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 Post subject: Re: Attempting to build a low voltage tube amplifier
PostPosted: Nov Fri 22, 2019 9:49 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 916
Location: Tokyo
According to your second schematic, the regenerative detector has a 1K plate load resistor. If so, that is far too small a value. Typical values would be in the 30K~100K range. The grid resistor for the following audio amp stage would typically be 500K~1M.

The following schematic is an example of a very typical two stage regen. Notice the component values. The entire web page is here:

http://www.members.iinet.net.au/~cool386/kits/ace.html


Attachments:
12AU7 regen.jpg
12AU7 regen.jpg [ 61.8 KiB | Viewed 750 times ]
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 Post subject: Re: Attempting to build a low voltage tube amplifier
PostPosted: Nov Fri 22, 2019 9:58 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 916
Location: Tokyo
As for heater power consumption, you should be able to get away with 12V/150mA for a two stage receiver. I assume you want to use the 12V to also provide B+. BTW, the actual B+ current drawn in these simple regens is extremely low. Typically less than 500uA for the detector, perhaps just 1 or 2mA for the audio stage. If you use several 9V batteries in series to raise the B+ to 45V or so, the radio will give you plenty of volume and the 9V batteries should easily last many months. It is the heater consumption that will drain your batteries quickly.

Rob


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 Post subject: Re: Attempting to build a low voltage tube amplifier
PostPosted: Nov Wed 27, 2019 4:34 am 
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Joined: Nov Thu 04, 2010 5:49 am
Posts: 615
Location: Albany, NY
I did some more experimentation with a 6BH6 and a 6AB4 and found that the 6AK5 was the best choice for an amplifier tube. It was able to produce the most gain which is what I am looking for.

Rob, I found your suggestion about the plate load resistor very useful. I tried substituting in a 47k resistor and there is now much more gain in the receiver. If I use any greater of a resistor value the set refuses to oscillate on the low end.

A few questions that came up in my experimentation were:

1) I've been having fun configuring pentodes such as the 6AK5 as triodes and noting the difference in audio reproduction. I'd like to include a pentode/triode switch in the build just for fun. My question is that I plan to mount the switch about 4 inches away from the actual tube itself meaning that the total length of wire for this circuit will be about 8". Will this cause any problems with oscillation?

2) As you can see in the diagram there are multiple antenna connections, some are connected to the coupling coil and one is not. I'd like to be able to switch between these so that, depending on which antenna I am using at the time, I will be able to have an appropriate amount of coupling to the receiver. Would it not be a wise idea to use a toggle switch for this function for fear of leakage from one selection position on the switch to the other?


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 Post subject: Re: Attempting to build a low voltage tube amplifier
PostPosted: Nov Wed 27, 2019 11:24 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 916
Location: Tokyo
The tube's plate resistance and the plate load resistor form, in effect, a voltage divider. Obviously, the larger the plate load resistor, the higher the voltage from the divider. However, the plate load resistor also obviously causes a voltage drop. Too large a drop will cause the detector to fall out of oscillation on the low end. Very common problem with regens. You usually have to play around with the plate load resistor value and the amount of feedback (ie, the tap point on the tank coil).

Your question 1: you want to switch the detector from triode to pentode operation. The problem is, proper operating points are different for triodes and pentodes. The proper screen grid voltage of a pentode detector will be approximately the same as a triode's plate voltage; however, the pentode plate voltage will be 2~3 times the screen voltage. This clearly complicates selecting the proper value for the plate load resistor. I know triode-pentode mode change is sometimes done with audio output stages, but it's different with an audio voltage amp or a regenerative detector. Pentode audio amps typically have much larger plate load resistors than triodes. A change in the plate load will definitely affect feedback and that will affect regeneration. You could experiment if you had a variable B+ power supply.

Question 2: a toggle switch is not really OK but may work for MW BC. A quality rotary switch should be used. Mandatory for SW.

Let me think more about that triode-pentode mode switch.

Rob


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 Post subject: Re: Attempting to build a low voltage tube amplifier
PostPosted: Nov Wed 27, 2019 9:09 pm 
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Joined: Nov Thu 04, 2010 5:49 am
Posts: 615
Location: Albany, NY
shinkuukan wrote:
The tube's plate resistance and the plate load resistor form, in effect, a voltage divider. Obviously, the larger the plate load resistor, the higher the voltage from the divider. However, the plate load resistor also obviously causes a voltage drop. Too large a drop will cause the detector to fall out of oscillation on the low end. Very common problem with regens. You usually have to play around with the plate load resistor value and the amount of feedback (ie, the tap point on the tank coil).

