Tweed Twi-Luxe, Gonna Do It! Need Help?

[Wally]

Gene, imho the amp you have built is better suited for the cabinet that it is 8n right now rather than in a tweed combo. In a tweed combo, won’t the tubes be horizontally mounted and toward the front of the cab? If so, then access to the tubes is less than desirable imho, and are you going to vent the top of that combo cab to allow heat to escape?
I personally would want it to be in the type of head cab it is in right now. Ymmv….but ease of access and heat would be prime considerations for me.

 

[The Ballzz]

Gene, imho the amp you have built is better suited for the cabinet that it is 8n right now rather than in a tweed combo. In a tweed combo, won’t the tubes be horizontally mounted and toward the front of the cab? If so, then access to the tubes is less than desirable imho, and are you going to vent the top of that combo cab to allow heat to escape?
I personally would want it to be in the type of head cab it is in right now. Ymmv….but ease of access and heat would be prime considerations for me.

Yes, good friend @Wally , I've gone down all those rabbit holes of thought! On the one hand, given the chassis mounting, I would bolt it securely to a slightly thicker, 1/2" thick rear panel and secure that panel with machine screws and T-nuts. So four screws would drop the whole amp out of the cabinet. As for venting, I've considered two of the Vox type gold vents. Thcute thing about them is not needing to monkey with repairing the tweed, other than cutting it and maybe gluing the edges. And the gold shouldn't look too shabby against the tweed!

On the other hand, it is a pretty cool little head, as it is!The Marshall logo needs to go though?

Again though, I need to confirm that this amp wants to be in regular rotation, as opposed to being a shelf/studio dweller! The chassis itself is pretty handy for prototyping, with many control hole, jack hole and tube hole options and plenty of room to work with.

Thanks For Your Thoughts,
Gene

 

[Wally]

Gene, I just had to resurrect one of the Limited Edition…250 made…Marshall JTM 45 amps. The chassis is mounted to a sturdy back panel, and that panel is mounted with six screws into vertical rails.….much like a Vox AC30 combo of which I had two Made in England models in at the same time.
The Marshall got dropped in shipment, and the entire chassis structure tore out from those screw mounts. The new owner is hoping that insurance will compensate him for the cab repair, the broken tubes, the damaged electrolytic cap can replacement, and the cosmetic damage to one of the OEM Celestion speakers. That damage to the speaker has an effect on the collector’s value,ime/imho.
From a repair person’s point of view, this type of construction does not make
the repair process easy. The chassis you have there would at least allow one to provide a back panel access to the circuit board with pulling the chassis…or so it appears to me. But….that type of access might weaken the mounting panel???

Logo for The head cab you have there?? ‘LEOJIM’????

 

[D'tar]

While it sounds amazing for many things, it sure could use a tad more gain!

At a glance is that tone stack not a large signal dump? And feedback to boot, even tho 10M. Perhaps a 5f6A tone stack would satisfy? Or some simple tweaks to pass more signal. Not sure how much would overdrive that PI though. Where's the math dept? Andrew, llc

 

[Wally]

That is my favorite tweed preamp….the same as in the 5E7 Bandmaster. Both of the feedback circuits are as is found in that Bandmaster, as well. I never wanted for more gain in my 5E7s…beautiful smooth, singing overdrive.
What settings do you use for volume and tones? Imho, the volume needs to be over halfway. Push the tones up a bit..especially the treble? This stack may prove to be surprising if you use it in a manner that is different from what one would do with the AB763 type of of preamp. Have you tried a 5751 in the input gain stage?

 

[The Ballzz]

@Wally & @D'tar ,

Notice in the schematic below that the 220K (red arrow) is different than the 100K of the 5E7 Bandmaster, as well as the default value of the Fender E-series of TSC in the web:

TSC in the web

I swapped in the 100K in both the calculator and on the amp and there was a noticeable difference in gain, as well as how the "mid scooping" occurred, through the travel of the pots. It is also interesting how trying a 50K and then a 10K affected the operation of the tone stack in that same location. So far, I've only done that with the calculator, but tweaking and twiddling that resistor value may produce some favorable results? What I think I'm shooting for is being able to achieve a balance between the Bass & Treble controls that gives the flattest response curve, with the least gain loss and gentlest mid scoop. Playing around with that calculator can be kinda fun and more importantly, enlightening!

Of course, another HUGE factor is currently only having a mixed bag of crappy current production 12AY7s on hand. Trying 12AX7s in place of the specced 12AY7s did NOT produce much love! It only recently came to my attention how important a really great 12AY7 is to the performance of my 5E3 and I'm simply extrapolating/assuming it will be at least as important in this TWI-LUXE! The differences between various CP offerings and the one old stock Baldwin branded Sylvania I have are staggering, to say the least!

Please also note that although the schematic does not show it, I've inserted a 1MΩ Master volume pot between the tone stack and driver stage V3b and this allows me to determine where the onset of power amp overdrive/distortion occurs. It also allows me to determine that there is no distortion in the preamp section, although the tonal complexity of said preamp is pretty stellar! I must say that the overdrive characteristics of this power amp section are quite pleasing indeed! Now if I can simply get the preamp to an "edge of breakup" level, all will be glorious. I may monkey around a bit with that "local feedback" loop and see what happens in the real world. I'm thinking that possibly separating the cathodes of V1 & V2 may help a bit. The totally passive effects loop (just before the Master) works wonderfully with my rack mount Lexicon delay/reverb!

