schematic critique

[centervolume]

Hello All

I am drafting schematics for a new build, parallel single ended with 5881s into a 10, voiced heavy bottom for harp.

Is this the right location to post a copy of the schematic for comments and suggestions etc?

[SilverFox]

General technical questions on amps. This is it. Can't wait to see...

silverfox.

[centervolume]

Ok let's see how this works. It's an early draft, but gotta start somewhere!

[centervolume]

This is from a buddy of mine who proposes adding a couple more 16uf caps to hold the voltage steady when the switch is flipped from full to half power. His description:

drop the screen voltage applied to *both* of the 6V6 power tubes in this amp (keeping the quiescent plate voltage constant, 325 Vdc - see 3rd schematic - *Screen_Mod.pdf), then the output power will decrease. If you drop the screen voltage to zero, there should be little/no output. So, looking at the 3rd schematic for this amp, the 6V6 screens are connected at the junction of 10K & 22K resistors in the DC power supply circuit chain (@ 250Vdc). At the B+ of this amp (pin 8 of the 5Y3 rectifier) e.g. you could add a simple 2-resistor divider to ground (call these resistors R1 and R2) and then add e.g. a 20 uF/450V electrolytic cap at the junction of R1 & R2 (the -ve side of this electrolytic cap is connected to ground, to hold the voltage at junction of R1 & R2 steady, DC voltage-wise). By suitably choosing the values of R1 & R2 (e.g. 220K and 100K) this DC voltage at the junction of R1 & R2 would be ~ 125Vdc (i.e. ~ half that of 250Vdc). Then you could use e.g. a (good-quality!) SPDT switch to simultaneously switch (both) of the 6V6 screen voltages from 250Vdc to 125Vdc, the latter setting would be ~ 1/2 power... An additional benefit of this method is that the overall impedance shouldn't change too much for full power vs. ~ 1/2 power switch settings...

[centervolume]

ok! hold your horses, make a single file line, there's time enough for everyone to chime in. Not all at one time!

The schematic is attached as a pdf (above) but you have to be logged in to get access to the download link to show up.

Would it be preferable for me to post it as an image? rather than requiring download?

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[guittech]

Lol it hasn't been 24 hours since you posted the schematic!!!!

Sometimes all that is hanging around are people like me who can say "Looks nice, but i have absolutely NO experience in single ended amps, let alone parallel ones!"

Even Tube Gurus gotta sleep man! And us hangers on don't always have much to say

[sluckey]

The DC-1 is missing some important components. V2B is missing a plate load resistor and coupling cap. Both output tubes need a grid leak resistor. V1A would be happy with a dedicated grid leak resistor.

PDF format is fine.

[Colossal]

Ok, I'll bite on a couple of things. Looks like a good start.

1) Why the triple input? Do you see that actually having utility?
2) On your schematic, for the third stage, the plate load resistor and output coupling cap are missing.
3) The grid leak to ground for the output tubes is also missing.
4) Screen grid resistors should be added on the power tubes. 470R - 1k.
5) The input grid resistor on lower power tube is only 470R, and missing on the upper tube. This may need to be increased to control oscillation as there is a high gain driver (post tone stack recovery) before the power tubes.
6) The 250uF bypass cap on the first stage will probably be a bit much. 1-22uF would be advised, probably 1-5uF in practice to keep the bass tight and not muddy
7) Similary, the second and third stage bypass caps will likely need to come way down. 0.47-0.68uF
8 ) Consider adding a Pi filter BEFORE the first B+ feed to the OT center tap. As shown, 16uF with nothing before is very likely to result in hum. 47u - 100R 10W - 47u - B+1 OT would be better.

[Stevem]

Where's the grid reference to ground for V1a?

[SilverFox]

I started a post right after your schematics yesterday but being also one with limited experience chose to hold my fire.

When I first viewed your schematic I found it a little bewildering to decipher and had a hunch something wasn't right. However with limited experience couldn't put a finger on it. This forum can be a great learning opportunity if you push yourself to wood shed some new skills. I would suggest redrawing the schematic if on nothing else than graph paper, like I did since I didn't want to learn a new program at the time.

My preference would be the Gibson schematic and perhaps change it over to 5881's. What I had put together but then discarded was: Since your friend has experience with that circuit it may be a good idea to build that so if there are any problems it could be easier to debug.





silverfox.

[JMFahey]

With due respect, I suggest you choose and build a tried and true circuit.

Start building it "by the book", practice and keep learning, and in a future you'll design your own, but somebody said: "you must know and practice the rules very well before you start breaking them".

It was said in an art painter's context, but applies here too.

[SilverFox]

My Uncle, a Jazz Pianist for over 40 years was teaching me basic music and piano. His educational background was Boston School of Music back in the 50's, if that was the school. Anyway, that's what he would tell me in regards to the rules: Learn the rules first, then you'll know how to break them.

silverfox.

[centervolume]

ok so I'm an eager beaver very good fellas, thanks for taking the time. Let me digest a few of these comments and recommendations and come to a second draft.

The reason for the triple input is simply because I am operating from a 5F11 tweed vibrolux chassis, this is one of the structural limitations of the build, it has to fit in that box.

I'll get my penmanship in order, graph paper is a good recommendation, or at least a ruler wouldn't hurt.

Thanks for pointing out the things I left out, I'm aware of the fact that my novice level warrants something a little simpler but this is what I want to build instead of just copying designs from A-Z as I have already done a few times.

I'm after a versatile 10-20 watt with a wide variation of tonal possibilities (bass, mid, treb). I think my transformers are good for this circuit, I feel I just need to get the bugs out of the schematic. If it's a complete p.o.s., I can take the heat for it; at this point its just a start.

[tubeswell]

JM2CW - if you're using two dual tridoes the preamp and two pentodes in the output stage, you'll get more bang for buck making a Push-pull amp.

Supro t-bolt circuit or early (wide-panel) tweed pro is a good circuit for harp if you want cathode based 5881s. The paraphase inverter really is magic here.

[centervolume]

that's cool, I'm willing to sacrifice efficiency for more pronounced even order harmonics. the "bang" I want is less about volume and more about tone.