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Post subject: Re: early breakup
Posted: Sun Feb 13, 2011 9:14 am
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430v at pin 3. so that would be 80 to 90mV bias to give me a 60 to 70 percent disapation? check my math.and spread the word that 430v seems to be the norm for bdri and hrd amps.


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Post subject: Re: early breakup
Posted: Sun Feb 13, 2011 12:10 pm
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You are on track, very close, and sounds like you understand how it's done. Using your numbers would be fine, on the conservative side, but just fine. Remember, you are calculating the wattage using the Plate current and the Plate Voltage. The test point reading is the Cathode current, which will be higher than the Plate current.

So, 60% of a 30 watt tube would be 18 watts. 18 watts divided by 430 volts is about 42 mA Plate current. The Cathode current will typically be about 5 mA higher than the Plate current, so add 5 mA to get 47 mA Cathode current for one tube. There are two tubes, so 2 x 47 mA = 94 mA, or 94 mV at the test point. Likewise, 70% in this case would be 49 mA Plate current for one tube, 54 mA Cathode current for one tube, 108 mA for both tubes 108 mV at the test point. So, 50% = 80 mV, 60% = 94 mV, 70% = 108mV at the test point.

Let us know how it sounds after you get it dialed in. After you play for awhile at the setting that you like, check the bias again before closing up the amp, it can drift a little and you might need to tweak it back to where you set it. If you use GTs, they aren't likely to drift much, but always a good idea to double check. :)

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Post subject: Re: early breakup
Posted: Sun Feb 13, 2011 3:32 pm
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you've been a lot of help. i knew i was on track once i found the plate voltage. i had calculated the wattage at 17.6 or something like that, so 18w is what i rounded to. once i get them changed and biased i'll find this post and let you know. i usually talk with my uncle about stuff like this, he was a tube guru from way back in the day but he passed away. i knew what to do but had to be sure i was on the right track. again thank you for you help.


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Post subject: Re: early breakup
Posted: Sun Feb 13, 2011 7:35 pm
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I'm with Shimmy. For most Fender amps, I like to setup the power tubes conservatively (running ~55-60%) for the best swing and most headroom. Esp at volume. Then, you can always get a clean tone, when you want it. Run effecient speakers (90+ dB).

I will play with the gain stages to get a bit of "brown" or overdrive in the pre-amp section. I love the 12AY7 in first stage for the brownest tone. Really, a good 6072 (mil spec 12AY7) can really sing, give a slower onset of overdrive, and brown out the tone to a Bluesier taste. 12AT7's are good too. But, subbing them into a 12AX7 may set the 12AT7 at too low idle for a good clean headroom (most 12AT7 like around 10mA whereas the 12AX7 likes aroudn 1-2 mA at idle).

A good 12AU7 can lower the gain of the PI. But, their just aren't many good new 12AU7's. I like the CBS-Hytron 5418A or 7618. Nearly impossible to find NOS these days. They used to sell 5 tube sleeves for $30, in the mid 1990's!

Then there's all the CRAZY coupling cap issues. Best left for another thread. Stomp boxes (liek Screamers and I like the old Yamaha MagicStomp) helps a lot, if you can get one that has minimal "digital edge." Too bad that Yamaha has stopped importing them. I got a new one from a friend in Thailand. For some reason they are still cranking them out in parts of the Orient.

One last important tube, in my opinion-- is the rectifier. Good, $$$ NOS GZ34 will stiffens the tone. Good, not so $$$ NOS 5U4GB gives you more sag. But, lasts forever. Mullard GZ37 is the cat's meow... but good luck there.

I know it's not popular for some reason, on this board... but 1970-1980's (pre-Gorbachev) Reflektor 6P3S-E (6N3C-E) is a pretty tough tube. Tone is more like a good 5881/6L6WGB. But it can take the plate voltages of vintage Fender 6L6GC amps. Bias it a bit lower--like 5mA per tube. Very nice midrange, not the bass of a an RCA 6L6GC. Worth a look at, for $56 per matched quad, IMHO.


