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Post subject: Hotrod Deluxe Bias pot mod?
Posted: Fri Dec 02, 2011 8:51 am
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Hi everyone!

I have a Hotrod Deluxe that I like to change the power tubes on for different flavors. I was wondering if I could move the bias knob from the PCB to the chassis so it doesn't have to be pulled from the cab every time I want to adjust bias. I would also add panel mount test lead sockets for metering.

Could the stock 25K linear plastic pot (Fender part #0027873000) be replaced with the metal 25K linear pot (Fender part #0026133000)? That pot looks as if it is threaded, so I could drill a hole in the chassis and mount it with a nut.

Thanks!

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Fri Dec 02, 2011 10:37 am
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What are you pulling from the chassis to adjust the bias pot? The PCB? Why? All you have to do is remove the back panel and the bias pot is sitting there staring you in the face. As far as mounting panel test points, just buy a bias probe and digital voltmeter. You need to measure plate voltage also and you sure as heck don't want that brought to the back panel. :shock:

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Fri Dec 02, 2011 11:33 am
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bluesky636 wrote:
What are you pulling from the chassis to adjust the bias pot? The PCB? Why? All you have to do is remove the back panel and the bias pot is sitting there staring you in the face. As far as mounting panel test points, just buy a bias probe and digital voltmeter. You need to measure plate voltage also and you sure as heck don't want that brought to the back panel. :shock:


I failed to mention that my HRD is not in it's original combo cab, but in a custom built head. I dont move the PCB, just pull the chassis out of the head case and adjust the bias pot using a multimeter on the test point. The only comment I made about the PCB was concerning the PCB mounted bias pot that is stock.

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Fri Dec 02, 2011 1:34 pm
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Photos would be useful. I am having a hard time visualizing what the amp looks like. If you don't measure plate voltage, you only have half the required information needed for a proper bias job. Running the plate test points to the cabinet would be a bad thing.

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Last edited by bluesky636 on Fri Dec 02, 2011 4:58 pm, edited 1 time in total.

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Fri Dec 02, 2011 3:54 pm
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You could probably move the bias test point and a ground and do it all from the outside, some of the higher end Egnater amps do it that way as well as a couple other amp manufacturers.

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Fri Dec 02, 2011 4:57 pm
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63supro wrote:
You could probably move the bias test point and a ground and do it all from the outside, some of the higher end Egnater amps do it that way as well as a couple other amp manufacturers.


How does Egnater handle measurement of the plate voltage since that can change as line voltage and bias adjustment changes? I really don't think it is wise to bring that to the outside unless some safety precaution is taken like a removable cover or something.

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Fri Dec 02, 2011 9:20 pm
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They have sockets mounted to the rear of the amp for a multi meter pin probe. It's still pretty much they have an average plate voltage range and you just to the recommended range for the tubes used. Here's the rear of the Renegade, Tourmaster series is the same.
It can be done. Unless you get really stupid and crank it enough to red-plate the tubes, you'll be fine. Being off a couple of MV isn't the end of the world.
http://www.egnateramps.com/Products/Ren ... Panel.html

I'd stilr use a Bias Probe/Bias Rite.

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Fri Dec 02, 2011 9:34 pm
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63supro wrote:
They have sockets mounted to the rear of the amp for a multi meter pin probe. It's still pretty much they have an average plate voltage range and you just to the recommended range for the tubes used. Here's the rear of the Renegade, Tourmaster series is the same.
It can be done. Unless you get really stupid and crank it enough to red-plate the tubes, you'll be fine. Being off a couple of MV isn't the end of the world.
http://www.egnateramps.com/Products/Ren ... Panel.html

I'd stilr use a Bias Probe/Bias Rite.


Ah. Ok.

Yeah, I too prefer using a bias probe and DVM with my BDRI.

I hope the OP posts photos of his amp. I'm interested in what it looks like.

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sat Dec 03, 2011 12:10 am
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No, the Egnater does not bring the Plate voltage out the back of the amp. Those are bias test points, measures Cathode mV just like any other amp test points. You don't adjust the Plate voltage on any amp, no such thing. The Plate voltage changes slightly with changes in bias current.

For the OP, it would be just fine to mount an external bias adjust, using a 25K ohm pot, mounted wherever you want. Make sure to keep the wires as short as possible, and away from other components so that you don't introduce noise. As for the Plate voltage, I agree with bluesky636, the Plate voltage should be read to get an accurate measure of bias setting, and it is not a good idea to bring that voltage to the outside of the chassis (430 VDC).

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sat Dec 03, 2011 12:34 am
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shimmilou wrote:
You don't adjust the Plate voltage on any amp, no such thing. The Plate voltage changes slightly with changes in bias current.


