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Post subject: BDRI Bias Current vs Plate Voltage Measurements
Posted: Thu Dec 22, 2011 11:07 pm
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Well, it has been a while since I last checked the bias on my BDRI, so I decided to do that tonight (technically last night since it is after midnight now :lol: ) and also measure the plate voltage change as bias is adjusted to satisfy my and Shimmilou's curiosity. It's a little long so here goes.

To refresh everyone's memories, Shimmilou and I had been discussing the change in plate voltage as bias current is changed. My amp is a late model Blues Deluxe Reissue in the following configuration:

V1 = NOS JAN GE 12AY7
V2 = JJ 12AX7
V3 = Balanced JJ 12AX7
V4/5 = JJ 6L6GC
Speaker = Eminence Cannabis Rex

The measurements were taken between about 7:00 pm and 8:30 pm on Thursday, 12/22. Line voltage at my wall outlet = 121.2 VAC (I have found that my line voltage varies between about 120 and 122 VAC during the course of a day).

I installed my bias probe

http://www.amp-head.com/product_info.ph ... ucts_id=70

and used it and my Ideal DVM for all measurements. I also used a cheap Radio Shack DVM for measurements at the Fender bias test point.

With the power tubes removed, the open circuit plate voltage measured 439 VDC (both tubes were within 1 volt of each other for all measurements). With the power tubes installed and bias current set to the minimum value (about 10 mA cathode current), the plate voltage measured 430 VDC. Both values are pretty close to the schematic value of 431 VDC:

http://www.blueguitar.org/new/schem/fen ... eissue.pdf

With the bias set to the factory recommended value of about 30 mA cathode current, the plate voltage dropped to about 420 - 425 VDC (I don't remember the exact value as I forgot to write it down).

Finally I set the bias to my desired value of 40 mA cathode current (About 38 mA plate current per the Weber bias calculator:

http://www.webervst.com/tubes1/calcbias.htm)

Both tubes were within 0.5 mA of each other. Pretty damn good. :D At that point the plate voltage measured 412 VDC. I also measured the voltage at the Fender bias test point for this set of values using my Radio Shack DVM and it was 89 mVDC. That works out to be about 44 mA cathode current (42 mA plate current) which is pretty close to that measured by the bias meter, taking into consideration that the bias meter uses precision 1 ohm, 1% resistors for each cathode while Fender uses a single 1 ohm, 5% resistor shared by both cathodes. Throw in the accuracy of the two DVMs and it looks pretty good. Using the Fender test point would result in a slightly hotter bias but still within the safe range of the tubes.

So there you have it. The tubes are running right at 50% max plate dissipation. I can't increase the bias much more than it is currently set to, and the plate voltage would probably drop a few more volts. The BDRI power supply does not seem very stiff.

So, how does the amp sound? In a word, terrific! With the controls set as follows:

Clean Volume = 6
Drive = 6
Treble = 8
Bass = 7
Mid = 6
Master = 5
Reverb off (I am now using a Boss FRV-1 63 Fender Reverb pedal which sounds great.)
Presence = 9

and my AVRI 52 Telecaster with Seymour Duncan Vintage Stack Noiseless pickups the sound is clean, rich and creamy. Playing with an E-H LPB-1 switched in boosts the mids somewhat for an even thicker sound. I use a Fulltone Full Drive 2 Mosfet for overdrive. The volume is loud but not overwhelmingly so. Note that using a 12AY7 in V1 turns the drive channel into a second clean channel with slightly reduced bass response which is great with humbucker equipped guitars.

So there you have it. The bias setting, although seemingly cold, gives a great tone and should be good for long tube life. Overall, I am extremely pleased with the sound. :D

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Post subject: Re: BDRI Bias Current vs Plate Voltage Measurements
Posted: Fri Dec 23, 2011 3:27 am
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Many fender amp have a great tone with a cold bias but it could be good thing to ear it with a medium or hot bias,
You said your amp bias adjustment can't give you these bias ?


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Post subject: Re: BDRI Bias Current vs Plate Voltage Measurements
Posted: Fri Dec 23, 2011 7:11 am
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stratele52 wrote:
Many fender amp have a great tone with a cold bias but it could be good thing to ear it with a medium or hot bias,
You said your amp bias adjustment can't give you these bias ?


