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 Post subject: Re: Mini Block class-A PP EL84 Amp projectPosted: 04 Aug 2018, 16:00
 Project Author

Joined: 04 Jun 2008, 20:59
Posts: 4023
Location: Arizona, USA
Hi, Good thinking, but not going to work. It is essential that the cathodes are connected much like shown. The second tube gets its signal from the cathode circuit as its grid is fixed at ground potential. Sort of a see-saw arrangement. I believe I explained that operation in either that project or one of the otherr that use the same design. The 25 ohm control allows for shifting the dc point of operation (essentially a shift of bias from one tube to the other). As the total current is a fixed amount this balances the current between the two tubes. Your meters will have some internal resistance and will certainly be more than 1 ohm plus the offset of the 25 ohm control. I would estimate about 50 ohms and would make the amp less linear in operation. Not the whole story though. The measurement of current is only valid at idle with zero signal input. When signal is applied there will be current swings in opposite directions across the 1 ohm resistors and the portion of the 25 ohm control. The meters will flail in response to the signal.

If what you are trying to do is eliminate the control...it can be done, but only by adding one elsewhere. I do that in some prototypes. It makes the circuit much more complicated and I don't recommend it. You still need the 1 ohm resistors as test points and the LM317 and the appropriate sense resistor.

You could use the meters if the resistance is below say 100 ohms in the anodes. That way you would be measuring the current going into the tube and not the current exiting it (yes I know that this is technically incorrect as the flow of electrons is in the opposite direction). The meters would need a bypass switch that shorts across them to operate the amp properly. This would effectively remove them from the circuit. The 1 ohm resistors would not be needed in this case, but the 25 ohm control, LM317 and sense resistor are still required. That resistor (the 15 ohm one) programs the LM317 for the current level. Replacing it with an adjustable resistor will cause the programming to be variable. The programmed value (using 15 ohms) is one that maximizes the amp output, but keeps the tubes in a safe operating region. That is approximately 85% of maximum dissipation.

Good listening
Bruce

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 Post subject: Re: Mini Block class-A PP EL84 Amp projectPosted: 04 Aug 2018, 20:18

Joined: 01 Nov 2017, 01:22
Posts: 6
Hi gofar99,

I was trying to do two things;

1. Provide an individual current control for each tube
2. Provide a way to visually measure that current at idle without the use of test points or a multimeter

So from what you say unfortunately it seems there is no easy way to do this? Due to the way the output stage is designed?

If the resistance of the panel meters is an issue, I thought that I could easily leave the 1R resistor, and simply use a panel mounted voltmeter instead across that 1R resistor, in the same way that one would use the test points with the multimeter. I understand the meter would move around once signal is applied, it's main use would be to set the current at idle.

gofar99 wrote:
That resistor (the 15 ohm one) programs the LM317 for the current level. Replacing it with an adjustable resistor will cause the programming to be variable. The programmed value (using 15 ohms) is one that maximizes the amp output, but keeps the tubes in a safe operating region. That is approximately 85% of maximum dissipation.

I would like this to be variable, because I want to adjust the current per tube - the idea being that you don't necessarily need to have matched valves. If you have a control per tube, then you can adjust for 85% of max dissipation for each valve individually - at least that was my thinking. I calculated that the adjustable pot for the control would need to be either say 30 or 50 ohms, as 30 ohms would be the roughly the value to set the control required for the current draw for a single tube (about 41ma). Or perhaps a 25 ohm pot in series with a 15 ohm fixed resistor, in case the pot failed short.

My goal was to not remove the control, but rather to add more fine grained control. Does that make sense?

If it's not easily possible then that's fine, I thought I'd ask.

Thanks for your help with this, much appreciated.

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 Post subject: Re: Mini Block class-A PP EL84 Amp projectPosted: 05 Aug 2018, 09:21
 Project Author

Joined: 04 Jun 2008, 20:59
Posts: 4023
Location: Arizona, USA
Hi, I believe you missed key aspects of the design. The CCS has two functions. One the CCS sets the total current for both tubes. The 25 ohm control allows you to shift it from one to the other to achieve balance at idle. The tubes do not need to be matched. It doesn't cost much any more and the will pair better. A tolerance of about 25% is OK. The second function is that the combination of CCS, 25R control and the 1 ohm resistors raises both cathodes at the same time above zero. Think of the array as a single resistor with the cathodes tied together and the resistor going to ground (that does work BTW and was used back about 1948 in a budget amp). When the tube that gets signal from the driver conducts (the grid gets a positive signal) the current will try to increase. At the same time the other tubes grid is stuck on ground. It must then conduct less current. It gets its drive from the cathode. Recall that the amount of current a tube will pass is directly related to the voltage difference between the grid and cathode. Back to the first tube. When a positive signal makes it conduct more, the cathode voltage (across all the stuff) will change as well. The CCS just keeps the current the same the voltage will actually mirror the input signal. The better regulated the current, the more linear the push-pull action will be. The circuit is not intuitive in operation but using the LM317 family of regulators works quite well. (the resistor deal is not very linear BTW) To maximize the way it works the two cathodes need to be closely coupled. A total in this case of about 27 ohms is fine. At these current levels up to about 100 ohm would be OK. For the bigger amps the current/voltage drop between the cathodes will cause non-linear effects. I have seen variations that use two CCS and have several large value non-polar caps between the cathodes. They ought to work, but IMO adding additional non linear components and complicating the circuit is contra productive. As designed the circuit is extremely linear and low distortion with a very wide bandwidth. Typical values for the EL84 versions are 10HZ to 35 KHZ +/- under 1 db, S/N about -90 db, distortion in the critical 1 watt range below 0.3% (some I have measured at 0.15%). The actual distortion level in that range is tube brand related surprisingly. None were were more than about 0.6% though. As part of my goal to encourage folks to get interested in diy and learn new stuff I see no harm in what you are contemplating. To satisfy your curiosity I would go and build it.

