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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sat Dec 03, 2011 10:07 am
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shimmilou wrote:
bluesky636 wrote:
...My question was "How does Egnater handle measurement of the plate voltage since that can change as line voltage and bias adjustment changes?" The answer is they don't have a means to measure it unless you go and open up the amp.


Yes, you got it. :) I had a feeling that 63supro didn't mean that, but I wasn't sure. :wink:


Well, now that we got that cleared up ..... :lol:

shimmilou wrote:
bluesky636 wrote:
...At my set bias point (which equates to about 83 to 85 mV at the Fender test point) my plate voltage measures about 413 VDC. At the factory bias setting of 60 to 65 mV (at the factory test point) my plate voltage is around 420 to 425 VDC (I forget exactly as it has been a while)...


That is where we were before, double check that Plate voltage over your bias adjust range and you will find that your 413 volts never even gets close to 420 volts, it can't change that much. If the voltage drop across your OT primary half was 7 volts, that would mean that the idle current for one tube would exceed 70 mA (140 mA at the test point), which is over 30 watts per tube, and that of course is not happening. If you had 13 volts on the OT primary half, the current through one tube would exceed 140 mA, which is over 60 watts per tube!

The voltage difference between the B+ and the Plate voltage is the few volts that are dropped on the OT primary, more idle current equals more voltage dropped on the OT. Check your OT primary resistance, and it is about 82 ohms or less for each half of the OT primary (maybe 58 ohms?). Your Plate voltage will only change by a few volts, and you will have somewhere around 3 volts dropped across each half of the OT primary. Not much difference between the Plate voltage and the B+ voltage.


I never worried about it because the situation you describe obviously does not exist. I looked at it from the standpoint that my B+ supply was probably low compared to the stated value on the schematic. If that is the case, it is only low by a couple of % and since all the parts tolerances are probably no better than 10% (and most likely closer to 20%), I didn't feel the need to investigate further. I may do so now, but just out of curiosity, not because of any concern over a possible problem.

I sure hope we didn't scare off the OP with our little "discussion". :lol: I'm still interested in seeing photos of his amp. It sounds interesting. :D

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sat Dec 03, 2011 11:18 am
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Cool stuff, but nothing scary. :lol:

The Egnater bias test points are a little different, reading across two points instead of referencing to ground as in some Fender amps, but the same concept. Not having prints available for the Egnaters, I can only guess what kind of circuit that they use to check bias current, but comparing the test point reading in my Egnater to a bias probe reading, leads me to believe that the Egnater test points are Cathode mV.

Actually, measuring the resistance and voltage drop across an OT primary half is a great way to get actual Plate current, especially if there is no test point. And the comparison of the Plate current and Cathode current will tell the grid currents also. Just another method to check bias, and I think that it is important to be clear about what is actually occurring. Don't want anyone to read this and think that their amp has a problem, or worse yet, tries to get a 17 volt drop across the OT primary half. :o

I remember being curious as to whether or not the OT primary halves were imbalanced resistances in the BDRI as they are in the HRDlx. The HRDvl and BDvl and BJr all have balanced OT primaries, can't remember the BD. :?:

And I want to see the OP's HRDlx head myself, I thought of doing the same thing, but took the lazy way out and bought a head.....or two. :)

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sat Dec 03, 2011 12:49 pm
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shimmilou wrote:
The Egnater bias test points are a little different, reading across two points instead of referencing to ground as in some Fender amps, but the same concept.


Looking at the photo that 63supro provided:

http://www.egnateramps.com/Products/Ren ... Panel.html

shows two test points, one for each tube pair, and a common ground. Page 12 and 13 of the owner's manual has the bias setting procedure. Page 13 states that the measurement is made across a 1 ohm resistor in the cathode circuit for each tube pair just like Fender.

http://www.egnateramps.com/manuals/RenegadeManual.pdf

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sat Dec 03, 2011 3:20 pm
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Yes, that amp is a little more user friendly compared to my Twkr 40. I have to remove two metal screens, to be able to remove the output tubes, to be able to pull the chassis out of the cab, to access the bias TPs and the single adjustment pot. In this one you read across only two TPs. Then with chassis pulled, put the tubes back in, check/set bias, pull tubes back out to reinsert chassis, replace tubes once again, replace screens, done...phew!

After all of that, I decided that the tube set that is in there now, sounds pretty darned good and won't need changed for awhile. :lol:

Here are the TPs in my 40.
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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sat Dec 03, 2011 6:28 pm
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shimmilou wrote:
Yes, that amp is a little more user friendly compared to my Twkr 40. I have to remove two metal screens, to be able to remove the output tubes, to be able to pull the chassis out of the cab, to access the bias TPs and the single adjustment pot. In this one you read across only two TPs. Then with chassis pulled, put the tubes back in, check/set bias, pull tubes back out to reinsert chassis, replace tubes once again, replace screens, done...phew!