Your question 1: you want to switch the detector from triode to pentode operation. The problem is, proper operating points are different for triodes and pentodes. The proper screen grid voltage of a pentode detector will be approximately the same as a triode's plate voltage; however, the pentode plate voltage will be 2~3 times the screen voltage. This clearly complicates selecting the proper value for the plate load resistor. I know triode-pentode mode change is sometimes done with audio output stages, but it's different with an audio voltage amp or a regenerative detector. Pentode audio amps typically have much larger plate load resistors than triodes. A change in the plate load will definitely affect feedback and that will affect regeneration. You could experiment if you had a variable B+ power supply.

Question 2: a toggle switch is not really OK but may work for MW BC. A quality rotary switch should be used. Mandatory for SW.

Let me think more about that triode-pentode mode switch.

Rob


Hello Rob,

Thank you for your insights. With regards to the pentode/triode switch this would be for the second 6AK5 only. The first 6AK5, used for the regenerative receiver will be left exactly as it is as shown (save for the change in the plate load resistor) in the circuit diagram. The pentode/triode switch will only be used on the second 6AK5 which is only used for audio amplification purposes.

One other question I had is that I am going to be picking off the signal from the collector of T2, the BC547, to feed into the control grid of the second 6AK5. I just don't have enough total gain in the circuit to pick the signal off from the plate of the first 6AK5. I'm okay with this compromise, though. I'd like to have a volume control between the collect of T2 and the control grid of the 6AK5. What value of potentiometer would be appropriate? The breadboard I have been using has a pair of built in 10k pots but upon connecting one of these in circuit the volume dropped even if the pot was at minimum resistance. Can a potentiometer cause loading of the input to a tube?


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 Post subject: Re: Attempting to build a low voltage tube amplifier
PostPosted: Nov Thu 28, 2019 12:34 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 916
Location: Tokyo
Could you post an updated schematic? I think the circuit I'm assuming is not the one you're working with. A quick sketch is fine.

Rob


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 Post subject: Re: Attempting to build a low voltage tube amplifier
PostPosted: Dec Sun 01, 2019 10:31 pm 
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Joined: Dec Sat 24, 2011 9:17 pm
Posts: 5460
Location: Vancouver Island, Canada
Since you are bread-boarding this? I would try some experimental wiring of switches.

I think a cathode resistor can be added or taken away with a double-throw switch. It could even be on a single-pole switch in parallel with a stationary resistor, I think.
Then a double pole switch would give you the pentode switching as well. But if that worked, I'd then search for a rotary switch. Just a thought.

Also you mention impedance matching between tubes, not likely to need it there, but using the output transformer is how you can achieve a good match to the headphones or buds.

How about a rechargeable Lithium battery pack for the heater supply? You could get a day's use easily from a rechargeable screwdriver.

_________________
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Burl Ives, RIP, oldtimer.
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 Post subject: Re: Attempting to build a low voltage tube amplifier
PostPosted: Dec Thu 05, 2019 8:46 pm 
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Joined: Nov Thu 04, 2010 5:49 am
Posts: 615
Location: Albany, NY
Hello Rob,

Here is a sketch of my current amplifier circuit. As labeled the input to the amplifier circuit is connected to the collector of T2 in the receiver circuit shown below. The only changes that I have made to this circuit is where it says 9vdc is connected just before P1 I now have 30vdc being applied there. Also, the plate load resistor has been changed from 1k to 100k. As much as I'd like to use the plate of the 6AK5 in the receiver as my input to the amplifier circuit there just is not enough gain there for adequate headphone volume.'

Receiver circuit:

Attachment:
image2.jpg
image2.jpg [ 60.66 KiB | Viewed 622 times ]


Amplifier circuit:

Attachment:
image3.jpg
image3.jpg [ 599.93 KiB | Viewed 622 times ]


Given my current amplifier circuit, is there anything that I can do to increase the gain of it? I'm maxed out in terms of the amount of plate voltage I can run due to space constraints. Should the screen grid be run at a lower voltage than the plate? Should I use a cathode resistor?

What would be an appropriate value of potentiometer to use with this circuit?


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 Post subject: Re: Attempting to build a low voltage tube amplifier
PostPosted: Dec Fri 06, 2019 1:29 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 916
Location: Tokyo
I think the problem is you're mixing high impedance tube circuits with low impedance bipolar transistor circuits and the resulting mismatching is killing gain. But I have very little experience with hybrid circuits. Or with pure transistor designs, for that matter. A pure tube design I can help with. Someone else needs to step in at this point.

Btw, why are you using R1 as a pull-up resistor instead of going to ground with it? I've never seen that with a tube regenerative detector. If it works, fine. I've just never seen it done that way. Yet it does go into oscillation.

