How do you parallel power stages into one speaker?

[guitarboy58]

I've just recently completed two amp projects; one with a 6V6 power stage and the other with EL84's.  Both have similar pre-amps and are running as cathode biased, push-pull configurations.  Every one who hears them thinks they sound great, as do I.  So one day a friend asked if I could build him a combo using both designs together so the two tones could be switched or blended. After some research I find that the Bad Cat Tone Cat and the Divided by 13 JRT 9/15 do something like this, but I'm having a mental block as to how it's done.  Anyone know how and care to share the secret?  Now that the idea has been suggested, I'd really like to try and build that combo version of my two designs.  Thanks.

[Geezer]

Weber has a switch that (I think) will do what you want  (look at the "high power" unit)

https://taweber.powweb.com/store/lpsw.gif

https://taweber.powweb.com/store/chansword.htm

I guess you'd need to have a dummy load for the OT secondary that's not being used(?)

[mackie2]

I have built several amps through the  years mixing preamp and power amp tubes on the  same OT.

To do this type of amp--Have a slightly oversized OT and and keep in mind like  "Z"  matching.
Separate coupling caps with  drive  limits for each type of tube.  They can be either fixed or self or both types biased  output ckts.

Keep in mind  6V6's will generally deliver more bass--EL 84's more mids  and treble.

Plate stop resistors of 470 ohms in 6V6's  and 1k in EL84's (Power resistors in series with plate to OT PP
winding)  Cuts power somewhat, but cuts out a tendency to oscillate under full power.

Consider increasing grid stopper resistors to paralleled PP  power tubes of dissimilar power types.

Good luck building and experimenting with this type amp--It should be a pleasant surprise.

Mackie2

[kagliostro]

you can do it very simply as in the Mesa Boogie Blue Angel

and you can have also a mix of the tubes

if you use only 2 x el84 (remember to modify the cathode resistor for that respect to the mesa schematic)

the primary OT impedance must be 4000 ohm and you must have (at least) 2 output impedances as to use the speaker in the right connection to have the desired reflected impedance



in my Vox mix amp schematic I do someting like that

see here

http://www.mediafire.com/file/w5nngmm3ugm/AC30

REVISED ==>  http://www.mediafire.com/file/mymhnyngozy/AC30

pay attention my schematic has mistakes and is not completed

however, the part of the final should be more or less correct

(due to the use of the doubled PPIMV may be is better to have 470k resistors instead of 220k resistors)

Have fun  

Kagliostro

[guitarboy58]

Thanks guys.  I'm new at actually building amps from scratch and tend to over think things.  I knew it had to be easier than I was imagining.  Any other comments and ideas are most welcome.

[guitarboy58]

I have built several amps through the  years mixing preamp and power amp tubes on the  same OT.

To do this type of amp--Have a slightly oversized OT and and keep in mind like  "Z"  matching.


So I have a question about this.  6V6 and EL84 push-pull amps usually have an 8k (or somewhere thereabout) winding on the OT.  Are you suggesting a different value because the amps are paralleled on that winding?  It makes sense to size the winding for more current (two amps pushing it), but I'm interested in your thoughts about the actual impedance.  Thanks.

[tubesornothing]

If we were to take a clue from randall smith and his Mesa amp, the impedance would not change if 6V6, or EL84 or both combined.

[guitarboy58]

That's the way I saw it; no change in the impedance for the amps where I could find a schematic.  It just didn't sound completely logical, so I raised the question.  Thanks for the confirmation.

