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EL84 Amp - Baby Huey
EL84 Amp - Baby Huey
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Old 8th April 2008, 04:19 AM   #421
gingertube is offline gingertube  Australia
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Guys,
My lastest Baby picture. Monoblocks.

Fixed bias, choke input power supply, SS rectifier, MOSFET (ZVN0545A) source followers AC coupled off the diff amp and the output tube grids direct coupled to the MOSFET sources pins. Output tube bias is applied at the MOSFET gates.

Cheers,
Ian
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Old 8th April 2008, 04:23 AM   #422
gingertube is offline gingertube  Australia
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And for Chrish and other "Downunder" tube nutters.

Dig those A&R Vintage Output Trannies made in Melbourne. I pulled some Hammond 1608 to put these in - much better.
And so as to not start that Sydney vs Melbourne mudslinging (being from Adelaide I'm above that) , the power trannies are Vintage Trimax from Sydney.

Cheers,
Ian
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Old 8th April 2008, 06:56 AM   #423
chrish is offline chrish  Australia
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Default Baby Huey Sings!

Finished wiring up "Baby Huey" Last night at about 1:30am, but it did not work

Left channel had a shorted out shield from the feedback. Fixed that, but was getting some nasty AC on the output of the coupling capacitor of that channel. Thought it might have been a faulty coupling cap (using Russian), so replaced - still same problem. Output from input tubes was OK when power tubes removed - kept looking for problems... Turned out that one of the output tubes was bad maybe from the shorted feedback shield. Had plenty of replacements as I am using 6P15P and 6P14P output tubes and 6N2P driver tubes purchased in bulk.

Anyway, working now. My modelling of the power supply was a little out, voltages a bit low. Not a real problem, I can change some resistor values and use GZ34 rectifier instead of the 5U4G. More to fiddle with in the future, but at least the basics are sorted.

No matter how hard I try, my patience gets the best of me and I can never get my wiring as neat as I would like

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Guess an appropriate CD for the first listening test!

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Nice work with the amps Ian. Speaking of transformers, in a weak moment on my last work trip to Singapore I bought a pair of these:

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I have some Russian 6L6GTs, 6SN7s and 6SL7s that need to be used too....

Cheers!

Chris
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Old 8th April 2008, 07:33 AM   #424
gingertube is offline gingertube  Australia
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Chris,
That wiring looks pretty good to me.
You should see some of mine - 1st build is generally OK but after the 5th or 8th mod !!!!
Let us know what you think of the sound. Hope you like it.

Cheers,
Ian
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Old 8th April 2008, 11:00 AM   #425
chrish is offline chrish  Australia
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Thanks Ian,

Have not done any serious listening tests yet, kids were complaining when I had my standard test tracks paying before dinner. I know that I have to sort out the voltages. I am using a valve rectifier and used PSU11 to model the power supply. I was worried my voltages would be too high, so from the rectifier I have used 32uF cap - 10H choke - 32uF - 60R 10W -200uF - 180R - 80uF motor run cap (80uF motor run is the 100uF cap shown at the output transformer in the schematic). I am just about to check the voltages with a GZ34 in place of the 5U4G that I had in it for testing.

Will give a better run down on the sound when I have sorted out the voltages, but I can say that I am happy so far and that it is hum, hiss and buzz free! It does sound different from my SE amps and better than my only other PP project (a cheap and simple 6BM8 posted in previous threads here).
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Old 8th April 2008, 03:53 PM   #426
chrish is offline chrish  Australia
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B+ with a GZ34 is 308 volts, not bad. Voltage at the input tubes is still a little low, around 145 volts, I think 170 volts is what is planned. Will have to work on it later.

Just a note, this is my 5th project using Edcor output transformers (project 4 used project 2s salvaged small 10 watt PP transformers). For the first 4 I followed the Edcor schematic for the connection of the feedback. For the first 4 I had to reverse the phase! This is the first using he enclosed XPP transformers, but I took a hunch and wired it up backwards to the schematic and it worked first up. I think TubeLab noted on his web site that when he wired up the Edcors they were out of phase compared to their schematic.

Cheers,

Chris
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Old 8th April 2008, 08:32 PM   #427
dupyv is offline dupyv  Luxembourg
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EL84 Amp - Baby Huey
Default Baby Huey El84 with source followers

Gingertube,

Concerning your new Baby Huey with EL84 & source followers:
- Did you notice any change in sound for the better / worse ?
- Did you already establish an attachable schematic for the above one ?

Kind regards,

Yves
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Old 9th April 2008, 12:21 AM   #428
gingertube is offline gingertube  Australia
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Yves,

I'm still listening critically and considering final tweaks, I guess I'm 95% happy with them.

I will sketch up the schematic and post it in the next day or so.

Cheers,
Ian
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Old 10th April 2008, 04:35 AM   #429
chrish is offline chrish  Australia
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Quick question re my amp.

I have on hand a few 6P15P (Russian EL83 equiv) and 6P14P (Russian EL84 equiv). I built this amp with the intention of being able to use either valve. Accordingly, I have put a bridge between pins 1 and 3 on the power valve sockets, as the 6P15P does not have an internal connection between the cathode and G3.

Have noticed that one channel might be a little unstable/oscillate with the 5P15P when I was checking things on the bench, but got no further than noticing there were no problems with the '14Ps. This might not have actually been the case, as I was fixing a couple of very minor things at the time. Anyway, I have read that the 6P15P has a slightly more fragile G2 than the '14P (1.5Watts v 2.2 watts) and care should be taken when substituting them for an EL84. The original schematic calls for 33R grid stoppers on the g2 pins. Should I try a higher value, say 1K? Would this adversely affect the performance?

Sorry for the questions, don't have enough time to experiment or do empirically right now as I am in the middle of university assignments (as well as working and being a part-time single dad...). Chewed up more time than I should have just building the thing!

Thanks,

Chris

EDIT: Just plugged some '15Ps in, and it seams to be playing OK, but still wondering if it might be prudent to increase the g2 resistor.

Cheers...
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Old 10th April 2008, 04:44 AM   #430
gingertube is offline gingertube  Australia
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Chris,
Yes - increase the G2 Grid Stoppers. I'm posting my monoblock schematic in a few minutes. You will see I used 150R. I would try 220R and 470R before going to 1K. Only make it as large as necessary to kill any parasitic oscillation. Of course you should make sure you have adequate grid stopper on Grid 1 first (4K7 is what I use).

Adjusting the G2 stopper resistor (Screen resistor) is a common guitar amp hacker thing. If you want a guitar amp to start to distort at lower power/volume levels you massively increase the screen resistor. Thats why I say, for HiFi only make it as large as you need to.

Cheers,
Ian
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