Fender Pre and Bassman Volume questions

[Subjecttochange]

That's what I was thinking. Does it look like it should work though? I thought for sure it would work as I currently have it... and I was surprised at how wrong I was.

Ok- I'll try using both sides of the relay

Thanks for looking those up Tom... they look pretty much the same arrangement as mine. Weird.

[Subjecttochange]

Not sure how... but that didn't work either. In fact- it killed the signal completely and all I get is crazy oscillations on each channel as I turn up the volumes... both do this regardless of the switch position.

What should be my plan of action... apparently this is a lot more difficult than a standard DPDT toggle. I thought it would be pretty staightforward... should I get a new relay?

I'd have to order one though...

I could pick this one up at radioshack... just to a little voltage doubler circuit to supply the coil?

http://www.radioshack.com/product/index ... Id=2062483

[Structo]

Have you tried it with a manual DPDT switch just to get it going before trying the relay?
That's what I would do.
To iron out the preamp issues and how they work with the power amp.
Then once you make that happy, set it up with the relay.

[Subjecttochange]

I think that is wise. I'm still stuck. I thought that it may be like a regular dpdt with the common in the middle instead of the top. so I switched the leads. The rocket channel works now... just not the fender channel.

I think that's probably the thing to do. Too many variables floating around for now.

[martin manning]

Looks like the cathode follower V2a is missing a ground at the bottom of its 56k cathode resistor, in your scheme, anyway.

What happens if you ground the output of the unused channel?

MPM

[Subjecttochange]

You're right... I'm pretty sure it's grounded on the amp... but now I have to check just in case.

I'm going to go try grounding one input... just to see

[Structo]

I'm not sure about that relay but the ones I use are in a 16 pin format and fit a 16 pin IC socket.

The switching is laid out like this diagram.

I would have the input to the phase inverter on pin 4 or 13 depending on what side you use.
Then have the two preamp outputs on the either side of that 6 & 8 or 9 & 11.

The coil is 1 & 16.

[Subjecttochange]

That's what I thought the relay was too... it didn't work like that though. Of course... it didn't work the other way either.

[Firestorm]

Have you tried it with a manual DPDT switch just to get it going before trying the relay?
That's what I would do.
To iron out the preamp issues and how they work with the power amp.
Then once you make that happy, set it up with the relay.

I'm with Structo here. There's something else wrong besides the relay not working. As drawn, this should put out at least 20 watts. It doesn't need a channel mixer stage -- the two 220ks should have worked fine. Look at a non-reverb blackgace Fender Pro: gain stage > tone stack >tone recovery stage > PI > 2 x 6L6 = 25 watts. I'd hardwire the Fenderish channel to the PI and see what you've got. If it ain't loud, I'd start looking for something hooked up wrong.

[Subjecttochange]

Thanks Firestorm and Structo. I think you guys are right.
One channel at a time it is fine- but both running into either a mixing stage or directly to the PI creates either some sort of nasty distortion or is just really quiet. SO I think there could be a problem in the PI.
I think I may try a PI arrangement similar to the 18W marshall design... having channel a and b run into C1 and C2 respectively- thus removing the NFB input.
How does this sound?

[Jana]

To add to what Martin mentioned, you are tapping the tone stack off of the wrong place on the cathode follower.

Details, pay attention to the details.

If it was me trying something like this I would: take the relay out of the place you have it now. Mix the two stages with mixing resistors. Do the switching on the gain pot of each channel--ground the wiper/grid of the channel you don't want to hear.

[Structo]

I think what Jana is saying is that you are tapping the cathode at the wrong place.
The stack should connect directly to the cathode, then the resistor to ground.

[Subjecttochange]

Yep. The drawing is wrong- the amp is right. I missed a few details on the schematic for sure... but I built the amp prior to drawing the schem. I needed to get my act together and draw it to better communicate it to you guys. So observant though- I thought I checked it all.

As of now the rocket input is grounded and the fender is hardwired just as Firestorm said. It sounds decent- but I am getting some slight distortion when the PPIMV is used and is anywhere other than 100% (not on schem). This is with the mixing stage included... with only the fender channel through the mixing stage.
Without the mixing stage the amp is quieter- but has no problems with the PPIMV. Still probably less than the volume of my 18W. Still hardwired to the PI- grounded rocket channel.

Thinking I may oust the PPIMV and bring in a VRM... but with an amp of this size- in as small of chassis as it has... it may be hard to find good place for a heat sink/place for the board.

I also changed the coupling cap to 0.002uF into the PI, which should give more balanced low-end while controlling possible blocking dist. as per Randall Aiken's short article on the LTPI:

"If C1 is made small (less than .01uF or so, with 1Meg grid resistors), it will improve the low frequency response balance between the two output phases if the second coupling cap, C2, is made at least ten times larger than the first cap, C1. "

Again thanks guys for being willing to help and patient. I've said this a few times... but it wouldn't be happening without you all.

[Subjecttochange]

Ok- got the fender channel sounding pretty good. I actually went to a non bypassed cathode resistor on V3b which keeps it a little cleaner all the way up. Which I think is better for the sound my friend is going for.

I also forgot to add that I have a 12AY7 there... not an AX7- keeping the gain down further. Basically he doesn't want any- or very minimal breakup- even on full volume.

Since- I have grounded the fender input and hardwired the rocket input. SO MUCH LOUDER. Wow- it goes from weaksauce with the fender channel to at least as loud as my 18W with the rocket channel (no mixing/recovery). Should I be using that "mixing stage" for the fender channel only? Afterall the fender channel is running a lower gain tube with two triodes- competing with three triodes all higher gain in the rocket?

[Firestorm]

The cathode follower doesn't have any gain (well, technically its has negative gain), so you more gain on the Fender channel because of the third stage (it isn't really a "mixer" because it doesn't mix anything.) Of course, the tone stack on the Rocket channel will be less "lossy" because of the lower source impedance of the cathode follower.

I still can't imagine why you'd need the third stage on the Fender channel if you're going for clean, but 12AY7 (or even 12AU7) is the way to go if so. Even then, you may need to tweak the plate and cathode resistors to set the gain where you want it (and you won't want too much).

When you hook the channels to the PI input, make sure the 220K isolation resistors really are 220K and not 22K or 2K2, by mistake. You can even try 470Ks as Fender did in the Brown amps; it will add some series resistance (high frequency loss), but further isolates the channels. Or go ahead and keep the third stage and adjust the gain to keep it clean

If you're going for minimal distortion, do not get rid of the NFB loop. The PI as you had it drawn originally.