Tremolux 64 Work Order

[GotA24Fretter]

You can also fit a LarMar very easy in the ground hole. Then you can crank and not get a visit from an angry neighbor that doesn't appreciate involuntary live streaming your performance into their home.

Change the cathode caps sooner than later. I've had some fail in such a way as they test as diodes on an M Tester unit. This would be mostly unnoticed as far as function because diodes can be used to bias a triode, but the bias will be shifted from the intended point. This can result in less/more gain from a stage and a noticable change in overall sound of the amp. A cathode cap that fails open will cause a roughly 6dB drop in gain across the board. Don't be surprised if you find your amp sounds louder and fuller (ie. As intended) once you perform this maintenance. And don't sweat if you can't find a 25μF cap. Anything larger than about 10μF will be sonically indistinguishable, as it fully bypasses all guitar frequencies. So the readily available 22μF is just fine.

As for the 100 ohm artificial center tap, you may hear a slight difference in hum between the two arrangements, especially if the filament CT isn't perfectly balanced, or the tolerance of the resistors used are wide. It's hard to say which would be quieter in your specific application. This is why hum balance pots exist.

Also, seeing this photo again reminded me of 3kV diodes from plate to ground. This is also cheap insurance for the output transformer if you're averse to adding MOVs. It's added to the socket, so easy to do while replacing plate and screen resistors. It adds a negligible amount capacitance to the plate, so it's sonically invisible--but it may add some stability to the power amp section if it is on the verge of ultrasonic oscillation. R3000s are a popular choice for this.

 

[Jasonpatrick]

2) To clarify, my advice would be to remove the death cap. It's a safety issue and provides no value. I was saying to leave the ground switch and convenience outlet alone. The ground switch will just do nothing once the death cap is removed.

This. No need to rewire anything. Just clip it out. Or like I said, just bypass that whole thing and leave it wired in place but not connected to anything live. But yours is way easier

 

[jaonline]

Thank you again friends.

Ok building my parts list. Main board capacitor options:

F&T MERSEN 25UF 70V AXIAL​

F&T Mersen 25uF 70V Axial Style Electrolytic Capacitor

F&T is now Mersen.Made in Germany. Founded in 1948 by Heinz Fischer and Alfred Tausche, F&T Capacitors has over 60 years of experience in producing some of the finest capacitors available today. F&T Caps are known for their great tone and reliability.

mojotone.com

Mod® Electronics, Aluminum Electrolytic​

https://www.amplifiedparts.com/sites/default/files/associated_files/c-et25-50-mod.pdf

Vishay BC, Aluminum Electrolytic, 021 ASM Series 63v​

https://www.amplifiedparts.com/products/capacitor-vishay-bc-aluminum-electrolytic-021-asm-series-axial-lead

Dont mind spending a few extra bucks for the five or so caps I need here.

 

[GotA24Fretter]

MODs are fine, Vishays are excellent. You'll probably never notice a difference between them though. I'd go with the MODs just because they have amp specific values and are very inexpensive. Buy a few 8μF and 250μFs, just in case. The 8μFs are great for killing bias supply hum at the pot wiper. And the 250μF will help tremolo intensity as mentioned earlier.

I've heard the F&Ts can be a bit too large, but I think that's mostly their double cap. Don't have personal experience with them.

 

[Pete Farrington]

BTW 'solder to chassis' was considered solid in the '60s

My understanding is that as far as the regs go, it's still fine provided it's done properly, suitably powerful soldering tool and a surface plating that takes the solder.
I suspect it's not used in modern production amps because it's inconvenient.

pos/neg voltage

I suggest to think of the incoming power as AC hot / live, and neutral.

I do wonder why the ground lift switch is not a problem because doesn't that flip the polarity of the wiring?

No, the switch connects the chassis death cap to one side (eg live) or the other (eg neutral) of the incoming mains power feed, or leaves it disconnected when set in the middle. The point being to set the switch to the quietest position, which would typically be with the chassis connected to neutral via the death cap. Because neutral is connected to planet earth back at the local supply distribution transformer, so the voltage on it at the wall outlet should be fairly low.

