Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pbman1953 on February 25, 2019, 09:18:14 am
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I've been living with the Pro 300 with KT88's now for a while. The reason for writing is that I've noticed the sound is crunchy and not as warm as I wanted. I thought that I'd take a moment to see where the bias was. In the past I've been sent the diagram on how to use the test points J's- 9, 10 & 11 to bias and balance the tubes. I know I could use J9 (the ground) and make sure that both J10 & 11 measure -100mv. After a private message, I was suggested tat I measure each bank to see where they are. The odd thing is that tubes V5-7-9 are hotter than tubes V6-8-10. But each side bank is very close in measurement using my Bias Rite but I can understand why the even number tubes are not thr same as the odd numbered tubes. Could there a bias resistor value be off?
Here's the breakdown-
I'm going to average the findings because the tubes are very close to each other per side bank-
Valve Art- KT 88 w/ 42 watts diss-
With a 100 mv setting-for 5,7 & 9
Average voltage - 740
Average Cathode Current- 46
Avg. result- 34 watts
With a 100 mv setting-for 6,8,10
Average voltage - 728
Average Cathode Current- 21
Avg. result- 15 watts
What is tough is that moving J10 effects the J11 reading and vice versa for the J11 to J10
Thanks for any input.
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With a powerful amp such as this I highly recommend you follow the manufacturer bias instructions. It a very good procedure.
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With a powerful amp such as this I highly recommend you follow the manufacturer bias instructions. It a very good procedure.
I'll go with what you suggest. To move forward, what setting in mv, could I try to get a softer warmer sound.
Do I go higher or lower?
A note on some recent postings out there- Since the introduction of the newer Fender Super Bassman 300, Fender has suggested to use 80mv instead of 100mv. What does the change do? And does it make sense?
Thanks!
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You got the amp. You tell me. :icon_biggrin:
I don't really think about warm sounds when you are talking 300 watts bass amp. Do you play bass through this amp?
Fender probably just wants you to run it cooler (80mV) to get through their warranty period with fewer failures.
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Yes I do. I figured, even with the 300 watts, that it could do "it all"
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Does the pb in your name indicate Precision Bass?
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Bingo!
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Thanks. I often wonder where people get their monikers. Your's makes perfect sense. Now to see how long I remember that! :l2:
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I got home and warmed up the amp.
Now I noticed that I'm blind and I see clear as day to bias to 80mv. Under J9 & J10 is marked 80mv.
I tried as hard as possible to use the factory method to get 80 and a zero balance. After setting the bias side with J9 & J10 its real hard to get J10 & J11 to balance to zero.
So far here's what I get -
Using Sluckey's way to set up , I used J9 (ground) as the fixed point-
J9 & J10- 77
J9 & J11- 76
J10 & J11 (for zero balance)- 1.7
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Anyone want to chime in?
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EZ PZ! :icon_biggrin:
Now that you got a good grip on this, let me offer another approach. Got a meter with a relative button? Good. Try this...
1. Connect your black meter lead to J9. Don't move it until you are done.
2. Connect the red meter lead to J10. Adjust bias pot for 80mV.
3. Press the "relative" button. Meter now indicates zero.
4. Now move red meter lead to J11. Adjust balance for zero reading.
5. Done.
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Thanks,
I have 2 meters. One is a basic digital and the other is newer. I'll have to check if the newer one has the "relative" button
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All set thanks!
There's so much written about cold setting vs. hot. To go in terms that I understand, I was taught to adjust tubes between 60-70% of the dissipation of a tube.
So If I want a warmer tone is that towards 60 or 70? Or lower or higher. I've seen some charts with 50% , 60 & 80
https://robrobinette.com/Tube_Bias_Calculator.htm
Thanks
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Let's do some math...
Max Plate power for 6550 is 27W. TP44 B+ is 742. Let's say there's no drop through the OT primary so 742 is the plate voltage. You set the bias for 80mA current. .08 x 742 = 59.4W. But that's for three tubes. Let's assume the three tubes (heck, let's say all six tubes) are perfectly matched. So, 59.4W ÷ 3 = 19.79W per tube. 19.79 ÷ 27 = .73 or 73%.
Is that close enough for you? Do you really want to be poking around with 742V just to set the bias to within a gnat's ass of 70%? Or would you rather be safe and just follow Fender's procedure?
BTW, how does the amp sound biased at 80mA?
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The amp sounds great, thanks for that!
I have apologize a bit only because my dissipation percentage question was not for the 300t.
It was really for my other amps without bias controls like the 300T. The others have a single bias. Voltage wise is in the mid 500 's with JJ 6550’s.
Where do you suggest I go from that info.
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Procedure is the same as for the 300 or any other amp.
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Ok, I need a little instruction on an amp with only 1 bias pot to do the same procedure like the 300.
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Here you go...
http://www.duncanamps.com/technical/lvbias.html
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Wow!
OK, possibly a silly question, couldn't I use my Bias Rite to achieve the results? It's alot safer
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Sure, if it has enough sockets for all power tubes. It's a real PITA to use a one socket tester with six power tubes.
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I have 2 more amps with 4 tubes and my tester has 4 sockets.