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Old 6th May 2013, 04:46 PM   #1
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Default 4tube Philco amp project...

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

Last edited by knockbill; 7th May 2013 at 03:16 AM.
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Old 7th May 2013, 01:30 AM   #2
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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.
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Old 7th May 2013, 02:24 AM   #3
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Quote:
Originally Posted by Eli Duttman View Post
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
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Old 7th May 2013, 06:45 AM   #4
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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+.
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Old 7th May 2013, 01:06 PM   #5
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Quote:
Originally Posted by Eli Duttman View Post
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

Last edited by knockbill; 7th May 2013 at 01:12 PM.
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Old 7th May 2013, 07:02 PM   #6
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Default 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...
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Old 7th May 2013, 09:07 PM   #7
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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".
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Old 7th May 2013, 11:48 PM   #8
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Quote:
Originally Posted by Eli Duttman View Post
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
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Old 8th May 2013, 12:44 AM   #9
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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.
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Old 8th May 2013, 12:56 AM   #10
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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
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