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Post subject: Hot Rod Deluxe-BIAS voltage is 6.8mV only (distorted clean)
Posted: Mon Oct 07, 2013 2:11 am
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Hello everyone.

I'm experiencing a distorted sound, most noticeable on high freq. with clean setup (only guitar plugged in) on my Hot Rod Deluxe amp. I also tried to bypass the preamp to check the amp section only... same thing, I plugged my GT-8 processor into AMP INSERT, set the processor for clean and I got out noticeable distorted sound. I checked the 12" loudspeaker, he was OK... tried with two different (external) cabinets also, cabinets and speakers are OK (all tested on my other Randall amp just to be sure).

I measured BIAS point, and did found out that voltage across BIAS-ing point is smaller then preferred... on 200mV multimeter region, measured value was only 6.8mV... on some forum posts I found out that this value need to be in 70mV-110mV range. Does anyone know what to check (and do) next to eliminate the problem?


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Post subject: Re: Hot Rod Deluxe-BIAS voltage is 6.8mV only (distorted cle
Posted: Mon Oct 07, 2013 2:48 am
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If you do your bias reading at TP 39 ( see schematic ) fender write 68 mv not 6.8mv

1- You do a mistake with reading or ,

2- Tubes are defective or

3- Bias pot is set too cold, bias circuit is défective , too much negative voltage at ( -C )

Take a reading M.V. at -C
6L6 grid ot bias pot wiper . Schematic said - 50 volts ( minus 50 vols DC )



SHEMATIC ;

http://support.fender.com/schematics/gu ... ematic.pdf


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Post subject: Re: Hot Rod Deluxe-BIAS voltage is 6.8mV only (distorted cle
Posted: Mon Oct 07, 2013 3:14 am
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No, I think that this is not reading error... on 200mV= range multimeter displayed 006.8 ... so I assume that this is indeed 6.8mV. I did not mess up with the BIAS pot at all, this is factory preset (btw. the pot is still sealed up). Amp was never opened before (I'm first owner)... thanks for advice and schematic, I'll check the voltage at -C as you proposed. :)


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Post subject: Re: Hot Rod Deluxe-BIAS voltage is 6.8mV only (distorted cle
Posted: Mon Oct 07, 2013 3:28 am
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You are sure 006.8 , do another reading .

6.8 MV at the RIGHT test point could mean 6L6 's tubes are not conducting , not working properly .


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Post subject: Re: Hot Rod Deluxe-BIAS voltage is 6.8mV only (distorted cle
Posted: Mon Oct 07, 2013 10:17 am
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I did the readings again, all measurements are with respect to ground... BIAS test point on resistor R66 has voltage potential of minimum 5.4mV and max. 11.2mV when I mess up with the TP30 BIAS-ing pot. So, yes... the values are very small (and unfortunately) correct. On 'C-' spot (resistors R59, R60) the voltage is -48V. On R76 the voltage is -52V. on resistors R78 and R79 the voltage is +48V and -48V like on the schematics, so all that voltages are OK... but... on spots SP1 and SP2 i couldn't do the measuring as my digital multimeter has gone to max (showing digital 1 on LCD display) ... so, value can not be measured, but it should be as my multimeter is for DC voltages ranged at 600V DC. So... it seems, something is really wrong here... and as far as I can see from the schematic, this spot with SP1 and SP2 is the high voltage section with rectifier. :?


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Post subject: Re: Hot Rod Deluxe-BIAS voltage is 6.8mV only (distorted cle
Posted: Mon Oct 07, 2013 10:46 am
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What is the DC Plate voltage at pin 3, and DC screen voltage at pin 4 of each output tube? You should get somewhere close to 430 VDC. If the voltage is low at pins 3, the high voltage (B+) is missing. If the voltage is correct at pins 3, but low at pins 4, the screen grid resistors (R61 and R62) are likely blown. If the SGRs are blown, that indicates bad output tubes. If you have the correct voltages, you should try a new set of tubes.

Also, what kind of output tubes do you have installed?

While you are checking, look at the voltages on the other side of R78 and R79, should be approx + and - 16 VDC.

BTW, the TPs indicated within ovals are AC, with a signal injected into the amp input. The TPs indicated within the rectangles are DC. For now, ignore the AC TPs, they won't give you any useful readings.

