EL86 / 6CW5 + 6BQ7A amp design

[kem69]

Hi. I am from Brazil, and i am newbie on tube amps.
I am trying to design a PP amp with EL86 power tubes + 6BQ7A pre / inverter with "18W Marshall Lite Style."
Can someone check this schematic to see if there is something wrong? The voltage chart was estimated.
Thanks a lot.

[Miles Prower]

Originally Posted by kem69

Can someone check this schematic to see if there is something wrong?
Thanks a lot.

I take it this is supposed to be a push-pull design? One final doesn't even have a signal applied to it. It's just hanging there, doing nothing. If you want the finals to self-split, you need a substantial (preferably a CCS) tail load, and you have none.

Four sections of two 6BQ7s in cascade is going to give way more gain than you'll need here, especially if you aren't using gNFB.

6BQ7s aren't the best type to use here either.

Either the design is badly messed up, or the schemo is.

[radiostar]

Do you really need two volume pots?

[Cookj]

There is a phase splitter:-

33k Anode load
Cathode 2k2 + 33k to gnd, 820k Bias, 10n cap coupled

[Blaireau]

Originally Posted by Cookj

There is a phase splitter:-

33k Anode load
Cathode 2k2 + 33k to gnd, 820k Bias, 10n cap coupled

Right! But a quick redraw of the schematics would make it easier to understand at first glance.

[Yvesm]

Miles was probably at the end of a long day
And it's obviously a guitar amplifier, that justifies the large gain and the two volume contols

Nevertheless, it should work, go for build it !

[Blaireau]

By the way, 25W with a couple of EL86... The designer was not really pessimistic!

[davorin]

You need coupling capacitor(5-33nF) between plate of 3rd triode and 2nd volume pot.

[davorin]

Originally Posted by Blaireau

By the way, 25W with a couple of EL86... The designer was not really pessimistic!

Yes, 25W is possible, but must rise first 32uF(68uF or more!) in HV-filter, and remuve 180ohm/5W, or substitute with much lower value (10-50ohms).

[kem69]

Originally Posted by Miles Prower

I take it this is supposed to be a push-pull design? One final doesn't even have a signal applied to it. It's just hanging there, doing nothing. If you want the finals to self-split, you need a substantial (preferably a CCS) tail load, and you have none.
Either the design is badly messed up, or the schemo is.

Originally Posted by Cookj

There is a phase splitter:-
33k Anode load
Cathode 2k2 + 33k to gnd, 820k Bias, 10n cap coupled

Originally Posted by Blaireau

Right! But a quick redraw of the schematics would make it easier to understand at first glance.

Originally Posted by Yvesm

Miles was probably at the end of a long day

Sorry for the poor design. It is my first, i tried to make the things easy to me.
The inverter is there, a little hide, i admit, but it is there... It will work???
My biggest doubt is about the value of the resistors on power supply. I need 250V to the plate, but 200V to the screen. 8k2 + 1K is good enough to decrease 50V at that point? Zero-signal screen current = 4mA and maximum-signal screen current = 23mA (G&E datasheet).

Originally Posted by Miles Prower

Four sections of two 6BQ7s in cascade is going to give way more gain than you'll need here, especially if you aren't using gNFB.
6BQ7s aren't the best type to use here either.

Originally Posted by radiostar

Do you really need two volume pots?

Originally Posted by Yvesm

And it's obviously a guitar amplifier, that justifies the large gain and the two volume contols
Nevertheless, it should work, go for build it !

The problem is... i have a LOT (more than 500 pieces) 6BQA7A... So, i want to use them.
And yes... the ideia is a guitar amp "18W Marshall Lite Style". The first volume is the gain control.
Good to know that i will have too much gain... Decrease gain is the easy part. I was afraid about do not have it... 6BQ7A has a Mu of only 35, less than a 12AV7.

Originally Posted by Blaireau

By the way, 25W with a couple of EL86... The designer was not really pessimistic!

I used the data on G&E datasheet... 25W for a pair of EL86 in PP...
But a good advertising is all.


Ps. My english is not so good. Sorry if i made any mistake.