Blending two circuits

[TIMBO]

Hi guys, this build has been very much an experiment and the circuit combinations in tube amps is ENDLESS but the 6AN8 pentode was great but made the amp a bit lifeless and very loud, maybe works better with bigger tubes 

Anyway, I have rebuilt it into a Train Wreck as per schem A5a and has a lot better response and touch with a nice tone and some breakup when cranked. The ELs are a bit bell/chime sounding as expected.

After seeing this http://www.el34world.com/Forum/index.php?topic=16391.0 tweaked a thought, could the OD circuit be hard wired as per the schem and a balance pot used to blend the two circuits.

OR add a blend resistor as per Sluckeys schem and use the OD LEVEL to increase/decrease the amount of OD added back into the circuit. Just some thoughts 

[tubeswell]

I think any pentode will do best if its running directly into a stage with a really high input impedance (like a bootstrapped cathode-biased stage, or a bootstrapped cathodyne). Anything you do that loads the pentode (like adding mixer resistors or blend pot after the pentode to mix with another channel is going to cause noticeable loss. If you had a spare triode you could make a cathode follower or just use a mosfet source follower (like an IRF820), or if you had room for a spare dual triode, you could use a mixing stage (where the stage following the pentode was bootstrapped) where your blend control could be a variable bypass boost for each side of the mixing stage. YMMV

Oops! I just peeked at your schematic after all that.  You don't need the blend control if you have the 470k and pot for the level control of the OD stage(s)

[terminalgs]

maybe You don't need a blend pot, just do it like a reverb circuit:



R1 and R2 are mix resistors.  you might as well use a pot instead of R1 and R2 to figure the ratio, but I'd think it'd be static once you figured it out.

the reason I don't think you need a blend pot is the combination of  P2 and "V" pot will determine the levels of the two.

I'd make the R3+P1 series resistance be high, at least 1M, and make R1 and R2 be higher as well, like (if its 50/50 mix) 1M/1M, or if end up being a 1:4 mix, make R1 be 1M and R2 be 4M.   This is so V1b's load will be high. I think the load of V1b is (R3+P1||R1)  (is that correct?)

also, I'd try moving the .02/4.7M pair between P1 and the 68K.

and I don't see a pentode anywhere.  If V1b is a pentode, then like tubeswell says, make load the of V1b high, so that R3+P1||R1>=1.5M  or >=2M

edit: that 56K grid leak on the next stage, V3a might need a tweak when adding mix resistors (or a blend pot).  i'm not sure what the original schematic looked like, if there was  series resistor between the .001 coupling cap and the grid of V3a, then the ratio of the voltage divider should be taken into account as the next mix resistors will form voltage dividers with that 56K, you might want to play with that valve as well.

[TIMBO]

Thanks Gs,The build is now a Trainwreck as per schem A5a from Dougs library. I have a spare socket that is right for a OD circuit. I am trying to reduce the amount of switches and knobs that are on the front panel. I main reason for this is (I hope that I don't offend anyone) but I found that musio's aren't very technical folk and when you put a bunch of knobs and switches in front of them, well do I need to say more.

With the Hudson SJ and TOSII it sent the guys into a tail spin on how to set it to their desired sound and when they found the desired sound it was never changed from that. We as more technical craftsman like to TWEAK because we CAN 

SO, I found with the TOSII much of the clean and overdrive came from the volume on the guitar, this is why the setting on the amp was left once set (but you guys already know this)

SO by having the MIX through either a POT or RESISTORS means that the clean channel can be mixed with the OD channel to any varying amount mainly for effect, by using the MIX pot to clean or OD and something in between (no volume pot on OD). OR with resistors doing the mixing, the volume pot on the OD then would increase the amount of OD to the PA

I was hoping to remove the footswitch part completely, is what i am trying to do.Thanks

[tubenit]

I personally like the idea of the mix/blend pot. 

You may find a need to use a 5751 or even down to a 12AY7 in the V2 overdrive (due to the EL84's)?  Let's us know what you come up with when you get it done.

I sure like hearing reviews of an amp after a number of guitarists have plugged into, so thanks for sharing that info.

With respect, Tubenit

[terminalgs]

hi Timbo,

I'm suggesting no blend pot, just mix resistors for that exact purpose: reducing knobs.  Treat it like a reverb circuit,  instead of "Volume" and "Reverb", you've got "Volume" and "OD drive".  I'm guessing  "OD trim" is a trim pot, accessible via a screwdriver and through-hole?  you could make the  mix resistors be a be a similar adjustment pot accessible via a through-hole/screwdriver.   

which triode is being overdriven in this circuit?  V3a?

[tubenit]

If your amp is going to be used by different players in different venues playing different types of music, then I would lean towards a blend knob so the degree of mix between clean/OD could be dialed in ..... vs. set/fixed by resistors.

Doug Hoffman uses a blend knob on his "add reverb to a western circuit" & it worked quite well when I tried that approach to his reverb.

Just a different viewpoint and not necessarily better than fixed resistors for the mix.

With respect, Tubenit

[TIMBO]

Thanks guys,I can wire it so that I can experiment a bit with the two options. Trim pot can be mounted on the back panel for easy adjustment. Thnaks

[TIMBO]

The blend pot works to some degree, but it seems to be unstable. I can get the mix ok but when turning up the DRIVE it starts to squeal. I put in a 12AU7 in V2 but it stayed the same.

