4tube Philco amp project...

[knockbill]

Hi guys,,,
I found another little, hot chassis amp (Philco H1410), to modify and rebuild,,, It has a 35W4 rec, 12AX7, and 2x 35C5 power tubes,,,
I have a SS power supply schematic, that was sent to me after I finished the 3tube Zenith amp, and I wanted to try it out...
The plan for this amp, is to replace the 35W4 1/2 wave, with a SS PS, and an isolation transformer and replace teh 35C5 tubes with 50C5's, which should up the power output about .8 Watt...
This amp has a separate OPT feedback winding,,,,

I'll post a schematic, and a couple pics of teh filter caps clipped in, if its allowed.. Its connected to an Isolation Transformer and DBT,,, and I got it working well enough to check voltages...
So, thats my new project, appreciate any help and advice....

Regards,
John

 

[Eli Duttman]

Just because you can squeeze a bit more power out of the 50C5 doesn't mean the O/P "iron" can handle it, especially in the deep bass.

The amp is a contradiction. Dangerous, cheap, circuitry, including a series heater string and transformerless B+, was employed. However, money was spent on the O/P "iron". Tertiary windings don't come gratis. It's possible that the "bean counters" worked out that "fancy" O/P trafos would cost less than a power trafo. Philco relied on the O/P trafos to isolate the end users from the AC mains. Notice that the O/P "iron" secondaries "float".

JMO, place a high pass filter at the I/P of each channel to protect against O/P trafo core saturation. Regulate O/P tube g2 B+, to obtain maximum open loop linearity.

 

[knockbill]

Just because you can squeeze a bit more power out of the 50C5 doesn't mean the O/P "iron" can handle it, especially in the deep bass.

The amp is a contradiction. Dangerous, cheap, circuitry, including a series heater string and transformerless B+, was employed. However, money was spent on the O/P "iron". Tertiary windings don't come gratis. It's possible that the "bean counters" worked out that "fancy" O/P trafos would cost less than a power trafo. Philco relied on the O/P trafos to isolate the end users from the AC mains. Notice that the O/P "iron" secondaries "float".

JMO, place a high pass filter at the I/P of each channel to protect against O/P trafo core saturation. Regulate O/P tube g2 B+, to obtain maximum open loop linearity.

I noticed that the OPT's were floating when I couldn't read tube voltages with teh meter grounded to teh chassis...
I did a search for teh OPT's get specs on them, but couldn't find any info,, I have the Philco part numbers..I can't imagine they will have trouble handling a couple watts, tho???
Regards,
John

 

[Eli Duttman]

Pictures, please! We need to see the O/P trafos in relation to the "fire bottles".

The series heater string is easy enough to execute; from the power trafo winding: a 39 Ω/3 W. resistor, a 50C5, the 12AX7, a 50C5, a 39 Ω/3 W. resistor, and back to the power trafo winding. If you spend some money and place the heater string and the B+ PSU on separate windings, the CT of the 12AX7's heater can be biased off B+ at about 80 V. Doing so will reduce hum.

It's safe enough to exceed the 50C5 data sheet max. anode voltage by a fair amount, IF you are "religious" about observing the max. anode dissipation limit. Bridge rectifying "120" VAC yields an approx. 165 VDC rail.

An 0B2 gas discharge regulator would be "ideal" handling the 50C5 g2 B+.

 

[knockbill]

Pictures, please! We need to see the O/P trafos in relation to the "fire bottles".

The series heater string is easy enough to execute; from the power trafo winding: a 39 Ω/3 W. resistor, a 50C5, the 12AX7, a 50C5, a 39 Ω/3 W. resistor, and back to the power trafo winding. If you spend some money and place the heater string and the B+ PSU on separate windings, the CT of the 12AX7's heater can be biased off B+ at about 80 V. Doing so will reduce hum.

It's safe enough to exceed the 50C5 data sheet max. anode voltage by a fair amount, IF you are "religious" about observing the max. anode dissipation limit. Bridge rectifying "120" VAC yields an approx. 165 VDC rail.

An 0B2 gas discharge regulator would be "ideal" handling the 50C5 g2 B+.

