Lunchbox ECL86

[zaphod_phil]

Transformer cutouts done,just enough room for the 5 tubes,tubes & pots are going to be at the front.
16F5C06D-7A77-4E43-AA80-5CF3C8AF60FE.jpeg

Five tubes?? My Lite IIb only has three tubes, with solid state rectification

[geoff 1965]

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?

[Bieworm]

Transformer cutouts done,just enough room for the 5 tubes,tubes & pots are going to be at the front.
16F5C06D-7A77-4E43-AA80-5CF3C8AF60FE.jpeg

Five tubes?? My Lite IIb only has three tubes, with solid state rectification

ZP... is that a SE output amp?

[geoff 1965]

Bieworm the ecl86’s have one pentode and 12ax7 triode in them,if you use those for the phase inverter you only need one more preamp tube for a lite2.

[Bieworm]

Bieworm the ecl86’s have one pentode and 12ax7 triode in them,if you use those for the phase inverter you only need one more preamp tube for a lite2.

ok.. my bad

[TriodeLuvr]

Nice chassis work!

Jack

[zaphod_phil]

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.

[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

[zaphod_phil]

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.

[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.

[TriodeLuvr]

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

[TriodeLuvr]

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



Jack

[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.

[TriodeLuvr]

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

[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.

[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

[geoff 1965]

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

[TriodeLuvr]

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

[zaphod_phil]

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

[TriodeLuvr]

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