Attempted Simplification of the Express - Non Traditional

[dorrisant]

rj,
Yes I did use the Presonance control (thanks Martin). I need to play around with it before I make an assessment though.
Mark,
How hot are you running? How hot are you thinking of going?
I didn't use anything different than the original schematic except for the PS dropping resistors. I used 8.2ks instead of 9.1ks... so my voltage chart will probably be a little different than the other three. I'm hoping that might turn out to be a side benefit in our assessment of the project as a whole... don't think it will throw anyone too far off.
I slipped this amp in on a player last night at a live show. I leaned into him on a live stage and warned him to start off low and work up. He gave a contorted look, like he knew what to expect. He obviously didn't though. When be did turn it up he made an even funnier face as he quickly fumbled to turn back down. Good laugh for me and mine. He recovered quickly though, as he is very professional. Once he found his way, he played and toyed with the audience for the better part of an hour. Needless to say we had much more attention leaving than we did bringing in our equipment.
It certainly cut the mustard... with a razor sharpness.
I still need to play it more myself. Probably will today.

Tony

[dorrisant]

After further review...

I think my amp is developing an oscillation... I get a low rumbling noise that ramps up to a level that makes the amp unusable. Does anyone have any clips of bad oscillations? I would like to hear so I could confirm it. It does not seem to be any specific frequency. Just a sputtering rumble. I've changed out tubes in all three preamp positions... some seem to make it worse than others. Then sometimes it just goes away on its own. I realize that someone here may spank my ass with a very simple explanation or fix... but that is ok... I like to learn, even if it is the hard way.

I'm thinking of installing a grid stopper on the input to first half of V1 to see if it will help. I used shielded cable there already as well as the other half of V1. Any suggestions or links to read would be much appreciated.

While I'm under the hood I have a couple of things I want to do. First is to rewire the tone stack so that I can chopstick it into harmonic bliss. I think some of the wiring is crowded to close and the harmonics could be better.

I also want to try a type of power scaling. It is an idea that ampdoc posted about a couple of times... here:

https://tubeamparchive.com/viewtopic.ph ... c&start=15

https://tubeamparchive.com/viewtopic.ph ... highlight=

As he stated, it is just dropping the filament voltage to the power tubes... seems pretty easy to implement. According to Merlin's Power Supply book, I would need to lift the 6.3v center tap and use something like his "humdinger" in its place if the resistance is added to only one of the filament taps. Guys... here is another chance... correct me where I'm wrong.

Maybe Ampdoc or others who have tried this mod would want to chide in...

Tony

[martin manning]

Can you verify that the rumble is not caused by the presonance mod? Colossal and M Fowler seem to have no issues, but maybe just disconnect that flying cap from the pot and see what happens just to be sure.

[dorrisant]

Martin,
I will try that and report back before doing anything else.

[Colossal]

After further review...

I think my amp is developing an oscillation...
I'm thinking of installing a grid stopper on the input to first half of V1 to see if it will help. I used shielded cable there already as well as the other half of V1. Any suggestions or links to read would be much appreciated.

Tony,

On my hot rodded Liverpool (which now has a switch on it that makes the stock Volume=8 Trainwreck distortion seem pretty tame by comparison), I did add 820R on the input grid to the second stage as per Ken Fischer's recommendation. Although others have not noticed blocking distortion with their amps, I am hedging against the possibility. I play 7 strings tuned to B on the 7th and I have had a player test drive my amp with a low A. It stays tight and defined and no blocking distortion. With the gain mod on, I am most definitely pushing the second stage but still free of blocking distortion.

The description of your rumbling sounds to me like you might have a bad solder joint or something else going on. Do you play with hot pickups? Just curious about that and how hard the front end of your amp is being pushed.

While I'm under the hood I have a couple of things I want to do. First is to rewire the tone stack so that I can chopstick it into harmonic bliss. I think some of the wiring is crowded to close and the harmonics could be better.

Cool. Others have noticed this (was it Fishy or Jackie Treehorn?) and moving the treble lead subtly resulted in a big change for the better in the amp's bloom and harmonics.

I also want to try a type of power scaling. It is an idea that ampdoc posted about a couple of times... here:

https://tubeamparchive.com/viewtopic.ph ... c&start=15

https://tubeamparchive.com/viewtopic.ph ... highlight=

As he stated, it is just dropping the filament voltage to the power tubes... seems pretty easy to implement.

I have done this and it does indeed work. By limiting cathode emission, you are reducing the output.

According to Merlin's Power Supply book, I would need to lift the 6.3v center tap and use something like his "humdinger" in its place if the resistance is added to only one of the filament taps. Guys... here is another chance... correct me where I'm wrong.

Maybe Ampdoc or others who have tried this mod would want to chide in...

Tony

Ampdoc and I both did this by modding the filament supply to highly regulated DC. If you want to do it simply and by brute force, you could take a big fat rheostat on the order of a few ohms and place that inline with the filament lead. The humdinger (500R) pot could be used to dial out any imbalance, although if you have a quality transformer, you shouldn't have a noisy filament winding to begin with. At 2.0VDC or so, it is at full bedroom level and the volume is so low as to not move the speakers (very much the same effect as a VVR at low levels).

