Bias Resistor Value OK?

[Apexelectric]

I just finished a Hoffman 5F6-A build and the PT I used is giving me about 20 less volts B+ than typical (about 410V) and 50V on the bias tap.  All my other node voltages are a little lower as well. When I went to bias the amp I was gettting only about 14ma dissipation on the 6L6 tubes via the reading across the 1ohm resistor. This is with the 50k pot maxed out. In order to get a proper bias I swapped out the stock 15K bias resistor for a 47K. Now I can get them biased in the 30 to 60ma range. My bias voltage at the mixing resistors is around -42V now instead of -49V which was what I was getting before the value swap.

Does this fix make sense or am I missing something that I am not considering?

Thanks in advance

[sluckey]

Makes perfect sense. That's why Hoffman calls that resistor the "range resistor".

[Apexelectric]

Thanks Slucky! Had a feeling I was on the right track but figured I'd ask anyway.

[Apexelectric]

Does it matter which resistor value you change to get the bias range, the one before or after the diode?

[sluckey]

I prefer to change R35 and/or R36 to get the proper bias voltage. I rarely mess with the 15K (R33) that you changed.

[Apexelectric]

With the 20 less volts of B+ than typical, would this affect my overall clean volume capabilities of the amp? It seems a bit weak. Tone is good and output dissipation seems proper but I'd like to coax a little more volume out of it. I do have another higher voltage secondary tap on the PT that will give me about 360-0-360 instead of 310-0-310.

Or is there a better way to maximize the volume output without affecting the overall tone.

Thanks much

[tubenit]

IF you are using a GZ34 (5AR4) rectifier, then a loose guesstimate would be that you'd have:

310vac x 1.3 = 403v on node A.  You are indicating that in reality you're getting close to 390v or so with tubes in?

310 x 1.4 (solid state rectifier plug-in) would guesstimate to be about 434v on node A   (31v more then you have now)

Hoffman sells the solid state plug-in's.  I have one, it works fine and they're reasonably inexpensive. So that might be a consideration?

IF you are using the GZ34 rectifier with 360, then a guesstimate would be:

360vac x 1.3 = 468v on node A

IF you used a 5V4 rectifier then a guesstimate would be

360 x 1.2 = 432v which is about the same as using 310-0-310 with a solid state plug-in.

I personally would not use the 360-0-360.   I'd probably use the GZ34 or the solid state plug-in.

With respect, Tubenit

[tubenit]

IF you built it with a 12AY7 in V1,  you might find more "perceived" volume and overdrive using a 5751?  If you have a 5751 around, that would be an easy thing to try out.

Obviously, speakers & efficiency make a difference also. 

With respect, Tubenit

[Apexelectric]

I've used the GZ34, 2x10" Celestion Gold speakers.

I guess I was wondering, with all other factors remaining the same, if I would have chosen a PT with 325-0-325, if there would be a noticeable change in volume and tone. Or if the effect would be almost impercievable.

I realize that the interpretation is somewhat subjective depending on your frame of reference but I am not the over anylitical type and coming from the perspective of the average listener and not the critical gear head.

I am learning lots of great stuff after doing my first 6 builds but I have so many questions that I would love to get some experienced opinions on to speed up the learning curve and minimize the amount of times I get to learn from mistakes, especially the time consuming, expensive types.

Thanks guys!

[tubenit]

Both my amps have a PT that is 300-0-300.

On both of those amps, I've tried a

5Y3GT with 6V6's  at 330v on plates
5V4  with 6L6/5881 at 360v on plates
GZ34 with 6L6/5881 at 390 on plates
& solid state with 6L6/5881 at 420 on plates.

I certainly hear a significant difference running 6V6 vs. 5881/6L6 especially with differences in the voltages. 

I hear some difference between 5V4 and GZ34 if I switch them out in a relatively small time frame trying to A/B them. The GZ34 has a dash more headroom.

IF someone switched back and forth between the 5V4 or the GZ34 and didn't tell me which was in the amp & I hadn't played the amp for a couple of days, ................ I doubt I could tell which one is installed in the amp (without them A/B-ing them for me)?

It seems like the solid state rectifier causes the notes to jump out and be instantly there with no sag.  I might be able to tell the difference between tube rectification and solid state without knowing which was installed?

Having used VVR's on a number of amps,  I was somewhat surprised how much of a voltage drop it took to change the volume.  That's about as much as I can offer on my subjective observation and/or opinion on this.

With respect, Tubenit

[Apexelectric]

Thanks for the info. Appreciate the insight.

Mike

[tubenit]

I honestly spend more time with preamp plate voltages then power tube plate voltages.  I hear far more a difference in raising a 12AX7 plate voltage 20 volts then a 6L6 plate voltage 20 volts.

You can always experiment with the B+ dropping resistors.

with respect, Tubenit