• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Making a console stereo to a speaker amp?

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The warnings of another poster about that unit being all cosmetics and lacking substance seem to be spot on. :( For it to be a stereo setup, the audio voltage amplifier in 1 channel will come from the EABC80 and the EBC91 will provide the 2nd channel's audio voltage amplifier. :down:

A pentode mode EL84/6BQ5 yields approx. 5 W. Speakers of true mid 90s or higher sensitivity are needed and they should have a "flat" impedance curve. Speaker manufacturers don't outright lie in their published specifications, but the claims they make frequently need to be adjusted, especially when tubed power power amplification is part of the equation. For instance, a nominally 4 Ω speaker is said to produce an 89 dB. SPL when driven by 2.83 VRMS. As 2.83 V. into 4 Ω is 2 W., not 1 W., the "true" sensitivity of the speaker is 86 dB.

I find myself concurring with Dave (planet10) in thinking use the power trafo and EL84s to build up something new. Perhaps the O/P transformers can be used too.

BTW, there's a good chance that unit uses a selenium rectifier, as a vacuum rectifier is not present in the tube complement. Selenium rectifiers are ticking, toxic, time bombs, which must be replaced by modern silicon parts.
 
The warnings of another poster about that unit being all cosmetics and lacking substance seem to be spot on. :( For it to be a stereo setup, the audio voltage amplifier in 1 channel will come from the EABC80 and the EBC91 will provide the 2nd channel's audio voltage amplifier. :down:

A pentode mode EL84/6BQ5 yields approx. 5 W. Speakers of true mid 90s or higher sensitivity are needed and they should have a "flat" impedance curve. Speaker manufacturers don't outright lie in their published specifications, but the claims they make frequently need to be adjusted, especially when tubed power power amplification is part of the equation. For instance, a nominally 4 Ω speaker is said to produce an 89 dB. SPL when driven by 2.83 VRMS. As 2.83 V. into 4 Ω is 2 W., not 1 W., the "true" sensitivity of the speaker is 86 dB.

I find myself concurring with Dave (planet10) in thinking use the power trafo and EL84s to build up something new. Perhaps the O/P transformers can be used too.

BTW, there's a good chance that unit uses a selenium rectifier, as a vacuum rectifier is not present in the tube complement. Selenium rectifiers are ticking, toxic, time bombs, which must be replaced by modern silicon parts.
OK, thanks.
I will probably build a new amp with it eventually. I have never really done anything like this, so I will need to learn some more first. Ive just done a few electronics projects, mostly from kits, where you just have to solder all the parts together.
 
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BTW, there's a good chance that unit uses a selenium rectifier, as a vacuum rectifier is not present in the tube complement. Selenium rectifiers are ticking, toxic, time bombs, which must be replaced by modern silicon parts.

I believe that the OP chassis has a Siemens rectangular rectifier, such as the B250C125. It must be replaced with a silicon rectifier and a 5W dropping resistor of course, this also will reduce hum because the first electrolytic capacitor could be increased, but I don'do this straight away on my restorations. I only change immediately the earlier cylindrical rectifiers; they are dangerously degraded by now. Siemens rectangular selenium rectifiers are usually still OK, they will fail eventually but probably not today.

The preamp section of early stereo european radios is a total disaster by modern Hi-Fi standards. The dissimilar tubes on left and right channel isn't the worst thing: this was a common arrangement, and most small signal triodes of the period have similar parameters. The real letdown is the widespread use of grid leak polarization implemented in the absolutely worst way. The cathode is directly connected to the chassis near the tube socket togheter with one filament lead, and a 10M resistor between grid and chassis provides the grid leak polarization. This way the polarization is more or less random and hum is almost always a issue. Widespread adoption of better designs started only on the second half of the '60. Some manufacturers such as Saba adopted HiFi concepts sooner (also in the loudspeaker), but it is almost impossible to find a unrestored stereo Saba at 45$ today - they go at 2-4 times as much or even more.

People without experience on tube circuits often have the misconception that tube technology is always the same and never evolved, this way the mere presence of a tube in a device should ensure the sound we now associate with Hi-Fi.
 
pcan said:
The real letdown is the widespread use of grid leak polarization implemented in the absolutely worst way. The cathode is directly connected to the chassis near the tube socket togheter with one filament lead, and a 10M resistor between grid and chassis provides the grid leak polarization. This way the polarization is more or less random and hum is almost always a issue.
This arrangement was adopted to reduce hum! The idea is that the cathode can be grounded so heater-cathode leakage does no harm.

Having one side of the heater supply grounded is usually necessary in radios in order to prevent RF getting where it shouldn't.
 
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