Class A Cathode & Class AB Bias Mod

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by letloverule, Jan 29, 2008.


  1. letloverule

    letloverule Member

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    hey guys, i'm currently thinking of doing this mod on my DRRI and wanted to know if you guys thought it was worth it. Basically the tech installs a switch to switch between the two biases. I know very little about biasing and am wondering if I should just do a regular bias with one of the Weber Bias-rite kits or is this a worthy improvement?
     
  2. vibroverbus

    vibroverbus Member

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    Well if your tech thinks Cathode Bias = Class A, you might want to start shopping for a new tech. Bias is one thing, Class of operation is another. He's probably talking about a bias switch, which you might very well like quite a bit - Cathode bias tends to have some nice dynamic differences when driven hard (compresses/damps a bit differently when near clipping).

    Doubtful that he's talking about really doing a Class A conversion, and putting in the circuits to make that work. Most (not all) 'Class A' advertised amps are marketing Bull$h!t (although any single tube amp - for example but not limited to - is inherently Class A).

    Here's some good techno info for you so you can be down with the Truth, hit his other pages on biasing and single-ended vs. push-pull because all these things are related but NOT the same...

    http://www.aikenamps.com/ClassA.htm
     
  3. letloverule

    letloverule Member

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    Ahh man and here I am all happy that i'll be making my amp sound sweeter! SO i guess there's no advantage to this then just getting a bias kit from weber or so and doing it myself. I guess that's that. I guess the only way to have a sweeter sounding deluxe is just to save up some money and get the real thing.
    thanks you I think you might have saved me some money and aggravation. Anyone else have experience in getting their DRRI modded with a cathode bias?
     
  4. AdmiralB

    AdmiralB Silver Supporting Member

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    Huh? Cathode bias would be more forgiving, since the cathode voltage 'subtracts' from the plate potential.

    You certainly aren't going to get a DR into class A, but cathode bias is doable. Probably won't sound much if any different, but it's doable.
     
  5. letloverule

    letloverule Member

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    Actually the tech I am talking to said it would be a rebuilding of the circuit and adding the components required for running the DRRI in Class A. Actually If I understand correctly he would be adding a switch where i can operate the amp in either Class A or Class AB. Anybody ?
     
  6. Blue Strat

    Blue Strat Member

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    Definitely possible, still a PC board unless he replaces it with a tag board (figure at least $500 for parts and labor), and depending on the cathode resistor value used it may or may not sound different.

    It probably won't be radical, but he could get it so the amp would compress and break up a bit sooner.
     
  7. 57paf

    57paf Member

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    I've added a mid-pot (Tonequest), a variable negative feedback pot, and a DPDT switch to go from fixed to cathode bias on my SFDR. When playing a low volume levels, I love boosting the mids and cutting the NF to make it sound like it does at higher levels. When I switch bias, however, I don't really hear an appreciable difference. You can hear it in an A/B comparison, but normally you wouldn't really be aware of it unless you do a direct comparison.
    I used to own a Groove Tubes Soul-O-30 Class A amp. About 2 years later, GT came out with a Soul-O-50 which is the same amp, but could switch to fixed bias mode. Takes a big power supply to do Class A w/6L6s. As one would expect, this amp was very saggy. Personally, I prefer a real Vox for creamy, squishy tones and a Marshall Plexi type amp for the dynamic punch. Which is what I have now.
     
  8. AdmiralB

    AdmiralB Silver Supporting Member

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    Then your tech doesn't understand class A. He/she would not be first -what you're probably talking about is simply toggling between cathode- and fixed-bias. That could be done pretty easily, even on a reissue.

    But cathode-bias isn't necessarily class A. The plate voltage in a DR is at least 100V too high to get anywhere close to class A.
     
  9. letloverule

    letloverule Member

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    Either way, from the comments here it sounds like it wouldn't result in a big tonal improvement in the amp. I wanted something to make the clean fuller, more alive. I'm guessing now that this isn't really worth it and i'd be better in investing in transformers / better tubes. I know this isn't really the topic of the thread but any of you tried the mercury magnetics transformers? Would NOS tubes be better than the JJ's I have. Thanks again.
     
  10. Blue Strat

    Blue Strat Member

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    NOS Brimar 6V6GTs are big sellers for these amps where the user desires a fuller sound. Replacing V2 and V4 preamp tubes can help too.

