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Post subject: Hot Rod Deluxe bias issue
Posted: Thu Feb 13, 2014 11:55 am
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I recently replaced my power tubes with EHX 6l6GC's, and my preamp tubes with tung-sol 12ax7's. I went to set the bias pot, and right out of the gate I was getting a reading of about 150 mV. So I cranked it down, and down, and down, and the lowest reading I could get was 99 mV. I was definitely turning the bias pot the right way, and eventually it just stopped and wouldn't go any further.

I know there is a thread already on this issue, but I just wanted to see if there was any other feedback, or success with this issue.

I'm fairly certain my fuse is is good working order, and it just seems unlikely that a dirty pin contact or pot would cause me to get a reading SO high. HELP!


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Post subject: Re: Hot Rod Deluxe bias issue
Posted: Thu Feb 13, 2014 12:04 pm
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I should note, and I don't know if it will make a difference in the answer, that my HRDx is a 2004 limited edition, with a Jensen special design.


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Post subject: Re: Hot Rod Deluxe bias issue
Posted: Thu Feb 13, 2014 1:04 pm
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Hi BWhitehurst,

It sounds like your new output tubes might have a very high rating/grade. Do you know the rating numbers on your output tubes? The downside of using the test point is that you can't tell if the output tubes are matched, and it could even be that one of the tubes has much more current draw than the other.

Just for reference, what was/is the bias with the old output tubes?

BTW, 99 mV at the test point on this particular amp with 6L6 is just at the high end of the 50% to 70% range for bias, about 21 watts per tube of idle, so it will be good to use if needed.

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Post subject: Re: Hot Rod Deluxe bias issue
Posted: Thu Feb 13, 2014 1:41 pm
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When I bought them, I bought them as a matched set. The only number I saw on the packaging was "41" which was on both tubes. I was under the impression that the recommended bias setting for the HRDx was between 60 and 90 mV. so would'nt 99 be exceeding that limit?
And shouldn't I have more control over my bias setting?


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Post subject: Re: Hot Rod Deluxe bias issue
Posted: Thu Feb 13, 2014 2:06 pm
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One of the old tubes was going out. I can go back and test the bias with the old ones, but will testing it with a failing tube affect the bias?


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Post subject: Re: Hot Rod Deluxe bias issue
Posted: Thu Feb 13, 2014 3:10 pm
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BWhitehurst wrote:
When I bought them, I bought them as a matched set. The only number I saw on the packaging was "41" which was on both tubes. I was under the impression that the recommended bias setting for the HRDx was between 60 and 90 mV. so would'nt 99 be exceeding that limit?
And shouldn't I have more control over my bias setting?


In order to properly bias an amp, you need to measure the power tube cathode current (which you are doing by measuring the voltage at the test point) as well as measure the plate voltage of the tube. This thread describes how bias should be properly measured and set:

viewtopic.php?f=13&t=88475

Sometimes it is necessary to modify the bias power supply to get the desired voltage range depending on the tubes in use.

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Post subject: Re: Hot Rod Deluxe bias issue
Posted: Thu Feb 13, 2014 3:13 pm
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BWhitehurst wrote:
One of the old tubes was going out. I can go back and test the bias with the old ones, but will testing it with a failing tube affect the bias?


You can try, but depending on the failure mode of the tube you might not get useful results.

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Post subject: Re: Hot Rod Deluxe bias issue
Posted: Thu Feb 13, 2014 5:18 pm
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Bill,

I watched the video and I have a little better understanding of the concept of plate voltage, but I still don't understand why I can't lower my cathode current below 99 mV. Even in the eurotube video he was able to get his cathode current down to a reasonable level.

Will adjusting my plate voltage alter my cathode current reading? And if so, how do I adjust the plate voltage?

Thanks,

Brooks


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Post subject: Re: Hot Rod Deluxe bias issue
Posted: Thu Feb 13, 2014 5:37 pm
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BWhitehurst wrote:
Bill,

I watched the video and I have a little better understanding of the concept of plate voltage, but I still don't understand why I can't lower my cathode current below 99 mV. Even in the eurotube video he was able to get his cathode current down to a reasonable level.

Will adjusting my plate voltage alter my cathode current reading? And if so, how do I adjust the plate voltage?

Thanks,

Brooks


You don't "adjust" plate voltage. Plate voltage changes as a function of cathode current (actually plate current but we won't quibble). Cathode current changes as a function of the bias voltage which you are adjusting with the bias pot. They are all interrelated.

shimmilou asked about the rating of the tubes you purchased. Too high a rating and they could require more current at idle than the bias supply can provide in its present configuration. Or, as he also mentioned, you could have a problem with one tube of the pair. Since you are using the Fender bias point, you are stuck measuring both tubes cathode current. That is why I use a dual socket bias probe. This allows me to see any imbalance between tubes. Your best bet would be to see if you can exchange the tubes you have for something rated a little lower.

