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Old 9th July 2008, 08:24 PM   #521
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Thanks Diomedian
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Old 9th July 2008, 08:28 PM   #522
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Geez I hate seeing excellent builds like that..!!!! It makes me feel so mediocre!!! Grrrrrrr.....

Especially the electrical bit is exemplary!!!
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Old 9th July 2008, 08:53 PM   #523
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Bas,
Thanks, you are generous!
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Old 9th July 2008, 11:59 PM   #524
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Quote:
Originally posted by albertli
Ian,

Thanks for the suggestion.
I have a 100R pot there for easy adjust the balance since I found none of my 12ax7 performed as the way I'd like them to.

132v on both the anode of 12ax7
37mA on bias of EL84
22K 2W on anode to anode (NFB)- I'd play with the value a little for better performance. It already shows really good square wave form from 20 Hz to 20K Hz on my scope. Ah.......... I'd be happy if I could tweak the band wave wider.

Albert
Duh ?

20Khz square means 100Khz sine, at least !

Yves.
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Old 19th July 2008, 06:18 PM   #525
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Default I think my HT is too high...

I've just finished building a monoblock baby huey as a breadboard that I plan on using to learn the basics, then build a pair of mono Hueys. I'm using a hammond 270EX PS transformer (275-0-275 @ 125 ma) in the original baby huey circuit, and my voltages are running somewhat high. My B+ is around 360V and the diffamp anode voltages are 220V.

The bias blocks are measuring 635mv across the 16 ohm R

Here is what my ultra-newbie brain is contemplating:

Up the 16 ohm resistors to 18 ohms or so in the bias blocks to reduce power dissipation in the EL84's to around 12W. This was a suggestion by Ian in an earlier post, but I can't see how it will help the diff amp voltage.

Rebuild the power supply as a choke PS per Ian's suggestions for the monoblock build. I have a 8H choke and the req'd caps. I'm hoping that this will reduce the B+ voltage somewhat.

My output tubes are JJ's and I think I also have a set of EI's, so I'm assuming that my B+ needs to be below 350V...My OPT's are rated at 20W.

Any suggestions as to the correct way to solve this problem are greatly appreciated.
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Old 21st July 2008, 12:19 AM   #526
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Boywonder,
Do the Cathode Bias Block resistor change from 16 to 18 Ohms and then have a listen to it. That mod will reduce your output tube combined anode and screen dissipation from

0.635/16 x 360V = 14.3 watts (102% of rated 14W)
down to
0.635/18 x 360V = 12.7 watts (91% of rated 14W)

OR even use 20 Ohms for extra tube life
0.635/20 x 360V = 11.4 Watts (81% of rated 14 W)

The amp will run quite happily at this although you will get more like 12 watts rather than 10.5 watts output.

See what you think of the sound first - if not real happy then contemplate the power supply change. If you are happy just leave it that way. EL84s are a tough tube and don't seem to be too fussed about having a bit extra voltage across them. There are a lot of EL84 amps around running 380V.

Don't mess with the diffamp. 200V there is OK and if you do change the main B+ later this will drop as B+ drops.

Cheers,
Ian
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Old 21st July 2008, 01:47 AM   #527
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Thanks Ian: I fired it up earlier for about 3 minutes and it plays rather well! I was just a little paranoid about melting my anodes.

I will swap the resistors out after a little listening, since I'm only running 102% or so. I am planning on building the second breadboard as the fixed bias mono version, and then modifying the original amp to be fixed bias.

Thanks again.
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Old 23rd July 2008, 09:51 PM   #528
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I finally got my single channel monoblock breadboard up and running for a few hours, and it sounds great, better than I expected. This has been an extremely satisfying project.

So far, I've only let the smoke out of one component; unfortunately, it was one of my input transformers That was before I figured out the PSUD software, another very cool learnig experience.

Actually I forgot about the smoked heater resistors when I accidentally mixed up the output and driver tubes in their sockets. (The sockets are mirror imaged when looking from the top of the amp!! ) I won't be doing that again...

Now I am planning on building the second monoblock as the fixed bias version, and then updating the first mono to fixed bias, after some listening comparisons.

Here are my next batch of beginner questions:

The ZVN0545A mosfets appear to be three-legged critters; how do I interpret the "middle arrow" connection between the source and the drain on the schematic?

Are the only parameters that are important for selecting the Green and Blue LEDs the voltage drops?

It appears that I will need another transformer to generate the -45V rail; does this have any current requirements? I am using the hammond 270EX for each monoblock for everything else.
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Old 24th July 2008, 12:26 AM   #529
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The "middle arrow connection" in the mosfet symbol represents the "substrate".
In all of the mosfets you are ever likely to come across it is never run out as a separate connection but is connected to the source internally.

ASIDE: There are some very specialised mosfet types usually intended for VERY high speed analog switches where the substrate connection is run out separtately and these devices have 4 leads. They also have absolutely no anti-static protection on the gate, are supplied with a conductive rubber ring around the 4 leads which has to be left in place until after the devices have been soldered into the circuit, and die if you "look at them sideways". I actually use some of them in the day job to implement analog switches which switch in about 1 nano second.

The -45V (nominal) supply needs to be able to deliver about 6 mA.

Note that you also need a +ve low voltage supply which can be any where between about +15 and +75V but only needs to supply about 1 mA

You could have a separate tranny for these.

My suggestions:
Use a zener regulator off the existing High Voltage for the anywhere between +15V and +75V supply. Make it say +33V, you need about 100K from the high voltage rail to the zener cathode. Zener anode to 0V. Use a 1.5 Watt Zener. The resistor will dissipate nearly 1 watt. so use 2 off 47K 2W metal film resistors in series.

The approx -45 supply can be done with a separate tranny in 2 different ways. Note that you are drawing only 6 mA. 6mA x 45V is 0.3 VA so a really tiny transformer will do.

1) Use a tranny with 18-0-18 volt secondary and bridge recify across the the full secondary leaving the"0" centre tap unconnected. This is waht I did for my prototypes.

2) IF you are not using the 270EX 5V winding for a tube rectifier then:
Use a 15 volt secondary, 115V primary tranny. Wire it in reverse to the 270EX 5V winding. That is connect the 15V secondary to the 270EX 5V winding. You will then get about 36 to 38 Volts AC on the primary. Bridge rectify and filter that.

The Blue LED is the reference for the diff amp current source. Keep the leads to it short and away from any AC wiring so that it doen't pick up any noise.


Cheers,
Ian
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Old 24th July 2008, 12:52 AM   #530
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Thanks Ian, still digesting the above....

I like 2) above, if I get a tiny 15V tranformer, am I wiring it's secondaries in parallel or series with the 5V 270EX secondary winding?
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