Attempted Simplification of the Express - Non Traditional

[RJ Guitars]

Danny,

Very cool to have you in the builder lineup, I'll be curious how these compare to some of the original recipe Express builds. I have not tried those old school tubes I got from you - I sorta forgot I had them. I had set them aside for something special - this might the time to feel special?? We practice tonight so I'm going to give them a shot.

I have a few guys signed up for the next bare bones iteration of the PCB Express. I am going to make the hardware so that the same board and chassis frame will work for either a 15 watt Liverpool or a 35 Watt Express. It takes a swap of the top plate and different trannies between the two, but otherwise same hardware. The board will offer either fixed or cathode biasing options on the output tubes so a few things to play with here as well... Liverpool with Fixed Bias and Express with cathode bias for example.

Thanks for the interest everybody, I'll plan to submit the board design this week and we can probably look at building amps by the end of the month. I am playing with the effects loop on my build as the final step in doing the board layout. The effects loop works OK, but as built it loads the 2nd gain stage and changes the dynamics of things. Simply, I loose some of the mean in the setup. I need to use the extra cathode for a cathode follower to feed the effects loop - thanks to whoever mentioned that by the way. I am 99.9% of the way there but I want to hear it live at practice tonight before I finalize the rev 1 board design.

rj

[dorrisant]

This is the first Express I have built. That said... correct me where I'm wrong.

I'm getting a white noise that is rendering this amp unusable. I didn't have any issues at first. Then after playing on it for a few hours one day, white noise crept in to a dull roar. This happens even with nothing plugged in. I have scoped it since and do not see any rouge waves riding on anything. I used my amp stethoscope and found the noise after the first stage and getting louder through to the power section.

I talked with rj about it and he sent me a tip on a snubber for the power tubes. He also reminded me that I may have to roll some tubes until I find the right combo. I hope this might be all there is to it,but I have swapped about 30 different tubes around in all sorts of combinations and permutations. I am currently rounding up the snubber parts to add this... but I have another question...

Why am I not seeing the HT voltage at one of the power tubes. I get roughly 380v on pin three of the power tube closest to the PI. With tubes in of course,I can turn up the volume and see this voltage stay relatively the same all the way up to full volume. The other power tube is showing different... roughly 380 with the volume all the way down. When I crack the volume just enough to get signal (or just noise),my meter shows the voltage drop to zero and then the meter acts a little funny... like its trying to switch ranges and the mA indicator lights up. I don't understand this and compared to my other amps I've never seen this. I wanted to check to see if this would happen if the noise was not present,but it seems to be present all the time now. Looking at the voltage charts... that is not right... I should be seeing 415v or so. I will have to look at this area more.

The bias is about 6mA higher on the tube next to the PI ,and the other tube is showing the 6mA lower draw even while the volume is up and pin 3 shows no voltage. This will not follow the tube if I swap them. When I did swap them,V4 for V5, the voltage and current draw remain about the same... 380 next to the PI and 0 on the other. I have another Fluke that I will bring home today so I can verify. I will also try a 12AU7 at V1, but I don't want to leave it that way as a fix.

I have completely rewired the tone stack,installed a new input jack and swapped the shielded inputs to V1 with solid core PVC coated wire, (no extra noise wired like this,but I wanted to remove any doubt). I have reflowed every solder joint even though I didn't think it was the cause. Also different power tubes will not change anything. I checked all the component values again as well. One thing I didn't do was check for DC being blocked through the signal caps. I will do that later today. All the voltages look pretty good except at that one power tube. I wish I understood more of what is going on here.

Could this be just the preamp tubes? School me guys, please. Apparently I need it! Frustrating lessons are always the ones I seem to learn the most from.

Oh yeah... rj, I don't know how I will pay for or explain, but... put me down for the 15 watt Liverpool... the disease is spreading!

Feeling slightly retarded,
Tony

[Cliff Schecht]

With the Express the trick to taming the hiss is cutting the treble and presence all of the way down and bringing them up only to where they need to be. Treble on my Express never goes past 3 and presence stays at around 3.5-4 (both out of 10).

With that said it sounds like you have a bad or incorrect part, or a bad tube somewhere. Is there anything in there that you heated too much or was a recycled part? Leaky caps can cause bias to shift all over the place but bad resistors are a bit trickier to track down. Most you can measure in circuit though with the tubes removed.


The Express is super finicky with tubes. Sometimes you'll find one that's low noise but microphonic, sometimes they'll be non-microphonic but noisy as all get out. Have you tried known good tubes throughout the amp? Are the power tubes new? Do they check out ok in other amps?

[dorrisant]

Have you tried known good tubes throughout the amp? Are the power tubes new? Do they check out ok in other amps?

Yes, I swapped out tubes with my Rockster with no change in results.

I did find the 200k resistor just before the reservoir cap has opened up to about 1.5 meg... changed that... noise was waiting for me as soon as I fired it back up. Will do more checking when I get home.

