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Mini Console iPod Dock Project

My gift for my Granddaughter will be a mini-console iPod dock fashioned after the stereo  consoles of the mid twentieth century time frame. I have been around the block a few times  deciding exactly how to structure it. I considered all kinds of options like push pull triode  outputs v.s. single ended pentode/UL outputs, full range mains with integral subwoofer v.s.  external sub connection etc. 

In this document I intend to chronicle the progress of the design and construction. 


I have decided to go with the Mark Audio CHR-70 4” full range driver and supply an  output for an external subwoofer. I installed the drivers in a temporary test enclosure for  the purpose of break in and initial testing. The final version will be a ported  enclosure of somewhat larger size so that we can get useful output down to about 40 Hz. 

Test Speaker Enclosure

The preamp section will include a second order high  pass filter for the mains feed to the power amp section to  protect the main speakers from over excursion below the  tuning frequency of the enclosure. It is likely that I will  tune the enclosure to have a slight bump just above the cutoff  frequency to help maintain flat response as long as  possible while maintaining a steep enough roll off.  The preamp will also provide a full range output that can  be used with an external subwoofer amplifier. The  external amplifier must provide its own low pass  filtering. 

Below we compare the response simulation of a sealed  alignment v.s. the ported design I am considering. In the  sealed version you will note that the response is down  3dB by 80Hz and at 40Hz is down by about 12dB.  Under normal circumstances one might be tempted to  EQ for flat response and retain the advantages of a sealed alignment but we will be using a  relatively low powered amplifier so it makes a bit more sense to go with the more efficient  tuned enclosure shown next.

Sealed Response

In the ported response you see a bump at the cut off.  This intentional misalignment is  designed to flatten the overall response after the high pass filter is applied. 

Response Ported


The Power Amp section is a KT-88 Single Ended Ultra-Linear with Schade feedback. I  originally intended to use 6N1P as the driver but after coming up with the preamp section I  realized that I did not need all the gain of the 6N1P and really would prefer to have more  input voltage swing headroom so I switched to a 6CG7. I am using two separate chassis for  the preamp and the Power Amp both of which were salvaged from separate electronic  organs. Below you see the Power Amp chassis which I took from a solid state organ. I stripped all of the electronics and drilled it for the tube sockets. Here you see a test fitting of components. Note that the 6N1P shown is shorter than the 6CG7 that will be used in the final design.

Test PA Chassis Layout

The preamp section has a gain section followed by an active bass and treble control section  after which a 2nd order high pass filter feeds the power amp and a cathode follower feeds  the subwoofer output. The 22k resistor from the 6CG7 anode to the return point for the feedback is included to  increase and somewhat linearize the impedance into which the feedback operates.

Mini-console Schematic

I have begun wiring up the beta test version of the power amp so we shall see how it all  comes together in the real world.

PA Chassis Gutshot

I still need  to add some more power supply parts and wiring but will soon be ready to test. I plan to  use a little 80s vintage Radio Shack microphone mixer as a preamp. Not exactly golden ear  but should allow me to determine if anything is way out of whack.

In the above picture you can see the rubber grommets used to protect the output transformer  wires where they pass through the side of the chasses. The 100 ohm power resistors on the anodes of the rectifier are there to protect the modern  rectifier tubes from arcing. The can cap will be in the right most cap cutout to keep it as far  away from the heat as possible.

Below you can see the new paint job that I did on the chassis. I used a base coat of  Rustoleum self etching primer followed by a top coat or two of metallic sage rustoleum. The preamp chassis from a Hammond tube organ is similar to the one shown below. It is  the same size and shape but has a somewhat different socket layout etc.

PA Chassis Paint

Preamp Chassis
Preamp Chassis Inside

Well, we have made quite a bit of progress on the power amp section. After initial wiring and testing I decided with a little help from my friends to re-bias the 6CG7 driver fo higher current operation. Initially we were running at under 1mA per channel so I reduced the plate load resistor to 39k and the cathode resistor to 1k. I also paralleled a second 22k resistor with the existing 22k feedback stabilization resistor (R18) as well as adding another parallel resistor to the feedback resistor R4 to bring the total feedback resistor value down to about 120k to increase the FB ratio by a couple of dB. The lower R18 should give the driver an easier time of it.

Finished PA Guts

Those two white 100 ohm power resistors on the plates of the rectifier tube were getting quite hot so I got some 50 ohm 10 watt ones (which are quite a bit larger) and replaced them. These run much cooler, though still warm, and should still provide some protection for the rectifier. The two little carbon film resistors piggybacked on the yellow and black caps are the 6CG7 bias resistors and have been replaced by 1k metal oxide versions.

Test Glowing


When I figured that everything was hooked up right I lit her up and measured voltages while looking for traces of the magic smoke escaping. When everything appeared OK I played some music through her and it sounded pretty good given the limitations of my test setup. The speakers in my work room are some old 8 ohm tri-ax car speakers in some random sized mostly sealed enclosure. Bass was solid and midrange and treble as clear as these speakers could do.

So after playing it about 4 hours and not noticing any real problems I took it downstairs and hooked it into the Mark Audio speakers in the un-optimized test boxes.

Up and Running


Not much bass in these little boxes but the clarity and imaging is pretty good. I apologize for the grainy picture but consumer grade point and shoot digicams can only do so much in low light and you have to see the pretty heaters glow right?


So here is some proof that it actually makes noise. :) Of course the microphone on my cheap digital camera doesn't tell you a whole lot about the quality but in person it does sound pretty good to my ears. Don't know why the imbedded player is so small. I will try to get that fixed. In the mean time if you right click (control click for mac) you can select watch on YouTube to get a full size version.