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A Beginner 6BQ5 SE Amp


hanksjim1
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Very simple design, since there is no negative feedback there are better driver options than a 12AX7. Or keep the design and add 6-10db of global feedback for better damping and lower distortion. There is no rule of "no feedback with output triodes", yes they are more linear and have less distortion but that just means you need LESS feedback for great results.

 

I know I just keep repeating myself but I refuse to build a low powered amp that can't drive the output tube grid positive. Low powered amps like 2-3 watts even if using on very efficient horns just don't have enough headroom for dynamic music and often run into short duration bias excursions and blocking distortion. You'll get more power out of the EL84 pushing the grid positive. On the last note I usually don't advertise the increased output power, I'd still call it a 2-3 watt amp but know it will have higher output power handling for transients. The reason is I'm not sure how much the EL84 grid can dissipate, I'm sure it's not a lot so it's best to leave the extra headroom for music reproduction transients and not for full RMS sine wave testing. E.g. you won't melt the control grid with music

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The 6BQ5 will  be driven to full power by 7.7v peak at the control grid. (50mA bias through 150r cathode resistor)

 

The 12AX7 has a mu of 100 but because they are using no cathode bypass capacitor there is local degeneration making the first stage have a gain of 20. The amp is driven to full power by only 1/4 volt input RMS. Many sources easily get to 1Vrms or more. Too much gain mean high sensitivity which will just be noisy. Also the 12AX7 has a very high plate impedance so the power supply ripple rejection of the stage isn't great.

 

A 6SN7 could even drive the amp to full power with a gain of 10 with just 1/2v rms input. Cut's down on noise and also it has better PSRR from the lower plate impedance.

 

You could really step the performance up by using a CCS for the input stage plate load, with a 6SN7 you'll achieve a gain of 20 which is the same as with the stock 12AX7 setup. The CCS affords you extremely good PSRR with the 6SN7. Use the extra gain for negative feedback. The amp has an open loop gain of 24db. To get to full power with 1v input the closed loop gain needs only be 12db so one could easily add 12db of negative feedback and get good sensitivity of 1v input drives the amp to full power.

 

Too many tube amps use old designs that have too much gain. The sources today put out plenty of signal, on the low side is 1v output for sources but many output 2v output. Many people run preamps that could probably drive the 6BQ5 to full power themselves alone!!!!! Think about it, preamp has a gain of 4x and your DAC outputs 2v, right there the output of the preamp is enough to drive the 6BQ5 to full power and no input tube is needed. BUT, of course that's ridiculous and it's a good goal to aim for full output power with 1v input.

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With the stock arrangement you get 2.3% THD at 2 watts output.

 

Just adding feedback gets you to .8% THD at the same power output. The feedback will also make the amp quieter, have higher bandwidth, and lower output impedance. All a win if you ask me.

 

All I did was split the 12AX7 cathode resistor into two values, 2k and 220 ohms in series. I ran feedback from the output through a 1.2k resistor to the junction of the two cathode resistors on the 12AX7. The 2k resistor gets connected to the cathode and the 220 goes to ground side of the junction.

 

The Hammond 125se line is a universal mid-fi transformer with bandwidth from 100Hz-15kHz. I'm okay with using these for guitar amps but not a hifi amp, especially one with no feedback around it. First thing as a build upgrade is better output transformers.

 

 

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Since this build isn't pushing 10 watts the response of the Hammond looks very respectable. Would different iron make that much difference in this configuration?

 

This is the frequency response George found at 1 watt. "Specs at 1 watt: frequency response is 11.7Hz to 42.9KHz with 50mA" 

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50 minutes ago, mojomc said:

Since this build isn't pushing 10 watts the response of the Hammond looks very respectable. Would different iron make that much difference in this configuration?

 

This is the frequency response George found at 1 watt. "Specs at 1 watt: frequency response is 11.7Hz to 42.9KHz with 50mA" 

 

The concern isn't so much that the low end is rolled off, it's the distortion figure at low frequencies from core saturation. But yes since the circuit is only 2 watts (4Vrms/Secondary) the core isn't going to saturate as badly down at 20Hz compared to maximum rating of 8 watts (8Vrms/Secondary).

 

They are probably okay enough for the circuit, especially if you add feedback it will help get them most out of them. I know tinkerers like these transformers because they are "universal" but I remember not really liking them when I tried them a long time ago. They sell for $60 each! For $2 more you can get a nicer 80mA transformer from Edcor with end bells. Just my opinion, I'd take the Edcor over the 125CSE any day of the week.

 

https://edcorusa.com/collections/tube-amplifier-single-ended-output-transformer/products/gxse10-5k-10w-5k-ohms-single-ended-guitar-tube-output-transformer-1

 

 

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14 hours ago, hanksjim1 said:

 

 

14 hours ago, hanksjim1 said:

Lot's of posts lately about inexpensive (cheap) forum amps and the 6BQ5 or EL84 tubes in triode mode (a la Decware Zen)

 

Here's a write up on simple construction for beginners:

 

https://audioxpress.com/article/a-beginner-6bq5-se-amp

 

Any comments?

 

 

 

AA_Photo1-A-Beginner-6BQ5-SE-Amp.jpg

I glanced over the article this morning and I like it. Looks like an excellent little simple circuit and easy to build with a good write up on how to. Great find Hanks. I like to find articles like this one. New one for me and I am looking forward to reading the article more closely later. I like these kind of things. 🙂

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