If the neutral connection to planet earth there wasn't good, it also may be noisy, hence the facility to set the switch in the middle, leaving the chassis floating.

Remember that the mains connectors were unpolarised, so it's invalid to think of one of the incoming power wires as being 'live' and the other 'neutral'. Rather it was a coin flip which was which every time it got plugged in.

So the idea some guys propound, that old Fenders put the power switch in the live feed, fuse in the neutral feed, is invalid.

Mod® Electronics, Aluminum Electrolytic​

https://www.amplifiedparts.com/sites/default/files/associated_files/c-et25-50-mod.pdf

I'm not keen on the 50V rating there.

 

[Pete Farrington]

it's super logical to replace the important resistors that sit baking over the tube sockets with modern, high-wattage, heat-resistant versions

I don't think there's a need / benefit from increasing the ratings of the 1k5 grid stoppers and 470R screen grid resistors?
Unless there's a weird valve short, the grid stoppers will never dissipate more than a few milliW. And 1W screen grid resistors are fine until their valve shorts. Then they typically blow and shut off tte fault current. Uprating them without also adding a HT fuse seems to me to put more risk on to the transformers, which in this case are valuable vintage.

But I suspect what is important is to fit them with some 'give' in their leads, so that thermal effects, vibration etc doesn't put mechanical stress on the resistor body.

And keep the screen grid resistor body well clear of any wiring, as they can run hot.

Simply adding diodes to the rectifier, an MOV across the courtesy outlet, another off the plates to the center taps is both easy to do and leaves the tone and performance intact.

3kV diodes from plate to ground. This is also cheap insurance for the output transformer

In my experience, adding anything to the super sensitive output anode circuit changes the frequency / phase response.

However I'm a strong advocate for adding protection silicon diodes in series with valve rectifier anodes.

It adds a negligible amount capacitance to the plate, so it's sonically invisible--but it may add some stability to the power amp section if it is on the verge of ultrasonic oscillation

I don't think it's safe to assume that adding capacitance to the output anodes will necessarily improve stability. Because an LCR resonant circuit is formed, depending on how everything plays out, it may push things in the opposite direction.

As a new bias board is to be made, I suggest that this idea for heater elevation is considered https://el34world.com/Forum/index.php?topic=30961.0

 

[King Fan]

My understanding is that it's still fine provided it's done properly, suitably powerful soldering tool and a surface plating that takes the solder.

You may well be right; I *thought* that US code now requires a mechanical connection. I'm not sure. Anyway, I'm not digging back into the NEC now, because 1) I'm lazy, and 2) I'll lean into the other thing you note: I doubt many home hobbyists can "do it properly".

I don't think there's a need / benefit from increasing the ratings of the 1k5 grid stoppers and 470R screen grid resistors?

I'm sure you're right that upping the wattage (beyond 1W?) isn't needed, and if you want the resistors to fail in overload, it's not even 'can't hurt.' My increased wattage was probably incidental. I set out to replace the old twice-baked CC resistors I found sitting over vintage sockets with something heat resistant, so grabbed metal oxides, which I happened to have in 2W and 3W sizes. Then, for the screen resistors, I started using these (relatively low-resistance for my little 6V6 amps) 470R wirewounds because they're heat resistant and very light for hanging off the socket; they happen to be 3W.

 

[Pete Farrington]

An HT fuse holder can be used in place of the unused ground switch

Yes, that's a great suggestion, I've fitted them there previously.

 

[sudogeek]

A bit OT but I’m a great fan of the ‘convenience’ outlet. I almost always play through my tube reverb unit which is a required match for a vintage Tremolux, btw. And in those amps with effects loops, it’s convenient to have a power tap for these effects in the loop - often sitting on the amp. I often cut a hole and wire one in on my new builds. You can wire a proper grounded receptacle when you re-do the power cord.

 

[tele_savales]

You

Hi Friends.

Really appreciate all the great replies here. So as way of an update I have been working hard at finishing the Mojotone Tweed Princeton kit as I noted above. After a slow week of the build (my first build) and three evenings trying to figure out why it only made no sound or a squeaky, feedback sound, I am happy to report the MT Princeton is up and running. It sounds great through the T-Lux cabinet (4ohm 2x10).

