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Old 9th October 2012, 05:23 PM   #821
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The amp is apart on the work table.

The toroid for 6C33C heaters is making a loud buzz.

First i took out the small PCB transformers i hooked up to the heater windings for the bias supply.

Most of the buzz gone ??

Im not overloading the toroid, i checked my connection to the bias transformer everything in phase no mixed up windings etc.

What is going on, what did i miss ??

Or is it so simple that it cant take the rated load and stay quiet.

New standalone bias toroid and new heater toroid is going to be hammered in during the evening .
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Old 9th October 2012, 05:59 PM   #822
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New heater and bias toroid.

Old bias PCB transformers unit.

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Old 9th October 2012, 07:17 PM   #823
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Quote:
Originally Posted by flatheadmurre View Post
The amp is apart on the work table.

The toroid for 6C33C heaters is making a loud buzz.

First i took out the small PCB transformers i hooked up to the heater windings for the bias supply.

Most of the buzz gone ??

Im not overloading the toroid, i checked my connection to the bias transformer everything in phase no mixed up windings etc.

What is going on, what did i miss ??
Or is it so simple that it cant take the rated load and stay quiet.
Are you running the PCB transformers off of the secondary of the toroid? If they are presenting an uneven load to the toroid, that could cause the core to vibrate, even though it appears to be within spec.
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Old 16th October 2012, 11:06 PM   #824
M Gregg is offline M Gregg  United Kingdom
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More trial and error..

Well initial tests are interesting, I'm Running 410V across the rails. 205v app across each 6C33C. I am running mixed bias 42V on the top tube. the bottonm tube is adjustable to get a balance across the 150 Ohm cathode resistors. The PSU caps are balanced with 10K and 22K.

I am still running the MU sections with the FETs at the moment..I will look at how low I can get the cathode resistors and still have some sort of control. Idle current is set at approx 200mA. So I'm getting about 25V single heater and 30 Volt dual heater across the 150 Ohm Cathode resistors.

The supply is now not CT and floating..bass is still good! Some more tweeking to do..

I still have the Auto bias sound at the moment, however its a bit louder..Perhaps more open as well.

Its all Good fun.... The only problem is the stink off the power resistors as they burn in....or Out depends on your point of view....


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M. Gregg
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Old 16th October 2012, 11:32 PM   #825
M Gregg is offline M Gregg  United Kingdom
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I should add,

The choke psu is staying...

The bias was created with a voltage doubler giving about 100V from a 50V winding on the new higher voltage power Tx.

The top tube bias from the bottom tube power rail..

So the saga continues...

Nothing like playing for fun is there.. Oh well bed I suppose...more playing tomorrow....

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M. Gregg
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Old 19th October 2012, 03:36 PM   #826
M Gregg is offline M Gregg  United Kingdom
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Well,

Discussion in the background....
The 1st stage is still MU, 2nd stage is resistor anode, direct coupled to PI no grid stopper.

Combined with the mixed bias and increased main HT (410V) gives me switchable one and two heaters. Volume is way better. Using Ansar coupling between PI and 6C33c, Robert Hovland super cap between 1st MU stage and 2nd stage driver direct coupled to PI. Bass drive is good..

I think there is still room to reduce the cathode resistors and increase DC bias. Its run in time again and test (on full power at the moment you can't shout and be heard)(mozeners still in place) so the saga continues...
Hum is ear in the speaker low...sound stage is good.

It would be interesting to try some kind of direct drive with the 6c33c's.
Perhaps another project...
Something I found strange is it sounds better with the Goss band removed from the main power TX..wonder why<<perhaps its imagination...

The cork pads work well under the toroid for isolation...(just for interest).

Regards
M. Gregg
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Old 19th October 2012, 06:20 PM   #827
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Quote:
Originally Posted by Matthewong View Post
Hi, would like to ask whether the vacuum tube OTL power amp worth to build? How is the sound performance if compare to normal output transformer type?? Does anyone has any good schematic diagram for OTL power amp, can share out, thinking to try to make one.
I made OTL - 6AS7 /sorry, very ugly, but this is only try/according this schematic:
diytube.com :: View topic - OTL amplifier
Before phase splitter I put preampl. 1/2PCC88.
Current plate every 1/2 6AS7 - 70 ma. -Ubias about 90 V.
U under 820 + 820 ohm is 0 /no more 100 mV/, because currents upper and down shoulders in different direction
must be the sames, for avoid U under speakers. You can connect fast fuss 300-500 ma to save speakers and voltmeter. In the begin, after switching, there is some U, after few minutes U must be min. and You can connect speakers. There isn't cathode shunts for 6AS7 /like for 6S33S, I think that its sound is some sharp/ that will paint the sound, that is very important!
Sound is cool, awesome! The scene is very deep, bass is giant, details are unbelievable. /Now I can't listen my SE 300B, 2A3....with output transf., because every OT press Low Freq. and the scene, IMHO /.

Last edited by azazello; 19th October 2012 at 06:42 PM.
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Old 20th October 2012, 03:24 PM   #828
M Gregg is offline M Gregg  United Kingdom
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Hi azazello,

I dont seem to be able to find the modified schematic of the preamp ecc88?
Did you put it after the volume control or before it?

Did you use any speaker protection? (on the psu or other)
and are you timing in the connection to the speakers? (after bias offset settles at start up).

Regards
M. Gregg
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Old 20th October 2012, 05:48 PM   #829
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For preamp. PCC88: +U = 270v, Rplate=20kohm, +Uplate=138v, Rcathode=390ohm, I plate=about 7 ma, Ucathode=2.7v., Rgride= about 500 kohm. Interstage cap about 470 nF. Potentiometer /about 2 - 10 kohm/ is in input.
My speakers are Fostex 206E - 8 ohm. 96 dB, for protection I use fast fuss in serial to speaker, depending how I can hear the level of the sound in my room. My speakers are with High sensitivity, volume is small and I use fuss 300 ma /with 100ma fuss is blowing/. I know that speakers never will die. For speakers with low sensitivity, fuss will be more, You will try.../I don't know how is the diameter of wire of Fostex 206E and how is max. current across the coil of speaker to decide what kind of fuss use/. For ex: If one from 4 triodes will die, U cathode res. will be 5-6 v, Current across 8 ohm will be about 700 ma, there are more wrong situations and wrong currents are different.
Waiting time to setting common current across common cathode resistor is about 3 min. After this I will connect speakers./Common current is +- about 100 mV. If all triodes are identical, U will be about zero/.

Last edited by azazello; 20th October 2012 at 05:53 PM.
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Old 20th October 2012, 06:25 PM   #830
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Sorry, I will add: Cathode shunt cap of PCC88 is 1000-1500 uF Nichicon or Elna, or Rubycon.
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