Bogen CHB10A conversion

[angelodp]

Hi, I have converted a Bogen CHB10A per this enclosed schematic. My tone stack is more Tweed, with a 500pf and .0047uf. I also have an on/off/on switch on the V1B cathode which allows for

1.on - 27k R in the NFB loop (clean)
2. off - NFB for some extra gain
3. on - 22uf 35v e-cap for the V1b cathode - increased gain

This all works quite well when I use a Boss Blues driver as a buffered input. I am not using the pedal in the on position (no light on ), however the pedal is powered. It appears that without the pedal in line I am lossing both hi end and general dynamics of the signal as a well as volume decreasing. So when I remove the pedal the amp is meh, a bit lifeless and with the pedal as a buffer it comes to life.

I have checked

1. 1Meg input correct
2, 68k input to grid - good
3. Ecaps all good
5. Grounds all good
6. Volatges very close to those on schematic.
7. Cable good and checked
8, wall voltage 115vac
9. SC guitar in good order
10. Tone circuit on amp works
11. Volume on amp works
12. Heaters have 100ohm virtual CT that is terminated at the cathode of the 7868.


Why is this amp lifeless without the buffer? With buffer frequency response way broader and gain is also increased.

It is a lovely sounding amp with the buffer. Have no encountered this aspect before/

[sumgai]

Jul 5, 2024 16:10:59 GMT -5 angelodp said:

Why is this amp lifeless without the buffer? With buffer frequency response way broader and gain is also increased.

The simple answer is an impedance mismatch. Your axe is probably exhibiting somewhere between 8 and 15KiloΩ, whereas the 1Meg resistor across the amp input expects to see a much higher value, somewhere in the vicinity of 1MegΩ. The buffer (a device that is called a buffer in the first place because it, usually, translates impedance from a source (your guitar) to somewhat near what a target (your amp) expects) is doing it's job correctly. Even when unpowered, it is connected in-line between the two endpoints, and thus has an effect.

Now, if I were you, I'd say to myself "I just got through modding this amp, why don't I just insert a buffer circuit at the input, which will fix it right up!"

Jul 5, 2024 16:10:59 GMT -5 angelodp said:

12. Heaters have 100ohm virtual CT that is terminated at the cathode of the 7868.

Virtual center tap, eh?    I wouldn't mind seeing a diagram of how that's done, if you please.

HTH


EDIT: You posted that pic after I started my reply. All I can say is, it looks like it came out of a factory, and one with pretty high standards at that. Nice job!

However, a quibble.... those holes in the chassis where wires pass through? They should have rubber grommets, because if I'm not mistaken, some pretty high voltage is being carried over the red and green wires in particular, and that's a potential safety hazard, if you get my drift. Your call.

FURTHER EDIT: By the way.... did you happen to trip across this thread, from 2013:

Bogen amp project on the way

Teleblooz didn't live too far from me, so I visited with him a couple of times, and I have to say, in a blind test I could've sworn that I was hearing a Fender Deluxe Reverb, it really was that good!

Just food for thought, that's all.






sumgai

[angelodp]

Mis-spoke its an elevated center tap with 100ohm resistors on each leg of the heater supply, going to the 7868 cathode.  Much like this. 

[angelodp]

Thank you for your kind words. I have never noticed this effect before as when I have a pedal in front of an amp it seems transparent until I engage the unit.

So how might I add a buffer to this amp without the pedal in play?

Best A

[Yogi B]

Jul 5, 2024 20:48:32 GMT -5 sumgai said:

Jul 5, 2024 16:10:59 GMT -5 angelodp said:

Why is this amp lifeless without the buffer? With buffer frequency response way broader and gain is also increased.

The simple answer is an impedance mismatch. Your axe is probably exhibiting somewhere between 8 and 15KiloΩ, whereas the 1Meg resistor across the amp input expects to see a much higher value, somewhere in the vicinity of 1MegΩ. The buffer (a device that is called a buffer in the first place because it, usually, translates impedance from a source (your guitar) to somewhat near what a target (your amp) expects) is doing it's job correctly.