Can you verify that the rumble is not caused by the presonance mod? Colossal and M Fowler seem to have no issues, but maybe just disconnect that flying cap from the pot and see what happens just to be sure.

My original test of the presonance circuit was wired point-to-point and it is as ugly as they come. That said, I have no oscillation issues and the amp can even idle at high volume without any nonsense.

Can you probe each of your grounds for stray voltage?

[M Fowler]

I have a 33k grid blocker on V1a/shielded wire and also used the 820R grid/shielded wire on second stage per A0 schematic.

My bias only goes as high as 33mA so I am increasing my bias adjust resistor. I have not written down my voltages yet. Got busy with pedals and a Princeton Reverb I had sitting around for too long.

I'm going to add a PPIMV, change slope resistor to 56k, try increasing 10k to 100k on V2a cathode, reduce the V1a cathode to 1uf/820R, and mess with the PI entrance ?470k. I want my power amp more Marshall flavor.

[RJ Guitars]

That's the stuff I was hoping we would be able to do on these. These amps are already a big departure from the traditional Express so exploring and personalizing seems appropriate. Tone tweaking, power scaling, and MV, all good additions and worth experimenting. I appreciate you guys sharing your info with us.

I am really intrigued with this power scaling by heater voltage option. I had always heard that was bad for tubes but I guess that depends on which tubes and how you do it?? If somebody has the rest of the story on this I think it would be worth learning more about it.... edit - never mind, i just read the links that were posted earlier. I am anxious to hear the results in this build.

I have been playing mine through an attenuator (home built Air-Brake) but by the time I dial it down enough to fit the occasion I'm not liking the tone as much as I do just raw through the speakers. I ordered an isolation cab and we'll see if that does the trick... not sure how I'll monitor it yet, I haven't learned to trust the sound man and I'm still adjusting to the in-ear monitors.

I am still enjoying mine and I actually used it at practice on Weds night... we'll see Sunday if I can handle an Express or if she takes me to school? that will be my first public outing with her.

[fishy]

The link to the tone stack wiring and harmonic bliss. Interesting to see an early version of an Express compared to where we are now. The Rikens look cool for sure.

https://tubeamparchive.com/viewtopic.ph ... ack+wiring

[RJ Guitars]

Fishy,

I am greatly pleased to see your posts here... I know you put a lot of effort into tweaking Express amps of your own and you can help us here. Those are some great old archives and I know many people have dialed in these amps and it's all posted if you can sort through the stack and find the gems.

In my tone stack layout I have kept all the wires close to or laying against something grounded anyplace where it's possible. With my rotated volume pot this allows you to put those brite caps and cap leads up near or against a grounded surface. This is the area that has caused me the most grief in past Express style builds and I had not really found what to do with those leads previously.

thanks very much for the info,

rj

[dorrisant]

It was V1... finally found a tube that the amp likes for that position.

I was trying yo be more patient than I wanted to be, but it has paid off. I didn't have to disconnect the Presonance... I've left the amp running with no input for quite a while now with no noise. I was having the trouble before even with no guitar plugged in so this is an apples-to-apples comparison. I am happy with the result and ready to proceed with other plans... on to the harmonic bliss!

Tony

[Cliff Schecht]

RJ good to hear you're trying out the Express live. Once you learn to play it live you'll have a hard time going back to other amps. It's rewarding being able to play an entire set with nothing but a volume knob, tone knob and your imagination!

Also keep us updated on your volume reducing experiments please. My Express is not getting used ATM because it overwhelms everything else at band practice when we mic everything while trying to keep the system volume down.

[Reeltarded]

Just a note, that rumbling.. is is howling? I get this when I am over the limit gain wise with one of my dirtier amps that has a very flat response with all tones at 5. If I get the amp singing really good, and add 2 clicks of bass, woooooooooooooo..

If I change out V1 to a little more reserved tube, no more woowoo. Not gonna happen though, instability rules. Right out there on the edge of destruction, ya know..

[RJ Guitars]

I looked over the entire amp to make sure it was safe to button her up. I did re-flow a couple solder joints that just didn't give me a smile when i saw them and made a few subtle changes... well one maybe not so subtle.

I added an effects loop by taking the signal into the third gain stage and running it out a jack on the rear panel and then back to V2. It appears to work just fine but I only had one pedal here (80's vintage "real tube" distortion pedal) to play with this afternoon so I'm not entirely sure if it's going to work the way I want it to. I wanted to get away from adding reverb or chorus at the input since I like to use those pedals occasionally. There isn't much good history with effects loops in an Express so consider this an experiment until it proves itself out one way or the other.

The other changes were the addition of 820R grid blocker resistors the input pins of each gain stage just as insurance should the beast try to make an appearance. Tomorrow it will be a live run with her and If it goes well, I am going to call her finished and leave her alone... except for metal stamping her name into the chassis.

[statorvane]

That is a really nice looking package; well thought out. If it sounds as good as it looks, it'll be dynamite.

[Bob S]

That looks Stunning!
Wow.
The TW has evolved.
Can't wait for tone reports.
Well Done RJ.