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Post subject: Re: Hot Rod Deluxe bias issue
Posted: Thu Feb 13, 2014 5:48 pm
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So if I stick with the current tubes do I run a risk of damaging my amp in any way, or do I just get a crunchier tone, or either?

As you can see I'm pretty limited in my technical knowledge of amps ha.


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Post subject: Re: Hot Rod Deluxe bias issue
Posted: Thu Feb 13, 2014 6:14 pm
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BWhitehurst wrote:
So if I stick with the current tubes do I run a risk of damaging my amp in any way, or do I just get a crunchier tone, or either?

As you can see I'm pretty limited in my technical knowledge of amps ha.


Hard to say without knowing the plate voltage. As shimmilou pointed out, if the plate voltage measures about 430 VDC, you are right at the maximum recommended idle plate output of 21 watts per tube. But, you can't go any lower if you wanted to. With good tubes, the bias supply should allow you to adjust the idle power output between 50% (15 watts) and 70% (21 watts). I could not do that with any tube I ran in my Blues Deluxe Reissue, so I chose to modify the bias power supply to achieve the desired working range. Unless you are able to try another set of tubes (and I would recommend buying a dual socket bias probe that measures both cathode current and plate voltage), you should take the amp to a good tech for a complete checkup.

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Post subject: Re: Hot Rod Deluxe bias issue
Posted: Thu Feb 13, 2014 7:28 pm
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BWhitehurst wrote:
One of the old tubes was going out. I can go back and test the bias with the old ones, but will testing it with a failing tube affect the bias?


It depends on what you mean by; it was "going out"? It is likely OK to put the old tubes back in just long enough to see if your current is in a more reasonable range, and this might give an indication if there is an issue with your new tubes. IMO, 40 Ip for the tubes are fine for this amp. Usually in these amps, the problem is the opposite, meaning that you sometimes can't get the current high enough. bluesky636 and I have both modified our bias circuits to get more current through the output tubes.

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Post subject: Re: Hot Rod Deluxe bias issue
Posted: Thu Feb 13, 2014 8:02 pm
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If you don't have bias probes to measure each output tube current individually, there is a way to check both with a meter. Keep in mind there will be high voltage and you must be careful, so don't attempt this unless you feel comfortable doing so.

With the amp off, unplugged from electricity, and caps discharged, check the resistance for each half of the output transformer (OT) primary, no need to disconnect anything, record the readings. The caps should discharge on their own, just measure across each cap (the big gray ones) with a meter to be sure that the voltage is low (under 20 volts or so), it will never be zero. The OT primary has three wires; a red one in the bottom middle of the main circuit board, a brown one and a blue one, both connect to the underside of the tube socket board in the bottom of the chassis. To check the OT primary halves resistance, measure from the red wire in the bottom middle of the main circuit board, to the brown wire that goes to the tube socket board in the bottom of the chassis, this is one half, then measure from the red wire to the blue wire, this is the other half.

Once the resistance of each half is recorded, turn the amp back on, standby switch in the play position (as if ready to play), and measure the DC voltage across each half of the OT primary in the same places that you checked the resistance. Then simply use Ohm's law to figure the idle current for each output tube (I = E/R, current equals voltage divided by resistance). For example, if one half of the OT primary reads 82 ohms, and the voltage across that half reads 3.28 VDC (yes that is three point two eight VDC), that gives you 40 mA of current through the corresponding tube. Each half of your OT primary will be a different resistance, something like 82 ohms on one half, and 112 ohms on the other half. The current that is calculated using the above method will be the actual Plate current at idle, instead of the Cathode current measured at the test point or with bias probes. The resistances of the OT primary halves will not change much over the life of the amp, so once the resistance is measured, no need to measure it again in the future with other output tube changes, just measure the voltage across each half of the OT primary after installing new output tubes and use the recorded resistances to calculate Plate current.

Unless something is wrong with your amp, the Plate voltage in these amps (HRDlx, BDlx, etc) should be about 430 VDC, varying somewhat with bias adjustments. The brown and blue wires mentioned previously are connected to the Plates of each output tube, and the voltage can be checked at each wire in reference to ground (chassis). Meaning, put the negative (black) meter lead on the chassis, and use the positive (red) meter lead at the point that you will measure, in the same way that you check the mV at the test point, but obviously a higher voltage on the Plates, so set your meter accordingly..

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