New Build Idea: 6G4 Based, Cathode Biased

[Chuck H]

If you only need to roll down a little volume a resistive attenuator works just fine and is cheap to make. I include them as an option for any custom builds. My own attenuator does have an active load and sounds a little better at high levels of attenuation, but at low to moderate attenuation you can hardly tell a difference. Lemme find the thread and I'll post a link. Super easy to make.

Ah... Found it. Just raise the watt rating of the 25W rheostat to 50W and use a pair of 50W 20R resistors in parallel for the load.

http://music-electronics-forum.com/t16468/

[Rocketrob]

Thanks Chuck! Thats very clever, and simple too. I'll have to make one and try it out. Thanks again for continuing to reply to my posts; it's been a big help. RD.

[Rocketrob]

Over 1100 views?! REV5A.

Wow, it's nice to know that this is getting some interest, even though there isn't a lot of comments lately. I hope someone is getting some use out of it; that was the point of posting everything. So to celebrate, here is REV5A, a slight revision including a change to a 5U4GB rectifier, 5881 power tubes, and a 300-10W cathode resistor. Nice usable power with a bit of hair to it. I like it alot, clean to dirty with a twist of the guitar volume pot. The only thing I might do next is contour the bass response a bit; still just a little too much low end. I have to keep the Bass control way down (just like my old Bandmaster Reverb). Enjoy, Rob.



Some numbers relating to the changes:

V5 5U4GB
pin 8: no tubes=486v, with tubes=403v

V3 & V4 5881
pin 3: 401v
pin 4: 394v
pin 8: 31.55

V2 12AX7
pin 1: 260v
pin 2: 21.26v
pin 3: 32.64v
pin 6: 240v
pin 7: 21.98v
pin 8: 32.64v

V1 12AX7
pin 1: 155v
pin 3: 1.25v
pin 6: 164v
pin 8: 1.33v

I figure it's dissipating about 18W per tube, biased at about 80% (cathode bias)

[Chuck H]

It seems like I always have to reduce bass way more than I feel is "kosher" when I compare to so many great stock amps. For aesthetics I try to make the reduction appear more subtle than it is (crazy, I know).

You can contour the bottom end by reducing the preamp tube cathode bypass caps. You probably only need to change one of them. Try a 2.2uf on V1b. If that's not enough change, put it back stock and try the 2.2uf on V1a for a little more effect. If you want to reduce low mids as well you can drop that cap value to 1uf (or .68 like a Marshall).

[Rocketrob]

Doh! I forgot to add the REV5A drawing. I edited my last post to put it in.

Thanks Chuck; yes that is what I am planning to do, just haven't got to it yet. I would think it is better to adjust frequency response early in the chain, before it gets amplified many times, although changing the coupling caps in the PI seems to be a popular tactic also. I'll post results, as usual. Rob.

[Chuck H]

although changing the coupling caps in the PI seems to be a popular tactic also.

Nah. Everyone's tastes are different though. For high gainers I have good results trimming the low end right before the preamp stages start to get clipped. For Fenderish or more vintage type amps I take the principal and extend it to the tone stack. If the tone controls are in the circuit prior to where gain stages begin clipping I say leave it alone to preserve the dynamics of the controls and trim after that. Leave the PI alone if you can. If the bass is trimmed before that it makes for a more "open" sound with more clear bass reproduction. So, IMHE... Trim bass after the tone controls and before the PI. If you must reduce more bass then V1 becomes the attack point though. Trimming bass late in the amp seems to constipate the tone and reduce that long hollow bass response that makes an amp sound bigger.

[HBamps]

Seems like a very cool project. I have built something very similar, a few times and really have liked the results. I got the same tip for the split load which is an awesome way to shave off gain (you'll need it there). ended up with a 75k/22k split with a .002 coupling cap.

please keep us up on how it turns out!

[HBamps]

one other thing; if you have the space, a 25k mid pot in place of that 10K is very handy.

[Rocketrob]

HBamps-Thanks for the comments. This is a good sounding amp, and alot of fun to play, thanks to the talented and generous people on this forum. This amp has also turned into a bit of a mule, for testing different ideas and techniques. The inside doesn't look as pretty as it once did, but it's solid and works well enough. Not really 6G4 based either; well, the pre-amp is, but the output section is closer to a tweed bassman with cathode bias. Hmm, maybe I'll try a fixed biased setup to see how it sounds.

The 25K pot for a mid control is a great idea, but there is not enough room for it. The chassis is the same size as a Epi valve jr head; in fact I use the Epi head cabinet to house it. It definitely gets a lot of second looks. I'll keep updating the drawings as I go along; maybe I can help some one like I've been helped. Rob.

[mazzbob]

can anyone forward PDF of the last (REV5) layout email mazzbob@gmail.com