Lunchbox ECL86

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TriodeLuvr
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Re: Lunchbox ECL86

Post by TriodeLuvr »

Below is a schematic showing how to create a non-intrusive effects loop. Resistor values will depend on the tube being used. This circuit works without loading down the TMB because the input impedance at the grid of the follower is several megohms. If you don't have space for the pot, the Send control can be replaced with a fixed resistor. The resistor is only needed to prevent DC on the output side of the coupling cap, so anything between 100K and 470K will work well. Gain of the follower is essentially unity, so whatever signal voltage comes off the Treble pot will appear at the Send jack.

Functionally, this circuit only disconnects the TMB from the Master Volume when something is plugged into the Return jack. This means you can use the Send output for almost anything, including an external power amp, without disrupting the 18W.

I only asked about the diodes because their function isn't at all apparent. I've seen quite a few instances of online schematics where diodes were misused in power supplies by persons who don't understand how they work. The most humorous of these was a schematic published several years ago by an amp builder. The circuit showed a diode in series with B+ after the main filter caps and included a caption claiming it halved the ripple. LOL. It think the company was Chimera Labs, now defunct. "Isolation" is another claim often made for PS diodes installed where they serve no purpose. At any rate, I'd be very interested to hear the explanation regarding the VVR circuit if there's a link to it online.

Jack

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Re: Lunchbox ECL86

Post by geoff 1965 »

I would think the diode that feeds the preamp prevents any surge of DC coming back to the vvr?
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Re: Lunchbox ECL86

Post by TriodeLuvr »

geoff 1965 wrote:
Tue 05/11/21 9:58 am
I would think the diode that feeds the preamp prevents any surge of DC coming back to the vvr?
What mechanism would cause that to happen? The preamp is preceded by a very effective decoupling network - 10K - 16uF - 8.2K - 16uF. And doesn't the VVR regulate its output voltage?

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Re: Lunchbox ECL86

Post by zaphod_phil »

The two SS diodes allow the PI and PA rails to be scaled, without the preamp.
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Re: Lunchbox ECL86

Post by TriodeLuvr »

zaphod_phil wrote:
Tue 05/11/21 5:16 pm
The two SS diodes allow the PI and PA rails to be scaled, without the preamp.
I don't know what you mean by "scaled." Can you give me an example?

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Re: Lunchbox ECL86

Post by zaphod_phil »

TriodeLuvr wrote:
Tue 05/11/21 6:47 pm
zaphod_phil wrote:
Tue 05/11/21 5:16 pm
The two SS diodes allow the PI and PA rails to be scaled, without the preamp.
I don't know what you mean by "scaled." Can you give me an example?
It means varying the power rail voltage to control the output level
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Re: Lunchbox ECL86

Post by TriodeLuvr »

zaphod_phil wrote:
Tue 05/11/21 10:32 pm
TriodeLuvr wrote:
Tue 05/11/21 6:47 pm
zaphod_phil wrote:
Tue 05/11/21 5:16 pm
The two SS diodes allow the PI and PA rails to be scaled, without the preamp.
I don't know what you mean by "scaled." Can you give me an example?
It means varying the power rail voltage to control the output level
The voltage to the PI/power stage can be varied with or without the diodes. The VVR provides that function. Let's be clear about the purpose of diodes in this circuit. If they never turn off, they're not needed. Is there a certain set of conditions when this circuit is powered on in which one or the other of these diodes turns off? If not, they're just extra parts.

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Re: Lunchbox ECL86

Post by TriodeLuvr »

Just to emphasize that the diodes in the VVR circuit have to turn off in order to be effective, here's a circuit with a diode after the rectifier that actually does provide isolation. I designed this about 15 years ago because I needed two different voltages from one transformer. The power loss in a dropping resistor/Zener/regulator would have been prohibitive.

The circuit takes advantage of the fact that a choke-input filter outputs about 0.9 X transformer voltage, whereas a capacitor-input filter outputs about 1.4 X transformer voltage. The diode allows connecting both types of filters to the same transformer, and it isolates them from each other. It does this by conducting only when the 120 Hz pulses from the bridge exceed the voltage on the caps (approx. +560V) by 0.6V. For any portion of the pulse less than that voltage, the diode turns off. This prevents the higher voltage on the cap-input filter from flowing back into the choke and charging the choke-input filter to the same high value.

Again, the isolation created in this circuit is only possible because the forward-biased current through the diode periodically turns off. I don't see that happening in the VVR circuit. If those diodes never turn off, they shouldn't be there.

It's not marked in the schematic, but I think the output of this transformer was about 400V AC.