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Post subject: Re: early breakup
Posted: Sun Feb 13, 2011 10:38 pm
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That's the way that I typically like my amps, very clean and I use pedals for drive. 8) Lately though, I have been playing around with the preamp tubes in my HRDlx, trying to get a good sound from the drive channel, so I wouldn't need pedals. I tried a few different types, one at a time, including US 12AX7 & 7025, GT 12AX7 (Sovtek 12AX7WA & 12AX7WC), GT 12AX7-C (Chinese), and GT & Jan Phillips 12AT7, in each preamp spot. The 12AT7 helped the drive channel a little in V1 or V2, not quite enough, but the clean channel sounded a little worse to me. The clean channel sounds the best with the US 12AX7A/7025 or the Sovtek 12AX7WA in all three spots.

I had been trying one tube change at a time, in each spot, then I finally tried GT 12AX7-C in all three spots at the same time, and now the drive channel is unbelievable. The breakup is fantastic, smooth warm and sustaining, especially on the neck pickup, and gets crunchier and squeals wonderfully when I change to the bridge pickup, with no harshness at all, just a killer tone. With the three GT 12AX7-Cs, the clean channel is a little warmer than before, but surprisingly still sounds great, and is easily brightened up with a little adjustment of the tone controls, maintaining a solid, clean, clear tone. I have heard people compare these GT 12AX7-C tubes to Mullards as far as the sound, and they certainly are great for that bluesy, creamy overdrive. I really can't say enough good about these Chinese tubes, they are amazing in my HRDlx, making an otherwise unpleasant drive channel sound great. :D :D

I also have been changing speakers around, but I guess that is also for another thread. :)

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Post subject: Re: early breakup
Posted: Sun Feb 13, 2011 11:52 pm
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blackstrat71 wrote:
430v at pin 3. so that would be 80 to 90mV bias to give me a 60 to 70 percent disapation? check my math.and spread the word that 430v seems to be the norm for bdri and hrd amps.


Hey Blackstrat71: Did you actually measure 430 volts at pin 3? I know that is what the schematic says, but as you crank the bias up to a hotter level, you will find that the measured plate voltage drops considerably. Using a bias probe, I have set my bias to 37.6 mA cathode current. At that level, the measured plate voltage on my BDRI is 406 volts. Using the Weber bias tables, this gives me a plate current of right about 36 mA and a 15W plate dissipation (running at 50%). This translate to 83 mV at the Fender test point. I have never been able to get to 60% regardless of how high I crank the bias. It tops out at about 95 mV at the Fender test point in my amp. The plate voltage drops accordingly.

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Post subject: Re: early breakup
Posted: Sun Feb 13, 2011 11:55 pm
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shimmilou wrote:
That's the way that I typically like my amps, very clean and I use pedals for drive. 8) Lately though, I have been playing around with the preamp tubes in my HRDlx, trying to get a good sound from the drive channel, so I wouldn't need pedals. I tried a few different types, one at a time, including US 12AX7 & 7025, GT 12AX7 (Sovtek 12AX7WA & 12AX7WC), GT 12AX7-C (Chinese), and GT & Jan Phillips 12AT7, in each preamp spot. The 12AT7 helped the drive channel a little in V1 or V2, not quite enough, but the clean channel sounded a little worse to me. The clean channel sounds the best with the US 12AX7A/7025 or the Sovtek 12AX7WA in all three spots.

I had been trying one tube change at a time, in each spot, then I finally tried GT 12AX7-C in all three spots at the same time, and now the drive channel is unbelievable. The breakup is fantastic, smooth warm and sustaining, especially on the neck pickup, and gets crunchier and squeals wonderfully when I change to the bridge pickup, with no harshness at all, just a killer tone. With the three GT 12AX7-Cs, the clean channel is a little warmer than before, but surprisingly still sounds great, and is easily brightened up with a little adjustment of the tone controls, maintaining a solid, clean, clear tone. I have heard people compare these GT 12AX7-C tubes to Mullards as far as the sound, and they certainly are great for that bluesy, creamy overdrive. I really can't say enough good about these Chinese tubes, they are amazing in my HRDlx, making an otherwise unpleasant drive channel sound great. :D :D

I also have been changing speakers around, but I guess that is also for another thread. :)


I'm getting ready to put a JJ 12AX7 back into V2 (one already in V1) and a balanced JJ 12AT7 in V3 in my BDRI. My rig has changed and I want to try for a little different tone.

