First clips of the 3 channel amp

[Bob-I]

Hmmm.... so if I understand you right, I'd move the dry line from the input of that last gain stage to the PI input right after the PI input cap. I'd also make the adjustments you mention to the resistors and add the series resistor, I'd assume AFTER the PI coupling cap, is that right?

Thx.

You want them to SUM BEFORE the PI input cap. With respect to ground there is -50 to -60 volts on the PI grids.

Take a look at this... same idea. Yours should sound better.



I've seen you single tube reverb before, thx.

I see that you use a 2M reverb mix pot. That seems like a lot of load on the output. With the 2 gain stages after the reverb I think that the 100K load Fender uses should be fine. What's your take?

The summing point goes into the PI input cap. With the 12AT7 drive and two stages of recovery you have should make a great sounding reverb.

Gary

[glasman]

I see that you use a 2M reverb mix pot. That seems like a lot of load on the output. With the 2 gain stages after the reverb I think that the 100K load Fender uses should be fine. What's your take?

For you circuit the 100K should be fine.


I use the 2M to minimize the load to the dry signal. I tried a 100K at that position and it swamped the signal as the control was rotated. If I had the space in my original chassis' I would have used a circuit very similar to yours. I actually may go to (glup) SS recovery amps in the future to add a liitle (or a lot) more reverb signal.

Gary

[Icetech]

Gary, i just built the ODreverb from norms layout using your single tube reverb.. and you might wanna try another tank before redesigning...

Ran it on my princeton tank (1970ish) and got very mild reverb but nice....

tossed in another old tank laying around and got almost no reverb, had to really strike the strings to hear it...

then grabbed a crusty old cobwebbed up tank and i feel like dick dale... just way over the top on reverb... all in all i like that single tube design very much:)

[glasman]

Gary, i just built the ODreverb from norms layout using your single tube reverb.. and you might wanna try another tank before redesigning...

Ran it on my princeton tank (1970ish) and got very mild reverb but nice....

tossed in another old tank laying around and got almost no reverb, had to really strike the strings to hear it...

then grabbed a crusty old cobwebbed up tank and i feel like dick dale... just way over the top on reverb... all in all i like that single tube design very much:)

I use 9AB3 style and there is a LOT of reverb. I just want a little more control of the sound. Currently it is a tad dark. I want to also try a 4 series tank to see how it sounds with a shorter dwell.

Gary

[Bob-I]

Another idea would be to mix the reverb and dry signals right at the input to the PI. Lower the 3.3M to somewhere between 100K to 470K and add a 470K series resistor on the output of the reverb recovery amp to the PI. This will lessen the effect on the "D" tone and still give a very usable reverb.

Gary

Gary,

I put a 100k in and found that even at the lower gain the reverb was over the top. I put a 100k pot in and adjusted until I felt it was good, then measured the pot. It was down below 10K!! I put a 10K resistor in and it actually balances well. Any reason I shouldn't leave it?

[glasman]

Another idea would be to mix the reverb and dry signals right at the input to the PI. Lower the 3.3M to somewhere between 100K to 470K and add a 470K series resistor on the output of the reverb recovery amp to the PI. This will lessen the effect on the "D" tone and still give a very usable reverb.

Gary

Gary,

I put a 100k in and found that even at the lower gain the reverb was over the top. I put a 100k pot in and adjusted until I felt it was good, then measured the pot. It was down below 10K!! I put a 10K resistor in and it actually balances well. Any reason I shouldn't leave it?

Well I really don't think it will hurt anything. IF it sounds good I would probably leave it alone.

Gary

[Bob-I]

Bob,

Try raising the resistor, over the 100k trim before the OD, to a 180k or 220k. Try a 470pf grid bypass of the CL2 220k grid resistor. Raise the snubbers tp 390pf to 470pf.

Try lowering CL2 to a .02uf

OD1 should be a .01 and OD2 a .005 (.0047)

The OD2 plate is 150k and the cathode 2.2k

Done....

Only CL2 gets a lower value bypass cap. 4.7 on CL1, OD1 and OD2 I like 1.2uf to 1.5uf on CL2

I can't find any 1.2uF or 1.5uF locally so I'm running some 1uF's in here.

100k over the OD drive pot and 150k over the OD level pot. Measure the pots. If they are slightly low, you can compensate by using a slightly larger resistor like a 110k or 160k.... The pots are linear taper. The schematics were deliberately wrong according to my inside source.

Gottcha... I'll have to stick with the Audio pots for now.

A .001uf cap across the bass pot will smooth the grain out. Try the Skyliner stack. I like to add a switch to go from .047 to .01 midcaps. 150k slope helps. Ditch the 70s stack IMO.

Measure some 330pf trb caps and choose one that is under 330pf. I like 315pf or so... It is noticeable.

I can't find any in my stash, I'll check some out later.

Don't worry about the CL2 local feedback. With the Skyliner stack, the amps sound great with none! Try it.....

Let me know how it goes!

So, I made most of these mods and I'm not thrilled. Right now the amp is too dark and dry. It needs more highs and smoothness.

I'm not thrilled with the .01 in the mid cap, I may just leave the .047uF in and forget the switch. Also it seems to override the bass controls effectivness.

I think the key here will be in getting the snubbers and grid bypass right. Somethings robbing my highs now.

It'll get there.

[dogears]

Bob,

What type plate resistors are you running???

[Bob-I]

Bob,

What type plate resistors are you running???

type? metal film

[dogears]

Bob,

Give me a buzz... I emailed you.

Thanks....

[dobbhill]

What kind of speakers? That can make a huge difference in the highs, IMO. Just trying to help......
D

[Bob-I]

What kind of speakers? That can make a huge difference in the highs, IMO. Just trying to help......
D

Agreed....

They're Celestion G12H80 in a 2x12" partially open back cabinet. I find this cabinet very good for the type of tone a Dumble should produce.

Actually dogears was a lot of help on the phone today. I also found a fractured wire in the tone stack, too much screwing around. It broke the bass control but when I fixed it the highs came back too. Interesting.

[llemtt]

Celestion G12H80s are very nice sounding speakers with this kind of amp but they have a somehow rolled-off high response. If you want sparkling highs you have to try some other speaker.