rlaury

Anybody have a full schematic for WSW 811301?
« on: February 15, 2009, 06:17:59 PM »
Hi Group:
Anybody have a schematic for the WSW 811301. I have THIS manual

http://amtecaudio.com/WSW%20811301.pdf



But no component values. It's really an oddball circuit with the
feedback circuitry. Notice the DC coupled R's from the E88C plates back to the cathode of the
ECC83. This is part of a nested feedback loop. Also note the R from the B+ at the output transformer
CT back to the grid network of the E88C. The output stage seemed to be biased up from ground.
Maybe the output is closer to CLASS B. I've never seen this before. Anybody got one for sale or trade?
Any ideas appreciated.

RonL
When all you have to work with is a hammer,
everything looks like a nail.


analag

Re: Anybody have a full schematic for WSW 811301?
« Reply #1 on: February 15, 2009, 07:54:03 PM »
What does it do?
Audio engineering suffers from misinformation, disinformation, and downright lying more than most fields of endeavour.

PRR

Re: Anybody have a full schematic for WSW 811301?
« Reply #2 on: February 16, 2009, 03:45:38 AM »
> Notice the DC coupled R's from the E88C plates back to the cathode of the ECC83.

That's a standard NFB affair. It lowers the impedance from the plate into the output transformer, extending local bandwidth.

Your gain control is, at lower gains, mostly the NFB from the tertiary winding. But you can't take huge NFB (wide range of gain) across the transformer unless you control the drive into the transformer. 

> Also note the R from the B+... output stage seemed to be biased up from ground. Maybe the output is closer to CLASS B.

Cap-bypassed cathode resistors, in an amp which may linger near full power (suchas broadcast tone-tests), "must" run Class A. The variable peaky current in class B would cause bias shift and various troubles.

The ECC88 is rated 130V max. The E88CC is rated 220V max (250V under 0.8W). They apparently had a lot of 300V supplies in the field.

Also the '88 is VERY high Gm, and surely pretty variable. And to get 0.5 Watts out, they must be running over 1W per plate, and likely pushing the 1.8W limit. Tube-to-tube variations could make this 1.3W or 2.3W, with high failures among the too-hot samples.

Tube will want ~~6V bias at the operating point. Perhaps 5V or 7V. By holding the grid up near say +60V, tube-to-tube variation will be reduced to 10% of a naked self-bias stage. They can nail it at 1.5W-1.7W no matter how much variation the tubes have.

And 300V supply minus 66V cathode voltage is 234V plate-cathode. Makes sense. Near-enuff the 220V spec. Maybe they hold the grids at +75V. Or there is resistance in the B+ which can shave 4V. They probably want 5%-20% drop in the B+ R-C network, so multiple modules don't leak through the common power and crap-up each other's signals.

It is not possible to derive exact values from the data given. But ECC83/12AX7 will tend to like 100K-220K and 470-1500 bias. The '88's specs and the 16mA total suggest 6mA per section of the '88. This and Rp at 7mA suggests ~~20K load per tube. Gain from '83 grid or cathode to '88 plate is about 500-1000. The total gain is 70dB or 3000 max, so the input iron somehow has 1:3 higher ratio than the output iron, and it can't have much effective NFB at this point (but gains 5-7 are not intended for good program). It is a VERY stringent design, and relies heavily on exceptional transformers.

rlaury

Re: Anybody have a full schematic for WSW 811301?
« Reply #3 on: February 16, 2009, 10:55:24 AM »
> Notice the DC coupled R's from the E88C plates back to the cathode of the ECC83.

That's a standard NFB affair. It lowers the impedance from the plate into the output transformer, extending local bandwidth.

PRR, Thanks for the comments.
I've used this in several other circuits but I've never seen it DC coupled. For example the McCurdy AU300 has a cap
in series with the R. I'm playing around with this in spice to get a feel for it.
« Last Edit: February 17, 2009, 02:38:32 AM by PRR »
When all you have to work with is a hammer,
everything looks like a nail.

Lukpio

Re: Anybody have a full schematic for WSW 811301?
« Reply #4 on: February 16, 2009, 01:48:33 PM »
Also note the R from the B+ at the output transformer CT back to the grid network of the E88C. The output stage seemed to be biased up from ground.
Maybe the output is closer to CLASS B. I've never seen this before.

The same trick was used in tube push-pull Pultecs (and a few other designs) and lets higher value cathode resistors to be used. Not only it lessens tube-to-tube variations, but triode-to-triode variations as well, as the cathode resistors with bypass caps are seperate. It's important in a push-pull output stage driving a transformer and no "balance" trimpot is necessary.

Hope that helps...
Lukasz Piotrowski
Lukasz Piotrowski
www.amtecaudio.com

analag

Re: Anybody have a full schematic for WSW 811301?
« Reply #5 on: February 16, 2009, 02:46:51 PM »
Let's put some numbers out there. I believe the E88C grids are lifted 100V above ground with 15K serving as cathode resistors. All decoupling caps can be 100uF (not saying they originally were, but can be made this value). B+ enters through 1K to the E88C and from there through 47K to the ECC83.  ECC83 plate resistors are 100K and cathodes are 1.5K. E88C anodes to ECC83 cathodes could be coupled with a value of 390K - 470K. Coupling caps might be 0.01uF , grid leak resistor looks good at 470K -680K. Grid leak is sent to ground with 120K and to B+ with 220K. Tertiary feedback resistance is variable and should be set up through experimentation. And the whole thing draws about 16mA with these values.
 
