Series heaters amp question

[AmberB]

I was recently given an amplifier board out of an old GE record player.  Of course, there was no isolation transformer, the original was wired straight to the plug.  I'm in the process of calculating the miliamp load of the amplifier so that I can buy an isolation transformer that has enough VA without buying something bigger than I need. 
There are 2 tubes in this amp, a 35W4 half wave rectifier and a 35EH5 pentode.  This little amp amplified the signal directly from  a crystal cartridge on the record player. 
Both of these tubes use a 1.5ma heater.  I understand how to do the math for the load resistor to bring the load to 120 volts at 1.5 ma.  My question, and perhaps the answer should be obvious, but for the sake of assurance, does the total load on the heater string stay at 1.5 ma for the purpose of calculating the isolation transformer rating?

The way this amp was originally set up, the heaters were in parallel and the heater power went through the turntable motor for the rest of the voltage load, first time I've seen a schematic like that.  I plan to add a 12AV6 for an input triode, and place all the heaters in series.  Then I just have to calculate the load resistor for the heater string to dump 120 volts at 1.5ma.

[Williamblake]

https://www.radiomuseum.org/tubes/tube_35eh5.html
The heater current is 150mA.

[AmberB]

Yeah, I had the heater current written wrong...

[AmberB]

I calculated the loads for the tube heater string and the volt-amp rating for the tubes for an isolation transformer.  Given 3 tubes, the 35EH5 pentode, the 35W4 half wave rectifier, and a 12 AV6, or 12AT6, depending on which one I choose to use, I'm dropping 82 volts on the heater string.  At .15 amps for the heater string, and 120 volt heater supply, I need to drop 38 volts at .15 amps.  That ends up being a 253 ohm, 5.7 watt resistor, more or less.  I think a 250 ohm, at least 6 watt, and probably 10 watt resistor should work for that.
As for the isolation transformer rating, I'm not sure that I got all the milliamp data for the 3 tubes, but I got what the RCA manual gave me.  Combining the tube plate currents and the heater current, I came up with approximately .3045 amps.  Figuring the 120 volt secondary of an isolation transformer and the .3 amps, I came up with 36VA. 
In my search for small isolation transformers, I found a 35 VA and a 50 VA transformer, and other larger ones.  I'm guessing that the 50 VA transformer will give me a decent margin of error for current capability.
Hopefully I did the math right...

[AlNewman]

I recently rebuilt a crosley all american 6 radio that had the heaters in series.  The voltages of the heaters added up to basically 120v.  I'm not sure how that calculates as far as amperage across the circuit, but I would think the amps are cumulative, but the voltage drop must be constant between heater windings to the point of ground.  Perhaps the difference is in the crosley circuit you have an unlimited (10-20) amp breaker to mains?

It must be somewhat the same idea as what you're starting with, although I'm not sure how it would have worked in parallel, do you have the original schematic?

[PRR]

In series, all the heaters "must" draw the same current. (There are tricks but add cost and heat.)

Your odd 6-tube is 118V 0.15A heater string.

The GE record player is a radio minus the tuner. The two big tubes are most of the demand so should be similar. However GE made a 70V supply (by over-winding the phono motor; they made their own windings). Without such a trick, as you calculated, there is 5.7 Watts of heat to shed to run it on 117V. Resistor catalog ratings are short-term, maybe a month straight. If you are just forking-around, a 5.7W resistor may last long enough to be fun. If you do not like smoke and repeated repairs, the usual rule is to "double and round-up". So 5.7W actual heat wants over 11.4 watts of resistor.

In some long-gone past a box of 47r 2W resistor fell off a truck in front of my bicycle. I did re-rig a radio to a phono with a huge string of 2W resistors nailed inside the case.

You could use a 6V transformer, silicon rectifier for B+, and 6Y6 audio output;
https://www.vivatubes.com/nos-ge-usa-6y6gt-black-plate-top-get-vacuum-tube
B+ from the small isolation transformer. You could even re-rig the 35W4 socket for 6SL7 etc, not have to bash a new hole in the chassis. GAH!! 6SL7 sells for stupid prices; yea, do the radio diode-triode in a miniature.