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Post subject: Re: early breakup
Posted: Mon Feb 14, 2011 12:50 pm
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I have a set of JJ gold pins in my HRD 410.
I played with the bias quite a bit, and settled on right about 87mv.
Sounds fabulous. Anything under 80 sounds clunky to me.
The drive/more drive channels are useless at any bias setting IMO.
Sounds much better on the clean channel with a little compression and a dist box. If you are not using a compressor with this amp, do your ears a favor and get one. Takes all the spike out, and you can actually hear the tone better.

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Post subject: Re: early breakup
Posted: Mon Feb 14, 2011 5:08 pm
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hey bluesky whats shakn', yes 430v at pin 3 and i did check both tubes. i finaly changed my tubes and biased them at 90mV at the test point. i ramped them up to 92mV and played a little then backed it down a bit to the 90mV. i was wrong saying they were rated with a plate current of 45 a piece it was 41. i use an 12at7(matched) in V3 and the 12ax7c in V1 and V2. i love the mids they put out and the o.d. . these new tubes with early breakup are fantastic. they almost give a class A feel to the amp. very responsive. i do use an efficent speaker of the alnico variaty. i put duncan aph 1n and 1b in my l.p. and they are awesome. i found a sweet spot with the volume on 4 and i can back off the volume on the guitar a bit and the amp will clean up but not loose punch. i run the normal channel only (plug and play) i even took the compressor out of the loop i normaly use. boys, it's valentines day and i am in love. i'm going to go play....well after i entertain the wife. again thanks for the help all you guys.


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Post subject: Re: early breakup
Posted: Tue Feb 15, 2011 12:34 pm
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blackstrat71 wrote:
hey bluesky whats shakn', yes 430v at pin 3 and i did check both tubes. i finaly changed my tubes and biased them at 90mV at the test point. i ramped them up to 92mV and played a little then backed it down a bit to the 90mV. i was wrong saying they were rated with a plate current of 45 a piece it was 41. i use an 12at7(matched) in V3 and the 12ax7c in V1 and V2. i love the mids they put out and the o.d. . these new tubes with early breakup are fantastic. they almost give a class A feel to the amp. very responsive. i do use an efficent speaker of the alnico variaty. i put duncan aph 1n and 1b in my l.p. and they are awesome. i found a sweet spot with the volume on 4 and i can back off the volume on the guitar a bit and the amp will clean up but not loose punch. i run the normal channel only (plug and play) i even took the compressor out of the loop i normaly use. boys, it's valentines day and i am in love. i'm going to go play....well after i entertain the wife. again thanks for the help all you guys.


I have to admit that I am really puzzled at how you can be reading 430 volts (full B+) at pin 3 of the output tubes with a bias setting of 90 mV. I measure 406 volts at pin 3 with a bias setting of 83 mV. Either you have a very stiff power supply or mine is very weak. Mine does sound fantastic, though. Can someone explain this to me?

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Post subject: Re: early breakup
Posted: Tue Feb 15, 2011 1:19 pm
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bluesky636,

Your voltage may just be low, some are a little lower than others. You might check your Plate voltage sometime with the output tubes removed, you'll find that if all is normal, it isn't that much different than when they are in. When the tubes have current flowing through them, a small voltage will be dropped across the primary of the output transformer, but only a few volts on each half. A change of 40 mA through one tube, only results in about 3-1/2 volts drop on the Plate voltage on that half of the transformer. 100 mA running through both tubes would only drop the Plate voltage by about 4 volts on each half of the transformer. The B+ should remain fairly constant also. :idea:

If your Plate voltage is 25 to 30 volts higher with the output tubes removed, then there is a problem with your supply. But if you only have a few volts difference, it is OK. The schematic shows what looks like the same power transformer for both the HRDlx and the BDRI, but they could be different anyway, which would account for the voltage differences between the two amps.

