beltone ap12

[olguy]

is their anyone who could direct me to a ap12 belltone skematic it would be appreciated

[DrummerDan]

Beltone AP12 AP14 schematic

Hey, I know the post is a few months old, but if you were like me you looked all over and couldn't find the schematic. I reversed engineered it and here it is (its not a professional version, but you'll get the basics). There's quite a unique tone stack on there which works surprisingly good. There's also a few noticeable mods you can also do.

Check my YouTube video to see a Beltone AP14 . My username is vintageamprevamp. Cheers!

[Jazz P Bass]

Cool.
Nice work.
I have attached a pdf file of the schematic.
JPG's suck.

[DrummerDan]

Beltone AP-12 / AP-14 schematic with mods

Here is the Beltone AP-12 / AP-14 schematic with the mods that I have made. Sorry, no scanner, just a picture of my hand drawn worksheet... in PDF!

Beltone AP12 with mods.pdf

[sparchead]

Hi DrummerDan! first thanks for drawing this schematic! I just get myself an old Beltone AP-A and the layout is exactly the same. I would like to install the three prong to make it safe, and I was wondering if you are around right now, I would really appreciate if you could explain me the mods you did.

[DrummerDan]

Hi DrummerDan! first thanks for drawing this schematic! I just get myself an old Beltone AP-A and the layout is exactly the same. I would like to install the three prong to make it safe, and I was wondering if you are around right now, I would really appreciate if you could explain me the mods you did.

Hey,sparchead, here are some of the mods i did: I gave the amp all fresh electrolytic capacitors (a must for these old amps). I beefed up the power supply caps a bit with an F&T 16uF x 16uF @450V capacitor, I put in a 25uF @ 50V cap on the 1st preamp for the cathode bypass cap instead of the thinner 10uF @ 50v. I replaced the paper in oil cap in the tone stack with a Mojo cap 4.7nF 630 V cap. I put in the Sprague Atom 8uF @ 450V for the last supply cap instead of the 2uF (it was the next available size up I could find in the Sprague). I eliminated the 4 input jacks and just put the one in its place. I only use switchcraft jacks. I replaced the power cord with a grounded cord. I added a 50uF @ 50V Sprague atom for the 6aq5 bypass cap. I also removed the ground lead on the one leg of the 6.3Vac windings and added an “artificial ground” using a 100 ohm resistor from each side of the transformer to ground…this is done to eliminate filament hum... i actually soldered it directly to pin 2 of the 6aq5. The speaker was blown (open) therefore caused the output transformer to fry… so I had to replace both, otherwise I would of left them in. The beefier 125cse Hammond must contribute to some of the great tone this amp has. I removed the “extra circuitry” after the 1st preamp cap: the 500k to ground, the 250k and .05uF to ground were all removed. I tied in the 100k plate load resistor directly to pin 7 of the 6av6. The 0.01uF (10nF) coupling cap from the last 6av6 to the 6aq5 could be upped to a 0.02uF (20nF). I just left the originals in. I had to add a small terminal strip to give me some room to solder the caps on. I’ll see if I have a picture of the back of the amp and I will post it.

Did you know, the 6av6 is exactly ½ of a 12ax7 with the same gain (100). The 6av6 only has a single triode stage and the 12ax7 has 2.

I hope I did not loose you here! If I were you, i would start with the 5 e-caps , the grounded power cord and go from there. It might be all it needs to please your ears!!!
Dan

[DrummerDan]

Hi DrummerDan! first thanks for drawing this schematic! I just get myself an old Beltone AP-A and the layout is exactly the same. I would like to install the three prong to make it safe, and I was wondering if you are around right now, I would really appreciate if you could explain me the mods you did.