I think I would use T1 as a plate load for the detector, add an emitter resistor to T1 and take the audio off the emitter to go to T2. The collector would go to a B+ around 12V. That would be low Z to low Z. But there's still the problem of matching the low Z output of T2 with the high Z input of the 6AK5 audio amp.

Sorry I can't be more helpful.

Rob


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 Post subject: Re: Attempting to build a low voltage tube amplifier
PostPosted: Dec Tue 10, 2019 4:19 am 
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Joined: Nov Thu 04, 2010 5:49 am
Posts: 615
Location: Albany, NY
Hello Rob,

I'm not sure why the circuit is configured as such. I built that schematic from a kit. The interesting thing is that, according to the documentation, they're using T1 as a switch for the B+ so that they could use a potentiometer with an SPST switch built into it.

Either way, after working with the circuit extensively I've decided to go in another direction. Although the 6AK5 section shows a lot of actual raw RF signal sensitivity now that it has a 100k plate load resistor the actual audio signal it outputs is really weak. All of the single stage amplifier stages that I've experimented with simply don't have enough gain to lift the output from plate of the 6AK5 up to a reasonable headphone volume. That and there will still quite a few unanswered questions about the circuit with regards to band switching possibilities, etc.

The "other direction" I'm going in is instead to use an existing varactor tuned solid state regenerative front end that I've constructed and make a two stage tube amplifier for it. There will be a lot less machining required for the enclosure, as well.

One basic question I have is that is it true if you increase the impedance on the secondary of an output transformer you also increase the impedance on the primary as well? Does that mean if you wanted to use a 3S4 output tube and have it drive a 64 ohm pair of headphones you'd actually want a transformer that is specified at ~625 ohm primary and 8 ohms secondary?


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 Post subject: Re: Attempting to build a low voltage tube amplifier
PostPosted: Dec Tue 10, 2019 1:29 pm 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 916
Location: Tokyo
Audio output transformers are specified like this: 5K primary, 8 ohm secondary. When the secondary is connected to an 8 ohm load, the amplifier plate will see a 5K load across the primary. If you use a 64 ohm load on the secondary, the plate will see a 40K load across the primary. That 20K:8 ohm output transformer is not a good choice for a 6AK5 (or a 3S4) used as a power amp, and would be even less so if you used a 64 ohm load across the secondary. Do you have any other output transformers on hand?

With 36V for the B+, you should be able to make a two stage, tube regen with sufficient volume for headphones. Not enough for a speaker or for power hungry stereo phones. Fair quality earbuds should work very well, just avoid the really cheap $1 type.

If this sort of receiver is what you want, I'll draw up a schematic. You seem to have 6AK5 and 6AK6 tubes. Any other tubes? Do you have the tuning coil (with tap) and a VC? What frequency range will this receiver tune?

If you prefer to use your SS regen to drive a two tube audio amp, remember you must properly match the tuner output impedance to the tube input impedance. And the output of the tube amp must be properly matched to your headphones.

Rob


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 Post subject: Re: Attempting to build a low voltage tube amplifier
PostPosted: Dec Tue 10, 2019 6:18 pm 
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Joined: Nov Thu 04, 2010 5:49 am
Posts: 615
Location: Albany, NY
Hello Rob,

Thank you for your continuing assistance.

Just to keep things simple I will be building a two stage amplifier based off of this circuit:

http://www.hi-ho.ne.jp/ux-45/smokey.html

I will be omitting the signal tracer portion of the schematic.

Questions:

1) If I wish to utilize this circuit with a pair of 64 ohm headphones, what impedance should the primary be specified for?

2) What would the correct primary impedance be if I wished to use a 6AK5 as a power amp with a 64 ohm secondary impedance?

3) I appreciate your assistance with a schematic. My ideal receiver would use something like a 3S4 tube and be varactor tuned as well as use molded coil inductors. I appreciate varactor tuning for the lack of hand capacitance and molded coils help on saving space. Is this type of circuit possible? I do not see many tube designs using varactors or molded inductors. I would use the amplifier that I am constructing above as the power section.


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 Post subject: Re: Attempting to build a low voltage tube amplifier
PostPosted: Dec Wed 11, 2019 6:00 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 916
Location: Tokyo
That amp circuit looks good to me. I'll just add that you don't have to use a 1S5 pentode since you won't be using the diode. 1T4, 1L4, 1U4 will be fine.

Question 1:

If you have a 5K:8 ohm output transformer, use it. Yes, the impedance seen by the 3S4 plate will be higher. Impedance varies with frequency; the 5K:8 ohm ratio is just a nominal working specification. As you change the ratio, some factors get better, some get worse (distortion and power output). We're talking small, cheap transformers; performance compromise is unavoidable. That said, your 20K:8 ohm trans ratio was too great, although it would work to some extent.

