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Post subject: Crackling HotRodDlx
Posted: Wed Nov 03, 2010 5:44 am
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Hi. I managed to overload my Hot Rod Deluxe when I forgot to check all the knobs were turned down before I switched it on. This caused squealing feedback etc, and since then the tone has been poor and it crackles, even when the guitar is not plugged in. I finally got round to replacing the stock power tubes with Sovtek 5881s, and the tone is now better than ever. However the crackling continues - could this be the pre amp tubes (which are the stock Sovteks) or might I be looking at replacing the speaker? Is there anything more complex and annoying that might be causing this?


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Post subject: HRDlx crackling noise
Posted: Wed Nov 03, 2010 7:11 am
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Hi suddentwigs,

It could be preamp tubes, and/or bad solder joints, or even preamp Plate load resistors need replaced, not likely a speaker problem. You can easily try replacing the preamp tubes one at a time to see if they are the problem. If the preamp tubes are OK, I would check for bad solder joints next, especially the ones on the tube socket boards, but also the ones on the main board. Sometimes, you can wiggle the preamp tubes in their socket to see if the crackling noise changes, if so it is likely solder joint problems. :idea:

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Posted: Thu Nov 04, 2010 2:15 am
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I agree with shimmilou

Do you check and reajust bias when you put new power tubes ? Very important

I agree what Tom Stewart of SpaceMan Music , Ottawa, said about bias :

" Under-biased amps will lack punch and the tubes will run noticibly hotter.
Over-biased amps will sound thin and brittle, with the tube running too cool for proper performance.
A correctly-biased ampsd will sound clean and tight at moderate volume, then at higher volumes break up and distort musicaly."


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Post subject:
Posted: Thu Nov 04, 2010 2:51 am
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As far as I have been told from a variety of sources, the hot rod deluxe has a fixed bias. Haven't got any problems of that nature with the tone, it's just the crackle that persists.


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Posted: Thu Nov 04, 2010 3:00 am
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Fixed bias does not mean it can't reajust .

You have 2 kind of bias : Fixed bias and cathode bias, and both need to be checked and adjust

You must check bias each time you change power tube if you want to save your power tubes, but you have to see a real tech to do a good job


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Post subject: HRDlx crackling
Posted: Thu Nov 04, 2010 6:06 am
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suddentwigs wrote:
As far as I have been told from a variety of sources, the hot rod deluxe has a fixed bias. ...


The HRDlx does have an adjustment, a blue pot right beside the two big white resistors on the main board. If you replace your output tubes with the exact same tubes, then no adjustment is required (presuming that it was right to begin with). The only way to know for sure is to check. But, if you are happy with the tone, it's probably OK. :)

Here is the schematic, in case you don't have a copy.

http://support.fender.com/schematics/guitar_amplifiers/Hot_Rod_Deluxe_Schematic.pdf

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Post subject:
Posted: Thu Nov 04, 2010 7:02 am
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shimmilou I would say if suddentwigs is happy with sound AND tubes are not red glow it could be ok.

But I should say he take chance with.


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Post subject:
Posted: Thu Nov 04, 2010 7:39 am
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Ah, bias adjustment. I was hoping to avoid it as I have never done it. But I guess I have to learn - how is it done?


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Posted: Thu Nov 04, 2010 7:45 am
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suddentwigs wrote:
Ah, bias adjustment. I was hoping to avoid it as I have never done it. But I guess I have to learn - how is it done?


Like this:

http://www.eurotubes.com/euro-Fender-HRDV.htm

I recommend that you invest in a bias probe and decent digital voltmeter. I use this bias probe, but there are many out there:

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

Any good DVM will work with it.

Do a search within this forum as there has been a lot posted by me and the others regarding proper amp biasing. It is not hard if done using the proper tools.

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Last edited by bluesky636 on Thu Nov 04, 2010 7:49 am, edited 1 time in total.