Also I increased the blend pot to 1M cause (turned to the clean) but the OD still has an effect on the clean

So if anyone has a suggestion I'm all eras. Thanks

[TIMBO]

STOP THE PRESS, I may have had misswire. 

[kagliostro]

Something like this ?

K

[TIMBO]

Hi guys, I rewired with the blend pot( after checking the wiring) and it works only to some degree, but the OD still inter acts with the clean and when adjusting the trim it howls and farts, similar to that when you have your OT wires back to front.

It works better with a 470K (blend pot removed)resistor in series. It still howls when the drive and trim are turned up but not as bad

[kagliostro]

And something like this with a pair of trimmer and a dual gang pot ?

K

[TIMBO]

Thanks K, I think the problem is in the (small)circled bit. If you think about it, whether a resistor or pot (tried 1M pot) the signal from the OD is not only going to the grid of V3 but it also is routed back into the OD circuit.

I rewired the OD in series and works great (on connection from clean to V3 grid) I was even able to have a 12AX7 in V2

Here is a CRAZY idea   Circled....................................

[terminalgs]

Thanks K, I think the problem is in the (small)circled bit. If you think about it, whether a resistor or pot (tried 1M pot) the signal from the OD is not only going to the grid of V3 but it also is routed back into the OD circuit.

Feedback. You need bigger mix resistors.  1M on each leg minimum, maybe 2M?  Do you have a 2M or 5M pot you can try as a blend pot?

[kagliostro]

On the Vox Cambridge the resistor that is used for the reverb insertion is bypassed with a cap but is a 3.3M resistor

http://www.el34world.com/charts/Schematics/files/Vox/vox_cambridge_reverb.pdf

K

[silverfox]

I've considered this in the past and don't understand the outcomes real well.

In mixing two signals either in this circuit or even two instruments plugged in to the inputs of a guitar amp, the signals at various points will either add or subtract, generate harmonics. Whenever I jammed with someone else using one amp there would be periods of near silence from the amp as the two guitars signals clashed.

Why  won't that happen in this instance?? Does the resistor act as a crude mixer. If so it seems to me that would result in crude outputs. What about a one tube mixer circuit?

As an experiment, is it possible to mix these two signals with a mixer and find out what it would sound like that way? Or are the signal levels incompatible with the mixer.

Silverfox.

[TIMBO]

Hey Fox, The two circuits do work but only to a certain point, that being the clean volume at any level and the od volume or mix pot at low level but when the OD volume is turned up this is when the problem occurs.

I looked at http://www.el34world.com/Hoffman/images/Hoffman_AB763.pdf and it uses the blend resistors to mix the two channels and it guess it works.

The only difference is the AB763 is two separate channels and in the other schems they DON'T have any mixing resistors at all and they all work fine.

Why does this not work.....

Feedback, the signal is being passed back into the OD circuit, even a small amount that is let through the mix resistor.
A clash of signals that don't agree with each other 
OR an out of phase thing  Thanks

[TIMBO]

Well to my surprise this works well.

No hums or buzzes squealing or magic smoke.
The OD level will do the same as the MIX pot, when it is dialled back to 0 the clean is loud and strong. When turned up the OD starts to kick in at about half way making a nice blend and proceeding up to full OD.

At full volume feedbacks nicely but starts to get unruly and is very LOUD 

probably just need to tweak the circuit a bit tone wise 

[kagliostro]

Interesting

---

Just a question

did you tried also this version ?

K

[TIMBO]

HI K, YES it was having the same howling problem when the OD was turned up. Again I think the signal was bleeding through the 470k resistor and feeding back into the OD circuit, this is the only thing I can think was causing the howl.

I did not try T's reverb mix resistor..........

Until someone says that what I have done is not a good idea I suppose it a goer.

I've added some smoothing caps and tone,seems to have improved the tone some.

[kagliostro]

What you did is to use two mix resistors, the 470k resistors

My knowledge is so poor that I can't figure how the cap you added at the input of the OD

can solve the problem, but if it works as you want .......

May be there is something that has to do with the phase of the AC signal at the moment

that it crosses the cap  

---

May be now you can try to add the blend pot at the junction of the two 470k resistors

Ciao

Franco

[TIMBO]

Hi K, I don't think that I need to add a mix pot as the OD level seems to do the trick, I'm thinking that with the extra gain stages in the OD makes it louder so when increasing the OD level it overrides the clean and with the OD level at about half way there is a nice blend/fatness of the two circuits.

K, I'm a bit like you, why it works I don't know. 

[tubenit]

Looks like a very cool build!  Congrats!

With respect, Tubenit

[TIMBO]

Thanks T, I'm not sure of the EL84s sound but I don't want to sub them for 6V6s and find that they don't sound any better. I know that your not a real fan of ELs.

[tubenit]

I'm not sure of the EL84s sound

Yeah, I've never been able to get an EL84 amp to work out for me.  I think 6V6's would sound "better".

Another option is to just use the pentode section on 6BM8 tubes which I like a lot.  They don't have the high end chimey hash that the EL84's have had in my experience.   Makes for about a 7-9 watt amp.

With respect, Tubenit