Hoffman AB763 troubles

[gheorge77]

So, the whole problem was that when you had the bias voltage at pin 5 set to -24v the tubes were biased so hot that the trem effect was smothered. Those tubes were probably not gonna last very long running that hot.

With this bias exercise I was trying to show how the tremolo strength is affected by the bias. With -24v on pin 5 the 6V6 is biased hot and the trem is weak. With -32V on pin 5 the 6V6 is biased cooler and the trem is stronger. IOW, hot bias gives weak trem and cool bias gives strong trem. So, there will usually be a compromise between your "optimum' bias and good trim.

Now back to my earlier statement, "That trem circuit is capable of producing a very nice trem effect without any modification." That's still a true statement. But sometimes you may not be able to get a good trem effect if you like to run your 6V6s very hot. The LED mod will allow you to run your 6V6s as hot as you like and still have good trem.

Hi Sluckey,

This post seems very relevant to my current build. I finished up a 2 Channel Hoffman AB763 and everything sounded great (normal/vibrato/reverb).  Both 6L6GCs were biased to 31 mA, with a Plate voltage of 476 and 478 on each.  I forgot to measure the negative voltage, but started the bias with pin 5 showing -65 volts prior to changing the bias.  After I biased the tubes up to 31 mA, I started playing with the amp on 5 or so and with both the reverb and tremolo on....After about 2 minutes the amp went silent and I thought I saw redplating on one of the 2 power tubes.  The fuse had blown as well.  My thinking at this point was that the bias fluctuation on the power tubes caused the redplating and fuse to blow. I am going to rebias and recheck all pins out this evening, since I just was able to get some extra fuses...If it turns out that the tremolo bias was the issue, if i were to mod to the LED in place of the 4k7 and 22uf r/c, would I not have to worry about even biasing the tremolo at all (i.e., just bias the power tubes based upon plate voltage and grid voltage)?  I can't seem to find the answer in any other threads.

[shooter]

just bias the power tubes based upon plate voltage and grid voltage

I would get that solid THEN worry about trem.

one of the things I like to do after I get the bias set for DC current;

monitor the bias VDC while playing, more often than not I typically dial the bias a smidge cooler, repeat til I'm confident the customer won't smoke tubes in a week 

[sluckey]

Adjust the bias with the trem off.

[gheorge77]

Will do and report back.

The separate question I had was whether changing to the LED mod changes the tremolo type to not rely upon a voltage fluctuation at all. I can't find a definitive answer on that and if the answer was yes I'd be more inclined to change the tremolo regardless.

[sluckey]

The LED mod will simply provide a stronger trem effect. That's all.

[gheorge77]

So I got some voltages on this build.  These are without any power tubes in (and I can't try the red-plating 6l6 because I dropped it and cracked it):

V1                  V2                  V3                    V4                  V5                    V6
1 - 295V          1 - 295V         1 - 495V           1 - 288V          1 - Fluctuating   1 - 285
2 - 0V              2 - 0              2 - 0V               2 - 0V             2 - Fluctuating   2 - 65.9V
3 - 2.21V         3 - 2.20V        3 - 10.54V        3 - 2.3V           3 - 2.13V          3 - 106.1V
4/5 - 3.27V      4/5 - 3.22V     4/5 - 3.28V      4/5 - 3.29V       4/5 - 3.34V      4/5 - 3.34V
6 - 292V          6 - 292V         6 - 494V           6 - 291V           6 - 503V          6 - 271V
7 - 0V              7 - 0V            7 - 0V               7 - 0V              7 - Fluctuating  7 - 103.7V
8 - 2.25V          8 - 2.24V       8 - 10.50V        8 - 2.26V          8 - Fluctuating  8 - 106.2V
9 - 3.22V          9 - 3.27V       9 - 3.23V          9 - 3.25V          9 - 3.31V         9 - 3.32V

V7                  V8                  V9
1 - 0V             1 - 0V     
2 - 3.34V         2 - 3.37V        2 - 506V
3 - 505V          3 - 506V         
4 - 503V          4 - 504V          4 - 374V
5 - -64V           5 - -64.1V
6 - -64V           6 - -64.1V       6 - 374V
7 - 3.36V          7 - 3.37V   
8 - 0V               8 - 0V            8 - 506V

I have no way of knowing if it was just the power tube or something else. It looks like the voltage on V1, V2, and V4 are normal, and I'm encouraged by the numbers on the power tubes.  I'm confused on V3, V5, V6, and the rectifier tube, especially since I'm not reading the heater values on that tube. I was getting about 120VAC at pins 2 and 8 on it. Thanks in advance!

[gheorge77]

Also, would running the Tremolo speed and intensity wires right next to the B+ nodes rather than over cause problems that I experienced?

[shooter]

guessing the redplating happened because the bias was hot and the trem swings it hotter.  One of the reasons I added the extra step of monitoring bias as I play, gives you a more "dynamic" view of the amp

[gheorge77]

Thanks. I'm going to pull some matched tubes from my Dual Showman to test out tonight to see what happens. I'm still pretty optimistic that it was just a bad tube.

Do the other voltages look more or less accurate?

[sluckey]

Since this amp has nothing to do with the Hoffman Princeton Reverb, I moved your content and all replies to this new thread.

[Joel]

I can recommend the LED mod.  I put it into my Hoffman Single Channel AB763 along with a cap change for slower trem.  It's strong and swampy.

[gheorge77]

Just updating on this. Was definitely a bad power tube. I've got the new one in and it's held a bias of 36mV and about -47V for about an hour now. With the tremolo on it fluctuates no higher than around 41mV ND -43V. Ive also got the plate voltage down to about 468V and everything sounds awesome. The intensity on the tremolo through the 15" Weber tone cult werewolf sounds awesome and looks great in the monotone charcoal tweed with oxblood cab.

The only other issue was a misplaced 470 ohm for 1 Meg resistor to pin 7 on the tremolo oscillator.

[davidwpack]

I built a 1x12 with a Weber Tone Cult Black Shuck and I have to agree those speakers are pretty badass.

[gheorge77]

I have a feeling I'm going to love them once I get a chance to really crank it. I'm willing to bet I'll want to get a 4x12 with those in them after finishing my next project (matamp gt120/orange GRO 100).