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


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Post subject: Re: Hot Rod Deluxe-BIAS voltage is 6.8mV only (distorted cle
Posted: Mon Oct 07, 2013 11:15 am
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I measured resistance of resistors R61 = 464 and R62 = 466 ohms... it seems that they are not blown out. Voltages on the other side of R78 and R79 are +17V and - 16V.

Voltage for tube V4 on pin 3 is 353V, on pin 4 is 355V.
Voltage for tube V5 on pin 3 is 356V, on pin 4 is 355V.

Output tubes are original Fender Groove Tubes GT6L6B. Amp is about 6 years old now, and has been played about 60 times a year for a 4-6 hours. :?


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Post subject: Re: Hot Rod Deluxe-BIAS voltage is 6.8mV only (distorted cle
Posted: Mon Oct 07, 2013 11:32 am
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It seems that your voltage at pins 3 (B+) is too low, it should be closer to 430 VDC. Perhaps your filter caps are bad, C31, C33, C35, C36, or at least some problem with the power supply. Any signs of leaking around the positive terminals of your caps?

With amp off and tubes cool, pull the output tubes and power the amp on and recheck the B+, it should be much higher, maybe 470 VDC.

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Post subject: Re: Hot Rod Deluxe-BIAS voltage is 6.8mV only (distorted cle
Posted: Mon Oct 07, 2013 11:59 am
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I pulled the output tubes V4 and V5 out, and B+ voltage (point SP1) is now 394 V.

How can i check for signs of leaking around the positive terminals of caps C31, C33, C35, C36 ... how to measure/check this... this may be the problem?


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Post subject: Re: Hot Rod Deluxe-BIAS voltage is 6.8mV only (distorted cle
Posted: Mon Oct 07, 2013 12:12 pm
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You might see a whitish/grey crusty substance on the positive lead of the cap, close to the cap body where the lead comes out. This could be on one or more of the caps. Also, check the values of the caps with a capacitance meter if available.

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Post subject: Re: Hot Rod Deluxe-BIAS voltage is 6.8mV only (distorted cle
Posted: Mon Oct 07, 2013 12:22 pm
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No, there is no whitish/grey crusty substance on the positive lead of the caps... all are very clean. :? Unfortunately, I don't have a capacitance meter. :(


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Post subject: Re: Hot Rod Deluxe-BIAS voltage is 6.8mV only (distorted cle
Posted: Mon Oct 07, 2013 12:44 pm
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At this point, I would thoroughly check out the entire power supply. If you can't test the caps any further, your only choice might be to replace them and see if it makes a difference. I would use some better quality caps, such as F&T.

With all tubes pulled, check the AC voltage, CP11 to CP12, with standby switch open vs standby switch closed.

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Post subject: Re: Hot Rod Deluxe-BIAS voltage is 6.8mV only (distorted cle
Posted: Mon Oct 07, 2013 1:06 pm
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With all tubes pulled out I checked the AC voltage between points CP11 and CP12... with standby switch open the AC voltage measured is 336V to 337V.

With standby switch on, the voltage between points CP11 and CP12 is 335-336V, and on B+ or to say voltage across capacitor C31 is now 447 V DC (all tubes are pulled out, of course). On C33 the voltage is 446V DC, and on C35 is 444-445 V DC.


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Post subject: Re: Hot Rod Deluxe-BIAS voltage is 6.8mV only (distorted cle
Posted: Mon Oct 07, 2013 1:16 pm
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Wow! That is quite a difference with the preamp/PI tubes pulled, +50 VDC. Maybe one of your preamp tubes, or the circuit around it, is pulling the voltage down.

Try putting the preamp/PI tubes back in one at a time, and see which one causes the B+ to drop. Start with V3, the PI, check the B+ voltage, then try V2, then V1 checking each time when/if the B+ drops.

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


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Post subject: Re: Hot Rod Deluxe-BIAS voltage is 6.8mV only (distorted cle
Posted: Mon Oct 07, 2013 1:45 pm
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Voltage is measured across B+ (actually the + pole of capacitor C31 cause it's handy to hook up) and ground. :)

And we have :

1) with only V3/PI tube in socket the voltage on B+ 413V DC
2) with only V2 tube in socket the voltage on B+ 424V DC
3) with only V1 tube in socket the voltage on B+ 424V DC


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