Good morning Eli,,,
I'd be glad to add more pics, the schematic I'm modifying, and teh slick PS supply that was sent to me right after I finished teh Zenith amp...
However, I got an email from teh moderators about this kind of (hot chassis) amps being discussed on this forum... I believe I complied with their concerns, but don't want to break teh rules,,,,

That being said, I am trying to make these plentiful, little, cheap, dangerous amps safe to use with the modifications I am learning, and passing on the info....

The SS PS I have places teh AX7 between teh 50C5's with a 33Ohm resister at each end...The AX7 heater CT will be tapped, thru a cap, and connected to Neg Grd...
A 1 to 1 Isolation transformer will be used to power teh amp...thru a bridge rectifier, thus raising the B+, and getting rid of teh "hot chassis" concerns,,,
This layout seems pretty similar to what you just described...
Also, as I did with the Zenith, the heater leads will be twisted and dressed away from teh inputs, and bigger filter caps will be used to further help quiet teh amp...

Do you have a schematic of teh 0B2 circuit, I can't find info in my RCA manual, but, if I make teh mods as I described, I will end up with a spare tube socket....
Regards,
John

 

[knockbill]

OPT's

Here is a pic of teh OPT's in this amplifier,,, The feedback coil is rated at 50Ohms,, and is tapped into G1 of teh power tube, and the vol/balance pot...

 

[Eli Duttman]

Read the GE Glow Tubes PDF.

With both heater string and B+ sharing a single winding, the ground connection you describe to the CT of the 12AX7's heater could be a variation on the, correctly, maligned "Death Cap." theme. Definitely use a film snubber part rated for a HIGH WVDC.

It's better if heater voltages are slightly on the low side, rather than slightly on the high side. 122.5 VAC is "perfect" for the 33 Ω dropping resistors you selected. That should prove quite adequate. When the mains voltage is 125 VAC, you'll be 2% high and that's OK. I'd be concerned, if things were more than 5% high. FWIW, 39 Ω dropping resistors work out to 124.3 VAC being "ideal".

 

[knockbill]

Read the GE Glow Tubes PDF.

With both heater string and B+ sharing a single winding, the ground connection you describe to the CT of the 12AX7's heater could be a variation on the, correctly, maligned "Death Cap." theme. Definitely use a film snubber part rated for a HIGH WVDC.

It's better if heater voltages are slightly on the low side, rather than slightly on the high side. 122.5 VAC is "perfect" for the 33 Ω dropping resistors you selected. That should prove quite adequate. When the mains voltage is 125 VAC, you'll be 2% high and that's OK. I'd be concerned, if things were more than 5% high. FWIW, 39 Ω dropping resistors work out to 124.3 VAC being "ideal".

Thanks for the reply, and the link...

Sorry, I don't understand your "death cap" statement,,, A snubber goes across the switch, correct? What value do I need to use? The Sansui amp I recapped used a .033 600V oil cap across teh switch,,, I replaced it with an orange drop. I believe...

My house is right off teh transformer,,, line in is right at 121V...Pretty sure the 33's will work,,, I may raise teh resistance if teh B+ looks too high....

I rewired teh heaters for teh 50C5's, got teh caps and resistors ordered,,, just need to find teh isolation transformers,,,

Regards,
John

 

[Eli Duttman]

The B+ can't come in too high. OTOH, if you lived in an area with out of spec., 130 VAC "HWM", mains 39 Ω dropping resistors would be needed.

Snubber caps. are used to suppress transients of various origins. The power switch situation you corrected is but 1 example. A "Death Cap." connects 1 side of the AC mains, or the equivalent, to the chassis. The unfortunate practice was common enough, before 3 wire (safety grounded) power cords became the norm. What's attractive about parts designated for snubber service is the high WVDC rating.

A Triad N-68X is very cost effective in the isolation transformer role. While I usually advise against reverse connecting the N-68's single secondary as the primary and its dual primaries as dual secondaries, it makes sense, in this situation. If a slight step down of the mains voltage occurs, it is of no consequence, but isolating the B+ PSU energy source from the heater string energy source is of considerable consequence. A superior and safe grounding of the 'X7 heater CT, via a 150 WVDC 'lytic, along with the noise suppression biasing off voltage divided B+ provides, becomes feasible.