Sure, I can increase the cathode current to about 45 mA, maybe even slightly higher, but why bother? The plate voltage drops to about 408 VDC at that point so that the percentage of maximum plate dissipation goes up to about 60%. Can I get it to 70%? I did not try. That is why I consider my bias to be "cold". However, with the cathode current at 40 mA (38 mA plate current), the amp sounds so much better than at the factory setting where the plate current is only about 30 mA (and plate voltage was about 425 VDC giving a % of max plate dissipation of about 43%). I did not run this little test because I thought that there was a problem with the amp, it was done only to satisfy my and shimmilou's curiosity. 36 to 38 mA plate current at about 412 VDC plate voltage seems to be this amp's "sweet spot", at least to me, anyway. YMMV. :D

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Post subject: Re: BDRI Bias Current vs Plate Voltage Measurements
Posted: Fri Dec 23, 2011 7:40 am
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Cool stuff bluesky636, I appreciate it! :D

Sounds as if you are calling the B+ the Plate voltage. That sounds about right, dropping the B+ by about 8 volts over the bias adjust range, which would be a 4 volt difference per tube Plate as the B+ is split for the two tubes through the OT primary. This means that the Plate voltage for each tube only changes about 4 volts through the bias adjust range, in other words, dropping 4 volts on each half of the OT primary with the extremes of the bias adjust (lowest to highest). That is close to what I thought it should be.

Any idea of the OT primary halves resistance? I think that the BDRI is more balanced than the HRDlx. If the resistance is measured, and volts drop across is measured, this gives actual Plate current. So if the OT primary resistance is 82 ohms per side, a 4 volt drop across the OT primary half would be 48 mA Plate current. Obviously, you can't drop 8 volts across each OT primary half, that would be an incredible 97 mA of Plate current per tube, and we know that isn't happening. I know I have measured the resistance on a BDRI before, but can't remember the resistance or balance between halves of the OT primary. I want to say that it is balanced (close as can be) at about 82 ohms per half. BTW, I like the approx 50% bias setting, perfect. :)

And to preempt the rest of you, this is as much fun for us as actually "playing the darned thing". :lol:

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Post subject: Re: BDRI Bias Current vs Plate Voltage Measurements
Posted: Fri Dec 23, 2011 8:00 am
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shimmilou wrote:
Cool stuff bluesky636, I appreciate it! :D

Sounds as if you are calling the B+ the Plate voltage. That sounds about right, dropping the B+ by about 8 volts over the bias adjust range, which would be a 4 volt difference per tube Plate as the B+ is split for the two tubes through the OT primary. This means that the Plate voltage for each tube only changes about 4 volts through the bias adjust range, in other words, dropping 4 volts on each half of the OT primary with the extremes of the bias adjust (lowest to highest). That is close to what I thought it should be.


Well, all measured values given are the actual plate voltage as measured at pin 3 of each tube by the bias probe. You are right though in that with this statement I was refering to the spec'd B+ value from the service manual:

"Both values are pretty close to the schematic value of 431 VDC"

I forgot about any drop through the OT.

shimmilou wrote:
Any idea of the OT primary halves resistance? I think that the BDRI is more balanced than the HRDlx. If the resistance is measured, and volts drop across is measured, this gives actual Plate current. So if the OT primary resistance is 82 ohms per side, a 4 volt drop across the OT primary half would be 48 mA Plate current. Obviously, you can't drop 8 volts across each OT primary half, that would be an incredible 97 mA of Plate current per tube, and we know that isn't happening. I know I have measured the resistance on a BDRI before, but can't remember the resistance or balance between halves of the OT primary. I want to say that it is balanced (close as can be) at about 82 ohms per half. BTW, I like the approx 50% bias setting, perfect. :)


No, I did not measure the OT resistance. Given that the tubes measured within about a volt of each other on the plate voltage and 3/4 mA of each other on the cathode current, I would say that the output tubes are really well balanced.