[TenderTendon]

In some long-gone past a box of 47r 2W resistor fell off a truck in front of my bicycle.

If this actually happened, I'd love to hear the whole story...

[kagliostro]

In some long-gone past a box of 47r 2W resistor fell off a truck in front of my bicycle.

Those are happenings that can change the cours of a life and I don't say it as a joke

Franco

[AmberB]

In series, all the heaters "must" draw the same current. (There are tricks but add cost and heat.)

Your odd 6-tube is 118V 0.15A heater string.

The GE record player is a radio minus the tuner. The two big tubes are most of the demand so should be similar. However GE made a 70V supply (by over-winding the phono motor; they made their own windings). Without such a trick, as you calculated, there is 5.7 Watts of heat to shed to run it on 117V. Resistor catalog ratings are short-term, maybe a month straight. If you are just forking-around, a 5.7W resistor may last long enough to be fun. If you do not like smoke and repeated repairs, the usual rule is to "double and round-up". So 5.7W actual heat wants over 11.4 watts of resistor.

In some long-gone past a box of 47r 2W resistor fell off a truck in front of my bicycle. I did re-rig a radio to a phono with a huge string of 2W resistors nailed inside the case.

You could use a 6V transformer, silicon rectifier for B+, and 6Y6 audio output;
https://www.vivatubes.com/nos-ge-usa-6y6gt-black-plate-top-get-vacuum-tube
B+ from the small isolation transformer. You could even re-rig the 35W4 socket for 6SL7 etc, not have to bash a new hole in the chassis. GAH!! 6SL7 sells for stupid prices; yea, do the radio diode-triode in a miniature.

I scrounged up a couple of 250 ohm, 10 watt resistors from my stash to use for the heater string for now.  If the resistor gets hot, I'll have to order a 15 watt resistor for a long term solution.
All the circuit parts for this little amp are mounted on a phenolic board and both tubes are on PC board 7 pin sockets.  Rewiring the socket pins for different tubes would be possible, but having to change socket types would basically mean starting over with a different board.  I don't actually have the old chassis, I was just given the circuit board mounted on a piece of wood.  I'll have to post a picture of it so you can see what I'm working with.
I do have a schematic and PC board layout for the thing.  The person who gave it to me had ordered the schematic from the Radiomuseum.org website, and he gave me that with the board.  He likes to tinker with vintage radios.  I guess he didn't have much use for this amp because it's not part of a radio...

[AlNewman]

It is an odd radio, but it is the best working and coolest looking one I have done to date.  I picked it up for $20 on marketplace.  Gave it a polish, redid the caps and dropping resistors, repositioned the shrunken cellulose viewscreen...
Works awesome. 

[PRR]

Works awesome. 

Not a shock. This has an RF stage before the converter and one IF stage. On today's crowded and crapped-up AM band, that extra RF filtering makes a major difference on marginal reception. From here (far coastal Maine), few radios will pull Boston, much less NYC. All intermodulation hash from other stations and also bad power lines. But I got a postwar radio which pulled Boston easy, WCBS in NYC by twilight. The Elders knew how to do it! But those who came after knew how to pinch a penny.

[AlNewman]

So just out of curiosity, as you mentioned in series all the heaters need to run at the same current.  In my mind that would make the current in parallel, with a series voltage drop across the resistors.

How would you run the same type of design with the heaters in parallel, to get the same result?  Would it be a separate ohms law calculation for each heater in the string?  With decreasing voltages as you went down the chain?

[PRR]

> In my mind that would make the current in parallel, with a series voltage drop across the resistors.

I don't understand what you are saying.

Series things should all use the same current.

Parallel things should all use the same voltage.

I'm sure you don't want to ask for a private lesson. This has been covered so many times before.
https://www.allaboutcircuits.com/textbook/direct-current/chpt-5/what-are-series-and-parallel-circuits/

[AlNewman]

I don"t mind taking some public lashings.

Series things should all use the same current.

And at that current, they drop voltage in the circuit according to their resistance.

Parallel things should all use the same voltage.

And at that voltage, they accumulate current in the circuit according to their resistance.

