Upgrade to AB763 reverb (no vibr) on bench

[pjd3]

Hello folks,

So finally, I'm looking at the scope traces (Input and dummy load output) and I believe I'm seeing good things However, I will need to tweak a thing or two and of course get to a real listening test.

This was a Rob Robinette Blackvibe 6L6 that although came out very good, was just way to clean of a clean tone. It was great with the pedals but I really wanted a bit more slice and grit to the basic clean tone - like a standard reverb/tremelo channel of a typical Fender AB763 circuit. I based this upgrade on a Deluxe Reverb preamp with a 6L6 output section. I have a matched pair of 6L6WGB's in there now, which I scored from KCA for the Blackvibe build. I like the extra stoutness that can be had from something a bit larger than a 6V6, I started with 10K/10K preamp power node resistors and plan to do some listening tests while trying say, 4.7K/10K, 4.7K/4.7K, and finally 1K/4.7K, usually suspects.

At this point the B+ is 465vdc on the plates of the 6L6WGB BUT, I have to dime the bias master all the way counterclockwise to see a 65-70% emissions, around 2.3volts to 2.5volts across OT CT to each plate. Although I'm comfortable with that method of setting bias, I am missing those 1 ohm resistors in series with the cathode to ground. I will likely try to find some 1 ohm resistors that I"m happy with and get those in there.

The bias board I built is one from Merlins sight, that uses the two 47uF caps with the 15K resistor. This goes out to both a bias level pot and bias balance pot. I just think its nice to have those adjustments plus what seemed like a more filtered and rectified bias voltage. But, as mentioned just above, I will need to see about getting the bias range more usable. Its not readily apparent which component in this particular bias circuit will be modified in order to do that. Its looking like I need to increase the negative voltage range as opposed to decrease it so, it won't be simple as placing a properly valued resistor before the diode. Its other way around I will be looking for. Please feel free to advise should you know what that entails.

A thing I found very odd when checking the power node and plate voltages of the 12ax7's. When I did a simple voltage test on I think the V1 plate, the oscilloscope waveforms would collapse, and wouldn't restore unless I went into standby momentarily and came back. I should mention at this point that I'm using 12ax7" in all preamp positions until I order up some 12AT7's for both the reverb driver and phase inverter. I hoped that for basic testing purposes 12ax7's in those postions would be OK. Again, feel free to put me in my place if missing something.
But, despite these things, I think we are OK. with a clean 100mV p-p sine wave I'm seeing a nice clean 5volt output on the 8 ohm dummy resistor load. I look at around 30Hz to 1Khz to examine waveforms for starters. For this new preamp board build, I did install 4.7uF cathode caps to gently lean out the low end of the guitar, and the scope does reflect this as I sweep down below 100Hz. Thats pretty much what I was going for and will ultimately judge that modification when listening time arrives.

Well thank you for hanging in for this long winded coverage but, basically, I believe it is a fair success so far. Still somethings to check and listen to but, its been a good time with this upgrade, if not a bit messy at times. As RG has said, "Its always a nicer build starting from scratch than to rebuild/upgrade". I'm experiencing that first hand with this one!

Thank you all for being here.
Best,
Phil D

[martin manning]

The bias board I built is one from Merlins sight, that uses the two 47uF caps with the 15K resistor.

So you have a CRC filter with a 15k from one 47u to the other? Reduce that 15k o get lower (more negative) voltage.[/quote]

[pjd3]

Thank you Martin,

Yes, it is a CRC filter, I believe with a 15K resistor. I will double check and also yes to wanting to get a more negative voltage from this. As a matter of fact, I will start by solder tacking some larger resistors onto the 15K until there is a better usable range to adjust within.

I'll be back to report findings !

Thank you so much,

Phil D

[pjd3]

So I piggy backed a 68K ohm resistor in parallel with the 15K resistor and I was able to get the -52 or so volts DC at about the 1/3 rotation - which is fine. That plenty of room for required range of negative voltage I would imagine. So, that was nice! Thank you Martin.

Now, to re-bias to say, 60% of 30watts - a little lower than usual for a 6L6GC but, some say these 6L6WGB's might be thought of as 25 watt max emission. I will still be in acceptable range I believe. That wouldn't be too much more than 70% of a 25 watt max emission.

Best,

Phil D

[rooster]

Scratch the 4.7uf pre cap if you want it to have any BF vibe at all, use the stock 25uf. The bass here is where the magic starts, the reverb circuit will thin that out. Also, you need the 12AT7s (reverb driver and PI). And remember that the 2nd input jack on a Fender amp creates 136K resistance before V1, while the 1st input is only 34K. A lot of pro cats playing HBs/P90s use/used the 2nd input so they could turn the amp up a bit more than into the 1st channel and get a different kind of overdrive. Options.

[pjd3]

Thank you Rooster,

And I will be keeping a Hawks ear on the 4.7uF that I placed in the preamp cathode bypassing. If I find anything down there is missing at all, I will pump those values right up. It will be interesting to see/hear/feel what those smaller values do to the sound. I know there are folks out there that have landed on those values for reasons that are understandable. But, I grew up on blackface sound so, I may just put those 22uF in if something is missing in my perception.

I believe the 2 input jacks are wired in the standard way and I did get 33k for the 1st (high) input to the first grid. (I believe it was double that for the 2nd input (low).

And there will be both 12AT7's for the PI and reverb driver. I only had one so, just put in a 12ax7 in to the reverb driver for power up and testing some things. As it turned out, I discovered that I missed sending the 2nd power node voltage to the plates of the reverb driver socket. That had me second guessing myself for a bit. But, carefully following the Voltage chart that I do for every build, I came right to it. Thats one of the reasons I do that chart, and of course to compare my Power node and plate voltages to classic design. I will be doing some light experimenting on this amp, just to hear differences in preamp voltages, say, Deluxe Reverb preamp voltages compared to Twin/Super/Pro voltages. Right now, with the PT thats in there plus 10K/10K node resistors, I'm getting somewhere between the design values - around 220-235 volts on preamp plates. I am interested to hear that before hearing the lower and higher plate voltages.

Thank you for reading up and giving some comments.

Best,
Phil D