does anyone know of a el86/6cw5 quad amp ?

rifraf50

Member
2015-09-19 10:28 am
hi good folk of diy.....does anyone know of a schematic for 4 el86's....guitar use of course,or can anyone give me a sketch that will work....cathode biased

I have already made a 17 watt amp using these tubes and quite like the sound but I want to put in 2 more tubes so that the other guys don't drown me out at rehearsals.

4 tubes....running 255 volts to the plates and 195 to g2 with 150r on the cathode per pair settling at around 17 v but howling loud

just cant seem to make 4 work....pull 2 and it sorts itself out....running 2k ohms on the OT
 

rifraf50

Member
2015-09-19 10:28 am
Power supply unstable? Feedback gone too far? EL86 is an EL84 with a different bias and current rating but pin for pin similar. More power than an 84 as well.
Have you got your circuit to hand and share?

I believe they are not the same....look up tube data and you will see, max plate = 230v......max grid 2 =200v and twice plate to plate resistance.....I'm asking for a 6cw5 / el86 circuit from the coupling caps past the phase inverter and onwards....this is where the problem lies.......2 tubes fine.....4 tubes problem......also the actual tubes I am using are PL84's/15cw5...basically an el86 with a 15v filament

I took it as a challenge to build the 17 watt and fell in love with it but now I want to add a couple more tubes for 30+ watts

checked all voltages and they are stable and pulled the preamp and phase inverter and problem still apparent...uncontrollable oscilation.....just don't know how to tame it
 

Printer2

Member
2010-04-02 6:34 pm
I believe they are not the same....look up tube data and you will see, max plate = 230v......max grid 2 =200v and twice plate to plate resistance.....I'm asking for a 6cw5 / el86 circuit from the coupling caps past the phase inverter and onwards....this is where the problem lies.......2 tubes fine.....4 tubes problem......also the actual tubes I am using are PL84's/15cw5...basically an el86 with a 15v filament

I took it as a challenge to build the 17 watt and fell in love with it but now I want to add a couple more tubes for 30+ watts

checked all voltages and they are stable and pulled the preamp and phase inverter and problem still apparent...uncontrollable oscilation.....just don't know how to tame it

I think JSE was trying to find out what it was that was happening, oscillations with an additional tube set. As far as the tube being similar to the EL84, the EL86 was designed to work at lower voltages but the designers used the EL84 as a starting point. They used the basic structure of the EL84 and increased the current rating and (I think) reduced the distance of the grid from the cathode. Basically it is a EL84 variant.

It would be good to see the schematic of the amp with the four tubes, even a hand drawn one . One thing that would be needed is grid and screen resistors on all the tubes. It would be best if the resistors were soldered right to the socket pin. The screen is an input to the pin and if a resistor is not there you can get feedback.
 
No, leave the dropping resistor and cap as is. Add an additional resistor in series with each screen grid mounted right at the pin of the tube. I use 100 ohms, but 220 or so might be a better choice.

I have wired 4 X 6CW5's together much like you have done, without any issues. I used a separate 270 ohm cathode resistor for each tube, but it shouldn't matter.

I have squeezed about 50 watts from 4 X 6CW5's with 200 volts on the screen, about 300 volts on the plate, and a 1650 ohm OPT. The tubes were all used GE's pulled out of old HP audio oscillators. I used the amp for a couple of years before giving it to a friend.....it never blew up.

If you have any feedback connected to the secondary side of the OPT, try disconnecting it.

I have another amp around here somewhere made for the Hundred Buck Amp Challenge. It got lost during a 1200 mile move, but I will find it eventually and continue tweaking it. That one used a pair of UL84's (45B5's) which are just 6CW5's with a 45 volt 100 mA heater. I ran about 320 volts of B+ with 165 volts on the screens through two tubes for about 25 watts output.
 

rifraf50

Member
2015-09-19 10:28 am
thanx guys

I seem to be slowly getting on top of it ,changed out the grid 2 setup....2k2 per pair then 150r to each....dropped the voltage about 3 or 4 volts......do you think voltage is lowish ? perhaps change 2k2 to 1k5 or 1k8 ?

also in order to utilise pl84 I used a 18v (2 x 9v) and used dropping resistors to get 14.5v then more dropping resistors to get 12.5v for 12ax7...I used the 9v as a centre tap ............where as prior I went 15v from 18v with 2 100r to create centre tap and then went 6v from 9v (using dropping resistors) and again 2 x 100r as centre tap this was fairly quiet but wondering if I should put artificial centre tap on 12.5v as it seems a little noisy ?
 

rifraf50

Member
2015-09-19 10:28 am
power supply

just wondering.....what are your thought on running 15v filaments on 12.5v.....I know from an experimental point of view I ran 6aq5's(6.3v) on 5aq5's (4.7v) voltage and both sounded the same to my ear and the filament glowed as red as if it were on 6.3v it was a class A super single ended amp....I just subbed 5aq5 for 6v6gt and lowered plate volts.....9 awesome watts ....a great little amp and simple to make

the reason for this is I am using a 18v toroid (2 x 9v) as a power supply for the 15cw5's and 12ax7's.......and I'm considering placing the dropping resistors on the centre tap .

