Cathode Bias on a BF Deluxe Reverb!

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Cathode Bias on a BF Deluxe Reverb!

Postby Peter Stroud on Wed Feb 18, 2004 5:55 pm

Being that I'm on a learning curve with electronics and amp mod's, I decided to convert my beloved Blackface Deluxe Reverb to cathode bias. I'm sure a few of you out there might be thinking "Blasphemy!! Why mess around with a perfectly good and original BF Deluxe!" and you're right. I thought about that right up until I hit the soldering iron to the chassis. But, I'm a big fan of that glassy compressed cathode bias tone and thought it would lend well to the Deluxe...and it did in a BIG way.

Also went a step further and disconnected the negative feedback circuit (one wire) and BaBOOM!! More gain and total harmonic richness. Scary. Took it into British tonezone. The clean sounds sparkle like your favorite Brian May moments and the gain is monstrous, especially run through a Marshall 412. The experiment was a total success IMO and is easily reversible back to it's original "fixed" bias (just two wires.)

I won't list all the details on how I did it, unless anyone wants me to. You can feel free to email me at whitesg@mindspring.com and I'll send them to you. I pretty much followed Dan Torres' cathode bias conversion mod in his book "Inside Tube Amps", except for the cap and resistor values he suggests.

Give it a try! It's well worth the comparison...After all, the tweed era Deluxe's were cathode bias.
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Re: Cathode Bias on a BF Deluxe Reverb!

Postby Baron Von Machinenmann on Wed Feb 18, 2004 10:48 pm

HERETIC!! :D
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Re: Cathode Bias on a BF Deluxe Reverb!

Postby Jlwctn on Thu Feb 19, 2004 4:04 am

Tell us about it; one of Gerald Weber's books goes into the same subject, but I'm just too lazy to get up and go find it right now. I'll look at it tomorrow, I'd bet that it's pretty close to the same mod.
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Re: Cathode Bias on a BF Deluxe Reverb!

Postby Peter Stroud on Sat Feb 21, 2004 4:15 am

Here's a photo of the cathode bias conversion I performed on my Deluxe Reverb... Look for the additional terminal strip soldered to the chassis with the 560 ohm/5 watt resistor paralleled with a 25uf/25volt electrolytic capacitor...Next in series before the ground connection is a 1 ohm/2 watt resistor for measuring the bias current. The red wire from the terminal strip connects to the cathodes of both power tubes (Pin 8.)

A complete detailed rundown of the conversion and disconnect of the feedback loop follows the photo.

<img src="http://img26.photobucket.com/albums/v79/Electricpie/Amp%20stuff/DlxRvbCathodeBias.jpg" alt=" - " />

CATHODE BIAS MOD:

1. Find the wire that feeds from the bias pot to the two 220K resistors in your bias network on the circuit board. Disconnect the wire from the bias pot and connect it to ground.
NOTE: This is assuming that your Deluxe (or other Fender) has an adjustable bias circuit and not a 70's-style hum-balance circuit.

2. Unsolder the ground from Pin 8 (the cathode) on each power tube.

3. Solder a jumper wire from Pin 8 of one power tube to Pin 8 of the other power tube.

4. For easy access, I soldered a short terminal strip to the chassis for mounting the cathode resistor and bypass cap as you'll see in the photo. The terminal strip originally had two screw down ends (ground), but one broke, so it is holding fine with one end soldered to the chassis.

5. Solder another wire from the right power tube Pin 8 to a terminal (NON-GROUNDED) on the strip.

6. Solder in place a 25mfd/25v electrolytic cap - negative end towards ground and the positive end connected to the wire from the power tubes' Pin 8 cathodes.

7. Solder a bias resistor (value to be determined) in parallel with the capacitor to ground on one end and the cathodes on the other.

8. ADDITIONAL RESISTOR PICTURED- This is a millivolt bias measure resistor, a 1 ohm/2-watt value. It is connected between the cap/bias resistor combo and ground. This will allow you to get a current draw reading of both tubes. (You can leave this out and connect the bias resistor/cap to ground)

That's it!! It's a bit of work initially, but if you want to reconnect the "fixed" bias to compare, just reconnect the wire from the 2-220K resistors back to the center post on the bias pot and then connect your new cathode wire back to ground...It's back to original.

Now...to determine the proper value bias resistor - The most accurate way to get your bias measurements is by the shunt method. If you're good at it, great, otherwise, let someone else do it, as I'm sure you know the scoop that it's very dangerous for you and the amp (stuff can blow up and people can get hurt.)

You should be able to bias it by ear... The resistor value should fall somewhere between 400 - 600 ohms...A safe level that shouldn't draw too much current. Your plate voltage (on a BF DeluxeRvb at least) should be around 410, maybe higher. Your plate current (per tube) should be safe around 30ma plus or minus, I'd imagine. That would give a reading of around 60 + across the 1ohm bias resistor.

