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.
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.
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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.
Right! But a quick redraw of the schematics would make it easier to understand at first glance.
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).
Sorry for the poor design. It is my first, i tried to make the things easy to me.Miles was probably at the end of a long day
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).
The problem is... i have a LOT (more than 500 pieces) 6BQA7A... So, i want to use them.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 !
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.
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.
I have seen 30 watts and that is without any glow or ill effects in a HiFi amp. This is a guitar amp and distortion figures at full power are meaningless as long as it sounds good cranked! 25 watts is definitely possible.
Beware some of the junk that has been stuffed into glass envelopes and called 6CW5 / EL86. I have seen Canadian made RCA's with guts about 2/3 the size of real 6CW5's that glow brightly when operated at 6CW5 specs and some Sylvania's that bear no resemblence to a 6CW5 at all.
Some 6BQ7's can be rather microphonic in a guitar amp but some are OK. 4 stages is not excessive gain either.
There is a $100 guitar amp design challenge on this forum. One of my designs started out as a "18 watt Lite" with 6BQ7's and 6CW5's. it now uses a series string heater circuit to fit in the $100 budget. It has also gone through a few other changes. I have changed input tubes to get more gain (I like excessive gain) but the output tubes are UL84's which are odd heater voltage 6CW5's. I run the plates at 320 volts and the screens at 160 volts unsing cathode bias for a really screaming 20 watts!
http://www.diyaudio.com/forums/instruments-amps/190738-hundred-buck-amp-challenge.html
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The second volume pot shouldn't be exposed to a high DC voltage, so I would want a cap before it. But you can't remove the cap after it, because the grid of the inverter is not grounded, so you end up with 2 caps. Not my favorite topology.
The screen bias has a common resistor and individual blockers. Not sure what that's all about.
If it's a guitar amp and the purpose of 2 volume controls is to overdrive it at different stages, I would use a higher gain tube for the gain stages.
Personally I don't think it's a very good design for a guitar amp. You're better off copying a recognized good design.
The screen bias has a common resistor and individual blockers. Not sure what that's all about.
If it's a guitar amp and the purpose of 2 volume controls is to overdrive it at different stages, I would use a higher gain tube for the gain stages.
Personally I don't think it's a very good design for a guitar amp. You're better off copying a recognized good design.
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Mine are RCA, but they have exactly the same size of a EL84. Is it good?
tubelab.com... do you have the link for for the schematic of this "18 watt Lite" with 6BQ7's and 6CW5's? And do you think that my schematic can work?
About coupling capacitor... I used one after the pot. Do you think is better to use just one before the pot or both?
I will put 100uF on the place of 32uF and 33 on the place of 16uF ones.
And about the EL86 cathode resistor (150R)... Higher values = lower power? If yes, can i try 100R and reduce if necessary?
I used the AX84 18w Daisycutter schematic to modify. These resistors were on the original schematic, and as i am in doubt of what value of resistor use to drop 50V (with Zero-signal screen current = 4mA and maximum-signal screen current = 23mA), i made no changes on them.
I tried to find one with 6BQ7A and EL86 /6CW5... but i can´t...
Do you know some?
Thanks a lot.
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davorin... Do you think it is better that i use, for example, the AX84 PP schematic?
My RCA EL86 (XL86 - 8V - same size of EL84) can delivery this power?
And now i realized that the 180R resistor you was talking about is the SAG resistor, not the cathode resistor.
I made some changes on schematics... Listening to what you guys said....
Someone can check again?
Thanks a lot to you all.
Regards,
Kem
My RCA EL86 (XL86 - 8V - same size of EL84) can delivery this power?
And now i realized that the 180R resistor you was talking about is the SAG resistor, not the cathode resistor.
I made some changes on schematics... Listening to what you guys said....
Someone can check again?
Thanks a lot to you all.
Regards,
Kem
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No that cap must be. The grid of the phasesplitter is not at gnd. You might want to use resistor values that give a little more juice since those tubes are a little more low voltage/higher current compared to 12ax7s. I assume the schem is based on those tubes.
I build guitaramps with 6922 and they have even less gain (33) so dont worry about that. The 6bq7 is pin compatible and I have just got me some (they are cheap). Besides, looking at the graphs they seem to be slightly semi-remote cutoff types, and I bet that can be really good for guitar duties. I havent had time to test them yet, but definately will use them.
I build guitaramps with 6922 and they have even less gain (33) so dont worry about that. The 6bq7 is pin compatible and I have just got me some (they are cheap). Besides, looking at the graphs they seem to be slightly semi-remote cutoff types, and I bet that can be really good for guitar duties. I havent had time to test them yet, but definately will use them.
Screen resitors of output tubes can be 100ohms 1/2W, no need to be 3W. You can connect each screen of EL86 via 2 Zener diode 48V/2,5W to center tape of output transformer, and will not need 6,8kohms/3W+100uF. In that case, plate voltage will be allways 48V higher than screen voltage. Also You can rise high voltage to 265V (screens via zener-diode!), any brand of EL86 can work with screen voltage <220V if plate voltage is higher. Last schematic is good, but You can also use full tone control (T-M-B), there is lot of gain there. Put grid stopers at third triode and phase splitter (~100k).
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I tried to find one with 6BQ7A and EL86 /6CW5... but i can´t...
Well, that's not how you design a good amp. Either you design it properly around the tubes you have, or you copy a good one if you can find one. If you can't do either, you're likely to end up unhappy.
If i remove the cap after tone/volume control, the pot volume will not groud the screen?
I use the resistor/caps values from RCA manual 6BQ7A table. I use first triode with 24 gain + Eo 70, second with 25 gain + Eo 45 (to saturate easier - if not, i will drecrease 1K2 resistor) and third triode with 25 gain + Eo 60 - all approximate values. Inverter have a unitary gain (i think ).
To conect the screens of EL86 to B+ via 2 zener, i have to remove the 100R resistors? I have to use only the zeners to provide voltage to the screens?
A friend of mine have a EL84 PP with 2 x 12AX7 and only one tone control... i liked it.
But i will try with full tone stack too.
Thanks.
I use the resistor/caps values from RCA manual 6BQ7A table. I use first triode with 24 gain + Eo 70, second with 25 gain + Eo 45 (to saturate easier - if not, i will drecrease 1K2 resistor) and third triode with 25 gain + Eo 60 - all approximate values. Inverter have a unitary gain (i think
).To conect the screens of EL86 to B+ via 2 zener, i have to remove the 100R resistors? I have to use only the zeners to provide voltage to the screens?
A friend of mine have a EL84 PP with 2 x 12AX7 and only one tone control... i liked it.
But i will try with full tone stack too.
Thanks.
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