Hi there! I've decided my precious designs were a bit to complicated for my skill level. I've changed course to a tube amp using 1930s vacuum tubes I have one hand. Hopefully I can use it for listening to music. Anyway I have a full design and was wondering if I could get a second opinion. Thanks in advance! The tubes are a pair of 26, a 80 rectifier, and a 41 pentode.
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Do you like ancient radio "tone"?
No bass, no treble, but distortion is high.
No free lunch. Three series 100nF capacitors in signal line and output tube's enourmous high Ri (even with 10k OPT) makes "music box" tone.
If you see "41" datasheet, even with optimum (8k) load the distortion is 11% (4.5W output).
The simulation (6K6 is "almost" identical to 41) shows similar values.
No bass, no treble, but distortion is high.
No free lunch. Three series 100nF capacitors in signal line and output tube's enourmous high Ri (even with 10k OPT) makes "music box" tone.
If you see "41" datasheet, even with optimum (8k) load the distortion is 11% (4.5W output).
The simulation (6K6 is "almost" identical to 41) shows similar values.
If it were me I'd build a 2 stage amp with a high mu output tube. My daily amp is 47 into EL12n which is a European tube. Since you're in the USA you can use a 7591. That makes your job much easier. The 26 will be fine as an input tube, and only one required. You can use the 7591 in triode if you prefer. I once had a McIntosh amp with 7591 outputs and it sounded great. Good tube.
I would definitely use Rod Coleman filament regs for the 26 and use it in filament bias. Rod is used to this and can give you the complete build instructions with the kit. Get it at Lyrima, not expensive and very, very good.
I would definitely use Rod Coleman filament regs for the 26 and use it in filament bias. Rod is used to this and can give you the complete build instructions with the kit. Get it at Lyrima, not expensive and very, very good.
Really cool!
What radio are we exactly talking about?
For these oldies photos are mandatory
I work on quite some old tube radius myself. I personally find them real pieces of art and will take you back into full nostalgia mode.
Besides for me it's very relaxing to work on, after a week of staring at complicated multi layer pcb designs.
What I personally like to do are two things
Either bring them back in original state, but only when (financially) possible.
Otherwise design my own working design around it with original tubes and parts (if possible) or similar at least.
Depending on the power supply and space I usually add an AUX input, phono as well as Bluetooth.
I can comment a bit better on your current schematic when I know more about the radio.
What radio are we exactly talking about?
For these oldies photos are mandatory
I work on quite some old tube radius myself. I personally find them real pieces of art and will take you back into full nostalgia mode.
Besides for me it's very relaxing to work on, after a week of staring at complicated multi layer pcb designs.
What I personally like to do are two things
Either bring them back in original state, but only when (financially) possible.
Otherwise design my own working design around it with original tubes and parts (if possible) or similar at least.
Depending on the power supply and space I usually add an AUX input, phono as well as Bluetooth.
I can comment a bit better on your current schematic when I know more about the radio.
I was thinking I could use one of those cheep Chinese 5k 5w output transformer but run the 4 ohm winding with a 8 ohms speaker for 10k impedance. https://www.amazon.com/QONIA-Amplif...6p1&qid=1692103028&sprefix=6p1,aps,144&sr=8-8
Yeah I was thinking of a single toob regenerative receiver. I'll need to look at designs for old receivers. I have a 71a triode and 6d6 pentode I could maybe use for that.
Yeah I was thinking of a single toob regenerative receiver. I'll need to look at designs for old receivers. I have a 71a triode and 6d6 pentode I could maybe use for that.
Those 70/100V transformers work pretty well, but are only useable for PP.
Otherwise Edcor has affordable things.
Or something like this:
https://www.amplifiedparts.com/products/transformer-output-8-w-single-ended
I haven't checked the datasheets yet, so first have to check the loadlines.
b_force; and unrealcrafter2,
That transformer does not specify how much quiescent DC current.
At 0.7 Pounds, with the output stage DC current, it might be saturated even before any low frequency signal is present.
With the 41 in Pentode mode, the damping factor will be non-existent.
Much much less than 1.
But old radios worked that way, with a single 'full range' loudspeaker (the full range of the day).
I like andyjevan's Idea of triode wired.
Old pentodes, like the 41 might not work very well that way because of the low maximum screen voltage.
The screen could work at a slightly higher than specified voltage (the max screen voltage spec is for Pentode operation).
Instead, in triode wired mode, when the control grid 1 voltage increases, the plate current increases and the screen current increases,
but the screen voltage is reduced, just the same as the plate voltage (versus in pentode mode where the screen voltage is constant, but the screen current increases).
An example of Beam Power mode (and Pentode mode) operation, versus Triode wired mode:
807 Beam Power mode, screen max voltage = 300V
807 Triode wired mode, screen max voltage = 400V
Quiescent specs:
41
max plate voltage, 315V
max screen voltage, 285V
You might be able to use the 41 in triode mode, with 300V plate & screen (voltage to cathode).
The B+ can be higher than 300V to make up for the voltage drop in the DCR of the output transformer primary, and if you use a self bias resistor and bypass cap (instead of the fixed adjustable bias), the voltage drop in the self bias cathode resistor.
Triode wired mode gives less power output, but often is lower distortion, and . . .