After all of that, I decided that the tube set that is in there now, sounds pretty darned good and won't need changed for awhile. :lol:



For all that effort, a good bias probe would probably be easier. :wink:

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sat Dec 03, 2011 6:49 pm
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:lol: I wish!

Doesn't matter, the adjust pot on this amp is near the test points inside the chassis, and you still have to go through the same procedure, even using a bias probe, to get to the pot. I actually thought about the same thing as the OP, bring the test points outside the chassis.....but the adjust pot would still be inside the chassis, so that would also have to be moved. :( I was a little jealous when I read about the amps that already have the TPs and pot on the outside, especially the pot, since you can use a probe instead of TPs as you pointed out.

Funny thing, I have nice bias probes, but I always use another method to double check the bias anyway. I have found that my probes are as accurate as any other method, and usually more accurate than the amp test points. But, I shouldn't have to change tubes for awhile in this one, so I'm set for now. 8)

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Last edited by shimmilou on Sat Dec 03, 2011 7:10 pm, edited 1 time in total.

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sat Dec 03, 2011 6:53 pm
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Hey everyone, I'm back! Got a little busy the past day...

Your discussions didn't scare me off. I'll read what people have to say and try to learn as much as I can. I had the idea about the bias test points mounted to the panel from seeing it on a few amps I've worked with, as supro mentioned seeing himself.

As for biasing an amp, I have asked various techs and read numerous things on the internet and everyone seems to vary a little in what's good enough. Though I do have some formal electronics training, none of it dealt with tubes, so I have a lot to learn. In the end, I have seen pros use every known method of biasing and get good tone without any amps dying or premature tube failure. I've poked around and read plate voltages before, but they dont seem to fluctuate much with bias adjustment. My method has been to talk with my tube supplier to see what mA range is suitable for a specific amp, then tune the bias within that range to what sounds good to me. I am ignorant here, but I dont understand why I need to read the plate voltage every time I do this, since in my experience, it doesn't move wildly and seems to keep in step with the mA rating predictably. Thus, please fill me in on what I am missing or how I could do it better.

I have considered buying a bias probe. What, in your opinions, is a good one and why is it better than others?

And now on to my little HRD head! I bought the HRD new in 2003 because I wanted a decent tube amp that wouldn't break the bank, not only for myself, but for those times I have a band in the studio and the guitar player wheels in his Crate solid state. :cry: Being both a multi-instrumentalist and a studio owner, my money has to cover a lot of ground! Upon first using it, I found I didn't care for the sound of the speaker. In actuality, I dont care for the sound of most combo amps, given that their cab design is serving multiple purposes instead of focusing on speaker tone and I generally lean to closed back cabs anyway (there are exceptions...I've recorded a number of vintage AC30s with blues and it's all part of THAT sound. Nevertheless, due to the well known AC30 combo "chamber of death to tubes and boards", I talked with a guy about pulling his AC30 and building a head for it. He's not had ONE failure or tube death since!). Anyway, I would sit the HRD combo in the control room and external out to the iso booth. I didn't like having a combo amp sitting there taking up unnecessary space. I pulled the amp, sold the cab/speaker and built a head.

It's a simple affair made of 5/8" thick solid pine. I routed the edges and applied green tolex, metal amp corners and feet. I installed the Accutronics verb tank and away I went. This was my first Tolex job, so it's not perfect, but overall I cant complain about how it turned out. Here are a few pics!

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Thanks for your input and help!

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sat Dec 03, 2011 7:24 pm
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Because every amp can have a different Plate voltage, there is not one standard voltage that all amps use. It isn't about the Plate voltage changing during a bias adjust, as you have seen it doesn't change much.

Telling you that a tube has a certain current range is wrong, because that current range can vary greatly, depending on the Plate voltage, so be wary of such claims. Even if they say a "certain amp", the Plate voltage can vary. The Plate voltage and Plate current determine Plate wattage. Tubes installed in and amp with 350 Plate volts will have a much different range of Plate current than in an amp with 485 Plate volts. In fact, the Plate voltage can be very different in the same amp depending on which tubes are used. Your amp's Plate voltage might be 430 volts with one set of tubes, and 425 with another set. I put 6V6 in my HRDlx and the Plate voltage went from 430 with 6L6, to 450 with the 6V6.