 

[knockbill]

Thanks for replying,,, I was looking at that transformer, but its only 50va IIRC, tahts about 50W,,, Is that big enough for this project? The other thing is Digikey had them for 7.50, but won't have them restocked til August!!
I guess I'll bite teh bullet, and order it someplace else....
Regards,
John

 

[knockbill]

The B+ can't come in too high. OTOH, if you lived in an area with out of spec., 130 VAC "HWM", mains 39 Ω dropping resistors would be needed.

Snubber caps. are used to suppress transients of various origins. The power switch situation you corrected is but 1 example. A "Death Cap." connects 1 side of the AC mains, or the equivalent, to the chassis. The unfortunate practice was common enough, before 3 wire (safety grounded) power cords became the norm. What's attractive about parts designated for snubber service is the high WVDC rating.
A Triad N-68X is very cost effective in the isolation transformer role. While I usually advise against reverse connecting the N-68's single secondary as the primary and its dual primaries as dual secondaries, it makes sense, in this situation. If a slight step down of the mains voltage occurs, it is of no consequence, but isolating the B+ PSU energy source from the heater string energy source is of considerable consequence. A superior and safe grounding of the 'X7 heater CT, via a 150 WVDC 'lytic, along with the noise suppression biasing off voltage divided B+ provides, becomes feasible.

I've read schematics that use this transformer in reverse, as you stated,,, however, my concerns are with an unbalanced load, with teh heaters on 1 side, and B+ on teh other, as it is teh same winding, I can't believe the B+ and filaments draw the same current....seems it could overheat the transformer, and/or cause distortion....

Now, if I'm incorrect, it seems like the way to go...
Regards,
John

 

[Eli Duttman]

The 50 VA N-68X is plenty. Reverse connected, each of the dual windings is 25 VA. 25 VA is (sic) 208 mA. at 120 V. The series heater string draws 150 mA., which is obviously OK. Squeezing 115 mA. peak of B+ out of a 208 mA. RMS winding is OK too. NET heating of the N-68X should be well within the part's limit.

Order from Mouser. By and large, they offer the best overall deal. At least for now, Mouser, Allied, and DigiKey is the parts vendor "pecking order".

 

[knockbill]

I was thinking the three tubes draw 150mA each, times 3 = 450mA,, guess I'm forgetting my Ohms law!!
I didn't know how to figure teh B+,,, Ohms law again....
Glad to know you EE types!!!!!!!!
I'll order from Mouser, if they have them in stock....
Regards,
John

PS That was the problem,,,they are out of stock...

 

[Eli Duttman]

Order the N-68X from Allied, who has 35 pieces available. Bundle any other parts you need into the order. Minor price differences between vendors are more than compensated for by holding shipping costs down. Shipping costs can be very painful.

Glad to know you EE types!

Me an EE type? No sir! I have a 60 year long hobby interest in electronics. I was educated as a Chemist. Professionally, I'm an IBM mainframe computer System Programmer, who specializes in CICS. Yes, I'm still in the labor force. Damn Disney and his blasted dwarves! "I owe, I owe. Off to work I go."

 

[knockbill]

Order the N-68X from Allied, who has 35 pieces available. Bundle any other parts you need into the order. Minor price differences between vendors are more than compensated for by holding shipping costs down. Shipping costs can be very painful.



Me an EE type? No sir! I have a 60 year long hobby interest in electronics. I was educated as a Chemist. Professionally, I'm an IBM mainframe computer System Programmer, who specializes in CICS. Yes, I'm still in the labor force. Damn Disney and his blasted dwarves! "I owe, I owe. Off to work I go."

Yep,,, the shipping kills teh deal, most of teh time,,, problem I have is, I ordered everything I could think of this morning... Even some stuff taht I Might have a use for!!! Didn't think the trans would be the problem,,, There's a big antique radio show on this weekend,, If i can't find a trans there,, I'll bite teh bullet....

Well, you're way closer to an EE then I am, you know the math... I've been an electrician for 45 years,,, I fooled around with stereo equipment on and off, but never completely rebuilt any until I found that little Zenith amp in January,,, Then I recapped my Sansui 800, dug an old Heathkit AA1515 out of a barn, and got it cleaned up and running, and think I'm hooked...
A few months later, and I'm trying to redesign cheap, dangerous tube amps!!!
I have too many interests... not enough time to indulge them all!!!!