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Post subject: Re: early breakup
Posted: Tue Feb 15, 2011 2:20 pm
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shimmilou wrote:
bluesky636,

Your voltage may just be low, some are a little lower than others. You might check your Plate voltage sometime with the output tubes removed, you'll find that if all is normal, it isn't that much different than when they are in. When the tubes have current flowing through them, a small voltage will be dropped across the primary of the output transformer, but only a few volts on each half. A change of 40 mA through one tube, only results in about 3-1/2 volts drop on the Plate voltage on that half of the transformer. 100 mA running through both tubes would only drop the Plate voltage by about 4 volts on each half of the transformer. The B+ should remain fairly constant also. :idea:

If your Plate voltage is 25 to 30 volts higher with the output tubes removed, then there is a problem with your supply. But if you only have a few volts difference, it is OK. The schematic shows what looks like the same power transformer for both the HRDlx and the BDRI, but they could be different anyway, which would account for the voltage differences between the two amps.


Hmmm. I've not measured the voltage with the tubes removed, nor have I measured it with the bias set at the factory 60 - 65 mV. I do know that with the bias set around 70 - 75 mV the plate voltage measures about 415 volts. At 80 mV the plate voltage drops to around 410 volts and at 83 - 85 mV it measures about 406 volts. This is all with an AC line in of right at 120 volts.

I'll be swapping some tubes in V2 and V3 later this week so If I get the chance, I'll take some more measurements. I'm not worried too much about it though as the amp sounds fantastic.

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Post subject: Re: early breakup
Posted: Tue Feb 15, 2011 3:01 pm
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When you first turn on the amp, the voltages will be a little higher, and will come down a little after warm up (ie, transformer resistance, and components values change with heat). Before pulling your tubes, play the amp for a few minutes to make sure it is completely warmed up, check the Plate voltage then turn off the amp and pull the tubes (might need an oven mitt), then turn the amp back on before it cools down and check the Plate voltage. Your initial measurement may have been when the amp wasn't quite warmed up, as the voltage difference that you noted is a little excessive. There is no way you are dropping 9 volts across 1/2 of the primary of the OT, unless there is a problem with the supply. Under normal operation, that would have to be 109 mA of current through each tube, or 228 mV at the test point.

Ohm's law is never wrong. Your output transformer primary has a specific resistance, and will produce a specific voltage drop with a specific current running through it, with only small differences due to heat. I believe that the resistance of each half of your OT primary is maybe 82 ohms, or maybe even 58 ohms. Going with 82 ohms will give the highest numbers (most change). At 70 mV at the test point, that is about 30 mA running through the OT primary, and will result in a 2-1/2 volt drop across the OT primary. (.030 A x 82 ohms = 2-1/2 volts). At 85 mV at the test point, that is about 37 mA running through the OT primary, and will only result in about a 3 volt drop at most. (.037 A x 82 ohms = 3 volts). So the difference in Plate voltage when changing from 70 mV to 85 mV at the test point should only be about 1 volt. :idea:

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Post subject: Re: early breakup
Posted: Tue Feb 15, 2011 3:14 pm
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shimmilou wrote:
When you first turn on the amp, the voltages will be a little higher, and will come down a little after warm up (ie, transformer resistance, and components values change with heat). Before pulling your tubes, play the amp for a few minutes to make sure it is completely warmed up, check the Plate voltage then turn off the amp and pull the tubes (might need an oven mitt), then turn the amp back on before it cools down and check the Plate voltage. Your initial measurement may have been when the amp wasn't quite warmed up, as the voltage difference that you noted is a little excessive.

Ohm's law is never wrong. Your output transformer primary has a specific resistance, and will produce a specific voltage drop with a specific current running through it, with only small differences due to heat. I believe that the resistance of each half of your OT primary is maybe 82 ohms, or maybe even 58 ohms. Going with 82 ohms will give the highest numbers (most change). At 70 mV at the test point, that is about 30 mA running through the OT primary, and will result in a 2-1/2 volt drop across the OT primary. (.030 A x 82 ohms = 2-1/2 volts). At 85 mV at the test point, that is about 37 mA running through the OT primary, and will only result in about a 3 volt drop at most. (.037 A x 82 ohms = 3 volts). So the difference in Plate voltage when changing from 70 mV to 85 mV at the test point should only be about 1 volt. :idea:


I always let the amp sit powered up for about 20 minutes before taking any bias readings. I also let it sit and stabilize for a like amount of time between any tweaks.

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Post subject: Re: early breakup
Posted: Tue Feb 15, 2011 3:21 pm
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Cool! Let me know what you find, you've peaked my curiosity (easy to do). :)

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