On the meters, I explored that concept and the meters need to be extremely sensitive if they are passive. You are trying to measure 0-50 millivolts across 1 ohm. Such meters tend to be costly. You can use digital meters and I do for set up. But they must be isolated from each other and not have a direct connection to the amps ground. I use two such meters in a plastic case with separate 9 volt batteries. As a test kit the batteries last forever. You could build isolated power sources or meters with isolated measuring capability into the amp, but since adjustments are only needed about every year or so after the first few weeks , why bother? The original stereo version of the amp from 2008 still is using the original tubes and they are fine. I checked it about once a year and it seldom needed adjustment. It ran for 8-10 hours per day, 5 days a week for 7 years in my office.

Good listening
Bruce

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 Post subject: Re: Mini Block class-A PP EL84 Amp projectPosted: 13 Aug 2018, 07:39

Joined: 01 Nov 2017, 01:22
Posts: 6
Hi Bruce,

The ammeters I have measure 1 ohm (+/- 0.1) - would this be ok to use? Or would it still introduce some non-linear behaviour? if I was to still use them for this project (I've already got my chassis plate set up for the meters), where do you think is the best place to have them - in place of the test points, or on the anodes as you mentioned before? I have ordered some switches so that I can switch out the meters as desired.

Thanks again for all your help with this and fast responses.

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 Post subject: Re: Mini Block class-A PP EL84 Amp projectPosted: 13 Aug 2018, 10:17
 Project Author

Joined: 04 Jun 2008, 20:59
Posts: 4023
Location: Arizona, USA
Hi, Even though I don't like to have high voltages on things.....I would put them in the anodes. Their purpose is to balance the tubes and while some current passes through the screens most is in the anodes. With a 10% tolerance in the resistance of the meters, it would mean that you can not balance the tubes better than 10%. This is actually at the limits of the best performance range. So putting them in either place would IMO be more for cosmetics and not for accuracy. I believe I would put test points out of sight, perhaps on the rear of the amps with one ohm resistors. Use them to calibrate the other meters so you would know when balance is achieved. It might be ok and might not be the same as the meters. You can put trim resistors in parallel with the meter that reads high to bring it into agreement with the balance determined by the test points. I personally use 1% resistors for my test points and check them individually for accurate matching.

Good listening
Bruce

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 Post subject: Re: Mini Block class-A PP EL84 Amp projectPosted: 05 Sep 2018, 04:22

Joined: 03 Sep 2018, 15:57
Posts: 2
I'm new to DIY hi-fi audio, but have built several DIY tube projects for recording audio, so I feel reasonably confident building this. What sort of speakers should I be looking for to pair with these?

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 Post subject: Re: Mini Block class-A PP EL84 Amp projectPosted: 06 Sep 2018, 08:51
 Project Author

Joined: 04 Jun 2008, 20:59
Posts: 4023
Location: Arizona, USA
Hi, Since it is a low power amp I would look for ones with a sensitivity of 92db/w or more for use with a normal sized room. If you intend to use them close like next to a PC then pretty much any will do OK. I have some that are at 92 and 95 db/w and either will fill a room to a satisfactory level. The 92s though will not "rock" the room. My room is about 12 feet wide by 17 long and 9 high. Go for decent full range speakers as the amps reach down below 20HZ and can make good use of such speakers.

Good listening
Bruce

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 Post subject: Re: Mini Block class-A PP EL84 Amp projectPosted: 06 Sep 2018, 09:23

Joined: 03 Sep 2018, 15:57
Posts: 2
Great, thanks Bruce.

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 Post subject: Re: Mini Block class-A PP EL84 Amp projectPosted: 28 Oct 2018, 01:38

Joined: 01 Nov 2017, 01:22
Posts: 6
Hi Bruce,

I finally completed my version of your Mini Block design. I made mine as a stereo rather than mono blocks, and I used a bit of a different power supply design, including a couple of regulated DC heater supply DIY modules from Pete Millet. I have to fix a couple of small things and get some knobs and feet for the amp, but it's pretty much done now. Am listening to it now and it sounds great. Thanks so much for all your help with this - very much appreciated - a very worthwhile DIY project!

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 Post subject: Re: Mini Block class-A PP EL84 Amp projectPosted: 28 Oct 2018, 14:17
 Project Author

Joined: 04 Jun 2008, 20:59
Posts: 4023
Location: Arizona, USA
Hi, Nice looking build. The meters look cool. I don't use ones in my amps for simplicity and the possibility of them introducing anomolies in the sound. Since their use in setting the idle current really means they need to measure in the 0-75 millivolt range they tend to be scarce. You can use meter amps but then it adds complications again. I am far from a minimalist in designs, but much prefer fewer components as they cause fewer problems and are less likely to mess with the sound.

The stereo version of the amp is a Poddwatt and comes in a number of variations. Even the earliest ones sound great. The latest dual mono block version is IMO about as nice as a small tube can sound. It is essentailly two mono blocks on a single chassis that share only the volume controls, input selection (if used) and a power cord/IEC entry filter.

Good listening
Bruce

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