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Old 11th August 2011, 06:32 AM   #1041
Rene is offline Rene  Netherlands
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Quote:
Originally Posted by gingertube View Post
.....Try 470 Ohms or even 1K in lieu of the 150 Ohms.
Ian,

I have tried your suggestion but unfortunately it didn't help. I tried 470 ohm and 1K but still with the oscillation.
The DC settings I think are correct. With an U+ of 306V the settings are as follows.
ECC803:
pin 1 and 6, 124V
pin 3, 0,95V
pin 8, 0,82V
Voltage CCS across 1K resistor is 1,3V. Current is then 1,3mA which should be fine.

EL84:
pin 7, 302V
pin 9, 301V
UL-Tap, 304V
pin 3, upper tube 10,9V, lower tube 11,3
Voltage bias block across 16 ohm resistor is 600mV. Current is then 37,5mA which is fine too, I think.

Both channels have about the same settings.

Heatings ECC803 and EL84 are ac 6,27V.

The chassis is connected with an 10 ohm resistor to earth.
Output is not connected to earth.

Everything should be all right. I am a bit lost. Could the ECC803 be the problem, since this tube has a high bandwith? Or maybe the UGT?

I have bought the UGT from a local store. He said this is a good UGT for an EL84 PP. But I forgot to ask what the Raa, % of UL connection and sec. impedance were.

Thanks in advance.

Rene
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Old 11th August 2011, 12:18 PM   #1042
UnixMan is offline UnixMan  Europe
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It may be a problem with the layout.

A few generic hints: make sure that all the grid stoppers (on g1 e g2 of all tubes) are mounted right on the tube socket. The by-pass capacitors for all the CCSs must be installed as close as possible to the cathodes and then joined together to the ground star point. Minimize the lenght of the feedback loop(s) connections. Be careful of parastitic couplings between different connections. Avoid wires running parallel to each other (except for those bringing "to and from" current path of the same signal, which should be twisted together).
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Old 13th August 2011, 06:20 AM   #1043
Rene is offline Rene  Netherlands
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Unixman,

The grid stoppers are directly connected to the tube sockets. I have a different lay-out for the CCS that might not be ideal. I have put the two 470 ohm resistor and the and 1k Bal. on the printcard instead of directly connected to the tube sockets. See attached image.

Eventually I put a capacitor of 1nF across the Anode (pin7) and grid 2 (pin9) which stops the oscillation. I am happy about that!
The sounds seems a bit more transparent, cleaner and the high's are brighter.

Regards,
René
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File Type: jpg baby huey el84_2.jpg (342.4 KB, 583 views)
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Old 14th August 2011, 11:26 PM   #1044
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Rene,
Somewhere earlier i this thread zobels from anode to screen on the output tubes were discussed. A 1k + 1n8 connected directly across the anode and screen transformer connections were IIRC what was used. The screen connection needs tgo be at the transformer side of the screen resistor. Try searching for it.
Cheers,
Ian
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Old 15th August 2011, 12:34 AM   #1045
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Rene,
I did the search for you - look back at page 83 and posts by myself and Yves "dupy", in particualr posts #815, #824, #827
These relate to UL instability and the adding of anode to screen zobels across the transformer connections.
Cheers,
Ian
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Old 18th August 2011, 06:20 AM   #1046
Rene is offline Rene  Netherlands
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Thanks Ian,

One question though.
The amp sound very nice now with only the 1nF cap between the screens and the anodes. But a zobel, like suggested in the earlier posts, means a resistor and C in serial. So is it really necessary to put the 1K resistor in as well? Maybe this benefits the stability.

René
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Old 18th August 2011, 07:28 AM   #1047
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Rene,
Yes - I would try adding the 1K in series with the 1nF cap. Easy to take out again if you don't like it or if the instability comes back.

This is an area where I need to do a bit more work to determine "optimum" result.

I had always intended to hook up the sig gen and oscilloscope and do full amplitude and phase plots of the amp with and without UL Zobels but got lazy and haven't done it yet (well actually I got sidetracked building a guitar amp).

I had halted mods on the most recent BH as it had reached the point where I need to upgrade my sources and preamp to tell if things were getting better or not.

A new CD Player - Onkyo with Terra Firma Clock and audio section upgrades arrived today (2 hour ago) so we are back in buisiness and I'm Aussie$1600 (about US$1700) poorer. Just need to get stuck in to the replacement preamp build. For a new preamp Allen Wright sent me his latest schematics for the SPV2 about 2 months before he died. They were provided on a "Comercial in Confidence" for my personal use ONLY basis, so please, no-one ask to have them posted because I won't.

Off for the next 4 days - will check back in after that.

Cheers,
Ian

Last edited by gingertube; 18th August 2011 at 07:30 AM.
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Old 18th August 2011, 10:53 AM   #1048
Rene is offline Rene  Netherlands
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Ian,

I am sorry to hear about Allen Wright.

I think I am just going to do that because it is easy to try it. And if the results are not satisfactory I indeed put it back again in the previous state.

Regards,
René
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Old 19th August 2011, 12:09 AM   #1049
TheGimp is offline TheGimp  United States
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There is an article hereon the SVP-2.:

http://www.vacuumstate.com/fileuploa...te_low_res.pdf

As complex as it looks I doubt I would ever attempt anything near that level.

Although I am tempted to build a version of his FVP5.

I too will deeply miss him in passing even though I never had correspondence with him. His influence is widespread.
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Old 21st August 2011, 11:32 PM   #1050
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Gimp,
The SVP2 is the "modern" FVP5A - very similar schematic.
The new Onkyo CD player with the Vacuum State Terra Firma Lite Clock and the audio section upgrade was a revelation - I don't regret spending the money. Hard to describe the sound change between this and my old Dennon, the initial word that comes to mind is "softer" but thats misleading - there is a harsh edge (which I hadn't really noticed) which is just not there with the Onkyo. Very articulate and just more "organic". This is my birthday present to myself.

Rene,
How did you go with the zobel mod?

Cheers,
Ian

No progress on the weekend - an emergency repair on a friends guitar amp instead.

Last edited by gingertube; 21st August 2011 at 11:34 PM.
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