Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: walkman on February 28, 2021, 11:21:53 pm

Title: Re-scale preamp gain for lower than typical B+ voltage
Post by: walkman on February 28, 2021, 11:21:53 pm
Rather than continue a post from 2010, I have created a new post instead.

To quote PRR (who with each of his posts I read makes me want to really listen and understand)

“The next step down from Champ/Junior ~~5W level, to my mind, is One Watt. And if you can score a quarter-decent 2K OT, then you go 110V B+ and either king 25L6/6Y6 or the killer-cheap 12FX5. However you have to re-scale preamp gain: a full-out 12AX7 preamp with 100V B+ will gag on hot guitar.“ https://el34world.com/Forum/index.php?topic=9655.0

Hoping for further discussion, both when it comes to triodes and small signal pentodes.
Title: Re: Re-scale preamp gain for lower than typical B+ voltage
Post by: tubeswell on March 01, 2021, 12:41:56 pm
So, if your question is 'what does a typical set up for a 12AX7 with an HT of 100V look like?', I've assumed a 100k load line working into a generous 330k (red is the DC load, green is the AC load) and aiming for the golden rule of 2/3 plate volts (67V), with centre-biasing (-0.79V @ 0.33mA*) here's a picture.


* Rk = 2k4 (4k7||4k7 would be close enough)


As you can see, you don't get much out for what you put in: for the first 21Vp-p input, a voltage gain of about 20 40. (Edit: Oops the blue grid curves are 0.5V intervals!)
Title: Re: Re-scale preamp gain for lower than typical B+ voltage
Post by: 2deaf on March 01, 2021, 01:10:33 pm
Quote from: tubeswell on March 01, 2021, 12:41:56 pm
As you can see, you don't get much out for what you put in: for the first 2Vp-p input, a voltage gain of about 20.

2Vpp is way too much so that the clipping gives an erroneous idea about the voltage gain.  Smaller input signals give voltage gains in the 42 range.
Title: Re: Re-scale preamp gain for lower than typical B+ voltage
Post by: tubeswell on March 01, 2021, 03:02:19 pm
Quote from: 2deaf on March 01, 2021, 01:10:33 pm
Quote from: tubeswell on March 01, 2021, 12:41:56 pm
As you can see, you don't get much out for what you put in: for the first 2Vp-p input, a voltage gain of about 20.

2Vpp is way too much so that the clipping gives an erroneous idea about the voltage gain.  Smaller input signals give voltage gains in the 42 range.


Indeed. At 0.79V bias voltage, the thing will 'gag' :-)
Title: Re: Re-scale preamp gain for lower than typical B+ voltage
Post by: walkman on March 01, 2021, 09:00:31 pm
This video while not the typical guitar amp build video https://youtu.be/HL5BIE5TD3s (https://youtu.be/HL5BIE5TD3s) suggest that for say a b+ of 24v on a preamp tube the plate load should be 1/10 of what it would be for 240v. To be fair the approach the builder is taking is more about demonstrating how tubes work and not about building the ideal guitar amplifier.

For a lower B+ wouldn’t you want to drop the plate load some even if it introduced some compression or did not provide the approximate maximum head room?