6
\$\begingroup\$

I have a tube radio that has really low output (I mean the sound is barely audible at full volume). Probably some capacitor in the LF amp failed. However, after I downloaded the schematic diagram, I was intrigues and confused by the design of the output amp. While I can (most likely) repair the amplifier by testing the capacitors, I want to learn about electronics too. In this case, why it was designed as it was.

Here it is: enter image description here

The left pentode is a 6ZH1P small signal pentode and the right pentode is the Soviet equivalent of EL84.

Capacitor value explanation - if the number is integer (2200) then the units are picofarads, if the number has a comma (25,0), then the units are microfarads. Resistor values are in ohms, unless there is a "k" or "M" written or the number has a comma (which means that the units are megaohms).

I understand how a basic grounded cathode amplifier works, but this one has some weird (to me) parts, for example:

  1. As I understand, C4-9 is for negative feedback, does it also behave as a cathode bypass cap? I mean, if I disconnected it, would the output increase or decrease?
  2. Why are the screen grids of both tubes and the plate of 6ZH1P connected to a tap of the output transformer? Why is that tap used to provide power to the detector and IF transformer (and their tubes). It is connected trough a 20uF cap to ground, so audio feedback probably would not be there. Is the output transformer used as a choke?

EDIT: The actual fault was a leaky C4-7.

\$\endgroup\$
  • \$\begingroup\$ Schematic: compact yes, readable no \$\endgroup\$ – stevenvh Apr 1 '12 at 10:37
  • \$\begingroup\$ @stevenvh, I replaced the schematic with a bigger one, which includes the power supply. However, I did not draw the schematic, I guess it was the style used in 1964 or whenever they were designing this radio. If you pen the schematic in a new tab, it will be bigger. \$\endgroup\$ – Pentium100 Apr 1 '12 at 10:45
  • \$\begingroup\$ I didn't mean to criticize you, I understand that you didn't draw it yourself. \$\endgroup\$ – stevenvh Apr 1 '12 at 10:47
  • \$\begingroup\$ The tubes do of course emit a nice orangey-red glow a few seconds after the filament voltage is applied, do they not? =P \$\endgroup\$ – JustJeff Apr 1 '12 at 13:33
  • \$\begingroup\$ Внешний громкоговоритель сломался!!! \$\endgroup\$ – user8459 Apr 2 '12 at 0:17
5
\$\begingroup\$

First, I think it's more likely these ancient tubes are no good anymore than the capacitors having failed. Except perhaps for the power, which is easy enough to check, these capacitors are probably wrapped foil or something else that is dry and should last a long time. Start by checking the supply voltages. Those appear to be well marked. If the power input diode has gone bad, then nothing else has much chance or working.

As for the circuit, I am somewhat confused too. I'll take a rough stab at it anyway. To really understand it would take more time working thru it than I want to spend on it.

The left tube seems to be a pretty straight forward amplifier. The amplified signal appears on the plate, which is then coupled into the power stage thru C4-4. Most of the mess between C4-4 and the control grid of the right tube looks to be a tone control. That's just from the general form. I haven't actually analyzed it. I think R5 is likely some sort of tone control. I'm less sure about R4, but R4 and R5 together may be something like bass and treble controls.

The strange part is how the two output transformers are hooked up. I'm guessing that the top two speakers are meant to be tweeters, the lower two the rest of the sound range, and the strange connections between the double transformer is like a crossover network. This also leads some credance to R4 being a treble control since its signal is driven from the feedback from the top transformer output.

C4-9 and R4-7 feed back a bit of the signal at TP3 onto the cathode of the power tube. This looks like classic negative feedback to provide predictable gain and a flatter frequency response.

The section of the circuit you show here can be easily enough tested in isolation. First, make sure the two power supply voltages are as marked, then feed a signal into the line you labeled as audio input. That should be clearly audible on the speakers.

