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Post subject: Power tube bias low?
Posted: Sat Oct 29, 2011 7:31 am
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I picked up a Blues Deluxe Reissue (yeah, I know you all aren't big fans ;) ) on eBay a couple months ago. It had a Celestion G12H30 in it and also had a full set of JJ's (12AX7, 6L6GC). They were "supposedly new." The cabinet, tweed and grill cloth is extremely clean so I was pretty happy about the deal I got. I got it with the intention of eventually putting in a Eminence Cannabis Rex and changing up the tubes. I'm happy with how it sounds right now so I won't be swapping anything until after Christmas.

I have a bad case of engineers disease (I'm a software engineer), and just can seem to leave well enough alone. I really enjoy taking things apart, tweaking, and learning as much as possible about anything electronic/mechanical. So with that in mind, one of the first things I did upon getting the amp was pop the back off. I wanted to check the bias at the test point. I got a reading of 45mV. Which seemed low, especially considering it was supposedly biased when the JJ's were put in by the previous owner. When I went to adjust the bias to stock 60mV, as a starting point, the trim pot was a little stuck from what I think was the little paint dab put on during assembly. So yeah, it was probably never adjusted with the new tubes.

Anyway, after a week or so of playing I opened the amp back up and re-checked the bias. It was sitting around 55mV. I bumped it back to 60mV which put the pot almost all the way "up." For curiosity sake, I wanted to see how much travel I had left. I could only get the bias up to around 65mV. The lowest point was about 18mV. At the moment it seems pretty stable at 60mV. I realize that the test point value doesn't mean much without knowing the plate voltage. But when I see comments about people biasing between 70-85mV's it makes me a little worried that something could be wrong.

I checked the power supply voltages and got the following. Actual values read are in parentheses. Does that seem reasonable? 2.5% error doesn't seem that bad to me.
B+ 431 (420)
Z 430 (421)
Y 392 (389)
X 345 (353)

I checked for a big voltage drop across both screen grid resistors and got the B+ 420 volts on both sides. I'm a pretty much a novice with tube amps but I'm not afraid to dig in and mess around. I'm not the most practiced with a soldering iron so I'd like to check as much as possible first before ripping stuff out. I didn't see any cracked solder joints or burned out components. Could this be just the power tubes wearing out? The amp sounds good and I've used it for practices and 2 gigs. But in the back of my mind I'm still worried about something failing in the middle of a gig.

I'd appreciate any ideas or things to check.

Even though this is my first post, I have read the forums for a while. I've learned a lot from the usual suspects just by reading through a lot of the old posts.
Thanks for the help!

Joel

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1997 Stratocaster Plus Deluxe
Blues Deluxe Reissue (Jan Phillips 5751, Tung-Sol 12AX7, SED 6L6GC and Eminence Cannabis Rex)
Peterson Strobostomp>Dunlop Crybaby>Keeley TS9 Mod Plus>Keeley Compressor>Keeley TS9 Baked>Ibanez AD9


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Post subject: Re: Power tube bias low?
Posted: Sat Oct 29, 2011 8:10 am
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Hello Twofootshunk,

What you did is not bad at all.

I've one question: where did you measure the bias voltage ? On 1 ohm resistor ?

On BDLx there is a resistor 1 ohm common to kathodes that "converts" current to voltage. Can you check if this resistor has been changed ?

What is the 6L6 grill voltage (supposed to be negative rougly -50V) ?
8)

Take care about lethal voltages when the rear is open !

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Post subject: Re: Power tube bias low?
Posted: Sat Oct 29, 2011 8:14 am
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Hi twofootskunk,

There are many BDlx and HRDlx fans here (including myself), and only a few that are haters, and even a couple with vendettas against these amps. For the most part these amps are pretty good. The complainers are obviously much more vocal than those with no problems. If you check the source of the hate posts, they are from a very few people posting thousands of times about the same amp ( :roll: )

It does seem that your bias was very cold and had not been adjusted, as evidenced by the seal on the pot not being broken. The drift in bias current is typical of many tube brands, try GTs if you want little-to-no drift after initial setting. GTs are usually rock solid as far as drift. I don't care for JJs.