I guess when you mentioned GE made a 70v supply, did you mean by leaving 70v behind for the 2 35v tubes by building a 50v phono motor?  I'm not sure what Maine english is, but you talk in it a lot.

[AmberB]

Actually, in this old GE record player amp, the heaters were run in parallel.  The voltage went through the phono motor to the heaters, and then to ground.  Since both tubes are 35 volt heater tubes, that would work.  How they dropped the rest of the voltage through the motor to get down to 35 volts for the heaters is beyond my understanding.

[AmberB]

I was looking for a series heater string type high mu audio service triode or twin triode that has a .15 amp heater to use in this little amp for the input triode.  I couldn't find one in the RCA manual, so I chose a 12AV6 to use.  If the heater string was .100 amp, there are a couple of choices that will drop more than 12 volts, such as the 20EZ7 twin triode, or the 18FY6A single triode dual diode tube.
Another thing that I noticed is that most of the series string audio output tubes use a relatively low impedance output transformer.  It seems that the beam power tubes mostly use a 2500 ohm primary, and the power pentodes mostly use a 3000 ohm primary.
I have not been able to find any low wattage replacement output transformers in those values.  You can get a 2500 ohm primary by changing the output load on a 5K output xfmr, but I also haven't found any 6kohm low wattage output xfmrs.  That makes me wonder, what do the radio rebuilders use if they have to replace one?

[sluckey]

I was looking for a series heater string type high mu audio service triode or twin triode that has a .15 amp heater to use in this little amp for the input triode.  I couldn't find one in the RCA manual, so I chose a 12AV6 to use.

Really! Did you consider the very rare 12AX7, or any of his cousins? 

[AmberB]

I was looking for a series heater string type high mu audio service triode or twin triode that has a .15 amp heater to use in this little amp for the input triode.  I couldn't find one in the RCA manual, so I chose a 12AV6 to use.

Really! Did you consider the very rare 12AX7, or any of his cousins? 

Why, yes, yes I did!  :-)
Actually, I was looking for a triode or dual triode that would dump more than 12 volts in the heater string.  The less volts left over from 120, the smaller load resistor necessary in the series heater string.
I think I only need 1 triode in the preamp for this little amplifier.  I dislike using a dual triode tube when I only need 1 triode.

[PRR]

...both tubes are 35 volt heater tubes, that would work.  How they dropped the rest of the voltage through the motor to get down to 35 volts for the heaters is beyond my understanding.

Make a simple induction motor. You have a laminated iron core, and a coil wound over it.

Wrap the coil with tough paper, and wrap another winding over the first. This will be a transformer.

If we want 120V line and 35V side-supply, we might wind 1,200 and 350 turns to get the right proportion. The actual turns count needs more math but the concept is there.

You note that this Dayton motor's coil is already pretty full. To also feed heaters we would pick a larger iron core and coil former. Say 10W for spin and 10W for heat, use a core for a 20W motor. (I have seen from 3W for a clock to 69W for a duct fan, and they may go higher.)

[AmberB]

I finally got around to taking a couple of pictures of the the "amp", which at this point is just the circuit board and attached output transformer.  The person who gave it to me had removed the circuit board from the original record player and put it on a board to experiment with it.  I'm guessing that he trashed the rest of the thing.
I wasn't sure of how useful the output transformer would be because of the way its shown on the schematic, so I tested it and came up with an impedance ratio of approximately 3300 ohms reflected into a 4 ohm speaker load.  That fits well with the 3Kohm suggested load for the 35EH5 in my RCA tube manual.  One less thing to have to find...
This design has a really weird tone control, I've never seen another one like it.

[AmberB]

I decided to make a custom board to mount a 7 pin PC mount socket onto to use for my single triode input preamp.  I had a couple of small pieces of what is probably phenolic board in my stash, so I marked and drilled holes in it for grommets to solder the various things to.  I'm not done with it yet, I still need several more grommets for the rest of the circuit that I want to mount on the board.  I'm going to add another filter section to the amp just for the preamp tube.  I'm going to mount the capacitor and resistor for that on the board along with the rest of the parts.
If it turns out that 1 triode is not enough to push the 35EH5, I'll just make another board with a 9 pin socket for that.
My hole drilling for the grommets for the 7 pin socket could have been a bit better, but with a few modifications of the grommets, none of them are too close together for my comfort...