\$\endgroup\$
  • \$\begingroup\$ Thank you, I understand it more now. But why the screen grids are connected trough Tp3? I mean the current from the power supply comes into pin 2, then out of pin 1 and goes to the screen grid of L4-2 and some other tubes (including L4-1)? Why connect it that way instead of just straight to the power supply? \$\endgroup\$ – Pentium100 Apr 1 '12 at 19:20
  • \$\begingroup\$ Huh? TP3 is connected to the cathode of the right tube, which is connected to the suppressor grid internal to the tube, neither of which are connected at all to the screen grid. \$\endgroup\$ – Olin Lathrop Apr 1 '12 at 21:24
  • \$\begingroup\$ The primary of Tp3 has three pins (ends and a tap), the power supply is connected to pin 2 (the tap), the anode to pin 3 and the screen grid to pin 1. The secondary (pin 5) is connected to the cathode. \$\endgroup\$ – Pentium100 Apr 1 '12 at 21:32
  • \$\begingroup\$ So TP3 refers to the transformer, not the bottom node? I suppose that is what is meant. I am used to "TP" meaning test point, but that probably has no relevance to a old russian schematic. \$\endgroup\$ – Olin Lathrop Apr 1 '12 at 22:55
  • 1
    \$\begingroup\$ In Cyrillic, "P" is read as "R", to that is "Tr", short for "transformator" (трансформатор). \$\endgroup\$ – Pentium100 Apr 2 '12 at 1:06
3
\$\begingroup\$

Just some general information about tubes that might help shed some light on this ..

Screen grids were normally connected to the positive supply to help provide a more constant electric field inside the tube, while the output signal was collected as anode current. The idea is that, as a relatively sparse electrode, the screen grid intercepts very little of the electron flow, so the voltage there will not fluctuate with the signal very much. The anode, by contrast, takes the full brunt of the electron flow, and so anode voltage can vary greatly with signal current. In a triode, the anode serves two functions: (1) to provide the large anode-cathode electric field in which the control grid has its modulatory effect, and (2) collect the resulting current, which provides the output of the tube. By separating these two functions, and assigning the former to the screen grid, the negative feedback effect of varying anode voltage was removed.

So it will be fairly common to see a screen grid attached in some way to the supply rail (oft called 'B+') while the anode current will be passed through a coupling transformer, or through a load resistor and coupled onward via capacitor.

In this case, the output of stage one is loaded by the 30K R4-4 and coupled to the 2nd stage by the 0.047uF C4-4. Then there's a big network that combines that, and some feedback, to drive the control grid of the 2nd stage. The output topology of this amp is kind of complex looking. It looks like they've integrated a cross-over circuit directly into the amplifier output. Looks to me like your lower frequency range is driven into taps 2 and 3 of Tp3, while the higher frequency stuff goes through Tp2. Someone with more time on their hands could probably work out the breakpoint between high and low frequency.

\$\endgroup\$
2
\$\begingroup\$

The first stage is straightforward. The only things to mention: the negative feedback by omitting the decoupling of the cathode and the physiologic volume control using a tap on the control potmeter. The tone control is also comprehensible: it is a Baxandall control. The high f control is R4 (controlling the ratio of the feedback voltage from the high f output trafo and the input stage) and R5 is the bass control. The output stage: the upper trafo is coupled in parallel with the bass and mid range trafo via C4.12. The C4.11 in parallel with the prime of the bass trafo is a bit funny: it short cuts to some extent the high frequencies so for the high trafo there is less left. I guess this is to balance the hi/low ratio of the speakers. The output stage feedback is also classical via C4.9 from the speaker output to the cathode. Removing will raise the amplification but it's not advisible: tube amps need feedback in spite of what some people say. The real unsure thing to me is the connection of the screen grid of the output stage with the output trafo. It is a positive feedback. I guess it is to boost the low frequencies. We have to bear in mind that there is a low pass filter between the output trafo and the screen grid by R4.6 and the elco C6. So the feedback works not for high freqs. Added: the last remark can be overruled: it is a anti-hum wiring. The 100 Hz ripple runs through the output trafo and cancels the hum from the plate.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.