So with that knowledge gained I am turning my attention back to the T-Lux. To answer @Jasonpatrick 's questions, I will not gig with this amp or very rarely will but no current plans to do so. The amp currently is very quiet and sounds great. I do not need to fix anything. So everything I am worried about right now is preventative and for my home studio use.

So using the same numbers as the original post here are my follow up questions and thoughts on the work order:

1. AC Cord - It is becoming clear to me the cord length is a real problem, it floats in the air from the amp to the outlet. I can run an extension cord, but the only place the amp can sit in my studio space means this would be a permanent 3 foot plus extension cord. Seems like a bad idea. The cord is obviously not original. So I am inclined to use the existing transformer lug connection and really bolt down the ground using @King Fan 's advice. So in this case it would be a one for one swap without any other changes.

2. Death cap, ground lift switch, outlet - Ok on this good advice I plan to leave all of this alone. I hear that the capacitor is not a danger but I do wonder why the ground lift switch is not a problem because doesn't that flip the polarity of the wiring? I guess amps back in the day did not care about pos/neg voltage but would appreciate if anyone could school me on that concept. I am also wondering if the convenience outlet on the back is an attractive nuisance since I am assuming the polarity of that outlet could be flipped as well? Again I plan to nothing here as suggested by @sds1 but would love to learn more.

3. Bias Board - I was thinking the concept of replacing the entire bias board was less invasive since the entire original bias board remains intact and safely stored. (In thew same vein, I had thoughts of pulling and storing the vintage Oxford speakers but they sound so good, so they stay fore now.) I understand from YouTube videos by Psionic that the bias diodes can fail and cause allow damaging current to flow out of the bias the wrong way and he recommends replacement when you recap. Replacing the diode aligns with my hope to protect other valuable parts of the amp. So the only component left on the AB763 bias board is the bias resister and Psionic seems to replace that as well. I am inclined to do the entire pre-populated board replacement over just replacing the bias cap.

4. Lift enter tap for the power trans., install a pair of 100ohm resistors. - Torn on this one. It aligns with the protect other valuable parts (transformer) of the amp with little to no sonic change (?) and is totally reversible. Any additional thoughts would be welcome. For what it is worth the Princeton I just built also has the same pair of 100ohm resisters, so the folks at Mojotone thought it was a value add to the 5F2a circuit. Maybe things are different with the AB763?

5. Electrolytic Caps - Overwhelming yes on this maintenance for all of you folks. To reiterate I am only talking about the brown Mallory caps. The blue ones stay, I promise . One follow up question, is there any guidance on "when" I should do this. The amp sounds great so should I let it ride? Or are they ticking time bombs and the sooner they are replaced the better?

So grateful for all of your advice so far. I will perhaps admit to a mod I made to the tremolo (could don't help myself), but the post is long enough as is so I will leave that subject for another post.

Thank you again friends.

You have mentioned the PT Center tap. If you're going to install a virtual center tap on the 6.3 volt secondary, that's not the PT center tap which is usually orange... It's the Green and Black wire only. Also, just because Lyle Caldwell does it on every amp that lands on his bench, doesn't really mean anything. It's just something that makes him sleep better at night. There are literal 100's of 1000's of old Fender's whose filament winding hasn't burnt out on.

 

[tele_savales]

You can also fit a LarMar very easy in the ground hole. Then you can crank and not get a visit from an angry neighbor that doesn't appreciate involuntary live streaming your performance into their home.
View attachment 1406695

Change the cathode caps sooner than later. I've had some fail in such a way as they test as diodes on an M Tester unit. This would be mostly unnoticed as far as function because diodes can be used to bias a triode, but the bias will be shifted from the intended point. This can result in less/more gain from a stage and a noticable change in overall sound of the amp. A cathode cap that fails open will cause a roughly 6dB drop in gain across the board. Don't be surprised if you find your amp sounds louder and fuller (ie. As intended) once you perform this maintenance. And don't sweat if you can't find a 25μF cap. Anything larger than about 10μF will be sonically indistinguishable, as it fully bypasses all guitar frequencies. So the readily available 22μF is just fine.