No one is saying that you adjust plate voltage. :?:

The B+ on my BDRI drops from 430 VDC (Per the schematic. I have never measured it at no load.) to 413 VDC at my chosen bias point (about 36 mA per tube).

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sat Dec 03, 2011 1:02 am
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You asked how the Egnater handled the Plate voltage measurement and the response was exactly what is said here.

63supro wrote:
They have sockets mounted to the rear of the amp for a multi meter pin probe. It's still pretty much they have an average plate voltage range and you just to the recommended range for the tubes used...


The Egnater amps have the test points labeled for a current adjust, no "Plate voltage range" given, or even mentioned at all on the amp or in the manual.

And we've been here before, there is no way that your voltage drops that much with a bias adjust. That much of a difference might occur with the tubes removed, but the entire bias adjust range will only change the Plate voltage by a few volts, not 17 volts, no way. You were going to check that before, but I guess that we forgot about it.

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sat Dec 03, 2011 7:00 am
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shimmilou wrote:
You asked how the Egnater handled the Plate voltage measurement and the response was exactly what is said here.

63supro wrote:
They have sockets mounted to the rear of the amp for a multi meter pin probe. It's still pretty much they have an average plate voltage range and you just to the recommended range for the tubes used...


The Egnater amps have the test points labeled for a current adjust, no "Plate voltage range" given, or even mentioned at all on the amp or in the manual.


I am still at a loss to understand your comment "You don't adjust the Plate voltage on any amp, no such thing." I am well aware that the plate voltage is not adjustable. 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.

shimmilou wrote:
And we've been here before, there is no way that your voltage drops that much with a bias adjust. That much of a difference might occur with the tubes removed, but the entire bias adjust range will only change the Plate voltage by a few volts, not 17 volts, no way. You were going to check that before, but I guess that we forgot about it.


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). The schematic shows the B+ voltage as 431 VDC. I have never measured it with the tube pulled. My line voltage is 122.9 VAC (measured 30 seconds ago). Maybe I will total voltage change today after I finish rearranging my pedalboard. I never looked into it being a problem since the amp sounds great as I have it set.

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sat Dec 03, 2011 7:38 am
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Sorry Bill I'm not explaining this right. I'm not the best at explaining tech stuff. There is no plate voltage adjustment. And I know that. You know how an amp has a plate voltage of say for the sake of argument 425 volts and you use that as the basis of adjusting the bias. That's what I meant. On my Rebel, there are bias settings that are recommended and silk screened for each set of tubes. The Rebel series have two bias adjustment pots. I checked the bias on my Rebel when I first got it and it was dead on to their recommendations. I went no further. I figure if it ain't broke don't fix it.
The bias test points for the Egnater Renegade and Tourmaster series are located on the rear of the amp. If you want to go and use a bias probe, that would really be the way to go. Sorry about the dipshit explanation. It was a long and seriously bad day. :oops:

Many amps are doing the test point thing. It's always better to check the plate voltage so you have a place to start, not just adjust to some recommended voltage.
Shim always has a problem with anything I say, that's why he's on my foe list. It's better that way.

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sat Dec 03, 2011 8:28 am
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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:

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.

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Post subject: Re: Hotrod Deluxe Bias pot mod?
Posted: Sat Dec 03, 2011 9:57 am
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63supro wrote:
Sorry Bill I'm not explaining this right. I'm not the best at explaining tech stuff. There is no plate voltage adjustment. And I know that. You know how an amp has a plate voltage of say for the sake of argument 425 volts and you use that as the basis of adjusting the bias. That's what I meant. On my Rebel, there are bias settings that are recommended and silk screened for each set of tubes. The Rebel series have two bias adjustment pots. I checked the bias on my Rebel when I first got it and it was dead on to their recommendations. I went no further. I figure if it ain't broke don't fix it.
The bias test points for the Egnater Renegade and Tourmaster series are located on the rear of the amp. If you want to go and use a bias probe, that would really be the way to go. Sorry about the dipshit explanation. It was a long and seriously bad day. :oops:

Many amps are doing the test point thing. It's always better to check the plate voltage so you have a place to start, not just adjust to some recommended voltage.


I understook EXACTLY what you said in your original post. :wink: It was the comment by shimmilou about adjusting plate voltage that threw me. When I talk about bias settings in my amp, I always list the cathode current and plate voltage as measured using my bias probe and DVM first, then the calculated plate current from the Weber bias tables. I also try to list the measured voltage at the Fender test point for those that do not have a bias probe and are possibly squeamish about measuring the plate voltage with a DVM directly. I figure that covers all bases. :D

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