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Old 6th November 2007, 04:02 AM   #321
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Nice -- not only a simple answer, but a simple explanation. Things are making more sense as time goes by.

Adam
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Old 6th November 2007, 09:27 PM   #322
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Another one...

Should the common speaker output on the OPT be left floating (only connected to the speaker terminal and not system ground) if I'm not using GNFB?

Takk Fyrir,

Adam
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Old 6th November 2007, 09:34 PM   #323
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Adam,
Theoretically it doesn't matter but I like to tie things down in preference to letting them float around. So a 0V connection for the OT secondary common is a good idea. If using global feedback the best place for this 0V connection is the 0V end of that 470 Ohm resistor which connects the input triode grid pin 7 to 0V (via the 1K grid stopper). So wire the secondary common to that point regardless of whether you end up using global feedback or not.
Cheers,
Ian
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Old 7th November 2007, 06:37 AM   #324
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I know this isn't a troubleshooting thread, but I've been posting here, and my issue is regarding this particular circuit, so post I shall...

Problem: 450V at GZ34 pin 8.

It is built exactly as the Baby Huey schematitic dictates with the following notable changes/additions:

Hammond 372JX 300-0-300 PT 250mA rated - without resistors in AC leads, should offload about 360-370V at rectifier.

Separate 5V transformer: Hammond 166J5 1A rated
I've wired the transformer's primaries (no center tap) with the PT's primaries at the power switch.
http://www.hammondmfg.com/166.htm

Power supply is shown below.

Power at the wall is a steady 117VAC.

I really don't know how to take this one. It just doesn't seem...possible. I used the 372JX before, with a GZ34 and the shown power supply circuit. All was OK.
I've triple-checked this circuit both while building and after.

Any advice is appreciated.

Regards,

Adam

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Old 7th November 2007, 07:42 AM   #325
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I did some more testing.
With the GZ34 disconnected from the circuit, I'm reading 295V at pin 8.
I've tested the PT's secondaries and all check out.
No sound comes over either channel. No hum, no hiss.

I'll refrain from posting this issue too extensively on this thread. This is the last mention I'll make of it for now.

Thanks,

Adam
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Old 7th November 2007, 12:24 PM   #326
chrish is offline chrish  Australia
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I am definitely a newb, so don't take my advice as anything special, but shouldn't you be getting 0 volts at pin 8 with the GZ34 removed? If you are getting 295 volts you probably have made a wiring error. If it is 295 volts AC it looks like you have pin 8 connected to the high voltage secondary some how.

You did not mention, but are you using the separate 5 volt transformer for the driver CCS and the 5 volt 4amp tap off the 372JX for the filament of the GZ34?

Any photos?

Good luck!

Chris
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Old 7th November 2007, 04:28 PM   #327
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Quote:
shouldn't you be getting 0 volts at pin 8 with the GZ34 removed?
What I meant is that I was reading 295V with the circuit disconnected from the GZ34.

I figured it out. As usual, I repeatedly overlooked something simple. I had connected all EL84 bias blocks to pin 1, not pin 3.

The amp is up and singing. I'm immediately impressed! I'll post my findings and some photos here later.

Back to tweaking!

Adam
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Old 7th November 2007, 07:04 PM   #328
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I've had the 'Huey up and running for a few hours now. I must say that this is one special amp. It has certainly met and exceeded my hopes. I am absolutely in love with its overall smoothness and listenability. The midrange is very pleasing in tone and nuance, which I think is the foundation for an emotionally involving listening experience.

I've tried two different feedback configurations:

- 22K shunt and no global
- 22K shunt and 18K global (into 8 ohm speaker load)

With global feedback removed, individual voices and acoustic instruments are more vivid and really jump out into the room but overall focus is compromised when multiple instruments are being played

With global feedback included, focus is improved but the midrange receeds - slightly but noticably. The amp also produces a mild amount of ear-burning high end hash. Would this be clipping?
These are very typical findings, I'm sure.
I'm going to order more resistors so that I can play with the feedback setting more. I'm sure there's a sweet spot to be found.

All in all, I think this is an excellent sounding circuit which deserves further attention. I'm glad I built it.

Photos to come.

Thanks again Ian for such a great circuit.

Ha det bra!

Adam

Current configuration:

-1000uF 50V Nicholson Muse KZ cathode bypass, all other small electrolytics are also Muse KZ, all are rated well over requirement
-22K Mills NIWW shunt feedback, 18K IRC metal film global feedback
-GZ34 rectified, 24uf input cap followed by a 1.5H choke into 100uf per channel
-Dynaclone Z-565 output transformers
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Old 8th November 2007, 03:09 AM   #329
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Quote:
Originally posted by LowRedMoon
With global feedback removed, individual voices and acoustic instruments are more vivid and really jump out into the room but overall focus is compromised when multiple instruments are being played

With global feedback included, focus is improved but the midrange receeds - slightly but noticably.
This matches my findings concerning NFB and its effects.

Quote:
The amp also produces a mild amount of ear-burning high end hash. Would this be clipping?
No. That would be your typical pentode nastiness due to high order harmonics. This is why you need some form of NFB to tame those beasts.

Quote:
I'm going to order more resistors so that I can play with the feedback setting more. I'm sure there's a sweet spot to be found.
Definitely do that. In fact, I include variable gNFB in my designs. I've found that the level of gNFB depends on program material.
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Old 8th November 2007, 04:25 AM   #330
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Quote:
Definitely do that. In fact, I include variable gNFB in my designs. I've found that the level of gNFB depends on program material.
I was thinking the same thing. Do you use carbon pots or attenuators?

Adam
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