4tube Philco amp project...

[Eli Duttman]

There is a way "to have your cake and it too". Buy catalog # 273-1365 at a nearby "Rat Shack". That part is a 450 mA/12.6 V. filament trafo. You use the secondary of the filament trafo to boost the single pseudo primary of the N-68X. You wire the secondary of the filament trafo in series with the single pseudo primary. That composite gets connected to the AC mains. The primary of the filament trafo gets connected to the AC mains too. Of the 2 possible ways to wire the filament trafo primary to the AC mains, 1 will raise the O/P voltage of the N-68's pseudo secondary pair and the 2nd will lower that O/P voltage. Obviously, you want the arrangement that boosts the O/P voltage.

 

[knockbill]

There is a way "to have your cake and it too". Buy catalog # 273-1365 at a nearby "Rat Shack". That part is a 450 mA/12.6 V. filament trafo. You use the secondary of the filament trafo to boost the single pseudo primary of the N-68X. You wire the secondary of the filament trafo in series with the single pseudo primary. That composite gets connected to the AC mains. The primary of the filament trafo gets connected to the AC mains too. Of the 2 possible ways to wire the filament trafo primary to the AC mains, 1 will raise the O/P voltage of the N-68's pseudo secondary pair and the 2nd will lower that O/P voltage. Obviously, you want the arrangement that boosts the O/P voltage.

 

[knockbill]

There is a way "to have your cake and eat it too". Buy catalog # 273-1365 at a nearby "Rat Shack". That part is a 450 mA/12.6 V. filament trafo. You use the secondary of the filament trafo to boost the single pseudo primary of the N-68X. You wire the secondary of the filament trafo in series with the single pseudo primary. That composite gets connected to the AC mains. The primary of the filament trafo gets connected to the AC mains too. Of the 2 possible ways to wire the filament trafo primary to the AC mains, 1 will raise the O/P voltage of the N-68's pseudo secondary pair and the 2nd will lower that O/P voltage. Obviously, you want the arrangement that boosts the O/P voltage.

I don't know if tahts having my cake, or teh whole bakery!!!!!!!!!!!
I got teh little Zenith amp, almost dead quiet, with caps, grounding, and using an iso trans for teh last little bit of noise... I can't imagine this one, with its "high end" OPT's can't sound even better... I rewired it to use normal pri-sec and it really sounds good, lots of range, good volume, even has decent bass, for a little 2W amp...
The hum is related to the volume, hardly audible to about 1/2 volume,,, and increases as it get louder... I still have 7 caps to replace,, and the ones I already changed were in bad shape,,, a couple were split... Then there are teh resistors,,, I changed teh output K resistors to 150 Ohm, and teh old ones were way out of range,,,

Of course, I could always get a separate trans for teh B+ and teh heaters,,, or even a 150V+12V PT. and use 12C5 tubes, or any other combo,, but thats not teh exercise... I want to see how good a 50C5 amp can sound, and get rid of teh hot chassis in teh process...

Right now, I have a well matched pair of output tubes, and am getting 145+V out... I could only get about 114V out of teh Zenith,,, I don't think teh hum is due to driving teh tubes, as it was humming teh same way with teh trans reverse wired, and only putting out 110V...
I think teh new caps will help...
Regards,
John

 

[knockbill]

I just read a thread about proximity of switches, fuses, and AC in general, to the signal path of an amp... One of the replies mentioned that bridges are a common cause of noise in amps,,, If that is true, how would I go about reducing hum, with teh Pos and Neg leads on a bridge so close together??

I have the heater string tightly twisted, and run against the chassis,, and tied to the AC leads on the bridge,,, it was the most convenient spot to put them, without running even more AC around the chassis....Is it possible this amp would have less noise with a half wave diode, or even the original rectifier tube?

Learning more every day!!!

Regards,
John

 

[knockbill]

You know,,, I've been searching for ground loops, bad caps, out of value resistors,,, and it still hums when teh Volume controls are in certain positions,,,Finally,, it dawned on me,,, this thing has bad VCs!!
Seems its another Philco glitch,, this amp has a clutch drive volume/balance controls, which seem worn out, to me, and, if used separately, to adjust balance, they have way to much play in them, and hum and static, when moved side to side,,, The amp is dead quiet when the VC is in the "right" position....

It is a two meg, ganged pot, with a 2 1/4" long stem,,, where can I look for something like that??? I was going to use 2 separate pots, but, I have a nice, brass faceplate for this guy, that I'd like to use...

Regards,
John

 

[Eli Duttman]

John,

Snip out the portion of the schematic containing the volume control circuitry and post it. If, as I suspect, the VC setup is "routine", the 2 MOhm value is non-critical. OTOH, the 2.25 inch shaft is something to deal with.

