Hi, I am looking for a schematic for a 300B single ended power amp circuit with about 2V sensitivity. All circuits I found so far either too sensitive (like 0.5V) or too insensitive (4-5V).
Thanks for any recommendation,
Mo
Thanks for any recommendation,
Mo
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You can always use an input level control on the amp, if it is too sensitive.
Or a fixed L-pad attenuator, if you want a precise sensitivity level.
Or a fixed L-pad attenuator, if you want a precise sensitivity level.
I do not want to use a voltage divider to then amplify again after it. My pre-amp has a max. output voltage of 2V RMS. I want a 300B power amp having that sensitivity.
Sounds like a driver tube with a mu of 20 to 30. Something like 6SN7 or 6N1P maybe?
Actually you probably don't if you play analog material with widely varying dynamic range in which case you want excess gain somewhere in the system to assure that you can still play at higher volumes if you are so inclined. (I have both analog and digital classical recordings that average less than -10dBr). This is less of a concern with digital recordings and even less so if you do not listen to music with a wide dynamic range.
I like a margin of at least 6dB. If you want the flexibility of tailoring input sensitivity to a specific requirement given the limited choices available in no feedback designs consider a high gain design with negative global feedback tailored to your sensitivity requirement. (Yeah I designed DHT amps for 30 years without and still don't but my signal chain has sufficient headroom to cope with the dynamic range problem depending on source.)
The Joe Roberts WE 91A clone from Sound Practices. A lovely amp with the right amount of sensitivity, IMO.
https://www.enjoythemusic.com/magazine/sound_practices/1/single_300b.pdf
https://www.enjoythemusic.com/magazine/sound_practices/1/single_300b.pdf
Need to be clarified
2V RMS input to drive 300B:
- up to A1 limit (depends of operating point: bias voltage of 300B);
- up to X Watt (depends of operating point, B++, OPT impedance).
If you can define the maximum swing peek of 300B grid, the gain of driving (VAS stage + optional driver) can be calculated, so tube selection is possible.
OK.
http://trioda.com/tools/triode.html
If you choose 360V, 80mA operating point for 300B (-71.57V bias), and 3k3 output transformer the required gain of voltage amplifier stage (VAS) and optional driver results this calculation:
peek driver voltage up to A1 (0V grid voltage): 71.57V.
2V RMS is 2.828V peek.
required gain: 71.57/2.828= 25.3
You must to choose such tube (and its loading) which is satisfies this condition AND guarantees -in my opinion- at least 2 (rather 3) dB headroom (it's the best, if output stage saturating first, not driver stage).
So 25.3 * 1.259 (+2dB) is 31.85.
You need such tube, which is has enough low output impedance (to drive capacitor coupled 300B grid up to A1, with enough slew rate -so mA anode current capacity tubes not recommended-), enough gain which is greater than 32.
http://trioda.com/tools/triode.html
If you choose 360V, 80mA operating point for 300B (-71.57V bias), and 3k3 output transformer the required gain of voltage amplifier stage (VAS) and optional driver results this calculation:
peek driver voltage up to A1 (0V grid voltage): 71.57V.
2V RMS is 2.828V peek.
required gain: 71.57/2.828= 25.3
You must to choose such tube (and its loading) which is satisfies this condition AND guarantees -in my opinion- at least 2 (rather 3) dB headroom (it's the best, if output stage saturating first, not driver stage).
So 25.3 * 1.259 (+2dB) is 31.85.
You need such tube, which is has enough low output impedance (to drive capacitor coupled 300B grid up to A1, with enough slew rate -so mA anode current capacity tubes not recommended-), enough gain which is greater than 32.
Hi
this is a driver I tested, with E88C, in srpp two tubes.
Gain = 42,5 times
Zout= around 1k2 ohm
for a s.e. with KT150 but easily used for 300B
frequency response of the input stage only
this is a driver I tested, with E88C, in srpp two tubes.
Gain = 42,5 times
Zout= around 1k2 ohm
for a s.e. with KT150 but easily used for 300B
frequency response of the input stage only
Start at the transformer. Work your way back from there to determine the gain you need from the driver tube.
5K primary load will be a large difference from a 3.5K primary load.
