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Post subject: Schematic Question
Posted: Mon Nov 05, 2018 9:21 am
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Here's a Hohner Contessa CA200 I'm restoring. My question is, R26 (47k) goes to the grid on the 12AU7 (V2a pin 2) according to the schematic. On the amp itself it connects to the cathode on pin 3 along with R16. This amp has never been touched. Schematic is wrong? I know opposite or same phase with the input signal come into play here when connecting to the grid or cathode.

Thanks, UN

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Fender Play November 2018
Post subject: Re: Schematic Question
Posted: Mon Nov 05, 2018 10:43 am
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It's a negative feedback loop resistor. Generally connected to the PI-grid but there are other ways of getting it there. I see them connected in series to the cathode resistor to ground in Fender amps, look at the Bassman circuit to see the varying ways Fender connected NFL. IMO, if it only connects to cathode it is used more to control the tubes operating parameters, whereas connected to grid it performs more cancellation of signal effects and decreases distortion(extending headroom). Removing the NFL is a temporary option for restoring, saving it for the last piece to install as its not highly critical to operation. If the amp needs it because of oscillations or heavy distortion, re-install it and adjust the Resistance as needed.
Whoever labeled the valves (in blue)did a poor job, as I've never seen an output tube labeled V1.
Does the un-molested amp you have work w/ the NFL connected to cathode?
Have you traced the rest of actual amp to the schematics?
Since you have the amp, you should be able to confirm the schematic's integrity. It is a fairly obscure amplifier and I have never seen one. The schematic otherwise looks pretty straight-forward to my eyes.
edited to try and make more sense.
Sorry, I'm just the layman-others w/ more knowledge will surely chime in.


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Post subject: Re: Schematic Question
Posted: Mon Nov 05, 2018 11:51 am
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It seems to work fine on the cathode so I'll leave it there. I was curious as to why the schematic was different. I haven't had a chance to go through the rest of it yet, that just stuck out to me.

My limitations are a great plenty as well. I planned on adding DC bias to the heaters via an RC network. It's not in the amp and was added to the schematic, R47, R48, C25. Right now the heater CT goes directly to ground and there's a little hum that may help to clear up. Maybe a 100 ohm pot would work as well. I'll replace the filter caps in the next few days so I can begin to narrow down where it's actually coming from.

UN


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Post subject: Re: Schematic Question
Posted: Mon Nov 05, 2018 12:08 pm
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I would wait to modify anything until after the caps have been updated. There is enough capacitance to filter the circuit. If the 6.3v pair has a center tap to ground, usually no artificial reference is required but they may help. Perhaps a shielded twisted pair wire is a better solution to reduce stray hum. Maybe renew the lacquer coat on the OT would help reduce hum/noise also, if it exists after the cap change.


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Post subject: Re: Schematic Question
Posted: Mon Nov 05, 2018 4:48 pm
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Looks like that amp has a cathodyne (Concertina) phase inverter (V2B). And V2A is a gain stage, posts tone stack. IIRC, these inverters have a set of issues, which differs from long-tail inverters. Including that cathode-tied GNFB resistor. Valve Wizard has a nice bit on these types of circuits. I'm not sure too much experimenting can be done, without researching the cathodyne circuit. Maybe, Morgan Jones might have such nice reviews and mods.

http://www.valvewizard.co.uk/cathodyne.html

https://www.amazon.com/Building-Valve-A ... 0750656956


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Post subject: Re: Schematic Question
Posted: Mon Nov 05, 2018 9:46 pm
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Very interesting low gain amp.
The tone controls don't seem to have a lot of range, but would be very clean by using the same values (.001) in the circuit. This prevents phase distortion in the tone controls.
The reverb is send is capacitor coupled. Don't unplug the a rca and touch it. It'll be hot.
Was this built for harmonica or accordian?


