4 Watt amp for critique

[MadMax]

Any of you experienced builders care to examine this schematic for efficacy? I have a transformer that's just right for something like this and most of the other stuff as well. Has anyone out there built this? Or can anyone see any fatal flaws with it, assuming a well built power supply?

http://www.lh-electric.net/projects/4w-SV83.html

[DummyLoad]

for guitar or hifi. seeks ok for hi-fi.


SRPP with 12AT7 gain is about 50 with 470K load - Vout is about 55Vpk.


--pete

[MadMax]

I was thinking low output bedroom guitar amp. What do you see that makes it more suited for hi-fi, is it the SV83 specs? Maybe I can modify it for guitar? Thanks.

-Mad Max-

[DummyLoad]

makes it more suited for hi-fi

SRPP and SE SV83.

--pete

[kagliostro]

What about a 5f1 with a bit of mod as to use a 6k6 tube instead of 6v6 ?



If you want to use it like an Herzog you need the bit of components that are on the place of the speaker

if instead you don't care about the Herzog, you can discard those components and attach a 4ohm speaker

here the tread about SILVERGUN's Herzog

http://el34world.com/Forum/index.php?topic=17218.msg172218#msg172218

K

[jojokeo]

There are a number of elements not consistent with a "guitar amplifier" such as the signal input jack should utilize a grid stopper resistor and a grid return resistor, not immediately a pot on the input. That should come after the preamp to control the signal prior to the output tube. You should design around using a 12ax7 or maybe 12ay7, not a 12au or at for V1. I would not use a 10k screen resistor, nothing higher than 1k here. Lower C2 down to no higher than .02uF and/or utilize a higher value grid stopper prior to the SV83 depending if you follow prior suggestions. Eliminate preamp from receiving power from C5 & PT's screen and follow a better more standardized voltage filtration string C R C type circuit. Just my initial suggestions...

[PRR]

> low output bedroom guitar amp

VERY low output. With just one stage of voltage gain, a typical guitar can't smack full output out of the power tube, without a LOT of arm effort.

What's wrong with a Champ? About the same number of parts. You can use the '83 power tube if you want. You can buy the whole thing as an Epi Valve Jr., and mod to sound better. If you means 4 Watts instead of 5.7 Watts, keep the B+ down to 300V, 250V, or so.

[MadMax]

WOW! you guys really stepped up. I just learned a ton from your comments. Your experienced eyes can see all kinds of things mine don't. I bookmarked that cool LB3 schematic for a later project. But since I already have the SV83, a transformer capable of about 300V with SS rectifier, AND a partially completed Champ board I think I'll head down that road (thanks PRR). If I find that the SV83 doesn't do it for a Champ, I'll just rewire the tube socket for a 6V6.
Many Thanks to everyone who gave this thing a look.

-MadMax-

[HotBluePlates]

In the end, the proposed application as a low-powered guitar amp was, I believe, an afterthought.

SRPP screams "audiophile fad" while the volume control between the input jack and the (low-ish gain) 1st stage implies this build was meant as a SE power amp when you already have an existing preamp out signal (or a hot output, as from a CD player).

I'm pretty sure you'll do well modifying t to a Champ (or a tweed Princeton, which gives you a tone control to play with).

[kagliostro]

Accidentally today I noticed that the FireFly uses an SRPP input stage (or something of similar), such arrangement is odd in a guitar amp, also if possible



Franco

[PRR]

> FireFly uses an SRPP input stage

Cascode.

[kagliostro]

There was something of different ......... it isn't an SRPP


Thanks


Franco

[HotBluePlates]

In a nutshell: Cascode operation is where you take 2 triodes and use the pair to mimic pentode operation. Overall gain is similar to using a pentode, so you have same gain-per-socket, but less total gain than 2x high-mu triodes in cascade.

[2deaf]

I used a cascode for the output stage of a microphone pre-amp to go into a low impedance load.

[PRR]

Cascode has poor performance into a low-Z load.

[2deaf]

I'm going off of memory (rarely reliable) but I'm reasonably certain that I stacked two 12AT7 stages and pulled the signal out from between the two and it was quite linear into a low impedance load.  Maybe I rigged in in an unconventional manner or maybe I call everything that has stacked triodes a cascode when really they are something else, but I wouldn't have done it if it didn't do what I wanted.

[DummyLoad]

SRPP would work better for low z load, SRPP is not desirable to drive high z loads.


--pete

[HotBluePlates]

I'm going off of memory (rarely reliable) but I'm reasonably certain that I stacked two 12AT7 stages and pulled the signal out from between the two and it was quite linear into a low impedance load.

Then you had a SRPP or some other totem-pole topology.

But look again at the Firefly schematic above; the output is from the top tube's plate rather than a point between the 2 triodes. That's part of the reason the cascode appears to have super-high internal resistance and mimics a pentode.

[2deaf]

Sorry.  I scrolled right past that schematic.  What is SRPP an acronym for?

[DummyLoad]

shunt regulated push-pull. there are many other names for it. it was first patented by marconi as a modulator.

[2deaf]

Thanks.  I wasn't going to guess that one.

[MadMax]

Yeah, that's where I originally found it. But apparently 'should work' doesn't equate to 'should work well for guitar'.

I like that switching in the Firefly, Kagliostro. SRPP aside, is that the proper way to switch in an extra gain stage? The switch is DPDT, correct? Any idea if that thing will 'pop' when switched?

[jojokeo]

is that the proper way to switch in an extra gain stage? The switch is DPDT, correct? Any idea if that thing will 'pop' when switched?

Yes, that's fine. DPDT switch must be used and no, it won't pop. You have pull down resistors or = in place. Plus - it's only a couple watt amp, you're not going to be switching in the middle of a song or playing a gig right!?