The amp sounds great at this setting. I need to get a digital recorder next year so I can record how things sound. Of course, that means ya'll would have to listen to my terrible playing. :lol:

shimmilou wrote:
And to preempt the rest of you, this is as much fun for us as actually "playing the darned thing". :lol:


Yeah, but unless a problem arises, I have put my bias probe and DVM away for the season (I like to check bias in the summer and winter as the load on the local power grid changes). Well, at least until I begin building my Weber 5F6A which arrived yeaterday. :D :D :D

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Post subject: Re: BDRI Bias Current vs Plate Voltage Measurements
Posted: Fri Dec 23, 2011 1:46 pm
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bluesky636 wrote:
stratele52 wrote:
Many fender amp have a great tone with a cold bias but it could be good thing to ear it with a medium or hot bias,
You said your amp bias adjustment can't give you these bias ?


Sure, I can increase the cathode current to about 45 mA, maybe even slightly higher, but why bother? The plate voltage drops to about 408 VDC at that point so that the percentage of maximum plate dissipation goes up to about 60%. Can I get it to 70%? I did not try. That is why I consider my bias to be "cold". However, with the cathode current at 40 mA (38 mA plate current), the amp sounds so much better than at the factory setting where the plate current is only about 30 mA (and plate voltage was about 425 VDC giving a % of max plate dissipation of about 43%). I did not run this little test because I thought that there was a problem with the amp, it was done only to satisfy my and shimmilou's curiosity. 36 to 38 mA plate current at about 412 VDC plate voltage seems to be this amp's "sweet spot", at least to me, anyway. YMMV. :D


You do a good job Bluesky.
I'm just checking with you because i'm not sure if I understand well what you write. I'm not english / american speaking.


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Post subject: Re: BDRI Bias Current vs Plate Voltage Measurements
Posted: Fri Dec 23, 2011 3:31 pm
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Location: Natural Bridge, Virginia
stratele52 wrote:
bluesky636 wrote:
stratele52 wrote:
Many fender amp have a great tone with a cold bias but it could be good thing to ear it with a medium or hot bias,
You said your amp bias adjustment can't give you these bias ?


Sure, I can increase the cathode current to about 45 mA, maybe even slightly higher, but why bother? The plate voltage drops to about 408 VDC at that point so that the percentage of maximum plate dissipation goes up to about 60%. Can I get it to 70%? I did not try. That is why I consider my bias to be "cold". However, with the cathode current at 40 mA (38 mA plate current), the amp sounds so much better than at the factory setting where the plate current is only about 30 mA (and plate voltage was about 425 VDC giving a % of max plate dissipation of about 43%). I did not run this little test because I thought that there was a problem with the amp, it was done only to satisfy my and shimmilou's curiosity. 36 to 38 mA plate current at about 412 VDC plate voltage seems to be this amp's "sweet spot", at least to me, anyway. YMMV. :D


You do a good job Bluesky.
I'm just checking with you because i'm not sure if I understand well what you write. I'm not english / american speaking.


Hey, no problem. Sometimes I get too wrapped up in "engineer speak" and tend to drone on. I do try and document everything as best as I can and am always open to questions or corrections. :D

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Post subject: Re: BDRI Bias Current vs Plate Voltage Measurements
Posted: Sun Dec 25, 2011 4:45 am
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Great post bluesky636.

I agree that once dialled in 'technically' speaking, always useful to compare with how the ear sounds and the amp performs.

I've posted about my DRRI and Tung Sol tubes before - now resolved through a swap-out to a pair of NOS JAN Phillips 6V6. However, although I could not tell much difference in sound between a hot & cold bias for straight clean chops and lead work, when using pedal derived overdrive/distortion sound I liked a hotter bias SO much more.

We currently play 'I Shot The Sheriff' in our set and at the end of the run-down riff (low G, 3rd fret, bottom E string), with a hot bias my Strat effortlessly coaxes into silky feedback while the drummer pads out to the chorus pick-up (you know the song).

However, with a cold bias setting (<19mA), there is no soul in the sound, producing said feedback is hard work and the harmonic content is lost.

This surprised me greatly, but all part of the fun of the set-up/tweak opportunities offered by tube amps.

Feed.

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Post subject: Re: BDRI Bias Current vs Plate Voltage Measurements
Posted: Sun Dec 25, 2011 7:12 am
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Feedbackboy wrote:
Great post bluesky636.

I agree that once dialled in 'technically' speaking, always useful to compare with how the ear sounds and the amp performs.
.
.
.
This surprised me greatly, but all part of the fun of the set-up/tweak opportunities offered by tube amps.

Feed.


Thanks.

Yep. Can't do that with some computer masqurading as a "guitar amp". :lol:

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