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Re: Lunchbox ECL86

Post by geoff 1965 »

At the point now of painting the chassis so in between coats then waiting for the paint to harden i’m going to do some research into the various TMB channels.i’ve already posted the SIII schematic but there is also the trinity,ceriatone,valvestorm etc
drawn out a couple for comparison and notes for tweaking,
20E28334-4D02-440A-B5BB-935BDE36FF6B.jpeg
The best overdrive i’ve heard was Luri’s ceriatone with the 12au7 cascade,so if I can get close to that without the 12au7 i’ll be happy.
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Re: Lunchbox ECL86

Post by zaphod_phil »

IMO the SIII's TMB preamp suffers from a severe lack of gain. I would recommend you try others, such as the Trinity "Plexi" 18W
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Re: Lunchbox ECL86

Post by geoff 1965 »

thank's for the tip ZP, yeah the ceriatone & trinity TMB's are very close but i'll check out the trinity plexi as well.i have the trinity plexi mk2 preamp in my 6V6 and the TMB channel was'nt as high gain as i expected but i have the normal channel cascade into it and that gives massive gain!
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Re: Lunchbox ECL86

Post by geoff 1965 »

Can someone tell me the purpose of the resistor in the TMB tonestack,i’m seeing 2 values here 33K and 47K?
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Re: Lunchbox ECL86

Post by TriodeLuvr »

geoff 1965 wrote:
Thu 05/13/21 1:58 pm
Can someone tell me the purpose of the resistor in the TMB tonestack,i’m seeing 2 values here 33K and 47K?
Here's a good explanation:

https://robrobinette.com/How_The_TMB_To ... _Works.htm

Jack
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Re: Lunchbox ECL86

Post by geoff 1965 »

Perfect explanation,thank’s Jack!
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Re: Lunchbox ECL86

Post by zaphod_phil »

TriodeLuvr wrote:
Thu 05/13/21 4:00 pm
geoff 1965 wrote:
Thu 05/13/21 1:58 pm
Can someone tell me the purpose of the resistor in the TMB tonestack,i’m seeing 2 values here 33K and 47K?
Here's a good explanation:

https://robrobinette.com/How_The_TMB_To ... _Works.htm

Jack
The 33k and 47k are the slope resistor, mentioned in that article. Higher values give more of a mid-range scoop. Therefore Marshall amp tone stacks tend to use lower values compared to F*nder
I strongly recommend playing with tone stack component values with the help of Duncan's Tone Stack calculator, to see how they all shape the sound - http://www.duncanamps.com/tsc/
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Re: Lunchbox ECL86

Post by geoff 1965 »

which is the higher gain between the trinity plexi and trinity TMB in your opinion ZP?
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Re: Lunchbox ECL86

Post by zaphod_phil »

geoff 1965 wrote:
Thu 05/13/21 7:06 pm
which is the higher gain between the trinity plexi and trinity TMB in your opinion ZP?
EditedI':It's been a few years since I was working with Trinity, which was before I had the surgery and radiation treatment on my brain. :? However, my recollection is that those two TMB variants from Trinity have around the same level of preamp gain. The "Plexi" 18W variant just has Bright and Normal channels in its TMB preamp, like a regular big Marshall Plexi (1959). It also has a Plexi-style 470 ohm cathode resistor in the PI, instead of the 820 ohm one of a regular 18W.
This gives increased gain in the PI, as well as a thicker PI distortion quality.
Hope that explanation helped!
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Re: Lunchbox ECL86

Post by geoff 1965 »

B6FE7042-ADBA-41E0-83C6-9843C890E163.jpeg
603D7A2E-47C4-489B-AD5F-067A578F6300.jpeg
Paintwork done,time to install the transformers & tube sockets “then” the dreaded heater filaments!
note;just redoing the front lettering,did'nt notice the slight slant until i posted pics!
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Re: Lunchbox ECL86

Post by geoff 1965 »

FE73B064-2BFE-419A-8C3C-82B684B41353.jpeg
657F98EF-2A45-48C6-98F3-A96A16203FE4.jpeg
Transformers & filaments wired in,vvr unit and switchable rectifier next.
I contacted trinity amps about the gain level of their 18W amps and had an awesome response from them,i’m leaning towards the plexi preamp,don’t want too much overdrive with this amp.
I’m going to bias the pentodes the same as el84’s at 180R and i’ve got 50,100,470 & 1000uf capacitors to experiment with.
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Re: Lunchbox ECL86

Post by geoff 1965 »

18WattPlexiSchematic9.pdf
here's the schematic for the plexi, with "slight mods!" i think this will be good.remove R8 and i won't be using the clean boost so i'll try 2K7 or 1K5 on that cathode and see which is best.not sure about the screen 100R's either with the ecl's will probably use a 1K5 dropper then 1K's.
6 X 22nf couplers in this amp,7 with the tube buffer!
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