Thanks for talking me thru some of this stuff, I really enjoy the challenge of hearing them come to life...
Regards,
John

 

[knockbill]

The 50 VA N-68X is plenty. Reverse connected, each of the dual windings is 25 VA. 25 VA is (sic) 208 mA. at 120 V. The series heater string draws 150 mA., which is obviously OK. Squeezing 115 mA. peak of B+ out of a 208 mA. RMS winding is OK too. NET heating of the N-68X should be well within the part's limit.

I got the transformer over the weekend, and did a couple of tests, to determine the windings and see what voltage I got from it, the house mains are 121V....
1. Fed the Red (secondary) wires and got 104.6V each out of each set of windings (primaries)...

2. Fed the Red (sec) wires and spliced the Red+Black wires and the Grn+Yel wires, and got 104.6V out (pri)

3. Fed the spliced Red+Black, and Grn+Yel (pri) wires, and got 138.5V out...

This being an Isolation transformer, I expected teh voltage to be teh same either direction, but there is a 17V difference... The 104V seems too low to feed the heaters,,, and probably also for teh B+.... I could put bigger resistors in teh heater string to protect teh tubes,, but what about teh B+?

I didn't expect this transformer to be a buck and boost...
Any thoughts or suggestions are welcome,,,
Regards,
John

 

[Eli Duttman]

I expected a step down to occur, but not as large as that observed. Triad allowed for droop under load, when the part is wired "normally".

Actually, things are quite OK, with a reversed N-68X setup. Just remove the dropping resistors from the ends of the series wired heater string. 104.8 is well within 10% of 112.6 and it's on the low side. Expect the tubes to have long service lives. Wherever the B+ rail comes in, will be OK. The North American standard for house current is 105 to 125 V. You measured 104.8, which is just outside of "acceptable" and the forward drop in the OEM 1/2 wave vacuum rectified setup is considerably greater than that in a full wave SS bridge rectified setup.
Bottom line, ALL IS WELL.

 

[knockbill]

I expected a step down to occur, but not as large as that observed. Triad allowed for droop under load, when the part is wired "normally".

Actually, things are quite OK, with a reversed N-68X setup. Just remove the dropping resistors from the ends of the series wired heater string. 104.8 is well within 10% of 112.6 and it's on the low side. Expect the tubes to have long service lives. Wherever the B+ rail comes in, will be OK. The North American standard for house current is 105 to 125 V. You measured 104.8, which is just outside of "acceptable" and the forward drop in the OEM 1/2 wave vacuum rectified setup is considerably greater than that in a full wave SS bridge rectified setup.
Bottom line, ALL IS WELL.

Thanks for the reply,,, I will try reverse wired, both with teh heaters and B+ on separate windings, and also with teh heaters and B+ on teh same winding, as I originally wired it to get teh base readings...

In a previous post, you said "teh B+ can't be too high",,, so, would I be better off wiring it to get teh higher voltage, and leave teh resistors in teh heater string??

I cut out a few caps, and realize I need a higher voltage, so I'll report back when its fired up again...
Regards,
John

 

[Eli Duttman]

Just remember that you can't bias the series heater string off B+, when the B+ PSU and the string share a winding.

The O/P voltage of a "normally" wired N-68X will be very close to that of the AC mains, when it's loaded down. The reason for a slight difference in turns count is to compensate for that drooping effect.

 

[knockbill]

Update,,,,

I tried running the amp, with the trans wired in reverse, with two separate windings and "normally",,, It sounds best wired normally,,, however, I can't separate teh B+ and heater string, with only one winding. Wired in reverse, I can only get 104V out, which is low for the heaters,,, and only about 110V B+... and it also has a lot of hum and distortion... Not gonna cut it, unfortunately...
Wired normally, gets rid of teh distortion, hum is still there, but its pushing teh outputs to about 144V,,, The heaters are a little high, 127V, but I think I can lower that with a pair of larger resistors,,, I still have 6-7 caps to replace, when they get here, and then I'll tackle the hum...
Thanks fro the help so far,,,
Regards,
John