high gain amp design

[big_teee]

Donīt entangle yourself with A and B which are just labels applied by you (not the tube manufacturer).
Point to remember is that pins 1/2/3 form one group, and 7/8/9 form another, do not mix elements from different groups.
Label them with any letter you fancy, donīt get carried away by it.

I think You had a Typo There My Friend on the 7/8/9 ?
How about 6/7/8
T

[beinz]

will do, i will have to go back and find who posted the schems, it was from the sloclone forum.

[big_teee]

will do, i will have to go back and find who posted the schems, it was from the sloclone forum.

So do you have that thing Running Yet?
What's the Hold Up?
T

[J M Fahey]

@ bigteee
Yes, of course, I slipped, I meant 6/7/8 .
Thanks for catching it.

[beinz]

thanks to memphiswillbelaidtowaste and defaced from sloclone forums for the schems!

[beinz]

i wish, these schems will help a tonne tho, not alot of spare time after work for "fun stuff" like this.
but this gets me there a bit faster.

[beinz]

this is what i thought, the diagram seems to jive with the schem then, for the most part.
im working on a diagram in inkscape, if life ever allows, i will finish it, and post it for scrutiny!
thx for your help guys. also, if anyone wants to chime in about standby switch wiring, please do.

[beinz]

looking over the avenger schem, the values for the mains and screens caps look significantly larger than what im used to seeing on other amps, and also larger that what i have read is necessary. seeing as you have experience with this particular circuit (thanks for the schem btw!) do you find this over kill (560u and 220u) or would values closer to marshall's pwr supply stage 50+50 and 50+50 be sufficient. im sure there is next to no sag with these, do the larger values sound "stiffer" (pardon the term) than the marshall values, or did soldano do this just "because" ?
from what ive read, it seems anything over 100u and 100u is over doing it...

[diagrammatiks]

those caps are in series.

the avenger is only 220uf and 100uf.

[defaced]

Above a minimum amount of needed to adequately filter the voltage rail, PSU filter caps in big AB amps affect feel more than anything else. Build, see if you like it, tweak. Use what you have on hand to start, and don't sweat this detail.

As a for instance, I have two amps with identical power amps, but very different power supplies. One is a CRC pi filter, 220u in series (110u effective), 390R, 33u. The other is a CLC pi filter, 680u in series (340u effective), 10H choke, 100u in series (50u effective). I might be able to feel a difference between them if I A/B tested them, but for what I'm after in an amp, I don't notice a difference if I just walk up and play one.

Oh, the 68u filter cap in the preamp can be smaller (if you happen not to have that value on hand). I'm using 22u in mine and have no problems from it.

Standby switch, put it wherever you like, there are many options: The Valve Wizard. I put mine between the rectifier diodes and the first filter cap with a resistor bypassing it to pre-charge the caps so I don't have problems with inrush current.

[beinz]

i stand corrected, duh!

[beinz]

thanks, 22u -68u and not much difference is good to know, lots of wiggle room.
i'll check out the valve wizard link on more bp options.
im sloooowwwwwly drawing this thing out in inkscape right now, scotch and schems!

[beinz]

so ive done some reading on valve wizard, and came across this

To find a suitable value for the grid-stopper, simply apply the formula for a low-pass filter, where C is the dynamic input capacitance and f is the desired low roll-off frequency- in this case 20kHz:
Rg = 1 / (2 * pi * f * C)
Rg = 1 / (2 * pi * 20000 * (100 * 10-12))
= 79.6k
You will often see old amplifier designs using a 68k grid stopper. This will provide a roll-off of approximately 23kHz, which is close enough. However, the guitar itself also contributes a series resistance, that may range from a few kilo-ohms to several hundred kilo-ohms if the volume controls are turned down. This can easily cause treble frequenices to be rolled off, losing some of the high harmonics and 'chime' of the guitar sound. In practice it seems that in most cases the grid stopper can actually be made quite a bit smaller than 68k, and 10k to 33k will do, unless you happen to be playing nextdoor to a radio transmitter.

It is worth noting that in pentodes Cag is very small, so the dynamic input capacitance can be assumed to be roughly equal to Cgk.

Increasing Miller capacitance: A further problem with the usual approach is that it places a very large value resistor in the signal path, which introduces noise. This isn't a worry in later parts of the preamp, but at the input we want to keep it to the bare minimum.
The grid stopper can be made smaller in value if the effective value of the input capacitance is made proportionately larger. This can be done by placing a capacitor between the anode and grid of the valve. This capacitor applies negative feedback of very high frequencies to the grid, and appears in parallel with the Miller capacitance, so its value is also multiplied by the gain of the stage, due to the Miller effect. Therefore the effective input capacitance is greatly increased and becomes equal to:
Cin = Cgk + (Cga * A) + (Cf * A)
Or: Cin = Cdyn + (Cf * A)

To use the previous example, a roll-off of 20kHz is desired, but this time with a grid-stopper of just 10k. The necessary effective input capacitance required would be:
Cin = 1 / (2 * pi * 20000 * 10000)
= 796pF

The value of the feedback capacitor would therefore be:
Cf = (Cin - Cdyn) / A
(where Cdyn is 100pF found earlier)
= (796 - 100) / 60
=11.6pF
In this case the closest standard value would be 10pF, providing an acceptable roll-off of about 23kHz. The capacitor used should be of high quality so as not to introduce its own noise, and should have sufficient voltage tolerance to withstand the anode voltage. A close tolerance ceramic capacitor would suffice.

For those afraid of the capacitor failing and placing a high voltage on your guitar strings, the capacitor could be placed between grid and cathode instead, although it will need to be a higher value since it won't be subject to the Miller effect. In this case:
796 - 100 = 696pF
The closest standard would be 680pF.
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The succeeding gain stages in an amplifier are normally enclosed within an earthed chassis where RF interference is unlikely to be picked up, therefore RF blocking is usually only used on the input where the guitar and guitar lead can act as antennae. However, if RF is getting into an amp somewhere, it may be necessary to add a grid stopper and/or feedback capacitor to the offending valve.

Of course, we don't just have to limit RF. Any roll-off frequency can be chosen for this method, and it is often used to limit treble frequencies in bright amplifiers, and this is why you may see grid-stoppers in later stages within an amplifier.


the last part has sparked my curiosity,
would a 10k grid stopper, and a 700pf cap across p7 and 8 be a better rf filtering option than the traditional 68k res?
have any of you tried this method?
as stated in the paragraph before, tying the pf cap between p6 and 7 leaves the opportunity for voltage across guitar strings if the cap fails, not so good...
the second method with this higher cap value across p7 and 8 seems safe, and is suggested to be quieter than the standard 68k.
what do you guys think?

[diagrammatiks]

you won't have any noise issues with a 68k gridstopper.

[beinz]

another thing i have stumbled apon is the nfb resistor value. the soldano avenger schem has it at 39k running 6l6gc tubes, the avenger layout has the same 39k with
6v6 tubes both to the 4ohm tap, but marshall uses a 100k with el34 tubes to the 4ohm tap.
i understand both the 2ndary tap and the tube type will affect the value of the resistor, but why the variance between soldano and marshall, or is this just part of what gives them their tone?

trying to determine if following the avenger schem, i will need to change this 39k as i will be running 6550 tubes?