Thanks For The Thoughts,
Gene

 

[D'tar]

I dont tinker at this level but Id sy youre in the right spot to experiment. Am I right that the 220k/220k there dumps 50% signal?

Also, sure would like to hear this monstrosity at some point!

 

[The Ballzz]

I dont tinker at this level but Id sy youre in the right spot to experiment. Am I right that the 220k/220k there dumps 50% signal?

Also, sure would like to hear this monstrosity at some point!

Well, first I'm gonna play with those resistor values in the tone stack and once I get that to my satisfaction, I'll go on to experiment with the local feedback values and even eliminating that loop, just for confirmation of what it does sonically. I will then play with splitting the cathodes of the first two pairs of triodes.

As far as hearing it, if I am at all proud of my work, I will share. I don't often share audio clips, as I'm such a hack at guitar, even though my tone is typically fabulous!

Just Tweakin' 'n' Twiddlin'
Gene

 

[D'tar]

Am I right that the 220k/220k there dumps 50% signal?

No. Lol never mind me!

 

[Wally]

Gene, it is interesting to compare these 2 x 6L6 tweeds….5E7,5E5A, 5F4, and 5E8/5E8A to the big tweeds…5F6A. In the first group, there are some difference in the NFB loops…especially the local. I have never been around a 5F4, but of the others, the Bandmaster 5E7 is my favorite. The 5E5A Pro has more cancellation in the local and less in the global Than does the Bandmaster, ime, the Bandmaster ‘sings’ in a way that the Pro does not. The Pro is a bit stiffer in the preamp, ime; and even with less cancellation in the output section, it does not ‘sing’ as early when pushed as does the Bandmaster.
The big tweeds have a different tone stack, no local feedback around those V2 triodes, and has greater cancellation in the global….it is much stiffer, ime. One would wonder if the difference in the tone stacks is the reason for the difference in whether or not the amp has that local NFB loop???
Keep up the research….

 

[The Ballzz]

@Wally

As you've obviously noticed, it is exactly those "subtle" differences I am endeavoring to track down and understand what they actually do from a sonic standpoint. Many folks may point out that the real differences are the varied speaker complements of each of those mentioned models, but I believe that is only part of the story. This is why when sonically testing and comparing amps I always use my '72, 4x12, 1960B Marshall cabinet with G12M/25watt/75hz greenback speakers. While not always the absolute "best" sounding cabinet for some amps, it is always consistent, I know "exactly" what it sounds like and it can easily handle the wattage of any amp, cranked, up to an 80 watt Showman or Twin Reverb. I would not however, crank a 100 watt Super Lead into this glorious old war horse!

For some reference, you should go to that TSC in the web calculator

TSC in the web

Plug in the value of the 220k resistor that I "arrowed" in the schematic above (R3 on the calculator) and watch the difference throughout the sweep of the pots compared to the 100K value of the 5E7 Bandmaster. Then try different resitor values, all the way down to 10K. When you get to those lower resistor values, then plug in different bass pot values (RB on the calculator), like 500k and 50k to watch what happens. You will find that different combinations of just these two values allows different overall gain levels with the pots set to provide as close to "flat" response as you can get. Then notice how the different values can be used to "move" the center point, width and depth of the mid scoop as you move away from that flat setting. I'm finding that R3 at 10k and RB at 500k may be fairly useful, by comparison, at least from my standpoint. These differences are anything but subtle! The feedback values will unfortunately be trial and error, as there does not appear a calculator out here for such! These values provide very little (if any) "scoop" but can provide a fairly accurate shelving of bass and treble frequencies, as well as an effective HPF for rolling off the tubby/flubby bottom end, if needed.

Just Sharin'
Gene

 

[The Ballzz]

Well Fine Folks, the words "Need Help" in the thread title were kind of a misnomer.

First> The Voltage survey above incorrectly showed a 5U4 rectumfryer, but was actually a 5V4. I have since bumped up all the voltages with a GZ34, as well as adjusting the dropping string to provide a bit more voltage to the screen, PI and preamp nodes.
Next> I changed one resistor in the tone stack to eliminate "mid range scoop" unless both Bass and Treble are nearly floored. The arrowed resistor is now a 10K, but I left the 1M pot. Treble around 8 or 9 and Bass at about 3 or 4 yields a fairly flat response and I simply add in a bit of the normal channel to thicken the bottom end. And yes, this amp loves being close to fully dimed! :naughty:
Next> I found that the passive effects loop send was just a bit too hot and upgraded the loop to a Granger unit. It works flawlessly and is as inert and transparent as could be! It didn't even shave any more than 1 volt off of the power rail and is dead silent. I can easily use the send as a "preamp out" to the effects return/power amp in of a different amp or use the effects return as a power amp in on this amp. Lots of cool options!
Then> The springy, bouncy character of the touch sensitive feel and response is absolutely amazing! It doesn't lose highs when the volume of the guitar gets rolled back, but simply softening picking attack works gloriously well also! I can go from clean to dirty to clean without touching a knob or stomping on a switch. It puts all control right in my hands, where I like it!
And Finally> The Master Volume is somewhat of a "white elephant" for now, but works as expected. It may find a use down the road, should I ever need to replace my Lexicon MX300 delay/reverb unit for something else. Did I mention that this amp provides no distortion in the preamp, but puts out a VERY strong, clear signal? I may monkey with splitting the V1 & V2 cathodes to color those two channels a little differently, but for now, This amp is truly amazing and exactly what I was shooting for!

Thanks Fine Folks!
Gene