Help with Bogen CHB 35A

[hearforever]

Hi again,

I posted about this a couple of weeks ago...
I've been tinkering with a Bogen CHB 35A Challenger that I picked up at a thrift store and trying to incorporate aspects of the Ampeg B-15/SB-12 circuits. I have only a pretty basic understanding of how tube circuits work and I'm pretty shaky on electronics in general.

Here's what I've wired up so far on the Bogen:
http://www.hereforever.org/chb35a.jpg

The tone control will not remain what it is, I wired it a couple of months ago when I was first learning with what I had on hand.

The Bogen's schematic is remarkably similiar to the 7868 version of the SB-12 (http://www.schematicheaven.com/ampeg..._portaflex.pdf) However, there are some differences that are confusing me and I have some questions.
1.) Why does the cathode of V2A and V2B connect to a 220K resistor and then the filter caps? In the SB-12 this doesn't happen...

2.) What affect would changing the values of the anode resistors in V1A , V1B, and V2A going towards the filter caps have? In the Bogen they are 470K, and 220K vs 68K and 220K.

3.) What is the 100K resistor in the SB-12 right before the .01u capacitor off of the anode of V1A?

4.) What is the whole circled purple area in the Bogen circuit? Can I simply remove this and replace it with resistors?

Finally, I've noticed with this amplifier that at certain volumes and/or tone settings I am getting a very harsh, high pitched squealing oscillation type effect. It happens at about 8 on the Master volume knob. What might be causing this?

thanks for your patience with all of my dumb questions!

[hearforever]

Anyone? am I on the right track here??

[tbryanh]

1.) I’ve never seen this before. I think it is part of the cathode bias. For V2A, it appears that a higher DC bias voltage is dropped across the 680R cathode resistor than would be if the 220K resistor was not there. This allows the cathode resistor to be smaller and still bias the stage yet not need a bypass capacitor to still get good gain from the circuit. (Likewise, the same for V2B.) I am pretty sure I have heard that capacitors at one time were more expensive than chokes, so I think the circuit here was finding a way to avoid using expensive bypass capacitors.

2.) It would change the gain and headroom of the stages. V1A and V1B would tend to stay biased in the center as their plate resistor values changed, but because of the 220R resistor connected to pin 3 of V2A (the resistor discussed in question 1), V2A might not stay biased in the center as the value of its plate resistor changed.

3.) This works with the 68K resistor to form a voltage divider to reduce the output of the stage.

4.) The purple area allows external circuitry to control mic precedence. The mic for V1A can be disabled by grounding the REM~1 terminal, and the mic for V1B can be disabled by grounding the REM~2 terminal.

The squealing is a parasitic oscillation. Adding a tiny capacitor across the 47K NFB resistor would probably eliminate the oscillation. You need to be carefull not to use too large of a capacitor, or you might attenuate the higher audio frequencies. (There are other ways to eliminate the oscillation.) If the amp sound good set below 8, you might consider not worring about the oscillation and leave the amp set below 8. Older Marshall amps do similar things and they are still considered to be some of the greatest amps.

You might consider using the CHB35 to clone the B-15/SB-12.

I am wondering why you want an Ampeg type amp? If I recall correctly, Ken Fischer of Trainwreck amps used to work for Ampeg before going off on his own. I think I read in one of his articles that he said the Ampeg amps were made to amplify acoustical instruments and were not very good amps for electric guitar.

[hearforever]

Thanks for the response. As for why I'm interested in Ampegs its simply that I'm looking for a decent recording/practice bass (or guitar) amp and the B-15 is such a great sounding amp

[PRNDL]

7868's are really cool tubes, and are part of the Ampeg tone.

They are, however, very sensitive to bias, which is most likely the cause of your harsh sounds. The schematic calls for -23 volts. I started there and fine tuned it while playing guitar (I added a pot).

The James tone stack in mine was tuned for a PA amp. I changed the values to make it suitable for guitar.