Triode wired mode, will give a damping factor greater than 1 (greater than unity).
That transformer does not specify how much quiescent DC current.
At 0.7 Pounds, with the output stage DC current, it might be saturated even before any low frequency signal is present.
With the 41 in Pentode mode, the damping factor will be non-existent.
Much much less than 1.
But old radios worked that way, with a single 'full range' loudspeaker (the full range of the day).
I like andyjevan's Idea of triode wired.
Old pentodes, like the 41 might not work very well that way because of the low maximum screen voltage.
The screen could work at a slightly higher than specified voltage (the max screen voltage spec is for Pentode operation).
Instead, in triode wired mode, when the control grid 1 voltage increases, the plate current increases and the screen current increases,
but the screen voltage is reduced, just the same as the plate voltage (versus in pentode mode where the screen voltage is constant, but the screen current increases).
An example of Beam Power mode (and Pentode mode) operation, versus Triode wired mode:
807 Beam Power mode, screen max voltage = 300V
807 Triode wired mode, screen max voltage = 400V
Quiescent specs:
41
max plate voltage, 315V
max screen voltage, 285V
You might be able to use the 41 in triode mode, with 300V plate & screen (voltage to cathode).
The B+ can be higher than 300V to make up for the voltage drop in the DCR of the output transformer primary, and if you use a self bias resistor and bypass cap (instead of the fixed adjustable bias), the voltage drop in the self bias cathode resistor.
Triode wired mode gives less power output, but often is lower distortion, and . . .
Triode wired mode, will give a damping factor greater than 1 (greater than unity).
Using 41 in triode mode, the developing is much easier: the distortion decreasing (due to the Ri decreasing) -even with 5k:8 OPT-, the bandwidth will be better.
But the output power in A1 region not too much, and linear range is narrow.
This tube "begging" for A2 region ... but this type 26 based driver stage is unfit for this.
But the output power in A1 region not too much, and linear range is narrow.
This tube "begging" for A2 region ... but this type 26 based driver stage is unfit for this.
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Biasing Simplified. In this one the bias for each tube is set by the network R10, R11 & R12, all bypassed by C3.
This kind of biasing was common during the late 1930s.
The amp is all 2V filament tubes. the filaments run from a common supply. The 33 PP 0utput can be driven into Class AB2.
Max output is a little more than 4.5W. This one built about 20 years ago & still sounds good.
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Typical Back Bias Circuit in Philco Radio about 1938. The back bias voltage is developed across the resistors 13 & 23 in the PS, lower RHS.
A 10 microF cap '6' provides bypassing so that the circuit does not oscillate. There were many of these, in some the back bias voltage was
developed across the resistance of the loudspeaker magnet winding thru a voltage divider.
A 10 microF cap '6' provides bypassing so that the circuit does not oscillate. There were many of these, in some the back bias voltage was
developed across the resistance of the loudspeaker magnet winding thru a voltage divider.
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Hi John,
I have tinkered with a few of those old radios, and noticed that often there was a choke and the voice coil in series, with a resistance divider across the voice coil to set the bias voltage. The interesting thing was that the choke and the coil were on the ground side of the power supply. I read that one advantage of this approach is the voltage across the components, that are also attached to the chassis, so they do not have 400Vdc across them, for example.
If you had a voltage doubler, then in theory there could be a choke on the common ground so it is used for both the normal and doubled HV outputs, but I don't think I have seen that.
Is there a catch to using the choke on the ground side?
Cheers, Richard
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In the normal kind of PS we see in common audio equipment, there is no problem with a choke in the negative return lead. That hookup would be OK for both ordinary, full wave CT & FW Doubler circuits.
However, if the PS uses choke input on the negative lead, the stray capacitances between the windings & core do create a problem. For a description of the strays, Etc refer to the page from Ryders text book.
One of the big advantages of the Back Bias Circuit is in the production phase of the radio or whatever it was part of. With the exception of the rectifier all the tube socket tabs of the cathodes could be bent over & soldered right to the chassis. The back bias resistances were combined in a short stick with tabs for the grid leads.
The Voice Coil you referred to is actually the loudspeaker magnet coil. Most circuits use that without the extra choke, the CLC combination formed is usually enough to filter the HV since most of those radio’s used pentode output. The pentode plate doesn’t need a lot of filtering. And the loudspeakers of that time were not as good as today.
The OP mentioned 1930s sound, for that he will need to find a 1930s loudspeaker.
However, if the PS uses choke input on the negative lead, the stray capacitances between the windings & core do create a problem. For a description of the strays, Etc refer to the page from Ryders text book.
One of the big advantages of the Back Bias Circuit is in the production phase of the radio or whatever it was part of. With the exception of the rectifier all the tube socket tabs of the cathodes could be bent over & soldered right to the chassis. The back bias resistances were combined in a short stick with tabs for the grid leads.
The Voice Coil you referred to is actually the loudspeaker magnet coil. Most circuits use that without the extra choke, the CLC combination formed is usually enough to filter the HV since most of those radio’s used pentode output. The pentode plate doesn’t need a lot of filtering. And the loudspeakers of that time were not as good as today.
The OP mentioned 1930s sound, for that he will need to find a 1930s loudspeaker.