Bias setup is all about the Plate wattage dissipated by the tubes at idle, not just the current running through them. If you don't consider the Plate voltage, the amount of current running through the tubes tells us nothing about where the bias is set.

Sweet cabinet! Very elegant solution, and looks as if no chassis modification was needed.

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sat Dec 03, 2011 8:11 pm
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shimmilou wrote:
Because every amp can have a different Plate voltage, there is not one standard voltage that all amps use. It isn't about the Plate voltage changing during a bias adjust, as you have seen it doesn't change much.

Telling you that a tube has a certain current range is wrong, because that current range can vary greatly, depending on the Plate voltage, so be wary of such claims. Even if they say a "certain amp", the Plate voltage can vary. The Plate voltage and Plate current determine Plate wattage. Tubes installed in and amp with 350 Plate volts will have a much different range of Plate current than in an amp with 485 Plate volts. In fact, the Plate voltage can be very different in the same amp depending on which tubes are used. Your amp's Plate voltage might be 430 volts with one set of tubes, and 425 with another set. I put 6V6 in my HRDlx and the Plate voltage went from 430 with 6L6, to 450 with the 6V6.

Bias setup is all about the Plate wattage dissipated by the tubes at idle, not just the current running through them. If you don't consider the Plate voltage, the amount of current running through the tubes tells us nothing about where the bias is set.

Sweet cabinet! Very elegant solution, and looks as if no chassis modification was needed.


Shim,

I was given a mA range for a specific amp with a known plate voltage. Do plate voltages vary widely from amp to amp of similar models? As in, will your HRDlx have a different plate voltage than mine if using similar tubes? My supplier knew the typical plate voltage of the HRDlx, then gave me a mA range for both my 6L6s and my 6V6s. I realize it's optimal to read plate voltage, but it never seemed to make any difference in amp well-being or tube life if I stayed within the range I was given for my specific amp and for the tubes I bought.

So, would this be the way I go about it?
1. Take a plate voltage reading.
2. Tube wattage dissipation / plate voltage reading = maximum current
3. Take around 70% of the maximum current to derive a median bias current

This is one way I was told to do it, even if I haven't been measuring plate voltages lately because the data from my supplier always seemed to work fine.

Since you brought up 6V6s (which is taboo among some HRDlx users), I just tried a set in mine and WOW...never going back to 6L6s! It "fixed" most everything I didn't like about the HRDlx's sound! I also swapped out both ECC83 preamp tubes and put an ECC81 in V1 and an ECC832 in V2, which made a drastic improvement in clarity and definition. With the the ECC81-ECC832-6V6 configuration, I have never heard any HRDlx sound as good...and with stock trannys at that! (also consider I'm running through closed back cabs with nice drivers)

And yes, the head design required no modification to the chassis at all. The head is a little deeper than most, at 12", but I decided living with that was better than relocating the tubes!

Thanks for your assistance!

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sun Dec 04, 2011 12:13 am
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Perhaps I was a bit narrow, but yes, if you are talking about a specific amp, the voltages will generally be very close from amp to amp, within tolerances. You can get close and guess, or you can be accurate and know for sure what is going on. It isn't unusual for an amp schematic to show 430 volts for the B+, but actually measure 420, or even less. This isn't necessarily a problem, but it can only be known if measured. You might note the increase in your Plate voltage when switching from 6L6 to 6V6.

Your procedure is a fine way to figure your idle setting. For a 30 watt tube, 70% (21 watts) being a maximum setting, and many like myself prefer a cooler setting in the range of 50% (15 watts) to 60% (18 watts). It seems that most like the range of 50% to 70%, being a balancing act of tube life vs sound, cooler giving longer tube life. My HRDlx is idling at about 15 watts per tube. Keep in mind that the test point, and most bias probes measure Cathode current, which is about 5 mA higher than the Plate current for each 6L6 tube. Only about a 3 mA difference for a 6V6. A slightly different Plate voltage, slightly different current, and it can add up to a few watts difference in your setting.

And, definitely dig the sound of the 6V6 in the HRDlx. I was blown away too when I tried them. I used EH, a couple of types of GTs, and JJs. The EH sound the best, really great, but the Plate voltage is at the limit of the tube, and the screen voltage is greatly exceeded (100 volts over). New Sensor said that they really advise against using the EH in the HRDlx, even though they do recommend it for the DRRI. The GT 6V6 S and the JJ 6V6 are both rated high enough to work in the HRDlx, and even the HRDvl with 485 Plate volts. I can understand the concern over the ratings (note Plate volts is important as well as wattage), but man the EH sound sweet.

I thought that the GTs and JJs both sound more like a 6L6 than a 6V6. The EH sound sweet but might be the wrong tubes for this application.