Thanks for the response, Cliff.

Tony

[martin manning]

Tony, the HT voltage has to go through the OT to get to the plates... I'm wondering if your OT doesn't have some kind of problem on one side of the primary?

[RJ Guitars]

Tony, the HT voltage has to go through the OT to get to the plates... I'm wondering if your OT doesn't have some kind of problem on one side of the primary?

I am thinking the same thought... if one side has the voltage then the other side has to have the same or there is an internal disconnect. If it's a bad tranny then Edcor will take care of that for us.... luckily you get the experience of diagnosing the problem and changing out the part if it turns out to be bad.

Diagnosing that - swap the leads on the output tubes and see if the problem tracks with the location. Also you have a set of UL taps on that tranny. You might or might not have the same problem with the UL tap on that same side of the transformer but you can hook those up as the primary taps and see if it's functional. Either way it gives you a data point you can use to sort out the problem. If this seems like the OT tranny is the problem, "clip lead" in another output tranny and see if it works. This might get you the info you need to understand what's up.

rj

[fishy]

If you want to switch over the OT leads just remember to consider the negative feedback.....

[RJ Guitars]

Yeah I think it becomes anti-negative feedback and rather than making sound come out the speaker it might actually suck sounds out of the air and your guitar will play itself... or something like that.

[RevD]

Yeah I think it becomes anti-negative feedback and rather than making sound come out the speaker it might actually suck sounds out of the air and your guitar will play itself... or something like that.

Might be good for some folks... Heh...

Regards,

Don

[dorrisant]

I did try out the snubber but it didn't help any.

It did have a few moments of clarity... but after a minute or two of warm-up it started up again. These brief moments sounded perfect, but short lived.

I turned down the treble all the way and played like that with the volume at about 9:00 and all others at noon. It seems like the noise would get loaded down and wasn't as noticeable until I stopped playing or turned the guitar signal down. You could hear the smaller noise level swell back up to swamp out the lower guitar level. This sound from this setup of the knobs was not what anyone would call useful other than to grab some troubleshooting data... which I'm posting here.

I will try swapping the leads, brown for blue and post that. I think I have an old tranny that I will clip lead in as soon as I can.

Martin, could I use the neon bulb trick to check for shorted windings? If that would work, I could easily check it by lifting a couple of leads. Maybe that would reveal some important info...

Thanks for your help guys... I will keep trying things and posting until it is working.

Tony

[martin manning]

The neon bulb test is exactly for that purpose (detecting shorted turns). Sounds like there is some temperature dependency, so it might have to be warm for the problem to show. Swapping in another OT would be the definitive test, I think.

[badtweed]

I would recommend that you isolate/disconnect the amp portion of your circuit from the preamp.

Next inject an outside line level source like an
iPod playing mp3s into your output circuit to troubleshoot the back end of your amplifier circuitry first before going through a lot of preamp rework that might not be the primary source of your headaches.

You might want to make sure that your tube socket contacts have good contact tension.
I ran into a problem with inadequate pin socket tension on an output tube
recently and I spent many hours troubleshooting to no effect until I finally discovered the ht voltage was simply not getting to the tube pin. I relied on the measurement of the ht at the solder joint to the tube socket and assumed all was well with the rest of the path to the tube itself.
This happens so infrequently that I first dismissed it as a source and subsequently spent plenty of time chasing my tail and blaming it on everything from cold solder joints to President Obama.

[Clyde]

Nice job Dave. I can only aspire to dream about doing a build like that.

[dorrisant]

Last night I finally got a chance to sit back down and check this thing out...

I checked the output TX by using the method outlined here:

www.premierguitar.com/Magazine/Issue/20 ... ester.aspx

Martin had suggested this method on another post... so I can't take credit for it. I built the bulb into a 1/4" jack... with the amp unplugged, the standby switch off and the OT feeds to the power tubes lifted, I plugged the bulb into the main output jack. I applied a 9 v battery to the lifted wires and pulled them off... the bulb flashed. So no bad windings here. This little test bulb setup was so cool... too easy to make and use to not have around.

I got out my stethoscope and started checking close to the input and found C2 to be the source of the noise. I had checked here before and not heard anything. This time it was obvious. I swapped it for new and tried it out. It was too late yo play but I left it on for about 1 1/2 hours with no noise other than a slight hiss. It still looks like I have an unbalance within the PI... so one problem down one to go.

I will dive into it again tonight.

Tony

[RJ Guitars]

Tony, any update on your progress?

I have Mark Fowler's PCB Express build here now and it sounds awesome... maybe better than the one I built - something isn't right here??? This amp will probably up for sale before too long if anyone wants to take the path of least resistance to Express tone... way too many amps in my life right now.

For the guys interested in building a PCB Express or Liverpool, I ordered the next iteration of the PCB boards. I expect they will be here in another week... I have the Liverpool chassis almost wired up and ready, looking forward to whatever challenges that lay in store?? So many builds and so little time.

rj