Lunchbox ECL86

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

Moderators: CurtissRobin, colossal, zaphod_phil, Daviedawg, Graydon

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

Re: Lunchbox ECL86

Post by TriodeLuvr »

Nice chassis work!

Jack
1 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 15208
Joined: Wed 03/19/03 2:00 am
Location: YYZ

Re: Lunchbox ECL86

Post by zaphod_phil »

geoff 1965 wrote:
Sun 05/09/21 4:57 pm
My 18W is lite2/cascade,and my 6V6 amp has the trinity plexi mk2 preamp so i’m going to do some experimenting with this one ZP, I know the tone & breakup is in those ecl’s and i’ve got the rectifier switchable between ez81 and solid state.the extra preamp tube gives me options as in TMB etc. I might try the hiwatt tube buffered effects loop with the extra triode?
Personally, I don't think FX loops are such a great idea with 18W amplifiers, since an 18W's distortion is all produced in the power stage, so you would need to take the FX Send signal from the power stage, across the speaker terminals.
0 x
Nature abhors a clean tube amp

geoff 1965
Verbose poster
Verbose poster
Posts: 1118
Joined: Mon 01/04/16 3:52 pm

Re: Lunchbox ECL86

Post by geoff 1965 »

97BA2CA3-8D49-4335-BA68-3D31724FD4BB.jpeg
This is what I was thinking of ZP,with the conventional TMB tonestack and the tube buffering patched in between the treble and master volume
You do not have the required permissions to view the files attached to this post.
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 15208
Joined: Wed 03/19/03 2:00 am
Location: YYZ

Re: Lunchbox ECL86

Post by zaphod_phil »

geoff 1965 wrote:
Mon 05/10/21 2:08 pm
97BA2CA3-8D49-4335-BA68-3D31724FD4BB.jpeg
This is what I was thinking of ZP,with the conventional TMB tonestack and the tube buffering patched in between the treble and master volume
Since you need to feed an overdriven signal into the FX loop and the tone stack attenuates your signal by quite a lot, you will need to have a complete TMB preamp feeding the loop.
Apart from that, an FX loop messes with signal levels ands spoils the 18W's gain structure.
0 x
Nature abhors a clean tube amp

geoff 1965
Verbose poster
Verbose poster
Posts: 1118
Joined: Mon 01/04/16 3:52 pm

Re: Lunchbox ECL86

Post by geoff 1965 »

18_watt_tmb_vvr_pa_pi_198.gif
this is the TMB channel i'm planning to use with vvr on the power & phase section and just one input,regarding the gain structure would'nt it just be a case of experimenting with the value of the master volume pot?
i like your earlier suggestion of the loop being in the output but i've never seen a schematic or example of that.
note; the tube buffer only has a 100K in series and we know 1K5 on the cathode will give good tone and the 22n is the same as the TMB into the phase inverter.
You do not have the required permissions to view the files attached to this post.
0 x

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

Re: Lunchbox ECL86

Post by TriodeLuvr »

geoff 1965 wrote:
Mon 05/10/21 2:08 pm
97BA2CA3-8D49-4335-BA68-3D31724FD4BB.jpeg
This is what I was thinking of ZP,with the conventional TMB tonestack and the tube buffering patched in between the treble and master volume
Use the wiper on the Treble pot to drive a unity gain cathode follower. The follower can drive the Send jack through a 25K or 50K pot, and the Receive jack can drive the Master Vol pot directly. Wire it so that when nothing is plugged into the Send jack, the TMB wiper is connected to the Master Vol pot. This arrangement will have no effect on the gain or tone of the amplifier when the loop is not in use. I can post a schematic if you're interested.

Jack
0 x

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

Re: Lunchbox ECL86

Post by TriodeLuvr »

What is the purpose of the two diodes after the filter cap?

Image

Jack
0 x

geoff 1965
Verbose poster
Verbose poster
Posts: 1118
Joined: Mon 01/04/16 3:52 pm

Re: Lunchbox ECL86

Post by geoff 1965 »

Not much room for extra pots Jack,but post a schematic if you don’t mind and I can keep it for reference.
I think the hiwatt tube buffer won’t affect the gain & tone that much and if it does’nt work out i’ll simply swap to passive series which works well in my 18W with reverb & delay,i’m not into modulating affects and any boost or drives have to go through the preamp to work best.
plan B for the extra triode is a dual input switch so I can have the high gain 820R/.68uf TMB on one triode and something like 1K5/3.3uf on the other.
the diodes are for isolation I was told when I built my other vvr unit Jack.
0 x

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

Re: Lunchbox ECL86

Post by TriodeLuvr »

geoff 1965 wrote:
Mon 05/10/21 5:54 pm
the diodes are for isolation I was told when I built my other vvr unit Jack.
What do they isolate? Are you sure they're not just extra parts? :)

Jack
0 x

geoff 1965
Verbose poster
Verbose poster
Posts: 1118
Joined: Mon 01/04/16 3:52 pm

Re: Lunchbox ECL86

Post by geoff 1965 »

one of the senior amp techs explained exactly what they do but i can't find it in my build notes,anyway i built the vvr unit in my 6V6 amp to them specs and it works dead smooth,the only alteration i made was setting the minimum voltage by replacing the 100K to ground from the pot.
0 x

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

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

Image
1 x

geoff 1965
Verbose poster
Verbose poster
Posts: 1118
Joined: Mon 01/04/16 3:52 pm

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?
0 x

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

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?

Jack
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 15208
Joined: Wed 03/19/03 2:00 am
Location: YYZ

Re: Lunchbox ECL86

Post by zaphod_phil »

The two SS diodes allow the PI and PA rails to be scaled, without the preamp.
0 x
Nature abhors a clean tube amp

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

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?

Jack
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 15208
Joined: Wed 03/19/03 2:00 am
Location: YYZ

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
0 x
Nature abhors a clean tube amp

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

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.

Jack
0 x

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

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.

Jack

Image
0 x

geoff 1965
Verbose poster
Verbose poster
Posts: 1118
Joined: Mon 01/04/16 3:52 pm

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.
You do not have the required permissions to view the files attached to this post.
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 15208
Joined: Wed 03/19/03 2:00 am
Location: YYZ

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
1 x
Nature abhors a clean tube amp

Post Reply