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RCA SE 6bq5 amp gain problems
RCA SE 6bq5 amp gain problems
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Old 13th September 2009, 05:11 AM   #1
ThSpeakerDude88 is offline ThSpeakerDude88  United States
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Default RCA SE 6bq5 amp gain problems

Hello all! Its been a while since I was last here, been extremely busy with projects and such.

Recently picked up a beat up RCA console with this nice SE 6bq5 stereo amp in it for about $30 USD.

Click the image to open in full size.\

A few bucks, some paint stripper, new paint,all stainless hardware, binding posts, and a volume control from radio shack later....

Click the image to open in full size.

Anyway I am impressed with this one. The power transformer loafs at the current demands by this amplifier and only gets warm due to ambient temps around the outputs. The output trannys are large and seem plenty enough iron for the power.

Tube lineup is two 6bq5's, and one 6FQ7, solid state rectification with a 100uf/100uf voltage doubler to get about 330v before the filters as per stock schematic.

It does sound quite nice as it sits, but I have a problem with the very low preamp gain. As the 6BQ5 is a very easy tube to drive, almost any preamp tube can be used at low gain to achieve desired goals.

As it sits, My computer sound card does not have enough output to drive this amplifier to clipping before the sound card clips. It does fine with my other amplifiers (two magnavox 8802's, an 8601, and a Zenith stereo, all with 100k input pots.)

I have two theories:

1) The input was designed for a high output preamp from the tuner section (original schematic shows a 470k-470k divider network on the input of the amp even!)
2) The amp has bucketloads of NFB that is eating away gain.

Here is a simplified schematic of the amplifier stock minus psu:
Click the image to open in full size.

Notice how the output tap goes directly to the bottom of the cathode resistor.

I drew up a similar schematic showing what I theorized to be an equivalent schematic in theory:

Click the image to open in full size.

Notice how in the original, it seems as if the cathode resistor supplies bias voltage AND provides attenuation for the level of NFB at the same time. It is also bypassed by a 100pf cap, which is commonly seen for stability reasons. Ideally it wouldn't need to be there.

In my equivalent schematic , ignoring the 47 ohm resistor, it shows the same amount of NFB going to the cathode of the 6FQ7, and uses one more resistor to bias the tube. Am I correct in thinking that one resistor kills two birds with one stone in the original? I have seen amplifiers before with a 47 ohm tail resistor AND a resistor on the order of 2.2k-10k between the divider, set up by Rk and the tail resistor, to the output tap, but never directly tieing the output tap to the divider.

SO any ideas here? If it is simply a matter of shifting the 6FQ7's operating point to match the amount of gain needed that should be easy enough. Its been a while since I calculated the gain of a cc stage though. Am I correct that the 6bq5 only needs about 12-14v P-P to drive to full output? This would mean a mu of 6-7 is needed for an input of 2v rms.

Of course I could always go with an RH84 conversion, but two of my SE amps have that already. I love the sound of the RH84 and it is one of the best designs I have found yet, but I want to see what the RCA can sound like with a few minor modifications first
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Old 13th September 2009, 10:18 PM   #2
coldcathode is offline coldcathode  United States
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First thing I would try is to eliminate the NFB altogether and see how it sounds.
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Old 13th September 2009, 10:32 PM   #3
ThSpeakerDude88 is offline ThSpeakerDude88  United States
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In pentode mode? I think that would not sound very good at all. I set it up like my second schematic, and it did not sound nice at all. Grainy, high noise floor.

Weird thing is, I looked up the RS199A schematic, and it seems like maybe there was a mistake made in the RS199B? The 47 ohm resistor is the cathode resistor, with the 3.3k/100pf NFB resistor going right to the cathode. Does not sound good this way either
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Old 5th April 2011, 02:59 AM   #4
ferronier is offline ferronier  United States
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Default rca amp

Hey Speakerdude,
Just found this post as I have acquired the exact same amp. It's still bone stock all original. I tried it with my tube preamp which is a pair of 12au7 and it sounds great. I would like to compare the layout in mine with your schematic and see if they differ-otherwise a good preamp seems to make all the difference with this little guy. BTW one of my old 6BQ5 gets a little glowing hot on the outer envelope, don't know if tube going or if bias needs adjusting...sounds great anyway.
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Old 5th April 2011, 03:51 PM   #5
grindstone is offline grindstone  United States
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Default one more fwiw

I'm not him but here's mine--no clue what's orig/not (haven't even looked at it yet). Guy I got it from had it running and it came well-regarded with the same surprise.
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Old 5th April 2011, 04:28 PM   #6
artosalo is offline artosalo  Finland
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I think that the reason for low sensitivity is because of the low amplification factor of the 6FQ7. The gain of this stage without NFB is 20 dB (A=10). 6BQ5 requires some 4,3 Vrms to be driven full. This means 0,43 V at the output and without NFB.
Now if the NFB were 20 dB, the input sensitivity of the whole amplifier would be again 4,3 Vrms, whis indeed is too low for most signal sources.

Change the voltage stage to 12AX7. Then you have spare gain.
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Old 5th April 2011, 04:57 PM   #7
mwiebe is offline mwiebe  United States
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You could try trioding the 6BQ5, remove feedback. Drop the plate resistor over the 6FQ7 to 35 to 40K. Change the 6FQ7 cathode resistor to 700 to 750 ohms (333uF bypass). Drop the pot value to 40 to 50K. If you still want more gain you will need to swap the first tube for one with more Mu. I wouldn't expect much 6BQ7 plate current on the second schematic.
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Old 5th April 2011, 07:20 PM   #8
dcgillespie is offline dcgillespie  United States
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SpeakerDude --Your revised circuit does two things you possibly have not realized:

1. It cuts the amount of NFB in half, since the two 3.3K resistors in your version form a voltage divider. In the original, there is no divider, so all available output signal is applied to the input stage -- albeit in series with the current FB developed across the original 3.3K cathode resistor.

2. Your version also cuts the bias for the input stage in half as well, as now there are two DC paths from the cathode to ground -- one through the actual cathode resistor to ground, and one through the FB resistor and OPT winding to ground as well. The original design accomplished providing maximum available NFB, without upsetting the bias of the first stage. The original 47 ohm resistor acted to maintained some load on the output transformer if the speaker load were removed, to maintain the target FB level, and therefore stability in that scenario. You could simply remove your added cathode bias resistor, which would return the bias and FB levels to normal, but would likely be unstable without a speaker load connected. Doing so would also basically restore the original circuit, where upon adding the 47 ohm resistor back in would in fact restore the original circuit in that case.

3. Simply replacing the 6FQ7 with a 12AX7 won't change the gain, as the NFB will simply compensate, and will possibly make the unit unstable in the process. The proper procedure would be to use a 12AX7, but also reduce the FB level by the amount of gain increase the 12AX7 provides. That way, the original FB level will be maintained, but the input sensitivity will be increased by a factor of the gain increase the new tube provides.

So for example, using the original circuit, if you replace the 6FQ7 with a 12AX7 (heater rewiring is required), the gain of the stage will likely increase by a factor of 4X with the current circuit shown. This means that the FB level will also incease by a factor of 4X as well. To compensate, simply install a 150 ohm resistor in series with the original FB connection from the output winding back to the 3.3K/47R junction point. This will effectively apply 1/4 of the original feedback -- but since the circuit has 4X gain over the original, the actual FB level remains unchanged. However, the input will now be 4X more sensitive than before as well. Therefore, if the original design required say 2.0 vac to drive the amplifier to full power, this new configuration will only require .5 vac, will still retaining all the characteristics of the original circuit.

I hope this helps!

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