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WHAT'S THE BEST CATHODE RESISTOR VALUE??


 :
10/22/1998 7:51 AM
Trace Allen
WHAT'S THE BEST CATHODE RESISTOR VALUE??
1.) This is a matter of opinion I'm sure but there must be a certian value most people prefer...right?  
Say for a 12AX7 on either an older Fender or Marshall amp (or something similar).  
2.) Does anyone like to use Caps along side the resistor for the cathode?  
3.) What's the favorite mid-range resistor on the fixed mids of an amp?  
 
Thanks so very much for all those who have been answering!!  
 
Trace  
http://members.aol.com/ItznotMe98/
 
10/22/1998 10:00 AM
Dave H.

These are the values I like to use in my homebrew amps. The speakers are V30s and guitars Strat/Tele.  
 
1.) 1k5 for the cathode and 100k for the plate. Iíve tried 2k7 and 220k for the first stage but it sounded about the same to me so I only use the 2k7/220k in the AC30 circuit.  
 
2.) So far Iíve only made traditional low gain amps (two gain stages before the phase splitter) and even then I sometimes end up with too much gain so I usually remove one or both cathode caps to reduce the gain so that the amp is clean up to half rotation of the vol pot. The value of the cap would depend on the speaker cab. For a 1 x 12" open back cab 22u or no cap. For a 4 x 12" closed back 0.68u on one or both stages to roll off the bass.  
 
3.) For tone controls I like 56k slope resistor, 270p treb cap. 220k log treb pot, 220k log bass pot, 22k lin mid pot (or 10k fixed) and 22n bass/mid caps. A 100k slope resistor is too thin sounding for me and 33k is too fat.  
 
I had better shut up now to make way for Steve Ahola.  
 
Dave
 
10/25/1998 3:52 PM
Steve A.

Dave:  
 
I had better shut up now to make way for Steve Ahola.  
 
 
    To quote Bruce Collins: "I resemble that statement, sir!"  
 
Steve  
 
P.S. Do you use different tone stack values for the BF style stacks (right after the initial preamp stage) and the 5F6A/Marshall-style stacks (immediately before the phase invertor)?
 
10/26/1998 8:22 AM
Mr Clean

Steve,  
 
In honour off your many posts on the subject I now call all Ck switches "Ahola" switches.  
 
My Blackface style amp has 100k,0.022u,0.022u with a pull mid boost which changes the 100k to 50k. It came with 100k,0.1u,0.047u. I thought the mids were lacking so I changed the caps but I only have a Strat and Tele. The mids may sound be OK with humbuckers.  
 
I find it depends a lot on where you are playing. When I was just playing in the bedroom I used the blackface tone circuit with bright cap and reverb but when I started playing with a band again I found that the bass and drums swamped it so switched to the 5F6A type circuit without bright cap or reverb. I donít need reverb because the church hall we rehearse in has enough natural reverb to satisfy even Dick Dale.  
 
I think I have good tone but the other guitarist in our band calls me "Mr Clean" because I have no low volume distortion which is fine by me but to shut him up I was thinking of building a Matchless Hotbox. What do you think of it ? Will it suit Mr Clean for occasional use ? He really only likes the sound of his 5F6A circuit cranked up and thinks TS9s make good paper weights.  
 
Dave
 
10/28/1998 3:11 AM
Steve A.

Dave:  
 
    Giving credit where it is due, I call the Ck switches the "Torres Triple Killer mod"... since I paid him at least 3 times what it was worth ($29 for a switch and a cap)! [ g ]  
 
    In comparing the tone stacks of BF and 5F6A amps, the values of the pots and slope resistor are quite different, presumably because of the location in the circuit. One question I've had is why the BF used audio pots for the tone controls while the tweed bassman and the Marshall derivitives used linear pots...  
 
    When you mention "5F6A type circuit" do you mean using a direct coupled pair with a cathode follower immediately before the tone stack, or just moving the tone stack to the end of the BF preamp (sans direct coupled pair)? I believe that watt-for-watt you get a more powerful guitar signal with the direct coupled pair (especially for leads) and that no adjustment of the tone controls will give you a 5F6A sound from a BF preamp (or vice versa). Both sounds are great, but they cover different territories.  
 
    If you are looking for more gain from a 5F6A circuit, you might want to look at the Pignose G40V schematic (with its added gain stage) and the adjustments I made to it when I transplanted it into my BF Pro: the "Pro Maniac" (I basically replaced the 0.047uF Ck for V1B with a 1.0uF cap in series with a 3k3 resistor to ground). At low gain settings it is very similiar to the tweed bassman circuit but with a bit more presence, and at the high gain settings you have plenty of real tube distortion.  
 
Steve Ahola
 
10/28/1998 10:45 AM
Mr Clean

Steve,  
 
My 5F6A circuit is stock except for the tone stack values. I have 250k log pots for bass and treb because I think it spreads the control over the whole rotation of the pot. With linear pots it all happens at the ccw end especially if you use 1M for bass. I donít suppose it matters where you put the tone controls as long as you donít overdrive any of the pre-amp stages but thereís not much chance of that in a guitar amp so position is important.  
 
I really like the sound of the stock 5F6A. Iím using 12" Celestions so itís more like ECís Beano amp than a Bassman. I never use the bright input so I could use that Ĺ 12AX7 for an extra gain stage but I would have to be able to switch back to the stock 5F6A because I donít want to loose THAT sound. I did think of switching in the extra gain stage with a footswitch and relays but I donít want to start modding my amps yet again so my preferred solution is something like the Matchless Hotbox with true bypass. Any opinions out there ? I could then use it with all my amps. A Rangemaster would be easier to build but I doubt it will be much use with a Strat or Tele.  
 
Mrs Clean has offered to buy me a digital multi effects unit thinking itíll stop me building any more equipment so Iíll be more use around the house but even worse than that last night my band suggested I get a chorus pedal. Yuk! Pass me a bucket quick. I feel sick.  
 
Dave
 
10/28/1998 8:02 PM
Steve A.

Dave:  
 
    I've heard a lot of great things about the HotBox but haven't seen one myself. Putting the power supply together for a single stomp box seems like a real waste to me; I'd rather put together a 2 or 3 space rack mount unit with maybe 3 or 4 different tube preamps and fx running off the same tranny (or at least double them up and make the HotBox and VibroBox in the same case/chassis). The VibroBox is supposed to have a great preamp even with the tremelo off- a cleaner sound much better than the clean channel on the HotBox.  
 
    So your amp is basically a 5F6A clone? Although there may be a subtle interaction between the two channels, adding a switchable bright cap should free up a tube stage without losing the basic sound of the bright channel. I've been wondering about using the trick in the new Victoria Tweed Twin amp: Input #1 would be connected to both stages of V1, while Input #2 would be connected only to V1A. One variation on that would be to use different Ck/Rk values for each stage and maybe add a switch to select which stage would be used for Input #2. (The Pittman book shows how identical stages were run in parallel on the 5E8-A Twin.)  
 
    From what I've read here, doubling up the initial preamp stage with an identical stage doesn't really boost the gain of the preamp much, but it is supposed to increase the signal-to-noise ratio. Any thoughts on that? (I'd think that tuning each stage with different Ck/Rk values would result in a fuller sound so that may be the way to go.)  
 
Steve Ahola
 

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