirror you might look under them with a flash light.

I can't remember if you posted voltages for each tube pin.

Point is your amp looks fine from what I/we can see, but it's not cooperating for some reason!

[Zippy]

Most every time that I've run into this situation (the first was a freshly mod'd Dan Torres Super Texan - grrrrrrrrrrrrrrrr), the issue has been the power supply.

Here's a quote from Aiken's "Technical Info: Q and A":

http://www.aikenamps.com/

Q: Some time ago I purchased an amp from a well-known boutique amplifier maker (whose name I won't mention here). This expensive amp has "new/old" switch on the back. The manual explains that the "new" mode corresponds to the pentode mode and the "old" to the triode one. The trouble is that in the "old" triode mode the amp produces ghost- notes that are out of pitch! Hearing out-of-pitch after-sounds really confuses my playing. The manufacturer says "that's normal" and points to the Matchless brand that "does the same"... I'd be curious to hear Randall's opinion on this subject."

Answer (clipped) "The tube acts as a "mixer" in this case, where the 120Hz (or 100Hz if you are on the other side of the pond) full-wave rectified ripple voltage mixes with the note you are playing, and generates sum and difference frequencies that are not harmonically related to the note. These "out-of-tune" frequencies sound like a second note following your playing, and can get really annoying. This is closely related to something called "intermodulation distortion" which occurs in all tube amps, regardless of pentode/triode mode, as a byproduct of the overdrive/distortion generation itself, but not at that great a level."

"The solution to your triode mode problem is more filtering in the power supply. Some pentode amps will also exhibit this problem if they have insufficient filtering in the power supply section, or in the screen supply. As mentioned previously, there will be some intermodulation distortion present in all amps, but you can usually easily distinguish between it and "ghost-noting" caused by a poorly-filtered supply. Improperly designed bias supplies can also cause this problem, again due to lack of filtering resulting in too much ripple voltage on the supply. In addition, "ghost-noting" can be caused by the speakers themselves. Some brands/models are worse than others."

Bottom line: I'd spend some more time in the power supply - maybe even rebuild it with fresh components if you have them. I am among the heretics that would use a board for the caps rather than stack and goop them.

[WhopperPlate]

Thanks Glen, funny how much I attention to detail I paid to this thing and it just turns around slaps me with oscillations.

Likely I will replace the power caps like I feared.

Like I said I previously, I tried dropping in a 16uf on the mains and v1 B+, but its probably be better to drop in at least 30uf or more. I don't suppose it might be a different part of the PS, like the bias?

Any ways, Those caps are next.

Charlie

[redshark]

Whopperplate:

I completed my build around december and I'm still dealing with the same problem. So far I've tried this ones....

-Correct lead dress issues and resolder some joints that looked "suspicious"
-Get 2 "prescreened" Ei preamp tubes.
-Use a grid resistor in V1 pin 7, I tried 10k and now I have a 15k. That thing solved the "radio station" problem.
-Used a second shielded wire from pin2 to the volume pot, I tried both ways grounding the shield to the volume pot and to the plate in pin1.
-I tried a grid resistor in pin2 820 ohms
- Used some super glue, tried Glen's foam under the board...

Still the instability things are there.....I mean in extreme settings only, my amp sounds very good with "normal" settings. I keep fighting it just because experienced people say it will sound better when issues are gone.
The one I succesfully achieved was the bloom, my amp sounded congested and now with grid resistor in pin7 and second shielded wire I get my transition from sustain to feedback very smooth.

So keep trying..... I will!!

[WhopperPlate]

Thanks Redshark,

For me I can hear the effects of the oscillations and ghosting at most all volumes. Even with cleans the note is hashed up on the high end and you can still here the modulation. When I here nothing but the notes, like in Glens clips, I'll know I'm there.

I can't give up on this thing either, I'm too much of a tone whore.

Charlie

[M Fowler]

Reminds me of the ghosting I had on a Liverpool for which I changed out the F&T filter caps and no problem since.

[WhopperPlate]

No shit! F&T caps huh? I know what's going first! Thanks Fowler!

Charlie

[redshark]

I'm using F&T in my build. I'm not sure if they are giving me the problems. How can I know if they are the problem? Mark did you changed them for Mallorys?

[RJ Guitars]

Most every time that I've run into this situation ... the issue has been the power supply.

Here's a quote from Aiken's "Technical Info: Q and A":
... "Some pentode amps will also exhibit this problem if they have insufficient filtering in the power supply section, or in the screen supply. As mentioned previously, there will be some intermodulation distortion present in all amps, but you can usually easily distinguish between it and "ghost-noting" caused by a poorly-filtered supply. Improperly designed bias supplies can also cause this problem, again due to lack of filtering resulting in too much ripple voltage on the supply. In addition, "ghost-noting" can be caused by the speakers themselves. Some brands/models are worse than others."

Bottom line: I'd spend some more time in the power supply - maybe even rebuild it with fresh components if you have them. I am among the heretics that would use a board for the caps rather than stack and goop them.

Zippy this is excellent technical info. As Glen mentions the wiring seems correct and it is sounding like similar problems have been seen and conquered by others. Aiken does a good job of explaining it and providing a couple solutions. I don't believe the Mallory filter caps are magic but they just might be doing something very right that is measureable and repeatable. I also think his note about the bias supply filtering is also noteworthy. Makes me wonder if cathode biasing it temporarily, would give a quick answer in sorting out the issues?

For those of us that ultimtely believe that Mojo is not only technically quantifiable but it can also be deliberately and reliably duplicated, here are a couple gems to keep in the tool chest.

thanks,

rj

[PlinytheWelder]

I'd still check for cold solder joints....That's what it sounds like to me.

[M Fowler]

PlinytheWelder

Yes I agree, it could be the soldering, but I would concentrate on the two 47uf main filter caps rather than the issue of poor caps themselves.

Depending if all connections were physically tied together at the main filter caps. If they were not physically tied then the solder probably is not giving good connection.

Mark

[WhopperPlate]

Thank you all for the comments.

I tied and wrapped the wiring on all the connections, though I have suspected cold solder joints by exhausting all other possibilities.

I have F&T caps all throughout the preamp only; the power section is all Sprague. I'll go over the solder connections again, but the F&T's are taking a hike.

RJ, I'll send you an email tonight.

Charlie

[redshark]

Maybe the Spragues are the problem then whopperplate....

but before you replace them.....

Check my new thread, see if it helps.....