ltp/driver

[andrew]

I want to build a stand alone power amp with a 275-0-275 PT, a 25 watt OT,and 6l6s or kt66s. Are there any advantages to using a LTP(6SN7) with a driver(12BZ7 or 6CG7) in this low power power amp? I see it in larger wattage amps and bass amps. Thanks for any advice.

[jjman]

A driver can overcome lack of adequate stamina from the LTP. My guess is drivers would not solve any issues in a 25w setup with a single output pair. Perhaps they would help if hifi performance is sought?

[andrew]

Thanks jjman, I don't know it might be for hifi use in a way. I have an older Groove Tube speaker emulator that is only a reactive load box with EQ and the signal out has to be amplified or run into a recording console . I would just use it at home. I have all the parts anyway and it will give me something to play with.

[andrew]

https://tubeamparchive.com/download/file.php?id=24372 anyone try this at lower power?

[gingertube]

Andrew,
The LTP/Driver combination is used when you need to drive multiple output tube pairs to high power.
For 25 Watts from a pair of KT66 or 6L6 then the LTP will be fine on its own. A 12AX7 or 12AT7 or 6SL7 will do the job just fine.

Can you give more details of the OT. If it is 25W HiFi then it is fine for a bit more wick as a guitar amp where extended bass response is not required. Do you know it's primary impedance? Many tubes actually sound better when running at lower supply voltages you will get from that power tranny (probably around 320V). I ask this so we can give better advice. Depending on the OT primary impedance you may wish to condsider a quad of 6V6 or EL84 instead but a pair of cathode biased KT66 would be lovely. Once you choose an output tube configuration then a sensible decision can be made about the phase splitter/driver.

Power Amp design always goes that way. Start with the output tranny and work backward.

Cheers,
Ian

[andrew]

Thanks gingertube. 275-0-275 Hammond PT and a Hammond 1650F 25 watt 7.6 k primary.

[gingertube]

OK that output tranny is rated for 25W 30Hz to 30kHz and 128mA max DC current per side. For Guitar Amp use you could easily screw 40W out of it.

The 275-0-275 power tranny is going to give you a B+ of around 320V. That restricts your options a little. I don't know what current rating it is:

So:
If rated for only 100mA then use a pair of 6V6 or EL84 for what the Guitar Amp DIY crowd refer to as the 20W designs (actually 14 watts and 17 Watts respectively).

If the power tranny is rated for 150mA then a pair of 6L6 will give you:

18 Watts in Class A1 push pull (Cathode Bias 270 Ohms each tube OR 120 to 130 Ohms shared) - probably needs a bit more than 150 mA

24 watts in Class AB1 push pull (Cathode Bias 470 Ohms each tube)

Adding a driver stage to run Class AB2 wouild take that power up to only
27 or so Watts and is hardly worth the effort.

KT66 will give very similar results to the 6L6 / 5881

If you want something different you could also try 807 output tubes (Watch out for the B+ on the anode cap - no good around kids and small animals).

A Power tranny with higher voltage would give you some more optiions but any of the above should be fine.

Cheers,
Ian