As for the 100 ohm artificial center tap, you may hear a slight difference in hum between the two arrangements, especially if the filament CT isn't perfectly balanced, or the tolerance of the resistors used are wide. It's hard to say which would be quieter in your specific application. This is why hum balance pots exist.

Also, seeing this photo again reminded me of 3kV diodes from plate to ground. This is also cheap insurance for the output transformer if you're averse to adding MOVs. It's added to the socket, so easy to do while replacing plate and screen resistors. It adds a negligible amount capacitance to the plate, so it's sonically invisible--but it may add some stability to the power amp section if it is on the verge of ultrasonic oscillation. R3000s are a popular choice for this.

What about swapping out all the Normal channel values for a Marshall high gain job?

 

[GotA24Fretter]

What about swapping out all the Normal channel values for a Marshall high gain job?

If that floats your boat, but the goals here aren't to change the tonality. Just protect/enhance what's there. Reread the first post.

 

[tele_savales]

If that floats your boat, but the goals here aren't to change the tonality. Just protect/enhance what's there. Reread the first post.

Pass.

 

[jaonline]

Hi Friends,

So spent a couple of hours just discharging caps, chop-sticking, measuring, and just tracing the circuit around. Based on that homework and your comments I am revising my plan as follows (in order of priority):

1. Death cap, ground lift switch - I am going to remove the cap from the amp and store it with any other original parts. I'll disconnect the ground switch but leave it amp.

2 AC Cord - Replace cord so I both have a longer ground wire that is not tight enough to pluck (it is now) and a cable that is long enough to comfortably reach my wall outlet.
Thank you @King Fan for the diagram. I am going to follow your advice here. I might wire the one lead of the PT Primary to the neutral wire of the outlet instead of the splice. Or does the twisting of the hot and neutral wires help with shielding? This digram is super helpful in organizing my thoughts.

I drew this conversion one time, let's see... assuming you have a vintage SPST power switch, something like this:

And I mentioned to save the cap? Well, likewise, the empty

3. Electrolytic Caps - Was leaning toward MOD 25uF - 50v caps. Except @Pete Farrington suggested the voltage is not correct. Original spec (AB763) is 25-25 which I read as 25uF 25 volts. What is considered best practice today for the voltage rating? (Thank you in advance!)


PAUSE HERE AND PLAY AMP FOR A TIME - I appreciate your advice @Supertwang and will do just that.

The “ground lift switch” turns into a 3-way NFB switch very nicely. I’d do that and replace those old brown Mallory cathode bypass caps. And rewire the incoming AC up to modern standards including definitely ditching the death cap. Then I’d give it a listen and if I liked what I hear I’d prob stop there. And if I owned a classic car that originally came with bias ply tires I’d update to radial tires in a heartbeat.

A few other notes I thought I would pass on.

Bias Board - So I found in my poking around that the Mallory cap on the bias board is an imposter. Hiding inside the cardboard is a small black cap (SURPRISE!). The bummer about this bit of trickery is I cannot see what the cap is or a date code. The Spaugue Atoms in the dog house are from 2021 so hooping the bias cap is of a similar date. I will have unsolder one end and lift the cap see what it is, so for now the bias board will remain unmolested (by me).

Artificial Center Tap - I am going to wait on the artificial center tap or MOV's or diodes to protect transformers. This type of work aligns with my stated goals but I am going to learn more before undertaking these changes. Thank you @GotA24Fretter, I am going to start my homework with your references.

To protect irreplaceable original transformers you can fit an HT fuse, MOVs across the OT primaries, and backup diodes on the rectifier tube. See RG Keen's "Immortal Amp Mods".

Tremolo - The slowest speed of the trem was way too fast and was bugging me. So I made my one and only physical change to this amp so far and replaced one of the .01 caps with a .022 (image 2242 below). I am so much happier the with the tremolo now. The bypass cap someone had added seems to be working so even though it is incorrectly installed. No noise yet @GotA24Fretter but will keep in mind.

Finally, the cap across the 10M resistor at the tremolo roach is an outdated mod.

OK that's it for now. Going to start ordering parts. If you have been reading all this and following along I really appreciate your time and insights.

