Hi. I built this amp with 45 output tube. The sound is so good but the gain maybe too high for 45 tube. Which is the best way to low the gain in 6sn7 circuit. Please help. Thanks
The easiest way is a level control at the input. If not that, maybe you have a 6SN7 cathode bypass capacitor that you can remove.
Possibly, but it's hard to know without seeing the circuit. Maybe he could post it?
If You remove cathode cap, You will decrease Low Freq. in amplified signal ....that depends from nominal of cap. /It is not very good idea....also adding R divider/.
Your ampl has 1 or 2 stages preamp? 1 stage doesn't enough and 2 stages are more for 45 with bias about minus 40-60 v....Try to use 1 stage preamp with 6SL7 in mode SRPP, but You need to change the schem of preamp.
Better, if You like the sound, don't touch Your amp and don't use potentiometer in full volume. With small input signal it will work in full...
Your ampl has 1 or 2 stages preamp? 1 stage doesn't enough and 2 stages are more for 45 with bias about minus 40-60 v....Try to use 1 stage preamp with 6SL7 in mode SRPP, but You need to change the schem of preamp.
Better, if You like the sound, don't touch Your amp and don't use potentiometer in full volume. With small input signal it will work in full...
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If you really want to lower the gain a lot, try removing the cap across the 27k on the second 6SN7 cathode. This will lower the distortion as well.
If that's too much gain reduction, instead remove the cap on the first stage cathode.
Why do you think there's too much gain, though?
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There is too much gain because the 45 requires less than half the drive voltage of a 300B which this circuit was intended to drive. I would start by removing the 100uF cathode bypass cap on the first stage. You can reduce the gain in the second stage by placing some resistance in series with the bypass cap.
If a signal level of 2Vrms is available at the input a carefully designed mu follower might have enough headroom to do the job. What is the available plate voltage?
If a signal level of 2Vrms is available at the input a carefully designed mu follower might have enough headroom to do the job. What is the available plate voltage?
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IMO, I don't like Your schem with 45 instead 300B. /This is like Trabant with turbo injection.../.
I have a cool ampl SE 2A3 RCA with preamp E180F Phillips in triod mode with gain about 50, that is enough for 2A3 / bias -43 v/ and for Your 45. E180F is awesome tube for HiFi audio. I'd like to advice it. /Every variant is not very good idea with caps, plate resistors, that changes mode and sounding/.
I have a cool ampl SE 2A3 RCA with preamp E180F Phillips in triod mode with gain about 50, that is enough for 2A3 / bias -43 v/ and for Your 45. E180F is awesome tube for HiFi audio. I'd like to advice it. /Every variant is not very good idea with caps, plate resistors, that changes mode and sounding/.
A few things to think about:
1- The original design, as noted by kevinkr, was allegedly to drive a 300B, albeit quite poorly. I've seen the same schematic with a 2A3 output with a few resistor changes.
2- Removing the cathode bypass cap on the driver basically renders the stage as a cathodyne phase inverter (ala Dynaco) with one side now unloaded... not a great fix but will certainly reduce the gain.
3- Depending on your OPT, you more than likely have an amplifier with inverted output phase, as most SE OPTs do not invert phase (Hashimoto SEs do) and you have 3 stages of phase inversion.
Piecemealing stuff together never works that great, but I wouldn't call it a Turbo-Trabbi either (the 45 is an excellent DHT). The 6SN7 is a good tube when employed properly, like a LTP phase inverter. Depending on the BIAS points, the 45 could easily require a Pk-Pk drive of 120 volts or more. My Hashimoto based 45 amps do have a similar topology and the input/driver stage yields about 43dB of voltage gain (150V/V) but no 6SN7.
As you're looking for lower overall gain, another option is to use a 12AT7 with the first stage as gain and the second stage as a directly coupled cathode follower (to the first stage and cap-coupled to the 45 grid). This will yield an overall gain of approximately 775mv input for 1-watt output. Make sure you bias the cathode follower around 2.5 - 3.0 ma to ensure good bandwidth. With good tubes, I've measured less than 0.4% THD at 1-watt with this configuration.