Some tubes will only draw so much current, and the bias adjust pot may max out at a low current level. This is just the particular tubes that you are using, and is normal. Other tubes might draw much more current, depending on the type. I have one set of GTs in my HRDlx (Sovtek 5881WXT) that will max out at 78 mV and goes no higher, but my new set draws well over 100 mA and more depending on the setting. I settled for between 85 mV and 90 mV with the new GTs.

If you like to tinker (like me), you might want to carefully pull the main circuit board and go over all solder joints thoroughly, with a magnifier, and touch up any questionable joints. Also check the joints on the tube circuit boards, particularly the tube sockets themselves.

As with most modern amps (not just the HRds), solder joints are probably the single biggest cause of issues, and a little preemptive soldering can go a long way. The main problem with the solder is that they are using the new 'lead free" crap that seems more prone to cracking. Couple crap solder with wave soldering that is done way too fast, and you have a recipe for joint failure.

Edit:
The test point mV can be directly translated to mA as the reading is across a 1 ohm resistor, and it is both tubes' Cathode currents, so divide by 2 to get the Cathode current of one tube. Subtract about 5 mA from the Cathode current to get the Plate current (Plate current times Plate voltage equals wattage dissipated at idle). With 430 Plate volts, and 30 watt tubes, the bias test point reading for your amp should be between 80 mV (50% - 15watts each tube), and 110 mV (70% - 21 watts each tube), depending on the sound that you want. A lower current will result in more clean headroom and longer tube life, while a higher current will result in earlier breakup and shorter tube life. I like mine at around 55% to 60% for headroom and long tube life. :idea:

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Post subject: Re: Power tube bias low?
Posted: Sat Oct 29, 2011 9:04 am
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I did measure at the test point. The resistor hasn't been changed and measures within spec.

Tis-san
Where would I measure the grill? (grid?) voltage?

Shimmilou
Where would I measure plate voltage?

It seems to me I should probably just try different power tubes and see what range I can bias them. If a new set can't be biased any higher than the 65mV at the test point, then I can keep hunting for the problem.

Bah, I had to give my wife's parents a christmas list (they shop early). So I gave them a list of different tubes. I guess I'll just have to wait unil then to try them out. I'm looking to put a Jan Phillips 5751 in V1. 2 Tung-sol 12Ax7 new productions in V2 and V3. And 2 SED Winged C 6l6gc's for the power tubes.

_________________
1997 Stratocaster Plus Deluxe
Blues Deluxe Reissue (Jan Phillips 5751, Tung-Sol 12AX7, SED 6L6GC and Eminence Cannabis Rex)
Peterson Strobostomp>Dunlop Crybaby>Keeley TS9 Mod Plus>Keeley Compressor>Keeley TS9 Baked>Ibanez AD9


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Post subject: Re: Power tube bias low?
Posted: Sat Oct 29, 2011 9:51 am
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The grid is pin 5, and the Plate is pin 3. The B+ is roughly the same as Plate voltage. There will be a small voltage drop across the output transformer primary, maybe 3 volts or so, leaving the Plate with the remainder of B+. Careful probing at the tube socket pins, it is very easy to slip and short two pins with the meter lead. I measure the Plate voltage at the brown and blue wires' connections to the tube socket board, more room there. :idea:

Seems like a solid line-up of tubes that you've chosen. Keep us posted. :!:

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Post subject: Re: Power tube bias low?
Posted: Sat Oct 29, 2011 10:20 am
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Grid voltage is -40V
Plate voltage is 419V
for both sockets.

I'm probably just making a mountain out of mole hill. Probably just weak tubes. I suppose the trimmer pot could be fouled up. Maybe I cracked it when breaking it free of the paint.

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1997 Stratocaster Plus Deluxe
Blues Deluxe Reissue (Jan Phillips 5751, Tung-Sol 12AX7, SED 6L6GC and Eminence Cannabis Rex)
Peterson Strobostomp>Dunlop Crybaby>Keeley TS9 Mod Plus>Keeley Compressor>Keeley TS9 Baked>Ibanez AD9


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Post subject: Re: Power tube bias low?
Posted: Sat Oct 29, 2011 10:43 am
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The trimmer pots resistance across lead 1 and 3 (max value) is 21.9k. I think these are supposed to be 25k pots. Access to lead 2 is under the board so I can't actually measure the value while turning without disassembling it.