Here's some photos if you are interested.
http://www.naturdoctor.com/Chapters/Amps/JazzTank.html

[hearforever]

Thanks tbryanh for your comments

From what you see, does it seem like I could just wire V1A, V1B, and V2A like V5 on the SB-12 and then keep the output section the same? Do I need the 220 K resistors after the volume pots before V2A? should they come before the pots?

PRNDL, can you explain for a novice how you would wire a pot to bias the 7868's?

thanks!

[tbryanh]

You might try removing the lead from the 0.01 coupling capacitor that is connected pin 7 of V1A and then hooking the guitar to the capacitor.

V1A is for a mic. A guitar has a hotter output than a mic, so you probably don't need V1A.

You might get pretty good tone without doing anything else.

[tbryanh]

does it seem like I could just wire V1A, V1B, and V2A like V5 on the SB-12

I think you mean to wire V1A, V1B, and V2A like V5A, V5B, and V4A on the SB-12.

That sounds like a good idea.

[hearforever]

There is no corresponding gain stage for V1B on the SB-12, but V5A and V5B correspond to V1A/V1B and V2A I was thinking of leaving V2B (bogen) and V4A (ampeg) as they were on the circuit. would it make sense to wire V2B like V4A on the ampeg? Is the 6C4 tube basically 1/2 of a 12AX7?

[tbryanh]

I wasn't looking that close.

Yeah, I think leaving the driver (V3) and the PA (V4 and V5) in the Bogen as it is is a good idea. Then work back from the driver to the input with the Ampeg circuit.

The 6EU7 has the same gain as a 12AX7, so they are probably compatible.

You might replace the 6EU7 with a 12AX7 if that is what Ampeg uses.

Yeah, the Ampeg has one less stage than the Bogen, so you will have an unused stage.

[tbryanh]

The Ampeg uses a tube rectifier, but the Bogen does not.

This means that the Ampeg might compress more than the Bogen when driven hard.

Resistors can be added in series with the solid state diodes in the Bogen to simulate the compression of a tube rectifier.

[PRNDL]

PRNDL, can you explain for a novice how you would wire a pot to bias the 7868's?

The bias is set by the 50K and 10K resistor network off of the red transformer lead.
Varying the 50K resistor changes the bias.

The simplest method would be to replace the 50K with a pot of a larger value, such as 100K.
Before turning on the power, set the pot to 50K, then turn on the power and make minor adjustments to set the bias to exactly -23 volts.

Most likely the amp will sound fantastic with the factory bias (-23 volts). You can make minor adjustments (0.5 volt or less either way at a time) and see if it sounds better.

Having a higher bias puts more current through the output tubes, which will begin to glow red. This shortens their lifetime considerably. A lower bias extends tube life, but increases distortion due to the crossover notch. These days, most recommend setting the bias on the normal/cool side.

You might want to do some more reading on how to bias an amp, since it is more involved than this simple method and really does effect the tone.

[hearforever]

Hi again,

I added a bias pot and fiddled with the NFB, adding a small capacitor across it but it didn't get rid of the shrieking/oscillating noise. I'm wondering if there is too much gain going into the power tubes. Any other ideas I could try? thanks

[tbryanh]

Maybe one of the tubes is microphonic. Try substituting one tube at a time with a known good tube.

Having known good tubes might require you to purchase another CHB 35A. Tube testers can tell you if tubes are bad, but they cannot tell you if tubes are good.

You might try removing V1 and see if the problem is still there. This will not tell you that V1 is bad (although it could be), but it will at least tell you that V1 is involved.

If the oscillation stops when V1 is removed, go ahead and complete your Ampeg conversion plans and see if the oscillation disappears. Changing the V1 circuitry might solve the problem.

[tbryanh]

Probably the best thing to do is do the Ampeg conversion and see what you get.

It makes no sense to fix something that you are going to almost completely take apart and then reassemble in a different way.