Sound checked Blackvibe 6L6 rebuild to reverb channel without tremelo

[pjd3]

Hello,

I did a first sound check on my Blackvibe 6L6 rebuild (6L6WGB, reverb, no tremolo single channel).

I seemed to be sounding fine through a Weber C12B, lots of Fender clarity and very loud.

A couple of concerns if you have the bandwidth. I don't suspect that anything is wrong (besides an incomplete reverb section) but, do you folks intentionally test for oscillations following a build just to be sure? And if you have contended with that before, where in the circuit would you go to check for supersonic oscillations? I would imagine that speaker taps are out do to limited frequency response but, I don't have high voltage probes to look at power tube plates. How about looking across the end of each primary to B+ at OT centertap?
How about across a one ohm resistor on the powertube cathodes?

Thank you!

Best,
Phil D.

[xtian]

For me, it's easiest to spot ultrasonic oscillations by watching power consumption, with the input jack shorted to ground. Watch your idle power consumption (25-50watts baseline?) as you turn up all the controls to max. Your power consumption should not rise.

[pjd3]

Thank you Xtian,

Where are you looking at the power consumption from? What part of the amp? I suspected that the output of the OT may not show a very high frequency oscillation but, that maybe a misnomer on my part. I always looking at power output with a 1Kh sine wave at the speaker, but I wouldn't have expected that way to work for very high frequencies.

thanks again!

Phil D

[GAStan]

Phil, I'm not sure what Xtian does, but I would personally monitor the overall input current to the amp. This would let me know if there is oscillation anywhere.

To monitor I would use either a clamp amp meter or for more accuracy an amp meter in series. Many multimeters are good for 10 amps. Not saying you should, but I would just pull the input fuse and connect my meter to the input and output of the fuseholder. Much care must be taken because my meter is fused, but with a 10 amp fuse vs the 3 amp slow blow I use in my amps. I would then conduct the tests previously mentioned. The meter would then be removed and fuse reinstalled before any other type of troubleshooting was performed.

The safest way would be disconnect the input wire somewhere and put the amp meter in series. All of my connections are soldered so I would opt for the fuse bypass method, but I understand and accept the risk involved.

[martin manning]

May be as simple as connecting the amp to its power source through a power monitoring device? Not super accurate, but it would show an increase where there shouldn't be any.

[johnnyreece]

Could one use a light bulb limiter for something like this? As in, bulb gets bright, then dims as caps charge, etc. Then, turn everything up an see if the bulb glows brighter as knobs are twiddled? I've never thought to test for oscillations, but if this works, it seems like an easy step to add to the fire-up procedures.

[martin manning]

Could one use a light bulb limiter for something like this? As in, bulb gets bright, then dims as caps charge, etc. Then, turn everything up an see if the bulb glows brighter as knobs are twiddled? I've never thought to test for oscillations, but if this works, it seems like an easy step to add to the fire-up procedures.

Absolutely, and you can learn a lot about how things are going on a start-up by watching the bulb closely. I was thinking about something like this: https://ampgarage.com/forum/viewtopic.p ... 10#p462410 , or the "Kill-a-Watt" meter that is used for monitoring power consumption.

[xtian]

Correct, I use a Kill-a-Watt meter to monitor power flowing into my devices.

Observing a bulb limiter might work, but your amp will be running at lower than normal voltages when inline with bulb limiter...might possibly hide an oscillation that happens at full voltage.

[pjd3]

A quick update on the Blackvibe 6L6 conversion to AB763 single channel (no Tremelo).

The B+MOSFET Reducer circuit is working like a charm. It allowed the B+ voltage to drop from 467vdc to around 442vdc. My hope is that it will allow those expensive little 6L6WGB' Phillips tubes to relax and last a little longer. And the amp may have even sounded better (probably sonic confirmation bias). I'm thinking of tossing a switch in there to bypass the B+MOSFET Reducer in the event that a pair of stout and robust 6L6GC's would make their way in there, one day. Ah, even they would probably be fine with a plate voltage of 445vdc. Lord knows I won't ever be in any state of needing more clean headroom as venues (and band members) no longer allow for 70's type dB levels.

And of course, my little 4024CV PI gives me a generous flash upon turn on every time. I chuckle every time I see it now.

I found a nice rotary Grayhill switch that will likely be the candidate for switchable bright caps. The amp, although sounding very good isn't full of that Fender high frequency "slice n' sparkle' that is nice to call on for certain songs so, that will probably make its way in there. I put a "Raw" midrange control in so, there won't be any deficit of mids to be had.

It's been a good venture. I learned a few more things as rebuilding forces you to have to address more things, than just doing a classic amp from scratch. One of those things is the relationship between plate voltage and bias. That really threw me for a while when after the installation of the B+ Mosfet reducer the B+ shot up to 500 volts. I was beside myself on that, until I realized it had offset the bias voltage range way way off from where it was needed to be. Once I began lowering the bias voltage (less negative) the plate volage began to drop significantly and all was well, except that I had to modify the bias circuit to re-establish a better range of control. And I see this shouldn't have really come as a surprise because that is how a power tube works. It modulates the bias voltage relatively small to make for a large swing in plate voltage. I just didn't expect to see that manifest so largely from "a little tweak of bias voltage adjustment".

Thank you again everyone for your valuable input, it always gives me much more confidence and always seems to lead to good results!

Best,
Phil D.