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Post subject: Re: Schematic Question
Posted: Tue Nov 06, 2018 6:35 am
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This amp surprised me to say the least. The cleans below 5 are good. After 5, the overdrive is amazing. Nice creamy, tight overdrive with plenty of girth. ("Make It Last" - Montrose by comparison) I have to say I didn't expect that from this amp, but on the other hand I didn't know anything about it or have I ever heard one in person.

From the very limited info available, it was designed for guitar with a Baxandall tone control circuit by Paul Barth and Ted Peckels who started Bartell.

I know this is the Fender Forum but here's a couple of gut shots anyway. I installed the grounding power cord. The PT looks like it also has 5v taps.

UN

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Post subject: Re: Schematic Question
Posted: Tue Nov 06, 2018 9:36 am
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That is a nice, pro-quality point-to-point wiring layout. Does it have a cap can, for the main PSU filtering? I'd only replace that (if it's original) and maybe some of the bypass caps.

Those are vintage Italian made Ducati electrolytic caps. IIRC, they are good stuff. Was the unit made in Italy?


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Post subject: Re: Schematic Question
Posted: Tue Nov 06, 2018 10:16 am
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Yes, the main filters are 40/20/20 450v and an additional 20uf/50v in the same can cap. I'll just use a 40/20/20 and add another axial 25uf/50v cap. I'll replace the C-By caps as well. I don't believe it was made in Italy, but in the US.

UN


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Post subject: Re: Schematic Question
Posted: Wed Nov 07, 2018 8:11 am
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Seen this YT clip? Very nice OD and reverb tone. Never heard of this model amp. I only have a faint recollection of Hohner amps. Wasn't that what the Beatles used in their Cavern days?




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Post subject: Re: Schematic Question
Posted: Wed Nov 07, 2018 8:36 am
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That is a really fine looking chassis, great find!! I'd clean up those sockets a bit and replace the caps and call it done unless there's something else going on with it.
Is that a 10" or 12" and what kind of speaker is in it?


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Post subject: Re: Schematic Question
Posted: Wed Nov 07, 2018 1:16 pm
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BMW2002Ti wrote:
Seen this YT clip?

Yes, I actually watched that one before I bought it.

sfceric64 wrote:
That is a really fine looking chassis, great find!! I'd clean up those sockets a bit and replace the caps and call it done unless there's something else going on with it.
Is that a 10" or 12" and what kind of speaker is in it?

I'll change out the needed caps, etc. shortly. It's a 12" speaker, the original Eminence alnico with an aluminum DC. I tried a couple of other 12's including a '66 C12NA. They all sound good, just different. I ordered a used Celestion G12M Greenback that'll be here tomorrow, we'll see how that does.

UN


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Post subject: Re: Schematic Question
Posted: Fri Nov 09, 2018 3:20 pm
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I'd like to get rid of some excessive low end on this amp. When I back the bass off to much I lose some gain so I was contemplating changing C3 to .01 and possibly adding a touch more mids by raising R11 to 15k.

Thoughts?

UN


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Post subject: Re: Schematic Question
Posted: Fri Nov 09, 2018 3:37 pm
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If I'm not mistaken that's a low block "high pass" RC circuit(resistor to ground).
Using frequency cutoff calculations(w/o regard to the pot), the schematic cutoff freq. is 723.79HZ
Changing the values to .01Uf and 15Kohm would change the cutoff freq. to 1061.57Hz
In this example it would block frequencies lower than 1.061KHz.
If you only change the cap to .01 the cutoff would be 1.592KHz more lower freqs. to ground.
You'r subs seem good to me, give it a try.

Edited as usual, had to do the math.


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Post subject: Re: Schematic Question
Posted: Fri Nov 09, 2018 6:07 pm
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Thanks much for taking the time to do that, I appreciate it. It just needs a little of the low frequency removed. The cabinet is larger than it needs to be and that could have the effect of "big bass". The Celestion G12M Greenback sounds really good. Even though it's made in China. I pulled the heater CT and used an artificial one instead which got rid of half the hum. I may redo the heaters with a re-positioned twisted pair and a 100 ohm balance pot when I do the PS caps.

UN


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