As can be seen from the Mouser catalog page, ganged log. taper pots by Alpha Taiwan are inexpensive, but the shaft is only 0.335 " long. Buy a set screw shaft coupling and rod stock from McMaster-Carr. You figure the rest out. Sorry, no balance trim capability, but you do get to keep the nice brass faceplate.

 

[knockbill]

John,

Snip out the portion of the schematic containing the volume control circuitry and post it. If, as I suspect, the VC setup is "routine", the 2 MOhm value is non-critical. OTOH, the 2.25 inch shaft is something to deal with.

As can be seen from the Mouser catalog page, ganged log. taper pots by Alpha Taiwan are inexpensive, but the shaft is only 0.335 " long. Buy a set screw shaft coupling and rod stock from McMaster-Carr. You figure the rest out. Sorry, no balance trim capability, but you do get to keep the nice brass faceplate.

Hi Eli,,,
Thanks for the help,,,The length of teh shaft is no worries at all!!!! I can machine my own shaft, coupling and all....I just need help with the electronics end of it,,, but, I think I'm gaining on it!!!!!!

Heres a pic of teh schematic, w/ VC showing,,, I also am including a pic of teh chassis, in progress, so teh mods can see I did replace teh hot chassis , with a PT...
If these pics aren't clear enough, just let me know....

Thanks again,,,
Regards,
John

 

[Eli Duttman]

OUCH!! The "Bean Counters" were working overtime. They used a shared RC bias network in the voltage amplifier setup. YUCK! The NFB is applied to the 'X7 grids. Just to avoid "accidents", use a 1 MOhm value for the volume controls and wire 1 MOhm fixed resistors in series with the pots.' "tops".

The tone controls are GHASTLY. IMO, the thing to do is eliminate the NASTY tone controls and install separate, L/R mono, volume controls. Devil take the ganged, stereo, parts.

Everything I see, except the tertiary NFB windings, in that amps' setup SCREAMS cheap. The presence of tertiary windings is incongruous. My guess is that fear of killing customers (bad publicity) accounts for the tertiary O/P trafo windings presence.

 

[knockbill]

Thanks for the reply,,,
We knew it was a "cheap amp", going in... if I can't get it to work properly, I guess I still have the parts...the little Zenith I rebuilt sounds great,,, I thought this one would sound even better with the OPT's it has,,,
Not sure how you mean to remove teh tone controls, what will replace them?,,,, I'm sure I can find 1M pots for teh VC, I don't understand the other 1M resistor the pots top....
Sorry for all the questions, I'm pretty new at this....
Regards,
John

 

[Eli Duttman]

Thanks for the reply,,,
We knew it was a "cheap amp", going in... if I can't get it to work properly, I guess I still have the parts...the little Zenith I rebuilt sounds great,,, I thought this one would sound even better with the OPT's it has,,,
Not sure how you mean to remove teh tone controls, what will replace them?,,,, I'm sure I can find 1M pots for teh VC, I don't understand the other 1M resistor the pots top....
Sorry for all the questions, I'm pretty new at this....
Regards,
John

There are 2 openings for controls in the chassis. Put a mono volume control in each opening. C6, C10, and R9 (A/B) constitute what passes for a stereo tone control. All those parts are capable of doing is rolling HF info. off. In short, they are GUANO. Remove means exactly that; excise those parts, for being the "cancer" that they are. C3/R2 and C4/R3 are frequency shaping networks that boost HF info. Those parts are "cancer" too and need to be removed. The less the I/P signal is "massaged", the better. That's HIFI.

Forget what I said about additional fixed resistance in series with the controls. Just use a pair of 1 MOhm mono pots. and connect them directly to the I/P signal wires.

Mouser stocks low cost Alpha Taiwan controls. Look here. You want audio, AKA log., taper parts. Your local "Rat Shack" may be a source too.

 

[knockbill]

There are 2 openings for controls in the chassis. Put a mono volume control in each opening. C6, C10, and R9 (A/B) constitute what passes for a stereo tone control. All those parts are capable of doing is rolling HF info. off. In short, they are GUANO. Remove means exactly that; excise those parts, for being the "cancer" that they are. C3/R2 and C4/R3 are frequency shaping networks that boost HF info. Those parts are "cancer" too and need to be removed. The less the I/P signal is "massaged", the better. That's HIFI.

Forget what I said about additional fixed resistance in series with the controls. Just use a pair of 1 MOhm mono pots. and connect them directly to the I/P signal wires.

Mouser stocks low cost Alpha Taiwan controls. Look here. You want audio, AKA log., taper parts. Your local "Rat Shack" may be a source too.

Thanks for the info,,, seems there is a lot more parts in here then need be,,,I think I got it now,,, remove the original tone pots and VC pots, and replace w/2 one meg audio pots, which will act as VC/Balance.... also remove the C & R associated with them...
I have to order some caps for the Motorola this morning, I'll get the pots also...
I appreciate all the time you've spent, tutoring me, as I'm sure all the other forum members you consistently help do also....
Regards,
John