Which 6V6 did you use?

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sun Dec 04, 2011 5:00 am
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shimmilou wrote:
I thought that the GTs and JJs both sound more like a 6L6 than a 6V6. The EH sound sweet but might be the wrong tubes for this application.

Which 6V6 did you use?


I have the JJs. I did a quick test recording that went HRDlx - 212 V30 cab - Soundelux U195 mic - Vintech preamp - ProTools. I set the amp clean channel and orverdrive channel, EQ flat, and didn't touch the amp controls. Then I went through all the tube configurations and recorded simple open and barre chords using an LP style guitar with coil tap, trying both bridge humbucker and single coils, with each tube set.

As a comparison to the stock GT/Fender tubes, which I found to have a somewhat hallow upper mid-range, spitty top end and loose bass, the JJs overall have a thicker, creamy, chewy mid range, smoother top end and more note definition with tighter bass. This is with ECC83s and 6L6s, basically the stock Fender configuration, but better with the JJs. Then I popped in an ECC81 and ECC832 into V1/V2. I couldn't believe the difference that made....even more punch and definition and the high end totally smoothed out and stopped being fizzy.

Next, I replaced the 6L6s with the 6V6s, keeping the same ECC81/ECC832 preamps. My jaw hit the console! The sound level drop from the 6V6s took a 2dB boost on the console fader to equal out volume with the 6L6 track, but holy cow, those 6V6s are the cat's meow! In comparison, the 6L6s sounded like there were holes in the frequency response (emphasized bass, cloudy lo-mids, a bit scooped in places throughout the mid range and a more brash top....even with the JJ 6L6s still being better than the GT 6L6s). The 6V6 tubes, on the other hand, were so balanced and neutral across the whole range of frequencies, with solid low end that wasn't boomy, super smooth high end and a ROCKIN' mid range RICH in harmonic definition with a fierce growl that sounded like it would eat you!

What I found interesting was just HOW much of a difference there was with the preamp tubes when using the 6V6s. The difference between the ECC83s and the ECC81/832 set was night and day with the 6L6 tubes. That same preamp tube difference with the 6V6 tubes made it sound like a completely different power tube. This might be why you didn't think the JJ 6V6s sounded as you would expect. When I put the ECC83s back in with the 6V6s, I got A LOT more front end gain/dirt, more so than with the 6L6s, and a totally different frequency response that resulted in a leaner low end, subdued mids and bright top (though still not fizzy). Going from the 83s to the 81/832s with the 6L6s just made the 6L6s better, IMO. That same preamp change with the 6V6s made them sound like totally different power tubes. In the end, I found the 81/832/6V6 set to be amazing. I didn't even know the HRDlx could sound like that.

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sun Dec 04, 2011 7:54 am
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Excellent writeup on your tube experiments, Musepro. I have done similar testing with my BDRI (except I don't have recording capability so all experiements were "real-time") geared primarily toward preamp tubes. I also favor JJ tubes with a couple of NOS tubes thrown into the mix. My current tube combination with my BDRI is:

V1 - NOS JAN GE 12AY7
V2 - JJ 12AX7 (I can never remember the European codes. :lol: )
V3 - Balanced JJ 12AX7
V 4/5 - JJ 6L6GC

This combo gives a very smooth and creamy sound all all levels.

I have also found the following combination to sound great:

V1 - NOS JAN Phillips or GE 5751
V2 - JJ 12AX7
V3 - Balanced JJ 12AT7
V 4/5 - JJ 6L6GC

I bias my amp to about 38 mA cathode current (about 36 mA plate current per the Weber bias tables) at a plate voltage of 413 VDC. This is about 50% max plate dissipation for the 6L6GCs and works out to be about 83 to 85 mV as measured at the Fender test point. I found that higher bias settings (90 mV as discussed on the Eurotubes website) did not really make an audible difference.

As far as a bias probe goes, I use this:

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

and a digital voltmeter from Ideal. There are numerous bias probes on the market and they all usually work on the same principle. They usually insert a 1 ohm, 1% tolerance resistor in the cathode circuit (note that the 1 ohm resistor that Fender uses is a 5% tolerance which accounts for most of the difference in readings) to measure cathode current and reads plate voltage directly. Very easy to use and makes the job quite safe. Is it better/worse than others on the market? I don't know. It was recommended to me by someone on TDPRI and it seems to work quite well so I am happy with it.

