Ao28

[Sveo100]

Hello
Is C2 and C7 kind of click filter on the AO28?

[Jyvoipabo]

Hello,

These capacitors are in the feedback loops of the amplifiers.
They are useful in limiting the gain at high frequencies, which improves stability and enhances low frequencies.

JP

[Sveo100]

Ok Thank you.

[kziss]

Hello
Is C2 and C7 kind of click filter on the AO28?

Hi Sveo100.
Yes the C2 is a treble cut feedback capacitor for the Non Vibrato channel, and the C7 is a treble cut feedback capacitor for the Vibrato channel so therefore the C2 and C7 do function as key click filters.

On my two C3 organs I have wired up switches which switch the C2 and the C7 on or off thus allowing the stock sound or a treble boosted sound.

Disconnecting the C2 and the C7 works best with the pre 1964 organs which have the aged wax paper capacitors on the tonewheel generator (TG) because the aged wax paper capacitors have caused the output levels of the TG notes 49 to 91 to be weaker so therefore when the C2 and the C7 are disconnected, the resulting treble boost effect creates a nice spitty key click as well as brightening up the main organ tone without the organ becoming too shrill sounding.

However on a pre 1964 organ with a recapped TG or with a red mylar capped post 1964 organ, the treble boost effect created by disconnecting the C2 and the C7 can make the organ very bright or shrill sounding which depending on your tastes you might like or you might dislike.

Please note that if you do disconnect the C7 and the C7, this only affects the main organ signal but it does not affect the Percussion signal so consequently the Percussion effect volume level will sound weaker compared to the volume level of the treble boosted main organ signal so therefore in order to avoid this issue, I also recommend that you disconnect the C33 0.0033 uF (3.3 nF) treble cut / key click filter capacitor from the Percussion output filter so that the Percussion effect also has the boosted treble response and the enhanced key click effect and therefore sounds better balanced with the treble boosted main organ signal.

If you still want the Percussion effect to be louder, you can also increase the value of the R51 grounding resistor of the Percussion output filter. The R51 is located between the terminal "P" and the ground inside the little rectangular box which is mounted on the middle section of the terminal panel side of the AO28 preamp. You can replace the R51 with a 100K small sized potentiometer or a trimpot mounted between the terminal "P" and the ground and thus be able to adjust the Percussion volume level as desired.

If you decide to wire up a DPDT switch to switch the C2 and the C7 on or off, then in order to minimize the likelihood of creating any hum noise, I recommend that you connect the switch wires in series with the sides of the C2 and the C7 which are wired towards the output sides of the V1 and the V2 6AU6 valves.

All the best.
Kon.

[Baldwin51]

the switch sounds like a good idea, did you use shielded wire?? my c-3 sounds pretty poor right now , the highest notes are weak and I think a recap is in order, I plan on rebuilding my AO 28 as well. still have the 1955 wax caps in the c-3 and the AO-28 has never been touched as it has sounded extremely nice until a couple yrs ago, it also has some funny sounding highs that sound a lot like a bad leslie treble speaker dose , but I ruled it out because I can hear it through all 4 tone cabinets, two are leslies.

[Sveo100]

Thank you Kon for taking time to write so much about this 😁. A switch is the best of course . I will so that. I didn't know about the C33. I will try that. The R51 is allready replaced with a pot.

Thnx again
Sven

[Tim_at_Jonas]

We are actually opening the series combo or R4+C2, and R13+C7, yes?

[kziss]

We are actually opening the series combo or R4+C2, and R13+C7, yes?

Hi Tim.
No the R4+C2 and the R13+C7 series combos do not need to be opened up, instead the capacitors to be removed for the treble boost effect are the C6 30 pf feedback capacitor which is wired in series with the R20 270 K resistor on the V2 6AU6 valve (Non Vibrato channel), and the C1 20 pf feedback capacitor which is wired in series with the R6 2.2 M resistor on the V1 6AU6 valve (Vibrato channel).

I have experimented with disconnecting the C2 and the C7 100 pf grounding capacitors but there was no noticeable tonal difference in the sound so l soldered the C2 and the C7 100 pf capacitors back in place.

All the best.
Kon.

[Sveo100]

I disconnected the C2 and C7 but I think that some bass disappeard. I think I will connect this caps again.

[kziss]

We are actually opening the series combo or R4+C2, and R13+C7, yes?

Hi Tim.
Yes that is correct. Opening up the series combo of the R4 ( 1.8 M ohms and the C2 ( 39 pf ) will create a treble boost effect on the V1 6AU6 valve (Non Vibrato channel), and opening up the series combo of R13 ( 4.7 M ohms ) and the C7 (24 pf ) will create a treble boost effect on the V2 6AU6 valve (Vibrato channel).

