Yes it was a question, meaning were we both looking at the same 340b schematic. See I made it very clear that although I am very confident in what I have done to repair the amp. I was a little confused when 6.6 ohms was mentioned. I didn't know, maybe there is another version that has an independent 3.3 ohm for each tube. just making sure I was on the right track. but instead of clarification of the 3.3 ohms vs 6.6 ohms I get a one word answer, Yep.
so thank you, and yep it was a question. and yes I did find the correct information so I could be 100% sure I was making the correct bias adjustment.
But thank you for clarifying that , yep means yes.
so thank you, and yep it was a question. and yes I did find the correct information so I could be 100% sure I was making the correct bias adjustment.
But thank you for clarifying that , yep means yes.
Directdriver was saying that a common 3.3 ohm resistor (1 for the pair of tubes) is equivalent to separate 6.6 ohm resistors (1 per tube). Let's say there is 40 mA per tube, so 80 mA for 2 tubes. That would result in 264 mV across the common 3.3 ohm (=80mA x 3.3). If you had separate 6.6 ohm resistors you would also get 264 mV across each resistor (=40 mA x 6.6). Or to make the maths easier use separate 10 ohm resistors and adjust bias for 400 mV (=40 mA x 10).
Thank you.
When I took on this project I was originally told the amp.work great. After I repair from the fall. I was 100% positive that the amp wasn't great.
The low voltage to the 12ax7s wasn't do to the drop nor was the open caps or the over heated 20 ohm resistors.
None of those straight forward corrections was an issue and once corrected I was absolutely positive that the amp was far better than purchased.
Understanding the bias was my issue, that's when Directdrive stepped in. I understood the math and ohms law, which as we all say, ohms law always applies. so my questions were geared more towards me Understanding that it was the amount of current that was key.
Yes it took me a little reading and studying but I ended up Understanding.
the end result was I decided to leave the 3.3s. I was able to find an excerpt from the manufacturer stating to adjust the bias as to the mv. that is now at 3411 and 3114. new tubes are on the way.
I still have low voltage to the 12ax7s 10.891 and about 2.5ac on each. new caps will arrive sometime tonight. the voltage will be a nice clean 12.3 with 0000 ac. with 4 new 7591s and 2 new 6u8 all new 12ax7s I will make the final bias adjustment and this amp should be good to go. If the owner decides to invest in a complete re-cap and cleanup ill be happy to do the work. Thanks to Directdrive, yourself and a few other very kind people I now have a much better understanding of this circuitry.
And I'm looking forward to my next. I haven't had to work on this type of circuitry in over 50 years it feels good getting back into it.
Thanks, Davr
When I took on this project I was originally told the amp.work great. After I repair from the fall. I was 100% positive that the amp wasn't great.
The low voltage to the 12ax7s wasn't do to the drop nor was the open caps or the over heated 20 ohm resistors.
None of those straight forward corrections was an issue and once corrected I was absolutely positive that the amp was far better than purchased.
Understanding the bias was my issue, that's when Directdrive stepped in. I understood the math and ohms law, which as we all say, ohms law always applies. so my questions were geared more towards me Understanding that it was the amount of current that was key.
Yes it took me a little reading and studying but I ended up Understanding.
the end result was I decided to leave the 3.3s. I was able to find an excerpt from the manufacturer stating to adjust the bias as to the mv. that is now at 3411 and 3114. new tubes are on the way.
I still have low voltage to the 12ax7s 10.891 and about 2.5ac on each. new caps will arrive sometime tonight. the voltage will be a nice clean 12.3 with 0000 ac. with 4 new 7591s and 2 new 6u8 all new 12ax7s I will make the final bias adjustment and this amp should be good to go. If the owner decides to invest in a complete re-cap and cleanup ill be happy to do the work. Thanks to Directdrive, yourself and a few other very kind people I now have a much better understanding of this circuitry.
And I'm looking forward to my next. I haven't had to work on this type of circuitry in over 50 years it feels good getting back into it.
Thanks, Davr
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I had always been a transistor technician and I looked at tube amps from afar until the day when a long-time customer asked me to look at his big Japanese tube amp which had passed through several hands without success and I competent people here kindly took me by the hand to explain the basics.
I have since come a long way on the learning path and this site has helped me a lot (in addition to very good books)
https://robrobinette.com/Amp_Stuff.htm
there is a very practical bias calculator, just enter the requested values and it works on its own.
I have since come a long way on the learning path and this site has helped me a lot (in addition to very good books)
https://robrobinette.com/Amp_Stuff.htm
there is a very practical bias calculator, just enter the requested values and it works on its own.
Sorry for the 2 word reply.
I went to trade schools for elections In the 70s..yep I'm that old. I went for engineering at Rutgers, to Glassboro for optical Engineering and design.
but elections never left me. I remember being fascinated the first time seeing a transistor. Most of my repair work was industrial or fixing just about anything for people. The fascination with tubes never left. And yea a bunch of books. I'll check your link out later.
Thanks again, Dave
I went to trade schools for elections In the 70s..yep I'm that old. I went for engineering at Rutgers, to Glassboro for optical Engineering and design.
but elections never left me. I remember being fascinated the first time seeing a transistor. Most of my repair work was industrial or fixing just about anything for people. The fascination with tubes never left. And yea a bunch of books. I'll check your link out later.
Thanks again, Dave
This suggested mod allows you to check bias of individual tube since the amp provides "DC Balance," hence TWO test points. You can of course keep the stock circuit but you have to make sure to use matching tubes.
The TUBE BIAS CALCULATOR recommended by huggygood is excellent. And based on its calculation, as long as you don't exceed 40mA, you are safe.
The TUBE BIAS CALCULATOR recommended by huggygood is excellent. And based on its calculation, as long as you don't exceed 40mA, you are safe.
The mod is excellent, and I get not to exceed 40ma. regardless. right now I do not have matched tubes. I have what looks to be one original or so pair and a used pair the owner bought to replace what he broke. I should have 4, 10ohm resistors in my stock. I'm still waiting for the caps to come in. I don't think the owner will care. make you wonder why build it with the one 3.3 in the first place.
in 1963 it wasn't money or labor it was design. I mean if not 10 why not individual resistors? I see these shared circuits in a lot of tube am schematic I reed ,and in almost all cases the mod recommends changing that.
it's a lot like the DC circuit going to the 12ax7s, the chain series of resistors and a 75mf cap between each. most of it to prevent a surge a current draw when the 12ax7s are powered up has been my thoughts.
but with that can, cap open I'm adding the capacitance before the resistors.
this project is almost over. my next is a fender Blues deluxe that was 100% working then the owner bought a mod kit and something went wrong.
I live in Central Jersey he lives in Brooklyn, no far just a pain of a ride.hes going to bring the amp and mod schematic and I'll start from there.
Thanks again Dave
in 1963 it wasn't money or labor it was design. I mean if not 10 why not individual resistors? I see these shared circuits in a lot of tube am schematic I reed ,and in almost all cases the mod recommends changing that.
it's a lot like the DC circuit going to the 12ax7s, the chain series of resistors and a 75mf cap between each. most of it to prevent a surge a current draw when the 12ax7s are powered up has been my thoughts.
but with that can, cap open I'm adding the capacitance before the resistors.
this project is almost over. my next is a fender Blues deluxe that was 100% working then the owner bought a mod kit and something went wrong.
I live in Central Jersey he lives in Brooklyn, no far just a pain of a ride.hes going to bring the amp and mod schematic and I'll start from there.
Thanks again Dave