Vibrodriverb special

[bepone]

some progress, wiring of preamp done.. remaining to do power supply switches, fuse, lamp, fsw, bias, and relays switching... reverb will be integrated in first powering, vibrato in future upgrade..

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[bepone]

pcb designed for tremolo..with LND150 FET, nothing special or new, oscillator and source follower, some zeners and gate stoppers for protection, activation by relay, local decoupling by local C. should be without any noise. on one side is close path to the speed and on the other close path for intensity pot. power will be stolen from some B+ line and reduced by onboard dropping resistor. currents kept in the short loop by onboard electrolytic.

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thinking about this pcb and where to do it.. all options are too slow. regardless the price.waiting 1-1.5 week from china or any other supplier is too much for me, i think i will do it by myself. in free time, classic way via photosensitive spray and acid. today for today is max (waiting) time assigned for this pcb

[bepone]

front and back plates designed, amp name font chosen to be llike silverface Fender, produced on the cnc milling machine. Aluminium black sheet is actually made for the laser burning, but....

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[bepone]

amp partially prepared for commissioning and first test run! finished bias, 12VDC, heater wiring, output sockets.
starting without reverb, vibrato. with EL34 some old worn tubes but still good. after switch to 6L6GC and finally to 7027 ( stronger 6L6GC). missing some knobs too, ordered!

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[bepone]

amp worked from the first powering on! i managed to connect NFB ok from the first , by luck !
finally loaded with Jensen Neo 65 Stealth, 7027 and some nice NOS 12AX7 tubes. some pictures:

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[pdf64]

amp worked from the first powering on! i managed to connect NFB ok from the first , by luck !…

No need to rely on luck with that. Just need to use a sig gen and scope to ensure the primary and secondary are in the same polarity.

[bepone]

amp worked from the first powering on! i managed to connect NFB ok from the first , by luck !…

No need to rely on luck with that. Just need to use a sig gen and scope to ensure the primary and secondary are in the same polarity.

if is not a problem, can you elaborate? you have 2 primary anodes and they are in opposite phase.
i believe you wanted to say, check PI input and speaker terminal, with NFB open, but then signals need to be in opposite phase. can do that way also, but there is no starting adrenaline then

[pdf64]

Assuming a standard power amp design, eg as per the AB763 Vibroverb, a positive going signal at the ‘first’ output valve anode (eg V7 pin3, fed from the common cathode section of the LTP) must be matched to a positive going signal on the OT secondary.
Hope that makes sense?
https://el34world.com/charts/Schematics ... _ab763.pdf

[bepone]

i understood what you mean..to avoid high peak anode voltages and destroy scope probe, maybe simpler to take speaker terminal and check PI input to be in opposite phase...

[bepone]

Here's a little bit about the sound..

Maybe the sound was a little warmer with the EL34s, which are old and worn out, pleasant... by switching to the new 7027, the sound is a little raw, but I'll attribute that to the new tubes, the running in.
When first turned on, the amp was quite present, but it was an instant fender tone. Solid bass, raised treble, beautiful. The neck sounded fantastic, but the bridge was a little too bright. Many people will leave it like that for live sound. The drive was also bright..as if I turned on the bright cap on the volume pot, but I didn't. By reviewing the scheme, coupling capacitor to the PI was too small.
Also in parallel with the vibrato rev. mixing stage resistor 4M7 i put 20pF instead of 10pF (I don't want to say that 20pF made such a dramatic difference, but it was noticeable). This is where my idea of ​​Vibroverb conflicts with the "dumble" standard, the difference in coupling caps is too big. Maybe i'll put a relay on the switch because I also like "bright mode"..

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I increased the cap to the PI to 2n2 and that was still not ok, so I put 10nF. Coupling cap after V1 increased to 47nF. Clean returned like nice, sparkle can be added on treble pot..

Then drive was still bright, and too "precise", if there is such a word! Adding a picofarads to the overdrive tube did not make such a big effect, so I decided a bit more radical, 10nF+47k series snubber connected to OD. VOL wiper to the gnd, the whole balance of the sound instantly returned to the bass side. Now it sounded very good, more with the strat than the LP, but for the LP you had to play with the controls a bit. Deep switch is wired like classic style with 270k resistors.. but is too radical also, adding too much of bass maybe i will do easy way with ther cap 390pF only.
JAZZ and ROCK in combination with Neo Jensen give a very wide palette of sounds. I'm currently trying to get a top OD on JAZZ mode, I think there is a lot of potential with this speaker in this small combo..

[pdf64]

i understood what you mean..to avoid high peak anode voltages and destroy scope probe, maybe simpler to take speaker terminal and check PI input to be in opposite phase...

I check everything I can about the transformers before fitting them. So OT winding resistances, ratios and relative polarities.
Then it can be wired up right first time, with no risk of the loop being positive rather than negative.
With the normal amp design, the LTP signal input must be in the same polarity as the speaker output (and hence the feedback signal at the other LTP grid).
ie the power amp block can be viewed as a non inverting opamp.

[martin manning]

I check everything I can about the transformers before fitting them. So OT winding resistances, ratios and relative polarities.

Me too :^) Some data sheets have the phasing marked so you have an excellent chance of getting the NFB right without measuring anything, but all of the above can be verified with the transformer on the bench or installed, using any AC source (heater voltage, e.g.) and a DMM.

[bepone]

soldered Tremolo effect as an addition to the existing amplifier..modern iteration uses a rather linear depletion mosfet LND150 which has nice "pentode" curves and can replace the ECC83 in various circuits, buffers, fx loops..and is not consuming expensive ECC83 ..

the circuit consists of an oscillator with LND 150 and a source follower for driving the optocoupler... which with its output can excite various parts of the amplifier and modulate their amplitude.
pcb designed, produced, assembled (with EJ parts), ready to release white smoke!

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[dorrisant]

Do you have a schematic for that LND tremolo circuit?

[bepone]

i took for the base this one:
https://www.projetg5.com/phpbb3/viewtopic.php?t=6387
all around the net you can find similar schematic.. first part is oscillator, and second is source follower..

didn't like the R4-C5 network, because on the gate how you increase the stopper you increase also the noise.
modified to 68k - 47nF, and on every fet gate added stopper, and zener diode limiter for protection (15V+15V ZD in antiseries)..