Here are my before and after pics. You can't see every component but you get a general view.
dan

[sparchead]

Hey,sparchead, here are some of the mods i did: I gave the amp all fresh electrolytic capacitors (a must for these old amps). I beefed up the power supply caps a bit with an F&T 16uF x 16uF @450V capacitor, I put in a 25uF @ 50V cap on the 1st preamp for the cathode bypass cap instead of the thinner 10uF @ 50v. I replaced the paper in oil cap in the tone stack with a Mojo cap 4.7nF 630 V cap. I put in the Sprague Atom 8uF @ 450V for the last supply cap instead of the 2uF (it was the next available size up I could find in the Sprague). I eliminated the 4 input jacks and just put the one in its place. I only use switchcraft jacks. I replaced the power cord with a grounded cord. I added a 50uF @ 50V Sprague atom for the 6aq5 bypass cap. I also removed the ground lead on the one leg of the 6.3Vac windings and added an “artificial ground” using a 100 ohm resistor from each side of the transformer to ground…this is done to eliminate filament hum... i actually soldered it directly to pin 2 of the 6aq5. The speaker was blown (open) therefore caused the output transformer to fry… so I had to replace both, otherwise I would of left them in. The beefier 125cse Hammond must contribute to some of the great tone this amp has. I removed the “extra circuitry” after the 1st preamp cap: the 500k to ground, the 250k and .05uF to ground were all removed. I tied in the 100k plate load resistor directly to pin 7 of the 6av6. The 0.01uF (10nF) coupling cap from the last 6av6 to the 6aq5 could be upped to a 0.02uF (20nF). I just left the originals in. I had to add a small terminal strip to give me some room to solder the caps on. I’ll see if I have a picture of the back of the amp and I will post it.

Did you know, the 6av6 is exactly ½ of a 12ax7 with the same gain (100). The 6av6 only has a single triode stage and the 12ax7 has 2.

I hope I did not loose you here! If I were you, i would start with the 5 e-caps , the grounded power cord and go from there. It might be all it needs to please your ears!!!
Dan

Very good job with yours

Yep! I know for the 6AV6 vs 12AX7...

Hey dude thanks for the reply! I was pretty lucky with mine, got it for free everything's working, speaker (3.4 ohm), OT, but the power tube (6AR5) was running pretty hot (11.5W is quite hot for this tube) so I swap a marconi NOS "6AQ5" in it, and now it's better! still biased at 11.4W but the 6AQ5 is rated at 12W in the datasheet (the 6AR5 is rated at 8.5W). Also the 6AR5 was not the best tube for the OT ratio/speaker impedance, I mean, the 6AQ5 is a better tube to match the OT primary impedance.

Since the tremolo didn't work, I replaced all the 0.02uF caps (3), now it works surprisingly well, also replaced coupling caps with 0.022 mallory yellow caps, rebuilt the big can caps (opened it and put some 450V e-caps inside, left them to the original values since it don't have any noise at all), the first preamp bypass e-cap was shorted so I put a new 10uF 50V, and a grounded power cord rewired so the black wire first passed the fuse, then the switch and finally go to the PS 120V lug, the white wire go directly to the 0V lug and the green wire is hooked on the transformer bolt. Also I left the heater as is, with the center tap...

The amp is working very well!! BUT.. sometimes when I touch the metal parts with my finger, I hear a little "tick" in the speaker, just like a static sound, and I was wondering if the grounded power cord was correctly instaled, I mean, I already did this mod in other amps, and I always had to remove a "death capacitor"... is it for this reason you removed the 0.05uF capacitor, which you called the "extra circuitry", is it the death cap? and why did you removed (bypassed) all those resistors?

also, what's the difference (in the sound) with or without the power tube cathode bypass cap?

I never played with it yet.. when I got it, it didn't work since there was too much dirt in the fuse holder, which is now cleaned... I put in all the caps today and it's too late right now, tomorrow I'm gonna make the first real sound test

P.S. When I look at your "after" pic, I see that the .05 grounded cap is still there (at the bottom right of the big Sprague atom e-cap...), did you remove it since or?

[DrummerDan]

Very good job with yours

Yep! I know for the 6AV6 vs 12AX7...

