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Old 29th November 2008, 07:35 PM   #611
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Quote:
Originally posted by adamus
I have a couple of options for the bypass caps, either silmic 1000uf (i have 4 in the parts bucket) or i have 4 220uf blackgates.

i assume i would lack bass with the 22ouf blackgate?

Use both the 1000uf and the 220uf blackgates. Bypass them with a 1-2uf film cap also. Gingertubes original schematic shows two electros + a small film bypass (I think threy were 4700u+470u+1u PP). Later posts in this thread show folks using 1000U or so. My Hueys are presently running 1000u BG+100u BG+ .1 PP

Once you get the amp up and running, dial in the caps to suit your taste.
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Old 30th November 2008, 05:15 AM   #612
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Hi,
Did some tinkering on maths and theory my own.

After studying the UL tab BH, It made me itch to put up something fast.... I only have a 185V power trans and some really cheap 0-3k5-5k SE OPT.

I calculated the %tage feedback of the latest BH circuit is
40% x 9k/19k= 18.9%

I recall that I have a pair of really cheap SE OPT with optional 3k5 & 5k tap, I did a quick measurement and know that they are on the the same coil. Then I calc the tap 3k5 on 5k as Sqrt(3k5/5k)=83.7%
so if I reverse the polarity of the SE primary coil, then we will have a 100-83.7%= 16.3% UL tab. near enough to Ginger's final test point.

I then recalled I have a power trans with 2 primary coil 0-185V & 0-6.3V, the 185V tap will give me ~250Vdc, should be enough for EL84...
If I use a 12AX7, set it at 0.5~0.6ma Ia with a CCS, I can feed the driver stage directly from the 16.3% tab!Use a large resistor to of 200K to bring it down to ~150V at the plate, and 0.5~0.6mA shouldn't cause much problem to the EL84 output stage as well...
BUT I start to puzzle over
1)what grid resistor should I use.
2) if I can drive the EL84's effectively with about 1/2 mA and very high Zo of the 12AX7 w/o a source follower??

This SE clearly looks like (at least to me) a kind of partial Voltage feedback rather then a I/V converter stage..

BTW, how would I know the Ro of the output stage? Is there any maths for this?

the 12AX6 give about 60V max onto the EL84 grid, looks like the EL84 will clip fairly early as well.

I'm new to these please point me the right direction.

Cheers

Ken

**ps, I'm even tempted to direct couple the two stages loftin-white style....
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Old 3rd December 2008, 04:44 AM   #613
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Ken,
Can't really comment on your proposed SE design.

Others,
More work last night on the working schematic of the fixed bias BH at post #604.

I kept pushing up the shunt feedback via the cross connect resistor but just could'nt get the sound I wanted. I needed a bit more bass roundness and tube warmth. That suggested the feedback was already too high so I went back to the 10K + 9K1 + 10K arrangement from the UL Taps and then started REDUCING feedback instead. 8K2 in place of the 9K1 gave me a fair amount of the sound I was looking for. May even try 7K5 BUT 8K2 is really lovely.

Lesson here for young players - If it doesn't sound "right" and pushing up the feedback does'nt get the sound you want, then try reducing the feedback instead.

I also played around with the common resistor in the cathodes of the EL84s. This was to see if I could implement that 3rd harmonic distortion reduction scheme posted earlier (which from Sheldons tests back on page 19, Posts 451 and 452 was also an intermodulation distortion reduction scheme).
According to Sheldons observations of his results and Allen Wrights published PPC1A circuit a Common 39 Ohm resistor would be ideal.
I fitted a 39 Ohm and then just experimented by shunting it with a 10 Ohm ( bias in the tubes set with the 10 Ohm shunt in place and just accepting that the idle current dropped with the shunt removed). The 39 Ohms I believe DID sound better so that is what I'm running now (with bias adjusted). This was a purely listening test. No empirical testing.

Its now well passed the stage that you need to listen to any changes over a few days with many different music selections. Some stuff which now sounds bad is because the "recording" is bad and we are revealling its "warts".

A vinyl session is now in order.

Cheers,
Ian
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Old 3rd December 2008, 08:51 AM   #614
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Ian,
Thank for the big red light, I get carry away easily.
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Old 5th December 2008, 01:07 AM   #615
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Hi Gingertube,

Did you receive my PM?

Cheers!
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Old 5th December 2008, 01:12 AM   #616
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Geek,
Yes - PM Receieved and I have (in the last 10 minutes) sent a reply.
Cheers,
Ian
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Old 5th December 2008, 03:18 AM   #617
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Thanks... seems our messages passed in the hallway

Cheers!
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Old 5th December 2008, 07:57 PM   #618
adamus is offline adamus  United Kingdom
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my baby huey is moving along...slowly...damn having so much work on.

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Old 7th December 2008, 06:17 PM   #619
adamus is offline adamus  United Kingdom
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any advice on layout... just playing around at the moment.

The white sheet is the same size as my chassis. I will use shieldd wire for inputs so are too bothered about the length to the volume pot.

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Old 8th December 2008, 02:07 AM   #620
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What I would do:
Put the two input RCA Sockets top left (insulated from chassis).
Run shielded cables down left side and around to volume pot.
Signal ground to chassis earth connection point near the volume pot
Power input rear right with chassis ground SAFETY connection in that corner.
Where ever possible use transformer mounting bolts and tube socket bolts to mount the boards to minimise the number of bolts through the top panel (aesthetics ONLY).

Looking good so far.

Cheers,
Ian
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