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Philco 91 ambiguities - Printable Version

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RE: Philco 91 ambiguities - Lead Fingers - 06-30-2026

In this photo of the restored Philco 91 code 221 (two speaker model) chassis you can see, in Wide Screen Technicolor, the darkened 136 ohm resistor on the BC resistor mounting rod, the one that is not shown in any of Philco's schematics, the missing information that started this thread.  The marked up schematic shows where the resistor properly goes - confirmed by rigorous testing.  Icon_eh

RE: Philco 91 ambiguities - morzh - 07-01-2026

If it were direct heated output tubes, I'd say it is the Cathode bias, but the Cathode are biased by the 4-5 section of the #46 resistor. Not sure why they separated the filaments and the centertap of the rectifier by that resistor. I guess they found out it somehow worked a bit better?

RE: Philco 91 ambiguities - Lead Fingers - 07-01-2026

Hi morzh,

Negative bias is supplied to the 37 detector amplifier, the 37 audio amplifier, and the 42 output tubes by feeding current from the negative side of the filter capacitors to a section of the 46A resistor.  The 42 output tubes have a combination of both fixed and cathode self bias.  It seems to be an exotic way of doing things but you know the old saying, "any port in a storm is what to look for."  It was a non intuitive problem to solve, a real head scratcher, and getting to it was less wit than luck. Icon_razz

LF

RE: Philco 91 ambiguities - Lead Fingers - 07-01-2026

See the photo for a better view of the invisible resistor in the Philco 91 code 221 two speaker model. Icon_e_ugeek

LF

RE: Philco 91 ambiguities - MrFixr55 - 07-06-2026

Hi Lead Fingers,

First off a beautiful job on what looks to have been a clean radio to begin with.

This is essentially cathode biased, but a little strange. For 2 42s or 6F6s in push pull class A or AB1, the cathode resistor is 340 Ohms. This may explain the 136 Ohms and the 205 Ohms in total. The strange part of this is that the difference between cathodes and grids is still only 205 ohms. If you add the 130 ohms between the HV secondary center tap and the 205 ohms, you get the required 340 ohms. Problem is that the extra 130 ohms is in the wrong place to add to the cathode resistor. I think that the important thing would be to measure between the cathodes and the driver transformer secondary center tap, and you have only -4.7V. It should be a lot more negative. Is the radio working? Is the speaker connected? Are the transformers good? You may be drawing too small a current. I would expect a grid bias between -16V and -24V depending on plate voltage. The more current, the more the voltage drop across the 205 Ohm resistor.

It is very important to not run a pentode with power connected to the screen but not the plate. Doing so will cause the screen to glow incandescent, then burn up, shorting the tube. This is visible as a bright light from the top of the tube that is not coming from the heater.

One possibility for where the r136 ohm resistor is placed is create a voltage difference that would prevent current flow from the cathode to the heater. This is a practice done with the introduction of the RCA 27 triode, the first indirectly heated cathode outside of the Western Electric / Bell System.

When 6V tubes were first introduced, around the time that this radio was introduced, the tubes were designed for car radios and rated for DC. Maybe this is an early set and they eliminated it in later models as they found that these tubes work just fine on AC without this scheme. Just my guess.

RE: Philco 91 ambiguities - Lead Fingers - 07-06-2026

Hi, Mr. Fixr55,

Your points are well taken.  Yes, the 42 cathode to grid voltage is in the vicinity of 15 volts.  Because of the 4th of July 250th anniversary of the Revolution hoopla Salute I put off doing a full voltage read out of the 91.  Yes, a bit odd way of doing things.  Perhaps the code 221 is a bastardized code 121, or the other way around.  Who knows?  Stay tuned for my next post in this thread.  Thanks.

LF

RE: Philco 91 ambiguities - Lead Fingers - 07-07-2026

Up coming is the voltage readings for my "ambiguous" Philco 91 code 221.  The values listed in Philco's voltage table for the model 91 receiver is part of the "ambiguity" narrative.

For those who have little or no experience accurately taking voltage readings in an operating receiver (Beware of deadly voltage levels!) the following is predatory:

How did Philco ascertain the voltage values for it service literature voltage tables for a correctly functioning receiver?  Was the receiver tuned to a signal generator signal of a certain level, or were the antenna terminals open, connected to a load resistor, or shorted?  What type of voltmeter was used to make measurements to establish the values for their Model 91 receivers?  Without any extenuating knowledge I'm presuming they used a typical technician grade volt meter such as their Model 020 volt meter (schematic attached).

Study the schematic to ascertain the meter's sensitivity and loading effect on circuits depending on the voltage range selected.  Knowing the loading effect when rationalizing the error the the voltmeter causes by its loading effect on high impedance circuits such as audio amplifier grid and plate circuits is the bane of fast and accurate circuit operating assessments for those with a weak understanding of electrical laws - present company excluded, of course. Icon_lolno

For example, on the 10 Volt range the loading effect is 50,000 Ohms, which translates to 5000 Ohms per Volt.  When measuring a few volts of grid bias the reading will be vastly incorrect and will require correction by Ohm's Law attention if perfection if preferred. Icon_cry

My next post will be the readings taken with my Knight Kit KG-620 VTVM referenced (calibrated) to my Fluke 77 DMM, or directly with my 77.  The loading effect of the Knight is 11 megohms per volt, the 77 is 10 megohms per volt.

OH!  By the way, a SAFETY BRIEFING is n order:  The common lead on a VTVM of the Heath and Knight variety is connected to the case.  Connecting that lead to anything other than chassis ground is begging for injury or worse.  To measure in circuit voltage differences take two readings referenced to chassis and subtract to get the difference, ....and stay alive. Icon_wave

Stand by for further...

LF

RE: Philco 91 ambiguities - Lead Fingers - 07-07-2026

Attached is the Philco 020 voltmeter schematic.

LF

RE: Philco 91 ambiguities - Lead Fingers - 07-07-2026

The Philco VOM model 020 schematic....

   

LF

RE: Philco 91 ambiguities - Lead Fingers - 07-09-2026

Speaking of test equipment, I thought this would be useful information, as it seems some of you do circuit analysis.  I find it often necessary to be able to measure current in a circuit, without having to make alterations that change circuit operating conditions, by use of a clip-on current probe.  For those who want to know what that is see the attached Icon_eek .

I've owned two HP-428A's for some years and find them necessary for doing serious audio work, both tube and transistor.  Here's where you can download the 428A manual:  

https://www.lost-manuals.com/en/manufacturer/hewlett-packard/428a#12901

LF

RE: Philco 91 ambiguities - Lead Fingers - 07-09-2026

This device:

LF

RE: Philco 91 ambiguities - MrFixr55 - 07-09-2026

Correct me if I'm wrong but I don't think that when the 91was made (1932) that anyone had produced a VTVM. The Rider Channelyst came out in 1940. the Voltohmyst much later. The Philco 20 VTVM was likely 1940s based on the Loctal tubes. I think that the best they had at the time was a 5,000 Ohm/Volt meter.

RE: Philco 91 ambiguities - Lead Fingers - 07-09-2026

Just for the sake of a sanity check these HP 428A/B meters are often available on eBay SANS PROBE.  However I did find a possible source for newly made replacement probes going for $170.  Icon_smile

LF

RE: Philco 91 ambiguities - MrFixr55 - 07-09-2026

Lead Fingers, please keep posting, The quality of your work is fantastic and is exceeded only by your knowledge.

RE: Philco 91 ambiguities - Lead Fingers - 07-09-2026

Thanks, friend.  It's what I do to stave off the grim reaper! Icon_lol