I really like the job you did on building the head cabinet. I'm not a fan of anything green, but that color looks good here. :lol:

Just a side comment here. I read all the time in print reviews and video reviewers always say that they "set the tone controls flat" in running their reviews/tests. With the exception of very few amps, the typical Fender/Marshall/Vox tone stack doesn't have a true "flat" setting, at least not in the context like hi-fi amps do. Here is a cool little tool that I like to use to "test drive" tone settings (Yeah, I'm an engineer. Does it show? :lol: ) when experimenting with tubes and tonal adjustments:

http://www.duncanamps.com/tsc/index.html

You can change component values to see the effect on the tone stack settings if you like to make mods.

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sun Dec 04, 2011 8:36 am
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I agree bluesky636, excellent write up. Excellent job on the head cab too.

In my best Pat Summerall voice; "What a huge day for the HRDlx!"

Musepro's HRDlx has got to be a one-of-a-kind amp, I'm jealous. :mrgreen:

I believe that a good name for the Hot Rod Deluxe (HRDlx) amp with 6V6 might be, "Hot Rod Deluxe Reverb" (HRDR). Is there anything that a HRDlx or BDRI can't do? :lol:

Thanks for sharing all of this Musepro, and enjoy your HRDR. 8)

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sun Dec 04, 2011 9:56 am
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Well....green would not have been my choice either, but tolex is expensive and the green was on clearance! :lol: I didn't want to dump much money into this project but wanted something better than a painted box.

Thanks for all the compliments, guys!

As for amp EQ, I use "flat" as a general statement, knowing they dont work in a traditional EQ fashion. As a professional record/live FOH mixer, I sometimes find myself frustrated with the interactive EQ controls on most amps. If I want to adjust the mids, I dont want the bass and highs going whack, dang it! I've considered looking into how I could mod this, but changing tone stacks can change the whole sound/dynamic of the amp, so I dunno. Vox has made numerous attempts to having their EQs switchable between interactive/isolated, but then, Vox hasn't made anything that sounds like a Vox for a decade! In the end, I rarely touch amp EQs unless it's a minor twitch up/down here and there. I do find the simple "tone" and "cut" knobs on many amps useful, however. That Duncan program looks nifty, so I'll have to play with it.

Webber likes you to have a probe under every power tube, which makes sense to me, yet many good amp techs and tube gurus dont find it necessary. Any comments concerning this?

And Shim, "one-of-a-kind" does not mean it's special or good! I had to build that silly box 3 times before I got something I could live with...and even then, I was just sick of dealing with it and wanted it to be over! Such is the life of a perfectionist. :|

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sun Dec 04, 2011 11:11 am
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Here is a simple mod to the tone stack, just a jumper wire, to give better control of the tone, especially the mids. Although the site is for the BJr, the same mod works well on the other HRd amps. I did this to my HRDlx, HRDvl as well as BJr. With this jumper you can completely dial out the mids for a great scooped tone, just for example of tone control. The description on the site says it all. Looking at the HRDlx schematic, jumper the top mid pot connection (the side of the mid pot connected to the bass pot), to the mid pot wiper as shown in the pictures. I had to remove the circuit board and jumper on the solder side in all but the BJr, as a cap is in the way for the HRDlx and HRDvl on the component side.

http://billmaudio.com/wp/?page_id=58

Having a bias probe, or better yet a dual or quad set, is very nice, especially to easily verify that a set of tubes is actually matched. Checking each tube, with a probe or other method, can verify whether matched or not. Some people trust that their tube set is matched and rely on the amp test points, some prefer to verify with a probe, and some amps have no test points. Some people don't even care and/or actually prefer mismatched tubes for one reason or another. I use a quad probe set and can check up to four tubes at once, which is much easier than swapping one probe four times with hot tubes.

With amps like the HRDlx that have only two output tubes, it is easy to verify a matched set of tubes without the need for probes. With amp off, unplugged from electricity, and caps discharged, measure and note the resistance of each half of the output transformer primary (red to brown resistance, and red to blue resistance), also noting that the brown wire is for V4 Plate and blue for V5 Plate (the brown and blue wires are the Plate voltage for each tube when amp is on). Then power up amp, in play and check the DC voltage drop across each half of the transformer in the same places as the resistance was measured. Then Ohm's Law gives the Plate current on each half for each output tube. You will have an unbalanced resistance measurement with the HRDlx OT primary, that is normal, but you can still get the tubes balanced very close as far as current through them, as the voltage on each Plate will also be slightly imbalanced.

I guess that it comes down to whether or not you will change tubes often, how accurate you want to be, and how much money you want to spend. I feel the need to verify balancing, and have found, and sent back, unbalanced tubes. And when dealing with many types of amps, and hundreds of tubes, bias probes are more than handy.

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


Last edited by shimmilou on Sun Dec 04, 2011 11:23 am, edited 2 times in total.

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