An alternate config is the Bugle-45 amp... a quick search will find a PDF with full design details and schematic. It uses a 12AY7/6072 biased around 2.8ma.
Regards, KM
1- The original design, as noted by kevinkr, was allegedly to drive a 300B, albeit quite poorly. I've seen the same schematic with a 2A3 output with a few resistor changes.
2- Removing the cathode bypass cap on the driver basically renders the stage as a cathodyne phase inverter (ala Dynaco) with one side now unloaded... not a great fix but will certainly reduce the gain.
3- Depending on your OPT, you more than likely have an amplifier with inverted output phase, as most SE OPTs do not invert phase (Hashimoto SEs do) and you have 3 stages of phase inversion.
Piecemealing stuff together never works that great, but I wouldn't call it a Turbo-Trabbi either (the 45 is an excellent DHT). The 6SN7 is a good tube when employed properly, like a LTP phase inverter. Depending on the BIAS points, the 45 could easily require a Pk-Pk drive of 120 volts or more. My Hashimoto based 45 amps do have a similar topology and the input/driver stage yields about 43dB of voltage gain (150V/V) but no 6SN7.
As you're looking for lower overall gain, another option is to use a 12AT7 with the first stage as gain and the second stage as a directly coupled cathode follower (to the first stage and cap-coupled to the 45 grid). This will yield an overall gain of approximately 775mv input for 1-watt output. Make sure you bias the cathode follower around 2.5 - 3.0 ma to ensure good bandwidth. With good tubes, I've measured less than 0.4% THD at 1-watt with this configuration.
An alternate config is the Bugle-45 amp... a quick search will find a PDF with full design details and schematic. It uses a 12AY7/6072 biased around 2.8ma.
Regards, KM
If You like the sound of Your amp.....at last...don't touch it, 6AS7, 45 /that is half of 2A3/ are tubes with cool quality.
Yes, that would be best. Why do you think the gain is too high? You can just lower the input pot setting.
If it must be turned down very far, it may be a linear pot instead of log taper.
On the contrary, I would encourage trying some other design topologies. A different design, or even a different set of operational parameters could yield a change for the better. After all, this is DIY.... I would simply suggest doing some solid design first, then prototype it and make changes based on measurements and listening tests. There's more than one way to skin a cat, and more than one way to drive a DHT.
Regards, KM
Regards, KM
Sorry, I have mistake...."If You like the sound of Your amp.....at last...don't touch it, 6AS7, 45 /that is half of 2A3/ are tubes with cool quality."....I mean 6SN7, not 6AS7.....
There are awesome pretubes for SE 45, like ECC88, 6N30P, 5670, 5687....my SE 300B has pretube ECC88.
In general, the 45 is not a difficult tube to drive (300B certainly is by contrast), so you can use quite a few different tubes as drivers and get very good results.
I used the 12AT7 (ECC81) for a simple 45/2A3 design which both measures well and performs well, topology noted earlier. For my Hashimoto based 45 and 2A3 amps, I'm using NOS broadcast versions of the 5814A, which have triple mica supports and are more tightly controlled on specs.
I don't have any 6N30P tubes but agree on 5687 and 5670 tubes as being quite good. I know many folks like the ECC88 (aka 6DJ8), but I've never been able to warm up to it.
Regards, KM
I used the 12AT7 (ECC81) for a simple 45/2A3 design which both measures well and performs well, topology noted earlier. For my Hashimoto based 45 and 2A3 amps, I'm using NOS broadcast versions of the 5814A, which have triple mica supports and are more tightly controlled on specs.
I don't have any 6N30P tubes but agree on 5687 and 5670 tubes as being quite good. I know many folks like the ECC88 (aka 6DJ8), but I've never been able to warm up to it.
Regards, KM
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