12AX7 heaters in paralell with bias resistors

[coman61]

Hi fellas
I got another newbie question. I'm working on an old fisher console pull. The amp has some 12AX7 heaters In paralell with the bias resistors for the EL84s. These 12AX7s are in the pre-amp tuner that I don't have.
Here is my question can I just measure the resistance between pins 4 and 5 on the 12AX7s and add these together for the missing resistor or is there more to it?
BTW the dynaco I was working on sounds quite good with the ADS cap mod board. Way better then I expected. TIA Yellow_Light_Colorz_PDT_20

[cartoonweirdo]

I'm not sure exactly what you mean by heaters in parallel with the bias resistor (at what point is the heater supply coupled to ground?) but wonder if someone didn't use the cathode (bias) voltage to reduce heater noise.

[coman61]

Well I think you may be right. All the EL84 cathodes are tied together. There is a 100uF cap going to ground. This is in Parallel with 68K resistor that is in series with the bias pot, 5K and that is also in series with a 10K resistor that goes to ground. The wiper of the pot connects to a voltage divider of 2, 330K that is tied to 1/2 the 12ax7 coupling caps from the plate. And all this is in parallel with 3 heaters in the the pre amp, that I don't have. 2 of the ax7s are for the phono input. the other is for bass control. These 3 ax7s heater are in series,... which end up going to ground.
I'm thinking that by leaveing these 3 heaters out of the circuit, this going to mess the bias up?? No? I measured the resistance across the heater of a 12ax7 and its 18 ohms. 18x3=56 ohms. I think this would be sugnificant?
Maybe I will need to send someone the schematic.

[cartoonweirdo]

I see... so the EL-84's are cathode biased, and you can adjust the bias via the pot. Feedback from the cathode of the power tubes is being aplied to the plates of the 12AX7's, and the heaters for the preamp tubes are connected in series with one end grounded, and the other end conected to the top of the cathode resistor......

I have two questions.

Is there a transformer winding conected to the heaters, or could it be that the voltage at the cathode IS the heater voltage (isn't three 12V heaters in series around 36 volts?).

What's up with the bias resistors? Last time I checked 68k, 5k, and 10k add to rather a large bias resistor on their own. However, in parallel with the theoretical 56 ohm load (did you measure when the heater was hot didn't you?) they basicly disapear as far as the tube is concerned. The bypass cap should still work. Even so I am a bit confused.

Perhaps this could be solved with a little expiriment. Put some crap 12AX7s in the socket (you only care about their heaters). Fire up the amp and measure the cathode voltage. Put the amp where you can get to the On/Off switch quickly and pull out any 12AX7 you want. Watch for fuses popping, tubes glowing red, big humming noises, strange smells etc. If anything untoward occurs imediately switch off the amp. If nothing bad happens measure the cathode voltage again.

If you would rather be as safe as possible take the time to disconect the 12AX7s and put a 100 ohm pot in where they used to be. Adjust the pot to read 56 ohms fire the amp and see what happens to cathode voltage with a small adjustment of the pot.

Let me know how it turns out.

[coman61]

Yes the heaters are connected to a Xformer winding. And according to the schematic the voltages on the cathodes of the EL84s should be about 33 Volts so you seem to be on the right track. I did not measure the resistance on the 12ax7 while hot, I didn't know that I should have. In fact I have not even powered this amp up yet as I'm still tring to gather information on it. There is very little information on the net that I can find. I got the schematic from some guy on audiokarma and I think he is struggling with his.
I do like the 100 ohm pot trick. I think I will give it a try.

[mesherm]

It was a common practice in the old days to use the preamp tube heaters as part or all of the cathode bias resistor(s). It was free DC current.
A typical 12AX7 filament is 84 ohms hot from 4 to 5 and 21 ohms hot from 4,5 to 9

[coman61]

So does this mean I will need to put a 250 ohm or there abouts back in the circuit if I don't use the Fisher pre amp. The goal here is to make the amp a stand alone amp so I can use any pre amp. But not the fisher, as I said I don't have one, just have the amp only. Hell this may all be for not. The OPT on this thing are kind of girly. Just wanted to see how it would sound.