Hey dude thanks for the reply! I was pretty lucky with mine, got it for free everything's working, speaker (3.4 ohm), OT, but the power tube (6AR5) was running pretty hot (11.5W is quite hot for this tube) so I swap a marconi NOS "6AQ5" in it, and now it's better! still biased at 11.4W but the 6AQ5 is rated at 12W in the datasheet (the 6AR5 is rated at 8.5W). Also the 6AR5 was not the best tube for the OT ratio/speaker impedance, I mean, the 6AQ5 is a better tube to match the OT primary impedance.

Since the tremolo didn't work, I replaced all the 0.02uF caps (3), now it works surprisingly well, also replaced coupling caps with 0.022 mallory yellow caps, rebuilt the big can caps (opened it and put some 450V e-caps inside, left them to the original values since it don't have any noise at all), the first preamp bypass e-cap was shorted so I put a new 10uF 50V, and a grounded power cord rewired so the black wire first passed the fuse, then the switch and finally go to the PS 120V lug, the white wire go directly to the 0V lug and the green wire is hooked on the transformer bolt. Also I left the heater as is, with the center tap...

The amp is working very well!! BUT.. sometimes when I touch the metal parts with my finger, I hear a little "tick" in the speaker, just like a static sound, and I was wondering if the grounded power cord was correctly instaled, I mean, I already did this mod in other amps, and I always had to remove a "death capacitor"... is it for this reason you removed the 0.05uF capacitor, which you called the "extra circuitry", is it the death cap? and why did you removed (bypassed) all those resistors?

also, what's the difference (in the sound) with or without the power tube cathode bypass cap?

I never played with it yet.. when I got it, it didn't work since there was too much dirt in the fuse holder, which is now cleaned... I put in all the caps today and it's too late right now, tomorrow I'm gonna make the first real sound test

P.S. When I look at your "after" pic, I see that the .05 grounded cap is still there (at the bottom right of the big Sprague atom e-cap...), did you remove it since or?

Hey, the 6aq5 is the better sounding tube here for this amp from what I read online. Every changes/repairs you listed sounds good to me.

The cathode bypass cap on the output tubes and the removal of "extra circuitry" are recommended mods in Gerry Weber's books...4th one i think. With the "extra circuitry" out, the preamp resembles more of a Fender preamp. The .05 cap in the "extra circuitry" was still present in that picture but was removed later. My youtube video of this amp was with it removed (per the mods in the schematic). You can temporarily tack a bypass cap on the outputs and do an A/B comparison. Let your ears be the judge.

I did not have a center tap on the 6.3v filament winding, so i made an "artificial ground". I wonder if this is why you get the "tick" sound when you touch the chassis. It might come from the "extra circuitry". You cantry to replace the 0.05uF cap with a fresh one, or remove the "extra circuitry" completely.
Have fun! Let me know how it sounds!
Dan

[sparchead]

Dan! You are correct! there's no 6.3V center tap my error sorry... so I did the artificial ground with 100R resistors and the tick sound is gone, the amp is much quieter now (hum), I also removed ground lead on the one leg of the 6.3Vac.. I did some test with(out) cathode bypass cap, which add some power and drive, I prefer without.. I also did test with(out) the extra circuitry, I must say that the difference is very subtle, so after one hour of test, I think Im gonna keep it, with a fresh .047 400V cap (I normally use 600V but I already had this one in my junk box). The amp seems very stable/reliable to me now, I like the sound so much. Done! Thank you so much for the help! Now this is the funny part.. I have some 6AQ5 tubes (5) so I'm gonna take the guitar and play until I hear the best one!!

[g1b3]

im tring to download this schema with no result.
someone can help ?
im looking for the ap-12 or ap-14 Beltone.
Thnks

[sparchead]

im tring to download this schema with no result.
someone can help ?
im looking for the ap-12 or ap-14 Beltone.
Thnks

I don't have 'em anymore.. but I found the one I drew based on DrummerDan's one. My amp is the AP-A model but I think it's almost the same.

Here it is:



hope this help!

[Rubber Soul]

Hi DrummerDan! Do you have a schematic of the mods you made?

[g1]

I think this is it.



( https://www.tdpri.com/threads/tech-q...-gurus.762000/ )