I chose a 560 ohm resistor and here are the readings. My Deluxe Rvb possibly has a hotter plate voltage than most...It was 450v on fixed bias.

Bias Resistor value: 560 ohms/5 watt
Plate current
(shunt method reading): 29ma per side (approx)
Plate voltage: 420v (approx)
Cathode 1 ohm resistor reading: 63.8 (average at idle)

TO DISCONNECT THE NEGATIVE FEEDBACK CIRCUIT:

Find the wire that goes from the external speaker jack to the circuit board. Disconnect the wire from the speaker jack and cap it or tape it up. This should be the only wire running from the speaker jacks to the circuit board. The other wires are coming from the output transformer.

There you have it! BaBoom - Big sound.
You can also refer to the AB763 Circuit layout..Here's the link (courtesy of Ampwares.com)
Good luck!! Anyone else who does it, let us know what you think!
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Re: Cathode Bias on a BF Deluxe Reverb!

Postby Peter Stroud on Sun Feb 29, 2004 1:54 pm

Cathode Bias on my Deluxe Reverb UPDATE:

Well, it ain't flameproof!!!

Yesterday I was recording with the amp. I had it cranked to 10 using a Strat and no pedals. Sounded amazing. But as I was dialing a tone, listening through my studio monitors, I heard the power drop. Sounded like one 6V6 failing, followed by a hum and a small "POP!" I ran into the amp room in time to see smoke coming out of the input jacks. This kind of thing doesn't scare me too much, since 6V6's blow fairly often in this amp.

Turns out a 6V6 must have gone bad. A 470 ohm suppressor grid resistor was cooked. The "pop" was caused by the 25mfd/25v cap on the cathode bias exploding!! Debris everywhere in the amp.

I'm thinking that the higher plate voltage in my Deluxe (420volts) is tough on the 6V6's when really cranked. Possibly the cathode bias is putting extra pressure on the tubes as well. I don't believe I'm pushing the amp into Class A mode with it's plate current at 29mA per tube. It should still be reacting as A/B.

Any tech' thoughts or opinions out there? Might try a set of 6L6's this week for comparison.
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Re: Cathode Bias on a BF Deluxe Reverb!

Postby Jlwctn on Mon Mar 01, 2004 2:15 pm

Be careful about replacing 6v6's with 6l6's, because of the heater current draw. If adding the 6l6's results in drawing more total current than the PT is rated for and can supply-- especially cranked-- a fried heater winding in the PT could result.
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Re: Cathode Bias on a BF Deluxe Reverb!

Postby barfoden on Tue Jul 02, 2013 10:31 am

I know this is a super old thread, and I am certain that you (Peter Stroud) is an expert after your venture into 65 amps with Dan.

I see that you used a 25µf/25V bypass cap on the cathode biasing. I think the 25V rating is too low for your application with a 560 ohm resistor.
When you try to bias a high plate voltage with a cathode resistor on those 6v6, the bias voltage will fluctuate and go high when driven hard. A guy meassured a rise in cathode voltage from idle at 11V to 18V when dimed and that is with a cathode biased EL84 amp with a 150 ohm resistor and 47µf bypass cap.

My guess is that your bias voltage is around 25V allready at idle and will go higher when dimed.

A vintage 5e3 deluxe get away with a 25µf/25V rated bypass cap as the resistor is lower at 250 ohm and the plate voltage in a real old vintage 5e3 with a nos 5y3 and a wall supply of 112V will be around 360V/320 plate and screen. That amp is biased very hot and the increase in bias voltage when dimed will probably be smaller than in your modified amp..
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Re: Cathode Bias on a BF Deluxe Reverb!

Postby barfoden on Tue Jul 02, 2013 10:31 am

I know this is a super old thread, and I am certain that you (Peter Stroud) is an expert after your venture into 65 amps with Dan.

I see that you used a 25µf/25V bypass cap on the cathode biasing. I think the 25V rating is too low for your application with a 560 ohm resistor.
When you try to bias a high plate voltage with a cathode resistor on those 6v6, the bias voltage will fluctuate and go high when driven hard. A guy meassured a rise in cathode voltage from idle at 11V to 18V when dimed and that is with a cathode biased EL84 amp with a 150 ohm resistor and 47µf bypass cap.

My guess is that your bias voltage is around 25V allready at idle and will go higher when dimed.

A vintage 5e3 deluxe get away with a 25µf/25V rated bypass cap as the resistor is lower at 250 ohm and the plate voltage in a real old vintage 5e3 with a nos 5y3 and a wall supply of 112V will be around 360V/320 plate and screen. That amp is biased very hot and the increase in bias voltage when dimed will probably be smaller than in your modified amp..
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