[PRR]

I was wrong. This is an 82V phono motor in series with 35V of heater. Never saw that done.

With that heater scheme I'm not sure how you are going to power an added triode, except with an added heater transformer (or these days, a 12VDC wall-wart).

That RC across the OT primary was THE tone circuit for millions of radios. The combination of pentode and voice-coil speaker tends to a rising "shrill" response. A cap alone is a dropping "dark" response. Adding a resistance can give a fairly good correction; variable resistor gives the user another knob, makes the set "more valuable".

We don't do this on "real" amps because at 300V the R and C would die quickly; it is practical at 125V as here.

[AmberB]

With that heater scheme I'm not sure how you are going to power an added triode, except with an added heater transformer (or these days, a 12VDC wall-wart).

My plan for the heater string is to cut the foil that connects pin 4 of the 35EH5 to ground and run a wire from there to one of the heater pins of the 12AV6, and the other heater pin of the 12AV6 goes to ground.  Heater pin 3 of the 35EH5 is connected to heater pin 4 of the 35W4 on the board, that is originally the parallel heater voltage connection from the motor.  I'm going to disconnect the wire that feeds that copper foil trace so it's only a connection between between pin 3 of the 35EH5 and pin 4 of the 35W4.  Pin 3 of the 35W4 is the other heater connection, that originally also goes to ground, but that copper foil trace is connected to ground with a jumper wire.  I'll disconnect that jumper to ground and feed the heater voltage from the dropping resistor to that connection.
So, the hot side of the heater voltage will go through a 250 ohm, 10 watt resistor to pin 3 of the 35W4, from there out of pin 4 of the 35W4 to pin 3 of the 35EH5.  From pin 4 of the 35EH5 a wire will go to one heater connector of the 12AV6, and the other heater connection of the 12AV6 goes to ground.  If I calculated the heater load resistor correctly to drop 38 volts at .150 amps, that should properly drop the 120 volts through the series string.  If I calculated incorrectly, things could get "interesting"...

[AlNewman]

My apologies.  My keyboard had too much to drink the other night.  I realized the next day what it said, and realized the motor must create an 85v voltage drop.  Bad keyboard, go home.
And although I would certainly benefit from a private lesson, I'm not gung ho about the 200 ohms law calculations per day.

[AmberB]

Just an update on this silly amplifier idea that I thought some of you might find amusing. 
I didn't know what I was going to mount these circuit boards in, as well as the other parts.  I was thinking about it and I thought about how a lot of the older heater string radios are in wood cabinets, so I decided to see if I could find a wood box to mount the parts into.  I found a nice sized wood box with a lid in the crafts section of the local big box store, so I bought one to see how stuff would fit in it.  I took a couple pictures of the results so far.  It's not finished yet, but it's mostly done.
One thing to wonder about, since there's no steel chassis to ground, is it still advisable to use a 3 wire power cord for the isolation transformer?

[kagliostro]

Usually double isolated apparatus don't require to have the earth connection wire

But, told that, I must say you are renouncing to the shield effect of a metal enclosure

A preamp can easily keep unwanted noise signals and amplify it

Can't you put some aluminium think foils on the inside of your wood chassis ?

Doesn't matter very much which is the thickness what is of interest is to form a shield around the circuit, the machanical stress is supported by the wood

Franco

[sluckey]

One thing to wonder about, since there's no steel chassis to ground, is it still advisable to use a 3 wire power cord for the isolation transformer?

Yes. Connect the green wire to the floating ground. The idea is to have the input jack connected to earth so your guitar strings will also be connected to earth.

[kagliostro]

 

I forgot the input,  this is a guitar amp not a radio or record player

Franco

[AmberB]

One thing to wonder about, since there's no steel chassis to ground, is it still advisable to use a 3 wire power cord for the isolation transformer?

Yes. Connect the green wire to the floating ground. The idea is to have the input jack connected to earth so your guitar strings will also be connected to earth.

Would it also be advisable to ground the frame of the isolation transformer and the output transformer to the main grounding point?