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1997 Stratocaster Plus Deluxe
Blues Deluxe Reissue (Jan Phillips 5751, Tung-Sol 12AX7, SED 6L6GC and Eminence Cannabis Rex)
Peterson Strobostomp>Dunlop Crybaby>Keeley TS9 Mod Plus>Keeley Compressor>Keeley TS9 Baked>Ibanez AD9


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Post subject: Re: Power tube bias low?
Posted: Sat Oct 29, 2011 11:06 am
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twofootskunk wrote:
...I'm probably just making a mountain out of mole hill. Probably just weak tubes. ...


Yes :) Your readings are fine. The more negative the grid voltage goes, the lower the Plate current. -40 volts at pin 5 will give a lower tube current than -20 volts. As the voltage at pin 5 gets closer to zero, the tube current increases. Too low of a negative voltage at pin 5 (too close to zero) would result in red-Plating many tubes, ruining them. Some tubes just pull less current than others at the same grid voltage.

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Post subject: Re: Power tube bias low?
Posted: Sat Oct 29, 2011 11:31 am
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Alrighty, thanks for the help and information. I need to tell myself, "more playing, less tinkering."

I'm looking forward to putting in the new tubes and speaker. I've heard the Cannabis Rex in a few different cabinets and love it. It seems to me that it helps tame that bright pick attack that stabs people in the head when they are directly on axis with the speaker.

I've also been kicking around the idea of trying out a "Mitchell Donut" to change the perceived directivity of the speaker. Most of the places I play and where we practice don't have deep stages and I hate having the amp at such a steep angle on the stand to put my ears on axis with the speaker. It should be interesting to see if it actually works. I've really noticed how directional a single 12" speaker can be. I played on a solid state Princeton Chorus with 2 10" speakers since 1994 and didn't notice it nearly as much.

Again, thanks for the help.

_________________
1997 Stratocaster Plus Deluxe
Blues Deluxe Reissue (Jan Phillips 5751, Tung-Sol 12AX7, SED 6L6GC and Eminence Cannabis Rex)
Peterson Strobostomp>Dunlop Crybaby>Keeley TS9 Mod Plus>Keeley Compressor>Keeley TS9 Baked>Ibanez AD9


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Post subject: Re: Power tube bias low?
Posted: Wed Dec 28, 2011 10:01 am
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UPDATE!

Well I got a full set of tubes, a new speaker, and some foam to make a Mitchell donut, for Christmas. I got everything installed last night. The low bias with the JJ's was nothing to worry about. I set the new tubes to the stock 60mv at the test point for now. I have plenty of travel to ramp up the bias if I want later.

Tubes:
I got a Jan Phillips 5751 for V1 and two Tung-Sol new production 12ax7's for v2 and v3.
I got 2 Winged C 6L6GC's for the power tubes. I'll keep the JJ's for backups and maybe switch some in and out to see what I like the best. It sounds good.

Speaker:
I installed a Cannabis Rex. The previous owner had put in a Celestion G12H30. To me the speaker seemed to accentuate lows and highs and wasn't very balanced. I also wasn't comfortable with it being rated at 30watts. I'm liking the sound of the Cannabis Rex a lot and I've still got a lot of break in left to go.

Mitchell Donut:
I read the 50+ page thread on the gear page about this. The physics made sense and I absolutely hate the directivity of a single 12" guitar speaker. Especially because I'm usually fairly close to my amp at both our practice space and the smaller stages we play in bars. The amp sits on an angled back stand. We do mic our amps but we tend to use our PA mostly for vocals with just a small amount of instrument reinforcement. Tonight's our first practice since putting it in, but in my house I can definitely tell the sound is much more uniform as I walk around the front of the amp in an arc. And also as I get closer and farther away. There is still a small amount of increased treble directly on axis, but the off axis sound is MUCH closer to it now. There's more clarity and less phaseyness (pretty sure that's not a word!) than with the duct tape X I had on my speaker grill before. I used the 1/2" firmess 4 foam. I have a piece of 3/4" which I would like to try but the tolerance is to tight and I don't want it to touch the speaker. All in all it's definitely a win for the cost ($16 foam/adhesive spray).


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Post subject: Re: Power tube bias low?
Posted: Wed Dec 28, 2011 11:01 am
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Sweet! Good job. 8)

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