    Speaker upgrades are also less common for players trying to remove the ice pick tendencies in the stock Jensen speakers.
     
  11. cochese

    cochese Member

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    I have an old Siverface Deluxe Reverb that was modified to Blackface specs and later had a Cathode Bias switch installed. I pretty much always use the Cathode bias setting. I just prefer the tone and response. It really makes the amp jump from the word go.
     
  12. AdmiralB

    AdmiralB Silver Supporting Member

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    I've never converted a DR to cathode-bias. I've built a couple 5E3s with higher-voltage iron, though. I'd probably start around 360 ohms for a shared cathode, and see what that gets. Assuming 6V6GTA tubes, I'd probably shoot for 10W dissipation, per.

    I've done some 5E3-types with switchable fixed/cathode bias, though, and the differences are REALLY subtle.
     
  13. letloverule

    letloverule Member

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    yeah for the speaker i'm leaning towards the Eminence speakers right now. I'll definitely be looking at the new tubes as well. I think that's pretty much all i can do with this amp. I'm still a little confused about the whole Class A/AB stuff. Most people seem to think that its just a switch for the cathode vs fixed bias. i'll ask. The tech tells me i'll get a more compressed sound, sweeter at low volumes in that mode. He says that the Class A mod is a complete rebuild of the bias circuit and adds power resistors, caps and other components in order to operate the amp in Class A. I still don't think from your feedback that it'll be worth the money. I hear from you guys that the difference will only be VERY VERY subtle.
     
  14. phsyconoodler

    phsyconoodler Member

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    There is a lot of real estate in the DRRI for placing a nice sag resistor.But the best tones you can get out of that amp come from a speaker change.
    Try an Eminence Red/White and Blues or a Govnor.Both sound killer in a DRRI.
    The JJ's are just fine the way it is,just make sure they are biased right.I like the DRRI a little on the hot side.
    If your tech is thinking of repalcing the entire circuit board,that is not a bad thing.Then he can put some decent parts in it.The factory DRRI has cheesy parts inside.
     
  15. cochese

    cochese Member

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    Why not just get a set of THD Yellow Jackets. Although I have the Cathode bias mod in my Deluxe and really like it I feel the Yellow Jackets will probably give you more of a drastic tonal change (with the EL84's) and will be a much cheaper alternative.
     
  16. Blue Strat

    Blue Strat Member

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    BTW, to achieve a similar effect to cathode biasing (more sag/compression) you can change rectifier types to a 5R4 or even a 5Y3. For best sag producing results, it's best to rebias but it's not absolutely necessary. It's a lot less expensive than modding the amp.
     
  17. AdmiralB

    AdmiralB Silver Supporting Member

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    That doesn't surprise me.

    The RIs I've been in have all been right around 420V, somewhat less than the originals.
     
  18. letloverule

    letloverule Member

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    That's a very interesting suggestion. I don't want my speaker to break up too much but this is definitely worth a shot. Always was curious about those yellow jackets. Which model do you suggest (if i'm not mistaken there are 2 models)? Thanks.
     
  19. cochese

    cochese Member

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    You should probably just contact THD or Ed DeGenaro is on TGP and he works for them. The YJ's are pretty cool and it's a totally passive mod. As I've said I do have a cathode bias switch on my converted Silverface/Blackface Deluxe and it was done a few years ago. I can't predict how it would sound in a reissue. I tend to run it in cathode bias mode most of the time. I would say if you are going to run the amp at lower volumes it's pretty cool. As for tona differences I listened last night and would say it's more of a response difference with the 6V6's. With the the YJ's it will probably give you a different tonal response as well.

    As for Cathode Bias versus Class A there are many opionions on this but in terms of guitar amps I think it's a bit silly. Class A in the Audiophile purest view is that it is an uncompromised signal and a very pure form of amplification. Guitar amps as we use them today are not typically used in this fasion. We "want" distortion and at this point the argument becomes somewhat moot. In the end it really comes down to what sounds good to you.
     
  20. Free

    Free Member

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    +1
    That's the way to go. You'll notice more sag from the voltage drop of a 5Y3 than cathode-biasing (cathode biasing imparts more of a tonal richness than sag necessarily), and it is indeed WAY simpler.
     

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