5K:4 Z = 35.35:1 turns
5K:8 Z = 25:1 turns
3.5K:8 Z = 20.9:1 turns
3.5K:4 Z = 29.5:1 turns
1W into 4 ohms is 2Vrms.
1W into 8 ohms is 2.828Vrms.
At the input of the transformer for one watt out:
5K:4 Z requires 70.7Vrms
5K:8 Z = requires 70.98Vrms
3.5K:8 Z = requires 59.1Vrms
3.5K:4 Z = requires 59Vrms
The speaker impedance has little impact on drive voltage from the output tube. The primary impedance of the output transformer does significantly impact drive requirements.
You need roughly 20% more voltage gain in the driver for the 5K primary impedance vs the 3.5K impedance. It would be even lower with a 2.5K primary impedance vs 3.5K.
5K primary load will be a large difference from a 3.5K primary load.
5K:4 Z = 35.35:1 turns
5K:8 Z = 25:1 turns
3.5K:8 Z = 20.9:1 turns
3.5K:4 Z = 29.5:1 turns
1W into 4 ohms is 2Vrms.
1W into 8 ohms is 2.828Vrms.
At the input of the transformer for one watt out:
5K:4 Z requires 70.7Vrms
5K:8 Z = requires 70.98Vrms
3.5K:8 Z = requires 59.1Vrms
3.5K:4 Z = requires 59Vrms
The speaker impedance has little impact on drive voltage from the output tube. The primary impedance of the output transformer does significantly impact drive requirements.
You need roughly 20% more voltage gain in the driver for the 5K primary impedance vs the 3.5K impedance. It would be even lower with a 2.5K primary impedance vs 3.5K.
...the 300b ...has Specified gain.tube which are voltage amplifiers, can make different sensitivities Depending on to the feedback. How do you say that there is no 2 volt sensitivity circuit?
Thanks @grovergardner
I played with the schematic in LTSpice and indeed it seems to be a nice amp. It has a sensitivity of 0.5V RMS, though.
The WE310 looks gorgeous and from what I can find is a terrific driver tube. The only new available seem to be from Psvane for about 300 EUR for the pair 😱. The Russian 10J12S seems compatible, but the only source being from Ukraine via Ebay, for a new amp to build, I am not sure choosing that tube is a good idea. Would like to be able to get replacements if needed.
The 6SJ7 can be gotten as NOS here, but I think supply is also pretty low.
Thanks, @waltube . I own an EL84 PP class A with SRPP front end and like that a lot. The gain of your E88C stage is almost perfect. So a driver like this is definitely a good option to meet my sensitivity "requirement". On the other hand, I'd like something "more different" to my EL84 amp and would prefer a single triode or pentode as driver.
According to spice, your schematic above is driving the E88C way too hard though. Are you sure all Rs are correct? The max. dissipation for both triodes used simultaneously is 2W, in the sim it is 7W. I tripple checked schematic and values, don't think I made a mistake.
When I change the values to the below (also lowered the anode voltage to 300V), the E88C stays in its boundaries.
The mistake you made is that an E88C is not a 6922 (E88CC), actually completely different.
Oh boy... 😆 , thanks for spotting.
E88C seems to be unobtainium as well. So I would need to use a diff one if I opted for the SRPP variant.
You're right, the 310 has more gain than I remembered. The best sub in this amp, IMO, is the RCA 5693, not at all difficult to find in the US (I'm not sure what country you're in) and not cheap but not more than a decent 6SN7 these days.
If you want less sensitivity maybe look at a 12AT7 SRPP.
What about RF triodes or triode-wired RF pentodes with mu of about 35 to 40?
5842
6S3P-EV
12GN7A-triode
6J9P-triode
6e5P-triode
Use a plate choke for max swing into the 300B grid.
For pentodes maybe
6SJ7 = Russian 6J8 (6Z8 or 6zh8)
6EJ7 (EF184) = Russian 6J51P (6zh51P)
There are many other possibilities...
5842
6S3P-EV
12GN7A-triode
6J9P-triode
6e5P-triode
Use a plate choke for max swing into the 300B grid.
For pentodes maybe
6SJ7 = Russian 6J8 (6Z8 or 6zh8)
6EJ7 (EF184) = Russian 6J51P (6zh51P)
There are many other possibilities...