Latest project - '90s HRM w/Dumblelator

[skyboltone]

Norm:
I don't understand the Bias supply. Is that the adjustment pot on the back panel with no connection to the wiper? Run the wiper to the center tip jack and lift the ground. The 15K tail is grounded and wires to the opposite end of the pot. Then, doesn't it feed to the board from there? Confused.

I'm amazed at how you guys keep pumping out these amps. I'm on my first and finding it's ten pounds of onions in a 5 pound sack. A 17" chassis is no place for my big iron, that's for sure. Anyway it looks really super. I would like to see the footpedal scheme. Is it posted elsewhere? Are the diodes across the coil for pop suppression or LED power?

Dan

[Tdale]

When you wire the relay supply like this, you can connect one side of the relay coils to ground, as opposed to the weber relay-kit that works off the 6.3V with center tap, where the coil cna't be grounded at any side?

Tommy

[Normster]

Norm:
I don't understand the Bias supply. Is that the adjustment pot on the back panel with no connection to the wiper? Run the wiper to the center tip jack and lift the ground. The 15K tail is grounded and wires to the opposite end of the pot. Then, doesn't it feed to the board from there? Confused.

I'm amazed at how you guys keep pumping out these amps. I'm on my first and finding it's ten pounds of onions in a 5 pound sack. A 17" chassis is no place for my big iron, that's for sure. Anyway it looks really super. I would like to see the footpedal scheme. Is it posted elsewhere? Are the diodes across the coil for pop suppression or LED power?

Dan

Wiper goes to bias splitter resistors. I guess I should at least label the connection so nobody gets confused. The 27k tail to the center bias test point is just a convenient place to ground the bias circuit. If it gives me a problem, I'll move the ground someplace else. (Oh, and 27k is just where I start when running EL34s. On one amp I've actually had to go as high as 47k!) Yes, the diodes are for pop suppression. IIRC, when the relay coil collapses, it creates a voltage spike which the diodes send to ground.

BTW, I built my first dumble clone into a Pignose G60VR chassis...17" x 6" x 2 1/4". Never again! (Although if you look at some of Gary's first Glaswerks amps, he managed to stuff an aweful lot of hardware into a 17" x 10" x3" chassis.)

[Bob-I]

BTW, I built my first dumble clone into a Pignose G60VR chassis...17" x 6" x 2 1/4". Never again! (Although if you look at some of Gary's first Glaswerks amps, he managed to stuff an aweful lot of hardware into a 17" x 10" x3" chassis.)

What's the matter, you got claws for hands? My Hybrid is in a 6x1.75x15" chassis. Honeymoon fit.... real tight.

[Normster]

BTW, I built my first dumble clone into a Pignose G60VR chassis...17" x 6" x 2 1/4". Never again! (Although if you look at some of Gary's first Glaswerks amps, he managed to stuff an aweful lot of hardware into a 17" x 10" x3" chassis.)

What's the matter, you got claws for hands? My Hybrid is in a 6x1.75x15" chassis. Honeymoon fit.... real tight.

Nah, I'm just having a hard time figuring out how to use opposable thumbs.

[skyboltone]

Wiper goes to bias splitter resistors. I guess I should at least label the connection so nobody gets confused. The 27k tail to the center bias test point is just a convenient place to ground the bias circuit. If it gives me a problem, I'll move the ground someplace else. (Oh, and 27k is just where I start when running EL34s. On one amp I've actually had to go as high as 47k!) Yes, the diodes are for pop suppression. IIRC, when the relay coil collapses, it creates a voltage spike which the diodes send to ground.

OK, unconfused. In my mind, (what's left of it) I was thinking current not voltage. We are looking at voltage across the one ohm resistor between cathode and ground. That's a nice trick. I've seen those one ohm resistors on all the schemes and not given it a thought. Are you buying precision shunts?

[Normster]

Oh, I see what you're talking about. Yes, the parts labeled "TP" are bias test points using 1% 1 Ohm resistors to convert current to volts. Since I do more than my fair share of tube swapping I wanted to make life easy.

I copied the layout for the test points from a Komet 60. Another cool feature that the Komet uses is a 10-turn 50k bias pot with precision dial. I've got a spare pot and dial so I may implement that as well. Hopefully, that will give me enough range to re-bias for 6L6, EL34, or 6V6.

The two outside tubes will be wired to a "half-power" switch to lift the cathodes from ground. If you look closely at the #124 pics (ODS_124044.jpg) it looks like Dumble powers the LED in half power mode by running cathodes through a pair of resistors and probably a diode as well, although I don't see one in the pic. Maybe he's using the LED as a diode and the resistors are just there to set forward voltage?

['67_Plexi]

For what it's worth on the efx jacks, you might want to use ground isolated cliff jacks and take the ground back to the preamp star point. I avoid using the chassis at all costs except for three or four designated points. I do use the single ended bus above the pots like you have, tied to the chassis only by the I/P jack. This is far superior to using the back of pots like many designs have.

[Luthierwnc]

I use a heavy (12ga romex) buss wire as well although mine are soldered to a spade terminal and tied to a PT bolt. I even try to keep circuits together on the same ground wire that ties into the buss. Each stage of the power supply has separate wires to the main buss.

The ground wires are more-or-less in order along the buss wire. The closer I get to B+ in the power supply, the closer I get to the base of the buss wire. For example, sometimes the OT is physically closer to the preamp than the power amp. I still run the OT ground to the isolated speaker jacks and another wire back to the buss near the PT bolt. I learned that trick from restoring an old V4. Ampegs are very touchy about ground loops. They all tie at the same point but there are different wires depending on the power amp stage.

The relay switching thing I mentioned in another thread is an aberation to my usual grounding schemes. I think I'll try the XLR method just to see what I am missing.

Skip

['67_Plexi]

I had this discussion with Dogears on Friday....

You might want to check the Dumbleator PS dropping resistors, they are a way off for any amp that has a B+ in excess of 400V, in fact the 2.2K is way off regardless of B+.

I ended up using 56K and 68K to replace the 22K and 2.2K. This gave voltages on the plate in the mid 200's.

[dogears]

And, I would power it in parallel from the screens. Get it out of series.

I had this discussion with Dogears on Friday....

You might want to check the Dumbleator PS dropping resistors, they are a way off for any amp that has a B+ in excess of 400V, in fact the 2.2K is way off regardless of B+.

I ended up using 56K and 68K to replace the 22K and 2.2K. This gave voltages on the plate in the mid 200's.

[Luthierwnc]

This got kicked around on an amp I was building:

https://tubeamparchive.com/viewtopic.ph ... ght=#13599

The thread includes some layouts, schematics and volts. Mine came out really well.

Hope it helps, Skip

['67_Plexi]

Scott, you misread it bro, it is in parallel. At first glance the ground connection off the two back to back caps in the efx power chain make it look like it's in series. 3A and 3B come off the screen via the resistor. The same screen connection also goes off to the right to the other B+ taps.

[dogears]

Doh! But the resistors are all wrong (as you found out). I'd also up the 1K that feeds the PI

Scott, you misread it bro, it is in parallel. At first glance the ground connection off the two back to back caps in the efx power chain make it look like it's in series. 3A and 3B come off the screen via the resistor. The same screen connection also goes off to the right to the other B+ taps.

['67_Plexi]

Once I had the amp all powered up with all stages connected and pulling current, the Dumbleator ended up with 200 and 237 on the plates. Sweet !! I have a tramsformer thats giving 500V on B+1, so some folks probably might not want to be as aggressive as 68K.