If you have an output transformer from an old AA5 radio, that will work fine.

Some say a 120V:6V step-down power transformer will work satisfactorily in this kind of a circuit since the power is so low.



Question 2:

Again, 5K:8 ohm would be a good choice. RF pentodes were not designed to be used as audio output amps, yet they have been used as such fairly frequently. For example, the pentode section of the 6AW8A.


Question 3:

Varactors and molded inductors are certainly convenient and inexpensive. However, both have considerably lower Q than standard coils and air VCs. I have a TenTec 1253 regen which uses them. The selectivity is very noticeably inferior to my other regens with air coils and air VCs. I do sometimes use a 1N4XXX diode as a varicap for fine tuning purposes, just a few pF; however, the main tuning and bandspread is always done with air VCs.

If you are worried about hand capacitance effects, just be sure to mount the VC behind a metal panel, preferably an inch or two behind it. Also, hand capacitance effects are often caused by overloading the regenerative detector. This leads me to a cardinal rule for regens: always have a signal attenuator between the antenna and the receiver, and adjust it so that band noise just slightly exceeds detector noise. This means when you attach the antenna, the noise level should go up just a little.

Rob


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 Post subject: Re: Attempting to build a low voltage tube amplifier
PostPosted: Dec Fri 13, 2019 8:35 pm 
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Joined: Nov Thu 04, 2010 5:49 am
Posts: 615
Location: Albany, NY
Hello Rob,

So, just to clarify, if I want to match either a 6AK5 or a 3S4 to a 64 ohm load then I would be looking for a transformer with a 5k primary?

I understand about the reduced Q of molded inductors and varactors. I'd be willing to use a polyvaricon style capacitor for the tuning capacitor but molded inductors make band switching so much easier. My main preference for molded inductors and varactors is space. I don't have the space in the enclosure I wish to use for a full size air variable capacitor and a large coil. I'm willing to sacrifice sensitivity for compact design.


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 Post subject: Re: Attempting to build a low voltage tube amplifier
PostPosted: Dec Sat 14, 2019 4:00 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 916
Location: Tokyo
Look for a 5K:8 ohm output transformer for either the 6AK5 or the 3S4. While these are very different pentodes, it will work with either.

Please note I'm not claiming such a transformer will 'match' your 64 ohm phones to the output tube plate impedance. Exact 'matching' is not practical and not necessarily desireable. For example, increasing the reflected load impedance (eg, using 64 ohm phones rather than 8 ohm) will usually reduce distortion at the expense of output power. Which is more important to you?

You've probably noticed that an output tube's plate resistance varies with plate voltage. Complicating this is the fact you plan to use a well below normal B+ voltage. Tube manufacturers did not provide operation characteristics for low voltage operation.

In short, you're somewhat on your own and will have to experiment to see what works best. Generally, that's the fun part. If you want to use a proven design, the manufacturers made them availble. The catch is, you'll have to use a much higher B+ voltage, typically 90 volts.



Rob


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 Post subject: Re: Attempting to build a low voltage tube amplifier
PostPosted: Jan Sun 05, 2020 10:58 pm 
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Joined: Jan Fri 03, 2020 7:54 pm
Posts: 22
Location: Malaysia Penang
Hi,
Project sound interesting, 6AK5 is good tube to work at low voltage , I have plot the 6J1 which is equivalent to 6AK5 curve at Vg2=6V and Vg2=12V and also triode at low voltage.

Triode curve
Attachment:
File comment: 6AK5/6J1 triode curve
6J1(T).GIF
6J1(T).GIF [ 7.32 KiB | Viewed 278 times ]


Pentode Vg2=12V
Attachment:
File comment: 6AK5/6J1 pentode Vg2=12V
6J1(P)_12V.GIF
6J1(P)_12V.GIF [ 6.96 KiB | Viewed 278 times ]


Pentode Vg2=6V
Attachment:
File comment: 6AK5/6J1 pentode Vg2=6V
6J1(P)_6V.GIF
6J1(P)_6V.GIF [ 6.78 KiB | Viewed 278 times ]


You can try to design the loadline , i think they only deliver 1-2mW when B+ =12V , 6V the output event lower.

I did use 6J2(6AS6) as mixer , 6J1 as IF AMP , and IN60 as detector and LM386 as audio output whole radio work at 12V. I was design for 6V , the 6J2 unable to oscillate well, when i increase to 12V it work very well.
https://youtu.be/pwyBCZ6hvdA


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