Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Apexelectric on August 28, 2017, 07:40:15 am
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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
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Makes perfect sense. That's why Hoffman calls that resistor the "range resistor".
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Thanks Slucky! Had a feeling I was on the right track but figured I'd ask anyway.
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Does it matter which resistor value you change to get the bias range, the one before or after the diode?
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I prefer to change R35 and/or R36 to get the proper bias voltage. I rarely mess with the 15K (R33) that you changed.
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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
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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
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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
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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!
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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
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Thanks for the info. Appreciate the insight.
Mike
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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