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Post subject: '93 Blues Deville Clean headroom mod
Posted: Fri Mar 06, 2015 5:56 am
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Hi all!
I recently have purchased a second hand Fender Blues Deville 4x10. I love the amp, but I wish I could get more headroom from the clean channel, as I bought the amp hoping to get those sparkly cleans that Fender are renowned for. I've found that once I push the first channel above around four, I get a lot of power tube breakup, as if it has been biased quite hot. I am experienced enough with electronics through college not to kill myself while poking around inside the amp, so I thought "I'll just bias this thing a little bit colder", before examining the schematics for the amp before opening her up. To my surprise I found that R87 had a 10K ohm resistor soldered in series to the original 27K ohm one. Someone had already tried increasing the headroom in the power amp. I have been using both the internal pre, as well as two separate rack preamps and everything seems to drive those output tubes when pushed above around 3. I want nice cleans, maybe with a tiny bit of breakup, but can't get it in this situation, and maybe not from this amp! Can anyone give me advice as to what sort of resistor I could replace R87 with in order to bias this thing colder and get a nice clean sound with very little breakup??
Thanks in advance :D
Dave


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Post subject: Re: '93 Blues Deville Clean headroom mod
Posted: Fri Mar 06, 2015 6:35 am
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R87 is for bias , change value = change bias . 10K in series with 27 K = more negative voltage = colder bias.

Early breakup; look at the phase inverter circuit ; resistor value , if wrong = early breakup .
Try new phase inverter tubes and new output tubes

Try 12AT7 in V1 , clean channel ; gain is lower than 12AX7

A second hand amp need a good checkup for worn tubes


Last edited by stratele52 on Fri Mar 06, 2015 9:00 am, edited 1 time in total.

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Post subject: Re: '93 Blues Deville Clean headroom mod
Posted: Fri Mar 06, 2015 7:06 am
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I thought increasing the resistor value by adding an extra 10K ohms would make the bias colder, sending less current to the power amp tubes. Obviously I was wrong. I still notice the power tube breakup even when bypassing the pre. I don't think the preamp tube swap to a 12AT7 will fix the problem of power tube breakup. Will lowering R87's value make the bias colder then??


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Post subject: Re: '93 Blues Deville Clean headroom mod
Posted: Fri Mar 06, 2015 8:59 am
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DaveWrat wrote:
I thought increasing the resistor value by adding an extra 10K ohms would make the bias colder, sending less current to the power amp tubes. Obviously I was wrong.
No you are right , I do mistake when writing :( . I change my answer

I still notice the power tube breakup even when bypassing the pre. I don't think the preamp tube swap to a 12AT7 will fix the problem of power tube breakup.
You should try



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Post subject: Re: '93 Blues Deville Clean headroom mod
Posted: Fri Mar 06, 2015 12:09 pm
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The bias circuit in a Blues Deville is one of the few Fenders that use a charge pump circuit to power the bias.
C36, 100uf100volt is a high wear part that needs to be closly examined or just replaced to get the required bias voltage of -49.6VDC. This cap has AC applied to it and has high ripple currents that age it quickly. Replace it with a cap with high ripple capability.
Measure the bias voltage before and after cap replacement to guage bias voltage change.
You might be able to remove the extra series resistor at that point.

If it still breaks up too soon and a 12AT7 in V1 doesn't help, you might consider changing the gain of the power stage by adjusting the value of the negative feedback resistor.
R93 is a 68K resistor that controls NFB. Start by tacking on a 100k in parallel with R93. Then work your way in steps down to about 50K till you get the gain you want. This may cause side effects, so listen closley to the results.


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Post subject: Re: '93 Blues Deville Clean headroom mod
Posted: Fri Mar 06, 2015 3:08 pm
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First of all, checking the negative voltage for the bias is meaningless, as it depends on which output tubes you have installed. The best way to check bias is to check the actual current through the tubes. A specific negative bias voltage can give different idle currents for different tubes, so check the tubes' currents to be sure. The easiest way to check the current is with a bias probe. If you have no probe, with amp off, unplugged from electricity, and caps discharged, measure the resistance of each half of the output transformer (OT) primary (no need to disconnect anything). Then, plug the amp back in, turn it on and warm it up, put the standby switch to play and measure the DC voltage drop across each half of the OT primary. Then use Ohm's law to calculate the current through each tube (voltage divided by resistance equals current)

Second, the PI (V3) is in the output section, so a lower gain tube such as a 12AT7 will help with output headroom. When you plug into the amp's "Power Amp In" you are bypassing the preamp and feeding the signal to the PI (reverb and presence still in the circuit too).

I wouldn't worry about changing any components in the amp until you know the current through the tubes, and try a 12AT7 in the PI position.

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Post subject: Re: '93 Blues Deville Clean headroom mod
Posted: Sat Mar 07, 2015 9:21 am
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Comparing the bias voltage before and after changing C36 will tell you if you have made an improvement in charge pump performance. A weak capacitor may only produce 40 volts so if you get it up to 49 with a new cap, you'll know that you've made an improvement and the bias will be lower, regardless of actual value. I like the sound of hot bias, 30-35Ma. To my ear, low bias current sounds cold and brittle.
A 12AT7 will only drop gain a little, I prefer to adjust power stage gain with feedback resistor adhustment.
Feedback resistors will quiet the noise in the power stage and have much more flexibility in lowering gain .


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