Still working on the Bogen CHA33, updates and more questions

[MWJB]

Reduce the value of R37, say 10K?

Remove R15, replace the 470ohm bypass resistor (R14) at the 6AV6 with 1.5K/25uf cap.

Remove the NFB loop or put it on a switch? If you remove the loop try a 25/25 bypass cap at R21 (you can put one there anyway but the bypass cap will affect the impact of the NFB loop). By the way I'm assuming that R27 is 2.2K NOT 100K.

Really to get more gain you'd be looking to completely rewire the PI as a long tail pair.

[Regis]

Reduce the value of R37, say 10K?

Remove R15, replace the 470ohm bypass resistor (R14) at the 6AV6 with 1.5K/25uf cap.

Remove the NFB loop or put it on a switch? If you remove the loop try a 25/25 bypass cap at R21 (you can put one there anyway but the bypass cap will affect the impact of the NFB loop). By the way I'm assuming that R27 is 2.2K NOT 100K.

Really to get more gain you'd be looking to completely rewire the PI as a long tail pair.

MWJB, thanks for the quick reply and all your help.

Do you mean to reduce R37 by 10K, or to 10K?
Switchable NFB might be an idea, I don't want to lose my presence control you helped me with.
Yes, R27 is 2.2k, made a misteak on the schem, fixed.

I wondered about the phase inverter, that might be more than I want to mess with as I wouldn't know what values to use.

Any comments on my tone stack question?

I'll try the other changes tonight or tomorrow. I haven't been out of the house for a week, going on a road trip to north Georgia today.

[MWJB]

You would reduce R37 TO 10K. Reducing BY 10K would make very little difference as preamps draw very small currents, thus there would be little subsequent voltage rise. Probably not an audible difference by the time you've heated up the soldering iron and changed the part.

Looking at your 6L6 cathode resistors, 1x680R (not K surely?) per tube looks a bit on the small side for 489v. I'd try 1x470ohm or 680ohm per PAIR (double ohms value if you want to stick to independent cathode resistors).

I don't really have any comments regarding the tone stack Q., are you not able to EQ the amp satisfactorily? I can't actually hear your amp, nor can I hear what the plate fed bassman tonestack will sound like in it. The bassman tone stack is a cathode follower in the original amp, it'll sound different & have more gain if you run it plate fed from the 6AV6. You're free to do what you like here and see if it works, but the Bogen is a cathode biased, paraphase PI amp with low gain & low preamp voltages...most 2x6L6 guitar amps sold today are fixed bias, long tail PI with preamp plate voltages over 200v on a 12AX7.

Basically, if you wanted typical 60's, or later, amp tone you wouldn't buy the Bogen.

Converting to fixed bias would mean that it would be easier to rewire the PI (late 5F6A, see the RI schem, or 6G12A PIs would work).

I'd tweak the preamp voltages (R37), get the bias right (50mA per tube?), maybe try an 470-820ohm shared cathode resistor (put another 1K in parallel?) at the preamp 12AX7 to increase distortion & keep using the overdrive pedal if you still need more gain. Halving the value of R30 (2.2 to 2.7K) may help too.

[MWJB]

A split load PI may be easier for you to implement & is equally at home with fixed or cathode bias...see push-pull 5E# Fenders.

[tbryanh]

"I wouldn't mind a little more gain a little sooner"

For more gain, consider inserting the second-half of V1 between the first-half of V1 and V2. This will add another preamp stage to the path.

Also, the amp probably needs to be setup for the stages to distort in the proper order.

From what I have heard, as the amp is driven harder and harder, the PA stage should distort first, then the splitter stage, then the last preamp stage, etc. on up to the first preamp stage.

This makes sense I think, because if the first preamp stage distorted first, the signal from the first preamp stage couldn't increase later to drive the other stages harder. So the other stages would never distort.

To set up the amp for the stages to distort in the proper order probably requires a scope and a signal generator. The interstage attenuators in the preamp stages would then be optimized. Not sure how to insure the PA distorts before the driver.

-Bryan

[MWJB]

I would say that normally the opposite was true, you determine the character of the distortion in the early preamp stages, later stages tend to be cleaner, better headroom so as not to compound buzzy SE class A preamp distortion as the amp is pushed and the PA should be distorting once the amp is being pushed beyond is clean W RMS rating.

