[jrebman]

Josh,

Thanks, and that actually makes sense to me now .  I have a 50 k PEC pot here that I was going to use in the UBM, but now that Broski has come out with the 24v Aikido preamp/headamp (and I managed to snag one of the boards), I'll probably just make the UBM a direct power amp and use the SB volume control until the 24 v Aikido is done, which I will build with an external PS so I can use a power brick to start and then eventually move up to a well regulated linear supply with the possibility of SLA as well.

-- Jim

[DaveC113]

Jim, do you have a link to details on the 24V Aikido? Looks interesting...

Dave

[jrebman]

Dave,

Just go to the tubecad.com web site and look for the January 27th blog entry.  There isn't a lot of information yet and he promises more in his next installment.

Go to the "New Hardware" link on that page and it will take you to the ordering page where you can click on the 24v Aikido and see if there are any more in stock -- $50 for the board and shipping.

-- Jim

[floobydust]

Kevin,

Nice -- thanks.  Lots of iron -- gotta like that.

Any recommendation for a level pot value in case somebody wants to build it as an "integrated"?

Would you happen to have the schematic as a PDF or something like it that I can try to make a tactile printout of?  These things just make so much more sense to me when I can see where things are .  Then I'm able to tell which resistors are what, etc.  A "budget" amp with hum balance pots -- nice touch.  I'm sure a few will want to know what changes to resistor values, etc. might be made when using a 2A3, but I'll let somebody ask before putting you to the trouble, because I don't necessarily want this myself.

Again, thanks, and I hope somebody can get to building one of these before I can.  At least I can start collecting parts piecemeal.

-- Jim

 Hi Jim,

 For a stereo volume control, Alps and Noble are decent and not too expensive. You could stick with a 100K or a 50K as Josh likes.

 And yes, anyone can swap over to a 2A3 output tube... the changes:

 Output Tube: 2A3 (doh!)
 Output Transformer: 3.5K load
 Cathode resistor: 825 ohm
 Cathode bypass: 33uF

 You would also want to buy a slightly higher capacity power transformer, say 150ma rather than 100ma. Not much else... perhaps a slightly larger fuse value.

 I can output the schematic to PDF for you.... I currently have the mono version which uses the low-cost power and choke with the voltage doubler supply fully drafted and in PDF format. I'll get to the stereo version (only difference is the power supply) soon and make it available as PDF also.

 Regards, KM

[jrebman]

Kevin,

Great.  No rush -- just appreciate it whenever you can manage it.

How's it sounding, BTW?

-- Jim

[jrebman]

Kevin,

Any updates on listening, especially with the Feastrex in the nbew cabinets?


-- Jim

[floobydust]

 Hi Jim,

 Been pretty swamped lately... and just returned from Singapore within the last couple hours.... however we have progress. I only did a single channel prototype with all electrolytic caps in the PS and bypass and NOS A-B carbon comp resistors using a James output tranny. It sounds very nice but not a final build.

 I have opted to build 3 variations for stereo operation. The first are the monoblock ones with the solid-state voltage-doubler supply using all Mallory CGS/TCG filter caps, Dale/Vishay resistors and Axon for circuit bypass/coupling. The power supply iron are used units pulled from an ancient pair of HP freq counters which were all vacuum tubes... very nice pieces and should last forever.The output iron is the entry level James H3115S pair with 3.5K/5K primary and 4/8 ohm secondary (hence no Feastrex driving). All parts are in and prep'd. Just need my son to get his butt up here so we can build 'em.

 The second is a Stereo all tube unit for my Dad. It uses larger Hammond iron (302AX) in the supply with dual (158M) filter chokes and caps for each channel, Magnequest TFA-204 outputs and 2A3 output tubes and a 5R4GY rectifier most likely. Power supply and circuit is all Axon caps and the resistors and such as above. All parts are in and prep'd.... just need to complete the mechanical layout and I can build it.

 The third is a pair of all tube monoblocks using Hammon iron and custom Magnequest RH-40 outputs with flying leads and endbells sans the UL tap with 45 outputs. I contacted Mike, showed him a custom mono schematic and he decided to do a run of these and posted on his forum... I also did an updated schematic and BOM specifically for this version. Again, all Axon caps, Dale/Vishay resistors and on 9x5x2 chassis with a 5V4GA rectifier. I just need the output iron (within 3-4 weeks) and the Hammond bits to complete this... plus the mechanical layout.

 I'll be posting final chassis layouts once they're complete as well for both MW output versions. I wish I had more time to get these moving quicker.... ugh.

 Regards, KM

[jrebman]

Kevin,

I completely understand the issue of finding time to build.  I'm making progress on my simplese but just ordered a full complement of custom iron from Jack Elliano, so once that gets here I'll be able to get going on the mechanical layout.  My audiophile/cabinetmaker friend is going to make me a killer chassis out of bird's eye South American Cherry (not at all like American cherry, but more like Brazillian Rosewood.)

Still trying to figure our if I can go with film caps for the cathode bypasses, and if I can only choose between gain stage or output, which would be best.  PS is all film/motor run caps though (and an Electraprint choke.)

The MQ iron sounds interesting -- may have to give Mike a call .

Now go rest .

