schematic Heaven "Dumble Mod"

[andrew]

I built the Schematic Heaven "Cool Dumble Mod" using a Trainwreck Rocket PI section with two EL-84s. With an attenuator it sounds good.It's touch responsive with good sustain but noisy if I bypass the attenuator. I'm using a Weber DeLuxe PT and my power section is CHOKE (315v) 4.7k(screens 314v) PI(296v) PREAMP(273v). Are these voltages OK? Thanks.

[Bob-I]

What are the voltages on the plates and cathodes of the preamp tubes?

[andrew]

V1a pin1=148 V1b pin6=178 V2a=234 V2b=238 The PI pins 3 and 8 is 75 Thanks!

[Bob-I]

V1a pin1=148 V1b pin6=178 V2a=234 V2b=238 The PI pins 3 and 8 is 75 Thanks!

I can't find the mod you're talking about, but assuming this is a pretty standard Dumble type the voltages are a tad odd. The plates should be closer to 200V and V1 should be more equal. 30V difference isn't normal. Check the cathodes and see if there's discrepency there too.

I'd adjust the dropping string to try to get these more like 190 to 210. The Overdrive will be smoother and the cleans crisper. With the clean side this low it may be a tad spongy, the OD this high will make it a bit brittle.

[andrew]

http://www.schematicheaven.com/dumbleamps/mod1.pdf Here is the mod.Thanks!

[Stanz]

It is basically the PAB (or is it the mid boost?) and Jazz/Rock features. Shouldn't really have any effect on plate/cathode voltages.

[andrew]

I do have a problem with hum. I notice if I touch the junction of the22M and .05 with the voltage meter(one end to ground) the hum goes away. Any ideas on what I could do to get rid of the hum? Thanks again!

[heisthl]

I don't know who created this mod but the 22m/.05 cap is just plain stupid if you don't have overdrive, it's only purpose in life was to keep the mud out of the OD stage and regular non OD clean tone is much better without it. If you still have the hum with those parts removed you could start by moving the 68K off the input jack and attaching it directly to pin 2 of V1. This will keep the radio stations out of your amp. You will probably like the tone better if you parallel a 500pf cap on the 220K resistor feeding pin 7.

[andrew]

Well I did away with the 22M-.05 thing and added the 500pf across the 200k. There is a little less hum. I wonder if the physical orientation makes any difference in hum? I could try moving it but room is pretty tight in there! Thanks abain!

[andrew]

Sorry about that last post . What I wanted to ask was did the angle of the choke make a difference in the hum. I moved it around and it did not affect the hum. I've changed the tone section to the 10k to ground on the bass and the .001 across the bass, 150k slope resistor,ect. and it was a great tonal improvement over the layout in the earlier post. Back to the hum problem if I touch the front of the chassis it cuts down on the hum. The input resistor is on the tube with insulated wire one end grounded. Could it be a bad tube. I'm running it ( two EL84s ) cathode biased with a 150ohm and a 50uf. Any help please?

[andrew]

Sorry about that last post . What I wanted to ask was did the angle of the choke make a difference in the hum. I moved it around and it did not affect the hum. I've changed the tone section to the 10k to ground on the bass and the .001 across the bass, 150k slope resistor,ect. and it was a great tonal improvement over the layout in the earlier post.. The input resistor is on the tube with insulated wire one end grounded. I'm running it ( two EL84s ) cathode biased with a 150ohm and a 50uf. Also I got the plate voltage on preamp pins 1 & 6 to 189v and 217v. I still get that hum but it goes away if I touch my voltmeter + to pin 6 and no ground. Is there a way to replicate this humbuster? Thanks.

[Structo]

I'm not sure if this will help but in my case, I had some low hum and it was related to lead dress and wire routing.
I had the B+ from the standby switch running too close to other wires in the power supply which carry AC.
One I routed that separately between the tranny and PS board it went away.
Also check out the lead dress to the preamp stages.
Use a chop stick to poke around.

How are your heaters wired?
Overhead (Fender)?
Or on the floor ( Marshall) ?

[andrew]

Hey! I fooled with it all morning and made some changes. First of all since I had a small chassis I thought I would save space and use combination on-off /volume pot. I ditched that and put in a normal volume pot. I installed a tube rectifier and now I use the preexisting standby as the new on-off switch.All of this helped but the hum is still reduced if I touch the front chassis. My filaments wires are on the floor. I use the same grounding scheme on all my amps and this is the only one that hums. I also tried attaching the heater CT to the power tube cathodes but it made no difference at all. Thanks for the reply!

[andrew]

I tried a lift circuit on the heaters but still no change. I touch the front chassis and it goes away.