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Old 20th August 2019, 09:27 PM   #61
liquidair is offline liquidair  United States
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So, I just wanted to give everyone an update. I did some simulating and in my attempt to get a bit more gain from the PI to overcome the additional load of the master volume, what I didn't factor was the PI's reduced output swing with that circuit. It looked like it would be enough still to overdrive the output tubes, but the traditional circuit still gave like 10V more. So I reverted to standard Ra=82.5k/100k with a 470 Rb, and ran my tests again...45W clean with 6L6s!

So ya, the PI just couldn't hang, but I'm happy with that number!

Originally Posted by MelB View Post
Do you have a 12AT7 to try in place of the 12AX7? You almost have a Fender Twin circuit there. They use a 12AT7 and slightly different resistor values but you are close. Maybe just parallel the 1K cathode resistor with another 1K to get 500 ohms there for a down a dirty test. It is a guitar amp after all.
I didn't, but I was going to do this if tweaking didn't work. And you're right, it's Fender derived. That's why I was being stubborn about the output power, the Bassman is supposed to put out 45W clean with 5881s...and I used a solid state rectifier (i.e., should be less sag than a GZ34 supply = more power).

Originally Posted by JMFahey View Post
And 340V peak into 1050 ohm would require 340mA peak ... a tall order for almost any 6L6.

Think 250/280mA peak and we are talking, at least with run of the mill 6L6 .
This set me straight. I went and checked the 400Vgs curves and ya, you can't even get 340mA unless your load line goes way above the knee (Vg1=0V)!

Originally Posted by Tubelab_com View Post
After turning a few internal adjustments up all the way, I can get 650 volts at 1.7 amps out of it which will power 4 X 35LR6 sweep tubes to 525 watts at the edge of clipping.....and a dull red glow.
That's insane (in a good way)! I've been wanting to build a bench supply, but the sheer power is a bit unnerving...You aren't careful and with 1100W (!!!) there's going to be trouble, lol.

Originally Posted by Tubelab_com View Post
The peak current is related to how much can be emitted by the cathode. Look at the cathode size in the 6L6, then compare the cathode in the EL34......it's about twice as big, and therefore can pass more current for a given tube drop......Got some 6550's or KT88's?

Remember that bigger cathodes need bigger heaters, which eat more current. Make sure that the heater winding in your power transformer can safely power those EL34's or KT88's before you leave them in there for more that a few minutes.
No, I don't have any 6550's or KT88's. But I did plan to try them at some point, so I made sure I had enough current for them!

Originally Posted by Tubelab_com View Post
Some tubes respond well to AB2 operation, others don't respond, while some others just don't like it. All of the 6L6 types DO like it, but they respond so well that it's easy to push them too far......100 watts from a pair on 500 volts into 3300 ohms IS too far.

TV sweep tubes generally don't like it, but some of them have peak current ratings in the 1 AMP and over range, so it's not necessary.
I'll have to get my sea legs before I try AB2, but I've definitely thought about using drivers to see what that and class B is like tonally. The idea of using a transformer to drive the output tubes also seems fun!
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Old Yesterday, 03:07 AM   #62
kodabmx is offline kodabmx  Canada
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Originally Posted by Tubelab_com View Post
100 watts from a pair on 500 volts into 3300 ohms IS too far.

TV sweep tubes generally don't like it, but some of them have peak current ratings in the 1 AMP and over range, so it's not necessary.
My 6P45S PPP triode amp makes 112W RMS sine with a B+ of 300V-320V into 1k3 Peak current ratings tell me that I can do this with one pair if I can deal with increased THD.

After finding sweep tubes, I'll never design around audio power tubes again
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Old Yesterday, 04:02 AM   #63
stocktrader200 is offline stocktrader200  Canada
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It likely becomes non linear at high currents when driving 100w from a single pair.
A BJT boosted 6922 has better curves, Think 75x current and 6922 curves.
I got 30w from a 150v B+ 2.5K tranformer. A 1.3K transformer and 180v would deliver 100w.
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Old Yesterday, 05:33 AM   #64
PRR is offline PRR  United States
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> get my sea legs before I try AB2

A handful of *guitar amps* have worked in this direction.

Before 1940, driver transformers were common, but the driver typically did not have the grunt to get far into grid current. This also covers the Fender Silverface Musicmaster Bass Amp with a puny 12AX7 driver.

Older Ampeg SVT and Fender 300 pushed real grid current. SVT had cathode followers. 300 had a 6V6 pushing a heavy transformer.

99+% of *guitar* amps use cap-coupling and can't sustain grid current even a few cycles. The cap(s) charges-down and throws the power tube toward cut-off. To a point, this may be The Sound Of Rock And Roll-- slamming the caps against the grids. Neil Young takes it to an extreme. Some designs (especially EL84) are too-prone to grid-fart/block and need the driver "lamed" so they don't block-up totally too easy.

Anyway, if you have a Bassman making 45 solid Watts, what more could you possibly want? The SVT/300 "work" because when pushed they are really Too Loud to stay on the same stage, and gross overdrive is self-punishing. The puny amps are both puny and not the overdrive flavor we love. The cap-coupled DeLuxe and Bassman inspired 80% of all the amps people actually play.
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Old Yesterday, 05:48 AM   #65
baudouin0 is offline baudouin0
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You could use 12AT7/ECC81 for your PI in the simulation - they will give about 2x more gain.
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Old Yesterday, 05:34 PM   #66
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by PRR View Post
The cap-coupled DeLuxe and Bassman inspired 80% of all the amps people actually play.
A nice '59 Bassman demo: YouTube

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Old Today, 07:42 PM   #67
mondogenerator is offline mondogenerator  England
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I recall playing a bassman head and 4x12(?) Cab set up in school. It was ancient, but what year I couldn't be sure.

The thing was the sweetest sounding amp, despite back then, me and my band mates were into a lot of metal.

When the head went away for repair (not my fault I promise) the cab was left.

Even that alone, blew a modern Marshall cab away, driven by a keyboard combo and (often) a Boss OD1
Testing everyone's patience since 1978.
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