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Olson/Dolly clone SE 6BQ5 build

willyrover

willyrover

Super Member
I've been trying to design a power supply that is SS rectified to build a clone of Redboy's Olson amp.

http://www.audiokarma.org/forums/showthread.php?t=125219&highlight=dolly

http://www.audiokarma.org/forums/showthread.php?t=190322


The transformer I have is a 1:1 (120v secondary with 6.3v heater) so I have been looking at voltage doublers.

I came up with this circuit using Duncan's PSU Designer II but to be honest, I don't know what half the numbers mean other than it didn't show any errors and the voltages look correct.

Any comments about this design would be great! I think I need to add another filter to supply the 12AX7 as well.
 

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Replacing that big power resistor with a choke would probably be a good upgrade. Better regulation,and better filtering (less humm).
 
I'll be of no help to you here willyrover, but I am curious to see how this works for you.

I've never gotten the hang of that PSUD program, mostly because I'm not really savvy enough to know what it's asking of me.
 
Make sure the transformer is good for about 3 times the DC current that you need (250 mA or more). A winding resistance of 25 Ohms or less should be OK.
 
Yes, the transformer should be fine. It was removed from a 6BQ5 SE console amp that also powered all the radio tubes.

I revised the schematic with slightly lower value caps. This arrangement gives 250VDC at 90mA for the 6BQ5s and 230VDC at 10mA for the single 12AX7.

Anyone see any obvious flaws? Are the polarities correct on the caps in the voltage doubler?

Thanks!
 

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The build is underway! The only pic at the moment is the chassis with holes drilled.

Spent the better part of the afternoon and evening working on it and got far enough to give it a test run. The power supply was being a pain in the ass. Voltages way too high. Humming. Distorted sound with no treble.

Also managed to crack a tube somehow.

I reworked the power supply and when I pick up some higher wattage resistors, I think it's going to work. :yes:
 

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I'm curious about how the tube amp builder starts on a project like this. Do you narrow it down to a particular type or output wattage and proceed from there? Do you choose a design from published, well known amp designs or is the recipe much of the builder's choosing? Just curious.
 
willyrover said:
The build is underway! The only pic at the moment is the chassis with holes drilled.

Spent the better part of the afternoon and evening working on it and got far enough to give it a test run. The power supply was being a pain in the ass. Voltages way too high. Humming. Distorted sound with no treble.

Also managed to crack a tube somehow.

I reworked the power supply and when I pick up some higher wattage resistors, I think it's going to work. :yes:
Click to expand...
Excellent!

That picture is a bit of a tease... Give us the rest! :D
 
westend said:
I'm curious about how the tube amp builder starts on a project like this. Do you narrow it down to a particular type or output wattage and proceed from there? Do you choose a design from published, well known amp designs or is the recipe much of the builder's choosing? Just curious.
Click to expand...

I usually think about the speakers first, then go from there. Once I decide how much power I need, I look at topologies (single ended or PP). Then I make a post in tubes to get ideas and help from AK gurus. I can't design from scratch yet, so I'll often find a circuit on the web, then modify it to my liking or to fit parts (iron) I have on hand. The big one though, is help from the AK gurus.:yes:
 
westend said:
I'm curious about how the tube amp builder starts on a project like this.
Click to expand...

I'm not even close to being an expert or even completely understanding how these things work but I'm interested to learn and I guess the best way to learn is to just do it and see what happens.

I don't really want to throw big bucks into a project so when I come across some good parts, I copy a working design. The challenge for this one was a redesign of the power supply. Pretty much learning as I go through trial and error.


Here are some more pics of the amp. My wiring is a bit embarassing. :D

All it needs now is binding posts, RCA jacks, proper resistors in the power supply, and a good polish.

I could not find 250pf caps so I used 270pf throughout. Not sure what effect this is going to have.
 

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Well, I can't for the life of me figure out what's wrong with the amp. When switched on I get music, then a low pitched hum after a few seconds (say 120hz) that slides down fairly quickly about an octave as the tubes warm up.:rant:

There is still sound but it's lacking in high frequencies and has lots of hum.


I've tried lifting grounds, moving grounds, grounding heaters, not grounding heaters, adding resistors from heaters to ground, reversing the leads on the primary side of the OTs. Double checked and triple checked wiring against schematic.

I even completely rebuilt the power supply and had the same problem.

Unless someone has a suggestion, it's going on the shelf for a while.
 
Have you flipped tubes left to right? Does it change when you plug it into another circuit socket?
 
Yep...flipped tubes and tried a good set from another amp. Also, tried a different circuit.

When one OT is connected to the power supply at a time, the hum is significantly less. Once the other OT is connected the hum is very loud in both channels.
 
djnagle said:
Does the OT have a good ground?
Click to expand...

The OT is isolated from the chassis.

On the primary side: 1 lead goes to B+ and the other goes to PIN 7 of 6BQ5.

On the secondary side: 1 lead goes to negative speaker terminal (isolated from chassis) the other goes to positive speaker terminal as well as providing feedback to the 12AX7.
 
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