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Post subject: Fender Hot Rod Deluxe. Distortion on clean channel.
Posted: Thu Jul 14, 2011 10:36 pm
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I have issues with my Fender Hot Rod Deluxe amp.

Symptoms: Distortion on clean channel.

I know very little about electronics however I decided to try to fix it myself ... I tested screen resistors and found 0Ω resistance on R62 instead of 470Ω. I replaced it but it didn't help, still getting distortion. I decided to replace some other parts that potentially could cause this problem. This is what I replaced.

1. Replaced R57 with new Vishay 1/2W 82KΩ resistor (as per specs)
2. Replaced R4, R11, R16, R22, R58 with new Vishay 1/2W 100KΩ resistors (as per specs)
3. Replaced R61, R62 with new metal oxide 3W 1KΩ resistors (specs: 1W 470Ω, recommended by J.Holton: 3W 1KΩ)
4. Replaced R78, R79 with new XICON 5W 470Ω resistors (as per specs)

Triple checked resistance and continuity of each component I replaced. Tried different bias settings (60 mV - 75 mV at test bias point). Retouched all solder joints of all tubes.

... still getting distortion on clean channel even when plugged in to POWER IN which tells me that problem in somewhere around power amp section.

I have 2 sets of power tubes: original Grooves and pair of JJs that are approx 6 months old and they were in the amp when it started to sound "bad". I tried original Grooves tubes and they gave me even worse sounding distortion on clean channel. I alto tested various preamp tubes in different positions.

Tested voltage on R61 and R62 and they both gave me ~420V from one side of resistor to ground (both ends of resistor). Is this normal?

Tested voltage on power tube pins, both gave me same result:

Pin 1 - ~0V (~0.3-0.4 mV)
Pin 3 - ~420V
Pin 6 - ~0V (~0.5 mV)

Is that OK to get 0v on Pin 1 and Pin 6? Looks suspicious to me :)

I don't really know where else to look. Any ideas?

Do I need to buy new set of power tubes and try them? :)


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Post subject: Re: Fender Hot Rod Deluxe. Distortion on clean channel.
Posted: Thu Jul 14, 2011 11:18 pm
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Pins 1 and 6 are unused in the 6L6/5881. In fact, many 6L6GC's omit pin 1 altogether.

Have you checked in and around your phase inverter tube?

Arjay

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Post subject: Re: Fender Hot Rod Deluxe. Distortion on clean channel.
Posted: Thu Jul 14, 2011 11:32 pm
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Thanks! I replaced R57 and R58 resistors that were around V3 (phase inverter). Checked voltage drop on R58 but a bit scared to check it on R57 as it is in difficult to reach spot. Resistance is as per spec on both R57 (82K) and R58 (100K).

If resistance OK, does that mean that resistor OK?


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Post subject: Re: Fender Hot Rod Deluxe. Distortion on clean channel.
Posted: Thu Jul 14, 2011 11:34 pm
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Do I need to check PINs on phase inverter tube? Negative to ground (chassis) and check each PIN with positive? Or I need to check specific PINs?

Thanks.


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Post subject: Re: Fender Hot Rod Deluxe. Distortion on clean channel.
Posted: Thu Jul 14, 2011 11:57 pm
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If the static check of R57 and R58 showed no anomalies, they're likely okay. How about the soldering job in and around that socket -- no joints that look "cold" or fractured? If the phase inverter is operating correctly, the only other section that I can think of that would cause distortion in the "clean" mode is V1 and its related components. I believe V2 is bypassed unless the "drive" or "more drive" function is selected.

You may need Billm or shimmilou to weigh in on this one -- they both own these amps and are well acquainted with their architecture.

Arjay

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Post subject: Re: Fender Hot Rod Deluxe. Distortion on clean channel.
Posted: Fri Jul 15, 2011 7:57 am
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JChr wrote:
I know very little about electronics however I decided to try to fix it myself ...


:shock: :shock: :shock: :shock:

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Post subject: Re: Fender Hot Rod Deluxe. Distortion on clean channel.
Posted: Fri Jul 15, 2011 8:07 am
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I can only agree with what you've suggested Retroverbial. :)

I would suspect that if one of the screen grid resistors was bad, that output tube is suspect, and there could still be other problems. But, "0 ohms" for the resistor doesn't sound right, maybe you mean "open" (OL) as is usually the case. I can't recall ever seeing a resistor that actually shorted (0 ohm). Resistors usually change value, or open, but don't short.

Half of V2 is used only for the drive channel (V2B), it is added when switched to drive, and the other half is used in both channels.

It is possible that the screen grid resistor was bad with the original tubes, and then the new tubes installed with the bad resistor. If you still have the distortion with new tubes and the resistor replaced, you may have a filter cap problem in the power supply. You can visually inspect the caps to see if any crusty substance coming out of the positive side, indicating a bad/leaky cap. Just a thought. :idea:

BTW, does the channel switching seem normal? Is there a noticeable change in sound when switching channels?

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Post subject: Re: Fender Hot Rod Deluxe. Distortion on clean channel.
Posted: Fri Jul 15, 2011 8:10 am
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bluesky636 wrote:
JChr wrote:
I know very little about electronics however I decided to try to fix it myself ...


:shock: :shock: :shock: :shock:


I know! He can solder, read schematics, use a meter, etc. Sounds very knowledgeable to me too. He should be helping us! :)

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Post subject: Re: Fender Hot Rod Deluxe. Distortion on clean channel.
Posted: Fri Jul 15, 2011 3:29 pm
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Thanks, shimmilou! Sorry, I meant OL not 0 ohms :) Resistor was definitely fried (black burned mark in the middle).