-- Jim

[floobydust]

 Well, I finally got some parts in and did an initial chassis layout. It'll be a tight fit underneath but a 9x5x2 chassis will be it. Here's a couple pics... still making a few minor changes so this will be a sacrificial chassis. I also changed a couple parts.... power transformer and choke... more details to follow later.





 Sorry for the blurry pics.... camera battery going dead and too lazy to wait and charge. Will try and have the first one wired up by end of the month.

 Regards, KM

[floobydust]

 Fitting parts... it's already getting crowded 

 There will be a 12 ga solid copper buss running side to side for a common ground point. The standoffs on the hex spacers hold it.



 Regards, KM

[floobydust]

 Completed the mechanical layout for the lowest cost version. Parts cost (sans tubes) is about $225 per chassis (less any tax/shipping).



 All that's left now is to wire it up and fire it up....

 Regards, KM

[jrebman]

KM,

Sounds good -- can't wait to hear the listening report.  Still don't have my RH-40s, but there's still a lot I don't have, so that's fine.

Looks like it won't be too terribly long until my F120A "Uber Fonken" will be coming to life too.

JKeep us posted,

Jim

[floobydust]

 Well, I've completed the first amplifier. Had an interesting problem.... laying on it's side, all was well. Once upright, it blew the fuse as soon as the (5V4GT) rectifier warmed up. Turns out one leg of the 5V filament was shorting inside to the endbell on the power transformer. The weight of the core just managed to let it move away slightly when on it's side. I would also mention that in one last cost savings attempt, I swapped to an Allied 6K3VG power transformer.... very inexpensive and apparently made by Hammond, but... I'll be checking more closely as I build it's mating amplifier.

 So, based on initial testing, it does meet or exceed all specs, but with the Allied power transformer, voltages are a bit high so I swapped to a 5Y3GT rectifier which dropped it down. At idle, I measured 63.5 volts across the cathode resistor (1.5K) for ~42ma of current and 297 volts from the cathode to the plate for a total of 12.5 watts dissipation. This is a bit high so chances are a 1.74K cathode resistor would promote better tube life. Anyway, it will run around 3-watts output before clipping and is below 0.5% distortion at 2-watts into 8- or 16-ohm loads (at 1KHz). Bandwidth with the 125ESE is decent... within 1dB to 25KHz.

 I'll burn it in for a while and do some additional checks and tests. Assembly into a 9x5x2 chassis might be a bit challenging for a novice, but if done in sequence it's not too bad. The Axon caps are just large, but in this design should last indefinitely. Attached is a pic of the wiring:



 Regards, KM

[jrebman]

KM,

Cool. Still watching and waiting to get started with something here.  I got my RH-40s with the brass bells and they're really nice, and I've got a couple pairs of 45s here as well.  Still have to clean up some of te backlog, including the Simple SE before getting into this one.  Parts collection is well underway though.

How's it sounding?

-- Jim

[floobydust]

 Hi Jim,

 I also received my end-belled/flying-leads RH-40s from Mike last week... I just got black painted ones. Very nice. I did some initial bench testing and they are better for bandwidth and square-wave response over the Hammond iron which is no great surprise. I've ordered some additional components to trim the current down a bit as well. Unfortunately I've not done any listening yet... as I want to have a second matching amplifier first. Once I get things trimmed a bit better I'll publish an updated BOM and such. In the meantime, I need to get the second chassis started.

 Regards, KM

[jrebman]

Ok, time for a question about the circuit.  Specifically the hum balance circuit.  What are you trying to balance against what?  In other words, what does the schematic of the heater circuit look like?  Where does the pot have it's connections?

I'm trying to figure out if it would be possible to use a 5 vct transformer and still be able to hum balance the tubes individually (talking about a single chassis stereo amp, that is).

Never dealt with a hum balance circuit before, so really don't know what's going on here.

Thanks,

Jim

[floobydust]

 Jim,

 Hum balance... I use AC on the filament for the 45. My preferred approach is a 2.5VCT transformer winding with the cathode bias resistor from the filament center-tap to ground. This provides a very low impedance for the filament and equal DC balance across the filament. As the filament is the cathode and AC operated, you need the ability to compensate for the non-linear conduction of the filament. Basically, it's a coated filament and as such is not perfect, ie, you'll get some AC hum on the output as a result. The hum balance pot is in parallel with the filament and the bypass capacitor from the wiper to ground which provides an adjustable AC balance to null out the resultant hum. I use a 100 ohm wirewound pot padded with a pair of 12 ohm resistors, one from each end to the wiper. This widens the null point so it's not a "knife-edge" adjustment and provides an effective series resistance of less than 5 ohms.

 As for using a 5VCT winding, not a good idea... You really want complete isolation of the 45 filaments so each has it's own self-bias and AC balance. You can get the basic Hammond 2.5VCT@3amp filament transformers (p/n 166M2) for $9 each at Angela. These are pretty small overall so stuffing them under the chassis should be pretty easy. Hope this helps.

 Regards, KM

[jrebman]

KM,

Helps a lot, thanks.  Just don't quite see where the wiper goes -- probably because I am also reminded that I don't really know what the cathode circuit for DHT looks like .

It's clear why the 5vct idea won't work though.

Thanks,

Jim

[jrebman]

Kevin,

Never mind -- figured it out.

-- Jim

[jrebman]

Can I also assume that it is best to locate the balance pot fairly close to the tube?

-- Jim