But it's a 'mismatch' in the 'good' direction: 1Meg input impedance will do very little to load down 15k output impedance. Additionally a buffer (typically) will have even lower output impedance (and, possibly, even higher input impedance) so by your logic should make the problem worse, not better.

If, say, the input impedance of the amp was significantly lower then that would load down the output from the pickups — particularly the treble (since the pickups are inductive and, to a point, have greater impedance at higher frequencies). Whereas a buffer with lower (and comparatively flat) output impedance would be less affected when driving a lower input impedance.

As such...

Jul 5, 2024 16:10:59 GMT -5 angelodp said:

1. 1Meg input correct

I know you said you've checked this, but lower than expected input impedance would be consistent with the issues you describe.

Although it could be the lighting or low resolution of the image, the bands on the input resistor don't look right — especially in contrast to the 1Meg on the power tube (which is quite clearly brown black green red).

I'm obviously misreading it since it looks like black black brown red or maybe black black brown yellow red, both of which would be nonsense. The second is close to brown black black yellow red which would be correct for 1Meg, but first two definitely look darker than the third.

[kitwn]

I did wonder if it was the input capacitance of the tube in series with the 68K that was the problem, but this article suggests not and I don't have time today to do the maths. It's a useful read none the less.

Note that ECC83 is English for 12AX7.

www.valvewizard.co.uk/gridstopper.html

Kit

[angelodp]

Hey all, thanks for looking at this. I could not find the emoji, the one where you bang your head against your open hand!! So some time ago I purchased some off brand (read made in China) 12A jacks where the switch and signal are in opposing positions. I was aware of that and wired it accordingly..... but after the comment on the 1Meg, I ripped out that 12A (to check the 1Meg) and put in a switchcraft and its fine now!!
I am still puzzled, but it does work fine with or without the pedal up front. I suppose that cheaper switch had a poor quality of metal which altered the impedance (wild guess) and so the Boss cleaned that up. Why I ever had that 12A around ?? I had an issue with that jack before with the switch/signal in opposition..... should have thrown it away.

So I fall on my sword and thank you for your attention to this matter. Does this all make sense?

best Ange

[angelodp]

Now another issue has arisen. When I engage the pickup sector on ANY guitar I am getting static (DC I think) noise as I change position. I do not get that noise when switching through the pickups when I am using the Boss pedal up front. So the volume and tone issue has been solved but now getting this noise when switching through the pickups?

1. Checked all grounds

2. Separated the tone stack ground from the input jack ground

3. Triple checked the input jack and 1M

4. Tried a different cable

5. Gone through all connections and reflowed any questionable joints (very few).

What the heck??

[angelodp]

What are the odds that three guitars in a row all need to have their switches cleaned (dioxit).....solved. Occam's razor! Onward

[kitwn]

Jul 7, 2024 11:46:54 GMT -5 angelodp said:

Now another issue has arisen. When I engage the pickup sector on ANY guitar I am getting static (DC I think) noise as I change position. I do not get that noise when switching through the pickups when I am using the Boss pedal up front. So the volume and tone issue has been solved but now getting this noise when switching through the pickups?

1. Checked all grounds

2. Separated the tone stack ground from the input jack ground

3. Triple checked the input jack and 1M

4. Tried a different cable

5. Gone through all connections and reflowed any questionable joints (very few).

What the heck??

That sounds a bit dodgy! Maybe you're getting a DC offset out of the input jack due to rectification in the tube? Some form of DC isolation at the input might be a good idea in case of an internal short in the tube or around it's base. A non-polarized, high voltage capacitor perhaps or an audio isolation transformer, or even a wireless link to replace the cable.

You can spend a lifetime working with high voltages but you will never make more than one fatal mistake.

[angelodp]

Thanks for the reply. It turns out that the several guitars I had tried all had selector switches that need some TLC. A bit of dioxit on the switches solved the issue. Amp is working and all well now.

Best A

[stevewf]

Jul 7, 2024 12:26:55 GMT -5 angelodp said:

What are the odds that three guitars in a row all need to have their switches cleaned (dioxit).....solved. Occam's razor! Onward

Good thing you tried that avenue. I'll use this episode as a reminder to check the unlikely!