 

[GotA24Fretter]

3. Electrolytic Caps - Was leaning toward MOD 25uF - 50v caps. Except @Pete Farrington suggested the voltage is not correct. Original spec (AB763) is 25-25 which I read as 25uF 25 volts. What is considered best practice today for the voltage rating? (Thank you in advance!)

I'm sure he'll chime in for himself, but I took that to mean he didn't believe the caps were actually capable of the stated voltage, not the they wouldn't be suitable in this circuit. They are physically significantly smaller than similarly rated caps from historically reliable companies, so I can see the apprehension.

There are some applications like a cathode bypass cap in a power amp where the voltages would be in the high teens to low 20s where you would want some margin and a more rugged capacitor than the MOD 25μF-50v.

In your amp the highest voltage will be the 2nd triode of the tremolo circuit, and either a 25v or 50v cap would be fine there. 63v+ is overkill unless you just got a smoking deal on a bulk buy or just need to stock up on versatile components for your shop.

One other consideration is lead thickness. The MOD leads are noticably thinner than comparable caps. This cuts two ways as it's easier to fit two thinner leads in the eyelet where a single lead once was, but leads are also used for heat dissipation. Probably neither here nor there in this circuit either as currents and voltages are low.

Finally, I've read electrolytic caps that are run well below their voltage rating will degrade more rapidly than a more appropriately rated cap of the same value, all else being equal. I'm not convinced it makes a huge difference in guitar amps, but something to consider for best practice.

 

[Pete Farrington]

Main board capacitor options:

3. Electrolytic Caps - Was leaning toward MOD 25uF - 50v caps. Except @Pete Farrington suggested the voltage is not correct

Oops, apologies, I missed the context of 'main board'.
With the bias board discussion, I was thinking of the cap there, original stock rating typically 50V, whereas the actual voltage could often be above that.
Thanks for raising the query!
I'll blame this oversight on the positive covid test result I just got

 

[King Fan]

I might wire the one lead of the PT Primary to the neutral wire of the outlet instead of the splice. Or does the twisting of the hot and neutral wires help with shielding?

This is actually a great idea -- a tie point is easier than a splice. For EMI cancellation *or* just plain wiring neatness and grouping, *if* the PT primary was long enough, I'd just wrap the PT primary with the hot back to that point. But if the PT primary is short, cancellation and neatness might favor a splice further down the 'long and twisted path.'

BTW, when you replace the 3-prong cord, several tips.
1. Get some generic 'strain relief pliers' off eBay if you don't own them. They make placing the new strain relief a picnic instead of a PITA.
2. You don't need the fat SJT cable -- unless they're running heavy dollies across your cords at the Hollywood Bowl, 18ga SVT is plenty sturdy and ampacious enough, and infinitely easier to run through a strain relief.
3. When you're fashioning your new green ground/earth wire, make a loop so it's longer than the black or white. The idea is it'll be the last wire to pull loose if your amp ever falls off the tower at Woodstock...

I found in my poking around that the Mallory cap on the bias board is an imposter. Hiding inside the cardboard is a small black cap (SURPRISE!).

Interesting. I take it you mean this one.

People do 're-stuff' cardboard tubes with new caps to maintain the vintage appearance of an amp. Is there any chance the cardboard caps on the main board were also re-stuffed? The tubes do look kinda slick and clean. Of course it'd help to know how new the inside caps are, but it would almost certainly change the urgency (and the cost/resale balance) of a cap job.

 

[GotA24Fretter]

I might wire the one lead of the PT Primary to the neutral wire of the outlet instead of the splice.

This is actually a great idea -- a tie point is easier than a splice.

Both on the courtesy outlet is my favorite way to do this. No splicing and you can run everything along the back wall save the primaries, which you can twist (See LarMar photo above). Note: I'm one of those heathens that prefers the Power switch before the fuse.

 

[jaonline]

Oops, apologies, I missed the context of 'main board'.
With the bias board discussion, I was thinking of the cap there, original stock rating typically 50V, whereas the actual voltage could often be above that.
Thanks for raising the query!
I'll blame this oversight on the positive covid test result I just got

Feel better soon and thank you for the valuable guidance on thinking about pos/neg.

 

[jaonline]

(Deleted accidental blank reply)