[erichayes]

Do you have a model or chassis number?

The 12AX7s are going to want to see somewhere around 30~36VDC across them. That's a given. Another given is that the heater current is 150 mA. Therefore, the total resistance of the heaters is 36 (nominal) divided by 0.15, which is 240Ω. I'd use a 240 or 250Ω 20W wirewound to be on the safe side.

The four EL84s will draw a combined cathode current of around 230 mA with 300 volts on the plates, so that pot might be to rope in the correct current.

[EWBrown]

I've seen this approach of using a 12AX7's filament as an EL84 cathode resistor, this accomplishes two things, it sets the cathodes at approximately 12.6VDC and 150 mA for the four conjoined cathodes, whuich sets their combined cathode current around 140-150 mAm so matched tubes are a must, and it supplies "free" DC filament power for the phono stage filament, a cost effective (read that as chapskate's) method of redicung AC hum in the high gain stage.

There are variations with other resistors / pots for bias tweeking and balancing , but most of those that I've experienced simply used the 12AX7 filament and a bypass cap.

I suppose that a "less than perfect" 12AX7, 12AU7 or 12AT7's (or any other 12.6V / 150 mA tube) filament could serve as the 95 ohm, 5W resistor as used in the original ST35 and SCA-35 designs. No sense tossing out a perfectly good "ballast" resistor.

As Gary Kaufman says, there is no totally useless tube, even a 1B3GT or 2X2 filament can serve as a fuse :o Yellow_Light_Colorz_PDT_04

/ed B in NH

[coman61]

Eric
the amp is a chassis #480-A and the pre\radio is a Chassis #590-T. Even after I get this straightened out, I have to wonder how the rest of my voltages will be seeing as this units PT was also used to power the Pre\tuner, a TT and a MPX adaptor.
This just a trainning exercise as I'm prepairing for my first scratch build. I have been collecting vintage iron for awhile now and it time to put some of it to use rather then collecting dust.

[coman61]

So Ed your saying that this method of design is not the most desirable and not something I want to copy in my future scratch build, Yellow_Light_Colorz_PDT_02

[EWBrown]

I would probably skip this method for a scratch build, the manufacturers were always trying to scrape a few cents off the production costs, so they cut corners and somehow managed to squak by as "good enough" for most consumers' tastes.

I do go with the more "standard" and recognized as technically sound methods of cathode (or even fixed)biasing fo rthe EL84s and power the low signal / high gain filaments with DC voltage, 12.6VDC at 150-300 or so mA is pretty easy and inexpensive to add in.

The rest of the filaments can be AC, there should be no 60 Hz AC problems.

/ed B in NH

[coman61]

Do you have a model or chassis number?

The 12AX7s are going to want to see somewhere around 30~36VDC across them. That's a given. Another given is that the heater current is 150 mA. Therefore, the total resistance of the heaters is 36 (nominal) divided by 0.15, which is 240Ω. I'd use a 240 or 250Ω 20W wirewound to be on the safe side.

The four EL84s will draw a combined cathode current of around 230 mA with 300 volts on the plates, so that pot might be to rope in the correct current.

Dang Eric, ya sure that sucker has to be a 20W resistor? :o

[EWBrown]

230 mA seems very high for a cluster of 4 EL84s, as that is 57.5 mA each, assuming a good matchintg balance. typically they cruise along around 35 to no more than 45 mA each, and 45 mA is really beating on them on them pretty hard (that is entering into 7189A or 6P14P-EV territory)..

The voltage typically would be 12-15V above ground. for cathode biasing EL84 / 6BQ5s.

36V and 230 mA sounds more like the correct figures for EL34s / 6CA7s, or perhaps 6L6GCs.

/ed B in NH

[erichayes]

That's what I thought too, Ed, and that's one of the problems when using heaters as cathode resistors in AB output stages. If you use quiescent current to calculate--Ik = 160mA--you come up with a completely different set of numbers, but more in line with 3 12AX7s in series.

And that, in itself, bothers me: 36 volts on the cathode? Those models aren't in my schematic library, but I might go through the photofacts to see if I can find something similar.