 
Audio engineering suffers from misinformation, disinformation, and downright lying more than most fields of endeavour.

rlaury

Re: Anybody have a full schematic for WSW 811301?
« Reply #6 on: February 16, 2009, 04:36:02 PM »
Thanks all.
This should get me in the ballpark!


RonL
When all you have to work with is a hammer,
everything looks like a nail.

moamps

Re: Anybody have a full schematic for WSW 811301?
« Reply #7 on: February 16, 2009, 05:17:05 PM »

Hi,
The Siemens V72b preamp looks very similar to the preamp you're inquiring about.
The V72B schematic is available from the Kubarth site.

Regards,
Milan


PRR

Re: Anybody have a full schematic for WSW 811301?
« Reply #8 on: February 17, 2009, 03:10:30 AM »
> I've never seen it DC coupled

Depends on the AC ratio you need.

If this were the only NFB in a "stiff" amp, say closed-loop gain of 30, and you DC coupled, you'd put about 300V/30= 10V on the first cathodes, which would upset them.

In this case this NFB loop is probably setting closed-loop gain just a hair less than open-loop gain, say 1000. So 300V DC in puts +0.3V DC on first cathodes....

ah-HAH! Voltages like this are "about right" for 12AX7. It is "fixed bias", but a triode has large tolerance for fix-bias since the plate also influences current. It won't vary nearly as much as a pentode.

Now if we have suitable fix-bias, we don't need "cathode bias resistor". We'd normally run ~~1K here, which cuts 12AX7 gain in half. But with DC already there, we want a much lower resistor. Say 100 ohms. The plate NFB resistor is 50K-100K, acceptable. The tertiary winding (assuming it mirrors output winding) NFB resistor could get as low as 3K, which seems low..... split the difference. Cathode resistors are 300r, from-plate resistors are well over 100K, from-winding resistors are maybe 10K to 60K for gains 1 to 4 (tapering to infinity at gain 7).

'83 plate-cathode voltage will be 0.3V*Mu for the fix-bias, 68K+300r*Mu for the self-bias. du-du-du... assuming 100K plate load for good bandwidth, the plates will sit about halfway up the 300V supply, a good point. We could aim higher for a bit more swing, but the '88 only needs 7V of grid swing and the '83 can do 60V, very ample. We could aim lower for a very slight increase of gain, though hardly worthwhile. If you discover it really sits at 100V or 200V, I'll say "OK", but finding it near 150V is OK.

> E88C grids are lifted 100V above ground with 15K serving as cathode resistors. All decoupling caps can be 100uF...

Generally agree. For a "large system" the high-Gm '88 might like >100uFd for near-nothing loss at 30Hz (rated 1dB but probably better, and we have three other poles).

Whether '88 grid resistors are 270K or 820K matters little, as long as their coupling caps are sized accordingly. They could even be above the "max" rating, because the huge cathode resistor will force design current even with volts of grid leak offset.

Any "sound" is very much about the transformers. It may not be possible to build an amp with all these specs or performance using available modern parts. The best source would be the cellar of old German radio buildings, or whoever salvaged that junk when the stations modernized.

EmRR

Re: Anybody have a full schematic for WSW 811301?
« Reply #9 on: February 17, 2009, 10:05:11 PM »
> Notice the DC coupled R's from the E88C plates back to the cathode of the ECC83.

That's a standard NFB affair. It lowers the impedance from the plate into the output transformer, extending local bandwidth.

PRR, Thanks for the comments.
I've used this in several other circuits but I've never seen it DC coupled. For example the McCurdy AU300 has a cap
in series with the R. I'm playing around with this in spice to get a feel for it.

I won't be able to spot it quickly, but there's a Radio News article from '47-48 written by a Gates Radio engineer going into various NFB variations, and compares DC versus AC coupled along with a lack of NFB.  Examples are generic versions of the preamps in the SA series consoles, and I've seen both AC and DC coupled examples in the field.  Gates also did the non-standard thing of using a low 8 dB NFB value, and included a standard passive volume pot between the stages, right in the middle of the NFB loop. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde


analag

Re: Anybody have a full schematic for WSW 811301?
« Reply #10 on: February 24, 2009, 03:39:09 PM »
  Gates also did the non-standard thing of using a low 8 dB NFB value, and included a standard passive volume pot between the stages, right in the middle of the NFB loop. 

I've done that before...it isn't necessarily such a bad thing.
Audio engineering suffers from misinformation, disinformation, and downright lying more than most fields of endeavour.

roginator

Brain is CV controlled device!!!!!! so we all are analog sequenced machines!!!! " ROBOTS "


 

Related Topics

  Subject / Started by Replies Last post
22 Replies
13438 Views
Last post June 24, 2005, 01:34:50 PM
by CJ
4 Replies
1802 Views
Last post September 09, 2005, 08:57:56 AM
by Mendelt
4 Replies
3121 Views
Last post January 15, 2007, 02:25:09 PM
by lvg_stn
2 Replies
1067 Views
Last post April 10, 2009, 03:29:26 AM
by Johan