PLEASE NOTE: PLEASE IGNORE MY REPLY ON POST #7.
Last night I used my mobile phone to quickly check the latest threads on the Hammond Organs section of the Organ Forum, including this "Ao28" thread stated by "Sveo100", and the ""Hi everyone, I have the Hammond M102, Is there aTone control adjustment???" thread started by "Ork".

Because I was using my mobile phone with it's small and much more limited screen instead of my computer, I somehow ended up confusing the two above mentioned threads whilst reading your post #6 on the "Ao28" thread.

I mistakenly thought that your question on post #6 was about the M102 AO67 amplifier instead of the AO28 preamp and I consequently replied to your question accordingly.

Now that I have come back on the Organ Forum, I have just realized that because I got the two threads mixed up on my mobile phone last night, I have written an unintentionally erroneous and misleading reply which may cause confusion, especially for less experienced tinkerers or "newbies" so therefore you and anyone reading my post #7 need to disregard that reply.

I apologize for my erroneous information and any confusion caused by my reply on post #7.

However, as is the case with the C2 and the C7 0.0001 uf ( 100 pf ) capacitors on the AO29 and the AO67 amplifiers of the M3 and the M100 series organs, disconnecting the C3 0.0001 uf ( 100pf) on the V1 6AU6 valve, and the C8 0.0001 uf ( 100 pf ) on the V2 6AU6 valve on the AO28 preamp had no effect on the sound so therefore I recommend leaving the C3 and the C8 in place in the AO28 preamp even if you do open up the R4+C2, and R13+C7 series combos in order to produce the treble boost effect.

All the best.
Kon.

[kziss]

I disconnected the C2 and C7 but I think that some bass disappeard. I think I will connect this caps again.

Hi Sveo.
Disconnecting the C2 and the C7 does not reduce the bass response, but because disconnecting the C2 and the C7 does creates a treble boost effect, this means that the bass response appears to be reduced in the tonal balance because the treble response becomes louder, however you can compensate for this reduced bass by creating a bass boost effect by wiring up a 39 pf capacitor in series with the R5 10 M ohms resistor on the V1 6AU6 valve ( Non Vibrato Channel ) and by wiring up a 39 pf capacitor in series with the R14 10 M ohms resistor on the V2 6AU6 valve (Vibrato channel).

On my own two C3 organs I have wired up a bass boost switch which switches one 39 pf capacitor in series with the R5 and another 39 pf capacitor in series with the R14, and this does create a noticeably deeper, heavier bass response and I sometimes do switch this bass boost switch on when I switch on the treble boost switch which disconnects the C2 and the C7.

However if you consider the bass boost effect to be too heavy or strong with the 39 pf capacitors wired in series with the R5 and the R14, then you can experiment with other capacitor values such as 47 pf or 68 pf or 82 pf or 100 pf or 120 pf or 150 pf etc until you get the particular bass boost effect that you like.

If you want you can wire up one capacitor value in series with the R5, and a different capacitor value with the R15 in order to more closely match the bass response of the Non Vibrato channel and the Vibrato channel.

All the best.
Kon.

[Sveo100]

Hi Kon Thanks. I'll try this caps on 39 pf. If I think AO29 , I have two M100 as well, it will be C6 and R21 on Non and C1 and R7 on Vib right?
I've read somewhere , I don't remember where, that C14, intermediate amp in AO29, shall be changed for a new one ,with same value. Do I get a better sound with this change or why?

Reg Sveo

[kziss]

Hi Sveo.

If I think AO29 , I have two M100 as well, it will be C6 and R21 on Non and C1 and R7 on Vib right?

Yes the above is correct except that on theVibrato channel it is the C1 and the R6 instead of the R7.

I've read somewhere , I don't remember where, that C14, intermediate amp in AO29, shall be changed for a new one ,with same value. Do I get a better sound with this change or why?

It is a good idea to change all the electrolytic capacitors including the C14 which is a 30 uf cathode bypass capacitor which is simultaneously connected to the cathodes of the V1, the V2, the V4A and the V4B because the electrolytic capacitors tend to deteriorate with age.

Depending on the condition of your C14, there may or there may not be a difference in the sound if you replace it with a new 33 uf capacitor but even if there is no difference in the sound, it is always a good idea to replace the C14 and the other electrolytic capacitors simply because they are so old so therefore they can fail at any time without warning.

All the best.
Kon.