I visually checked filter caps and they all look OK to me.

What I noticed is that if I push bias to around 80-85 mV at bias test point, distortion is much less noticeable, I can hear it only if I strum hard which is good. But ... Is that safe to run (bias) power tubes at 85 mV? This amp used to be "clean" at around 60-65 mV with same power tubes.

At 60-65 or 80-85 mV drive channel sounds very bad but I rarely use it.

All solder joints looks OK to me but I retouched them on all tubes anyway.


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Post subject: Re: Fender Hot Rod Deluxe. Distortion on clean channel.
Posted: Fri Jul 15, 2011 4:03 pm
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JChr wrote:
Thanks, shimmilou! Sorry, I meant OL not 0 ohms :) Resistor was definitely fried (black burned mark in the middle).

I visually checked filter caps and they all look OK to me.

What I noticed is that if I push bias to around 80-85 mV at bias test point, distortion is much less noticeable, I can hear it only if I strum hard which is good. But ... Is that safe to run (bias) power tubes at 85 mV? This amp used to be "clean" at around 60-65 mV with same power tubes.

At 60-65 or 80-85 mV drive channel sounds very bad but I rarely use it.

All solder joints looks OK to me but I retouched them on all tubes anyway.


I run JJ 6L6GCs in my BDRI. I have them biased at 38 mA cathode current (about 36 mA plate current) at 413 VDC plate voltage. That works out to be about 83 to 85 mV at the Fender test point. They sound great. What is your plate voltage at a bias of 85 mV?

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Post subject: Re: Fender Hot Rod Deluxe. Distortion on clean channel.
Posted: Fri Jul 15, 2011 8:27 pm
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Visual inspection is only part of the check for the caps, value and/or ESR should be checked to be sure, but the fact that they look good is at least a good start, as many of those type caps physically leak before they electrically leak.

You may be on to something with the bias adjust. At 430 Plate volts (mine approx) and 85 mV at test point, that is about 15 watts idle for your tubes, 50% if 30 watt tube, 60% if 25 watt tube, very safe either way. Crank your bias up to about 90 mV and see if it doesn't sound better, that's only about 17 watts at 430 Plate volts, adjust according to your actual voltage. I treat my original GTs as 25 watt tubes (Sovtek 5881WXT, whatever those really are), and run my idle at about 15 watts, and IIRC the test point reading was 78 mV. The GT output tubes might actually be 30 watts, not sure though.

With either set of tubes that you have, 85 mV is more than OK and safe. :idea:

Also, I wouldn't want to use the tubes that were in the amp when the screed grid resistor burnt. It is possible that the tubes are OK, but also possible/likely that something caused the resistor to burn (caps, tubes?).

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Post subject: Re: Fender Hot Rod Deluxe. Distortion on clean channel.
Posted: Sat Jul 16, 2011 1:54 am
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JChr, Shimmilou, may be an advice:

On HRdx there is an "Power Amp In", Using another preamp or inject direclty your guitar in will allow to check if the amp is OK before investigate in the preamp section.
8) 8)

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Post subject: Re: Fender Hot Rod Deluxe. Distortion on clean channel.
Posted: Sat Jul 16, 2011 7:08 am
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Great idea Tis-san. I believe that the OP indicated in his first post that he did try that, and it helped him determine that the problem was in the output section.

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Post subject: Re: Fender Hot Rod Deluxe. Distortion on clean channel.
Posted: Sat Jul 16, 2011 7:25 am
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:oops: :oops: :oops: :oops: :oops: :oops:
Sorry for the disturbance. I'm going to investigate in the amp with you.

On PI stage:
Did you measure rougly +40V on kathodes (pins 3 and 8) ?
Did you measure rougly +230V on plates (pins 1 and 6) ?

On AB stage:
What is the voltage measured on Grills (pins 5) ? (rougly -45V but depending of bias adjustment)

Let us know. :oops:

BTW, take care about lethal voltage, let one hand in your pocket since the orher is trying to measure something (no electrical path through your heart).

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Post subject: Re: Fender Hot Rod Deluxe. Distortion on clean channel.
Posted: Sat Jul 16, 2011 11:20 pm
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Thank you all for your help!

I did two more measurements, one with JJ 12AX7 in V3 and other with NOS Mullard 12AT7WA.

-= Scenario 1 (JJ 12AX7) =-

Bias at bias test point: 89 mV

V3 (PI)
PIN1 - 195V
PIN3 - 50V
PIN6 - 190V
PIN8 - 50V

V4 (JJ 6L6GC)
PIN3 - 423V
PIN5 - -39.5V

V5 (JJ 6L6GC)
PIN3 - 427V
PIN5 - -39.5V

-= Scenario 2 (NOS Mullard 12AT7WA) =-

Bias at bias test point: 91 mV
(I didn't change bias, just swapped V3 and remeasured)

V3 (PI)
PIN1 - 255V
PIN3 - 34V
PIN6 - 252V
PIN8 - 34V

V4 (JJ 6L6GC)
PIN3 - 425V
PIN5 - -39.5V

V5 (JJ 6L6GC)
PIN3 - 425V
PIN5 - -39.5V

It looks like distortion in clean channel is gone after I rebiased JJs above 85 mV.

My HRDX sounds fantastic with 12AT7WA in V3! I could not believe my ears yesterday when I first tried. Rechecked today again, swapped AT7 with AX7 and it sounded OK. Changed back to AT7 and WOW...! I can't believe this is my Hot Rod Deluxe! 8-)


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