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Post subject:
Posted: Thu Nov 04, 2010 7:48 am
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Look on You Tube someting like " biasing tubes amp" or "bias hot rod deluxe"

But don't forget you MUST know what you are doing ; in tubes amps you have lethal voltage, it can kill you.


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Post subject:
Posted: Thu Nov 04, 2010 7:51 am
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stratele52 wrote:
But don't forget you MUST know what you are doing ; in tubes amps you have lethal voltage, it can kill you.


+ A lot!

Using the proper tools such as a bias probe will go a long way toward doing this safely.

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Post subject:
Posted: Thu Nov 04, 2010 8:07 am
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Lethal voltage, even when unplugged? The power tubes I removed are JJ Groovetubes, and I put in Sovtek 6L6GC-SOV/EH (http://www.watfordvalves.com/product_detail.asp?id=278)

If I decide not to adjust the bias (after all it does sound good where it is), what's the worst thing that could happen?


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Post subject:
Posted: Thu Nov 04, 2010 8:15 am
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suddentwigs wrote:
Lethal voltage, even when unplugged? The power tubes I removed are JJ Groovetubes, and I put in Sovtek 6L6GC-SOV/EH (http://www.watfordvalves.com/product_detail.asp?id=278)

If I decide not to adjust the bias (after all it does sound good where it is), what's the worst thing that could happen?


You have several large capacitors in the amp that will store high voltage for quite some time. There is no need to discharge them when biasing the amp, however, never put more than one hand in an amp when working on it. Keep the other hand in your pocket or behind your back. Proper use of a bias probe and DVM will eliminate much of the danger to yourself and the amp.

The very worst that could happen is that the power tubes die prematurely. "Good sound" is not an accurate measure of proper bias.

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Post subject:
Posted: Thu Nov 04, 2010 8:28 am
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suddentwigs, the easiest and most popular method is to check the schematic for TP 30. The physical location is on the outside corner of the big tube mounting circuit board (the one that the output tubes connect to). With voltmeter on DC volts, check from ground (chassis) to the TP and look for between 60 and 80 mV. This measurement is the voltage drop across a 1-ohm resistor that is there specifically for bias check. This mV reading can be directly translated to mA of Cathode current for both output tubes, 60 mV = 60 mA. This reading is divided by the two output tubes, so each tube will have 30 mA of Cathode current. This reading is for reference only, as the Plate current and Plate voltage are what is important to calculate the Plate dissipation in watts (tube watt rating), and set the idle power dissipation. A reading of 60 mV is about 30 mA Cathode current per tube, which is about 25 mA of Plate current per tube. Just subtract about 5 mA from the Cathode current to get the Plate current. (this is too low for good sound btw)

Then check the Plate DC voltage at CP 19 for the V5 tube. Multiply the approx Plate current by the measured Plate voltage to get the Plate wattage at idle for that tube. For 5881s you will probably want about 55 to 60 % of the tubes rating of 25 watts, which would be around 14 watts. So you would probably want a reading of around 70 mV at TP 30 for the clean headroom, or up to 85 mV for the earlier breakup. 70 mV at TP 30 = approx 30 mA Plate current per tube, and at 440 volts Plate, that's over 13 watts (close to 55%). But of course your Plate voltage may be different, use your measurements, the math is the same. Lower plate voltage would need higher Plate current to get the same idle wattage setting. My suggestions for settings are based on what I thought was your desire for cleaner sound.

Please make sure to be safe when working inside your amp, be aware of the dangers. And, If you adjust the setting too high, you can ruin the tubes and/or smoke some components. If you aren't sure, don't do it. :)

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Post subject:
Posted: Thu Nov 04, 2010 8:34 am
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Thanks guys. Clearly a bigger job than I thought, will attempt to rebias once I have the correct equipment. In the meantime, will I be able to keep using my amp at the current bias? Not really fussed if my power tubes blow, but I certainly don't wish to smoke any of the rest of the circuitry ... that sounds very expensive and annoying.


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