The amp should still be harmonically rich and full at moderate volume.

This is usually achieved by ear testing as a scope cannot tell you what the distortion sounds like.

In reality, all the stages should compliment each other. If the PA distorts first, at moderate volume, then that's it - you have no headroom.

[billyz]

Looks like fun. I would use a 1.5k cathode resistor on V1 a&b and for more gain use the original 220k Plate load resistor r7 on V1 as well. Definitely use a 10k or so for r37. as previously mentioned redo the V2 cathode to 820 with 25uf bypass cap. you don't need r16 maybe use a .1 coupling cap instead. 1 meg volume pots are the way to go as well. Did you implement the bassman tone stack? If so try a 1 meg treble pot too. I would really go with the 100k in the NFB loop and maybe drop the presence pot to 5k L , this gives a better ratio of 20:1. Or disconnect it all together.

[tbryanh]

For non-master-volume tweed-type amps such as the Bogen conversion here, I am pretty certain that most of the distortion comes from the PA. I am pretty sure that the heart of the classic rock sound is PA distortion.

Modern amps (master volume amps) are a different story. I am pretty certain that most of the distortion comes from the preamp stages in modern amps.

I am pretty sure that I read in one of Kevin OConnor's TUT books that amps are set up to distort in later stages first (be it preamp-only stages (as in master volume amps) or PA, splitter, preamp-stages (as in classic amps). The reason for this is it give more touch-control to the amp. It also allows the volume control on the guitar to act as a "tone" control.

I could be wrong, and I am having a hard time digging up the info in my collection of TUT books.

[MWJB]

I don't think it is helpful to really try and pinpoint where in a design distortion specifically occurs first. Most successful amp designs are quite balanced in this respect. I was really talking about typical amps from the late 50's onwards (many of which are still popular today). Master volume is not really a consideration as it can easily be added to or removed from any design, if you are clever you can design one that drops out of the circuit completely. Agreed though, that if you are the kind of player who keeps the master down and the gain up, you are introducing more SE preamp character.

The Bogen isn't really a tweed type amp, it's pretty typical of an early to mid 50's topology (though with higher PA voltages), note that the Bogen and most amps from that period ran very low preamp plate voltages, the lower the voltage you run at the plate the more distortion you get. With 90vdc on a 12AX7 in grid leak bias (original Bogen spec) the preamp will always be somewhat distorted, not a lot you can do at the PA to change that.

[Regis]

Updates:

Paralleled another 1k on top of the 1k V1 cathode resistor, it helped a little, left it in.

Did the 1.5k/25uf at the cathode of the 6AV6, took it out again. Hard to describe, it just sounded bad.

Paralleled a 15k on top of R37, which raised the voltage on V1 to a little over 200 volts, that helped a lot.

MWJB, I have separate 680/25 resistors and caps on each power tube, with the tubes in it now the bias is at 53ma each. I know it is on the edge dissipating about 23 watts but that is within spec? You guys told me not to worry about bias.

Billy, I forgot to bypass R16, still will try it. I like the presence as is, but still might experiment.

The voltages on the PI are off some, Pin1 = 223 and pin 6 = 264, is that peculiar?

At this point the amp is sounding very good. Changing the tone stack and PI will stretch my skills and abilities so I might wait on that.

Swapping out parts in this amp sometimes is a bitch because of the PTP design and they did a pretty good job back in the sixties building these things, the leads are physically secured well and depending on where the connection is are sometimes difficult to remove without risking damage to a tube socket or something.

I want to do clips but I am in the middle of a mix in my studio and can't change the settings on the mixer. Next week maybe.

I took a few pictures so you guys can see what you've been helping with. The clean spot by the 5U4 is where I started cleaning 48 years worth of crud off the thing.

Thanks again everyone!

[MWJB]

23W/53mA per power tube is OK, I was just expecting a high plate voltage - so after subtracting cathode voltage you have 433v at the plate?

[RogerWilco99]

[*]82k NFB resistor was actually 100k, added parallel 68k.

The 68K resistor paralleled with the 100K would yield only about 40K ohms (if my maths is correct) for your NFB. Is that what you wanted/intended?

Have you considered putting something like a 100K linear pot there for a presence type control (not sure that's what you'd call it)? Then you can vary the NFB to suite you. You might want different levels of feedback based on what you're playing. Sorry if that's already been discussed.