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Post subject: Low BIAS at measuring point - Blues Deluxe Reissue
Posted: Mon Aug 08, 2011 8:50 am
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First of all, sorry my english.
A friend has a 2004 Fender Blues Deluxe Reissue, and said me from a time to now it breaks so early, at 2 or so, and it's that.
With stock power tubes and with new TAD 6L6WGC-STR tubes, using in V1 a 5751 tube, and ECC83 (all TAD) in V2 and V3.
Adjusting BIAS was the option but i'm desconcertated because bias test point is to low, the max value i can set is 47mV. and min about 10mV with the stock (Sovtek) tubes, and about 70mV. and 25mV. with the TAD tubes. It's supposed the bias test point measures both tubes, so i don't know if it's something wrong, or i'm fail and it measures only one tube.
It's possible there is any broken resistor?, I revised socket solder points and it look good.
Ah, one more thing, in both cases, with stock or TAD tubes, V4 tube (the first power tube) turns with a blue light, more intense when more bias value.

Thanks from Spain !!!


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Post subject: Re: Low BIAS at measuring point - Blues Deluxe Reissue
Posted: Mon Aug 08, 2011 9:09 am
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Hi planetasur,

The bias test point is the Cathode current of both tubes (mV is the same as mA across a 1-ohm resistor). Seems that you may have a problem with your power supply, or output section of the amp. To make sure to get the correct reading at the bias test point, measure the millivolts (mV) at the test point with the negative meter lead on the chassis (ground) and the positive lead on the test point. Next, measure the power supply DC voltages and compare to the schematic listed voltages at B+, Z, Y, and X.

http://www.blueguitar.org/new/schem/fender/blues_deluxe-deville_reissue.pdf

Just a guess, but it seems that you have a large imbalance between the two output tubes, so you may have a bad connection there somewhere, solder joints being the first suspect. You should re-solder all of the tube socket solder joints even if they appear to be OK. Next would be to check the screen grid resistors, both for correct value and solder joints. :idea:

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Post subject: Re: Low BIAS at measuring point - Blues Deluxe Reissue
Posted: Mon Aug 08, 2011 9:48 am
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Thanks shimmilou, I'll follow your indications.
A pair of cuestions, what are the screen grid resistors?, and, how to measure the power supply DC voltages?
I'm not novate in soldering, reparing begginer/intermediate problems, I modified myself my own peavey classic 30, but i never did some simple things like measure the power supply DC voltages.


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Post subject: Re: Low BIAS at measuring point - Blues Deluxe Reissue
Posted: Mon Aug 08, 2011 10:58 am
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First I would really suspect a solder joint, or tube socket connection problem, re-solder all tube socket connections first, and make sure that the tubes are tight in the sockets, not loose.

Make sure to be safe, the voltages in the amp can exceed 400 volts, and the amp can hold a charge of electricity, even when turned off and unplugged. :!: If you measure the amp components with an ohmmeter, or do any soldering, unplug the amp from electricity and make sure that the capacitors have been discharged before attaching meter. To check the power supply, the amp will have to be turned on, use a DC voltmeter to check each DC supply voltage (VDC) as listed on the schematic.

The power supply is shown on page 12. There are two supplies shown, so be sure to look at the one labeled Blues Deluxe Power Supply. Test these voltages in the same manner as you also checked the bias test point mV, except that the power supply voltages will be 400+ VDC, set your meter accordingly. With negative meter lead on chassis, and the positive lead used to probe each test point, B+, Z, Y, and X, and compare your meter readings to those listed on the schematic (TP37 through TP40). The voltage readings should be close to that listed on schematic, within tolerances. The test points are not labeled on the amp circuit boards, locate the component lead to use for each test point. Example, the positive + side of capacitor C31 should be close to 430 VDC, this is the "B+" (TP37). The + side of C33 is the "Z" (TP38).

If the DC supplies are good, with power still on, check the resistors voltages. Find the output tubes on the schematic labeled V4 and V5, on page 11. The screen grid resistors are the 470 ohm Fp (flame-proof), labeled R61 and R62, just to the right of the tubes on the schematic. With negative meter lead still on the chassis, check the DC voltage on both sides of each resistor, and the reading will be "Z" on one side (as above), and about the same voltage on the other side, just a couple of volts lower. You would be looking for a large voltage difference here if the resistors are the problem. :idea:

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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: Low BIAS at measuring point - Blues Deluxe Reissue
Posted: Mon Aug 08, 2011 11:35 am
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Thanks again, i'll do it in these days, and i'll tell you the result.


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Post subject: Re: Low BIAS at measuring point - Blues Deluxe Reissue
Posted: Fri Sep 23, 2011 3:40 pm
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Hi again,
Now I could try to repair the amp, I resolded the power tube sockets, but the problem persits, but fortunately I saw the problem, the R61 (I suppose is one of the screen grid resistors) is burned!, so now i've got to get one and put it.
I'll tell you!


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Post subject: Re: Low BIAS at measuring point - Blues Deluxe Reissue
Posted: Fri Sep 23, 2011 4:34 pm
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Good job. Make sure to use good quality "Flame Proof" replacements for the screen grid resistors, as indicated by the "Fp" on the schematic. It might be a good idea to change both R61 and R62 while you are in there. :idea:

Let us know when you finish.

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---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

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


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Post subject: Re: Low BIAS at measuring point - Blues Deluxe Reissue
Posted: Sat Sep 24, 2011 4:12 am
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Well guys, that's right!!!
Resistors replaced, and amp working, BIAS perfectly adjusted!!!
Thanks everybody for your indications, I'm very happy and I learned a lot of things.
And one more time, excuse my english.


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Post subject: Re: Low BIAS at measuring point - Blues Deluxe Reissue
Posted: Mon Oct 17, 2016 10:46 am
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Hi everyone,just joined,
I need some advice on my Fender HR Deville-all voltages check ok with schematic and amp is working okay , problem: i do not get any reading on the bias test point it reads 0volts-or on pin 8?-re soldered power tube sockets on pcb

i do have some experience in amp repair :shock:
thanks

Phorton
Norway


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Post subject: Re: Low BIAS at measuring point - Blues Deluxe Reissue
Posted: Mon Oct 17, 2016 6:09 pm
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On HR Deville, you read across the F-pin (1-ohm resistor, R66) tied to the cathode ---pin 8. Red DVM lead on TP30 and black lead on chassis ground. Set to read mV DC.

http://ampwares.com/schematics/hotrod_deluxe.pdf


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