[kagliostro]

About the impedance

with 4 x el84 a 4000K OT is fine

with 2 x 6v6 also good a 4000k

with both 4 x el84 and 2 x 6v6 I think is better to connect the loudspeaker in a way as the reflected impedance will be adequate

Mesa in the manual of Blue Angel tell someting that I read so

Kagliostro

[tubesornothing]

Good catch Kagliostro.  Check the bottom page 4 of the user manual:

http://www.google.ca/url?sa=t&source=web&ct=res&cd=1&ved=0CAgQFjAA&url=http%3A%2F%2Fwww.mesaboogie.com%2Fmanuals%2FBlue%2520Angel.pdf&ei=w6S_S5WCIIKCngeC9ryCCg&usg=AFQjCNGv43FY6H2FvEooDiiwLucNKW-jSQ&sig2=LbvFO99Na1iQzGZU9Va1HQ

[kagliostro]

Exact that is the page which I referred

Kagliostro

[mackie2]

KAGLIOSTRO--

In my earlier reply, I was referring to OT being higher power rating--bigger iron on the hip.

Look to the left--that red cap amp, by my name, is a tri-amp all three on a single OT--The output tubes are 6SN7, 6L6, and EL84 all on same OT.  Variable bias (front panel) on the 6L6 and EL84 allow for blending of all three--6SN7 is fixed bias. I have it set for 1 watt min to max range bias controls 40 watts--the 6SN7 is studio level.  The 6L6 has more bottom--the EL84 comes in with mids and highs.

We are working on a Studio Amp called LITTLE BLUES TONE that will have this kind of arrangement-- no VVR.

No schematics at present.

Mackie2

[kagliostro]

Hi Mackie2

you make me curious

6SN7 as final tubes

how many ?

2 x 6SN7 with the two triodes in parallel than PP it ?

how many W do you obtain from that final tubes ?

which plate voltage and which OT primary impedence ?

screen and cathode resistor value ?

in your amp do you swap the different type of tubes or you parallel different tubes like in MB Blue Angel ?

do you change bias level in which way ? lifting the cathode bypass cap from ground ?

lot of question to put

can you give more info to us ?

THANKS

Kagliostro

p.s.: sometime I don't understand well english

[mackie2]

Kagliostro

6SN7 as final tubes----------Red cap amp has 1 in PP
how many ?-------------  I have used up to  4 in PP
2 x 6SN7 with the two triodes in parallel than PP it ?-------  YES
how many W do you obtain from that final tubes ?
which plate voltage and which OT primary impedence ?------340v
cathode resistor value ?---6sn7    500 ohms to 750 ohms
The driver is along tailed pair--With each tube having their own coupling caps--TRI AMP=6 coupling caps.
in your amp do you swap the different type of tubes or you parallel different tubes like in MB Blue Angel ? ----- yes  6v6 or 6L6--put 500 ohm WW PWR RES in plates of 6v6 and EL 84--none in 6L6 plates
Screen resistors  in pentodes (normal).  I always put a 1 ohm in cathode of each  PP power tube so I know what the current is in each PP pair--Don’t exceed the 12MA in each section of 6SN7--The other tubes will run less than full potential--you are more interested in blending and pwr level control of 6V6’s, 6L6’s, or  EL84’s.

The  designed bias ck. is variable for all but the 6SN7--which is fixed bias=ck. 500ohms.

6v6 or EL84 ck.=470 ohm (variable  bias for each--arrived  at experimentally for range to cutoff)
6L6 ---------ck.=300 ohms  (Experimentally for range to cutoff)
do you change bias level in which way ? lifting the cathode bypass cap from ground ?----  No switches at all. (variable bias only add more negative voltage, that sums  with CK Voltage.
***Var Bias is front panel  ADJ---------***********
lot of question to put --I built the 1st amp with--6SN7 1EA PP, 6V6
2EA PP, and EL84 2EA PP--

The 6SN7 alone is 1 Watt into a JBL 10” --  Pretty loud for studio--
Have    vol--tone-- mv

can you give more info to us ?  When we finish Little Blues Tone,

I can give more--Happy experimenting--YOU ALL!!!!!!

Mackie2

[kagliostro]

Hi Mackie2

Many thanks for the given info

waiting for your's further news 

Kagliostro