space is very limited in this unit and these resistors are getting a little hot so I am considering bigger resistors and mounting on top of chassis...chassis is 14" x 6" x 2.5" so you can see the battle I'm having.

dropping voltage for 2 tubes wasn't an issue but 4 = hot
 

rifraf50

Member
2015-09-19 10:28 am
anon...

well they seem to run just fine...I think.....they are running at 13v ac and everything is happy.......but !!!

the amp itself is very loud,man I triple checked everything.....I have replaced the PI with a 12au7 and put 82k load resistors on PI and V1 and the amp is still roaring changed cathode resistor to 172 ohms per pair on the pl84's and the bass is a little gravelly its gonna sound great when I get it sorted....the second gain stage cathode has been subbed from 820r to 1k5

the volts at V1 are 210v and PI is 250v (I think theyre suppose to be around 270v +/- 10v

I was thinking using smaller coupling caps but would the sound thin out ?

the grumbling bass has been consistant through all changes, but then i'm only a tinkerer...perhaps i'm missing something (a brain ?)

is there too much drive to the power tubes (I cant see it ) or perhaps bad bias in the preamp ?

i'll post the layout (scetching the schem is a pain but will do when happy with it),and maybe someone might see where I'm going wrong

and thanx for the support so far,you's have been helpful

regards rifraf
just wondering.....what are your thought on running 15v filaments on 12.5v.....I know from an experimental point of view I ran 6aq5's(6.3v) on 5aq5's (4.7v) voltage and both sounded the same to my ear and the filament glowed as red as if it were on 6.3v it was a class A super single ended amp....I just subbed 5aq5 for 6v6gt and lowered plate volts.....9 awesome watts ....a great little amp and simple to make

the reason for this is I am using a 18v toroid (2 x 9v) as a power supply for the 15cw5's and 12ax7's.......and I'm considering placing the dropping resistors on the centre tap .

space is very limited in this unit and these resistors are getting a little hot so I am considering bigger resistors and mounting on top of chassis...chassis is 14" x 6" x 2.5" so you can see the battle I'm having.

dropping voltage for 2 tubes wasn't an issue but 4 = hot
 

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Printer2

Member
2010-04-02 6:34 pm
If you want you could increase the bias resistance on the PI, some go up to 1.5k. The greater the tail resistor the less headroom on the PI stage and the lower the signal to the output tubes. Could also play with that. Otherwise can't see anything others have not done. Could also lower the resistance on the grids of the output tubes to load down the PI if you are hitting them too hot. Or lower the voltage to that stage. Wish I could give more concrete advice. If you could maybe join the 18watt forum.
 

rifraf50

Member
2015-09-19 10:28 am
resist,resist !

If you want you could increase the bias resistance on the PI, some go up to 1.5k. The greater the tail resistor the less headroom on the PI stage and the lower the signal to the output tubes. Could also play with that. Otherwise can't see anything others have not done. Could also lower the resistance on the grids of the output tubes to load down the PI if you are hitting them too hot. Or lower the voltage to that stage. Wish I could give more concrete advice. If you could maybe join the 18watt forum.

I have taken the two 470k's to 220k (for 4 x 15cw5)....

at about half volume i'm getting 215 ish volts on g2 ( amp is pretty loud by now) at 0 volume its around 170 v (with 12ax7 in PI )....the arrangement used for G2 is 255v >2k2 5watt per pair then 3k3 5watt each one

amp is pretty Quiet....only thing bugging me is the bass,it seems to have slight odd order harmonics and its persisted right through the build....on the cathode of V1 I have subbed the 2uf bypass cap with a 25 uf to increase bass response... maybe take it back to 2uf or something middle 10uf ?:confused:
 

rifraf50

Member
2015-09-19 10:28 am
output tranny ?

I have taken the two 470k's to 220k (for 4 x 15cw5)....

at about half volume i'm getting 215 ish volts on g2 ( amp is pretty loud by now) at 0 volume its around 170 v (with 12ax7 in PI )....the arrangement used for G2 is 255v >2k2 5watt per pair then 3k3 5watt each one

amp is pretty Quiet....only thing bugging me is the bass,it seems to have slight odd order harmonics and its persisted right through the build....on the cathode of V1 I have subbed the 2uf bypass cap with a 25 uf to increase bass response... maybe take it back to 2uf or something middle 10uf ?:confused:

changing 25uf to 2uf did improve it a little,begining to sound damn fine, the distortion is soo smooth.....just wondering ,is this low end bass growl at half to full volume possible transformer saturation ?.......Hammond 1760 j 35watts @ 4k

currently running reflected load of 2K.......16ohm to 8ohm speaker

would running at 4k decrease the load ? and thus possible transformer saturation ?:crazy:
 

Printer2

Member
2010-04-02 6:34 pm
changing 25uf to 2uf did improve it a little,begining to sound damn fine, the distortion is soo smooth.....just wondering ,is this low end bass growl at half to full volume possible transformer saturation ?.......Hammond 1760 j 35watts @ 4k

currently running reflected load of 2K.......16ohm to 8ohm speaker

would running at 4k decrease the load ? and thus possible transformer saturation ?:crazy:

Give it a try, not sure if it would be saturation but the tubes may sound different with a different load.